Teaching Secondary Biology [3 ed.] 9781510462885, 1510462880

Citation preview

Teaching Secondary

Biology 3rd Edition

Editors: Michael J. Reiss and Mark Winterbottom Series editor: Chris Harrison

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Titles in this series:

Teaching Secondary Biology 978 1 5104 6256 4 Teaching Secondary Chemistry 978 1 5104 6257 1 Teaching Secondary Physics 978 1 5104 6258 8

Acknowledgements

The Publishers would like to thank the following for permission to reproduce copyright material. Photo on page 177 © Juraj Kamenicky/Shutterstock.com Thanks to Wynne Harlen and The Association for Science Education for permission to reproduce extracts from Harlen, W. (ed.) (2010) Principles and Big Ideas of Science Education. Hatfield: The Association for Science Education and from Harlen, W. (ed.) (2015) Working with Big Ideas of Science Education. Trieste: InterAcademy Partnership on pages 4, 5 and 252 of this book. Thanks to Steve Tilling and The Association for Science Education for permission to reproduce the data in Table 11.5, which is taken from Tilling, S. (2007) Outdoor science. Linking trees with energy. School Science Review, 89 (327), 11–15. Thanks to The Association for Science Education for permission to reproduce an extract from Lambert, D. and Reiss, M.J. (2015) The place of fieldwork in science qualifications. School Science Review, 97 (359), 89–96 on page 254 of this book. Every effort has been made to trace all copyright holders, but if any have been inadvertently overlooked, the Publishers will be pleased to make the necessary arrangements at the first opportunity. Although every effort has been made to ensure that website addresses are correct at time of going to press, Hodder Education cannot be held responsible for the content of any website mentioned in this book. It is sometimes possible to find a relocated web page by typing in the address of the home page for a website in the URL window of your browser. Hachette UK’s policy is to use papers that are natural, renewable and recyclable products and made from wood grown in well-managed forests and other controlled sources. The logging and manufacturing processes are expected to conform to the environmental regulations of the country of origin. Orders: please contact Hachette UK Distribution, Hely Hutchinson Centre, Milton Road, Didcot, Oxfordshire, OX11 7HH. Telephone: +44 (0)1235 827827. Email [email protected] Lines are open from 9 a.m. to 5 p.m., Monday to Friday. You can also order through our website: www.hoddereducation.co.uk ISBN: 978 1 5104 6256 4 © Association for Science Education 2021 First published in 2000. Second edition published in 2011. This edition published in 2021 by Hodder Education, An Hachette UK Company Carmelite House 50 Victoria Embankment London EC4Y 0DZ www.hoddereducation.co.uk Impression number 10 9 8 7 6 5 4 3 2 1 Year

2025 2024 2023 2022 2021

All rights reserved. Apart from any use permitted under UK copyright law, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying and recording, or held within any information storage and retrieval system, without permission in writing from the publisher or under licence from the Copyright Licensing Agency Limited. Further details of such licences (for reprographic reproduction) may be obtained from the Copyright Licensing Agency Limited, www.cla.co.uk. Cover photo © Leigh Prather – stock.adobe.com Illustrations by Integra Software Services Pvt. Ltd. Typeset by Integra Software Services Pvt. Ltd., Puducherry, India. Printed in the UK. A catalogue record for this title is available from the British Library.

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Contents

xi

2 Cells



1 17

Chris Harrison and Rachel Waterhouse



3 Energy and materials



44

Jeremy Airey and Elizabeth Lupton



4 Exchange



73

Ann Fullick and Indira Banner



5 Transport



97

Mark Winterbottom and Dan Jenkins



6 Communication and control



127

Mike Cassidy and Beverley Goodger



7 Reproduction



155

Mary Berry and Michael J. Reiss



8 Variation



181

Paul Davies and Neil Ingram



9 Evolution



207

Alistair Moore and Chris Graham

10 Biodiversity



228

Marcus Grace and David Slingsby

11 The environment





Acknowledgements and Dedication Michael J. Reiss and Mark Winterbottom





iv

1 The principles behind secondary biology teaching





Contributors

251

Melissa Glackin and Steve Tilling

12 Microbiology and biotechnology



John Schollar and Jenny Byrne

Index

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281 313

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Contributors Michael J. Reiss is Professor of Science Education at UCL Institute of Education, a Fellow of the Academy of Social Sciences, a member of the Nuffield Council on Bioethics, Visiting Professor at the Universities of York and the Royal Veterinary College, Honorary Fellow of the College of Teachers, Docent at the University of Helsinki, President of the International Society for Science and Religion, President of the International Association for Science and Religion in Schools and a Priest in the Church of England. He was Director of the Salters-Nuffield Advanced Biology Project (2000–15), a member of the Farm Animal Welfare Council/Committee (2004–12), Director of Education at the Royal Society (2006–08), a member of the GM Science Review Panel (2002–04), Specialist Advisor to the House of Lords Select Committee on Animals in Scientific Procedures (2001–02) and Chair of EuropaBio’s External Advisory Group on Ethics (2000–01). His academic interests are in science education, sex education and bioethics. Books of his include: Barmania, S. & Reiss, M. J. (2018) Islam and Health Policies Related to HIV Prevention in Malaysia, Springer; Abrahams, I. & Reiss, M. J. (Eds) (2017) Enhancing Learning with Effective Practical Science 11-16, Bloomsbury; Reiss, M. J. & White, J. (2013) An Aims-based Curriculum, IOE Press; Jones, A., McKim, A. & Reiss, M. (Eds) (2010) Ethics in the Science and Technology Classroom: A New Approach to Teaching and Learning, Sense; Jones, L. & Reiss, M. J. (Eds) (2007) Teaching about Scientific Origins: Taking Account of Creationism, Peter Lang; Braund, M. & Reiss, M. J. (Eds) (2004) Learning Science Outside the Classroom, RoutledgeFalmer; Halstead, J. M. & Reiss, M. J. (2003) Values in Sex Education: From Principles to Practice, RoutledgeFalmer; and Reiss, M. J. (2000) Understanding Science Lessons: Five Years of Science Teaching, Open University Press. Mark Winterbottom is a Senior Lecturer in Science Education at the Faculty of Education, University of Cambridge. He leads the science/biology PGCE course, as well as being course manager for the Secondary PGCE. He is a Fellow of the Royal Society of Biology, Editor of the Journal of Biological Education, Chair of the judging panel for the Biology Teacher of the Year Award in the UK and a member of the judging panel for Cambridge University Press’ dedicated teacher awards. Mark’s research interests are in Science and Biology education, teacher education and classroom environment. He has a large group of PhD students researching diverse aspects of science education, in both formal and informal contexts. He is a founding member of the Centre of Excellence in Technology Education and has been an active participant in the Cambridge–Africa Partnership for Research Excellence. Mark has authored and contributed to a wide variety of Biology textbooks for 11–14, GCSE, A level, and BTEC students, along with a number of books for teachers and Masters students in Education. Mark is a senior examiner at GCSE and A level, for both UK and international qualifications, and regularly iv

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Contributors

runs continuing professional development courses on behalf of domestic and international examination boards. He has been involved in a number of Biology and Science curriculum-development projects, as well as teacher education and development projects in the UK and around the world. Before working at Cambridge, Mark was Head of Biology in a UK upper school. He undertook his own PGCE at the University of Cambridge, and gained his PhD from the University of Sheffield, spending several months following a small bird around the Namibian veld. Chris Harrison is Professor of Science Education at King’s College London where she researches in the areas of assessment and teacher professional learning. Chris started her professional life as a Science teacher in London schools for 13 years. On joining King’s, she began her PhD focusing on classroom assessment, supervised by Professor Paul Black. Following the publication of Inside the Black Box (Black & Wiliam, 1998), she worked on the innovative KMOFAP study which looked at assessment for learning in Science, English and Mathematics classrooms. Several studies and consultancies developed from this work and the collaborative action research approach that she developed through these projects has informed many national and international projects. Chris’ research has transformed the ways teachers use assessment in their classrooms through diagnosis and feedback to learners. She encourages teachers to use classroom talk to tap into children’s thinking and use the evidence of where they are in their learning to inform next steps. Chris’ recent work has investigated STEM inquiry learning, preservice teachers’ assessment capabilities and assessment of practical skills in science. Chris is known, both nationally and internationally, for the ways she can relate research to practice, made possible through extensive experience of teacher education from both a research and a teaching perspective. She is an enthusiastic member of the Association for Science Education, working on several committees and projects and was Chair of ASE in 2015. Jeremy Airey has been a biologist in immunology research and a secondary school Science teacher. For several years, he was a senior professional development leader at what is now the National STEM Learning Centre, with a brief that included Biology, Psychology and early career secondary Science teachers. He is now a lecturer in the University of York Department of Education, where he enjoys being a tutor on the Science PGCE programme and directing the studies of the department’s undergraduates. Indira Banner is a lecturer in Science Education at the University of Leeds. She taught Biology in secondary school before becoming Head of the Biology PGCE at Leeds. Her research interests include students’ attitudes to science in schools and using art in science teaching. She has several PhD students including current school science teachers. Indira has co-edited and contributed towards books written to support science teachers and teaching in schools. v

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Contributors

Mary Berry has taught all stages of secondary science, with A level specialism in Biology for over 20 years. With 12 years’ experience as a Curriculum Leader for Science, Mary has been a coach and mentor to trainee, newly qualified and experienced teachers, as well as an examiner. Prior to teaching, Mary completed a PhD in ecology at Wye College, University of London having previously worked for the Ministry of Agriculture, Fisheries and Food. Her experience includes research on seeds and weed ecology in organic farming systems. Mary has been involved with the Gatsby Plant Sciences Summer School, Science and Plants for Schools (SAPS), and the United Kingdom Plant Sciences Forum (UKPSF). Mary is a Fellow of the Royal Society of Biology and a Fellow of the Chartered College of Teaching. She strives to encourage a love of science, and an appreciation of the role and beauty of plants. Jenny Byrne is Associate Professor in Education at Southampton Education School, University of Southampton, with expertise and research interests in biological science and health education. Jenny graduated from Birmingham University with a B.Sc. in Bacteriology. After completing a PGCE at Durham University, Jenny taught secondary biology, eventually becoming Head of Science. Jenny has taught in all phases of education including a pupil referral unit. She also worked as a health education officer and adviser before moving to Higher Education. Since then, she has taught and led undergraduate and PGCE primary and secondary science programmes, as well as supervised masters and PhD students. Her research interests include children’s knowledge and understanding of microbes and effective teaching strategies related to microbes and health education; this includes exploring the connections between science and health education, including scientific and health literacy. Mike Cassidy has taught in schools, colleges, and universities. He is a biologist and educator and currently Teaching Fellow in Science Education at Durham University. Mike previously taught Science Communication, Evolutionary Biology and Education at the University of Warwick. He has worked extensively with the Royal Society of Biology and is a Fellow both of that Society and the Linnaean Society. Mike has appeared on TV and written textbooks and Biology publications, including his book on Biological Evolution (2020). His interest in whole organism biology (particularly animal behaviour and evolution) has led to a strong interest in matters of body coordination. Paul Davies has been involved in biology education for over twenty years. He started his career teaching biology in secondary school before moving to work in the field of biology education research and teacher training, working at University College London Institute of Education (UCL IOE). Here, Paul was the leader for the Science PGCE programme and carried out research into the teaching of evolution and using technology in learning biology. Paul moved back into the school system five years ago and is currently the Head of

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Contributors

Science and Director of Teaching and Learning at Queen’s College, London. Still an Associate Lecturer at UCL IOE, and co-leader of the Biology Education Research Group, Paul enjoys bringing aspects of educational research into the school setting as well as continuing with his own research into biology education. Ann Fullick graduated in Natural Sciences from Cambridge University and taught biology in a variety of schools, eventually running a large science department and becoming an A level examiner. She has written almost 200 textbooks, widely used both in the UK and in many countries around the world. Her titles cover Key Stage 3, GCSE Biology, IGCSE Biology, A level Biology, GNVQs, BTECs, 21st Century Science, Science for Public Understanding and CSEC (Caribbean GCSE), along with around 30 books on topics ranging from infertility treatment and organ transplants to biographies of Charles Darwin and others. Ann is a very active Fellow of the Royal Society of Biology, working on their curriculum group, and was awarded the RSB President’s medal in 2020. She is also a member of the Biological Education Research Group. Her work gives her a deep insight into teaching and learning Biology both nationally and internationally. Melissa Glackin is senior lecturer in science education in the School of Education, Communication and Society at King’s College London. Melissa’s research and teaching interests include teaching and learning science outside the classroom, teachers’ beliefs and self-efficacy, in-service and pre-service teacher professional development and outdoor science and environmental education curriculum development. She is programme director for the MA in STEM Education and supervises PhD candidates in topics relating to learning inside and outside the classroom. Melissa is a trustee of London Wildlife Trust and an invited fellow of the National Association of Environment Education. Beverley Goodger has taught Biology in secondary schools in the UK since 1983. She was the inaugural winner of the Royal Society of Biology’s School Biology Teacher of the Year Award in 2013 and has been a member of the judging panel for the Award since 2014. Her other roles within the RSB have included membership of the Editorial Board for the Society’s Journal of Biological Education and of the Biology Curriculum Committee. Beverley is an Ambassador for Science and Plants for Schools (SAPS) and has worked with them to develop plant science practical protocols and resources and to write, review and facilitate online courses developed by SAPS and STEM Learning. Marcus Grace is Professor of Science Education and former Head of the Education School at the University of Southampton, where, until recently, he also coordinated the Science PGCE courses. He taught for many years in comprehensive schools in London and was a Headteacher at an international school in Tokyo. One of his main interests is in teaching and learning about socio-scientific issues, particularly those relating to biodiversity, conservation vii

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Contributors

and the environment. He is Chair of the Academic Committee of European Researchers in Didactics of Biology (ERIDOB), and Chair of the UK Biology Education Research Group (BERG), a Royal Society of Biology special interest group. Chris Graham has been teaching Biology since 2004. He is currently Head of Biology at Hills Road Sixth Form College in Cambridge. Between 2007 and 2015, he was seconded to work for one day a week on the Science PGCE course at Cambridge University. He is now seconded for one day a week to develop education resources for Science and Plants for Schools (SAPS). He is passionate about supporting teachers to open students’ eyes to the wonder of biology, and helping students to develop a deep understanding of, and great appreciation for, biology. In 2009, he led a project that won the Eden Project Environment Award and was runner up in the Rolls Royce Science Prize. He was filmed for a video supporting teachers to run a lung dissection demonstration as part of National Science and Engineering Week in 2014 and he has been involved in developing teaching resources for SAPS for many years. Neil Ingram has many years’ experience as a biology teacher in a variety of secondary schools, Head of Science and an A level examiner. He has written extensively for Nuffield curriculum projects, co-authored with Michael Roberts, and has recently co-authored a book on evolution for Oxford University Press. He is interested in curriculum development for middle years biology and is a member of the Royal Society of Biology education committee. He is senior lecturer in science education in the University of Bristol, where he teaches the biology programme. He is interested in the impact of genomics on society, and runs a course on Genetics, Society and Education in the University of Bristol. Dan Jenkins is Director of the Science and Plants for Schools (SAPS) project and Head of the Gatsby Plant Science Education Programme, based at the University of Cambridge. As a passionate botanist and science educator Dan has spent over 10 years enthusing teachers to engage with plant science teaching resources. Science education research informs the SAPS approach to design and implementation of new teaching resources to support the 11–18 curriculum. Dan’s work focuses on students’ interest in plant biology topics and supporting the development of pedagogic content knowledge with pre-service teachers. Dan has developed teaching resources to consider, for example, alternative conceptions in plant science topics, developing animations and practical lab-based activities to address these concerns. Elizabeth Lupton taught in a comprehensive school in Gateshead in North East England for 21 years. She was Head of Biology, Head of Post-16, and was part of the school’s teaching and learning team. Elizabeth worked with the University of York Science Education Group, producing materials for the Best Evidence Science Teaching project, and she was lead biologist for the ASE’s BEST STEPS diagnostic materials. She is a senior examiner for GCSE viii

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Contributors

Biology and when not working on something to do with science education or assessment, she can be found riding (and taming!) her beloved horses, Amy and Saffron. Alistair Moore is a research fellow in science education at the University of York Science Education Group with many years of experience in researchinformed development of curricula, teaching resources and assessment materials for school science. He regularly works with teachers to undertake research in schools, most recently as part of the Practical Assessment in School Science (PASS) research project, undertaken in response to reforms of the assessment of practical skills in GCSE and A level science. He also believes in applying the findings of research to transform evidence into practice and is the lead biologist for the Best Evidence Science Teaching (BEST) project, which is developing hundreds of research-informed teaching resources and making them freely available to teachers. In his spare time, he is a senior examiner for GCSE Biology, and sits on the RSB Curriculum Committee and the ASE Research Committee. John Schollar started his career as a Biology teacher in London and then as Head of Biology at a large Berkshire comprehensive school. After a secondment to a biotechnology project at the University of Reading to develop in-service training resources, John moved to the National Centre for Biotechnology Education (NCBE) as the Development Officer before taking on the role of Co-Director of the Centre. He designed and developed practical workshop activities in microbiology, biotechnology and DNA technology for teachers and technicians, plus hands-on DNA workshops for pupils. For many years, he delivered microbiology workshops for teachers, technicians and PGCE students across the UK for the Microbiology Society. His involvement in two European projects developed and disseminated biotechnology resources across Europe and he was honoured with a Doctorate from Gothenburg University for his design and delivery of biotechnology training courses for Swedish teachers. He is Vice-Chairman of Microbiology in Schools Advisory Committee (MiSAC). David Slingsby gained a PhD in plant ecology at the University of Bristol and is Fellow of the Royal Society of Biology. He combined being a biology teacher with plenty of field work at both GCSE and A level with a variety of other ecological academic activities. These included a long-term study of a classic serpentine site in Shetland from 1970 to 2010 (with the help of his A level students) resulting in peer-reviewed papers published in international journals (www.unst-serpentine.uk), serving as Chair of Education of the British Ecological Society, as Editor of the Journal of Biological Education (Royal Society of Biology) and as an A level biology Principal Examiner in SNAB biology. At the time of writing, he works as a tutor for two Open University level three modules, both in ecology, ’Terrestrial Ecosystems’ and ‘Environment – responding to change’. ix

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Steve Tilling is an ecologist with a particular interest in promoting, supporting and protecting fieldwork education. He is currently an Honorary Senior Research Associate at University College London’s Institute of Education. Until his recent retirement, he was the Director of Communications at the Field Studies Council (FSC). Throughout his FSC career, he worked in many countries with teachers, students and community groups, including some global biodiversity hotspots. That experience has reinforced his belief that science education should include strong elements of sustainability education with opportunities to explore 21st Century environmental issues through fieldwork, including experiences in distant locations as well as those closer to home and school. Rachel Waterhouse is an experienced Head of Science who has worked across a range of secondary schools in London for 16 years. She has taught across all stages of secondary science including A level Biology and Chemistry. Since completing her MA in Science Education in 2008 at King’s College London, Rachel has worked regularly as a visiting lecturer on their PGCE Biology course. Her work with PGCE programmes also extended to Roehampton University and the Sutton Schools Alliance, which led to her receiving an Inspirational Educator Award in 2015. Rachel is currently the Head of Science at Manor House School, Bookham, where she is passionate about inspiring the next leaders in science through her delivery of the Leadership Skills Programme and development of the use of technology to support learning.

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Acknowledgements The authors and editors are very grateful to the following for their advice during the preparation of this book: Chris Harrison (Subject Advisor) ASE Safeguards Committee ASE member reviewers Thank you to the reviewers of this book: Andrew Chandler-Grevatt, Dave Dennis, Pat Dower, Sofia Castro De Luz, Matthew Livesey, Linda Needham, Richard Needham, Uzma Sarwar, Greg Seal, Paul Spenceley and James Williams. Thanks also to the authors of the previous edition of this book: Nigel Skinner, Jennifer Harrison, Jenny Lewis, Neil Ingram, Susan Barker and Roger Lock. Thank you to Ralph Whitcher and the ASE’s Health & Safety Group. Finally, thanks to Marianne Cutler for project management on behalf of ASE.

Dedication

This book is dedicated to Tim King and Stephen Tomkins.

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1

The principles behind secondary biology teaching Michael J. Reiss and Mark Winterbottom

Introduction In this book, our aim is to help biology to be taught so that students at secondary school (we have in mind particularly the 11–16 age range) build an excellent understanding of the subject, enhance their interest in it and learn to connect ideas from disparate areas of biology. There are twelve chapters, this one and then eleven that look at particular areas within biology – such as ‘cells’ or ‘evolution’ – and discuss how each might be taught. This book is one of a series of three Association for Science Education handbooks, the others being parallel volumes in chemistry and physics. The first edition of this book was published in 1999, over 20 years ago; the second edition in 2011, a decade ago. This third edition retains the basic structure of the previous editions but includes a number of new authors and all chapters have been substantially revised and brought up to date. The author team has kept in mind a secondary teacher confronted with the task of teaching a specific topic, such as respiration or ecosystems, and the preparation they would need to undertake. What does such a teacher need to produce a series of effective lessons, that will also engage learners and enhance or sustain their curiosity? Some teachers will approach this task with an excellent understanding of the topic. However, we have kept in mind that not all teachers of secondary biology have a degree in the subject and that, even if they do, very few degrees cover all of secondary school biology. We hope that all teachers of secondary school biology, even if they have been teaching the subject for some time, will find much of value in here. This chapter examines the discipline of biology and discusses approaches to teaching which enable students to engage in the discipline, to build their identity as biologists and to learn conceptual ideas in the subject.

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1 The principles behind secondary biology teaching

1.1 What is biology? It is standard for biology textbooks to assert that biology is the study of life – and this is indeed the case if we avoid a narrow interpretation that might exclude the inorganic building blocks of life, important non-living features of the environment (temperature, salinity and so on) and the fate of dead organisms (decay and/or fossilisation).

Key concepts in biology There are a number of key concepts within biology, perhaps more than in chemistry or physics. There is no particular order that has been shown to be best for introducing these to students; indeed, many of them are specific to particular areas of biology. For example, a crucial realisation in cell and organism biology was that the cell is a fundamental unit. This insight is usually attributed to Rudolf Virchow who, in 1858, coined the epigram Omnis cellula e cellula (‘all cells come from cells’). Virchow was a polymath; in addition to being a biologist (with some 2000 scientific publications to his name), he was also a prehistorian and a politician. He was the first to describe and name a number of diseases and other pathological conditions, including embolism, leukaemia, spina bifida and thrombosis; he introduced hair analysis into forensics and was the first to systematise how autopsies were undertaken. At the same time, he was an antievolutionist and called Charles Darwin an ‘ignoramus’, which seems a touch harsh. Another key concept (or pair of concepts) within biology – but found in the other sciences – is to do with the flow of energy and the circulation of materials. Many students find it difficult to ‘get’ that while both energy (the law of conservation of energy) and matter (the law of conservation of mass) are conserved, there is a fundamental asymmetry in that energy moves in one direction, continuously dissipating, whereas matter circulates; this is true whether we are thinking at the cellular or ecosystem scale. Other key concepts that are restricted to biology, and which will be treated in more depth in succeeding chapters, include: ➜ Reproduction.

No organism is immortal and so all organisms need to give rise to individuals in future generations or become extinct. ➜ Heredity. In giving rise to the next generation, organisms may split into two (asexual reproduction) or produce specialised structures that enable either sexual or asexual reproduction. Sexual reproduction means that offspring typically differ from either of their parents. Information from one generation to another is carried in genes.

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1.1   What is biology? ➜ Evolution.

Over time, organisms change. A key insight of Charles Darwin and some other biologists, notably his contemporary Alfred Russel Wallace, was that natural selection is an inevitable consequence of the overproduction of offspring, what we now call genetics and the pressures exercised on organisms by the environment. ➜ Homeostasis. All organisms are able to regulate their internal environments to a very considerable degree – though this is more apparent in some (including homeotherms, such as most mammals and birds) than in others.

History of biology Not all students enjoy learning large amounts of history, but small amounts can enliven the teaching of a topic (think Mendel and genetics, van Helmont and plant growth, Francis Crick, Rosalind Franklin and James Watson and the structure of DNA). More importantly, the inclusion of history can help students get a better understanding of a topic or of the nature of science. For example, thinking about why Mendel’s work was under-appreciated for some 40 years can help students to realise that one really can be ‘ahead of one’s time’ and to appreciate the way in which understanding in science (not just in biology) depends on the social and scientific context in which a discovery is made. The story of the way in which the contributions of Francis Crick, Rosalind Franklin and James Watson to the elucidation of the structure of DNA were differentially recognised has been a feminist trope for decades. Almost every student of biology in the 14–19 age range would benefit from reading both James Watson’s eminently readable, autobiographical The Double Helix: A Personal Account of the Discovery of the Structure of DNA (Watson, 1968) and Anne Sayre’s feminist reclamation of Rosalind Franklin’s contribution, Rosalind Franklin and DNA (Sayre, 1975). The story of how van Helmont disproved the idea that plants grow by eating soil provides a simple yet effective context to learn how scientists can change scientific understanding through providing evidence to contradict current ideas. Van Helmont weighed a willow tree and some dry soil. He planted the willow tree in the soil and added water. Five years later, the willow tree had substantially gained in weight, but the weight of the dried soil was pretty much the same. He had used evidence to disprove the theory that plants gain mass by eating soil. He suggested that trees gain mass by taking in water. One hundred years later, Nicolas de Saussure provided evidence that trees gain mass from a gas in the air (that we now know is carbon dioxide).

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1 The principles behind secondary biology teaching

Big ideas in biology An influential pair of reports that link the big ideas of science to the science curriculum have been produced by Wynne Harlen and colleagues (Harlen et al., 2010, 2015). In the 2010 report, Harlen and her colleagues came up with ten big ideas of science, of which four were of biology: ➜ organisms

are organised on a cellular basis require a supply of energy and materials for which they are often dependent on or in competition with other organisms ➜ genetic information is passed down from one generation of organisms to another ➜ the diversity of organisms, living and extinct, is the result of evolution ➜ organisms

and four were about science – which apply to biology and to the other sciences: ➜ science

assumes that for every effect there is one or more causes explanations, theories and models are those that best fit the facts known at a particular time ➜ the knowledge produced by science is used in some technologies to create products to serve human ends ➜ applications of science often have ethical, social, economic and political implications. ➜ scientific

The big ideas in the science movement started because of a wish to address what was perceived to be a fragmentation of students’ learning experiences as a result of standard methods of summative assessment. Too often, it was felt, science is seen by students as requiring learning about a mass of information with many students having little appreciation of why they are learning what they are – or of how different topics aggregate into significant big ideas. Each of these big ideas was then spelt out in more detail and, in the 2015 report, progression was addressed explicitly. For example, the big idea that ‘organisms require a supply of energy and materials for which they are often dependent on or in competition with other organisms’ was organised across the 5–17 age range as follows:

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14–17

11–14

7–11

5–7

1.1   What is biology?

All living things need food as their source of energy as well as air, water and certain temperature conditions. Plants containing chlorophyll can use sunlight to make the food they need and can store food that they do not immediately use. Animals need food that they can break down, which comes either directly by eating plants (herbivores) or by eating animals (carnivores) which have eaten plants or other animals. Animals are ultimately dependent on plants for their survival. The relationships among organisms can be represented as food chains and food webs. Some animals are dependent on plants in other ways as well as for food, for example for shelter and, in the case of human beings, for clothing and fuel. Plants also depend on animals in various ways. For example, many flowering plants depend on insects for pollination and on other animals for dispersing their seeds. Interdependent organisms living together in particular environmental conditions form an ecosystem. In a stable ecosystem there are producers of food (plants), consumers (animals) and decomposers (bacteria and fungi which feed on waste products and dead organisms). The decomposers produce materials that help plants to grow, so the molecules in the organisms are constantly re-used. At the same time, energy resources pass through the ecosystem. When food is used by organisms for life processes, some energy is dissipated as heat but is replaced in the ecosystem by radiation from the Sun being used to produce plant food. In any given ecosystem there is competition among species for the energy resources and the materials they need to live. The persistence of an ecosystem depends on the continued availability in the environment of these energy resources and materials. Plant species have adaptations to obtain the water, light, minerals and space they need to grow and reproduce in particular locations characterised by climatic, geological and hydrological conditions. If conditions change, the plant populations may change, resulting in changes to animal populations. Human activity which controls the growth of certain plants and animals changes an ecosystem. Forestry which favours the growth of certain trees over others removes the food plants of certain animals and so reduces the diversity of species dependent on these plants and on other organisms in the food chain. Modern farming is designed to reduce biodiversity by creating conditions that are suited to particular animals and plants in order to feed the human population. The widespread use of pesticides to preserve one type of crop has widespread effects on pollinating insects on which many other plants depend. Human activity of this kind creates a simple and unnatural ecosystem which limits biodiversity and results in the loss of culturally valuable landscape and wildlife. (Harlen et al., 2015: 27)

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1 The principles behind secondary biology teaching

The Harlen reports have had considerable impact in the UK and in a number of other countries. In England, Northern Ireland and Wales, it is hoped that work by the Royal Society of Biology, along with the equivalent professional organisations for chemistry and physics, will mean that the next version of the science National Curriculum is informed by them. For science teachers, one of the benefits of the Harlen reports is that they can facilitate departmental curriculum planning, helping to ensure that there is coherence in student experiences. As can be seen from the above, the Harlen progression goes to post-16, and good 11–16 teaching should prepare the groundwork for post-16 biology.

1.2 Doing biology Scientists are always asking why things happen or how things happen. By asking questions like this, they may be able to come up with new theories to explain new findings, and then test those theories. Scientific ideas can never be said to be proven: every idea is potentially falsifiable if data eventually contradict it. But learning how to ask ‘how’ and ‘why’ is fundamental to educating new biologists. Such biologists may go on to an extraordinarily wide range of careers, many with people (particularly in the medical professions), some out-of-doors, and some in laboratories or other specialised sites such as zoos. Biologists are employed in an enormous number of different jobs and at every level, whether a student leaves school at the first opportunity or goes on to take a master’s degree or even a doctorate in the subject.

Practical biology Biology is a practical subject, as much as any other science. It is therefore a matter of deep regret if students sometimes experience substantially less practical work in biology than in other sciences, instead too often spending long periods of time making notes on the structure and function of organs or specialised cells. At the same time, there are a number of distinctive characteristics about biology that mean that practical work in biology differs from practical work in chemistry or in physics. For a start, many organisms are sentient, that is, capable of experiencing pleasure and of suffering (experiencing pain). This means that they cannot be used for certain experiments, whatever the educational benefits might be. Indeed, there is a move to be respectful to all living organisms even if, as in the case, for instance, of unicellular organisms, it seems certain that they are incapable of suffering.

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1.2   Doing biology

Then there is the fact that organisms, even within a species or local population, are rarely identical. At school level, a chemistry teacher does not have to worry about the possibility that different samples of copper will have different thermal conductivities; biologists cannot make comparable assumptions about their objects of study. Related to this is the issue that it can be difficult in biology to control variables in a way that physical scientists would expect. Often, with care, this can be done, even when there are multiple variables, using appropriate data collection design to remove any systematic bias. Even if it is difficult to control variables, their effect can also be accounted for in analysis through use of appropriate statistics. Nevertheless, biology does sometimes require more interpretation of data than in other sciences. Furthermore, there are times when biology can, with hindsight, be seen to have been more subjective. There is a long history of white, male biologists gathering data that ‘showed’ that women and people of other ethnicities were less intelligent than they (Gould, 1981). Much of this bias was probably unconscious – but bias it was. Finally, although all the sciences can profitably be studied out of doors (Braund and Reiss, 2004), it is especially important that such study be undertaken in biology. Although much ecology can valuably be undertaken in the laboratory, the subject comes alive when studied out of doors, whether in school grounds or further afield. It is a matter of deep regret that fieldwork is increasingly threatened in school biology in the UK (Tilling, 2018).

Mathematics in biology Mathematics is important in all the sciences but the various sciences are not the same in the use that they make of mathematics (Boohan, 2010). At secondary school level, even up to age 19, there is no need, for example, for calculus in biology, whereas chemistry and physics are helped if simple differential equations can at least be introduced (for example, when studying rates of reactions and changes of momentum). However, it is not the case that school biology always requires simpler mathematics than do the other sciences. As outlined above, biology has an especial need for statistics. Nowadays, calculators and online software can take much of the drudgery out of statistics. Furthermore, there are many excellent introductions to mathematics for biologists – though these are hardly needed pre-16. Nevertheless, what is valuable is for students to have an understanding as to why they are taking the measurements and using the statistical tests that they are.

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Biological reasoning Much of science is about reasoning: formulating hypotheses, making deductions, developing an argument and being able both to buttress it and to critique it. In biology, there is an especial need for students to develop the capacity to appreciate the importance of probability. Of course, probability plays an important role in other sciences (such as when a particular radioactive atom decays) but in biology probabilistic reasoning is important in many areas (mutations, independent assortment, whether a predator catches prey in a particular hunt, whether a tree is killed by lightning or not, whether succession takes one direction or another, and so on). One of the difficult things we want students to appreciate is when we can be pretty sure about what will happen next in biology as opposed to when there are a number of possibilities. Biology is also noticeable (though it shares this feature with parts of earth sciences and cosmology) in the importance of historical reasoning. To get a good understanding of the history of life over the last three and a half thousand million years or so requires the ability to imagine and then to reason historically.

Biology in context Some students love ‘pure’ biology but most are fascinated by biology in context. The student who may have little interest in the semi-permeability of membranes may become captivated by the realisation that the various problems that result from having cystic fibrosis can all be traced back to damage to certain proteins that carry ions across such membranes. As a science, biology is fortunate in that so much of it can be taught in context. Perhaps two contexts stand out: health and the environment. There was a time when biology teaching about health for 11–16-year-old students consisted of little more than diatribes against cigarette smoking and the use of illicit drugs along with a litany of things that could go wrong with various parts of the body (everything from vitamin deficiencies to cheerful lists of sexually transmitted infections). Plus ça change – and yet the difference now is that there is far more of a link from molecular biology through cell biology to whole organism biology, as in the cystic fibrosis example above. Teaching about the environment has changed too over the years. No longer are contexts dominated by oil spills, the grubbing out of hedgerows, acid rain and the hole in the ozone layer. Nowadays, two anthropogenic instances of environmental damage stand out: climate change (including global warming, more extreme weather events, ocean acidification and rising sea levels) and the ever-accelerating loss of biodiversity.

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1.3   Learning biology

Ethics in biology Every science needs to take account of ethical implications (Jones et al., 2010), but no science more so than biology. Indeed, almost every biology topic seems to throw up ethical issues. Should we change the genes of individuals to prevent genetic diseases? Is it right to exterminate certain species, such as the Anopheles mosquitoes that transmit malaria? At what stage during development does an embryo become a person or does this happen at fertilisation? How much money should we spend conserving a species so that it does not go extinct? Should badgers be culled to prevent the spread of bovine tuberculosis? And so on. Too often, biology courses simply raise such ethical questions. While this is useful, it can overwhelm students. To help them move forward, they may benefit from being taught one or more ethical frameworks within which to consider such ethical questions (for example, Levinson and Reiss, 2003).

1.3 Learning biology When learning biology, students should not only learn conceptual ideas, but also get a good understanding of what biology is and how it is done. By designing teaching and learning approaches which enable students to learn, and which enable learners to experience ‘doing’ biology, we should give students the chance to develop the feeling that biology ‘is for me’ and is also of broader value to society. This means that teaching and learning requires individuals to think.

Constructing understanding Building learning by requiring students to think ideas into existence complies with constructivist ideas about learning. The theory of social constructivism says that such building of ideas happens better in social interaction with others, such as a teacher or a student’s peers. Such interaction scaffolds students’ developing understanding. The way in which learning activities are designed by a teacher enables such scaffolding to take place. Hence, teachers have to consider a learner’s starting point, and how best to enable (or scaffold) them to build up ideas. Engaging in biological reasoning provides an excellent framework, where students make deductions from observational and experimental evidence that they can share and hone with others. Thinking together through a rich diet of talk is essential for developing successful learners and building biological reasoning skills. When educational dialogue is working well, students listen to each other, they share their ideas,

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they justify their ideas and engage with each other’s views. The teacher’s role in this is important, and includes inviting students to build on each other’s ideas (‘Do you agree?’, ‘Can you add?’), challenging ideas (‘Are you sure?), inviting reasoning (‘Why?’, ‘How?’), co-ordinating ideas (‘So we all think that …’), connecting (‘Last lesson …’), inviting reflection (‘What have you learned?’), guiding the dialogue or activity (‘Have you thought about …?’), and inviting original ideas (‘What do you think about …?’).

Conceptual change Rich dialogue can also help students to engage with their own prior knowledge, drawing on different funds of knowledge across increasingly diverse classrooms. Indeed, some of learners’ prior ideas can be very different to scientifically accepted knowledge. These ideas can be labelled misconceptions or, perhaps better, alternative conceptions, as many such ideas are simply learners’ attempts to make sense of their world using ‘common sense’ rather than scientific logic (Driver, 2014). It is difficult for students to give up their alternative conceptions, so lessons and learning activities need careful design. Teachers need to know the alternative conceptions their students hold, and students need opportunities to make their ideas explicit, to encounter alternative ideas, and to assimilate such ideas into their thinking. Diagnostic tasks are useful tools to help teachers uncover alternative conceptions, some examples of which are listed below: ➜ Multiple

choice questions, if well-designed, may have options which include the scientific view, alongside common alternative conceptions. ➜ Sometimes a student’s understanding may be more nuanced, and asking them to express how confident they feel about each answer may give the teacher a better insight into their understanding, and a better starting point for addressing their difficulties. ➜ Asking students to identify whether statements are correct, partially correct or incorrect, justifying their ideas with reasons, can help explore the cause of students’ difficulties, as can more open-ended questions. ➜ Refutation tasks are also useful, asking students to explain why a particular alternative conception is wrong. Making students realise that their ideas may be naive, by generating conflict in their mind between their own ideas and evidence, is one approach to changing their ideas. For example, a teacher may ask students to make a prediction before a piece of practical work. Their prediction is based on their prior ideas, and makes those ideas explicit to the teacher, but also to the students themselves, because the data they collect may conflict with

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1.3   Learning biology

those ideas. You can do the same with simulations: ask students to make predictions, run the simulation, and generate that kind of conflict in their minds. Concept cartoons can help achieve the same aims, but through dialogue (Keogh and Naylor, 2000). A concept cartoon provides a picture of a scientific phenomenon, with different people giving alternative explanations of that phenomenon. Inviting students to say what they think or to decide how much they agree with various statements, and then justify their position to each other, creates dialogue that can help students to unpick their current understanding. However, such cognitive conflict is not the only way to think about conceptual change. An ‘evolutionary change’ model views conceptual change as being a more gradual and ongoing process, where students’ prior conceptions are used as resources for learning, regardless of whether these prior conceptions are scientifically accurate notions or misconceptions. You can think of this as the step-by-step development of understanding, building pieces of knowledge upon existing understanding. Whichever model a teacher exploits (and many would adopt both, depending on the circumstances), the teacher has to structure ideas in sequence, ensuring good progression, with ideas building on each other over time, and making connections to other curriculum areas, both within biology, within science, and in other subjects.

Problems with language Language is one of our main tools to help students learn biology. But it can also be our biggest obstacle. This is not just because of unfamiliar complex scientific words (like homeostasis or nephron), but because some words often have different meanings in biology. For example, many people would talk about artificial respiration when referring to the ‘kiss of life’, but respiration in a biological sense is the chemical process which releases energy from glucose (or other substrates) and occurs in cells. Even common, everyday words like random or abundance may have very particular meanings in biology. Because we are biologists, it can be very easy to use such complex vocabulary without even thinking about it. As a result, we need to focus on clear, logical explanations, rather than defaulting to specialist language. For example, talking about ‘more stomata on the lower epidermis’ makes no sense unless you have explained stomata and their role, have spent time getting students to examine the structure of a leaf, and have discussed with them how all these relate to the processes of transpiration and evaporation.

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Even then, some of the grammatical conventions we use in biology can be a challenge. Logical connectives are essential to constructing a logical scientific argument because they help make links between claims and data, and because they can be used when making comparisons and when highlighting differences. These are words which suggest cause (for example, because), addition (for example, and), time (for example, after/before) and opposition (for example, otherwise). However, students can find their use difficult. Finally, science does not just communicate through words, but through diagrams, physical models, graphs, symbols, mathematics and equations. Some of these may make ideas easier to understand (imagine trying to learn the cross-sectional structure of a leaf through reading about it without a diagram). But others, such as mathematics, may present obstacles to students’ understanding.

Helping students with language Because language can be a challenge, it is important to plan for talk, reading and writing.

Talk We wrote above about encouraging dialogue, but that can happen as a whole class, in small groups, or in pairs. Groups tend to work better when each student has a particular role, and there are various grouping strategies which can encourage talk in different ways. For example, jigsaw (students team up in expert groups and then split apart into new groups, with one member from each expert group), snowball (pairs discuss an idea, then team up into groups of four, then into groups of eight, etc.), envoys (a group discusses an idea and then sends an envoy to explain their ideas to a different group), and spokespeople (one person summarises a group’s discussion) are easy-toimplement ways to encourage talk.

Reading Reading is important; so much so that students should be able to read, re-read and reflect upon text, building connections with prior knowledge. Constructing and deconstructing text to help understand it can help in this process, and may include strategies like: ➜ completing

text, tables or diagrams (for example, completing missing words, completing missing labels on diagrams or completing a compareand-contrast table) ➜ sequencing and labelling (for example, labelling the digestive system, or ordering key phrases which describe the process of natural selection) ➜ matching (for example, matching key words to definitions) ➜ predicting (for example, predicting the final words of a sentence, or the question, given an answer) 12

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1.3   Learning biology ➜ labelling

and highlighting (for example, highlighting words which relate to a particular organ system in a piece of text) ➜ processing a piece of text (for example, using information given in a text; answering questions about a text; converting a piece of text to a flow chart).

Writing Writing has a number of functions in biology teaching and learning. Writing is first and foremost a tool to help prompt students to think and to learn. Writing helps students to formulate their ideas (for example, creating a graphic organiser or preparing a summary of the key points). Writing helps students to engage in biological reasoning (for example, writing up a practical inquiry, setting out methods, prediction, findings and conclusion). Writing helps students to communicate their understanding, not just in conventional ways, but in more creative ways, such as stories, newspaper articles or blogs. Allowing students to engage with language, and to use language to help develop and communicate their skills and understanding, is a challenge, but essential to effective learning of biology.

Practical work and inquiry Practical work is part of what biologists do, and engaging with it (including fieldwork) gives students a sense of what it means to be a biologist. It can also help students to learn. It may help students to develop: ➜ essential

practical skills (for example, using a measuring cylinder or a micropipette) ➜ their observation abilities (for example, in dissection or drawing activities) ➜ problem-solving skills (for example, working out which urine sample is from a diabetic) ➜ classification skills, by recognising that groups of organisms share similar structures ➜ conceptual ideas through investigation (for example, using bicarbonate indicator to investigate the effect of light intensity on photosynthetic rate). Practical work may also be used by a teacher as a demonstration to support an explanation (such as the way valves work in the heart) or simply to illustrate a phenomenon (for example, testing a leaf for starch). When you use a piece of practical work as part of a learning activity, make sure you understand the purpose. Think to yourself, ‘What are you asking students to do or see?’, ‘Will they do/see it?’, ‘What will they learn how to do?’ and ‘What conceptual learning will take place?’ (Reiss and Abrahams, 2016). If you don’t think about these questions, you won’t consider how to draw learning from the practical. For example, a student can test a leaf for starch, but unless you ask the right questions, they won’t learn anything 13

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from what they observe, and may get lost in the complexity of what they are doing, rather than spending time thinking about and understanding their results. If the practical work you choose involves investigation, then your students may be doing inquiry. In biology, good inquiry is sometimes hard to fit into an hour lesson block, and so teachers often default to more illustrative practical tasks. Biological inquiry may involve extended project work, data collection over time, and inferences from observation, rather than just from experimentation. Inquiry does not have to involve practical investigation, but may feature any of the following components: ➜ a

question to investigate of evidence ➜ analysis of evidence ➜ explaining the evidence ➜ connecting their explanation to existing scientific knowledge ➜ communicating and justifying their explanation ➜ reflecting upon and evaluating their inquiry. ➜ collection

Teachers can choose whether students need to undertake an inquiry which involves all of these skills, or whether they could simply provide data to be analysed, or an experimental protocol and findings which students then reflect upon and evaluate. Often, teachers provide a mixture of inquiry types over the years of secondary education so that students experience open, closed and guided inquiry work. Some inquiries may fit alongside physical science inquiries quite well (for example, osmosis and enzymes), while others require a different approach (such as estimating the diversity index in a meadow or designing a 75 g ‘healthy’ snack bar).

Learning in context The inquiry approaches above can be particularly effective when asking students to learn biology through consideration of biological contexts. As biology teachers, we are fortunate. Our students feel a connection to biology because they themselves are living things, living in their own habitat and ecosystem. Because of this, when students learn about biology through contexts, they are often more motivated, and their interest in biology lessons increases. Contexts also help students perceive relations between science and everyday life, enhancing the relevance of biology lessons. Contexts are often introduced through the use of media, including newspapers and magazines. Topics which are conducive to a context-led approach include health, agriculture, genetics, global warming, sustainability, disease, habitat destruction and drug abuse. However, it is

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1.4   Final thoughts

possible to use contexts across the curriculum to foster students’ motivation and their perception of the relevance of biology to everyday life. This is important because biology teaching should foster students’ sense that biology ‘is for them’, and creating a motivating and relevant classroom is important.

Using digital technologies Inquiry can also be made easier by the use of digital technologies. Being able to observe changes in variables in biological contexts can take time (for example, growth of bamboo over a week) and can involve lots of laborious data collection. Data logging offers a solution, by automating data collection and offering more accurate data collection, allowing time in lessons to undertake analysis, interpretation of results, formation of conclusions and general critique. Data-logging sensors can measure changes in atmospheric oxygen, light, humidity, temperature and carbon dioxide, and can investigate biological processes as broad as respiration, pollution, osmosis, transpiration, photosynthesis, homeostasis and circulation. Mobile phones can be useful as there are numerous apps for supporting identification of organisms, etc. Mobile phones can also be used to record data such as the growth of bacterial colonies on a Petri dish or by taking photographs or videos down a microscope. Small attachments can be purchased that enable mobile phones to function as microscopes. Given the microscopic and sub-microscopic nature of biological processes (such as membrane transport), animations are essential to help students visualise such processes, as are simulations, which allow students to undertake inquiry on such processes by altering independent variables and observing the impact on dependent variables. The use of collaborative technologies, such as WhatsApp, wikis and blogs, can help students to work together remotely on extended project tasks.

1.4 Final thoughts What you have read above sets out a vision for learning biology. Learning biology is not about learning the contents of a textbook. Learning biology is about conceptual learning, learning about what it means to do biology and learning what it means to be a biologist. We hope that this book helps you to achieve these aims.

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References Boohan, R. (2010) The Language of Measurement. Hatfield: The Association for Science Education. Braund, M. and Reiss, M. J. (eds) (2004) Learning Science Outside the Classroom. London: RoutledgeFalmer. Driver, R., Squires, A., Rushworth, P. and Wood-Robinson, V. (2014) Making Sense of Secondary Science, 2nd edn. London: Routledge. Gould, S. J. (1981) The Mismeasure of Man. New York: W. W. Norton. Harlen, W. (ed.) (2010) Principles and Big Ideas of Science Education. Hatfield: The Association for Science Education. Available at: www.ase.org.uk/bigideas Harlen, W. (ed.) (2015) Working with Big Ideas of Science Education. Trieste: InterAcademy Partnership. Available at: www.ase.org.uk/bigideas Jones, A., McKim, A. and Reiss, M. (eds) (2010) Ethics in the Science and Technology Classroom: A New Approach to Teaching and Learning. Rotterdam: Sense. Levinson, R. and Reiss, M. J. (eds) (2003) Key Issues in Bioethics: A Guide for Teachers. London: RoutledgeFalmer. Naylor, S. and Keogh, B. (2000) Concept Cartoons in Science Education. London: Millgate House. Reiss, M. and Abrahams, I. (2016) Enhancing Learning through Effective Practical Science. London: Bloomsbury. Sayre, A. (1975) Rosalind Franklin and DNA. New York: W. W. Norton. Tilling, S. (2018) Ecological science fieldwork and secondary school biology in England: does a more secure future lie in Geography? The Curriculum Journal, 29 (4), 538–556. Watson, J. D. (1968) The Double Helix: A Personal Account of the Discovery of the Structure of DNA. London: Weidenfeld & Nicolson.

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2 Cells

Chris Harrison and Rachel Waterhouse

Topic overview Cell biology is the study of cell structure and function, and it revolves around the concept that the cell is the fundamental unit of life. Focusing on the cell permits a foundational understanding of the processes that tissues, organs and organ systems are involved in. While most students have a concept of what ‘living’ means, they often find it difficult to articulate and tend to define living in terms of what they believe a living organism similar to themselves is capable of: mainly movement, growth or needing food. Life can be thought of as the result of the various interactions between the many different chemical substances that make up a cell and the processes that allow an organism to function.

2.1 Cell biology Cell theory is one of the basic principles of biology. Cell theory states: all living organisms are composed of cells and all life comes from pre-existing cells. Organisms may be unicellular or multicellular. Credit for the formulation of this theory is given to the German scientists Theodor Schwann, Matthias Schleiden and Rudolf Virchow. The term ‘cell’ today describes a microscopic unit of life that separates itself from its surroundings by a thin partition, the cell membrane. The two primary kinds of cells are eukaryotic cells, which have a true nucleus containing DNA (deoxyribonucleic acid) surrounded by a nuclear membrane, and prokaryotic cells, in which the DNA is coiled up in a region called the nucleoid. DNA is also present in smaller pieces of nucleic acids called plasmids spread through the cytoplasm. The DNA and RNA (ribonucleic acid) carry the genetic information necessary for directing cellular activities, mainly through the control of protein synthesis in the cytoplasm. Cells contain organelles (literally ‘little organs’) that carry out specific functions necessary for normal cellular operation. One of the largest organelles in the cell is the mitochondrion. Mitochondria contain enzymes that drive the reactions of respiration (see Chapter 3). Most organelles have a surrounding membrane but some, like ribosomes (the site of protein synthesis), do not. The organelles are situated in a jelly-like substance called cytoplasm and surrounded by the cell membrane.

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This chapter begins with a discussion of some of the characteristics of life that are displayed by whole organisms and how these processes relate back to cells and the many chemical reactions that occur within them. We then provide guidance on using microscopes to view cells and tissues. The remainder of the chapter is mainly concerned with life processes at the molecular and cellular level. An understanding of this from a cell biology perspective underpins the study of the many other aspects of biological science that are addressed in other chapters (Chapters 3–7).

Prior knowledge and experience Some students will know that living things are made up of cells but they are unlikely to have studied this topic in any depth. Research has shown that younger students sometimes think the words ‘molecule’ and ‘cell’ have the same meaning and this can give rise to a generalised concept of living things being made up of ‘very small units’ that can be molecules or cells. In addition, students sometimes think that many of the non-cellular things studied in the context of biology lessons (such as proteins, carbohydrates and water) are actually made of cells.

A teaching sequence The activities described in the first three sections are aimed mainly at students aged 11–14 years old. The suggested approach is intended to help students gain an overview of what biology is about through encouraging them to observe carefully and note similarities and differences. The Characteristics of Living Organisms (COLO) outlines some of the processes that are recognisable in living things and also highlights the differences between living and non-living things. To maintain links between ideas, teachers might consider ‘How small can a living thing be?’ and use practical microscope activities to achieve this. Cell theory is a key idea in biology and realising that what happens at cellular level has implications for organisms is important. The final part of the teaching sequence picks up on ideas of what is happening at molecular level in cells and how these processes support life in organisms; this work is more suitable for 14–16-year-old students. This includes an introduction to enzymes and the processes of gas exchange, respiration and excretion, which are dealt with in more detail in Chapters 3 and 4.

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2.2   Characteristics of living organisms (COLO)

2.2 Characteristics of living organisms (COLO)

KEY ACTIVITIES

Most children’s concept of living is in relation to themselves. They ‘know’ that they are alive and so if something ‘does’ things similar to them, then it must be living too. Pets, garden birds, worms, insects and spiders all move and feed and grow and so are recognised and categorised as living. However, because almost all plants and some animals, such as barnacles, coral and limpets, seem to stay in one place, children are less sure whether to class these as living. Young children often believe that the wind is living because it moves, and fire is alive because it consumes material; while such ideas might be considered naive, stories and mythology they have encountered at home sometimes support these ideas. The standard way in which COLO is introduced is using the acronym MRS GREN (M = movement, R = respiration, S = sensitivity, G = growth, R = reproduction, E = excretion, N = nutrition). While this approach does help many students to remember the names of the seven characteristics, it does not necessarily help them understand the processes that go on in living organisms.

Observing living organisms It helps to give students an opportunity to observe a range of living organisms in settings which are as natural as possible. These might be: 1 setting up a large tank with a layer of woodland soil or garden soil and/or a pile of fallen leaves for students to search for any insects, worms, spiders, woodlice, snails, slugs, mites, etc. 2 pond dipping or stream dipping 3 a field trip to an unkempt patch of land, a rotting log in the school grounds, a local park or a wood. With each of these activities, students can observe what the animals and plants are doing, possibly using a magnifying glass or capturing the animals from each habitat using pooters, nets, plastic spoons, etc., and observing their living organisms back in the classroom with a binocular microscope. While the students are likely to observe animals moving or feeding or possibly reproducing (either directly or by finding eggs), they will also gain some ideas of how these organisms respond to light, moisture or other organisms. Their observation of plants may illustrate seeds, fruits and possibly reproductive structures of mosses, and they may find plant leaves with holes where animals have fed on them.

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2 Cells

While you will be able to find video clips of living organisms, it is advisable to do ‘live’ observations, as these help students in developing their observational skills and in building a respect for working with living organisms. The time needed for careful observation is rarely scheduled into video footage, plus being so close to the organisms and realising they can interact with you is a different experience to seeing them on a screen. In each case, try to return the animals to their original site and move logs, stones and leaves gently back into position. The aim of this type of activity is for students to develop their observational skills and relate their findings to COLO, rather than getting preoccupied with identification or classification. However, it does help if you know the difference between a slug and a snail, and an insect and a woodlouse. The Field Studies Council do free ‘bug hunting’ sheets and sell some reasonably priced identification guides for different habitats.

Tip A good resource to be used both for diagnostic assessment and to support learning in this area has been produced by the Best Evidence Science Teaching project (BEST) at the University of York, whose website address is given at the end of this chapter. It is worth making clear to students that animals that do not move from place to place, including many marine organisms such as adult coral, barnacles and mussels, live in an environment that is moving; their food comes to them and they filter it out of the water around them. Students could compare the characteristics of living organisms with some non-living things that ‘do’ things that might initially suggest that they are living: for example, battery-powered toys or robots that move and appear to be responsive. The observable key ‘living’ processes that they do not display are reproduction, respiration and growth.

Science in context Many living processes can be discussed in connection with life cycles. Animals such as stick insects are easily maintained in school laboratories, and students can study their growth and development from the egg to the adult stage. This could include a consideration of their food preferences and patterns of movement. Plants that complete their life cycle in a relatively short space of time can also be studied. For example, pea or bean seeds planted in early spring will become mature and produce seeds before the end of the summer term. White

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2.2   Characteristics of living organisms (COLO)

icicle radishes can complete their life cycle in around 60 days. ‘Fast plants’ (more correctly called rapid-cycling brassicas) complete their life cycle in about 35 days, if kept under a light bank. Details of how to grow and use these plants in schools can be obtained from Science & Plants for Schools (SAPS) whose website address is given at the end of this chapter. So, in the early years of secondary school, students should understand that living organisms have seven characteristics in common: ➜ Movement:

they can move and change their position, usually to find food, a mate, conditions they prefer or to escape from predators. ➜ Reproduction: they can make more of the same kind of organism as themselves. ➜ Sensitivity: they can detect stimuli, such as light, dampness, texture or gravity, and respond to them. ➜ Growth: they can permanently increase their size by increasing the number and/or size of their cells. ➜ Respiration: they have chemical reactions that break down food molecules inside their cells to provide energy (in the form of ATP) for movement or making new biological molecules. ➜ Excretion: they can remove waste products of metabolism, some toxic chemicals and excess substances which might stop cells working properly. ➜ Nutrition: they can take in and absorb nutrients (food), such as organic substances and mineral ions. These nutrients contain the raw materials or stored energy needed for making new cells in growth and tissue repair. It is worth discussing the similarities and differences for COLO between plants and animals. Green plants make their own food (glucose) using energy from sunlight to combine carbon dioxide and water in a process called photosynthesis. Most animals respond quickly to stimuli by moving towards or away from the stimulus, while most plants take much longer to respond. Speeded-up video can show plants moving in relation to light (think about a plant on a window-sill leaning over towards the light). Some plants, such as sensitive mimosa (Mimosa pudica) and Venus flytrap (Dionaea muscipula), move faster in response to touch, while fake shamrock/wood sorrel (Oxalis triangularis) moves relatively quickly in response to light intensity. It is sometimes difficult to get students to understand that plant cells respire and need oxygen in exactly the same way as animal cells; they often confuse respiration with photosynthesis or believe that plants only respire in the dark. It is important to stress that the cells in living things respire all the time; if they didn’t, the cells, and hence the organism, would be dead! One way of addressing this problematic area is to demonstrate that plants respire. Hydrogencarbonate indicator can be used to demonstrate that these organisms

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KEY ACTIVITY

respire and produce carbon dioxide. Hydrogencarbonate indicator is a very sensitive pH indicator (details for how to make and use this very useful indicator can be found on the CLEAPSS website, the address of which is given at the end of the chapter). It has a red colour when in equilibrium with the carbon dioxide levels of atmospheric air. When the carbon dioxide level rises slightly, it changes through orange to a yellow colour. This happens because the extra carbon dioxide that becomes dissolved in the indicator solution forms a very weak acid called carbonic acid.

Investigating respiration One way to challenge misconceptions about living/non-living things, and also about plants carrying out COLO, is to take five boiling tubes with gauze platforms above a 6–8 cm depth of fresh, equilibrated hydrogencarbonate indicator. In tube 1, put a small invertebrate such as a woodlouse or maggot. In tube 2, a chunk of apple. In tube 3, a chunk of mushroom. In tube 4, a couple of cress or other seedlings. Tube 5 has no organism in it and is the control. Place a bung in each tube and start a stopclock. Ask students to predict in which of the tubes the hydrogencarbonate indicator will change colour and to suggest an order from fastest to slowest of those that will change. Ask students to discuss with their partners how they have made their decisions and then facilitate a class discussion, while giving each tube an occasional swirl to encourage the air in the tubes to mix with the indicator. Check the colour changes by holding all five tubes against a white background and comparing each of tubes 1–4 with the control tube. If you use freshly made up indicator, you can get a change in colour in around 20 minutes.

woodlouse - hydrogen carbonate turns yellow

apple - hydrogen carbonate turns yellow

mushroom - hydrogen carbonate turns yellow

seedlings - hydrogen carbonate turns yellow

control - hydrogen carbonate remains red

Figure 2.1  The results of the hydrogencarbonate practical activity

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2.2   Characteristics of living organisms (COLO)

With this practical, students often feel that tube 1 should change first because they recognise that the invertebrate is an animal and so it respires. They often believe that tubes 2 and 3 will not change because they contain plants (younger students often think of mushrooms as plants), or because they contain only part of the plant (and so in the students’ view are dead). Some even believe that the mushroom is non-living because of its appearance. So, students may be surprised when the indicator in tubes 2 and 3 does change colour, and often (because of the greater number of cells in a chunk of apple or mushroom than in the invertebrate) these change quicker. Tube 4 with the seedlings also changes, though generally slower than tubes 1–3, and this can spark an interesting discussion about respiration and photosynthesis occurring at the same time and allow you to begin to work on misconceptions in this area. The indicator will turn yellow when the respiration rate of the seedling is above that of the photosynthesis rate and so you can ask students to predict what would happen to the colour if the seedling developed more leaves, or if you covered the tube with black paper, or if you increased the light intensity. These relatively fast changes from low carbon dioxide (red) to higher (yellow) to very low (purple) with the hydrogencarbonate indicator enable you to discuss which processes are happening at the cellular level in terms of the net outputs of respiration and photosynthesis. (Respiration produces carbon dioxide; photosynthesis requires it.) As students build their biological knowledge, the characteristics of living organisms can be expanded to the following. Living things: ➜ are

highly organised and more complex in structure than non-living things ➜ take energy from their environment and transform it from one form to another, through respiration ➜ respond to stimuli ➜ grow, develop and reproduce ➜ through evolutionary pressures, become adapted to their mode of life. A key idea that needs emphasising is that, in multicellular organisms, life processes are supported by the organisation of cells into tissues, organs and organ systems. It might also be worth pointing out here that many organisms, particularly birds and mammals, regulate their ‘internal environment’; the technical term for this is homeostasis.

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Enhancement ideas Living things that do not show any obvious signs of life, such as frog spawn, seeds and lichens, could be brought into lessons and students asked to explain how they would try to find out whether they are actually alive. Students are often intrigued by the possibility that life exists on other planets, so they could be asked to devise a series of investigations that could be carried out to find out whether life exists elsewhere in the Universe. Younger students could draw and label an imaginary animal or plant that displays the characteristics of life and is adapted to living in particular conditions, such as very hot or very cold climates. You could then get the students to swap their drawings with each other and draw potential predators or prey for the imaginary animals or plants. Their ideas could be displayed as posters.

2.3 Using microscopes Microscopes are essential tools in biology and, if used appropriately, can play a vital role in developing students’ knowledge and understanding of many aspects of living processes and organisms. All students will have seen magnified images in books, magazines and on television. Many will have used magnifying glasses to study small things and some will have used (and may even own) a simple microscope. Very few students will have received systematic instruction in the use of microscopes. To capitalise on the learning opportunities that microscopes offer, it is important for students to be motivated by the experience of using a microscope. A disappointing first use, where little is observed, can permanently disengage even the keenest learner. Most schools have sets of optical microscopes. Many schools also have digital or USB microscopes, which are easy to use and, with the appropriate software, enable images to be captured and annotated on computers and used in conjunction with interactive whiteboards. USB microscopes use incident, rather than transmitted, light so are good for looking at surface features of specimens and will also provide good images of slides placed on a white background. Students can also take photographs with their mobile phones directly through the eyepiece lens (where they put their eye). This works surprisingly well, although they have to move their phone very carefully to get it in the right position. In fact, most students are fully aware of magnification, and ‘resolution’, from using phone cameras.

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2.3   Using microscopes

Technology use There are a range of apps and games that ask the player to guess an object that is very ‘close up’. This is something your students may be familiar with so it makes a good learning hook as the starter for this topic, as well as an opportunity for challenging the use of non-scientific language. Start lessons by displaying high-quality images that are likely to engender interest, and discuss how these images were obtained and the value of being able to magnify things when trying to find out more about them. Students can try to guess what the projected image depicts as it is gradually revealed. This can lead into discussions about working scientifically and the application and implications of science, by consideration of the relationship between technology and science, including the way that developments in microscope design and microscopy techniques have been so important in helping biologists to understand the cellular basis of life. Robert Hooke (1635–1703) was a scientist who developed scientific instruments and apparatus to extend human perception. He developed an improved microscope and in Micrographia (1665) was the first person to use the term ‘cell’ in a biological context. Students are usually unaware that Hooke named them cells because they looked like monastery bedrooms! Hooke and his contemporaries believed that cells were only found in plants and it was not until the 1830s that cells began to be considered as fundamental units of life. The Nobel Prize website has some very useful information about the history of microscopy with useful links to the Nobel winners responsible for some of the advances in microscope technique.

The functions of each part of a microscope The majority of schools now have electric microscopes with built-in lamps; however, younger students may use microscopes with mirrors beneath the stage for reflecting light from a bench lamp or the sky (not direct sunlight) towards the object being viewed. Eyepiece lenses usually magnify ×10 and there are often three objective lenses with magnifying powers of ×4, ×10 and ×40. When first using microscopes, it is sensible to use only the low- and medium-power objective lenses. You should stress to students that microscopes are complicated and delicate pieces of equipment. It is a good idea to spend a whole lesson helping students become familiar with

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the particular type of microscope that they will be using. A useful exercise is to ask students to look for similarities and differences between their microscope and a diagram of a microscope from a textbook. The functions of each part should be explained and a set of ‘rules for using microscopes’ formulated jointly with the students.

A set of rules for using microscopes Microscopes are expensive and delicate instruments. To ensure students look after them properly, instruct them to do the following: l carry

them carefully with one hand beneath and the other supporting the body of the microscope l place them in the middle of the bench and away from sinks but in a position where you can look comfortably down the microscope eyepiece l never touch the end of the lenses or try to take the microscope apart l when finished, leave the microscope set on the lowest power objective lens with the body (or stage) racked fully down so that no strain is put on the cogs that move these parts l pick up microscope slides and coverslips by their edges to keep them fingerprint free l always clean away any spillage on the slide or stage so that no solutions get onto the lenses.

Using microscopes for the first time First use needs to be exciting so it is wise to use specimens that can be seen with the eye and do not require mounting under a coverslip. This will help to ensure that students achieve visual success straight away. Suitable items to view include small crystals, such as salt or sugar, strands of students’ own hair or small pieces of cloth, where students can see the weaving patterns and textures. They can also view ‘secret’ writing, printed on good-quality paper in three-point font. A further task is observing prepared slides of whole small animals such as fleas, or parts of animals such as insect mouthparts or wings. Before viewing these objects with a microscope, students could make observations with the unaided eye, hand lenses, mounted magnifiers and low-power binocular microscopes. The best way to use a hand lens is to hold the lens about 3 cm from one eye and bring the specimen into focus close to the lens. This technique enables students to concentrate their attention on the specimen being observed.

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An important aim when using microscopes for the first time is to impress on students the usefulness of microscopes for extending the power of our sense of sight. To help develop this idea, students could write descriptions of what the objects look like when viewed without being magnified and then when magnified using a hand lens and, separately, a microscope.

Teaching points when using microscopes Students should follow these instructions when using a microscope: l Using

the lowest power objective lens, adjust the angle of the mirror (or the intensity of the built-in lamp) to obtain a uniformly bright (but not too bright) field of view. l Place the prepared microscope slide on the stage with the specimen directly beneath the lowest power objective lens. l Looking from the side, use the coarse focus control to rack the low-power objective downwards (or the stage upwards; it depends on the microscope) until the lens is as close as it will get to the slide. On some microscopes, there is a stopping mechanism that prevents the lenses from touching the slide. l While looking through the eyepiece lens, slowly move the lens away from the slide with the coarse focus control until the object is in focus. l Slowly, move the slide across the stage to centre the object in the field of view. The image will move the opposite way to the object; this may take students a little while to get used to. l Adjust the illumination to gain a better image. With either too little or too much light, less detail will be seen. This is particularly important with very thin specimens, which may not be seen at all if the illumination is too bright. l Rotate to the next power objective lens. When rotating to the highest magnification objective lens, encourage students to look at the slide at eye level, just to be sure that the rotating objective lens will not collide with the slide. Most microscopes used in schools are parfocal, which means that if the object is in focus when viewed under one objective lens, it will remain in focus when a different lens is used. If the object does not appear in the field of view when a higher power objective lens is moved into position, this is usually because the object was not accurately centred in the field of view with the lower power lens. When the object appears visible, use the fine focus control to sharpen the image and adjust the illumination to improve the image contrast.

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Making temporary slides

KEY ACTIVITY

When viewing biological material with a microscope, it is usual practice to mount the specimen in water (or a stain) and place a coverslip on top. Temporary mounts that are prepared properly have just enough fluid to fill the space between the slide and coverslip. If too much fluid is used, the coverslip floats on top of the specimen and moves around. If too little fluid is used, the air bubbles that are left interfere with the image and may be mistaken for the specimen. The technique for making a temporary mount and lowering a coverslip is illustrated in Figure 2.2. If you do not trust your students to use mounted needles, a sharp pencil can be used instead.

How to lower a coverslip onto a temporary slide a

tip of mounted needle coverslip

slide

specimen in drop of water (or stain) b

tissue paper

drop of stain

water

Figure 2.2 a  Technique for lowering a coverslip onto a slide. Place the tip of a mounted needle onto a slide next to the specimen. One edge of the coverslip is placed on the slide with the opposite edge supported by the mounted needle. Slowly moving the needle in the direction shown by the arrow will lower the coverslip onto the specimen without trapping air bubbles. b Tissue paper can be used to soak up any excess water, as shown. A stain (such as iodine in potassium iodide solution) placed next to the coverslip can also be drawn under the coverslip using this technique.

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Tip Stains can help make structures or tissues clearer on a temporary mount. In general, potassium iodide solution (often referred to as iodine solution in school prep rooms) is used for plant material, and methylene blue solution is used for animal material. When using methylene blue, make sure students avoid inhalation and wash thoroughly if the stain comes into contact with skin or eyes.

Helping students to see what you want them to see When students begin to use microscopes they will need help in finding what you want them to see and in making appropriate observations. If they are not given help, you will find that they mistake such things as dirt, air bubbles, the interesting patterns left when water or a stain evaporates and even the edges of the coverslip or slide, for the things that you want them to look at. Drawing attention to the appearance of these artefacts is a useful way of helping students to avoid making such mistakes. Ensure students are aware that the cell membrane cannot be seen in a plant cell because it is pushed up tight against the cellulose cell wall. Also because the cell is transparent, it is not possible to see the cell vacuole, which is full of cell sap, in the centre of the cytoplasm. Having diagrams or models of cells around while students are using microscopes to observe cells helps students realise what they are looking at.

Further activities ➜ It

is a good idea to put clear plastic rulers onto the stage of a microscope and measure the width of the field of view at different magnifications using the millimetre divisions. The size of structures being viewed can then be estimated by judging how much of the field of view they occupy and doing some simple arithmetic. There are a range of excellent websites that show size comparisons. This also provides an opportunity to introduce standard form as another mathematical skill. ➜ A huge variety of interesting microscopic organisms live in ponds or containers of water (such as cattle troughs) that have been left standing for some time. Hunting for these organisms using the microscope can engage students, develop their microscope techniques and demonstrate the usefulness of the microscope in biological studies. Good hygiene is needed and staff should make students aware of the guidelines from CLEAPSS when handling microscopic organisms.

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Enhancement ideas Further ideas for improving communication skills and helping students to appreciate the implications and applications of this aspect of science could include projects in which students have to use microscopic analysis of specimens such as hairs, fibres and powders to help solve a fictional crime. These types of activities can be very motivating. Students could work together as teams of investigators to produce an illustrated scientific report that could be used in the trial of a suspect.

2.4 Studying cells Many different organisms exist as single cells, but others are made up of many cells. These multicellular organisms are composed of a variety of different types of cell that work together to maintain the life processes of the organism. This section suggests some teaching approaches that will help students to understand the structure of plant and animal cells. Most cells are too small to be seen with the unaided eye, so the use of microscopes to magnify is essential. Good thinking skills can be developed from asking questions like ‘What is the largest biological cell?’. Many think it is the ostrich egg at an impressive 15 cm but in fact the longest (not heaviest) cells can be up to 12 m in organisms like the giant squid. Giant cells in peppers are visible to the eye, with instructions available from Science & Plants for Schools. It is a good idea to wait until students are proficient at using microscopes before beginning practical work that is aimed at developing their understanding of the structure of cells. It is best to begin with plant cells because they are generally larger and have a more distinct structure than animal cells. Using living material will help students to relate the structures they look at under the microscope to the organism from which they came. In addition, preparing slides for themselves will help students develop their manipulative skills.

Studying plant cells The easiest plant cells for students to study under the microscope are those that form single layers. The bulbs of onions and related species (such as shallots and leeks) are a good source of single layers of skin (‘epidermal’) cells. Red onions are particularly useful because the cells in the outer epidermal layers contain a red cell sap that makes them easier to see. The technique of obtaining epidermal cells, mounting them and viewing them under a microscope is described in student textbooks, on YouTube and on the SAPS website.

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One disadvantage of using onion epidermis as an example of plant cells is that these cells do not possess chloroplasts. The epidermal layer of privet, iris or lettuce leaves is a good source of cells that do possess chloroplasts. Whole leaves of pondweed such as Elodea, ivy-leaved toadflax and young moss leaves are thin enough to enable cells to be seen when viewed under a light microscope.

Studying animal cells Studying human cheek cells is an interesting and motivating activity to try with students who are skilled at using microscopes. There is a very small theoretical risk of transmission of the viruses that cause AIDS and hepatitis B associated with this practical, but most employers allow students to carry it out provided they follow a strict safety procedure (see CLEAPSS sheet PP033 for advice on the procedure and appropriate safety measures). Care needs to be taken that students do not share cotton buds, and so either do this technique as a demonstration or ensure students get their own cotton bud, which they place in disinfectant immediately after use. If cotton buds are not available, they can be substituted with interdental sticks or coffee stirrers from newly opened packets. These are then used to collect the cheek cells by gently swabbing the inside of the mouth around the gums. The saliva (which will contain the cells) is smeared onto a slide, a few drops of suitable stain (such as methylene blue) are added and a coverslip is placed on top. After use, the cotton buds, slides and coverslip must be put into a disinfectant such as 1% sodium chlorate(I) (hypochlorite). The cells are very small and students will need to magnify them at least 100 times in order to see them clearly.

Drawing and interpreting microscope images of cells The drawing of images viewed using microscopes is an important skill in biology since it encourages careful observation and thus helps students to understand the images they are looking at. When drawing from the microscope, students should be taught to follow these procedures: ➜ Write

a clear heading. a sharp HB pencil. ➜ Make your drawing either half or a whole page. ➜ Draw firm, continuous lines and avoid using shading. ➜ Include the magnification the specimen was drawn under (objective lens power × eyepiece lens power) or a scale line. ➜ Use

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label lines with a ruler. These must touch the structure they are labelling and should not cross each other. ➜ Write labels horizontally on the page using the book lines (students often write along the same angle as the label lines) and arrange them neatly around (not over) the drawing. Examples of good and not so good drawings are given in Figure 2.3. nucleus nucleus

cell membrane

cytoplasm

cytoplasm

  

cell membrane

Figure 2.3  Good and bad drawings of a human cheek cell

When they attempt to draw cells for the first time, students often tend to draw too many cells. Instead, they should be encouraged to draw either a single cell or a few cells in detail and to use at least half a page for their drawing. To help interpret images of cells, it is a good idea for students to draw what they can see under the low-power objective lens, then using the middle- and high-power lenses. When using high-power lenses, students may be able to investigate the three-dimensional nature of larger cells (for example, plant epidermal cells) by carefully focusing on different planes of the cell. Drawing or projecting diagrams onto a board and using models will help students to interpret the structures of the cells they are viewing and see that these structures are three-dimensional.

Cell structure and function You will want students to learn the names and functions of the main parts of cells. These are described in student texts. Some of the technical terms are difficult to spell and pronounce so it is worth going over them carefully. Getting students to make three-dimensional models of specialised cells (such as red blood cells, neurons, sperm cells, root hair cells, guard cells) using

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easily obtainable items, such as shoe boxes, polythene bags and small balls, to represent structures such as cell walls, membranes and nuclei is a very useful way of illustrating the structure of simple cells.

KEY ACTIVITIES

Since students often have to share microscopes, it is useful to adopt teaching techniques that take account of this, such as taking turns to use the microscopes. An activity that can be used to develop observational and descriptive skills, and that can be done in pairs or threes, involves one student looking down a microscope and describing the appearance of a specialised cell (permanent cell mounts are best for this). The other student(s) have to draw and label what is being described. Encourage the students who are undertaking the drawings to ask questions to gain further information to enhance their drawings. They can try to predict what sort of cell is being looked at and then compare the drawings with the real thing. Students then swap roles and look at a different type of cell.

Investigating specialised cells l Draw

diagrams of plant and animal cells deliberately labelled incorrectly. Get the students to work individually to spot and make a note of the mistakes. Students then pair up or take part in a class plenary to correct all mistakes. l A game of ‘10 questions’ or ‘Taboo’ could be created, in which one student thinks of a specialised cell and their partner has to guess what the cell is in just ten questions; the questions can only be answered ‘yes’ or ‘no’.

Enhancement ideas Pictures taken with scanning electron microscopes illustrate the external appearance, shape and three-dimensional nature of cells very clearly. Show students scanning electron microscope pictures of a selection of cells, including red blood cells, unicellular organisms and the xylem cells that form wood, to demonstrate the variation shown by cells. Longitudinal sections of vascular bundles from celery have lovely spiral lignification of xylem, which strengthens these ‘water pipes’ of the plant. Showing students the sort of images that can be produced using transmission electron microscopes (these show details of structures found inside cells) will help them to understand that cells are more complex structures than they appear to be when looked at using light microscopes.

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Maths Basic micrometry can be used to indicate to students the actual sizes of cells. Simple ‘measuring’ slides can be made by sticking a small, stamp-sized piece of graph paper onto a slide and working out the diameter of the field of view. Alternatively, view the scale on a small transparent ruler and work out the diameter of the field of view. When students observe a specimen in the field of view they can estimate how much of the diameter is covered and so work out the size of the specimen.

2.5 The chemistry of life In the first section of this chapter, life was described as arising from interactions between the chemical substances that make up organisms. Chemical reactions inside cells occur in sequences called metabolic pathways. These pathways involve either making or breaking down the large molecules that are found in living things. The steps along the pathway are catalysed by biological catalysts (enzymes), and the specific structure of each enzyme enables it to catalyse a particular chemical change.

Enzymes Enzymes are proteins and the temperature and pH of their surroundings can affect their structure. If these vary outside certain limits then the shapes of enzymes change so much that they are unable to perform their functions and metabolic pathways are disrupted; the enzymes are said to be ‘denatured’. This is one reason why it is important for organisms to maintain relatively constant conditions inside their bodies and cells (in other words, to show homeostasis). As well as synthesising, and so producing products that are needed by organisms, metabolic reactions often produce by-products that need to be removed. The removal of the waste products of metabolism is called excretion.

Scientific literacy Research has shown that students often think that molecules, which they have been told are ‘large’ (such as proteins), are bigger than cells, which they have been taught are very ‘small’. Careful use of these relative terms is therefore needed.

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2.5   The chemistry of life

Science in context The names of some of the types of molecule found in living things will be familiar to students from advertisements associated with food and cosmetics, while enzymes will often be associated with washing powders. (Tell students that the names of most enzymes end in ‘-ase’, such as sucrase, aminase, lipase, invertase.) Such advertising often contains somewhat questionable ‘pseudoscience’ that can give rise to many misconceptions. Students could be asked to use their scientific understanding to critique the claims advertisers make for various products. Examination specifications and student texts often include many details concerning the complex molecules that are needed by all living things, and enzyme action in the context of the human diet and digestion. However, before covering diet and digestion in detail, it is a good idea to go over some more general ideas about the chemistry of living things. The activities suggested below could be used in a variety of contexts and are not set out as a continuous teaching sequence. The idea that the complex molecules found in cells are assembled from smaller molecules is important in many contexts. For example, carbohydrates such as starch and cellulose are built up from simple sugars such as glucose; proteins are built up from amino acids; and nucleic acids are made up of sugars, phosphates and bases. The steps involved in the formation or breakdown of molecules in cells can be illustrated using models. This could be done using either physical models or a molecular-modelling package for use with computers. It is important that students do not relate enzymes simply to digestion, and so it is helpful to provide them with an example of enzymes building up molecules. For example, a good demonstration is to show that an enzyme found in potato will catalyse the formation of starch from glucose. The enzyme is obtained by grinding up a small piece of potato with water. When the enzyme extract is mixed with glucose monophosphate, the iodine test can be used to show that starch is produced.

Metabolic pathways and excretion Cells manufacture new chemicals in their cytoplasm, changing glucose, produced by plants in photosynthesis, into amino acids and fats. All parts of the cell are made from these chemicals. Some reactions produce useful chemicals for the cell, while others produce chemicals that can be toxic to the cell processes and so need to be excreted.

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Excretion is often defined as being the release of waste products from living things. Two important ideas to emphasise when discussing excretion are, firstly, that the waste products released are made inside body cells and, secondly, that they will disrupt metabolism (they are poisonous) if they accumulate inside the cells. In single-celled organisms and many small organisms, such as flatworms, waste materials can diffuse out of the cell along a concentration gradient and easily pass into the environment. Plants can get rid of much of their waste gas in this way because they have such a large surface area. In larger animals, excretion is more of a problem as the waste cannot easily diffuse out of the cells into the environment and so accumulates. Some metabolic waste products are molecules that can be used by the organism that produces them. For example, in plants the carbon dioxide produced by respiration can be utilised in photosynthesis and only at night is there a net release of carbon dioxide. Similarly, some of the oxygen produced by photosynthesis is used by plants for respiration. However, when the rate of photosynthesis is high, excess oxygen must be released. In contrast to plants, animals need to get rid of excess carbon dioxide all the time. The other main metabolic waste products produced by animals are nitrogenous compounds, such as ammonia, that are formed from the breakdown of amino acids. Ammonia is very poisonous and in some animals is converted to urea, which is less toxic than ammonia, and filtered out of the bloodstream by the kidneys.

Cell division Cell division is the basis of growth in multicellular organisms and of reproduction in all organisms. Students can be taught many things about growth and reproduction without discussing cell division in detail. However, to gain a deeper understanding of these processes, students will need to learn more about what happens when cells divide. The type of cell division that the zygote (fertilised egg) undergoes many, many times is called mitosis. This type of cell division also enables multicellular organisms to grow and to repair damaged tissues.

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2.5   The chemistry of life

Science in context Students may be fascinated to hear about a recent experiment carried out by astronauts on board the International Space Station. The experiment is an attempt to grow human tissue by sending adult human stem cells into space and allowing them to grow. Eventually, it is hoped, the stem cells will develop into bone, cartilage and other organs. If that is successful, the scientists involved say that the discoveries could be used to try to grow organs for transplant. The lack of gravity on the International Space Station is used to encourage the stem cells to grow into tissue in three dimensions, rather than the single-layer structures that form on Earth. This pioneering work should enable the tissue to develop into organs for transplant. Mitosis should not be confused with binary fission, which is the mechanism by which many single-celled organisms, such as bacteria and yeast, reproduce. Binary fission occurs primarily in prokaryotes, while mitosis is only found in eukaryotes, such as plants and animals. Cell division involves studying structures (chromosomes) which are very difficult to see, even with the aid of high-powered light microscopes. The sequences of events that occur are complex and, when describing them, student texts often use many technical terms. The approach suggested below involves the use of video footage, diagrams and models to help students understand the principles of cell division.

Science in context Each species has its own number of chromosome pairs: humans have 23 pairs, fruit flies 4 pairs and crayfish 100 pairs. Apart from gametes, each cell in a particular organism contains the same number of chromosomes. Students may need help to understand that even the cells in your big toe have the DNA code for your eye colour and the gene which determines whether your hair is curly or not!

Mitotic cell division Mitosis should be covered before introducing meiosis. It is advised that mitosis and meiosis are studied in separate topics because they often get confused. Sometimes students are asked to compare the two processes in examination questions, but this is better approached if they have a good understanding of each process separately to start with. Mitosis is a fascinating process, and

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a good starting point is to show students a video of actual cells dividing and an animated sequence of this process (see websites listed at the end of the chapter for good sources of these). This can be followed by using a sequence of diagrams similar to that shown in Figure 2.4 to explain the important features of the process in a simplified representation. When a cell is not dividing, its chromosomes exist as very long, thin structures that are only just visible using electron microscopy. Stage 2 in the figure (when the chromosomes make copies of themselves) actually occurs when the chromosomes are inside the nucleus in their long, thin conformation. When cell division starts, the nuclear membrane breaks down and the chromosomes become shorter and fatter (and visible under a light microscope) after this has taken place. A disadvantage of using diagrams to illustrate mitosis is that it is difficult to show the behaviour of the chromosomes in three dimensions. Models can be used to help students visualise the three-dimensional nature of the process, and students could also make models using plasticine, wool or pipe cleaners to aid their understanding. Flicker books or stop-frame animations also provide an opportunity for some creativity as well as an opportunity to describe and explain mitosis.

Meiosis and gamete formation The type of cell division that produces sex cells (gametes) is called meiosis. Gametes differ from other types of cell (somatic cells) in organisms in two fundamental respects. First, they contain half the number of chromosomes (denoted by the haploid number, ‘n’) compared to somatic cells, which contain the diploid number (denoted by ‘2n’). If this were not the case, the zygotes (fertilised eggs) formed when sex cells combine (fuse) would contain twice as many chromosomes as the parental cells, which would be unviable. Secondly, many of the chromosomes present in gametes contain a mixture of genes derived from the male and female parents of the individual that is producing the gametes. Figure 2.5 illustrates the important events that occur during meiosis. Comparison with Figure 2.4 shows that the initial stages of meiosis and mitosis are essentially the same. A crucial difference occurs after the chromosomes have made copies of themselves. In mitosis, the copies are separated.

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1 A cell containing two pairs of (homologous) chromosomes. One of each pair comes from the original female parent and is shown as a continuous line. The other comes from the original male parent and is shown as a dotted line.

2 Each chromosome makes an exact copy of itself. (The copies are called chromatids.)

3 Chromosomes (now pairs of chromatids) line up in the middle (on the equator) of the cell.

4 The copies (chromatids) separate (and are now renamed chromosomes in their own right) and move to opposite ends (poles) of the cell, which then divides to form two (daughter) cells. Each of these has exactly the same genetic make-up as the original cell. A crucial point to establish is that the cells formed as a result of mitotic division contain exactly the same complement of chromosomes (and hence genes) as the original, undivided cell.

Figure 2.4  A diagram summarising the events that occur during mitosis in an organism with a diploid number of four

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1 A cell containing two pairs of (homologous) chromosomes. One of each pair comes from the female parent and is shown as a continuous line. The other comes from the male parent and is shown as a dotted line.

2 Each chromosome makes an exact copy of itself. (The copies are called chromatids.)

3 Homologous chromosomes (each made up of two copies or chromatids) line up alongside each other in the middle (on the equator) of the cell.

4 Homologous chromosomes are separated and pulled to opposite ends (poles) of the cell and two new cells are formed. This is the first division with the result that there is already a mixture of the male parent and female parent chromosomes in each cell. 5 The chromosome copies (chromatids) now separate (becoming chromosomes in their own right) and move to opposite ends (poles) of each cell. The second division nowoccurs forming a total of four new cells. The number of chromosomes in each is therefore half the number found in the original cell.

Figure 2.5  A diagram summarising the events that occur during meiosis in an organism with a diploid number of four

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The gametes that these cells produce will be genetically different from each other because of the random arrangement of chromosomes from the male and female parents in stage 3 above. This is referred to as ‘independent assortment’ of the chromosomes and leads to independent segregation of alleles (see Chapter 8). Additional variation between gametes occurs because genetic material is exchanged between chromosomes when they pair up with each other. This process is called ‘crossing over’ and is illustrated in Figure 2.6. Awareness of this can help students to understand the variation in gametes, however details of the process are not needed until post-16 courses. This, together with the variation resulting from the independent segregation of chromosomes, results in an almost infinite amount of genetic variation between the gametes produced by an individual. This contributes to the greater genetic variation that occurs in the offspring produced by sexual reproduction, and this variation has important consequences for the evolution of species, as discussed in Chapter 9.

Figure 2.6  Crossing over in meiosis. As the homologous pairs line up, the chromatids intertwine, break and reanneal having swapped some sections.

Further activities ➜ An

activity in which students play the role of chromosomes and enact cell division provides a useful way of reinforcing learning about this process. To provide guidance on how you might do this, go to YouTube, where you will find various approaches, including one involving synchronised swimming! ➜ Older students with good microscopy skills could look at prepared microscope slides showing the different stages of mitosis and meiosis. They could also prepare root-tip squashes (see the SAPS website for practical details) to look for different stages of mitotic division, although this is generally done by students aged 17–19.

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Careers The study of cells is fundamental to understanding the functionality of a living organism and so this topic links to all biological careers from the obvious medical routes of doctors, nurses and technicians to the slightly more unusual tissue culture specialist. The topic also links to the work of recent astronauts who are growing human organs and plants in space using tissue culture, so showing the crossover between physics and biology. Other careers where this becomes interesting include biotechnology for medicine production or, more recently, food production. There is a growing movement towards meat substitutes, and the use of tissue culture to produce ‘animal-free meat’ can help combat climate issues. Additional career paths where cellular biology plays an important role would be: palaeontology, vaccine development, pathology lab diagnostic work, genetic engineering and gene therapy.

2.6 Resources Generic websites ‘Practical Biology’ is a website that is a joint project of the Nuffield Curriculum Centre, the Royal Society of Biology and CLEAPSS. It is intended to encourage teachers to carry out more practical biology in schools: http://practicalbiology. org/ Science & Plants for Schools (SAPS) has a range of materials and suggestions for practical work, including investigating mitosis in root tips and viewing micro-organisms using the ‘hanging drop’ technique: www-saps.plantsci.cam. ac.uk The Best Evidence Science Teaching (BEST) website has a range of diagnostic questions and outlines of research focused on science teaching: www.stem. org.uk/best-evidence-science-teaching The Nobel Prize website, www.nobelprize.org, has information on the international recognition of important research. For example, the 2019 Nobel Prize in Physiology or Medicine was awarded jointly to William G. Kaelin Jr, Sir Peter J. Ratcliffe and Gregg L. Semenza ‘for their discoveries of how cells sense and adapt to oxygen availability’. They identified molecular machinery that regulates the activity of genes in response to varying levels of oxygen.

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Some useful activities for introducing classes to biology outside the classroom can be found at: www.pstt-cpd.org.uk/ext/cpd/thinking-beyond-the-classroom/ activities.scientific-observations.asp The website of the Field Studies Council can be accessed at: www.field-studies-council.org

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3

Energy and materials Jeremy Airey and Elizabeth Lupton

Topic overview This chapter is concerned with the idea, introduced in Chapter 1, that organisms require a supply of energy and materials for which they are often dependent on other organisms. This relates to Harlen’s fourth ‘big idea’ (2015): that the total amount of energy in the Universe is conserved but energy can be transferred when things change. Learners will have encountered energy outside science lessons, and also (if not already, then soon) in physics and chemistry. It is incumbent on biology teachers to teach about energy in biology in ways that take account of learners’ prior knowledge and that are consistent with chemical and physical accounts of energy. We are also concerned here with two related ideas about biological systems that were noted in Chapter 1: the circulation of materials within them, and the transfers of energy into, through and ultimately out of them. Both energy and matter are conserved, but they differ in their patterns of transfer (movement) in biology. These ideas may not be easy for learners to grasp; they warrant clear attention and explanation in lessons. A unifying thread through this chapter is food – also, helpfully, a familiar context for learners. In their primary education, children are likely to learn that food provides living things with materials for growth, reproduction and repair, as well as energy resources to fuel biological processes. They may also have learned, through studying food chains and webs, that animals are – directly or indirectly – dependent on plants for their food supply, and that plants manufacture their own food. To many biologists, a logical progression through ‘energy and materials’ would start with photosynthesis, which represents both the principal transfer of energy into biological systems and a cycling of materials from non-living to living components of ecosystems. However, the challenging counter-intuitive origin of biomass (from a gas and water) in photosynthesis, together with the familiarity of food, makes diet a better place to start in this phase of our spiral science curriculum. The components of this theme – nutrition, respiration, photosynthesis, nutrient cycles and energy flow – all hold their challenges for learners. Furthermore, even if learners have a good understanding of each component, they may not appreciate the links between ideas. We need to emphasise these. Learners may also struggle as a result of anthropocentric views, including the view that

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plants depend on humans or exist to meet human needs (for example, that the function of photosynthesis is to produce oxygen for us to breathe). Some students may find it difficult to describe biological phenomena in terms of physics and chemistry, or be reluctant to accept such ‘reductionism’. We need to address these conceptions and be clear that biology remains awesome however we explain it!

3.1 Respiration Energy and life Energy is a tricky concept to discuss. It is an abstract idea that is hard to define precisely and many students have ideas about energy that have come from everyday life. For example, if they feel tired, they may say that they have run out of energy, which they might then replenish with an energy drink. Many words have different scientific and everyday meanings, and we should be alert to the different ways that children encounter and use such terms. Trying to distinguish scientific meanings from everyday meanings is likely to be helpful.

Science in context Red Bull Sugarfree is marketed as an ‘energy drink’ but contains no sugar. It works by containing enough caffeine to act as a stimulant. It is worth discussing with students in what sense such products are called ‘energy drinks’. Energy is also a pervasive idea in biology, but Needham (2014) has pointed out that in school biology, we have tended to use mixed models of energy, including a ‘stores and transfers’ model and a ‘transformations’ model. In this chapter we have tried to use the former model consistently.

Cross-disciplinary If science is studied as separate disciplines rather than as combined science, speak to colleagues in the physics department to ensure that there is a co-ordinated approach to the subject of energy and the way it is taught. Before being taught about respiration, students are most likely to have studied energy in the context of physics. They may have developed some understanding of the idea that energy is always conserved, but a very common misconception in a biological context is that energy is a physical substance that can be ‘used

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up’. Transformation models of energy can reinforce this notion. Needham recommends distinguishing carefully between ‘fuels’ (including biomass and food) and energy. We have tried to do that here. Ross (2013), however, cautions against the ways that fuel and food are sometimes described as if they are ‘made of’ energy. Needham also suggests using biomass and productivity as an underpinning theme. We have taken a similar approach here, using food as a thread through the topics in this chapter. There is ongoing debate and discussion about the best ways to use energy ideas across the school sciences. We recommend continuing discussion between physics, chemistry and biology teachers, so that science education presents young people with a consistent energy story.

What is respiration? The energy needed for all living processes is made available inside cells by a metabolic pathway (a linked series of chemical reactions in living systems) called cellular respiration. This process must take place inside every living cell. Many students have difficulty understanding what is meant by the term ‘respiration’, and research has shown that they often retain many misconceptions after it has been taught: for example, that ‘respiration’ and ‘breathing’ are synonymous, that plants do not respire or only respire when in darkness, and that respiration ‘creates’ energy for living processes. Cellular respiration acts on molecules originating in the food that plants make for themselves or that other organisms consume. Many components of food can be respired, including fats and proteins, but in many organisms, carbohydrates are the principal ‘substrate’. The archetypal reactant in respiration discussed in 11–16 biology is glucose. There are two forms of respiration: aerobic respiration and anaerobic respiration. Aerobic respiration occurs when sufficient oxygen is available and can be summarised, for glucose, by the equation: glucose + oxygen

energy transferred

carbon dioxide + water

If there is insufficient oxygen available, anaerobic respiration occurs. For example, if exercise levels are vigorous or sustained, the circulation may not supply oxygen to muscles quickly enough to maintain aerobic respiration. When this happens, the muscle cells begin to respire anaerobically. This releases less energy than aerobic respiration and, in animals and some bacteria, can be summarised as: glucose

energy transferred

lactic acid

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The lactic acid produced in muscles is broken down into carbon dioxide and water when enough oxygen is available to repay the ‘oxygen debt’ that results from anaerobic respiration. In fungi (for example, yeast), in low-oxygen conditions, anaerobic respiration can produce ethanol and carbon dioxide, summarised as: glucose

energy transferred

carbon dioxide + ethanol

Baking and brewing use this fermentation process.

Careers Food microbiologists study the effects of different micro-organisms within food; by gaining an understanding of these effects, they can find ways of using these microorganisms in the food production process, such as brewing and baking. Food technologists focus on ways of maintaining food at its freshest and consider the best ways to convert raw materials into food products. This is a thriving industry and one which is set to get bigger. Two vital (and linked) industries which depend on understanding of decomposition, decay, energy flow and materials cycles are the sustainable energy industry (for biofuel production) and the water industry (sewage works). Few cells or organisms respire anaerobically for long; the products are toxic in large quantities and little energy is released from food stores, making it unsustainable. Exceptions include some pathogenic bacteria (for example, those causing tetanus) and methane-producing bacteria that humans can use in sewage treatment.

Food; digestion and absorption; from food to ATP Photosynthesis and respiration are both complex chemical pathways that we explain in 11–16 biology with relatively simple models. (Post-16 students start to break down the processes into, for example, the so-called light and dark reactions of photosynthesis, and the stages of glycolysis and the Krebs cycle in respiration.) Starting with simpler ideas, it is important to ensure that learners are clear about principles. For example, all cells which do not photosynthesise – including many cells in plants – require a supply of food molecules that can easily be converted to glucose, to be used in cellular respiration. Some cells can store these away for later by converting them into starch, its equivalent in animals (glycogen) or fats. But ultimately, all cells need a supply.

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The molecules supplied need to be sufficiently small and soluble to be transportable to the cells and across their membranes. (In plants, the photosynthetic product exported from leaves is sucrose.) This is the principle behind consumers’ digestion of their food. The macromolecules that comprise much of our food are broken down physically and chemically into much smaller molecules that can be absorbed into the consumers’ transport systems and into their respiring cells. In the case of many animals, digestion and absorption are functions of the gut. Fungi and many other consumers secrete digestive enzymes into the local environment and absorb the products directly into their cells. Food is not only used as an energy resource for respiration, but also as raw material for growth, reproduction and repair. Cells construct their own macromolecules, to meet local requirements, from the ‘building blocks’ supplied by digestion and absorption of food molecules (in consumers) and by photosynthesis and the absorption of minerals from the environment (in producers). Students aged 11–16 should come to understand the relationships between food macromolecules and their constituent parts (for example, proteins and amino acids). There are many examples of practical activities for investigating the products and processes of digestion. In our simple model, respiration is an oxidation reaction, in which organic (carbon-containing) molecules are oxidised. Complete oxidation, to carbon dioxide and water, transfers the maximum possible energy out of the reactants (food molecules and oxygen) and into the wider system. This is analogous to (but not the same as) combustion. When you come to teach food and cellular respiration, you should find out what your students have been taught about combustion reactions in chemistry. Draw links between the two topics, emphasising that respiration is not a burning reaction, though the overall chemical change is similar. Indeed, the amount of energy stored in various foodstuffs, such as crisps, is often estimated in schools through combustion and simple calorimetry. Place 10 cm3 of water in a boiling tube and record its temperature. Ignite a crisp or snack biscuit and use the burning food to heat the water. Record the final temperature. Energy stored in the foodstuff can be calculated using the formula: energy (J) = mass of water (10 g) × specific heat capacity of water (4.2) × temperature change (°C) You can refine this by weighing the crisp and working out energy per gram and then comparing with the energy value on the food label.

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Incomplete oxidation results in some (or all) of the carbon atoms in the food molecules not becoming chemically bonded to oxygen in the form of carbon dioxide. This is the case in anaerobic respiration and results in less energy being transferred out of the food. Some food molecules, notably fatty acids, are in a less oxidised chemical state than carbohydrates such as glucose. Students can be shown the chemical formula of a fatty acid, noting the small proportion of oxygen in it (you could also note the similarity in formula and structure between fatty acids and alkanes, such as those in petrol and similar fuels). Full oxidation of these molecules to carbon dioxide and water results in even more energy being transferred out of chemical stores. It is all too easy to exceed your food energy intake requirements if your diet is rich in fats. Again, some simple food calorimetry could help to demonstrate the ‘energy-dense’ nature of fatty foods; compare, for example, baked and fried snack foods.

Science in context Students may be aware that camels’ humps and small seeds can be valuable stores of fat. Fat provides a lot of energy per gram, and produces a lot of water, when aerobically respired. However, water loss through additional breathing when camels respire fat may exceed water generated in respiration. Thermal insulation provided by fat is an unrelated advantage. In photosynthesis, the energy acquired by absorbing light is linked to the reduction of carbon, from carbon dioxide and water to glucose and oxygen. The energy is thereby transferred into a chemical store. Older and higher-attaining students should be introduced to ATP (adenosine triphosphate), a small, mobile molecule used as an energy store in cells. The reaction that results in the conversion of a molecule of ATP to ADP (adenosine diphosphate) and a phosphate ion, making an amount of energy available, can be linked in cells to other molecular processes that take an ‘inward’ transfer of energy. Thereby, muscles can contract, ions can be pumped across membranes, and macromolecules can be built. In a more sophisticated model of respiration, learners can understand the function of cellular respiration as being to restore the ATP molecules, by recombining ADP and phosphate. The net energy transfer of cellular respiration is from biomass stores (glucose) to more usable stores in the form of ATP.

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Science in context ATP and muscle contraction can be seen in rigor mortis – the stiffening of muscles after death. This typically starts 3–4 hours after death, but may be faster in those who have exercised strenuously before death or who have had convulsions leading up to death. Many students are aware that ATP is required for muscle contractions but not that ATP is also required for a muscle to relax. Respiration ceases upon death, so no more ATP is generated and muscles stiffen. Rigor mortis passes after 2–3 days as enzymes break down and soften muscles.

Prior knowledge and experience of respiration Learners bring to their science lessons ideas that they have acquired elsewhere, or indeed through previous science teaching. These can sometimes be barriers to further learning. Although living organisms are respiring all the time, they do not usually show obvious physical signs that this is occurring. Something that some animals obviously do is ‘breathe’, which is perhaps why respiration and breathing (or ventilation) are often thought of as being the same thing. Breathing may be regarded by some students as an end in itself, rather than being linked to the needs of cells. Some learners may think that cellular respiration only happens when an organism is physically active. Oxygen may be regarded as synonymous with air. Respiration in plants is particularly problematic. Some students may think that it does not happen or that it only happens at night. They may believe that respiration takes place only in leaves because they have stomata for gas exchange, and that leaves alternate their function between day and night. Learners may appreciate the role of photosynthesis in plant growth but not the role of respiration in growth and in the many other energy transfer processes in cells.

A teaching sequence Practical investigations concerning the reactants and products of respiration can be used to help students understand the process. The fundamental importance of respiration to all living things can be emphasised by using a variety of different types of organisms in such investigations. Further ideas and more detailed explanations of the procedures that are suggested below can be found in many student texts and on websites recommended at the end of this chapter.

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Energy flow in living and non-living systems Before teaching respiration, it is important to review students’ knowledge of what is meant by the word ‘energy’. We cannot see or feel energy – the physical experiences that we associate with it occur only when an energy transfer takes place. An ‘energy circus’ is a useful way of helping students appreciate that the concept of ‘energy’ can be applied in many situations. Students can be asked the question, ‘Where is the energy now?’ when presented with examples of living and non-living systems in which energy transfers are occurring. The utilisation of energy transfers by organisms of all kinds could include generation of movement (such as plant tropisms), growth, sound, heat, light (in bioluminescent algae and fungi, for example) and electricity (as in nerve impulses and electric eels). After considering situations in which a few energy transfers occur, students could then trace the many energy transfers that occur from the Sun, to their food, to their own life processes.

Food as a source of energy Making the link between the high sugar (or starch or fat) content of ‘high energy’ foods and the photosynthesis that produces the glucose food energy store will help students to appreciate the idea of energy flow from producers to primary consumers. Give your students some food labels to study. Ask them to determine whether there is a connection between high energy ‘density’ (kJ/g) foods and the sugar/starch/fat content of the food.

Careers Dietetics is the study of nutrition and its effects on health, focusing on the nutrition required for good health and how to help those who are not in good health through the construction of specialised diets. Dieticians can specialise in many areas, including renal, oncological and paediatric. Career opportunities for dieticians are growing and include working in scientific research, in the food industry, with athletes and even with the media. The idea that respiration involves food being ‘burned’ inside the body to release energy is often used when introducing respiration. Respiration is an oxidation reaction rather than a simple combustion reaction. Be sure to emphasise the difference if burning foodstuffs. It should be made clear that respiration involves a series of small steps that transfer energy gradually whereas combustion is a much more rapid reaction. Dramatic demonstrations

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of the energy released when cornflour is burned or ‘the screaming jelly baby’ can provide good stimulus material for introducing the idea that food provides the source of energy for consumers (see websites listed at the end of this chapter for details of these demonstrations). Cellular respiration as a fundamental process of life can be enhanced by making explicit links with topics such as: movement involving muscle contraction, movement of chromosomes during cell division, the synthesis of large molecules from smaller ones and the active transport of materials in and out of cells. The importance of respiration in the carbon cycle should also be highlighted and this provides another opportunity to challenge the misconception that respiration does not occur in plants.

KEY ACTIVITY

Demonstrating that oxygen is used in aerobic respiration Using respirometers to monitor oxygen uptake in small organisms Respirometers can be used to measure oxygen uptake by small invertebrates or germinating seeds. The invertebrates or seeds are placed inside a container (such as a boiling tube) with a substance (usually pellets of soda lime) which will absorb the carbon dioxide produced by respiration. A bung connected to a U-shaped capillary tube with water inside is used to seal the boiling tube. The air pressure in the sealed container drops since oxygen is being used up in respiration and the water in the attached capillary tube is drawn along the tube. In practice this type of respirometer is fiddly to set up and may not provide reliable results unless it is completely airtight and maintained at a constant temperature. Depending on your students and the time you have available, it may be more sensible to use a respirometer as a demonstration rather than for a whole class practical activity. Note that soda lime is corrosive. Alternatively, the CLEAPSS procedure ‘GL159 Removing oxygen in a seed germination practical without using alkaline pyrogallol’ can be used to demonstrate the need for oxygen in aerobic respiration. Consider asking and discussing with students what happens to the mass of a fertilised bird’s egg as it is incubated, and why. Many students will tell you that the mass increases, when in fact it drops – a common misconception – because respiration must occur. Oxygen diffuses in and carbon dioxide diffuses out of the egg. Water is also lost through the eggshell.

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Demonstrating that carbon dioxide is produced in aerobic respiration The well-known limewater test for carbon dioxide can be used when the amount of carbon dioxide being produced is relatively large, for example in air exhaled from the lungs. If only small amounts of carbon dioxide are produced, however, hydrogencarbonate indicator may be a better choice. As mentioned in Chapter 2, hydrogencarbonate indicator is a very sensitive pH indicator that can be used to demonstrate carbon dioxide production by respiring plants and small animals (details for how to make and use this indicator can be found on the CLEAPSS website). It has an orange/red colour when in equilibrium with the carbon dioxide levels of atmospheric air. When the carbon dioxide level rises slightly, it changes to a yellow colour. Placing pondweed (for example Elodea) into a boiling tube containing the indicator and excluding light to prevent photosynthesis will produce a colour change in about one hour. Pond snails are not harmed by the indicator and will cause a significant colour change in about 30 minutes. Leaves of terrestrial plants (such as privet) or small invertebrates (such as woodlice) can also be suspended above the indicator solution in the dark, producing a colour change after a few hours.

Measuring heat production in germinating seeds Germinating seeds transfer energy by cellular respiration from their food stores, typically oils (liquid fats) or starch, into the biochemical reactions giving rise to growth. As in most energy transfer processes, a proportion of the energy is transferred into random thermal motion of particles – in other words, heat – and the system warms up. This temperature rise can be measured by placing a thermometer or a temperature sensor (attached to a digital meter and, if possible, a data logger) into a thermos flask containing germinating seeds. Soaked dried peas are typically used as soaking triggers germination. Unsoaked peas and boiled soaked peas are important controls. A rinse of the soaked and the boiled peas in mild disinfectant solution will inhibit microbial activity, which can interfere with this demonstration.

Anaerobic respiration in bacteria Students are likely to be familiar with the smell or taste of milk that has ‘gone off’. The sour taste results from bacteria in the milk respiring anaerobically and producing lactic acid. The fall in pH can be monitored over a number of

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days using a pH sensor connected to a meter with a digital readout, which can be connected to a data logger. Comparing results obtained using pasteurised and long-life milk can help illustrate the role of bacteria in this process. Lactic fermentation can use other substrates (food sources) for the bacteria too, for example cabbage or flour (in sourdough bread).

Anaerobic respiration in yeast When yeast respires anaerobically, it produces ethanol (alcohol) and carbon dioxide. We use yeast in food production. Yeast is used in the baking process (the carbon dioxide produced makes the dough rise) and in the production of alcohol such as in wine and beer. Fermentation by yeast is easily investigated by younger students with a conical flask of yeast suspension in sugar solution. The sugar solution should have been made up with freshly boiled water to drive off much of the dissolved oxygen. Floating a layer of cooking oil on top prevents more oxygen dissolving in the suspension. The gas given off can be used to inflate a balloon stretched over the mouth of the conical flask; draw the gas in the balloon up into a regular syringe and gently push it from the syringe, through a narrow rubber tube, into limewater, to test for carbon dioxide. CLEAPSS have a more ambitious procedure for older learners: ‘GL089 Measurement of anaerobic respiration in yeast’.

Respiration and cells This would be a good point at which to make links and remind learners of cell theory, having just considered respiration in (unicellular) bacteria and yeasts. Every living cell, including in multicellular organisms, must transfer energy from food supplies as required by other energy transfers happening inside the cell (for example, as new complex materials are made). All cells do this through respiration. This implies that all cells need a supply of the raw materials (substrates) for respiration and to be able to lose harmful products (including carbon dioxide). The transport, exchange and other processes that secure these in various organisms are the subject of other chapters in this book. In prokaryotes (cells without nuclei, such as bacteria), respiration happens within the cytoplasm, with some stages occurring at the cell membrane. In cells with nuclei, only early stages happen in the cytoplasm, with respiration predominantly taking place in membrane-bound organelles called mitochondria. Students may recall learning about these, and perhaps modelling them, when they studied cells. Excellent photomicrographs of mitochondria can be accessed at the website of the Science Photo Library.

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3.1   Respiration

Enhancement ideas Mitochondria have two membranes around them (the inner being deeply folded, providing a large surface area). They are about the same size as many bacteria, with a small genome of their own. You could discuss with older students what this might suggest about the origins of mitochondria in evolution and, in principle, how this could be investigated further. (Are there any other similarities between mitochondria and living bacteria?)

Anaerobic respiration in muscles The rapid exercising of muscles quickly results in a build-up of lactic acid, and most students will be familiar with the sensations of tiredness, cramp and stiffness that follow strenuous exercise. A simple procedure that illustrates one of the reasons why these sensations occur is to clench and unclench your fists two to three times each second with one hand held above your head and the other by your side. The raised arm tires more quickly: it is harder to pump blood carrying oxygen upwards to the raised hand, so more anaerobic respiration occurs in the muscle cells; the build-up of lactic acid becomes uncomfortable.

Enhancement ideas Much of our knowledge about metabolic processes such as respiration comes from experiments using radioactive tracers, for example following the path of a radioactively labelled carbon atom from glucose dissolved in an animal’s drinking water to the carbon dioxide that the animal produces by respiring. With more able students some of the techniques involved and the results obtained can be discussed to help them understand the importance of such experiments in modern biology. Molecular models or ‘student modelling’, in which the students themselves represent different atoms (labelled and unlabelled), can be used to provide a physical representation of the process.

3.2 Photosynthesis Prior knowledge and experience of photosynthesis At age 11, students will probably have grown plants in pots and in gardens and will know that plants need water and light to grow, but they are unlikely to know much more about photosynthesis. A common misconception that learners may hold is that the purpose of photosynthesis is to produce oxygen 55

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for ‘us’ to breathe. Photosynthesis may be regarded mistakenly as a ‘special form of respiration’, or as a substance rather than a process. The function of chlorophyll may be misunderstood (for example, as absorbing carbon dioxide). The belief that only plants photosynthesise can inhibit learning about classification or understanding that many single-celled organisms can also photosynthesise.

A teaching sequence The teaching sequence suggested here starts with the photosynthesis equation and uses that as a basis for investigating the formation of the products (carbohydrate and oxygen) and the uptake of the reactant carbon dioxide. Some students are notoriously unenthusiastic about plant biology, but they may be more enthusiastic about practical work. Most of the practical activities advocated here can easily be carried out by the students themselves. Indeed, the science could be derived entirely from investigative work, with students supported in constructing the summary equation themselves from practical results. The Science & Plants for Schools (SAPS) website has details of most of the investigations suggested here, as well as further background information on plants and photosynthesis.

Plant growth A good way to test students’ existing understanding of plant nutrition is to show them a plant seed and a fully-grown plant (the bigger the better) and ask ‘Where did most of the mass of the adult plant come from?’. Most students are likely to answer from soil or from water, or even from the Sun, but very few give the correct answer: from the air. It is counter-intuitive for some students to believe that solid wood was built from gases in the air, and students may like to investigate the history of discoveries about plant nutrition, such as the experiments of Jan Baptist van Helmont (Figure 3.1), Jan Ingenhousz and Joseph Priestley. Such research can add a human element to the subject and can support deeper understanding. Consider discussing with students what plants take from the soil through their roots, besides water. Particularly important elements to mention are: nitrogen (in the form of nitrate ions), used as part of amino acids for protein production; phosphorus (as phosphate ions) for DNA and similar molecules; and magnesium ions to become part of chlorophyll. Some plants are grown commercially, for example tomatoes, cucumbers and peppers, with their roots in solution rather than soil and this is called hydroponics.

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3.2   Photosynthesis

Science in context Simple investigations into the growth of plants without soil, a technique known as hydroponics, can be carried out in class. It is possible to do investigations into the growth of plants such as Tradescantia, radishes or duckweed in soil-less conditions by growing seedlings in a container of water with minerals/fertiliser added. Students can also use this approach to investigate what mineral salts plants need for growth, production of chlorophyll, flowering, etc. by supplying mineral solutions deficient in particular elements.

5lb shoot

+

200lb dry soil

+

5 years with only a supply of rain water for growth

169lb 3oz tree

+

199lb 14oz dry soil

Figure 3.1  Van Helmont’s experiment

The discoveries of Priestley and others led to this summary equation for photosynthesis: carbon dioxide + water 6CO2 + 6H2O

energy transferred energy transferred

glucose + oxygen

C6H12O6 + 6O2

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Carbon dioxide is taken from the air into the leaves via the stomata; water is taken in through the roots from the soil. The glucose produced is a carbohydrate, used as raw material for growth (biomass production) and for cellular respiration in the plant. The waste product, molecular oxygen, exits the leaf through stomata or is used in respiration by cells in the leaves. Younger students should know the word equation while older students may need to know the symbol equation as well. It is fairly simple to balance the symbol equation, but this balancing is actually misleading. The photosynthesis ‘equation’ is a summary of some 30 separate steps, rather than a simple chemical reaction. As was the case with the respiration equations, this is an important point to make. All the carbon and oxygen atoms in glucose come from carbon dioxide, while all the oxygen atoms in the product oxygen come from the water, and to show this correctly it would be necessary to include water on both sides of the equation. Experimentally, this was demonstrated using different isotopes of oxygen. Students will encounter this in post-16 biology, but a qualitative treatment could be suitable for higher-attaining students in the 14–16 age group.

Leaves Photosynthesis mostly takes place in leaves, which are well adapted to this job. Leaves are thin and flat to absorb as much light as possible, and they often turn to face the Sun. Time-lapse videos of this heliotropism (first noted by Leonardo da Vinci) can be found on YouTube or the BBC video The Private Life of Plants. A transverse section of a leaf can be studied under the microscope. Stomata can also easily be observed under the microscope: suggested methods are given on the SAPS website. Examples include observing stomata in Tradescantia zebrina, observing stomatal opening and closing in Commelina communis, and the use of a graticule to measure the density of stomata on a leaf surface and to investigate how this stomatal density varies in plants from different environments. Many students are familiar with the structure of a typical palisade cell, but some may think that this is a rigid structure. Cytoplasmic streaming can be observed in African violets using a light microscope. The chloroplasts can be seen to move around the cell’s cytoplasm. Alternatively, the internet has many videos of this fascinating process.

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3.2   Photosynthesis

Chlorophyll Photosynthesis depends on chlorophyll, a green compound containing magnesium found inside chloroplasts. Light transfers energy to chlorophyll, which causes water to split. The energy is eventually transferred into a chemical store, glucose. Students can extract chlorophyll from leaves easily, using thin-layer chromatography to separate the pigments (see TLC of photosynthetic pigments protocol on the SAPS website for detailed instructions). It should be possible to see chlorophyll a (dark green), chlorophyll b (pale green), carotenoids (yellow and orange) and lutein (brown). Different-coloured leaves can also be used for comparison. The chlorophyll solution can also be used to investigate the absorption of light. An intense white light can be split into a spectrum using a prism in the dark and then a cuvette of chlorophyll inserted into the light path (Figure 3.2). The chlorophyll absorbs the red and blue light but not the green and yellow. This can help students appreciate that photosynthesis uses red and blue light, but not green light. prism

screen

chlorophyll cuvette

slit

red (dim) green

white light source

blue (dim)

Figure 3.2  Absorption of light by chlorophyll

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KEY ACTIVITIES

3 Energy and materials

Investigating the factors required for photosynthesis Students can test for the main product of photosynthesis: carbohydrate. They can investigate production of starch (a storage product made from glucose in leaves) using the iodine test on laboratory plants such as Pelargonium. The plants need to be destarched first (in a dark cupboard for two days, so that pre-existing starch is used up in respiration). Then the plants can be set up to test the importance of factors such as light (by covering a leaf with foil, perhaps with a hole cut in it), carbon dioxide (by placing the leaf in a flask containing potassium hydroxide to absorb the carbon dioxide), stomata(by painting one or both surfaces with petroleum jelly) and chlorophyll (by using a variegated leaf). Expose the plant to bright artificial light for several (at least 1–3) hours (Figure 3.3a) and then test the leaves for starch using iodine. See the SAPS website for further details. a

b

A

variegated leaf

A

orange black

C B

orange cotton wool bung

B flask with KOH solution to absorb CO2

D C lower surface of leaf painted with petroleum jelly

orange black

D

orange

Figure 3.3  Investigating the factors required for photosynthesis: a setting up the experimental plant; b the results after testing the leaves for starch

Typical results are shown in Figure 3.3b. Students should be supported to draw their own conclusions. Be aware of potential pitfalls with this common approach. It is easy not to spend long enough on helping learners to understand why the practical is designed as it is. In terms of investigation, it is best regarded as a test of the model. Given students’ new knowledge of photosynthesis, what results would they predict and why? It is important to make the links between the starch (which is being tested for) and the glucose (which is produced in photosynthesis). Direct students’ attention to the key part of the practical – the starch test – rather than to the earlier softening and chlorophyll removal that facilitate the starch test. Consider with the learners the role of the prior de-starching of the plants and of the subsequent illumination. The latter promotes photosynthesis, with starch accumulating in those areas exposed to light, in large enough amounts to yield a distinct result in the iodine test. 60

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3.2   Photosynthesis

Measuring the rate of photosynthesis We can also measure the production of the other product of photosynthesis: oxygen gas. The most common way to do this is to use an aquatic plant. Some tropical pondweeds work better than temperate Elodea species (Eldridge, 2005). CLEAPSS suggest, for example, using Cabomba aquatica and advise on safe disposal of potentially invasive species. A length of pondweed is placed upside down in 0.25 M (2%) potassium hydrogencarbonate solution, which can be explained to students as providing dissolved carbon dioxide. The oxygen produced by photosynthesis appears as bubbles from the cut end (Figure 3.4a). The number of bubbles emerging in a minute can simply be counted and used as a measure of the rate of photosynthesis. Alternatively, the gas can be collected by upward delivery and the volume measured that way (Figure 3.4b and 3.4c). The collected gas can be tested with a glowing splint to demonstrate that it is oxygen. a

b

c oxygen bubble collects here

oxygen bubble

oxygen

ruler

pondweed

Plasticine weight

measuring cylinder

oxygen bubble moved here to be measured, using the syringe

funnel

2% KHCO3 solution

Figure 3.4  Measuring the rate of oxygen production by pondweed: a counting bubbles; b measuring the volume using a photosynthometer; c collecting the gas in a measuring cylinder

The plants should be kept in large sunny aquarium tanks and short lengths cut cleanly with sharp scissors underwater immediately before use. The 2% potassium hydrogencarbonate solution is needed to provide plenty of carbon dioxide for photosynthesis, and the potassium salt is reported to be more reliable than the sodium salt. A very bright light source is needed to encourage a good rate of bubbling. Normal room lights or desk lamps are inadequate. Halogen lamps can be used, with an intervening heat filter, or fluorescent tubes (including energy-saving bulbs) work well. For a controlled experiment, the plant material should be carefully shielded from other light sources. If the bubble rate slows significantly, cut the end again and wait a couple of minutes.

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Technology use Many data-logging systems include oxygen and carbon dioxide gas sensors (Delpech, 2006). These sensors can be used to demonstrate the changes in the gases due to photosynthesis in leaves and small plants (Figure 3.5).

gas sensors

CO2

O2 O2

glass jar

CO2

fresh leaves

Figure 3.5  Using gas sensors to demonstrate photosynthesis

Changes in both gases can be observed in real time as conditions, such as light intensity, are changed. The gas sensors have the advantages that familiar land plants can be used, both oxygen and carbon dioxide can be monitored simultaneously and the changes can be projected onto a screen.

Light intensity The rate of photosynthesis is affected by light intensity. If light is limiting, a plant which is further away from a light source will have a lower rate of photosynthesis than a plant which is closer to that light source. We can use the inverse square law to explain this. As light waves move away from a light source they spread out: the total amount of light stays the same, but it is spread out over a greater area. This means that each unit of area receives a smaller proportion of the light; in other words, the light intensity decreases. We can calculate the relative light intensity at any distance from the light source using the inverse square law: light intensity (LI) ∝

1 d2

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3.2   Photosynthesis

Investigating the effect of light intensity on the rate of photosynthesis Students can investigate the effects of distance (d) and light intensity (LI) using the technique described in Figure 3.4. A plant can be placed at a set measured distance from a light source and the rate of photosynthesis calculated. The distance between the plant and light source can be altered (and measured) and the rate of photosynthesis recalculated. Alternatively, the intensity can be measured with a lux meter. Students can then use the inverse square law to explain why the rate of photosynthesis is lower when the plant is at a greater distance from the light source.

Investigating further factors in relation to photosynthesis l The

colour of the light can be changed by placing coloured filters in front of the lamp, with discussion about how to keep overall light intensity constant (using light meters and changing distances could offer a solution). l The carbon dioxide concentration can be altered by changing the amount of potassium hydrogencarbonate added to the solution. l The temperature can be changed by placing the pondweed tube in a glass water bath, such as a large beaker. All these investigations can be planned by students themselves, choosing which variables to control and how to quantify the outcome. Another way of visualising the oxygen produced in photosynthesis is to use floating leaf discs. Small, standard-sized discs are punched from leaves and placed in a potassium hydrogencarbonate solution. The oxygen produced by photosynthesis in the leaf discs causes the discs to rise to the surface and float, and the time taken to rise can be used as a measure of the rate of photosynthesis (details on the SAPS website). In this experiment students may observe some discs floating and others sinking. Ask students what must be happening at the point where half the discs are floating and half are sinking. The answer is that this is the compensation point, where the rates of photosynthesis and respiration are equal. Carbon dioxide is about 40 times more soluble in water than oxygen is, so the buoyancy of the leaf discs is dependent on oxygen gas in the leaf’s inner spaces.

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Science in context Many students hold the misconception that all seeds require light to germinate. This is easily challenged with a packet of cress seeds and a dark cupboard! But, more interestingly, cress seedlings in the dark grow more quickly than cress seedlings in the light, at least at first; they etiolate (grow tall, spindly and do not turn green). Gardeners and farmers use this idea with ‘forced’ crops such as chicory or early rhubarb. Commercially, rhubarb plants are grown outside for a few years, then brought inside in the winter where, in the dark, they grow rapidly into tall, pale pink leaf stalks (with yellow leaves). After heavy harvesting of these stalks, the plants are almost dead. Challenge the students to explain the processes involved.

Limiting factors Experiments such as the ones above show that the rate of photosynthesis depends on a number of factors, including light intensity, temperature and carbon dioxide concentration. But at any given time there can only be one factor that is actually controlling the rate: the limiting factor. This is the factor that is in shortest supply. Students should be able to interpret graphs such as the one shown in Figure 3.6.

Rate of photosynthesis

B

A

Light intensity

Figure 3.6  Graph used to identify limiting factors. At low light intensities (A), the rate of photosynthesis increases as the light intensity increases, so light must be the limiting factor. At higher light intensities (B), the rate of photosynthesis stays the same even if the light intensity increases. This means that light is not the limiting factor, and the rate of photosynthesis must be limited by some other factor.

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3.2   Photosynthesis

The fate of glucose Plants are amazingly self-sufficient. They use the glucose they make in photosynthesis as the basis for making everything else in their cells, in other words, their biomass. This includes: sucrose, easily transported around the plant; starch, stored in roots, leaves and many seeds; fats for energy stores and for cell membranes; cellulose to make cell walls; and proteins for growth. For some of these compounds, plants need other elements, such as nitrogen and sulfur. They get these two elements in the form of the minerals nitrate and sulfate, which they take up from the soil by active transport through their roots. Students should be encouraged to use the word ‘minerals’ rather than ‘nutrients’ or ‘food’ for these inorganic substances absorbed from the soil.

Science in context Many garden centres sell products that claim they are plant ‘food’; one example of such a product is Miracle-Gro. Ask students whether this is an accurate reflection of what the product is. Does it fit their understanding of the term ‘food’? Ask students what they would call it.

Photosynthesis and respiration Plants use some of the glucose they have synthesised in the process of photosynthesis as a substrate in cellular respiration. This occurs throughout the plant, releasing carbon dioxide and water as products. The energy made available by this respiration drives biochemical reactions necessary for life processes, such as growth, repair, movement and reproduction. All living cells respire all the time, including plant cells. So, in the dark (and in non-photosynthesising parts, also in the light), plants give out carbon dioxide. In favourable conditions in the light, photosynthesising parts of plants (typically leaves) take in carbon dioxide and release oxygen at the same time. Overall, growing plants take in more carbon dioxide than they give off: they are net photosynthesisers, not respiring all of their photosynthetic product.

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3 Energy and materials

Investigating photosynthesis and respiration using hydrogencarbonate indicator Students can investigate changes in carbon dioxide concentration to show that carbon dioxide is taken up by plants in photosynthesis and given out by plants in respiration. The most common way to do this is to use hydrogencarbonate indicator solution. Since carbon dioxide forms a weak acid in solution, its concentration can be detected using this very sensitive pH indicator. Figure 3.7 shows the colour changes. hydrogencarbonate indicator. Yellow

Orange

carbon dioxide released by plant to surroundings

Red

Magenta

0.04% carbon dioxide in atmospheric air

Purple

carbon dioxide taken up by plant from surroundings

Figure 3.7  Colour scale for hydrogencarbonate indicator

A typical experiment is shown in Figure 3.8. After a day in the light, tube A remains red, tube B turns purple as carbon dioxide is, overall, taken up by the plant for photosynthesis and tube C turns yellow as carbon dioxide is released in respiration. Students should be able to explain the colour changes in terms of photosynthesis and respiration. A different and very successful variation on this is to use algae instead of pondweed (Eldridge, 2004). The algae are easily entrapped in alginate forming green beads or ‘algal balls’. (See the SAPS website for further details.) The light intensity that gives no change in colour of the indicator represents the ‘compensation point’ (as noted above with floating leaf discs); the rates of photosynthesis and respiration are equal, so there is no net change in carbon dioxide concentration.

hydrogen carbonate indicator solution

tube A control

foil

tube B light

tube C dark

Figure 3.8  Investigating changes in carbon dioxide concentration using hydrogencarbonate indicator

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3.3   Energy and materials in ecosystems

3.3 Energy and materials in ecosystems A teaching sequence It makes sense to begin the topic considering energy and materials in ecosystems once again with food. Discussion of food chains and food webs is followed with an introduction to ecological pyramids: numbers, biomass and (in later studies) energy. The topic concludes with a section on decomposition and recycling.

Food chains and food webs A good starting point for considering energy and materials in ecosystems is with food and, when teaching food chains and webs, to describe feeding interactions in habitats with which learners are already familiar. Getting students to construct food chains that explain the origin (ultimately in producers) of a meal they have eaten can be a useful activity to get them to use secondary sources to link themselves with the global ecosystem. What, for example, do tuna fish eat, and what are the conservation issues of commercial tuna fishing? What kind of food is fed to the animals that many people eat as food, and what are the ethical issues that this knowledge raises? Revisiting this when learners have considered ecological pyramids and energy flow gives an opportunity to consider why vegetarianism can be energy efficient, and what might compromise this (for example, eating airfreighted out-of-season fruit and vegetables). One very common misconception about food chains and webs concerns the arrows; learners may get them the wrong way round as they have not appreciated that they show the direction in which biomass is transferred. When learning about nutrient cycles, children typically bring to the carbon cycle any misconceptions that they hold about photosynthesis and respiration; it may be useful to revise key ideas briefly. The stochastic nature of nutrient cycles also causes confusion: learners may believe that carbon (or nitrogen) atoms always follow a fixed route round their respective cycles. Various game-like simulations of the cycles (perhaps designed by the students themselves) may help learners to understand that there are many chance events determining which way a particular atom moves, if, indeed, it moves through the cycle at all.

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Ecological pyramids and energy flow One of the reasons why food chains are quantified and represented as pyramids (numbers, biomass and – beyond 11–16 biology – energy) is to demonstrate that the amount of material or energy available in the food chain decreases with increasing trophic level. The only way in which energy is made available along food chains is through biomass being transferred as food for the next trophic level. The energy in the biomass store is only made available to a particular trophic level if the biomass is respired at that level. Hints on teaching energy flow: ➜ Energy

in biology is the same as energy in physics and chemistry. ➜ The Sun is the ultimate origin of energy for almost all food chains. ➜ Energy is not recycled in an ecosystem; a continuing input to photosynthesising producers is required to sustain it. ➜ At each trophic level, some energy is transferred out of the living components of the ecosystem, heating the environment. This energy is then not available to the food chain for further life processes. ➜ Therefore, food chains rarely have more than five links, as too little stored energy becomes available for the needs of the top consumer. Learners should consider all the ways in which biomass in one trophic level might not reach the next trophic level: including by not being eaten (and going instead to decomposers), by not being absorbed by the gut (and going as faeces to decomposers) and through being respired and the subsequent energy transfers warming the environment directly (conduction, convection and radiation) and indirectly (through the work of muscles in generating movement and sound). This could be a good opportunity to discuss the practices and ethics of modern intensive farming of livestock, specifically the various approaches it uses to avoid biomass energy stores being transferred into anything other than food for humans (such as losses to pests and to heat).

Careers It is worth reminding students that with an ever-increasing population, science is continually trying to find solutions to the problems we face regarding food security. Many roles exist in agricultural technology, plant biotechnology and applied food science which may be of interest to them.

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3.3   Energy and materials in ecosystems

Constructing pyramids of numbers It is possible for students to count producers, herbivores and carnivores in a sample from pitfall traps, from leaf litter or from a pond and thus construct a pyramid of numbers. This usually only demonstrates the principle of a pyramid of numbers very crudely. In pitfall traps, the carnivores may eat the herbivores before they can be counted! In leaf litter, many of the herbivores, such as springtails, are very small and easily overlooked. In pond water, the countless millions of microscopic algae on which most of the food chains rely for energy are not normally noticed at all. It is, however, well worth carrying out such an exercise because it makes the theoretical treatment, which must inevitably follow, much easier to understand by making some difficult concepts more concrete. Good hygiene is essential when handling leaf litter and pond water. If students grasp the difficulty of producing reliable and accurate pyramids of number for an ecosystem, they are likely to appreciate how much more difficult it is to generate good pyramids of biomass. And even these are crude ways of estimating the amounts of energy transferred through trophic levels (for a given volume of ecosystem over a particular time period) that are the real measures of how productive ecosystems are. Learners should be able to explain in energy terms why pyramids of number are usually pyramidal, and pyramids of biomass even more so. Exceptional pyramids of number are typically those where a single producer is very large (such as a tree) or where there are consumers that are smaller but more numerous than their food organisms: for example, aphids or fleas. These anomalies generally resolve when numbers are converted to biomass.

Decomposition and recycling Substances needed by plants, such as carbon dioxide and mineral salts, rarely run out because they are continually recycled, owing to microbial action with the help of detritivores. Animals depend on plants to recycle oxygen and to produce food by photosynthesis. Detritivores are animals, such as earthworms and woodlice, that play a part in decomposition. When dead or waste organic material passes through a detritivore, it is partly digested and absorbed; what remains comes out in the faeces. Digestion of the remains is then completed by soil microbes. Decomposers are key to the cycling of materials in ecosystems, as their actions return materials from organic phases to the non-living parts of the ecosystem, where they are available again to plants.

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Some students find decomposition difficult to understand. Some reasons for this are: ➜ Microbes

are too small to see. Many fungi are exceptions to this; their mycelia and reproductive bodies (mushrooms and toadstools) can often be seen on rotting vegetation. ➜ Lack of understanding that plants need carbon dioxide and mineral salts from the soil and that continued plant growth depends on the recycling of these substances.

Science in context Students should consider why arable farmers must use fertilisers to replace the minerals taken away in the crop. Reflect on how the use of manure as a fertiliser on mixed farms represents a cycling of nutrients in the farm ecosystem. Note the simple meaning of ‘fertile’ (i.e. ‘productive’) in this context. Nutrient cycles contain some very difficult concepts; be careful not to teach more theory than necessary. You do not have to teach every nutrient cycle for students to get the idea of cycling. The overriding challenge is to find exciting and motivating ways to teach nutrient cycling. Practical activities that you could do, with suitable risk assessment, include setting up a pile of rotting logs outside for future fungus forays (and a good supply of woodlice for choice chambers!), keeping a wormery or making compost. SAPS have a simple activity for making a ‘compost column’, which collects a liquid product that makes a very good plant ‘food’, closing the loop!

Science in context The Venus flytrap, sundews and pitcher plants are examples of insectivorous plants. Ask students to identify the key materials they gain from their prey and suggest why this behaviour gives a survival advantage. A clue comes in the fact that their natural habitat is boggy, waterlogged ground. The water in this soil fills the spaces occupied by air in drier soil. Denitrifying bacteria are adapted for anaerobic conditions, making it difficult for these plants to obtain nitrogen. You could even get students to grow some of these plants in your classroom.

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3.4   Resources

3.4 Resources Suppliers Blades Biological (www.blades-bio.co.uk) for pondweed, light banks, algae and growth media Lee Filters (www.leefilters.com) for light filters Progrow (www.progrow.co.uk) for hydroponics supplies including grow lights Sciento (www.sciento.co.uk) for algae and growth media

Websites Generic websites CLEAPSS is a subscription service with definitive advice on safe and effective practical science for schools, available at: http://science.cleapss.org.uk eChalk provides excellent diagrams, animations and images for use with an interactive whiteboard. A subscription is needed to access these resources: www.echalk.co.uk/Science/biology.aspx ‘Kings Science’: Richard King has put together a very useful set of free resources, including flash animations and worksheets, which can be found at: www.kscience.co.uk/index.htm Practical Biology is a website that is a joint project of the Nuffield Curriculum Centre, the Society of Biology and CLEAPSS, and is intended to encourage teachers to carry out more practical biology in schools: https://pbiol.rsb.org.uk/

Websites related to photosynthesis Science & Plants for Schools (SAPS) has a range of materials and suggestions for practical work: www.saps.org.uk/ The Tomato Zone offers free resources of tomato-based information: www. thetomatozone.co.uk/

Websites related to respiration Practical instructions and a demonstration of the screaming jelly baby experiment can be found by searching for ‘screaming jelly baby’ at: www. stem.org.uk/elibrary/ The Science Photo Library website provides a vast number of high quality photographs. The images of tissues, cells and organelles (including mitochondria) are excellent: www.sciencephoto.com

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References Delpech, R. (2006) Making the invisible visible: monitoring levels of gaseous carbon dioxide in the field and classroom. School Science Review, 87 (320), 41. Eldridge, D. (2004) A novel approach to photosynthesis practicals. School Science Review, 85 (312), 37–45. Eldridge, D. (2005) Cabomba – a reliable alternative to Elodea? SSERC Bulletin, 215, 10–12. Harlen, W. (ed.) (2015) Working with Big Ideas of Science Education. Trieste: InterAcademy Partnership. Available at: www.ase.org.uk/bigideas Needham, R. (2014) Using ‘Energy Ideas’ in the teaching of biology. School Science Review, 96 (354), 74–77. Ross, K. (2013) Fuel and food are not made of energy – A constructive view of respiration and combustion. School Science Review, 94 (349), 60–69. Tomkins, S. P. and Miller, M. B. (1994) A rapid extraction and fast separation of leaf pigments using thin layer chromatography. School Science Review, 75 (273), 69–72.

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4 Exchange

Ann Fullick and Indira Banner

Topic overview The principle of exchange, at different levels of organisation, runs through many aspects of biology. Students may have met the process of diffusion and the movement of substances across cell membranes in their work on cells. Exchange surfaces can be thought of as the cell membrane, where substances pass in and out of the cell, or two organ parts separated by the one-cell-thick membrane of each, as in the lung capillaries and the lining of the lung alveoli. Clearly the exchange point has thin membranes for the substances to pass through and these tend to be wet surfaces to support diffusion. When you discuss exchange, consider what students have – or have not – met before. Weave in as many examples as possible, while focusing on those organisms highlighted in curricula and specifications. This helps your students see that this is a ‘big idea’: a biological principle that can be applied in many different systems, as important in plants as in animals, and which can be considered when analysing any new situation (Harlen, 2010). The principles of exchange play a part in developing an understanding of many biological processes, from the sense of smell to excretion, photosynthesis and respiration. The most important thing when teaching this topic is to ensure that your students understand the principles of exchange and can apply them in a number of different systems. There are several ways you might teach these ideas: 1 All of the topics relating to exchange can be taught as a linked and integrated whole, building around the principles of exchange, drawing on the different systems where they are relevant as examples. This approach has many advantages, ensuring that students focus on the process – exchange – rather than specific examples. The disadvantage is that students may become confused by dipping into each system, and lose the coherence of a particular plant or animal system. This approach needs careful planning. 2 Different topics involving exchange are covered as they appear in the planned, taught curriculum, often built around the demands of examination specifications. It can be easier to engage students in factors that increase the rate of diffusion/exchange when talking about their own lungs and the effects of different activities on the process. On the other hand, this approach may result in students failing to see the big idea, and being unable to transfer their understanding of exchange to other organisms or 73

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systems. The exchange process becomes an add-on, tackled only briefly in the context of specific exchange surfaces such as the alveoli. Students may also get that ‘done this before’ feeling when exchange is revisited throughout a course. 3 A combination of these two approaches can be an ideal way to cover exchange. Beginning with the big ideas approach for exchange, you engage students with the over-arching principles, as well as illustrating the many places it is relevant in organisms. This can segue into an in-depth study of particular systems where exchange is key, focusing on the principles they have already come to understand. Each time you introduce a new biological system or process that depends on exchange, revisit the key principles. Once you have weighed up these ideas, you will find your own way of delivering exchange, bearing in mind: ➜ the

demands of the curriculum demands of the examination specification for older students ➜ the order in which other topics are to be introduced ➜ personal preference ➜ departmental policy. ➜ the

One final point is that as teachers we must remember the demands of a spiral curriculum. Students are often revisiting a topic when we ‘introduce’ it to them, or we in turn may be revisiting it later in their school careers. Tempting as it may be to tell all our best anecdotes and introduce all the most fascinating ideas to our students lower down the school, if we ration them and keep some things fresh for later, we – and our students – will reap the benefit.

4.1 Principles of exchange Prior knowledge and experience Students in primary school are only likely to have met the lungs as one of the organs of the body. They may know that living things need oxygen and produce waste products including carbon dioxide. Progression in this topic comes with age and with coverage across both different areas of biology and the other sciences. For example, students may meet the particle model of materials in chemistry along with the basic concept of diffusion. When they learn about cells (see Chapter 2), they will meet the basic principles of diffusion and movement across cell membranes. When they look at transport systems (see Chapter 5) they will have the importance of diffusion reinforced.

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4.1   Principles of exchange

A teaching sequence Your desired learning outcomes will depend on the age and capabilities of the students you are working with. It is a good idea to start off with some questions, to give you a feel for what the students have met before: ‘What does the term “exchange” mean to you?’, ‘Can you think of situations in a plant or an animal where exchange might be important?’, ‘What can you tell me about diffusion?’, etc. It is useful to go into the first lesson with plenty of material prepared, in case the students have a good grasp of the basics from earlier studies. Most students learn best in context. A session where everyone is encouraged to input ideas, building up a map showing where exchange is important in living things, can involve students actively from the start. A presentation illustrating different areas where exchange is important (not simply gas exchange) can help give the ‘big ideas’ (Harlen, 2010) approach more relevance. The level of understanding you expect from students will depend on their age and what they have been taught. In the early years of secondary biology, students should understand the basic concepts of diffusion and exchange surfaces. Progression includes a fuller understanding of the processes: how adaptations of exchange surfaces have evolved to maximise exchange, a mathematical understanding of surface area : volume ratios, and an ability to apply the principles of exchange to new situations in both animals and plants.

Physical principles There are three physical principles needed for complete understanding of exchange: ➜ diffusion ➜ surface

area ➜ surface area : volume ratios.

Cross-disciplinary It is worth checking with colleagues whether you can reasonably expect some understanding of these areas from other subjects such as mathematics, physics and chemistry. It can be helpful to run a quick experiment or demonstration of each principle when you need it, as students do not always transfer learning between subjects.

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Diffusion There are many demonstrations of diffusion, from potassium manganate(VII) crystals dropped into water to thought experiments about cooking smells in the home. It is useful to emphasise that diffusion takes place down a concentration gradient: from where there are a lot of particles of the diffusing substance to where there are relatively few. This is also a good opportunity to point out that diffusion ‘just happens’; it is due to the random motion of particles which end up spreading out equally and is not an active, energy-consuming process. A common misconception among students is that particles somehow move from A to B with intent. If possible, take students to an empty space and put the whole class in one corner. Ask them to walk about gently with their eyes closed, changing direction when they bump into another person or the walls. After a few minutes students will see that they have spread randomly about the room. Releasing a puff of perfume or aftershave at one point in the classroom and asking students to indicate when they can smell it also shows the random movement model; ask students to explain what is happening and, importantly, why it is happening. There are some effective animations showing how random movement results in a relatively even spread of particles, giving the overall effect of diffusion down a concentration gradient.

KEY ACTIVITY

In biological systems, exchange usually takes place across membranes. Students may have already met this idea when they looked at cells, so you may be reprising ideas, not introducing them. Meeting important ideas several times in different contexts is valuable for students, but they may not think so!

Diffusion into a model cell

To help picture what happens at cell level, you can fill lengths of Visking tubing with starch suspension, tying knots at either end to produce a ‘sausage’ shape. Half fill a boiling tube with dilute potassium iodide solution and drop the ‘model cell’ into it. Remove the tubing every thirty seconds or so to see how the starch suspension gradually turns dark blue as the iodine passes through the Visking tube membrane into the ‘model cell’. This demonstration helps mimic the process of diffusion through a membrane into a cell. Challenge the students by asking what you could do to see if the membrane is only allowing small molecules through or that the membrane only allows movement of molecules through in one direction.

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Surface area The idea that the larger the surface area of a membrane, the more diffusion will take place may be obvious to you, but it is not always obvious to students. Exchange surfaces often have structural adaptations to increase the surface area available for diffusion, as in the spongy mesophyll of plant leaves or the alveolar structure of mammalian lungs. Folding, either in the overall structure or in the membrane itself, is another common way of increasing surface area. This is a clear example of how structure is related to function. Relating surface area to everyday examples can make it more comprehensible. (Current thought: surface area of a pair of human lungs is 70 m2 or half a tennis court.) Understanding surface area is something students often find difficult, so do not be afraid to revisit it every time it is relevant in different aspects of biology.

Maths Physical models are very important in helping students to understand this surface area principle, whether you demonstrate or they make the models themselves.

Surface area : volume ratio Students may well have met this principle when looking at cells. The relationship between surface area and volume is a key biological principle which has relevance in many different biological contexts. Chapter 2 has more details and some useful practicals. You can also visit the Nuffield website for practical advice and guidance (see Resources section at the end of the chapter).

Further activities If you have time, get students to carry out a physical demonstration of the surface area : volume principle: 1 Provide groups with one large potato and several little ones with (between them) approximately the same mass as the large one. 2 Ask the students to peel the potatoes and use graph paper to work out the approximate surface area of the peel of each mass of potatoes. The small ones should (between them) have a larger surface area : volume ratio. Ask students to draw on their learning to explain why lungs contain many tiny alveoli, rather than being one big air sac. 3 Remind students not to eat the raw potato because of contamination risk in the laboratory.

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Enhancement ideas Introduce students to one or more unusual examples of exchange in action. Some good examples include ‘bum-breathing’ turtles or insect larvae, salt glands in some sea birdauminous plants. Task them to find out more, summarise how the organisms are adapted for the exchanges and report back to the class.

4.2 Gas exchange in animals and plants As mentioned in Chapter 3, students often get confused between breathing, gas exchange and respiration. Be constantly aware of this and aim to keep the different concepts separate in their minds. One way is to teach breathing and gas exchange at a different time from respiration; another is to help students recognise clearly how the different processes are interdependent. To reduce confusion, students must understand that breathing movements cause air to travel into and out of the lungs, maintaining a steep diffusion gradient between the lungs and blood vessels so that gas exchange can take place. Gas exchange means that the net movement of oxygen is into the blood from the air in the lungs and net carbon dioxide movement is out of the blood into the air in the lungs. The oxygen brought into the body in this way is needed in the cells for respiration, and the excreted carbon dioxide from respiration is removed from the cells and, eventually, the body because if concentrations rise too much, carbon dioxide becomes poisonous.

Science in context One of the joys of teaching this topic is that students can use their own bodies as an experimental system. As long as teachers are aware of safety concerns, and the need to control any competitive comparisons of performance, this area of biology can be full of interest for students because it relates easily to themselves, their families and sporting personalities. Be aware of medical conditions that affect the ability of certain students to perform exercise; students with asthma, for example, may need to use their inhalers first. Exclusions from practical work must be handled carefully to avoid ‘labelling’ students with disabilities. However, all students breathe, and most students increase the rate at which they breathe with some sort of exertion, so it should be possible to include all students in some way.

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4.2   Gas exchange in animals and plants

Another useful aspect of teaching gas exchange is that it takes place in plants as much as animals. This provides another huge practical resource, along with a prime opportunity to reinforce the idea that all living things face similar problems. You can highlight the similarities in their solutions – in this case, for example, looking at leaves as well as lungs to show structural adaptations to make gas exchange as efficient as possible – and the differences. While doing this, encourage students to think about how plants produce their own oxygen in photosynthesis when it is light, and how animals are able to move air over their exchange surfaces in a way not available to plants.

Prior knowledge and experience Students will probably have met material specifically linked to breathing in primary schools. However, they are unlikely to have considered gas exchange in plants in any detail beyond – hopefully – recognising that plants need oxygen, for respiration, and they produce carbon dioxide. A common misconception about all organisms, but often particularly related to people, is that we breathe in oxygen and breathe out carbon dioxide! Always reinforce the idea that it is air that moves in and out of the lungs. The oxygen and carbon dioxide that are exchanged only make up part of the air. It is simply that more oxygen goes into the lungs than comes out and more carbon dioxide comes out of the lungs than goes in. Give students a table showing them the differences in oxygen and carbon dioxide concentration in inhaled and exhaled air to make this point more clearly.

A teaching sequence As always, before beginning this topic you need to decide your desired learning outcomes. For example, younger students might know and label the main parts of a leaf and the human breathing system; with older students you may want them to understand how the structure of the leaf and the lungs are related to their functions in gas exchange, how breathing movements in mammals, fish and insects increase diffusion gradients, and the physical effects of exercise. Starter questions can give you an insight into the existing understanding of the whole class, so you might ask: ‘Do animals and plants have the same requirements for oxygen and carbon dioxide?’ or ‘Why is breathing so important?’. An important understanding is that plants do not breathe but, like animals, they respire all of the time.

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Science in context Students often love the stimulus and challenge of social, moral and ethical issues associated with a specification area; articles linked to topic areas like this are common in the media – both print and electronic – and it is a good idea to save them when you see them. For example, material on the role of plants in removing carbon dioxide from the air, asthma, cystic fibrosis, health risks from air pollution and smoking, and green areas as the ‘lungs’ of a city often appears in the media. You can use these resources in teaching as a basis for a discussion, a class or homework exercise, or a starting point for individual research.

Air movement in response to pressure changes Cross-disciplinary Students may have met the effect of pressure changes on air movements in physics or geography. However, be aware that you may be introducing the idea for the first time. When demonstrating the movement of air in response to pressure changes, you can use a model chest. Many school laboratories have a ‘balloon in a bell jar’ model chest ready made for this part of the course (Figure 4.1). Movement of the sheet at the base (the model diaphragm), starting with it domed and then pulling it down, creates a bigger space around the balloon, reducing the pressure and causing air to rush into it. Try the apparatus out beforehand. All too often the seals are leaky or the balloon has perished and it will not work! A cut-open pink bathroom sponge is useful here too. It can be used to show students the spongy structure of air sacs. Explain that the balloon in the bell jar represents one sac. The model therefore illustrates the way in which air moves into and out of a single alveolus as the pressure changes. Students should understand that this is one of millions of alveoli within their lungs. This is preferable to having two balloons set up as model lungs. Many students already have the misconception that their lungs are like a pair of balloons inflating and deflating in their chest, and the traditional ‘model chest’ reinforces that misconception.

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Science in context Ask students to identify helpful points and limitations of the model, which can expose misunderstandings. For example, a limitation of the ‘two balloons equals two lungs’ model is that the lungs are not actually like balloons; the ‘ribs’ of the model are a limitation because they do not move. air out

air in

glass tube tight seal

pressure falls

pressure increases

balloon inflates

balloon deflates

bell jar

‘diaphragm’ pulled down rubber ‘diaphragm’ must be attached very tightly to bell jar, ideally with handle for pulling it

rubber ‘diaphragm’ pushed upwards (domed)

Figure 4.1  An artificial chest with the ‘diaphragm’ in different positions showing air moving into or out of the balloon.

The need for gas exchange Living things need a molecule called ATP to carry out all the processes of life. As outlined in Chapter 3, respiration is a reaction which releases ATP. This reaction requires food (glucose) and oxygen and releases carbon dioxide as a waste product. Respiration is the reason why living organisms need oxygen and must get rid of carbon dioxide. This is a good opportunity to make sure all students are still comfortable with the idea of diffusion as described above, and recognise that gas exchange takes place in all organisms, including animals and plants.

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Organs for gas exchange The idea that some organisms need specialised organs to bring about successful gas exchange is an important one. Your students must understand the concept of surface area : volume ratio (see page 77) and its implications for breathing systems as animals get bigger and more active. Plants have relatively low energy requirements and make their own oxygen in photosynthesis. The spongy mesophyll layer in leaves, with its large surface area, allows efficient gas exchange and so plants do not require special organs for ventilation. Single-celled organisms and very small animals do not need specialised exchange surfaces because their surface area : volume ratios are sufficiently large for all the exchange they need to take place by simple diffusion through their external surfaces. However, as animals get bigger, the diffusion pathways to the cells inside the body get too long, and diffusion alone cannot supply all the oxygen the animals need to support their much more active lifestyles, or remove all the carbon dioxide they produce.

Gas exchange in plants The important aspects of the process of gas exchange in plants are detailed below.

1 The need for gas exchange in plants Students often get very confused about the fact that plants photosynthesise, using carbon dioxide and producing oxygen, and also respire, using oxygen and producing carbon dioxide. They are often semi-oblivious of the latter. Emphasise that plants need to respire 24 hours per day, as animals do, and that gas exchange is a necessary process in the leaves.

2 Structure of the gas exchange surfaces related to function Students may have already looked at the structure of a leaf under the microscope when they studied photosynthesis (Chapter 3) or looked at cells (Chapter 2). In this topic, they should think about leaf structure related to its function of gas exchange, focusing on the spongy mesophyll, the stomata and the lenticels. Lenticels are useful because they emphasise the need for the living cells in the trunk and branches of a tree to exchange gases, and they can be seen easily without a microscope! SAPS (Science & Plants for Schools) is the outstanding place to find ideas for practical activities involving plants. Students can look at the internal structure of leaves using microscopes and prepared or projected slides, focusing on the large surface area of the cells

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exposed to the air and any other adaptations for gas exchange, including the short diffusion pathways between the cells and the air. They can also carry out practicals which involve looking at stomata, using a variety of different techniques (search ‘measuring stomatal density’ on the SAPS website (detailed at the end of the chapter)). Students should develop a clear picture of the numbers and positions of the stomata, their ability to open and close, the factors which impact stomatal opening and closing, and how this affects gas exchange.

3 Factors affecting gas exchange in plants It is important to help students consider the different requirements of plant cells for different gases through a 24-hour cycle, and how the products of photosynthesis and respiration interact. More able students will recognise that gas exchange takes place into and out of chloroplasts and mitochondria as well as into and out of individual cells (see Chapter 3).

Gas exchange in humans The important aspects of the process of gas exchange in humans are detailed below.

1 Gross structure of the breathing system The structure of the breathing system (as opposed to the microscopic structure of the lungs) is important biologically to help students understand the movement of air into and out of the lungs. A model chest with removable parts demonstrates how the breathing system and thorax fit together. Students can observe slides of lung tissue using microscopes themselves, projected slides or appropriate web resources to make the spongy structure clear. The C-shaped cartilage rings of the trachea and the way food is swallowed past them often causes interest, and an explanation of choking can lead to a brief description of how to help someone who is seriously choking – an aside that may save lives! With older students, the beating of the cilia of the ciliated epithelium in the trachea and the valuable role of this process is usefully mentioned, not least as this will be referenced later when talking about smoking.

2 Method of ventilation This involves looking at how air moves in response to changes in pressure. There are a number of ways of approaching this.

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If you start by simply asking the students how they breathe in and out, you will learn their misconceptions. You can then describe what happens in the body when we breathe in and out. Putting their hands on their own ribcages and taking a deep breath in and out usually enables students to work out how the ribs move. The movement of the diaphragm as it flattens needs describing, followed by an explanation of the effect of this movement on the volume and air pressure inside the chest. Then, using the demonstration in Figure 4.1, the movement of air into and out of the lungs can be discussed. Students should recognise that breathing in is an active process (energy is expended on muscular contraction) whereas breathing out is normally passive as the intercostal and diaphragm muscles relax, reducing the volume of the chest cavity and so increasing the pressure of air in the lungs and forcing it out of the system. You can also explain that breathing out can be active too, in forced exhalation, and students can try it themselves.

3 The exchange process in the alveoli/adaptations to function This is where the concept of diffusion becomes very important. Students need to know the structure of an individual alveolus and its close association with blood vessels. If they have not yet looked at the circulatory system, it is a good idea to explain that the blood carries the oxygen to the cells where it is needed (see Chapter 5). It is difficult to talk about the structure of the alveolus and its capillaries without dealing with the issues of adaptation for function. Looking at the shape of the alveoli, single cell layers and the close proximity of the blood vessels makes it easy to point out how this system is so well adapted to the movement of oxygen from the air in the lungs into the blood and the movement of waste carbon dioxide from the blood into the lungs. ➜ The

alveoli have a large surface area for gas exchange to take place (see Physical principles, pages 75–77). ➜ The alveolar walls are thin (a single cell thick) so diffusion pathways are short. ➜ There is a good blood supply to carry carbon dioxide to the lungs and oxygen away from the lungs, maintaining a concentration gradient to aid diffusion in both directions. ➜ The lining of the alveoli walls is kept moist, so the diffusing gases dissolve easily, helping them to pass across the gas exchange surface. The diffusion gradient, maintained by blood flowing through the vessels and the changing of the air in the lungs, can be explained to older and more capable students.

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Technology use There are a large number of video clips and animations available. However, many online resources reinforce common misconceptions by talking about oxygen coming into the lungs and carbon dioxide leaving the lungs, rather than air with a higher concentration of oxygen or carbon dioxide. They may also show the lungs inflating and deflating without showing the movements of the ribs and diaphragm, so students think that the lungs self-inflate. A useful exercise is to ask students to find several online resources and look at them critically, writing a review and recommending the best teaching aid. Students can observe the effect of gas exchange on the air they breathe in and out using the apparatus shown in Figure 4.2. Either limewater (a clear liquid that turns cloudy when carbon dioxide reacts with it) or hydrogencarbonate indicator solution (a red liquid that turns yellow when carbon dioxide dissolves in it) can be used as indicators. For the limewater, prolonged exposure to carbon dioxide may make the cloudiness disappear, so stop once it has gone cloudy. Eye protection is needed when handling limewater. Some schools suggest doing this with the apparatus joined together so students breathe in and out, squeezing various bits of tubing to direct the flow of air. The apparatus shown in Figure 4.2 is simpler, avoids confusion and also helps to prevent a lot of spluttering and limewater getting everywhere! straw A

straw B

glass tube

glass tube

limewater

Breathe in through straw A – inhaled air bubbles through limewater.

Breathe out through straw B – exhaled air bubbles through limewater (can be just a straw into limewater).

Figure 4.2  Apparatus to show testing of inhaled and exhaled air for carbon dioxide

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It is also possible to show the difference between inhaled air and exhaled air by measuring the time that burning candles will stay alight in gas jars of each type of air. Both of these practicals involve students putting their mouths in contact with the apparatus; care must be taken to make sure that each student uses an individual straw or tube and that they are told very clearly to avoid getting liquid into their mouths. Students should breathe steadily while doing these investigations; rapid breathing can cause risk of hyperventilating. A circus of these practicals, including breathing on mirrors and cobalt chloride paper to show warmth and water in exhaled air, exposes students to different ways of thinking about inhaled and exhaled air. (NB. Use forceps or disposable gloves when handling cobalt chloride paper and avoid skin contact with it.)

Gas exchange in other living organisms Some examination specifications demand that students study gas exchange in different groups of animals, others do not. But at every stage of secondary biology, for completeness and interest, a lightning tour of some of the ways in which a variety of animals other than mammals manage gas exchange can be extremely useful for highlighting those all-important principles of exchange. Fish (gills), insects (tracheae), frogs (skin, simple lungs), etc. are good examples. Consider each gas-exchange system with reference to adaptations to function, both broadening the students’ knowledge of a function common to all living things and reinforcing the ‘large surface area, thin walls, concentration gradient’ ideas fundamental to an understanding of gas exchange.

Effect of smoking/pollutants/allergens on the functioning of the breathing system Cross-disciplinary When considering this topic, it may be useful to take into account the fact that you may be asked to support work on this in Personal, Health, Social Education (PHSE) or similar courses. The effects of smoking on the lungs can be dealt with very effectively here as part of the work on gas exchange, or when dealing with the effects of drugs on the body. One advantage of considering smoking here is to look at smoking alongside the effects of other air pollutants and allergens in causing asthma.

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The main effects of smoking on the breathing system include the following: ➜ Smoke

anaesthetises the cilia in the trachea and bronchioles, allowing mucus, bacteria and dirt to accumulate in the lungs and so making smokers more open to infection. ➜ Smoke contains a number of known carcinogens (chemicals that can cause cancer/increase the risk of developing cancer) which can trigger changes in the cells of the lungs, turning them cancerous. ➜ Tar and other chemicals that are part of cigarette smoke build up in the lungs on the surface of the alveoli and make gas exchange less effective. ➜ Many alveoli break down in response to the irritant chemicals in smoke, leading to fewer, bigger air spaces. There is less surface area for gas exchange to take place and the large spaces may fill with fluid, a condition known as chronic obstructive pulmonary disease (COPD). Students may have heard of an example of this disease called emphysema. The smoke from a cigarette can be drawn through a simple filter of glass wool and then bubbled through Universal Indicator. Students are often appalled at the level of tar collected from even mild cigarettes. The indicator goes yellow in colour to show acidity. Check local health and safety regulations for this demonstration and carry out in a fume cupboard. Take care handling glass wool.

Science in context Inform students that many of the problems with the breathing system caused by tobacco smoke are also seen as a response to air pollution. Breathing in dust from industrial processes is still a common cause of lung cancer and COPD in many countries, although health and safety legislation in the workplace has made this less of an issue in developed countries. Industrial pollution, particulates from diesel engines and everyday allergens such as grass pollen and pet hairs can affect the breathing system in very immediate ways. In sensitive individuals they trigger a release of histamine from the cells lining the gas exchange tract, causing the tissue to swell and narrowing the tubes leading down to the lungs. This in turn increases the resistance to air flow, making it very difficult to move air into and out of the lungs and giving the symptoms of asthma.

Treat your students sensitively When dealing with this topic it is important to be sensitive and noncondemnatory. Many students will have family members or others they know well who smoke, or who are affected by smoking-related diseases.

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A number of students in most classes will carry asthma inhalers. If the students are confident and outgoing, and you have a good relationship with the class, some of your students with asthma may be prepared to explain to their classmates what an asthma attack feels like and how they use their inhaler. However, it is never a good idea to spring a request to share personal information on a student in the middle of a lesson – ask in a prior session so, if they are happy to share their experiences, they are well prepared.

Data research There are many opportunities for students to research and discuss the implications of the scientific knowledge and the role of science in society. The internet, libraries and doctors’ surgeries are all places where students can research the effects of smoking, asthma and air pollution, including the mortality and morbidity (ill health) of smokers. Questions such as: ‘Does the available evidence support the idea that public smoking bans reduce the incidence of smoking-related diseases?’, ‘Is asthma becoming more frequent and, if so, why?’ or ‘What does the available evidence tell us about the effect of smoking on the risk of being hospitalised or dying from COVID-19?’ can be posed for research, analysis and comment.

Scientific literacy While smoking tobacco is a known risk factor for severe respiratory and circulatory disease, the impact of smoking on infections is not always so clear. In the first wave of the COVID-19 pandemic in 2020, there appeared to be some anomalies in the way smokers were affected by the virus compared with non-smokers. Two main questions emerged – does smoking affect the susceptibility of an individual to infection by the virus, and does it affect clinical outcomes? There were some indications in the data that smoking might reduce the likelihood of becoming infected with COVID-19, whilst increasing the risks of a severe outcome once an individual was infected. It is worth checking on the World Health Organization website regarding the current scientific thinking, and to discuss the responses to health and scientific advice throughout the pandemic.

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Science in context This area of biology provides an excellent opportunity to set up discussions, role plays or other forms of interactive involvement for the students. It can be used to reinforce the skills of questioning the evidence used in decision-making and the way scientific evidence influences society. It also provides an opportunity to look at the difference between causal links and associations by drawing on the data on smoking and cancers. Students can also look at the evidence for the impact of smoking and other lung conditions on the risk of severe infection or death from COVID-19 infection. As always, sensitivity is needed in these discussions.

Exercise and fitness When you exercise and your muscles work, the demand for oxygen goes up. Practical work measuring the response of students’ own bodies to exercise is a really useful teaching tool here, and there are a number of important ideas to get across. This work can be done in the laboratory, in a sports hall or outside, depending on the nature of the group, space available and your own preference.

KEY ACTIVITIES

It is very important before students undertake exercise to check if any of the class do not do PE for health reasons, or if individuals need to use asthma inhalers before exercising. Also, pre-warn students to bring in PE shoes to avoid any possible injuries. Students can all exercise within their capabilities. Any students who cannot exercise or who would prefer not to exercise can be involved in timing, recording the breathing rates of small groups of students, etc. so that they are included in the practical.

The effect of exercise on breathing rate or heart rate

1 It is necessary for each individual to measure their own resting breathing rate. They must sit still and in silence for a few minutes, breathing, and then count the number of breaths they take over each of three 30-second periods. They must not move or talk during the measuring, just note down the number of breaths at each count. Each result can then be doubled, and the mean of the three numbers found. This will give them their average breathing rate per minute. It is important to stress the need for them to be completely at rest when they are measuring their breathing rate, both at this stage and after exercise. This practical may give you one of the quietest lessons on record! 2 Students should then undertake a minute of gentle exercise and then, staying still and quiet, record their breathing rate at the end of that exercise and for each of five subsequent minutes, by when, for most students, it will have returned to the resting rate.

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3 Students can then undertake a more vigorous minute of exercise, or a longer period of gentle exercise, and repeat the process of measuring their breathing rate for five minutes afterwards. The same format can also be carried out with students measuring their heart rate instead of their breathing rate. Many students will have an app on their phones which will measure heart rate. Students can be asked to compare the results, encouraging them to think about the links between breathing and the circulation system and the maintenance of the diffusion gradient. A clear worksheet with a results table to fill in will help some students to cope with this, and you may need to go through how to calculate the average breathing rate step-bystep on the board. This type of experiment provides students with real raw data which they can use in a number of ways, either using their own data or combining class data. Combining data raises important discussion points about producing reliable data and then interpreting these data, as well as the satisfaction of seeing patterns emerging. Encourage students to interpret their data in terms of gas exchange in the lungs.

Maths Increased fitness can result in increased health. Students can be given data both on the effects of exercise on health, and on the numbers in the population who take part in regular exercise, as a stimulus to discussion. Students could use internet resources to help them identify this link.

Science in context There is clear evidence that people who exercise more are typically less likely to be overweight or obese than people who do not do much exercise. This means they are less likely to suffer the diseases associated with obesity: heart disease, high blood pressure and Type II diabetes, conditions that also increase your risk of being badly affected by COVID-19. A common misconception among students is that exercise/ sport will undo the effects of smoking. This is not true. Explore this with students.

Treat your students sensitively This is a topic with great potential for benefiting students, but also to cause embarrassment, for example if students are uncomfortable exercising in front of class mates or are very unfit. It is important to curb competitive instincts so it can be useful to set a limit – for example, students sit down and count their breathing rate once they have completed a set number of actions or spent a certain amount of time – whichever comes sooner. 90

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Enhancement ideas Work can be done on diseases of the breathing system, looking at ways in which diseases are spread and the role of social improvement, drugs and vaccination in the reduction of many such diseases. Looking at the impact of certain diseases on gas exchange – for example the causes and effects of reduced lung surface area in COPD, TB, COVID-19 or pneumonia – can reinforce students’ understanding of the key principles of gas exchange. There are great opportunities for data handling and for developing an awareness of how selective use of data can slant a picture; for example, the causes of the decline in incidence of TB in the UK (effects of vaccination, antibiotics and public health), and comparisons between outcomes from COVID-19 in different countries.

Technology use Students can investigate the various measurements taken to assess fitness (for example, the many wearable fitness devices/smartphone apps available) and consider the accuracy and validity of these measures.

Further activities ➜ Adaptations

of the gas exchange system in response to regular exercise, living at altitude, etc. ➜ Using a spirometer to compare the rate of gas exchange before and after exercise, etc.

Technology use Students could use sensors and data loggers to help measure the effect of exercise on the breathing rate, the strength of the lungs, the levels of oxygen and water in inhaled and exhaled air, etc. These topics can be brought in and dealt with here, or in a separate section looking at the concept of cardiovascular fitness as well as how the gas exchange system is affected. The response to exercise can also be treated independently under a more general ‘Keeping healthy’ umbrella, which might also include diet, infectious diseases, etc.

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4.3 Exchange and excretion Prior knowledge and experience Students will have met the basic idea of excretion – getting rid of waste products – at primary school, but there are often many misconceptions, not least that passing faeces is excretion. Students are also often unaware that they are excreting every time they breathe out! If students have already covered respiration and gas exchange, and the excretory aspects of removing carbon dioxide have been covered well, then the production and removal of carbon dioxide as a waste product will need only a light touch. Similarly, if students have already covered photosynthesis, they will know that oxygen is a waste product of that process and is excreted by plants into the atmosphere. On the other hand, if you teach excretion relatively early in the course, then you may have to start from the ground up, and cover the content on the principles of exchange from the early sections of this chapter as well.

A teaching sequence Excretion can be taught in a variety of ways. It may be used to introduce exchange, illustrating it through excretion via the leaves of plants and the lungs and kidneys of vertebrates. More frequently, students will have met the principles of exchange and excretion before; here, excretion will focus on the removal of urea in animals. The big biological principles to introduce/ revisit here are exchange, along with the factors which affect the efficiency of the exchange process, and excretion as the removal of metabolic waste. Students need to understand how the structure of excretory organs is related to their function and how concentration gradients affect the exchange process, both in the kidneys and in dialysis machines used to replace kidney function when needed. Start by developing learning outcomes and give a broad overview of the work to come before asking questions to find out the level of prior knowledge of the group you are teaching: this will set the scene for the topic to come and enable you to pitch it at the right level. Ensure that students end up with a clear understanding that excretion is a process that happens in all organisms, and in every cell, not just something that takes place in human kidneys.

Physical principles When dealing with excretion it is important to discover how well your students understand the principles of exchange and how they interpret the

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term excretion. If necessary, cover the principles of exchange as described in Section 4.1. If students are comfortable with those concepts, move on to excretion.

Excretion in animals and plants It is important to help students understand the difference between egestion – the process by which animals remove undigested material out of their bodies – and excretion, the process by which organisms remove the waste products of their metabolism. Plants produce carbon dioxide and water as waste products of respiration, oxygen as a waste product of photosynthesis and a wide range of complex compounds as waste products of other reactions. The carbon dioxide and water produced during respiration are often used up by the plant in photosynthesis and, if not, are lost by diffusion. The oxygen produced as a waste product of photosynthesis is partly used by the plant in respiration and partly removed from the leaves and other structures by diffusion. If new to students, this is the place for the relevant parts of Section 4.2. Students do not need to know the huge range of other waste products made by plants, some of which are highly toxic, but it can be interesting for them to investigate these. Web resources can be really useful here. Search the SAPS website using the search term ‘plants, waste compounds and the vacuole’ for some interesting information. Animals produce carbon dioxide and water as waste products of cellular respiration. Mammals are the usual group considered until the final years of study, and humans are the most widely used example, and often the most interesting to students. If gas exchange has been covered, this needs only the briefest of reprisals. If it has not, this is where you teach Section 4.2. Mammals also produce nitrogenous waste from the breakdown of excess proteins in the form of urea, which is toxic to the body.

Exchange in the kidney The starting point of the excretion of urea with secondary students tends to focus on how the excretory system connects together in their bodies – the physical structures of the kidneys, ureters, bladder and urethra. Good questioning can help them realise that they already know something about this system, in terms of how the amount of urine produced changes in relation to environmental factors, such as the amount of liquid taken in or the ambient temperature.

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Technology use Students can use models to help them understand the anatomy of excretory systems and there are lots of good photographic images, diagrams and animations of the urinary tract online. The ABPI schools website has a whole section on the kidney and its role in homeostasis (use the search term ‘homeostasis – kidneys and water balance’). Be careful when using drawings to make sure that students can see the different structures clearly, especially if using a longitudinal section of the female urinary tract, as it is not always clear that the vagina and urethra are separate structures. It can be a common misconception, perhaps especially (and understandably) with boys, that the tubes for urination and for sex are the same in both males and females. Make sure the anatomical differences are clear and understood. This needs sensitivity as some students may be relatively unaware of their own anatomy, let alone that of the opposite sex.

Science in context This is also an opportunity for students to look at a kidney and dissect it, which they can do in groups or you can do as a demonstration. If you have access to a friendly local butcher, you may be able to obtain kidneys still surrounded by fat and with the tubes relatively intact, which helps put them in a whole body context more effectively. The tissue you are using is material that is eaten by many people, but remember that some students will have religious or other reasons for not being prepared to handle tissues from pigs or cows. Lambs’ kidneys are likely to be acceptable to most students. Some students, though, may be uncomfortable with any dissection, and this should be respected. The next stage is to develop an understanding of how the structure of the kidney is related to its function. The emphasis here is on the basic structure of the tubules and the role of exchange in the formation of urine. Examining stained slides showing the kidney tubules can help students see the different regions they are discussing and the complexity of the organ.

Dialysis and exchange While the exchange processes of the kidney tubule are very complex for students under the age of 16, the difficulties of replacing kidneys when they fail are much easier to understand, as is the exchange process required during

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dialysis. Thinking about the problems caused when kidneys fail, the loss of balance of electrolytes and the build-up of toxins can be a powerful way of demonstrating – or even introducing – the importance of exchange in the kidneys. The way dialysis is used to replace the kidneys – the diffusion gradients, short pathway and the surface area of the dialysis membranes – really helps students get to grips with the idea of what happens in the kidney itself. The limitations of the process offer great opportunities surrounding a lack of health resources. This in turn leads on to the issue of organ transplants, where there is scope for data analysis on the success of transplants. Students may also explore the possibility of growing replacement organs in other species of animals (xenotransplantation) or using stem cells as a starting point.

Treat your students sensitively This topic offers a rich seam for work on the role of science in society and the ethical aspects of scientific and medical knowledge. However, it also touches on some very complex and serious issues. Students may have lost relatives, or have family members on dialysis or in transplant programmes. Tread lightly.

Enhancement ideas Link this topic with students to work on co-ordination and control, looking at both the neurological and chemical basis of thirst, and the roles of vasopressin – still often referred to as ADH (antidiuretic hormone) in specifications and therefore school textbooks – and aldosterone in controlling urine production. This topic can be reprised or introduced from work on stem cells/gene editing and the potential to grow new body parts.

4.4 Resources There are many official and unofficial ICT resources to enhance your teaching, support weaker students or stretch more able ones. The internet provides you with many data sources and www.ase.org.uk has done the necessary shortlisting of reliable sites for you. The ASE has links to many organisations that provide useful resources, and some are mentioned below. The ASE website enables you to search by topic area and also by age range of students: www.ase.org.uk/resources

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SAPS – Science & Plants for Schools – has an outstanding website (www.saps. org.uk/) which provides free-to-use resources providing well-tested, exciting practical opportunities illustrating many biological principles using plants. The ABPI – Association of British Pharmaceutical Industries – has a range of very varied web resources (www.abpischools.org.uk/) aimed at supporting teachers and students in many areas of human biology. The Nuffield Foundation produces online resources supporting practical work in exchange (www.nuffieldfoundation.org/practical-biology/exchange-materials) and diffusion (www.nuffieldfoundation.org/practical-biology/effect-size-uptakediffusion). Additional resources include: www.nuffieldfoundation.org/teachers Students can learn a lot about exchange using sensors and data-logging technology. STEM Learning (www.stem.org.uk/) is full of ideas and resources to help you teach exchange. Harlen, W. (ed.) (2010) Principles and Big Ideas of Science Education. Hatfield: The Association for Science Education. Available at: www.ase.org.uk/bigideas

References Banner, I. and Hillier, J. (eds) (2018) ASE Guide to Secondary Science Education (4th edition). Hatfield: The Association for Science Education. Sang, D. and Frost, R. (2005) Teaching Secondary Science using ICT. London: ASE John Murray Science Practice. (This book recommends a number of resources relevant to curriculum work.)

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5 Transport

Mark Winterbottom and Dan Jenkins

Topic overview This chapter deals with transport of water and other materials within living things, why such transport needs to happen, and the processes and systems which allow it to happen. To understand transport in living things, students need to understand diffusion. You can find some good approaches in Chapter 4. They also need to understand the need for a transport system and why such systems have evolved. A good way to do this is by looking at transport in unicellular organisms, and then exploring the limits of simple diffusion as an organism increases in size (see Chapter 2). In animals, secondary school students are usually happy with the role of the blood in moving the reactants and products of respiration around the body, and carrying other materials between organs. In plants, 11–14-yearold students are likely to realise that all parts of the plant need water and minerals, but for 14–16 year olds, building a more sophisticated understanding of the mechanisms of transport can be so dependent on their understanding of osmosis that you can easily trip up when trying to support their learning. Your students are probably most familiar with transport systems in their own bodies from their primary education, so you may want to focus on humans first. The purpose of the transport system here is to: ➜ supply

cells with raw materials for respiration, to carry waste products away from cells, and to transport materials between organs.

Alternatively, you could start with plants, where the purpose of transport is very similar, but just like in unicellular organisms, water has a more important structural function too. Transport in plants functions to: ➜ carry

water, sugar and mineral salts around the plant to where they are needed, along with other organic substances such as amino acids and plant growth substances.

In this chapter, we have gone with plants first, simply because they are fascinating, and they help to demonstrate key principles which are useful for students when thinking about humans.

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Science in context Biology is the study of living things. Teaching and learning key ideas with plants is an easy way to encourage students to interact with living things and realise both the similarities and differences between plants, animals and other organisms.

5.1 Why transport and transport systems? Prior knowledge and experience Students may have met the idea that smells spread through the air, and that solutes will spread out through a solvent. Depending on whether you have already covered the ideas about diffusion in Chapter 4, students may understand the particle model of diffusion and that some solutes can diffuse across a membrane. Some may know the definition of diffusion (the net movement of particles from a region of high concentration to a region of low concentration), but may not understand the idea of ‘net’ movement. Students will understand that unicellular organisms are very small and that the maximum distance over which exchanges take place is correspondingly small. They should realise easily that these unicellular organisms do not have specialised transport systems, although they may not immediately understand why. Some students may realise that many multicellular living things have a number of adaptations to increase surface area, often involving folding of membranes.

A teaching sequence To understand why transport is important, students must realise that all living things need to take in materials from their environment and eject waste materials into their environment. Because materials are often dissolved in water, they must also understand how water is transported. Begin by considering unicellular organisms; help students to understand that the organisms are very small, and that the maximum distance over which exchanges take place is therefore small. However, as multicellular organisms evolved, transport systems evolved in order to ensure materials were transported to and from cells efficiently enough to meet the needs of all cells; diffusion is no longer adequate.

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Transport in unicellular organisms To appreciate the importance of transport, ask students to look at some pond water, cultures of unicellular organisms or simple filamentous algae under the microscope. You can buy these from your usual laboratory suppliers. Ask the students to work in pairs, to consider the questions below and to write down their thoughts as they go along. Once they are finished, organise them into small groups to agree on their answers: ➜ What

do organisms take in from their environment? ➜ What wastes do organisms release into their immediate surroundings? ➜ Which biological processes take in or release substances into their immediate surroundings? ➜ How do materials move into and out of these unicellular organisms? ➜ Why do materials need to move into and out of these unicells and so why do they need to move in and out of any cell?

Science in context You can remind students of Robert Hooke, who discovered the cell by looking at cork under the microscope. The microscope had been invented by Galileo just 40 years beforehand (see Chapter 2).

Maths To help students to understand that unicellular organisms are very small, ask them to calculate the magnification provided by the microscope (see Chapter 2). Some groups may suggest that diffusion is involved in the exchanges between cells and their immediate environment; others may simply say that materials pass in and out of the cells. It is important at this stage to reinforce the particle model of diffusion. (You can use the teaching and learning ideas in Chapter 4.) Depending on when you teach this topic, some groups will understand the need for glucose and oxygen for respiration, and the need to remove carbon dioxide as a waste product. If they do, ensure that they realise that this applies to both plant and animal cells (see Chapter 2). Remind students that cells are membrane-bound and, through discussion, help them to realise that diffusion must be able to happen through the outer membrane. Drawing on their understanding of how diffusion works, ask them, in pairs, to make hypotheses about properties of membranes which are needed to allow diffusion to happen. As a class, arrive at a functional model

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of a membrane as a surface that has holes in, which will allow particles to pass through. You could use a colander, sieve or netting as a useful analogy of something that allows small materials through. You may want to use a simulation that shows diffusion across a membrane. Be careful to talk about diffusion of small particles such as oxygen and carbon dioxide rather than water; that comes later! As you work through the simulation, ask students to predict how the particles will move.

Scientific literacy Biologists often use models to help develop explanations. They are not always structurally accurate, but functionally correct, allowing a student to make sense of scientific phenomena. For example, the membrane does not really have ‘holes’ in, but the solute particles can still pass through it.

The need for a transport system in multicellular organisms The area of surface available for exchange and diffusion is enough to supply the needs of a unicellular organism. For larger organisms, as volume increases, the ratio between their surface area and volume decreases (see Chapter 2). The concept of decreasing relative surface area with increasing volume is always tricky for students; 14–16 year olds may find it easier than 11–14 year olds. The changes that take place in the surface area : volume ratio, as organisms of increasing mass and volume are considered, can be demonstrated quickly using model organisms represented by plasticine cubes: doubling the linear dimension increases the surface area by a factor of four but the volume by a factor of eight. Hence, the increasing distance between deeply seated cells and the ‘relatively smaller’ exchange surface makes diffusion alone too slow and haphazard as the sole means of exchange for larger organisms, especially as some are very active. The evolution of transport systems that give faster delivery, and delivery with direction, has overcome both these inadequacies.

Maths This is an excellent opportunity to develop some mathematics. Given that the relationship between linear dimensions, surface area and volume is not intuitive, allow students to use simple maths to build up the idea.

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Students might carry out an investigation into the effect of decreasing surface area : volume ratios on surface exchange using agar or gelatine cubes and an aqueous solution of a food dye. Cubes of differing sizes immersed in the dye can be sectioned after a fixed time to see the extent of ‘delivery’ by diffusion alone. Full details of a protocol are available at the end of the chapter.

Features of a transport system

KEY ACTIVITY

The transport systems of animals and plants have adaptations that help to maximise the rate of diffusion across a membrane, often involving folds in the membrane. If you think about a membrane as a surface with holes in, particles will diffuse through these holes. If you can fit more holes in the same space by making the membrane fold (hence increasing the area of membrane and the number of holes), this will increase the overall rate of diffusion.

Investigating the relevance of folds in transport systems One way to show this is using two different pieces of paper: 1 Take a sheet of A4 paper. Punch holes into the paper at regular intervals using a pencil, so you have 20 holes punched into the paper. Insert the paper into a cardboard tube which is exactly the length of the paper. 2 Take a sheet of A3 paper (double the area of A4). Punch holes into the paper at regular intervals, so you have 40 holes (double the holes for A4) punched into the paper. Insert the paper into an identical cardboard tube, squashing and pushing it to make it fit. 3 Take both pieces of paper out. Make clear that they are models of membranes, and that folding membranes (the folds will be obvious in the A3 paper) is a way of increasing available surface area. A second way is to do something similar with string. 1 Take a 1-m length of string and a 2-m length of string and mark the position of ‘holes’ on the string every centimetre with a pencil. 2 Lay one length of string along a metre stick. Loop the other piece away from the ruler at regular intervals. In class discussion, help students to realise that loops in the membrane allow greater surface area in the same space, and hence a higher rate of diffusion. This can be hard to understand, and it is worth coming back to this explanation wherever relevant below. To assess students’ understanding, you could ask them to storyboard an animation that shows the effect of a folded membrane on the rate of diffusion. If students work in groups to devise the storyboard, it would probably help to support the understanding of lessadvanced students. 101

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Science in context Students may have learned about emphysema when studying the breathing system (Chapter 4). Emphysema is a condition in which the air sacs of the lungs are damaged, causing breathlessness (see Chapter 4). The membranes in the lungs have been damaged by activities such as smoking, and there is not enough surface area for normal gaseous exchange to take place.

5.2 Transport in plants Prior knowledge and experience Given that plants are fixed to one spot, they have evolved unique mechanisms which allow them to survive. Students are likely to have met the idea that plants absorb water from the soil via their roots and that water passes up the stem to the leaves. They are also likely to be aware that plants photosynthesise to produce food, but that they also take in minerals from the soil (the reason we use fertilisers). This cannot happen by diffusion alone. Students can have a range of misconceptions about water and transport in plants. Some may think that water: ➜ enters

a plant through the leaves (when students water plants, they often pour the water on the leaves) ➜ leaves a plant only through the flowers ➜ taken in through the roots is retained (i.e. none is lost through the leaves) ➜ exits the leaves as a liquid (some may have seen guttation) ➜ is pumped around the plant in much the same way that a heart pumps blood.

A teaching sequence Water transport in plants evolved because diffusion is inadequate to supply plant cells with water and minerals. (Remind students that water is also important for support.) Students aged 11–14 will be able to learn about transport in plants with only a cursory understanding of osmosis. However, for older students to really understand transport in plants, an understanding of osmosis is essential. Following that, it is sensible to look at the vessels through which transport occurs. Finally, in the absence of a pump like the heart, it is important for students to understand how transport happens in plants, both of water (and minerals) from roots to leaves, and of sugars (and other organic molecules) from the leaves to the rest of the plant.

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Diffusion and osmosis

KEY ACTIVITY

Osmosis refers to the diffusion of water across a partially permeable membrane. Students find osmosis hard to understand because we don’t usually refer to diffusion of the solvent, and different textbooks define osmosis in different ways. It is important to build their understanding of osmosis gradually, so first remind them about diffusion (see Chapter 4). Then help them to understand how adding a solute can affect the concentration of water. Finally, explain that osmosis involves water crossing a partially permeable membrane from a region where water is in high concentration to a region where water is in low concentration.

Investigating osmosis using sugar solutions

1 Present two large measuring cylinders of the same volume (2 dm3 or 5 dm3). Label them A and B. Fill both with warm water to about two-thirds full (making sure the levels are identical in the two cylinders). Ask the class what will happen to the level in A if some sugar is poured in. (Likely answer: ‘It will go up’.) 2 Add 150 ml of sugar to A. Ask the class what actually happened. (Likely answer: ‘The level did go up’.) Ask the class what will happen to the level if the sugar dissolves. (Likely answers: either ‘It will stay the same’ or ‘It will go down’.) Work on this difference of opinion in discussion. 3 Shake the cylinder, or use a magnetic stirrer, until the sugar dissolves. The level does not go down. This means that the level of solution in A is now higher than the level of the water in B. 4 Pour off the extra volume from A into a small beaker until the levels in A and B are again identical. Ask the class which cylinder has more sugar in it. (Likely answer: ‘A’.) Ask the class which cylinder has more water in it. (Likely answer: ‘B’, although some may say they are the same.) Challenge those who get it wrong by pointing out the (sugary) water in the small beaker. To complete the demonstration, point out that water can diffuse from one solution to another from a high water concentration (B) to a low water concentration (A). Explain that such diffusion can take place even if B and A are separated by a partially permeable membrane. Finally, define osmosis as the diffusion of water from high to low concentration of water through a partially permeable membrane. Point out that water moves in and out of cells by osmosis because the cell membrane is partially permeable.

Scientific literacy Some students can struggle to get to grips with the way solutions are described. Make sure you give them clear definitions for key terms and that students understand how they are used.

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Some students can find osmosis difficult, partly due to the way in which language is used about solutions: ➜ A

‘dilute solution’ usually refers to a solution with low solute concentration (and high water concentration). ➜ A ‘concentrated solution’ usually refers to a solution with high solute concentration (and low water concentration). With these definitions, it is true to say that water molecules diffuse by osmosis from dilute (high water concentration) to concentrated solutions (low water concentration). You can see this is confusing, and so when talking about osmosis, always talk about the concentration of water, so students do not become confused. You can also refer to water molecules diffusing along a concentration gradient. For more advanced students at 14–16, it may be helpful to use the terms isotonic (solutions at the same concentration), hypotonic (more dilute solution; higher water concentration) and hypertonic (more concentrated solution; lower water concentration). You can demonstrate this movement using some or all of these models: ➜ Use

Visking tubing filled with black treacle and submerged in pure water so you can see the effect of water diffusing in. (The Visking tubing enlarges in size and the colour of the treacle pales as it is diluted.) ➜ The balloon in a ‘paper box’ apparatus models the effect of osmosis on turgidity and plasmolysis (Figure 5.1a). As you inflate the balloon, it pushes on the inside of the paper box, making it bulge. ➜ Put a mixture of large and small seeds in a box with an artificial partially permeable membrane. This can be as simple as a piece of cardboard with holes big enough to let the small seeds through but too small for the large seeds to pass through. ➜ A plastic bottle with holes cut in it can be used to model a cell with a partially permeable membrane (Figure 5.1b). ➜ Use one of the many ICT simulations of osmosis.

Technology use If you use an ICT simulation, try to get one with variable speed control, or one that moves slowly so students can follow individual particles.

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a

b

b

plasticplastic bottlebottle containing containing large large buttons buttons and and small small beadsbeads

cut into holes holes cut into the bottle the bottle large large enough enough only only the small to let to theletsmall escape beadsbeads escape

balloon balloon

box made box made with with flexible flexible card card

    

Figure 5.1 a  Balloon in a paper box model of a plant cell. Inflation of the balloon represents water entering the cell by osmosis. b Plastic bottle model of a cell with a partially permeable membrane.

Science in context Applying ideas in artificial models is a good intermediate step to help biologists understand phenomena they see in living tissues. Students need to explain the changes they measure or observe in terms of what is happening with the solvent (water) and solute (sugar or salt) at a micro-level between the cells, across the partially permeable membrane and with the cells’ immediate environment. Finally, show students how these models apply in living plant tissue. You could use some or all of the ideas below to help secure students’ understanding. Don’t forget to emphasise the importance of osmosis in maintaining support in tissues, cell expansion and survival responses. ➜ Measure

(length or mass), bathe in water and re-measure each of ten sultanas. Because the cells in the sultanas are relatively dried out, the concentration of water in the cells is low and so water diffuses in. The sultanas can be seen to expand as water diffuses into each cell. ➜ Examine giant red pepper cells under the microscope and pop them using a mounted needle to demonstrate the pressure of water inside the cells and the role of water in maintaining the structure of the cells (see SAPS website). ➜ Examine microscope slides of red onion cells or rhubarb stem cells in solutions of pure water and in concentrated sugar solution. In the former, the cell membrane pushes up against the cell wall. This is because water has diffused into the cell from a region of high water concentration (pure

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water) to lower water concentration (in the cytoplasm). In the latter, the concentration of water in the sugar solution is lower than the concentration in the cytoplasm. Water diffuses out of the cell and the membrane pulls away from the cell wall. Because the cytoplasm is coloured red in red onion cells and rhubarb, this effect is clear when viewed under a light microscope. ➜ Place potato chips in sugar solutions of varying concentrations; in those solutions with high water concentration (and low sugar concentration), water will diffuse into the cells and the potato chip will feel stiff (the turgid cells push up against each other). The opposite will happen in those solutions with the highest sugar concentrations (and lowest water concentration). It is worth emphasising again that osmosis occurs in all plant tissues and in animals too, not only in, for example, potato tubers. To make this clear to students, you could substitute other vegetables, such as carrots or beetroot, or even use the practical involving bell peppers (see the link at the end of the chapter). You can also demonstrate osmosis in a hen’s egg (see the link at the end of the chapter). Some misconceptions observed when using the activity with potato sticks include that the potato ‘acts as a sponge’ or simply ‘soaks up water like a piece of bread’. It is therefore important to give students an opportunity to explain their results by describing what is happening at micro-level to result in the changes they can measure or describe qualitatively. To assess students’ understanding, you could ask them to write questions and answers about osmosis, and to construct a table that compares and contrasts diffusion and osmosis. Do be careful though; some students can end up thinking that diffusion happens in animals and osmosis happens in plants.

Science in context Students may be interested to know that scientists are researching the potential role of osmosis in generating electrical power. Prototypes have been trialled in Norway and Canada. The energy source to be harvested is the salinity gradient between two liquids, that is, their osmotic pressure (OP) difference. A pilot osmotic power plant has been operating in Norway since 2009 driven by the salinity gradient between the sea and a fiord.

Demonstrating transport Having secured a good understanding of osmosis, it is now sensible to demonstrate that transport does occur in plants. Many students’ impression of plants is that they are relatively passive, so showing evidence of transport is important.

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An easy way to do this uses leafy celery stalks. Leave them until their leaves are wilted and then place in a stain during the lesson; colour will move up the petiole (leaf stalk) and into the leaves over just a few minutes. You can do something similar with the stem of a white carnation. Split the stem down the middle to about 3–4 cm below the flower and carefully place each halfstem into different coloured inks. The following day this gives a flower head that is twin-coloured and somewhat spectacular. This can also be a good activity for science clubs, allowing students to experiment with different inks, food dyes, etc.

Vascular tissue Vascular tissue comprises the vessels through which transport of water, minerals and sugars happens in plants. A shoot taken from a plant such as Busy Lizzy or a celery petiole will take up a stain (such as Indian ink or toluidine blue) in its vascular tissue and can be used to provide material for sectioning. Students can cut thin sections of the stained stem or petiole and prepare their own temporary slides (but remind them to take care with scalpels and razor blades; see link at the end of the chapter for a protocol). These will demonstrate that stain travels up the stem through particular regions or tubes (called xylem). Ask students to cut transverse and longitudinal sections. In the latter, they can see spiral lignin deposits, which gives a real ‘wow’ moment! You can extend this idea to suggest to students that sugars are also transported through different tubes (called phloem). Helpfully, toluidine blue differentially stains xylem and phloem, aiding their identification under a microscope: xylem stains blue-green, while phloem stains purple. A useful animation exploring the movement of water and sugars in a plant is available in the list of websites at the end of the chapter. The distribution of vascular tissue in plants is related not only to transport but also to support. In dicotyledonous plants, those primarily studied in schools such as sunflower (Helianthus) and buttercup (Ranunculus), the vascular bundles are arranged in a ring around the outside of the stem. In a root the vascular tissue is arranged at the centre of the root. A neat demonstration to explain the function of this structural difference involves using pieces of rolled up A4 paper to simulate the vascular tissue: 1 To simulate their arrangement in a stem, arrange the rolled-up pieces of paper in a circle and balance masses or textbooks on them to show that the circle can support substantial weight. (If you keep adding textbooks until the paper collapses it may be worth ensuring students move back a little!) Explain that this is one reason for their organisation in stems.

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2 The forces on a root are different; roots are pulled and pushed longitudinally when a plant blows in the wind. Putting all the vascular tissue in the centre of the root helps resist such forces. You can show this by asking students to pull on opposite ends of a bundle of rolled pieces of A4 paper to see if they will rip, which they don’t. You may want to ask students to draw a transverse section of a stem and a root, using either prepared slides or diagrams in a textbook, and to make and annotate models of the vessels using empty drinks bottles or other clean, recyclable household materials. You could ask students to research and record a video (using a digital camera or their mobile phone) about xylem vessels and phloem sieve tubes, using their models.

Transpiration and the transpiration stream So what makes water move into the roots, through the roots, up the stem and through the leaves? There is no heart, so the force that makes this happen has to come from somewhere else. Predominantly, it comes from transpiration (evaporation of water from the leaf surface). Before starting on transpiration, make sure students are familiar with changes of state (specifically evaporation). The fact that plants lose water from their leaves can be demonstrated by placing a clear polythene bag over the shoot, but not the soil, of a potted plant for a few hours in advance of a lesson. Condensation will accumulate inside the bag. More water is lost from the lower surface of most leaves than from the upper surface. Students can investigate this in two ways: 1 Use sticky tape to attach cobalt chloride paper to the top and bottom of a leaf. The paper turns pink when damp. As there is no wax (and more stomata, see below) on the underside of a leaf, this loses water more quickly and the paper on it turns pink first. (NB. Use forceps or disposable gloves when handling cobalt chloride paper and avoid skin contact with it.) 2 Detach four leaves from a plant. Leave one as a control. For the others, spread petroleum jelly onto (i) both top and bottom surfaces, (ii) top surface only, (iii) bottom surface only. When left pegged on a length of string, the leaves without petroleum jelly on their bottom surface will lose water (and mass) and wilt most quickly. You should help students relate these differences to the leaf’s adaptations to prevent water loss. Because the upper surface of the leaf receives more incident sunlight than the lower surface, water would evaporate more quickly, other things being equal. However:

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is a coating of wax on the upper side of most leaves. Wax is waterproof and so reduces diffusion of water. ➜ Stomata (tiny pores in the leaf) have an unequal distribution between the upper and lower surface of the leaf. Microscopic examination of nail varnish or Germolene New Skin peels of upper and lower epidermal tissue, produced by the students, are useful for demonstrating this difference. A neat way to visualise stomata is to make a temporary slide of a small piece of Tradescantia zebrina. When viewed under the microscope, the underside of the leaf clearly shows the stomata, with green guard cells (the rest of the cells are red). Further information is available on the SAPS website (see the list at the end of this chapter). For higher attainers, you could also ask students to investigate upper and lower leaf surface temperature and relate the temperature to stomatal distribution. Further information is available on the SAPS website.

Careers Water supplies are becoming more scarce and the climate is changing. European agricultural researchers are looking at the potential for growing crop plants which require less water. These include cool-season legumes such as peas, lentils and fava beans, and the brassica crops: Brussels sprouts, cabbage, cauliflower, kale, kohlrabi, mustard, broccoli and turnips. Farmers, agronomists and horticulturalists depend on an understanding of plant physiology, including plant transport, in order to implement appropriate water and fertiliser regimes. Knowledge of plant transport may also be useful to plant pathologists. Given that plants wilt if they lose too much water, it may seem unintuitive to some students that a plant should have stomata that let water out. There are two reasons for stomata. The first is that gaseous exchange also takes place through the stomata, so they have to be open during the day to let carbon dioxide in (see Chapter 4). Second, and more relevant to plant transport, letting water evaporate drives absorption and movement of water through the plant; this replaces water lost by evaporation, and enables absorption and transport of mineral ions. If the plant does become short of water, it begins to wilt and it can close its stomata. The cells on either side of the stomata are called guard cells. During the day (when plants need to open the stomata) they photosynthesise, increasing the concentration of glucose in the cytoplasm and hence decreasing the relative concentration of water in the cytoplasm. As a result,

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water diffuses into them by osmosis from neighbouring cells. Most students would predict the guard cells would bulge and so close the stomata as a result of this. However, the opposite happens: the guard cells open the stomata during the day and close them at night. This is because the inner wall of each guard cell is thick, and so doesn’t bulge out. You can model this for students by taking two long balloons and sticking sticky tape to one side of both. Put the balloons together with the pieces of sticky tape facing each other. Inflate the balloons and they will each form a semicircle shape, leaving a hole in the middle. The next step is for students to understand that water moves into the roots in response to loss of water from the leaf. There are some website animations at the end of the chapter to help you explain this but, essentially, follow the path of water starting at the leaves. Explain that water is lost from the mesophyll cells, reducing the concentration of water in the cytoplasm of those cells. Water then moves into those cells from neighbouring cells by osmosis, and this repeats itself all the way back to the xylem vessels. When water leaves the xylem in the leaf, water is pulled up the xylem from the xylem vessels in the root (which you can think of as continuous with the leaf xylem). With a lower water concentration in the root xylem, water moves out of neighbouring cells, and sets up a diffusion gradient, all the way back to the root hair cells, where water moves into the root from the water in the soil. This movement of water from roots to leaves is called the transpiration stream and is ‘driven’ by evaporation of water from the leaves. Although this explanation is simplified, and the level of your explanation will depend on the students involved, you would not usually be expected to discuss this movement in terms of water potential gradients. More complex explanation would usually be reserved for post-16 biology. It is important to stress that minerals, such as nitrates, phosphorus, potassium, sulfur, calcium and magnesium ions, are also transported in the xylem, essentially swept along by the flow of water. One way to demonstrate the transpiration stream is by using a potometer (Figure 5.2), practical details for which can be found on the SAPS website, included in the links at the end of the chapter. Good species to use include Buddleia, willow (Salix) and willow-herb (Epilobium). Make sure you practise setting it up beforehand, but the SAPS protocol is much easier than traditional bubble potometers, and can be used to measure change in mass as well. Using this potometer, students can investigate the effect of environmental conditions on transpiration and the transpiration stream, for example at different ambient temperatures or humidities.

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Science in context Ask students to make predictions about the rate of transpiration in a crop under different weather conditions. graduated pipette

shoot

syringe

rubber bung

water

Figure 5.2  Potometer for estimating the rate of transpiration

The effect of air movement can be demonstrated using an electric fan or hairdryer (on cool setting). Continuous readings of the mass of a plant on a balance (or of the SAPS potometer set up with a plant) experiencing known changes in environmental variables can produce some interesting graphs for interpretation. Students might have the opportunity here to employ datalogging devices linked to computers to obtain continuous readings of water loss from a whole plant. The penultimate step is to look at the root hairs as being adapted to maximise diffusion of water and transport of mineral salts into the root. You could ask students to examine the root hairs produced by germinated cress seedlings, using a hand lens or a low-power microscope, and then draw a diagram of the root hairs, explaining how their shape increases the surface area across which water and mineral ions can be absorbed.

Tip Using a microscope and a hand lens are essential tools in biology, but observation skills are even more essential. These kinds of open observation tasks allow students to come up with their own ideas about structure and function or biological processes, and experience being a real biologist. 111

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To assess their understanding of the role of root hairs, you could also ask students to write a mission statement for the root, to include an explanation of how exchange occurs. They could build a social photo-sharing site or Instagram profile, which they use to educate the reader about the role of root hairs, and of other membranous exchange surfaces in plants (such as the spongy mesophyll layer in the leaf) or humans (for example the villi in the digestive system or alveoli in the lungs). The last part of the story is examining how mineral salts enter the root hairs. If they diffused in passively, the plant would not get enough, so they must be pumped in actively (using energy) through tiny pumps on the root hair membranes. To assess their understanding of the differences between active transport and passive diffusion, you could ask students to design their own role play to perform to the class. Secondary data about active uptake are available for analysis in one of the websites listed at the end of the chapter.

Translocation Trans(change)-location(position) is just what it says in the name. It means moving the products of photosynthesis (and other organic molecules) around the plant. Getting first-hand evidence of the involvement of phloem in transport is difficult at this level. However, students may be aware of the damage that occurs to trees and shrubs if their bark is ‘ringed’ (see Figure 5.3). They may well have seen young trees in woods or parks with protective sleeves around them to prevent their bark being damaged. You could provide the group with a piece of continuous prose about translocation, written to suit their reading level and learning expectation, and set some specifically targeted questions to extract the desired information. Alternatively, present the information as a CLOZE activity, where after the initial sentences, the fifth word is removed in each sentence to encourage ‘making sense’ as they read. Make sure the fifth word is not essential terminology for this topic as you are encouraging reading skills here. At this level it is probably sufficient to bring out the following points: ➜ Phloem

cells are alive. ➜ Their walls and membranes are permeable to water. ➜ Phloem is involved in the transport of dissolved sucrose (formed from photosynthetic glucose) and other organic molecules. ➜ Transport in phloem occurs both up and down the plant, with the leaves as the source of glucose.

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exposed wood

lapse of time

bark and phloem cut away

Figure 5.3  Removing bark and phloem prevents downward movement of sucrose, which accumulates above the ring.

Further activities ➜ Students

could research the ways in which plants are adapted to desert environments, reporting their findings as a podcast ‘from our own correspondent’. ➜ Students could be challenged to play the role of xylem in a tree, and see how far they can suck water up a straw (given that water moves over 100 m up the xylem of a giant redwood tree). Further information is available on the SAPS website (see the list at the end of this chapter). ➜ You could present scenarios to students, which they have to make a judgement on, based on their knowledge of transport. Examples could include a planning application to fell trees and build houses in a wetland area, a proposal to use plants to absorb toxic minerals from the soil, or the stresses to water transport which climate change may bring to plants.

Science in context Many of these ideas can be set in a commercial context for students, with the physiology of transport related to commercial growing of crops and maximising profit for farmers and horticulturalists.

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5.3 Transport in humans Prior knowledge and experience Students should have already met the heart as an organ and perhaps blood as a tissue in primary school. Most students will recognise that the food they take in through their mouths and the oxygen in the air they breathe must get around their body somehow. They are likely to know that the heart functions as a muscular pump, which circulates blood to all parts of the body. They may have met the idea that blood flows around the body in blood vessels. Students are also likely to have some idea about the effects of different lifestyles on the health of the heart.

Science in context Setting your teaching of the circulatory system within a health context can be very motivating for students. However, be sensitive to those students who have relatives with heart disease.

A teaching sequence Given that students will be familiar with organ systems, it is sensible to introduce the circulatory system as a system within which blood (as a transport medium) is pumped around the body. Focus on the way in which the system functions as a whole (as a double circulation: one circulation to the lungs and a separate one to the body, with the heart operating almost as two separate pumps), and then focus in, in more detail. Look at the structure and function of the heart to appreciate how it operates as a pump within the double circulation. Then look at the ways in which the arteries and veins are adapted for their functions. Finally, look at the structure and function of capillaries in allowing materials to pass in and out of the blood. This is a good point at which to look at the constituents of the blood and their role in transporting oxygen and glucose to cells, and taking away carbon dioxide and water.

Careers An understanding of the circulatory system is essential to any student who wants to study medicine or nursing, or who will work as a healthcare assistant of some sort. Remind students that the circulatory system is a key focus for doctors, who may specialise in its treatment. Those who do so are called ‘cardiologists’. Veterinary surgeons and veterinary nurses can also build upon an understanding of the human transport system. 114

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Science in context Giving blood saves lives as blood is always in high demand. Most people can give blood. Men can give blood every 12 weeks and women can give blood every 16 weeks. Tell students that you can give blood if you: l are

fit and healthy aged between 17 and 66 (or 70 if you have given blood before) l have a mass between 50 kg and 158 kg (7 stone 12 lbs and 25 stone). l are

You will face a number of common misconceptions in this topic. Students can think of the heart as a pulsating bag (a single pump) and may find it difficult to visualise the double circulation; often they imagine that blood travels from the heart, through the lungs and directly to the body. Many students will think that arteries always carry oxygenated blood while veins only carry deoxygenated blood. In fact, direction of flow relative to the heart is the distinguishing feature between arteries and veins. (The pulmonary artery actually carries deoxygenated blood and the pulmonary vein carries oxygenated blood.) Some students may mistakenly think that oxygen is carried in the blood plasma, and some students may think that deoxygenated blood is blue (because it is often depicted that way in textbooks).

The circulatory system: structure and function Understanding the need for a double circulation can be difficult. To ensure students are familiar with the structure, you may want to give them a diagram of the human circulatory system (Figure 5.4) and ask them to suggest what happens to the level of oxygen and carbon dioxide in the blood as it passes around the two circuits. Then, in pairs, give them the equivalent diagrams for fish (single circulation), amphibians, birds and reptiles (incompletely separated double circulation) and ask them to list any differences they can see (compared to the human circulation) and to propose any consequences which may arise from those differences. For less advanced students, try to focus their attention on whether oxygenated and deoxygenated blood mix together, and the pressure at which blood can be pumped. Put pairs together into a group of four and ask them to summarise the benefits of a double circulation.

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Figure 5.4  A blank flow chart of the circulatory system

The heart Dissection can be an emotive subject and one about which a school science/ biology department will have a policy. You may wish to do a dissection alone and take photos to show the class. If possible, though, real material is the most interesting for students to see. Depending on your class, you could do a demonstration dissection of the heart (with a webcam pointing down onto the dissection and images projected onto the whiteboard), or you could let students dissect hearts in small groups. Some students may object to dissection and it is important to respect their views. Some students may feel nauseous and could even faint. Ensure adequate provision for washing hands within the laboratory after the practical work. Take care with scalpels and scissors; undertake a risk assessment, drawing on appropriate advice (see link to CLEAPSS at the end of the chapter). You can obtain hearts from supermarkets (although they are usually trimmed up too neatly) or from a butcher or abattoir (where you can ask for the main veins and arteries to be left protruding from the heart). 116

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Technology use Virtual dissections on the internet may provide useful alternatives (examples are included at the end of the chapter).

Careers Explain to students that trainee doctors and vets use dissection to help them understand anatomy, how organs work, and what happens when they stop working properly. The dissection should establish the features shown in Figure 5.5; give students a diagram and help them to understand the relationship between the living tissues and the diagram. You can find protocols on the internet to help you dissect well enough to yield the ‘textbook’ picture (see the list of websites at the end of the chapter). However, don’t forget to look at the outside of the heart first, and don’t be afraid to poke your fingers down tubes before and during dissection to see where they lead to! You can even mimic the action of the heart by filling it with water through the arteries that come out of the top, and squeezing the base to force the water back out. The first cut you make should remove the bottom 2 cm from the base of the heart. By looking at the cut end, you can see the distinction between the wall thickness of the left ventricle (which pumps blood to the whole body) and right ventricle (which only pumps blood to the lungs), providing a useful link back to the double circulation. Doing this also helps you to understand the orientation of the heart, which assists you in knowing where to cut to open up the ventricles and the atria. Once you have these open, keep reminding students that they are looking at ‘chambers’ through which the blood would pass. It is quite hard for students to relate the ‘two-dimensional’ structure of the heart when cut open, to the intact, undissected heart. If students do their own dissection, ask them to label their dissected structure to show the direction taken by the blood through the heart and the position of the valves. If you project the dissected structure onto a whiteboard, ask students to draw in the route taken by the blood.

Science in context You may want to talk about the ethical aspects of using tissues and organs for dissection. Be careful to be respectful of students’ views, while enabling them to think carefully about the ethical issues.

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anterior vena cava posterior vena cava right atrium tricuspid valve

pulmonary artery aorta pulmonary vein

left atrium bicuspid valve

valve tendon valve muscle right ventricle

left ventricle

Figure 5.5  A cross-section through a human heart

To ensure students understand how blood is moved through the heart, ask them to find an animation of the cardiac cycle on the internet and write a commentary for it. The commentary should indicate at each stage which muscles are contracting, which valves are open and closed, and in which direction the blood is flowing. Students should listen to some of the commentaries as a class, giving each other feedback; then, as a follow up exercise, you could ask them to create a flicker book to summarise the process. For less advanced students you could provide pictures and descriptions of each stage of the cardiac cycle for them to match up.

Arteries and veins As an introduction, ask students to build a montage of images of blood vessels; there are some great pictures of ‘isolated’ blood vessels on the internet (search ‘blood vessels’ to find them) which show blood vessels becoming smaller and smaller as they get further away from the heart. At this stage it is sometimes helpful to ask students to write a design brief for the vessels that will carry blood immediately away from the heart and a design brief for the vessels that will bring blood back towards the heart. This is a really good opportunity to use a snowball technique, where students start working on their own, then in pairs, and then in fours, gaining new ideas at each transition stage. Given that this is quite a challenging task, using group work in this way also supports differentiation for some students. Its aim is to focus students on what adaptations each type of vessel requires. 118

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To help them understand the structures of each vessel, you could ask students to examine prepared slides of transverse sections of arteries and veins under the microscope (many biology departments have a store of prepared slides). Ensure you spend some time explaining that a transverse section has cut across the vessel, and that they are looking at the cut end through the microscope (as above, it can be difficult for students to relate two-dimensional images to three-dimensional structures). Projected images of the same sections using photomicrographs would be helpful to make sure that students are actually focused on what you want them to see. You can find good examples by using the search terms ‘transverse section vein’ or ‘transverse section artery’ on the internet. Ask students to draw what they see and use their drawings, or prepared diagrams, to build models of the blood vessels (you could ask them to bring in appropriate materials). Students should copy and complete a table like Table 5.1 (with a description of the structure and its function) and then evaluate each other’s models. Hopefully, students should recognise that blood vessels bringing blood to the heart (veins) differ in structure from those taking blood away from the heart (arteries) and that the differences reflect the differences in blood pressures that they have to accommodate. You can test their understanding of the adaptations of the vessels by asking, ‘What would happen if an artery had the structure of a vein?’ and ‘Why can you feel your pulse?’. Table 5.1  Table to show the differences between arteries and veins Feature

Artery

Vein

no valves present except at the base of the aorta and pulmonary artery

pocket valves present

relative thickness of wall amount of elastic tissue, including muscle relative size of lumen valves

The reason we can feel our pulse is because of the recoiling action of the artery wall as the heart pumps blood through. One novel way of demonstrating the pulse is to attach a drawing pin to the base of a safety match. If the drawing pin is delicately balanced over the radial pulse with the arm resting firmly on a flat surface, it is possible to actually see the pulsating action of the left ventricle (Figure 5.6).

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oscillation

matchstick

drawing pin

wrist

Figure 5.6  A drawing pin and matchstick can be used to demonstrate the action of the pulse.

Science in context William Harvey graduated in 1597 from the University of Cambridge. He worked at the University of Padua in Italy for Fabricius, who had discovered how valves work. Harvey himself was the first to identify the double circulation.

Maths Introduce some simple calculations by asking students to work out their beats per minute. To do this, ask students to check the pulse at their wrist by placing two fingers between the bone and the tendon over the radial artery (which is located on the thumb side of the wrist). When students have located their pulse, ask them to count the number of beats in 15 seconds. This number is then multiplied by four to calculate beats per minute. If students struggle to find a pulse in their wrist, it is also possible to find one at the side of the neck or just below the collarbone.

Capillaries Having established how blood is moved around the system, the next step is to understand how things ‘get on and off’ the transport system. Students know that arteries and veins work as tubes carrying blood from one place to another and to do so efficiently, they presumably must not leak. However, if some molecules, such as oxygen, carbon dioxide and glucose, are going to enter and leave the blood, there must be a third type of vessel that does allow molecules to enter and leave. This is the capillary. It is possible to

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see capillaries by placing a drop of cedarwood oil on the skin of one finger, just below the cuticle of the fingernail (do this yourself!). By looking at the skin under the microscope, the tiny threads of surface capillaries are visible. You can project the image, or simply ask one of the students to take a picture of the image down the microscope with their mobile phone (it works pretty well!), and then share the image with their classmates.

Technology use Mobile phones work relatively well in photographing microscope images. Simply hold the mobile phone lens above the objective lens. Students may need to rest the phone on an extra finger to ensure it is still enough to view the image clearly (see Chapter 2). Capillaries not only let things in and out; capillary beds provide a much greater surface area for exchange of materials than a single artery would have done (even if it had a very thin wall). This is fairly obvious (if you split one tube into many smaller tubes, the surface area is increased). However, you could use a model to help make it clear. The capillaries are in fact so narrow that the red blood cells have to squeeze through, pushing up against the capillary wall and minimising the diffusion distance for solutes.

Blood as a transport medium Discussion about materials entering and leaving the transport system in capillary beds provides a natural progression to the nature of blood and the way it functions as a transport medium. Always take advice from the head of biology or head of science before engaging in any practical where you intend to use your own or your students’ blood. Advice is also available from CLEAPSS Student safety sheet 3: Human body fluids and tissues, or your local science advisory authority. You may be surprised at what is possible (and you may find rules and regulations are different between schools and over time), but unless you take advice at an early stage, you could easily put your own health or that of your students at risk. The first step is to look at what is in blood, and then to look at how it transports materials. It can be difficult to persuade some students that blood is not just a liquid. If you order some mammalian blood from your local butcher or laboratory supplier and centrifuge it, it is possible to demonstrate that there is a yellow fluid (plasma) along with some thicker material (the blood cells). If you can’t get access to blood, you can see this on YouTube (simply search ‘centrifuge blood’).

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You can find some useful animations of red blood cells on YouTube, and your department may have some prepared slides of blood that students can look at under the microscope, to establish that the ‘non-plasma’ part of the blood is made of red blood cells, white blood cells and small fragments of cells called platelets. See Chapter 2 for more on the structure and function of specialised cells. At this point, you could ask students to work in groups, listing the things that must be transported around the body in the blood. Invite them to record where these things come from, where they are going to, and what they are needed for/produced by. Having spent a few minutes setting out their ideas in a table, they could swap sheets with another group who then add to their ideas or correct them. Set up a game of ‘Blind Date’, where three members of the class have role cards describing their adaptations (as white blood cell, platelet or red blood cell). In terms of transport, the white blood cells and platelets have no role. However, the red blood cells are adapted to carry oxygen. The class has to ‘choose’ which of the Blind Date participants is involved in oxygen carriage. To reinforce the ideas on their role card, ask students to produce a job advertisement in groups, advertising the role of ‘oxygen transporter’ and seeking applicants who have the appropriate adaptations (being small, having a biconcave shape, lacking a nucleus, having a thin and permeable membrane, being flexible and containing haemoglobin). To help students understand how the biconcave shape increases surface area, you could ask them to make a model of a red blood cell by cutting into a conventional bathroom sponge. The surface area of the cut sponge can then be measured by sticking squared paper to the sponge and measuring the total area of the paper. By comparing this to an intact sponge of the same dimensions, students can demonstrate to themselves that biconcave cells have more surface area and can therefore absorb and lose oxygen more easily. As the difference between the surface area of a biconcave sponge and a normal sponge is not large, you may need to collate data from each student to find a class average in order to make the difference clear. To help older students understand why haemoglobin takes up or loses oxygen in different conditions, introduce the reaction as an equilibrium, in which oxyhaemoglobin is on one side and haemoglobin and oxygen are on the other. Where there is lots of oxygen (such as in the lungs), the equilibrium moves towards oxyhaemoglobin. Where there is little oxygen (for example around the body’s cells), the equilibrium shifts to release more oxygen, which then diffuses out of the red blood cell into the plasma, and out of the capillary into the tissue fluid that surrounds the cells. If you can obtain blood from the butcher you can demonstrate this. As soon as you get it, add 5 cm3 of 0.1%

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sodium oxalate per litre of blood to prevent clotting. Place equal amounts in three flasks and bubble oxygen through one, carbon dioxide through another and leave the third as a control. The blood in the oxygen flask will turn red, showing that haemoglobin binds to oxygen in high oxygen conditions. The blood in the carbon dioxide flask will turn dull red/purple, showing oxygen being released from haemoglobin in high carbon dioxide conditions. Finally, ensure students know that glucose and carbon dioxide are transported around the body in solution in the plasma. These substances diffuse out of the capillaries and into the tissue fluid on their way to and from the body’s cells. You could ask students to draw a flow chart depicting the journey of glucose from the blood plasma to the cells, or carbon dioxide from the cells to the blood plasma. Oxygen is transported around the body bound to the haemoglobin in the red blood cells (to form oxyhaemoglobin), but diffuses from there, through the plasma and tissue fluid, on its way to the body’s cells.

Careers The cardiovascular and respiratory systems work together to get oxygen to the working muscles and remove carbon dioxide from the body. During exercise there is an increase in physical activity and muscle cells respire more than they do when the body is at rest. The heart rate increases during exercise; this ensures that blood moves more quickly through the arteries. The rate and depth of breathing increases and more oxygen is absorbed into the blood, and more carbon dioxide is removed from it. Students may be interested in the idea of a career as a sports physiologist or trainer. Such professionals need to understand how oxygen is transported around the body, in order to set up the correct training regime for athletes. They also require good knowledge of the circulatory system in order to provide appropriate training approaches.

Lymph as a transport medium The lymphatic system operates alongside the blood system. Some students will be aware of swollen lymphatic glands in their neck and elsewhere when they are ill, or they may be aware of the role of lymph in the transport of digested fats in the digestive system. Given that little detail is usually required, it is commonly adequate, and indeed interesting, for students to identify similarities and differences between the blood system and the lymphatic system, both in terms of structure and function. The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the circulatory system and the immune system. It is made up of a large network of lymphatic vessels, lymphatic or lymphoid organs, and lymphoid tissues. The vessels carry a clear fluid called lymph towards the heart.

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Further activities ➜ To

bring together their learning so far, ask students to create a board game that shows how the transport system works either in plants or in humans. ➜ There are numerous opportunities to link social, moral and ethical issues to students’ work in this topic, and your teaching of any of the ideas above could be set in any of the following contexts: – valve bypass operations – heart transplants – use of pigs’ valves as a means of treating cardiac disease in humans – the ethics of developing transgenic organisms to provide organ banks – blood donation/transfusion – blood transfusion and HIV – use of blood for teaching and learning.

Careers Some students may have experience of having a blood sample taken at hospital. The person who takes your blood in a hospital is called a phlebotomist. It is such an important job, that they have their own special name!

5.4 Resources Standard laboratory equipment, including microscopes, is important in this topic. Further guidance on resources required is included above, and in the website protocols below.

Websites Websites relating to safety The CLEAPSS website provides clear guidance on practical procedures and safety for all science teachers: www.cleapss.org.uk SSERC is an organisation that supports science teaching in Scotland: www. sserc.org.uk

Websites on why transport and why transport systems A joint website of the Royal Society of Biology, the Nuffield Foundation and CLEAPSS (under the umbrella name ‘Practical Biology’) provides many useful resources. For this topic, search ‘effect of size on uptake by diffusion’ from the homepage: https://pbiol.rsb.org.uk

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5.4   Resources

Websites on plant transport Some practical experiments to investigate osmosis can be found on the Practical Biology website (https://pbiol.rsb.org.uk) by searching ‘osmosis’ and then ‘investigating osmosis in chickens’ eggs’. From the Practical Biology homepage, search ‘a window on the past: measuring stomatal density’ for an interesting practical on change in stomatal density with changing conditions over time. The Practical Biology website also has some resources around active uptake of mineral ions. Search ‘tracking active uptake of minerals by plant roots’ from the homepage. Science & Plants for Schools (SAPS) has some excellent resources for visualising stomata and investigating stomatal density. From the homepage (www.saps.org.uk) search ‘observing stomata in Tradescantia zebrina’ and ‘measuring stomatal density’. A further investigation into the temperature around the leaf surface can be found by searching ‘an investigation into leaf surface temperature’. The STEM website (www.stem.org.uk/) has some useful information on transport in plants. Try searching the terms ‘plant transport’ and ‘transport systems in plants’ from the homepage. Some simulations and animations about movement of water and sugars in plants can be found at the following websites: ➜ Search

‘animation – transport of water and sugar into plants’ from the SAPS homepage (www.saps.org.uk) ➜ https://go.unl.edu/ewi2, a webpage from the New Mexico State University The activity ‘Can you beat the Giant Redwood’ on the SAPS website provides a good starter activity for the topic of xylem. The activity can be found by searching from the homepage: www.saps.org.uk

Websites on transport in humans Some good virtual heart dissections can be found at the following two sites: https://biologycorner.com/virtual/heart and http://thevirtualheart.org The Practical Biology website, mentioned above, has some good information on various aspects of transport in humans. Search ‘looking at a heart’ from the homepage for some detailed information on class dissections. The same website has a practical activity involving students’ blood (please see main text for safety warning). The activity can be found by searching ‘a closer look at blood’ from the homepage. A final suggestion can be found by searching ‘observing blood circulation in asellus’. Asellus is a small, freshwater crustacean. It is possible to see the blood moving in its limbs under a lowpower microscope. 125

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Further reading Ainiyah, M. et al. (2018) The profile of student misconceptions on the human and plant transport systems. Journal of Physics: Conference Series, 947. (Can be found on the website of IOP Science: https://iopscience.iop.org/) Lester, A. and Lock, R. (1998) Sponges as visual aids – bath time fun for biologists. Journal of Biological Education, 32, 87–89. Pelaez, N. J., Boyd, D. D., Rojas, J. B. and Hoover, M. A. (2005) Prevalence of blood circulation misconceptions among prospective elementary teachers. Advances in Physiology Education, 29, 172–181. (Can be found on the website of Physiology.org: https://journals.physiology.org/) Vitharana, P. R. K. A. (2015) Student misconceptions about plant transport – a Sri Lankan example. European Journal of Science and Mathematics Education, 3 (3), 275–288. Yip, D. Y. (2010) Teachers’ misconceptions of the circulatory system. Journal of Biological Education, 32 (3), 207–215.

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6

Communication and control Mike Cassidy and Beverley Goodger

Topic overview The use of a stimulus–response chain will form the organising principle for this chapter. It will look at teaching both the process of co-ordination and the mechanisms of control and communication.

Cross-disciplinary The concept of control systems should already be familiar to students through their Design and Technology experiences and their study of the human body. Control and co-ordination is also recognised as a characteristic of living things and something that separates the living (or animate) from the non-living (or inanimate). However, students will be less familiar with the mechanisms of control (nerves and hormonal secretions) and the notion that all living things, including plants and single-celled organisms, will exhibit some sort of control, both intracellularly and extracellularly.

Prior knowledge and experience Students may be familiar with the various behaviours of animals and plants and their responses to various stimuli – again, necessitating the need for some sort of control mechanisms. In primary education, children are introduced to forces (action and reaction), to the differences between things that are living, that are dead and that have never been alive, and a recognition of the impact of diet, exercise, drugs and lifestyle on the way their bodies function. The topics in this chapter are more likely to have been encountered by older students.

A teaching sequence A teaching sequence might begin with the complexity of living organisms: either body complexity (for example, the trillions of cells in the human body) or cell complexity (the thousands of biochemical activities taking place in the living cell at any moment in time, along with the variety of intracellular structures).

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That such complexity requires control is not always self-evident. Students can be introduced to complex machines (anything from washing machines to automobiles) and the need to integrate the component parts. A jumble of ‘spare parts’ cannot function; only when assembled correctly are they potentially capable of functioning. And there needs to be a stimulus (switching on, turning the ignition key) to begin an operation. All control systems, either animate (such as animals and plants) or inanimate (such as washing machines and automobiles) operate along the same lines. This commonality of approach is a useful adjunct to study, emphasising how engineers gain ideas from biology (such as aeroplanes and bird flight) and, conversely, how biologists gain ideas from engineers (by, for example, incorporating concepts of systems control into biological thinking). By the age of 16, students should have explored principles of both nervous and hormonal control in humans (including the reflex arc and hormonal control of contraception). Progression will respond to the ‘what’, ‘why’ and ‘how’ questions relating to body control systems. The teaching sequence will adopt a pattern, starting with the known (organisms are complex and require co-ordination) and moving towards the unknown (the mechanisms of control and co-ordination). A process of ‘bridging’ will use inanimate examples (thermostats, robotic control) to explain the principles of control, namely: (sensory) INPUT → (stimulus detection) DETECTOR → (control centre) REGULATOR → (mechanical response) EFFECTOR → (behaviour or response) OUTPUT A typical teaching sequence might then include: 1 Why control? 2 Mechanisms of nervous and hormonal control (nerves, reflex arc, endocrine glands, plant hormones) 3 Efficient body functioning (homeostasis) 4 Applications (use of human hormones in contraception, use of plant hormones in regulating flowering, fruiting, etc.) A useful addition to the teaching sequence above is that the teacher can underscore both the similarities and differences between animals and plants. Animals and plants are similar in that they show a range of behaviours that are carefully co-ordinated and possess survival value. They are different in that animals are motile and use both fast-acting (nervous) plus slower, more generalised (hormonal) control systems. Plants, on the other hand, are sessile, without muscles and so rely on growth behaviours (tropisms) regulated by a variety of chemical compounds (plant hormones).

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6.1   Communication and control (a study of control systems)

6.1 Communication and control (a study of control systems) Why control? Biological systems are complex. In order to ensure survival, the bodies of living things must acquire and maintain a source of energy. They also need to carry out all the other life processes such as growth, reproduction, removal of metabolic waste, etc. The bodies of multicellular animals and plants are hierarchical: cells → tissues → organs → body systems This complex of trillions of cells (as in humans) requires a high level of control both for efficient functioning and to ensure that organisms seek out favourable and avoid unfavourable circumstances.

Control systems Mechanisms of regulation in living things show features in common with the regulation of machines. Both organisms and machines achieve stability by control. The science of control systems is called cybernetics. Communication in living things is achieved either by chemical means (hormones) or electrical means (nerves). The basic components of a control system are: input → detector → regulator → effector → output The detector (or in the case of sense organs, the receptor) detects the stimulus while the effector delivers (or effects) the desired response. Stability is achieved by establishing a standard operating level (the norm) and thereafter correcting any deviation from this. The efficiency of the system is determined by the degree of deviation from the norm. For instance, we might set a room thermostat at 25 °C. The control system then attempts to maintain this temperature, turning radiators on when too cold and turning them off when too hot. Its efficiency is determined by how closely the ‘set’ temperature is maintained. In the human body, the temperature norm is around 37 °C (slightly higher for children). Temperature control (or thermoregulation) is achieved by the hypothalamus in the brain that is responsible for monitoring and controlling

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core temperature. If there is a deviation from the norm, then various mechanisms (both behavioural and physiological) come into play (see Section 6.4). It can be useful to ask students to name the component parts of a room’s heating system (or air conditioning system) and to compare them with the body’s heating and cooling system (see Table 6.1). Table 6.1  Comparison of body temperature control with a room heating system Component

Thermostatically controlled room heater

Body temperature control

input

the room temperature

body core temperature

detector

thermometer

specialised blood temperature receptors

regulator

thermostat

the brain (hypothalamus)

effector

room heater (or cooling system)

muscles (dilating or constricting superficial blood vessels and shivering response); sweat glands

output

heat generated and emitted (or turned off)

heat generated, heat conserved or heat lost

Effective control systems rely on their components being linked together. Information flows from detector to regulator to effector. Communication between component parts is achieved by nerve impulses or hormones. Feedback is used to inform the control system as to how effective these corrective mechanisms have been and whether further effort is needed to return to the norm. A feedback loop is built into most control systems, both living and non-living. Positive feedback will exaggerate the direction of the response (for instance, make the room hotter); negative feedback has the opposite effect (that is, make the room cooler). Feedback loops are found throughout the animal body, for example in glucose control in the blood where there are feedback loops with responses when glucose is too high (increase in blood insulin) and too low (increase in blood glucagon). The basic components of an animal control system are analogous to those of a machine: input → detector → regulator → effector → output stimulus → receptor → central nervous system → effector → response

➜ Machine: ➜ Animal:

The components are linked in a specific way forming a stimulus–response (S–R) chain (Figure 6.1).

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6.2   Nervous control in animals

Stimulus: a change in the external or internal environment.

Receptors: detect the stimulus, transmitting ‘information’ to the central co-ordinating region. Receptors are the sense organs.

Co-ordinator: a central, co-ordinating region. Receives information from the receptors, makes decisions and contacts the effectors.

Effectors: respond to signals from the nervous system, effecting (making) a response. Effectors are usually muscles or glands.

Feedback

Response

Figure 6.1  The stimulus–response chain

Animal behaviours can be studied as a sequence of stimulus–response (S–R) chains of increasing complexity, for instance, behaviour of woodlice in choice chambers or bees visiting different coloured flowers. Perhaps the stimulus–response chain can be exemplified most easily by the ‘dropping ruler’ investigation where (working in pairs) the experimenter drops a ruler, 30 cm or longer in length, between the open fingers of the subject. The stimulus is the sight of the ruler falling, the response the closing of the subject’s fingers. Different sensory parameters may be used, such as the touch of a ruler on a blindfolded subject. Human learned behaviours (such as mirror writing and maze activities) are more complex than simple S–R routines, but still demonstrate S–R chains; only this time the (learned) response is mediated by previous experience. Students can recognise that a ‘practice effect’ is commonly seen in these situations.

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6.2 Nervous control in animals The nervous system One of the characteristics of the animal kingdom is that all animals possess a nervous system. A nervous system is a collection of excitable cells arranged in a meaningful way. If students are asked to name parts of the nervous system, they often give responses such as ‘brain’ and ‘nerves’. Show students a diagram of the human nervous system. It is made up of two parts: ➜ a

central region containing brain and spinal cord peripheral nerves (emanating from the spinal column).

➜ associated

The term ‘central nervous system’ is used to label the central region of brain and spinal cord. The nerve cell (neuron) is specialised for conducting impulses. Like all body cells, neurons are microscopic but some (such as those coming out of the spinal column) can be very long, reaching over a metre in length. A nerve impulse is actually caused by a change in electrical potential (due to movement of ions across the outer nerve cell membrane); this is called an action potential. Nerve fibres are often covered in a fatty sheath for (electrical) insulation. Neurons: ➜ are

separated by gaps (or synapses) at their ends ➜ can make many connections ➜ can modify their connections (the basis of learning). The speed of a nerve impulse can be estimated by having a group of students standing in a circle. One student ‘passes on’ a message by squeezing the hand of the individual to their left, who immediately does the same. The time taken to pass the message between students is recorded while distance taken (hand to brain to hand for each person) is estimated by tracing the nerve pathway using string. Velocity can then be calculated using distance divided by time (in metres per second, m s–1). Body reflexes are another way of investigating the nervous system. A reflex is a rapid, involuntary response to a stimulus. The eye-blink reflex, knee jerk reflex and swallowing reflex are good examples (though the swallowing reflex is quite complex, and the knee jerk reflex is unusual in not having an interneuron (sometimes called a connector neuron)). Using an example such as removing the hand from a hot object, the survival value of body reflexes can be seen. One of the characteristics of the animal kingdom is that all animals possess a nervous system. Even jellyfish have a simple nerve net. 132

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6.2   Nervous control in animals

Nerve impulses are fast and localised. Different neurons have different roles: ➜ The

sensory neuron takes impulses from the receptor (sense organ) to the central nervous system. ➜ The motor neuron takes impulses to the effector (muscle or gland) from the central nervous system. ➜ The interneuron joins the input component with the output within the central nervous system. sensory neuron stimulus

central nervous system

receptor

connector neuron response

effector motor neuron

Figure 6.2  Components of a reflex arc

Scientific literacy Students are tempted to refer to neurons as ‘nerves’ (sensory nerve, etc.). Remind them that a neuron is a single cell, whereas a ‘nerve’ is a collection of neurons. Typical examples of reflex arcs that might be studied include a range of human reflexes such as: ➜ a

puff of air in the eye causing blinking the hand from a hot or sharp object ➜ the knee jerk reflex ➜ the swallowing reflex. ➜ withdrawing

Swallowing (as mentioned earlier) is a rather complex procedure involving muscles raising the tongue, moving the epiglottis, etc. Although it begins voluntarily, it requires a stimulus at the back of the throat to complete. (Try swallowing continuously. After a while you will run out of ‘spit’ and so there will no longer be any stimulus at the back of the throat.) Interestingly and unusually, the knee jerk reflex involves only a sensory and motor neuron (no interneuron within the central nervous system) and there is no direct communication with the brain. 133

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Body senses Students can discuss the responses made by animals to a variety of stimuli: ➜ bees

visiting brightly coloured flowers (colour) hunting by smell (molecules in water) ➜ houseflies landing on food and ‘tasting’ food with their feet (molecules on the surface). ➜ sharks

Each of the above examples requires a receptor (a part of the body that is adapted to receive stimuli). A receptor may be a single cell or a complex structure (such as the eye); it detects stimuli from both the external and the internal environments. Essentially, a receptor works by altering the concentration of sodium and potassium ions on either side of the neuron’s membrane. This then causes the production of a nerve impulse (or action potential). Nerve impulses are: ➜ very

fast

➜ electrical

in nature with an ‘all or nothing’ response (no half measures; they either ‘fire’ or they don’t) ➜ unidirectional (travel in one direction only). ➜ generated

We are generally thought to have five senses (sight, hearing, touch, taste, smell) but others have been identified, most notably proprioception (knowing where your body is in relation to itself) and sense of equilibrium. There are many different kinds of receptors in the human body and they are classified in several ways (Table 6.2). Students usually do not need to know the names of specific receptors, simply that the body responds to many different kinds of stimulation, both internal and external. Table 6.2  Classifying body receptors Classified by

Receptor name

Stimulus detected

Example

general senses (simple)

single cells or small groups of cells; respond to a variety of stimuli

pain sense endings, touch receptors, pressure receptors in blood vessels

special senses (complex)

complex sense organs; respond to specific stimuli

eye, ear

photoreceptor

light

eye

chemoreceptor

chemical

taste buds, receptors in the nose

receptor complexity

stimulus type

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6.2   Nervous control in animals

Classified by

Receptor name

Stimulus detected

Example

thermoreceptor

heat

simple receptors in the skin and in the hypothalamus

mechanoreceptor

mechanical stimuli (such as touch)

pressure receptors and touch receptors in the skin

baroreceptor

pressure

pressure receptors in blood vessels

Special senses: the eye Eyes are complex structures adapted to receiving light and transmitting information to a central co-ordinating region (the brain). Eyes range from simple eye cups in flatworms (that simply indicate light and shade and the direction light is coming from) to the complex vertebrate eye with structures for focusing light into sharp images and detecting different wavelengths as colours. In vertical section, the eye is seen to be made up of a spherical bag of jelly separated by a crystalline lens. Light is focused by the lens (it alters its shape due to the contraction and relaxation of a muscle ring) onto the light-sensitive layer, called the retina, at the back of the eye. At the front of the eye (Figure 6.3) the muscular iris (the coloured part) contracts or relaxes, decreasing or increasing the size of the pupil (the black dot in the centre), thereby altering the amount of light entering the eye. Too much light and the retina can be irreparably damaged. eyelid eyebrow

eyelash iris pupil sclera (white of eye)

Figure 6.3  External (visible) features of the human eye

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Students find it difficult to appreciate the eye’s internal structure through diagrams alone. The suspension of the lens is a particularly troublesome area and it is important to establish that the flexible lens is joined by many fibres to the ciliary muscle, rather like a circular trampoline is joined to its frame. The difference here, though, is that the fibres are strong and rigid so that when the ciliary muscle contracts, it pulls on the lens. Model eyes and eye dissection are particularly useful in this respect. Check whether your school has a policy on dissection. Note that the optic nerve connects the eye to the visual centre of the brain (located at the back of the brain). The optic nerve is a large nerve. If the human skull is examined, a hole at the back of the eye socket indicates where the optic nerve leaves the eye on its way to the brain. At a point just below the centre of the retina, the optic nerve collects together sensory fibres from retinal cells. There are no light-sensitive cells (rods and cones) here. So, in this area, light falling on the retina cannot be detected. We call this region the blind spot. If you draw a cross and a large dot 10 cm apart on a sheet of white paper and hold it at arm’s length, both symbols can be clearly seen. Focus on the cross and close your left eye. Bring the paper slowly towards the face and you will see the spot disappear. At this point the image of the cross is picked up by the rods and cones but the image of the spot is focused on the blind spot, which has no light-sensitive capability.

Cross-disciplinary The physics of the eye may be studied with reference to work on lenses, refraction, reflection, wavelengths and energy transformation. If your school teaches separate rather than combined science, an effective strategy is to plan joint sessions with physics teaching staff in order to reinforce the basic principles of optics. Students should cover accommodation (the process of changing the shape of the lens to focus on near or distant objects) and eye conditions such as myopia (short-sightedness) and hyperopia (long-sightedness) in which rays of light do not focus on the retina. Ray diagrams can be used to display these conditions effectively. Vertebrate eyes (especially those of birds and mammals) are particularly good at ‘seeing’. They: ➜ have

a lens that can form an image adjust the lens to focus on near and distant objects ➜ can generally control the amount of light falling onto the retina ➜ can often work in both low and high light intensities ➜ can

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6.2   Nervous control in animals ➜ can

generally distinguish different colours have sharp (acute) vision ➜ come in pairs to provide three-dimensional vision and depth perception. Horses, for example, and prey animals, such as antelopes and deer, have eyes on the side of their head, which gives a greater all-round vision. ➜ usually

The retina is made up of light-sensitive cells. Cone cells provide us with colour vision and, because they are densely packed, give us especially acute (high-resolution) vision. Rod cells work at lower light levels than cones and provide us with simple monochrome (black and white) vision. Ask students to consider what happens to the colour of trees, hedges and flowers as light falls; everything becomes more monochrome as only the rods work in low light intensities. Incidentally, rod cells are concentrated at the edges of the retina (that is, at the periphery of our field of view). Hence, sentries posted at night in army barracks learn to look at objects through the corners of their eyes. Students may also appreciate that we have different types of cone cell (responding to the three colour wavelengths: red, green and blue). Some students may not be able to discriminate between certain colours due to lack of a particular type of cone. We call this colour blindness. Models of the eye can be employed profitably to explore structure, while functioning can be relatively easily studied, for instance: ➜ Photochemical

reactions are commonplace, from the bleaching of coloured fabrics in sunlight to the action of light on silver nitrate (as used in old style, pre-digital, photographic films). So, students should not be surprised to learn that light falling on pigments inside receptor cells (rods and cones) causes these pigments to break down (a reversible reaction, of course), thereby causing an action potential and releasing a nerve impulse. ➜ The eye responds to light stimuli, but the eye does not ‘see’. It is the brain that makes sense of the information from the eye and it is therefore the brain (or person with the brain) that sees. Students are often excited to view visual illusions. There are many of these to choose from and all illustrate the principle that our perception can be tricked into seeing things that are not there. ➜ What is the function of eyebrows, eyelids and tears? These are interesting questions for students to consider. Consider their protective functions. ➜ Students could find out about health issues that affect eyes, such as cataracts, glaucoma and diabetes.

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Science in context Explain to students that birds and mammals achieve accommodation by altering the shape of the lens in their eyes, while fish and amphibians normally adjust focus by moving the lens closer to or further from the retina.

Special senses: the ear The human ear has a dual function as both an organ of hearing and an organ of balance. It has a three-part structure: ➜ the

outer ear, comprising a twisted ear canal and ending in the tympanum, or eardrum ➜ the middle ear, comprising three small bones (ossicles) that transmit vibrations across this air-filled space ➜ the inner ear, a complex, fluid-filled region made up of an upper ‘looped’ region controlling our sense of balance and a lower coiled region where vibrations in the fluid are detected by sensory cells. These structures are very small and they are best illustrated by use of models, posters or diagrams in textbooks. Most of the structures have both common and scientific names (such as eardrum/tympanum, ear lobe/pinna). Younger students use the common names; older (examination) groups may need to use the scientific terms. Hearing may be best approached through revision of the physics of sound. Noise comprises waves of compressed air (sound waves) created by a vibrating object. Reference to guitar strings, drums and ‘twanging’ rulers can be introduced here. Properties of sound include amplitude of the sound wave (loudness) and frequency of the sound wave (pitch, high notes/low notes).

Technology use Students can be shown sound waves visually with a demonstration of the coloured images found on most amplifiers, MP3 players or music computer programs. (The oscilloscope found in the physics department is also very useful in the teaching of sound and hearing.) How we hear is complex, but in essence it relates to detection of sound waves. These: ➜ enter

the ear to be collected by the ear lobe ➜ are channelled down the ear canal

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6.2   Nervous control in animals ➜ cause

vibration of the eardrum vibrations are transmitted through the middle ear via three small bones (ear ossicles) touching both the eardrum and the inner oval window.

➜ ensure

Then: ➜ vibrations

are transmitted through the fluid of the coiled cochlea from the oval window ➜ sensory cells lining the cochlea respond to loudness (how much these small cells are displaced) and pitch (different patches of cells responding to different note frequencies) ➜ the sound wave is dampened by hitting the round window (a membrane at the far end of the cochlea). The ear also provides a sensation of balance; how we sense position in space is achieved by: of fluid in the ‘looped’ region, the semi-circular canals (together with the sac-like region below), depending upon the direction in which our head is tilted ➜ fluid displacement affecting patches of sensory cells which send information to the brain.

KEY ACTIVITIES

➜ movement

Exploring hearing and balance

Hearing can be studied indirectly by asking students to respond to various sound stimuli. Some examples are given below. l A

signal frequency generator (borrowed from the physics department) connected to a loudspeaker can generate sounds of varying frequency. Young people can generally hear notes of between 20 and 20 000 Hz (cycles per second); (older) teachers have reduced frequency discrimination. l Sensitivity to loudness can be determined by holding a ticking watch at varying distances from a blindfolded subject. l Ability to sense the direction of sound can also be determined using a blindfold, a subject and a ticking watch. l Use of an ear trumpet (made from thin card) is seen to increase the sensitivity to sound and directional ability. Ask students why this would be the case. l Use of a tuning fork can demonstrate how vibrations can travel through air, water and solids. Tapping a tuning fork and then holding it against a lab bench demonstrates how well sound travels through a solid. Talking into an inflated balloon (feel the vibrations) is used to demonstrate this feature to children, particularly those with hearing difficulties.

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Balance can be explored as follows: l Have

students stand still for three minutes (they will sway slightly). Notice how they correct their balance. With eyes closed, students find this task much more difficult (no reference point to focus on). l A model of the ear’s three semi-circular canals can be made from stoppered, clear plastic tubing. The three semi-circular canals can be constructed separately but attached to each other at right angles, as would be the case in real life. Trapped air bubbles will demonstrate displacement of the fluid as the head is tilted from side to side.

General senses: touch, smell and taste The skin is the largest organ in the body and home to several senses: pain, touch, pressure and heat. Taste and smell are chemical senses located in the human tongue and nose, respectively. All are known as general senses and detection is brought about by a relatively small cluster of cells. Little detail is required in most specifications, but some misconceptions may be addressed here: ➜ What

we think we are ‘tasting’ (such as the taste of an onion) is actually a combination of taste and smell. When wearing a nose clip, an onion tastes sweet! Students can rarely tell the difference between a slice of apple and a slice of onion gently placed on the tongue (providing a nose clip is in place, and there is no looking and no chewing). Do this activity in the dining room or food hall; there should be no food or drink in the science lab. ➜ The little ‘bumps’ on the tongue are not taste buds. Rather, they are papillae, designed to roughen the tongue’s surface. (Just think how difficult it would be to chew and swallow with a smooth, shiny tongue!) Ask students if they have been licked by a cat or a large herbivore such as a cow! ➜ Our skin is not equally sensitive over the body. Using a small piece of card with two pins (placed 1 cm apart), ask students to test different skin areas for sensitivity. This is achieved by students working in pairs, with the experimenter applying either one or two pins to the skin surface and the subject responding. (Can they recognise if they are being touched by one pin or two?) After 20 presentations, the number of correct responses is recorded. Which areas do students think are more sensitive: fingertip, back of hand, back of neck, lips? Check your department’s or employer’s risk assessment before undertaking this activity. ➜ There are now thought to be five taste sensations: the original sweet, salt, sour and bitter along with umami (a sort of savoury/meaty flavour). ➜ The classic taste map of the tongue (where different tastes are located in different regions of the tongue surface) is now known to be invalid. Recent research has shown that there is sensitivity to taste across all regions of the tongue. 140

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6.3   Chemical control (in animals and plants)

6.3 Chemical control (in animals and plants) Animal hormones Chemical communication is universal in animals, plants and microbes. Chemical signals, though, are usually relatively slow and diffuse. They work well over small distances (as in cell-to-cell signalling) or for slow, measured responses such as growth and reproductive cycles. Chemicals, though, cannot mediate fast responses such as reflexes; they are also persistent and therefore need to be broken down. Students may be familiar with specific examples of hormone action (insulin to lower blood glucose, sex hormones to control puberty and reproduction) but are less likely to have an integrated picture of the role chemical signals play in control and co-ordination. A hormone is a secretion, released into the bloodstream, which has an effect on a distant structure (target organ). A gland is any body structure that produces a secretion. An endocrine gland does not possess a tube or duct to release its secretion (unlike, say, a sweat gland). The secretion, a hormone, is released directly into the bloodstream. A hormone works by attaching to the specific receptor molecule for that hormone. The receptor molecule is found on the outer cell membrane of the hormone’s target cells (although the specific receptor for steroid hormones is found inside the cell). This generally sets up a chain of biochemical reactions within the cell, causing the alteration of its chemistry together with the production, and maybe release, of a cell-produced compound. The hormonal control system generally follows the stimulus–response chain (as seen earlier). Examples of hormone action for several of these functions are provided in Table 6.3. Table 6.3  A summary of major hormones and their effects Endocrine gland pancreas

adrenal gland

thyroid gland

Hormone produced

Effects

insulin

lowers blood glucose

glucagon

raises blood glucose

adrenaline

gets the body ‘ready for action’ by raising blood glucose and increasing chemical activities and general awareness

cortisol

helps the body resist stress by raising blood glucose

thyroxine

increases the body’s general metabolic rate (and causes metamorphosis in frogs)

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Endocrine gland

Hormone produced

testosterone (produced by the male in the testes and by the female in the ovaries)

Effects promotes formation of male secondary sexual characteristics including sperm formation involved in male courtship behaviour promotes healthy musculoskeletal and reproductive systems in males and females promotes development of female secondary sexual characteristics

sex organs

oestrogen (produced by the ovary and the testes)

stimulates growth of the uterine lining during early parts of the menstrual cycle important in the development and production of sperm in the male

progesterone (produced by the ovary during the menstrual cycle and by the testes)

pituitary gland

completes the development of the uterine lining and maintains this lining if fertilisation takes place with an embryo implanted there important in testosterone production by the testes

growth hormone

increases growth rate of young animal

thyroid-stimulating hormone (TSH)

stimulates hormone production in the thyroid gland

follicle-stimulating hormone (FSH)

stimulates production of follicles within the ovary (resulting in the shedding of eggs)

anti-diuretic hormone (ADH)/ vasopressin

stimulates water reabsorption in the kidneys in times of water deficit

oxytocin

stimulates muscles of the uterus during childbirth and milk release during suckling

Specific information can be introduced at the relevant syllabus point: ➜ Reproductive

hormones can be addressed when discussing growth and development, the menstrual cycle and birth. ➜ Control of blood glucose is explored when discussing transport of the products of digestion (although this is a popular example when describing principles of homeostasis). ➜ The action of pituitary growth hormone is included when describing patterns of vertebrate growth and development. However, the pituitary gland has such an important co-ordinating role (it affects other endocrine glands) that it should be treated separately. Students should be encouraged to locate and identify endocrine glands on an outline diagram of the human body. Students might wish to explore a case study of a patient suffering from diabetes, describing both the symptoms and the mechanism of insulin

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6.3   Chemical control (in animals and plants)

treatment. (Be sensitive to the fact that you may have students in your class that suffer from this condition or have relatives who do.) You can also ask students to compare and contrast endocrine and nervous function (see Table 6.4). Table 6.4  A comparison of nervous and hormonal function Factor

Nervous response

Endocrine response

speed of effect

generally, very fast (a fraction of a second)

relatively slow (hours, days, weeks); the exception to this is adrenaline, which acts in seconds

area of effect

localised effect

general effect around an organ or around the body

timing

short-lived effects

long-term effects

blood supply

limited

very good capillary supply

Table 6.4 provides information about the similarities and differences between hormones and the nervous system. This should enable students to understand that slow, moderated and controlled responses such as growth, puberty and the menstrual cycle are controlled by hormones. Hormones can also influence the function of the immune system, and even alter behaviour. A gradual build-up of hormone is often required to activate change. Hormones travel in the bloodstream and pass through all organs but generally only cause changes in their target organ. Excess hormones are broken down after a period of time; some are broken down by the liver and others are metabolised by the cells that secrete them or the target organ cells. Hormones are involved in three main areas of physiological function: ➜ growth

and development ➜ reproduction ➜ maintenance of the internal environment (homeostasis). Nerve action potentials move rapidly along nerve fibres and are associated with much faster changes than those determined by hormones. Action potentials stimulate muscles to move part of the body or the whole body. A useful set of questions to draw out some ideas from Table 6.4 are: ➜ Why

are nerves much faster than hormones in achieving their effects? there any fast-acting hormones? ➜ If hormones have long-term effects, what happens to excess hormones in the bloodstream? ➜ What types of body response are controlled by hormones rather than nerves? ➜ Are

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Pheromones are sometimes called ‘external hormones’ and are important in several areas of animal behaviour; for example, receptive female mammals in some species release a scent to attract males, and the queen bee releases a chemical to control the activities of the hive. In humans it is said that mothers can recognise their babies by scent alone.

Plant growth regulators (plant hormones) The term ‘hormone’ (from the Greek meaning to stir up or excite) is normally reserved for those secretions produced by animal endocrine glands. Substances that influence growth and development in plants are not produced by specific glandular structures; they may be transported from the point of synthesis via the phloem or xylem or they may diffuse locally into neighbouring cells. Plant biologists generally use the terms ‘plant growth regulators’, ‘plant growth substances’ (Biological Nomenclature, Society of Biology, 2010) or ‘plant growth factors’. Plant responses to the environment can be observed by growing plants on window ledges (they bend towards the light), by growing plants from seeds (the shoot always grows upwards, the roots down, irrespective of how the seed is planted) and by observation (ivy clinging to walls, pea plants growing along supporting wires). ➜ Phototropisms

are responses to light (plants on the window ledge). are responses to gravity (growth of seedlings). ➜ Thigmotropisms are responses to touch (ivy growing close to walls). ➜ Geotropisms

It is better that students observe these responses themselves. Note that a ‘tropism’ is a growth response. Plants do not have muscles; they can respond by growing towards (positive tropism) or away from (negative tropism) a stimulus. Ask students to examine a plant growing towards the light. They should hopefully see that greatest curvature (that is, greatest growth) occurs on the shaded side. Ask them, therefore, whether light inhibits cell growth and cell division. Do they know what happens when plants are grown in the dark?

Technology use A very useful animation for students on the action of auxin is available as a downloadable resource from the Wellcome Trust’s The Big Picture website, issue 24, May 2016 (Plants).

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KEY ACTIVITY

6.3   Chemical control (in animals and plants)

Demonstrating plant phototropisms In the classroom, plant phototropisms can be demonstrated by growing mustard or cress seeds on damp cotton wool in a small dish and exposing the seedlings (they germinate in two or three days) to: l unidirectional

light all around l no light. l light

Cardboard boxes can be used to exclude all light or (if openings are made at one end) to provide unidirectional light. Seedlings outside the boxes are bathed in light all around.

Technology use Some good video sequences on plant growth are available to show classes, and several excellent practical investigations can be found on the Science & Plants for Schools website (see Resources section at the end of the chapter). Student investigations into tropisms could include the use of time-lapse photography. The question arises, ‘What are plant growth substances?’. The answer is that they are compounds produced by cells in particular regions (often the growing points). Some of the major plant growth substances and their effects are given in Table 6.5. Table 6.5  Roles of the main plant growth substances Hormone

Effects

Examples

auxins

cause cell elongation and growth of stem and root

phototropism, cress bends towards light; used in hormone rooting powders

high concentrations disrupt plant growth

synthetic auxins are used as weedkillers

causes leaf ageing

important in leaf fall in some trees

ripens fruit

used commercially to ripen fruits, such as lemons and bananas

abscisic acid

controls bud dormancy and is involved in leaf fall (abscission)

can transform the growing tip of trees into dormant buds; used to speed up leaf fall

gibberellins

stimulate seed germination, stem elongation used commercially to increase the size of and induce flowering seedless fruit such as grapes

ethene/ethylene

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Rapid cycling brassicas (‘fast plants’) can be used to demonstrate plant responses to growth substances. Suggested investigations, along with details of suppliers, planting instructions and worksheets, can be found on the Science & Plants for Schools website. At this point it can be a useful exercise to ask students to compare and contrast plant and animal responses. Table 6.6  Comparing plant and animal responses Animal responses

Plant responses

usually rapid

usually slow

short stimulus needed

prolonged stimulus usually needed

effect normally temporary

effect usually permanent

behavioural responses involve movement

growth responses are produced

Garden centres are useful suppliers of selective weedkillers and hormone rooting powders. Hazard warnings, safety advice and safe disposal methods must always be followed. Commercial uses of plant growth substances may be investigated as a project, using product labelling and classroom investigations such as the effect of rooting powders on plant cuttings. A simple investigation into the growth of plant cuttings can be found on the Practical Biology website (see end of chapter for details). The investigation encourages students to nurture their plants, helping to develop the respect for living organisms that is a core value of any biology course.

Maths Practical investigations and the interpretation of primary and secondary sources of data allow students to develop the following mathematical skills: l the

construction and interpretation of frequency tables and bar diagrams, bar charts and histograms l the translation of information between graphical and numerical forms.

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6.4   Homeostasis

Careers A study of both endocrinology and neurobiology are useful for careers in laboratory biosciences, while research plant biologists are in demand by a range of agricultural and horticultural companies. This is also a particularly interesting area for those students involved in psychology. A very interesting and thought-provoking interview with a leading plant scientist about plant survival strategies, growth substances and responses can be found on the SAPS website (see end of chapter for details). After watching the video, older students could answer the questions that accompany the video and perhaps produce a magazine article comparing the ‘root or run’ survival strategies of plants vs (most) animals. As a postscript to this section on mechanisms, it is worth discussing briefly the role of vitamins. Vitamins are micronutrients (needed in tiny amounts) that play a role in controlling various biochemical pathways. For instance, vitamin A controls cell and tissue growth while vitamin D controls mineral (especially calcium) uptake in the maintenance of bones. Most vitamins are found naturally in the foods we eat and have multiple functions in the bodies of animals (and plants).

6.4 Homeostasis Introduction to homeostasis When observing body functioning, it is significant that humans (and other mammals) maintain a constancy of internal factors such as temperature, electrolyte balance, etc. irrespective of external conditions. Claude Bernard (1813–78), a French physiology professor, was one of the first to recognise and state this principle of internal constancy that we now know as homeostasis. The more we understand cell biology, and cell chemistry in particular, the more we realise that individual body cells are vulnerable to even slight changes in conditions. Protein molecules, at the heart of much of our biochemistry, are readily altered by temperature, while the outer cell membrane can be damaged easily by osmotic changes. Homeostasis is the ability of organisms to maintain chemical equilibrium. It involves feedback and self-adjusting mechanisms that return bodily function to a norm or set point. These mechanisms can be either physiological (sweating, shivering) or behavioural (standing in the shade, resting).

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The usual stimulus–response chain is important here. For example, in order to maintain water balance, receptors measure the viscosity (‘stickiness’) of the blood. Dilute body fluids (too much water) trigger a response leading to loss of water, while concentrated body fluids cause water retention and thirst.

corrective mechanism, e.g. lower body temperature or loss of body water

existing value

disturbance in either the internal or external environment

the normal value or set point

new value altered value reflecting the changes

feedback, e.g. inform brain of new temperature or water balance

Figure 6.4  Use of correction and feedback in maintaining a constant internal environment

Temperature control in animals Science in context Temperature control (thermoregulation) is commonly used to illustrate the principles of homeostasis to students. Students will be familiar with the terms ‘cold-blooded’ and ‘warm-blooded’, although these phrases can be misleading (the blood of a tropical fish is actually quite warm). The terms ‘poikilothermic’ (passive temperature control using a combination of adaptive behaviour in relation to external temperature) and ‘homeothermic’, or sometimes ‘homoiothermic’ (active temperature control using metabolic and physiological processes) are preferred. Alternative terms here (less technical but useful in some circumstances) include ‘ectothermic’ and ‘endothermic’. These last two terms reflect the fact that heat can be acquired externally (ectothermic) or internally, by physiological means (endothermic). 148

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6.4   Homeostasis

In humans, several mechanisms are used to raise body temperature: ➜ increase

in heat production through raising the metabolic rate of the liver and other organs; active muscle contraction (shivering) ➜ thermal insulation to maintain existing body heat by body fat and addition of extra layers of clothing ➜ reduction of active cooling: reduction in sweat production; constriction of blood vessels near the skin surface. Having explored these mechanisms, student discussion could then be used to suggest the inverse mechanisms to lower body temperature in humans. Inverse mechanisms used to lower body temperature include: ➜ decreased

heat production: lowering the metabolic rate of organs ➜ decreased thermal insulation: removal of clothing ➜ increase in active cooling mechanisms: increased sweat production; dilation of blood vessels near the skin surface; active ‘flapping’ to cool the body down. Table 6.7  Temperature control in a small mammal (homeothermic) External temperature low

External temperature high

narrowing of blood vessels near skin surface to conserve heat

widening of blood vessels near skin surface to lose excess heat

small (erector) muscles in skin cause hairs to stand on end, trapping a layer of air

hairs flatten against the skin

sweat glands produce very little sweat

sweat glands produce more sweat, which evaporates from the skin surface

general increase in metabolic rate (non-shivering heat response)

general decrease in metabolic rate

shivering of voluntary muscle (high energy cost, not efficient for long periods) behavioural responses, such as moving to a warmer spot

behavioural responses, such as moving to a cooler position

result = warming of the body

result = cooling of the body

In animals other than mammals and birds (in other words, the ectotherms, also known as poikilotherms), temperature control is effected by behavioural methods (see Table 6.8).

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6 Communication and control Table 6.8  Temperature control in a lizard (poikilothermic) External temperature low

External temperature high

aligns its body at right angles to the Sun to catch maximum rays from the Sun

aligns its body parallel to the Sun’s rays to reduce the amount of sunlight

changes to a darker skin colour to absorb more heat

changes to a lighter skin colour to reflect more heat opens its mouth (thermal gaping) to lose heat by evaporation displays burrowing behaviour to avoid high surface temperatures

KEY ACTIVITY

result = warming of the body

result = cooling of the body

Modelling heat loss using the ‘Beaker family’

The ‘Beaker family’ investigation can be used to model heat loss. Different-sized beakers (say 500 ml, 250 ml and 50 ml) filled with hot water can be used to compare drop in temperature (students need to draw cooling curves). A thermometer records drop in temperature while heat loss can be calculated mathematically knowing both the volume of water and the temperature drop: heat loss (measured in joules) = drop in temperature (°C) × volume of water (cm3) × 4.2 Students should note that the smallest beaker has the greatest drop in temperature, but the largest beaker has the greatest heat loss. Can they explain why this is? What significance does this have for large and small mammals?

Cross-disciplinary The physics of heat loss by the following methods can also be introduced to students here: l conduction l convection l radiation l evaporation.

Scientific literacy Many complex terms are fundamental to students’ understanding of the concept of homeostasis, such as stimulus, co-ordination, control, response, regulation, negative feedback, dynamic equilibrium, stable internal environment. It is useful to consider this particular ‘vocabulary set’.

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6.5   Disturbance of the body’s control systems (when things go wrong!)

6.5 Disturbance of the body’s control systems (when things go wrong!) This section continues the ‘homeostasis’ theme by looking at scenarios in animals and plants where co-ordination is interrupted or somehow disturbed. Different symptoms may appear depending upon the nature of the problem.

Use of drugs This section will look at the problems of drug misuse in a more general (homeostasis) context. Such compounds may severely affect the body’s chemistry and the body’s physiology, thereby disrupting the body’s normal internal constancy.

Cross-disciplinary Students will have some knowledge of the effects of smoking tobacco, together with an awareness of alcohol and drug abuse, from Personal, Social, Health and Economic (PSHE) studies in school. However, students gain as much knowledge on these products from friends, family and the wider media as they do from school. This is a fact we must not overlook. It is probable that their knowledge is detailed in parts but sketchy overall.

Scientific literacy The word ‘drug’ is often taken to mean ‘illegal substances’ but a technical vocabulary is necessary to enable students to use the terms correctly. l A

drug is any chemical, synthetic or natural, that alters the chemistry, physiology or behaviour of a person. l Drugs can be described as medicinal (used to treat illnesses) or recreational (such as stimulants, depressants and hallucinogens). l Drugs can be legal or illegal. Drugs provide little or no nutritional value and are taken either to benefit health (medicinal drugs) or to affect the body artificially (both legal and illegal drugs). The former (beneficial) category, sometimes referred to as medicines, include: ➜ quinine,

obtained from the cinchona tree, which prevents malaria obtained from the opium poppy, which provides pain relief ➜ digitalin, obtained from the foxglove plant, which is used in heart medication. ➜ morphine,

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The latter category includes: ➜ nicotine,

obtained from tobacco leaves, which stimulates heart rate, contributes to the build-up of fatty acids and causes physical and psychological dependence ➜ cannabis, obtained from the plant Cannabis sativa, which acts as a central nervous system depressant or hallucinogen and can result in psychological problems such as paranoia ➜ cocaine, made from the leaves of the South American coca plant, which stimulates the nervous system, rapidly raising the blood pressure; it can result in high physical and psychological dependence in a matter of days. Drug abuse is the deliberate use of a drug other than for its intended purpose. When drugs are misused (such as excessive alcohol consumption) or abused (for example use of illegal drugs) by someone, that person’s health is compromised. An individual can develop chemical dependence. Students could consider the effects of drugs: the huge medicinal benefits of prescription drugs or the cost of illicit drug use. They might also look at current trends in food and drink consumption, for instance the rise of decaffeinated drinks. (How is coffee, for example, decaffeinated?)

The introduction of disease organisms Agents of disease include microbes: bacteria, viruses and fungi. An infectious disease promotes a reaction as the organism disrupts normal body functioning. The body reacts by attempting to neutralise the effects of the pathogen (harmful microbe) by destroying the pathogen and breaking down any toxins it produces. These chemicals upset the chemical equilibrium of the infected host, causing symptoms. Common symptoms such as fever, lethargy and loss of appetite result; these can also have useful, adaptive functions, channelling the body systems to attack and overcome the pathogen and giving time for organs to recuperate and so aid recovery. There are links here with Chapter 12.

6.6 Resources General resources Simple neurobiology is perhaps best studied through images of neurons. Easily constructed reaction-time activities (reaction timers are now commonplace on many smartphones) can demonstrate the stimulus–response principle.

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6.6   Resources

Models of eye, ear and skin can be used to introduce structure, while simple activities can elucidate functioning. Plant hormones (rooting powder, etc.) may be purchased commercially and used to demonstrate growth regulators.

Websites Edexcel provide a useful website for this topic at various ages including word mats: www.twinkl.co.uk/resource/t4-sc-344-edexcel-biology-animalcoordination-control-and-homeostasis-differentiated-word-mat BBC Bitesize (www.bbc.co.uk/bitesize) includes this topic along with countless others. The ABPI schools website contains many interactive resources for schools: www.abpischools.org.uk/topic/hormones Zoo trips are useful opportunities to study animal behaviour. For instance, London Zoo indicates several activities for post-16 students: www.zsl.org/ education/animal-behaviour-study The tes website has a range of resources (including animal behaviour: www. tes.com/teaching-resource/animal-behaviour-resources-worksheetsactivities-6039022#) for use both inside and outside the classroom. Many sites (for instance: www.teachitscience.co.uk/) require a (free) membership for access. The STEM Learning Centre (www.stem.org.uk/) has developed a range of activities and (STEM) clubs in various topic areas. The Association for the Study of Animal Behaviour (www.asab.org/) has both primary and secondary (behaviour) teaching resources. The Science & Plants for Schools (SAPS) website (www.saps.org.uk) has some excellent resources on investigating plant tropisms. There are also suggested investigations, along with details of suppliers, worksheets and instructions, on plant responses to hormones. From the homepage, input the search term ‘fast plants – rapid-cycling brassica kits’. The SAPS website also includes a fascinating interview with a leading plant scientist about plant survival strategies, hormones and responses. Search ‘interviews with scientists – plant survival strategies: hormones and responses’ from the homepage.

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The SAPS newsletter from March 2013 has links to some relevant and interesting teaching resources for secondary pupils on the topic of tropisms. Search ‘newsletter March 2013’ from the homepage. The Practical Biology website (a joint project between the Nuffield Foundation, the Royal Society of Biology and CLEAPSS) has ideas and protocols for investigations into plant growth and plant growth regulators: https://pbiol.rsb. org.uk/. The site also includes a simple investigation into the growth of plant cuttings. It can be found using the search term ‘cloning a living organism’ from the homepage. All the learned societies, the Royal Society of Biology (www.rsb.org.uk/) and the Association for Science Education (www.ase.org.uk/), produce teachers’ materials and have knowledgeable education officers with whom to discuss issues. Interestingly, the Royal Society of Chemistry also explores the topic of nerves and hormones: www.rsb.org.uk/education/teaching-resources

Further reading A new (2019) series of Oxford Biology Primers (Oxford University Press) aims to look at cutting-edge biology from the point of view of the post-16 student thinking of studying biology at university. Books in the series may be of use to staff wanting up-to-date information. For example: Hinson, J. and Raven, P. (2019) Hormones. Oxford: Oxford University Press. Primary teachers often need a basic text in science in order to understand and develop ideas for their pupils. The following book by Chambers and Souter has an interesting chapter on animal behaviour (Chapter 7) with activities related to body senses: Chambers, P. and Souter, N. (2017) Explaining Primary Science. London: SAGE. A comprehensive summary of integration and control in the animal kingdom is found in: Jurd, R. D. (2004). Instant Notes: Animal Biology, 2nd edition. Oxford: Garland Science. The Association for Science Education’s publication, School Science Review, (www.ase.org.uk) contains short ‘Science Notes’ on a variety of topics. Those of relevance here include: ➜ Butler,

K. G. (2000) Demonstrating hydrotropism in the roots of mustard cress or cress seedlings. School Science Review, 82 (299), 95–96. ➜ Grant, P. (2006) A model of the ear’s central canal. School Science Review, 88 (322), 11. ➜ Klein, S. and Zion, M. (2015) The characteristics of homeostasis: a new perspective on teaching a fundamental principle in biology. School Science Review, 97 (358), 85–93. ➜ Thomason, B. (1992) Plant sensitivity, a historical source for teaching. School Science Review, 73 (264), 97–101. 154

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7 Reproduction

Mary Berry and Michael J. Reiss

Topic overview Reproduction is a crucial phase in the life cycle of any organism. It is the way in which individuals leave descendants and a species is perpetuated, given that individuals must eventually die. Most students will know that all living organisms grow and reproduce. However, they may have only limited understanding of reproductive processes, including asexual reproduction. Students’ previous experiences of practical work with plant materials, such as flower structures, can be developed and gaps in knowledge addressed. Extending students’ knowledge of different types of seed dispersal (to include wind, water and animal dispersal) is also helpful. Students frequently confuse seed dispersal with types of pollination, and they may also confuse pollination with fertilisation. Clarification can be followed up with more detailed work on the reproduction of a specific animal, such as a frog or fish. Care will be needed in planning for practical work with respect to the seasons and availability of specimens. Observation of the cycles of reproduction in plants generally requires long-term planning, though SAPS (Science & Plants for Schools) resources using rapid-cycling brassicas can make this more easily achievable. Find out if students have had the opportunity to grow or propagate plants or examine in detail a variety of living organisms. Providing students with this opportunity need not be expensive nor require a lot of space. It will be rewarding and inspiring if every student can plant a seed or take, and subsequently root, a cutting. Students should know the structure of a typical seed and appreciate how this relates to germination and the stages in a plant’s life cycle. The topic of reproduction provides abundant opportunities for bringing living things into your laboratory or classroom and providing students with handson experience. While many schools may not have a well-stocked school greenhouse or pond, windowsills or communal areas can provide access to a variety of seasonal plant species. The value of this cannot be overestimated, since this is a key opportunity to provide students with skills and understanding for life, as well as to fulfil aspects of the science curriculum. It can also engender in students a sense of success and manifest the ‘awe and wonder’ of how a single, small seed can give rise to something as impressive as an oak tree or the food we eat.

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Science in context Linking ideas of basic science to the wider issues of food security and the global environmental is desirable to aid students’ understanding. Animal sexual reproduction presents good opportunities for maintaining and observing a variety of organisms throughout their life cycle. Some species need little laboratory space and may have a life cycle that can be watched over just two weeks and independently of the seasons (such as the fruit fly). Others may demand more extensive maintenance, space and equipment, requiring observation over many weeks or months, and within specific breeding seasons. The whole topic of reproduction links directly with ‘growth and development’ in plants and in animals. Basic microscopy skills are assumed in this chapter, and students’ ideas to do with size and the concept of magnification can be reinforced and extended. It is useful for students to know how to estimate the size of the cells they are examining (see Chapter 2). It is important to include examples of plant cells and to reinforce the similarities and differences between plant and animal cells. Finally, your students need a firm understanding of the basics of nuclear and cell divisions, which lead to the formation of either genetically identical cells (as a result of mitosis) or gametes (as a result of meiosis) (see Chapter 2). They also need a grasp of the role of DNA in the control of the cell’s activities by the nucleus (see Chapters 2 and 8). Plant examples, such as growing cloves of garlic in the top of a test tube of water and using these for observing mitotic division of cells, can provide simple and effective practical activities.

Cross-disciplinary Teaching about human reproduction and other aspects of sex education can present challenges to any teacher. Schools vary in their delivery of this area, in some cases deploying a well-integrated, whole-school approach; in other cases devolving much of the area of study to the science department. It is crucial, as a teacher of biology, that you are well informed about your school’s policy for sex education and about the precise part that you, as a science teacher, are to play in this sensitive area of education. Science teachers not only need to provide accurate information to students about reproduction and sexual health, but also to have strategies ready to deal with questions, some of which may be awkward, that might arise.

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7.1   Asexual reproduction in plants and animals

7.1 Asexual reproduction in plants and animals Prior knowledge and experience Students are likely to have met aspects of reproduction in plants and animals but that will mostly have been concerned with sexual rather than asexual reproduction.

Advantages of asexual reproduction Using a variety of examples, teachers can show students that asexual reproduction has several advantages over sexual reproduction: ➜ no

energy is wasted in finding a mate offspring are produced rapidly ➜ favourable circumstances can be exploited very efficiently ➜ desirable traits are ensured in offspring. ➜ many

Examples of asexual reproduction A variety of organisms can be used to show that there is only one parent in asexual reproduction. In each case some part of this parent divides to produce an identical individual, which then separates from the parent. A circus of activities (suggestions are largely seasonal) can provide an overview of the range of mechanisms for asexual reproduction in plants and animals: ➜ Estimation

of the number of plantlets associated with a spider plant or Bryophyllum: ask students to suggest reasons for the production of so many. Refer to the common name of Bryophyllum (mother of thousands) and ask students why this might be relevant. ➜ Illustration of reproduction (fission) in a bacterium or a unicellular organism, such as Amoeba proteus: use a sequence from a video. ➜ Bread-mould cultures: observe the development of colonies of mould (a fungus) and the tiny black dots (sporangia) containing the spores. Ask students to suggest how these may be dispersed. Some fungal spores can trigger asthmatic and other allergic responses, so keep cultures in closed plastic bags.

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Technology use The use of a visualiser would be helpful for mould cultures. This can limit potential hazards associated with allergic responses, while allowing for whole class participation and engagement.

Mitosis Build on your students’ basic knowledge of mitosis. Explore with them the role of this division: it allows cells to reproduce themselves to form genetically identical ‘daughter’ cells. You can extend this to make links with the role of mitosis in allowing multicellular organisms to grow and to repair themselves. Mitosis should eventually be understood as the basis of asexual reproduction (see Chapter 2). Refer students to practical work on plants; this helps to link theory securely to context.

Cloning and tissue culture Asexual reproduction results in genetically identical individuals and means that a clone (for example, a clump of daffodils) results from a single organism. You can consider examples from horticulture and agriculture (including strawberries and pineapples) where the advantage is that particular genetically based characteristics of the crop plant can be maintained from one generation to the next. Consider also how a change in an aspect of the environment, such as a prolonged drought, can be a disadvantage to a species that is reliant on asexual reproduction. Notice how a range of wild plants (such as grasses) use both methods of reproduction, so that the disadvantages of cloning become less significant (see Chapter 9). Emphasise the fact that the term ‘clone’ applies to individuals produced naturally by asexual reproduction as well as those produced by artificial cloning. This is something students often do not appreciate.

Careers Tissue culture is a way of conducting asexual reproduction on a massive scale; the process is now a routine laboratory and commercial procedure, and examples can help students to appreciate the extent to which this is part of everyday life. This is an ideal opportunity to encourage students to discuss cloning in a balanced way in order to highlight and clarify some of the advantages to society of these techniques. Cloning animals is not as easy as cloning plants. The activity of cloning cauliflowers can be used here to show the advantages of cloning in context. If this is not available to do as a 158

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demonstration or class practical, use the SAPS video clip showing the technique (details at the end of the chapter). There are teaching notes, student worksheets and illustrated worksheets.

Science in context Few young people have a good understanding of what constitutes cancerous growth. You can provide help by making the link with asexual reproduction in terms of uncontrolled cell division (mitosis). A video sequence can demonstrate the irregular mass of cells (a tumour) that may have come about as a result of mutation in the genes that control cell division (see Chapter 8). Students need to understand that plants also show periods of unchecked cell divisions with the development of a tumour. Understanding this should help to reinforce the fact that plants have many of the same processes as animals. Treat the topic with some sensitivity since a student may have/have had a relative or friend who has suffered from, or is being treated for, cancer. It is unlikely that everyone in a class will have been unaffected, and shared experiences can also be an opportunity to highlight students who need additional support, but never pressurise students to speak of their personal experiences.

Science in context Useful secondary data can raise older students’ awareness of the ways in which scientists begin to correlate the incidence of a disease with a particular factor. This is also an opportunity to talk about different carcinogens (ionising radiation, certain chemicals) and some viruses, all of which are linked with the promotion of cancerous growths in both plants and animals.

Further activities To explore asexual reproduction in a practical context, record numbers of duckweed in a particular area of a pond over several days in spring to demonstrate the rate of increase. This can be done on a micro-scale in the laboratory, using an old ice cream container and samples of duckweed.

Maths The duckweed activity provides a good opportunity to focus on data analysis and representation with the use of graphs and tables.

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Investigating horticultural methods of plant propagation You could try some common horticultural methods with students. l Late

spring: stem cuttings (geranium, Coleus); leaf cuttings such as Begonia rex or African violets; layering (‘pinks’). l Early summer: rooting the ‘buds’ of runners (strawberry), or rooting plantlets from spider plants (Chlorophytum comosum) or mother of thousands (Bryophyllum daigremontianum). More advanced students could research tissue culture procedures, finding out how a callus is formed and how tiny plants are grown from subdivisions of this. Introduce this activity with a video sequence together with literature from commercial plant breeders (such as producers of hybrid orchids). The video clip from the TV series Botany: A Blooming History (SAPS secondary teaching resources: Genetic engineering to increase productivity in rice) could be used as it discusses the importance of genetic engineering for feeding a growing population.

Science in context Encourage students to make links between plant reproduction and the increasingly important issue of global food security. Further cultural context can be gained from considering how supermarkets can provide herb plants all year round.

Enhancement ideas Examine yeast cells for signs of budding. Under the microscope, students can watch cells reach a certain size and produce outgrowths (buds), which eventually split to form new individuals. The use of a graticule on the eyepiece lens of the microscope allows students to judge the size of the cells. A digital microscope linked to a data projector/screen or a visualiser could also allow whole class observation of yeast division. A deflated balloon can be attached to a vessel containing a mixture of yeast, sugar and warm water. Students are always entertained when the balloon is inflated by the gas produced (carbon dioxide), usually within an hour lesson. Time-lapse photography could be used to show the production of the gas over time.

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Maths A mathematical link could then be to provide students with data on volume of gas production over time, or students could plot data they have collected. Get students to think how the rate of production of gas is related to the number of yeast cells.

Working scientifically Extend the work for investigative activities in which advanced students might explore the effects of culture temperature, or the amount of sugar, on the rate of cell division in yeast. Withdraw a drop of culture from the starter flask every 15 minutes and count the number of cells in the field of view. Plot a line graph to illustrate the increase in numbers of cells over time or pool class data entered on a spreadsheet and obtain mean values and plots of rates of the increases in numbers. Using Figure 7.1, ask students to explain what might be happening at X, Y and Z. Explore possible reasons for why yeast eventually stops budding.

Numbers

Z Y

X Time

Figure 7.1  The growth curve for yeast

7.2 Characteristics of sexual reproduction in plants and animals Prior knowledge and experience Students will have met the idea of life cycles and will know the main stages of the human life cycle: humans produce babies who grow into children and then into adults. Similarly, they will have been introduced to the main stages of the flowering plant life cycle, including growth, pollination, seed dispersal and the germination of seeds to form new plants. However, students are frequently unable to relate the life cycles of humans and plants. Often, students are unaware of the structure of a seed and are unclear as to how and why seeds germinate. Similarly, students can often label parts of a plant and describe the role of 161

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pollinators in pollination but are not clear about the mechanism of fertilisation in plants, and how this leads to production of seeds. Students may have a broad grasp of the fact that seeds disperse, but not the detail of mechanisms. Introduce students to the intriguing and sometimes bizarre ways that plants ensure their seeds are placed in the most advantageous position for subsequent germination.

A teaching sequence The sequence should try to explore the question, ‘Why sexual reproduction, given the various advantages to asexual reproduction?’. The fundamental reason is that sexual reproduction gives rise to genetically based variation among the offspring of the individual that is reproducing. This makes it more likely, especially in a changing environment, that at least some offspring will survive and, in turn, reproduce themselves. By considering and comparing different reproductive patterns in a range of animal species, students can be helped to forge links with the human reproductive pattern (Section 7.3).

Examples of plant sexual reproduction Use every opportunity you have to check students’ understanding of plant reproduction and different plant life cycles, building on teaching sequences from earlier stages. The idea that many plants can reproduce both asexually and sexually should be reinforced, using examples from across the plant kingdom as well as flowering plants, including grasses.

Science in context This is a good opportunity to reinforce with students the role of grasses as food crops and make links to food security and global climate changes. Many flowering plants have interesting mechanisms for the production of specialised gametes; the male gametes are the pollen cells, found in the anthers; the female gametes are the ovules, found in the ovary. Meiosis is the special cell division that produces these gametes (see Chapter 2). Students could observe pollen under the light microscope and deduce whether the pollen is from a wind-pollinated or an insect-pollinated species. Students can be introduced to the idea that many (but not all) flowering plants are hermaphrodite, with the capacity to produce both male and female gametes. Try also to present examples of single-sex plants, such as the holly (Ilex) with its separate male and female plants. Students could compare these with other species of plants (for example birch, Betula) which are hermaphrodite. Students could then be asked 162

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to suggest mechanisms which prevent self-fertilisation in hermaphrodite species and explore the advantages of out-breeding.

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There are a number of films and practical protocols on the SAPS website to study pollen, its role in reproduction and its link with hay fever in more detail.

Germinating seeds Soaked white icicle radish or broad bean seeds provide younger students with independent, observational homework (over 4–6 weeks). Each student needs a jam jar and a roll of thick, absorbent paper, ensuring the seed is clamped to the side of the jar by the paper. The paper sits in 2 cm of water and acts as a wick. Ask students to keep diaries, supported by drawings, for the whole life cycle. For older students, this could be set up as a demonstration and shown each lesson with students making their own notes to add to ones from the previous lesson. Photographs could be taken and put on Google Classroom or similar sharing platforms. Students need to be able to label the main parts of a seed and appreciate its subsequent growth. Reinforce here the emergence of the radicle first and then the plumule.

Science in context Students might consider how fruits are formed without seeds (such as seedless grapes and satsumas). (These are generated by spraying with hormones that stimulate fruit production without prior fertilisation.) They can deduce that the normally prerequisite stage of fertilisation has been bypassed and the resulting fruit, while popular with consumers, is of no use in producing another generation. Introduce students to a range of seeds, fruits and vegetables. Many students do not realise how many of their foods come from plants, and this is an opportunity to make this clear. Show students multigrain bread, rolls with poppy seeds, and mustard with wholegrain mustard seeds. Help students to appreciate that chickpeas can be ground to make flour and hummus. Have a range of dried beans to show students along with various cans of beans, including kidney beans, baked beans and mixed beans. Get students to appreciate the plants and plant parts that are used for popcorn, sweetcorn and tortillas. Allow students to explore seeds and introduce some seeds more familiar to students from different cultural backgrounds, such as coriander seeds, cardamom seeds, black cardamom seeds, chilli seeds, onion seeds, chia seeds, seeds in various types of gourd, fenugreek seeds, chickpeas, nigella seeds, pomegranate seeds and dill seeds. 163

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With more advanced students, illustrate the starch food store in seeds and demonstrate the role of the (starch-digesting) enzyme amylase, using halved, soaked barley seeds placed with the outside in contact with starch agar in a Petri dish. This can also be done with broad beans, halved and placed surface downwards on the surface of the starch agar. After 24–48 hours at room temperature, flood the dish with iodine solution. The zone around each grain remains clear while the iodine on the rest of the starch agar is blackened. Ask students to account for the lack of starch in the clear zone. Eye protection is needed when handling iodine solution.

Working scientifically How do particular seeds fall? Ask students to use dandelion or sycamore seeds (technically, fruits) in order to examine the relationship between height above ground and rate of falling. Consider how this can be achieved safely with careful supervision. You can also ask students which variables (mass, shape, etc.) might affect the rate of fall of seeds. Data can be entered into a spreadsheet for further analysis. Model seeds, made from paper/card and ‘weighted’ with paperclips, can be made to test ideas further. Ask them why a slower rate may be an advantage to the plant producing the seeds. The video clip from ‘Earth Unplugged’ of exploding cucumbers (found at the end of the chapter) is an excellent clip to show seed dispersal by explosion.

Examples of animal sexual reproduction The idea that most animals reproduce sexually as a result of the production of specialised gametes needs reinforcing; the male gametes are spermatozoa (singular: spermatozoon), abbreviated to sperm; the female gametes are ova (singular: ovum). Meiosis is the cell division that produces the gametes (see Chapter 2). Students can be supported in discussing the reasons for sexual reproduction, particularly mammalian reproduction, which is technically more complex than asexual reproduction. Encourage them to deduce that it requires more energy and specialised organs that produce gametes, and also results in fewer offspring in a population. The crucial advantage is that individuals are slightly different from either parent and from other offspring of the same parents. In a changing environment some of these individuals may be better adapted and able to exploit new resources or colonise new areas. An organism’s ‘fitness’ for a particular environment is a useful concept since it links environmental features and the organism’s individual characteristics (see Chapter 9).

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7.2   Characteristics of sexual reproduction in plants and animals

Specialised sex cells, courtship and fertilisation Students should appreciate that most species can reproduce sexually, even those that also reproduce asexually. Examples might include species on a coral reef, including invertebrates such as sea anemones, corals and jellyfish. Show students some filmed sequences from Malice in Wonderland (1994), available from BBC Active Video. Such sequences present visually the specialised sex cells (gametes), one from each parent of the same species, which join to form the first cell (zygote) of the new individual. Examples should also show that there are some species where individuals are able to produce both male and female gametes; these are described as hermaphrodites (including earthworms and many molluscs, such as snails and slugs). By providing stimulating materials and sources of information including texts, images (both photographs and diagrams), data and video clips, your students can be encouraged to study the life cycles of certain animals in more detail (such as fish or frogs): ➜ Discuss

how eggs are fertilised externally/internally, pointing out the numbers of eggs that are fertilised at any one time. Students can be helped to think why species with internal fertilisation typically produce fewer offspring than do species with external fertilisation.

Enhancement ideas Ask students to identify any pattern across different species in the relationship between the number of eggs, whether fertilisation is internal or external, whether the period of post-fertilisation internal development is brief or extended, the extent of parental care once offspring are no longer inside the mother, and the chances of offspring surviving to maturity. Video sequences can helpfully illustrate courtship patterns, as well as the act of copulation and fertilisation, in a variety of animals, such as amphibians, fish and birds. BBC Trials of Life (1990) has two episodes, Courting and Continuing the Line, which illustrate a variety of breeding activity (BBC Active Video) and there are plenty of more recent examples that can easily be accessed online.

External versus internal development; aftercare Discuss the advantages of retaining the young offspring in the body of a parent; this is usually the female but note, for example, the use of the male pouch in seahorses (students can search for a video of this on YouTube).

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Science in context You can also draw on students’ own experiences of a variety of newborn pets if there are any. Students can discuss the extent to which humans and other mammals provide for emotional as well as physical needs of their offspring.

Further activities The advantages of sexual reproduction can also be seen as the disadvantages of asexual reproduction, where only a stable environment will ensure reproductive success. Students can compare and then summarise in a table the advantages and disadvantages of both types of reproduction.

7.3 Human reproduction and sex education Legal framework for sex education Countries, both within the UK and beyond, vary greatly in the role that they expect schools to play in sex education. In England a helpful summary of the legal framework is provided by a House of Commons Briefing Paper (Long, 2019). A DfE document (DfE, 2019) provides the statutory guidance. The key points for secondary schools in England are that from September 2020: ➜ All

secondary schools (whether state or independent) need to teach ageappropriate ‘relationships and sex education’. ➜ Schools must ensure that they comply with the relevant provisions of the Equality Act 2010, under which sexual orientation and gender reassignment are among the protected characteristics. ➜ Schools should realise that students may need support to recognise when relationships (including family relationships) are unhealthy or abusive (including the unacceptability of neglect, emotional, sexual and physical abuse, and violence, including honour-based violence and forced marriage). ➜ Parents still have the right to withdraw their children from sex education outside of National Curriculum science but all children, even if their parents disagree, are entitled to receive one term of sex education in the three terms before they reach the age of 16. As a science teacher, you are likely to be involved in teaching aspects of sex education whether or not you are a biology teacher, though the extent to which this is the case varies from school to school and between countries. You must be aware of your school’s policy on sex and relationships education and you must stick to it! Do not invite in outside speakers or take students out on visits unless you have cleared this with a senior colleague, such as the appropriate Head of Year. 166

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Cross-disciplinary In some schools, anything to do with ‘relationships’ is dealt with outside of biology lessons, often in a programme to do with Personal, Social, Health and Economic (PSHE) education issues. Your own classroom relationships with students are important for being effective in this sensitive area, and you should not be deterred from encouraging, wherever possible, open and frank discussion about issues, though never about particular personal circumstances. Where a student might approach you to share some personal information, you should use your professional judgement about keeping confidentiality, while at the same time never promising confidentiality. You can always provide details of alternative (non-school) sources of advice and of treatment and your school may have a school nurse who can be a tremendous asset. Developing your own sensitivities to the personal circumstances of individual students and their families is important in your class work, and teaching this topic in particular may expose you to potential signs of child abuse. Identification is often not easy and, in addition to more obvious physical signs such as bruises, burns, bites and scars, a general indicator is often neglect. Indicators of sexual abuse include sexually transmitted infections, recurrent urinary infections, inappropriately sexually explicit behaviour, young students with too much sexual knowledge, sexually abusive behaviour towards other children and pregnancy. Emotional abuse is often indicated by low self-esteem, lethargy or attention-seeking behaviour, and delayed social development. As a teacher, always take steps to share any concerns with the designated member of the school staff – the School Child Protection Officer (SCPO) – writing down dated, factual details as well as reporting them orally.

Prior knowledge and experience Students will almost certainly have been taught something about aspects of human reproduction and relationships, at primary school and/or in their families, but research shows such teaching is often patchy. In addition, at this age, students’ physical and emotional developments are highly variable and this partly determines the extent to which they will have absorbed or questioned information given earlier on. There will be differences in the provision from different homes, social and cultural backgrounds, different primary schools and different teachers within any school. National surveys to find out what young people really want in this area generally find that teaching is often provided too late. Targeting younger

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secondary students is, therefore, important. Boys generally have lower levels of knowledge of reproduction, contraception and contraceptive services than do girls, and so a school needs to take steps to ensure that this is rectified. Girls often say that they want more discussion and explanations in order to counterbalance an overemphasis on biological facts. Clearly boys require this too. Educational videos and other material in this area of the curriculum should be chosen with care for their appropriateness and always be used in such a way that there is time for reflection, clarification and discussion of sexual issues in the classroom. Many schools have a system where parents are able to come in and see materials that are used for teaching sex education and meet staff who will be teaching it.

A teaching sequence As a science teacher you can do a great deal of good when teaching sex education and you have the advantage when teaching in this area that your students are not likely to forget most of what you teach them! Many teachers initially find it embarrassing to teach sex education. However, this generally eases over time and science teachers can communicate a lot of extremely valuable information even if not everyone, in the first year or two of teaching, feels comfortable at handling discussions in this area. For younger secondary students, you are aiming to extend knowledge about human reproduction and to relate the ways the body changes in adolescence to your students’ developing understanding of human reproduction, growth and the menstrual cycle. Use terminology with care. In mammals, including humans, the fertilised ovum is called a zygote and it develops to form an embryo and subsequently the fetus. Students should appreciate the size and approximate number of gametes produced by males and females, and older students should identify the similarities and differences between the structure of an ovum and a sperm. Try to assess and then extend your students’ present knowledge of puberty, anatomy, conception and its prevention, the development of relationships and the medical and other problems associated with sexual involvement early in life.

Scientific literacy Bear in mind that some students may have the technical vocabulary but their actual understanding can be poor. They may need help in knowing where various anatomical parts are located as well as what the parts do.

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Aim to make your audience feel comfortable about not knowing things, while at the same time making it possible for them to find out. Laughter and appropriate joking can help to make everyone feel less awkward and more comfortable while at the same time enabling you to address every question seriously and with respect.

Physical and emotional changes at puberty and during adolescence Between the ages of 10 and 14, most young people will be entering puberty and will be interested in hormones, how they will be affected by them, the menstrual cycle, wet dreams, erections, fertility, pregnancy (including how it can be avoided) and safer sex. They may also be wondering if their physical development is ‘normal’. They may want to know about the difference between sexual attraction and love and whether it is normal to be attracted or in love with someone of the same sex. They are likely to be asking questions about relationships, when is the right time to have sex and where they can get more information if they need it, including the best websites, confidential services, etc. Ascertain what materials your school uses for teaching in this area and if you want to use different ones, check with your Head of Department first. An exploration with your students of how males and females differ physically should enable you to summarise the key changes at puberty. A useful icebreaking activity might be to place each key point describing a secondary sexual characteristic (such as breasts, wider hips, facial and body hair, voice changes, stronger body smell) on a separate card; small groups of students can then discuss and arrange the cards under one of the two headings ‘males’ and ‘females’. Stress the wide variation in the age of onset of puberty and the generally earlier age of onset for girls and the fact that puberty does not happen all at once but takes place over a number of years. Help students understand that changes in hormone concentrations result in the development of secondary sexual characteristics and emotional changes at puberty. Explore possible reasons for the earlier onset over the last 50 years (better diet, fewer infectious diseases).

Male and female reproductive systems Many younger students tend to have muddled ideas about how many urinary, genital and defaecatory orifices people have (females have three, males two) and often both boys and girls do not know from where a girl urinates.

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Students should understand that the urethra has a dual function in the male. In the female the urethra has only one function, which is in connection with the urinary system.

Scientific literacy The similarity in sound and spelling of ‘ureter’ and ‘urethra’ can also be confusing for many students. Older students need to know the structure and function of the parts of the urinogenital (= genitourinary) system; younger students can be presented with a reduced list of key terms or simplified terminology. It is generally best to provide accurate, but unlabelled, line diagrams that students can then label themselves. This activity can be extended into a card-sorting/matching exercise in which students are provided with key names on one set of cards and a second set of cards with the key functions of the parts. Help students to understand that the erectile tissue of the penis becomes firm as it fills with blood when the penis is stimulated either manually or indirectly through specific visual or other stimuli. Some male students may need reassurance about the normality of wet dreams and masturbation. The dual role of the urethra in the male will need some clarification: glands at the base of the bladder produce secretions that wash away the urine in the urethra (urine can deactivate sperm). Masturbation for female students also needs mentioning, particularly as it is sometimes omitted from school textbooks. Indeed, the clitoris is sometimes rendered invisible by being absent from diagrams as well as absent in any discussion about structure and function (see Cohut (2018)). Muscle rings in the sperm duct squeeze the sperm along the passage. This action can be simulated by pushing toothpaste along in its tube, which is similar to peristalsis in the intestinal tract. Further glands mix nutrient secretions with the sperm to form semen. Ask students why this is necessary. Illustrate the volume of the ejaculate: about one teaspoon of semen is produced at ejaculation. The prostate gland is often incorrectly referred to as ‘prostrate’! It is frequently enlarged in older men and students can deduce the effect of any enlargement on the frequency of and difficulty in urination. Since the female reproductive organs are largely invisible, and therefore particularly mysterious, ask students (male and female) to site the position of the ovaries by placing their fingers on their own abdomen. To do this, suggest they feel for the front points of the pelvis and move in towards the navel an

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inch or so. Ask students to feel the tip of their nose with a forefinger. Say that this feels rather like the cervix, the ring of muscle that closes the lower end of the uterus where it joins the vagina. Use a model of the human torso, and a skeleton, to help pupils understand the 3D arrangements of the key reproductive organs, since diagrams are presented as 2D arrangements, which can be confusing. There is a very small hole in the cervix to permit sperm to enter. The cervix dilates during labour; check that students understand why. Students can estimate the size of the adult vagina (which they often conflate with the vulva) and the size of the adult, non-pregnant uterus. For comparison use a mediumsized inverted pear (about 10 cm long) and tilt it backwards slightly to illustrate the angle of the uterus with respect to the vagina. Point out the need for a good blood supply to the uterus and explore why this is so. Ask students to suggest how long the egg takes to be moved from the ovary to the uterus (24–48 hours). The vagina is a muscular tube with sensitive nerve endings and glands that can secrete mucus. Explore with your students the reasons for these. Sperm swim towards the oviducts aided by movements of the female reproductive system. After an hour they no longer swim but they can survive in the uterus or oviducts for three or even more days. Explore with your students why it is important to be aware of this and how the timing of ovulation can influence the chance of fertilisation.

Scientific literacy One way to address questions from students is to provide them with a box in which they can place their questions at the end of a lesson. Read the questions away from students and deal with as many as possible the next time you teach the students. This can become an extended writing activity – an agony column in a teenage magazine – where you provide stimulus questions/issues that require answering. Students then write a response to provide factual and supportive answers. The extent to which you share this writing across the class should be considered carefully. Use video sequences and pictures to illustrate and discuss the processes which lead to fertilisation and implantation of the zygote. Students can devise flow diagrams (electronically or on paper/cardboard) to represent the key events leading to implantation. The emotional as well as the physical aspects of sexual relationships should be addressed through discussion, though only if the school sex education policy allows for this.

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Textbooks and other teaching materials sometimes assume intercourse is heterosexual and that sexual activity is penetrative (vaginal) intercourse. Such texts can also be sexist since intercourse is sometimes presented only from a male’s point of view. Ejaculation by the male is frequently mentioned; orgasm in the female less often. Accounts of loving relationships, or indeed the passions or pleasures associated with sex, are all too often notable by their absence. It is important to create a balance between an anatomical account and a psychological and emotional account. Again, you must act in accordance with your school’s policy. Finally, you can take the opportunity to review fertilisation in terms of fusion of male and female nuclei and discuss how this results in characteristics being passed from parents to offspring (see Chapter 8). Twin and multiple pregnancies are always of interest to young people: provide some statements containing some correct and incorrect explanations and ask them to select the correct explanations of how identical and non-identical twins can arise. Factual examples of conjoined twins and how they arise are often of interest. There are always examples to share with students. This is also an opportunity to establish that, generally, humans have one offspring at a time, and that the reproductive system and post-natal care, including breastfeeding, have evolved to make sure that the one offspring is as likely as possible to survive.

Enhancement ideas Encourage students to speculate about different causes of infertility in men and women, linking their ideas to the structures and functions of the reproductive systems. Causes can include low sperm counts, blocked oviducts and infrequent ovulation. This could be extended to include information on chlamydia, cervical cancer and testicular cancer. Students could be encouraged to find out more about the technological treatments now available, and some of the social and ethical issues surrounding the practices of fertility clinics.

Menstruation and the control of fertility Menstruation tends to remain a taboo subject in our society, which is unhelpful to an adolescent girl for whom it is acutely realistic. The physical, emotional and practical aspects of ‘periods’, particularly in the school setting, do very little to reassure girls of the positive experiences of becoming a woman. When young people are asked ‘What is menstruation?’, research has shown that over one-third of 13- to 14-year-old students do not mention menstrual fluid and, when questioned specifically, the actual source of the menstrual blood as the shedding of the uterine lining is frequently misunderstood. Try to aim for a balance between a purely physiological approach and a more personal account that goes some way to acknowledging the reality of this event for half the school population! 172

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Many schools have the ‘Red Box Project’ to provide free menstrual products within school. This is particularly important in ensuring that girls do not miss out on education because they cannot afford sanitary protection.

Science in context This is also an opportunity for students to think about how girls and women in other parts of the world deal with their periods. This can make cultural links more explicit.

KEY ACTIVITY

The teaching and associated discussion of the natural and artificial methods of preventing pregnancy (in other words, birth control or ‘family planning’) frequently fall to the science teacher. Much of what follows is for older students (14 years plus), so be guided by your school sex education policy and be mindful that the use of artificial contraception is not acceptable to all adults or young people. The health risks, both physical and emotional, of under-age sex should always be discussed. Students need both general and local guidance about how to seek information and advice (such as ‘drop-in sessions’ with a school nurse or at a youth clinic) and how to buy items or to access the particular medical services that supply them (the diaphragm, intrauterine device, contraceptive pill and the ‘morning after’ pill, usually referred to as ‘emergency contraception’). In relation to fertility, you may wish to point out that an unfertilised egg will not survive for more than three days, although sperm can remain alive for a day or two longer. Challenge any idea that fertilisation cannot take place if male ejaculation takes place outside of the vagina, and explain why.

Thinking about contraceptives Ask older students in groups of three to five to make a PowerPoint presentation advertising a particular contraceptive. They should consider how the contraceptive works, for whom it would be most useful, and present the chief risks. Alternatively, using desk top publishing, small groups of students might produce a leaflet concerned with one type of contraceptive.

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Science in context The control of fertility raises a number of points of social and ethical concern. The following technologies provide an opportunity for your students to think about the links between the science and society: l Artificial

insemination (AI) of a woman with her male partner’s sperm can allow an infertile couple to have a child without a third party being involved. If the male cannot produce fertile sperm, sperm from a donor may be used (AID). You can explore with students how this might be similar to, or different from, adoption of a child by the couple. Ask them to make a list of issues that might be of concern with AID. l In vitro fertilisation (IVF) (‘test-tube babies’) provides a particularly useful discussion topic. It is used to treat women whose ovaries are functioning but whose oviducts are blocked, or where sperm motility of the partner is poor. The woman is treated hormonally to super-ovulate. Ask students to consider advantages and possible disadvantages of IVF. l IVF for same-sex couples or for single women wishing to have a baby could be discussed.

Sexually transmitted infections A detailed study of sexually transmitted infections (STIs) may best be done within the topic of microbiology (see Chapter 12). The modes of transmission should be considered for the six commonest STIs: chlamydia (bacterial), genital warts (viral), gonorrhoea (bacterial), genital herpes (viral), hepatitis B (viral) and human immunodeficiency virus (HIV; viral). Other STIs that might be considered include syphilis (bacterial), pubic lice (insect), urethritis (bacterial), thrush (fungal), bacterial vaginosis and trichomoniasis (protozoon). Students should learn how STIs are passed on to another person and how they are best avoided. Offer the reassurance that, with early detection, almost all can be treated successfully. Aim to dispel any misunderstandings. Find out what your students do, or do not, know about STIs; challenge prejudices about those who may catch an STI. Students need to know that nowadays people infected with HIV generally look as healthy as anyone else. Human papilloma virus (HPV), sometimes called ‘genital wart virus’, can be passed from one person to another through sexual contact. Younger students (11 years plus) need to be prepared for the HPV vaccination programme, which is offered to all students in the UK, boys and girls. The programme is cutting drastically the rates of cervical cancer. Since the routes of transmission of STIs involve sexual contact, an important teaching objective is to raise students’ awareness about the need for safer sex. They should be informed about the risks of transmission through the mixing of 174

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7.4   Human pregnancy

body fluids, which include semen, vaginal secretions and blood. This may best be addressed when teaching about contraception so students can weigh up the ‘pros’ and ‘cons’ of the various contraceptives in terms of effectiveness in preventing transmission of STIs. Schools may decide to invite health professionals into the classroom to support the teaching in this area: SRE Advisors (local authority), Teenage Pregnancy Co-ordinators/Managers (NHS) or other personnel from GUM clinics can often provide expert help. There are good video sequences, too, though it is important to check that these are up to date and to plan carefully so students always have the opportunity to talk about the topics and issues that arise.

7.4 Human pregnancy Prior knowledge and experience Students are likely to have met some of the processes of fetal development; some understanding of the role of the parents and childcare will also be present.

A teaching sequence Your aim is to extend students’ ideas about how offspring are protected and nurtured following the fertilisation of the egg through pregnancy and up to birth itself. Students should learn that the fetus develops within a membranous bag, supported and cushioned by amniotic fluid. They should develop their understanding of the placenta: this supplies nutrients and oxygen to the fetus via the umbilical cord, and removes carbon dioxide and other waste products. Students will need help in making the links with other important biology topics such as the circulatory system, so they can appreciate the route taken by nutrients from the mother’s digestive system to the fetal brain and other tissues. They will need to link their broader knowledge of diffusion gradients to explain how oxygen, water and digested food pass from the mother’s blood to the fetal blood in the placenta, and, in the reverse direction, how carbon dioxide and other waste materials leave the fetal blood and enter the maternal blood. The frequent reality of miscarriage might be discussed. Ask the students for which body organs the placenta acts as a substitute. Stress that fetal and maternal blood supplies are very close but completely separate. Students need help to appreciate that harmful substances can cross the placenta to the fetus and affect development. 175

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The processes of birth can be summarised by students, using photographs and diagrams as illustration and alongside a flow chart that shows the main stages of labour. Describe how the newborn baby obtains nutrients and oxygen vital for survival and growth. The many excellent videos and photographs that are available should always be used with care, to avoid embarrassing or distressing those students who may find that some sequences make them feel squeamish or faint. Always be sensitive, where pregnancy and birth are concerned, to their impact on young people and their families (for example, adoption, miscarriage or neonatal death). Students should also be helped to understand the importance of breast milk in providing antibodies to protect against infection from common microorganisms. Be aware that some cultures do not encourage breastfeeding. Students could find out more about newborn reflexes, such as head turning when a newborn’s cheek is touched.

Further activities The whole topic of pregnancy, birth and neonatal care provides good opportunities for students to get to know more about the medical, maternity and welfare services provided by GPs and other parts of the NHS, as well as parenting issues.

Careers A school nurse, local midwife or health visitor may be helpful in providing your class with accurate information and detail about Caesarean sections, induction of birth, breech births, modern monitoring techniques, good-quality childcare and so on. In advance of a session with a visitor, let students draw up a list of possible questions they might ask. You can help students prepare for talking with professionals about healthrelated aspects with a range of suggestions. (Why are pregnant women offered additional iron supplements? Why can obesity be a health problem in pregnancy? Do fathers have to watch the birth? What is a good role for a new father?) Students can find out more about the composition of breast milk and the value of the colostrum that is produced immediately after birth as the first feed, and appreciate why some women use bottles for feeding.

Social and ethical issues Abortion is a particularly sensitive issue that needs careful handling, and with due consideration for some religious and cultural groups. Again, consultation of your school’s policy is essential here, as is discussion with relevant staff. You should endeavour to remain neutral at all times 176

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7.5   Equipment notes

when presenting the debate and allow your students to try to work out for themselves the position they may wish to adopt. Be particularly cautious about how you might use the materials and resources produced by some of the key pressure groups. Animal clones can be introduced as genetically identical individuals formed by taking a body cell, such as a skin cell, removing its nucleus and inserting this into an egg cell that has had its nucleus removed. The zygote that is formed grows into a ball of cells, as was the case in Dolly the sheep – the first mammal to be cloned from an adult cell. The ball of cells is planted into the prepared ‘pregnant uterus’ of the recipient. Ask students to summarise the advantages and potential issues of such processes in animal breeding.

Science in context Explain to students that the cloning of livestock embryos is an important technique in animal breeding. There have been claims that various research groups have cloned humans but none of these have been substantiated and accepted by the scientific community. From a technical perspective, cloning humans and other primates is more difficult than cloning other mammals. One reason is that the spindle proteins, which pull the chromosomes to opposite ends of the cell during cell division, are located very close to the chromosomes in primate eggs. Consequently, removal of the egg’s nucleus to make room for the donor nucleus can unintentionally remove the spindle proteins, halting the cell division process. Clearly, human cloning would raise huge ethical issues and students may enjoy discussing these.

7.5 Equipment notes Asexual reproduction in plants and animals Bread mould cultures require moistened pieces of stale bread in dishes exposed to the air for 24 hours. Keep the bread slightly moist to prevent it drying out, and then keep in closed plastic bags. Ensure that students do not open the bags.

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A cell suspension of actively growing yeast cells can be made at the bottom of a small flask, using 8 g fresh baker’s yeast and 10 g glucose (or cane sugar), made up to 200 cm3 with distilled water. Plug with cotton wool and leave in a warm room (22 °C) for 20 minutes before use with the class. Always pretest culture conditions before the lesson to ensure that cell division is taking place. In a small pipette, transfer one drop of the culture to a microscope slide with a small amount of methylene blue stain and cover with a coverslip. Use methylene blue for staining living cells as follows: 1 g methylene blue; 0.6 g sodium chloride; 100 cm3 distilled water. Methylene blue is harmful.

Sexual reproduction in plants A specially selected, mutant, rapid-cycling brassica provides a versatile and easily maintained resource to develop students’ understanding of the life cycle of a flowering plant in 4–6 weeks, when grown under a specially constructed artificial light bank in the school laboratory. Its versatility lies in the opportunity to study its life cycle at any time of the year; it can be ‘grown to order’ to obtain germinating seedlings, pre-flowering or flowering plants for a particular teaching day. There are numerous investigative ideas available for teachers as part of extensive resource packs. Contact Science & Plants for Schools (SAPS). Starch agar plates are made up as follows: make a starch suspension with 10 g starch and 1 dm3 distilled water. To do this, mix a little of the starch with cold water, bring the rest of the water to the boil and add the starch mixture to the boiling water. To make up the iodine solution use 3 g iodine crystals and 6 g potassium iodide. Dissolve the potassium iodide in 200 cm3 distilled water, add the iodine crystals and make up to 1 dm3 with distilled water. Make up 24 hours before it is to be used to allow the iodine to dissolve fully. Iodine crystals are harmful; use gloves and eye protection when handling.

Sexual reproduction in animals Using a scale of 1 : 200, students can make scaled, two-dimensional models of an egg and a sperm. To do this for the egg, draw a circle with a diameter of 20 cm. For the sperm draw a head of diameter 0.8 cm, and overall length including head of 10 cm. The actual dimensions are indicated on Figure 7.3.

diameter 0.1 mm

membrane jelly coat cytoplasm nucleus containing genetic material

the head of the sperm has a diameter of 0.004 mm while the overall length is 0.05 mm

Figure 7.3  Relative sizes of the human egg and sperm

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7.6   Resources

7.6 Resources Local organisations AIDS support services Department of genitourinary medicine (GUM) Family planning clinic and/or young person’s clinic Health promotion centre Lesbian and gay communities resource centre Rape crisis centre

National organisations AVERT: information and education on HIV and AIDS: www.avert.org/ Brook: advice and information for under 25s within the UK on STIs/ contraception/relationships/pregnancy: www.brook.org.uk Sex Education Forum: collaborative network, representing many organisations in England, all involved directly or indirectly in the provision or support of sex and relationships education (SRE). It provides SRE resources, web materials, and training: www.sexeducationforum.org.uk

Websites The SAPS website (www.saps.org.uk) has many valuable resources, including a video clip showing the technique of cauliflower cloning. From the homepage, use the search term ‘cauliflower cloning – tissue culture and micropropagation’. YouTube has a video clip from ‘Earth Unplugged’ showing exploding cucumbers. This is an excellent way to show seed dispersal by explosion: www.youtube.com/watch?v=wOIHzl2h9a8

References and further reading Callahan, G. N. (2009) Between XX and XY: Intersexuality and the Myth of Two Sexes. Chicago: Chicago Review Press. Cohut, M. (2018) The clitoris: What is there to know about this mystery organ? MedicalNewsToday. Available at: www.medicalnewstoday.com/articles/322235. php

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Department for Education (DfE) (2019) Relationships Education, Relationships and Sex Education (RSE) and Health Education. London: Department for Education. Available online via the government website, www.gov.uk Halstead, J. M. and Reiss, M. J. (2003) Values in Sex Education: From Principles to Practice. London: RoutledgeFalmer. Long, R. (2019) Relationships and Sex Education in Schools (England). London: House of Commons Library. Available online from the website of the House of Commons Library: https://commonslibrary.parliament.uk/. Reiss, M. J. (2018) Reproduction and sex education. In: Kampourakis, K. and Reiss, M. J. (eds) Teaching Biology in Schools: Global Research, Issues, and Trends. New York: Routledge, pp. 87–98. Reiss, M. J. (2019) Thinking like a fox: Queering the science classroom when teaching about sex and sexuality. In: STEM of Desire: Queer Theories and Science Education, Letts, W. and Fifield, S. (eds). Leiden: Brill | Sense, pp. 255–267. Stanger-Hall, K. F. and Hall, D. W. (2011) Abstinence-only education and teen pregnancy rates: why we need comprehensive sex education in the U.S. PLoS ONE, 6 (10), e24658. Yip, A. K.-T. and Page, S.-J. (2013) Religious and Sexual Identities: A Multi-faith Exploration of Young Adults. Farnham: Ashgate.

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8 Variation

Paul Davies and Neil Ingram

Topic overview The differences that exist between living things are termed ‘biological variation’, and this occupies a central part of how biologists think about the living world. Variation is important to many broad areas of biology, from the molecular level of how genes function to the diversity of species and the vastness of geological time and evolution. This makes teaching and learning about variation exciting but also potentially daunting for teachers and hard for students to access. For example, some concepts are very abstract, while others require students to bring together different knowledge from across the curriculum in order to understand complex ideas properly. Considering some of these challenges is important for teachers when thinking about a teaching sequence and the approaches they will take in the classroom. Central to this thinking is helping students make links between the abstract and invisible nature of how genetics explains variation, including the structure and function of DNA, and the observable features in nature. Students also need to be able to explain variation in terms of genes and the environment interacting together, often in ways that cannot be observed directly. Overarching all of this is the complex language and specialist vocabulary used to talk about inheritance and variation. Given these challenges, it is important to think about how a route through the topic might best suit students. It is probably most sensible to begin by considering what inheritance means and what material is passed on from one generation to the next. This then leads on to considering the nature of genes, how they function and their relationship to the rest of the cell. Once that is understood, students can begin to consider the types of variation that are observed in biology and reasons that help to explain this. This leads on to thinking about the complexities of how genes interact with the environment and how selection acts on populations (see Chapter 9 Evolution). Finally, we come to the exciting developments to do with humans controlling genetics for themselves (see Chapter 12 Microbiology and biotechnology). Most of this chapter concentrates on variation in eukaryotic organisms, that is organisms which have a nucleus and other membrane-bound organelles. This reflects the content of the school curriculum, where most of the discussion of variation is focused on animals and plants. Prokaryotic organisms (those

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8 Variation

without membrane-bound organelles) are only briefly considered here. However, they do have an important role in biotechnology, which is the focus of Chapter 12. The topic of variation is both wonderful and mysterious to students and provides opportunities to explore complex biology and the history of science and introduces students to an array of scientists, all of whom have interesting stories to tell. We begin this journey by considering what is meant by inheritance and the concept of the gene.

8.1 Inheritance and genes Prior knowledge and experience All students will have a rudimentary understanding that offspring ‘get’ something from their parents which explains why they look similar. It can be useful to introduce this topic by considering some historical ideas to explain this phenomenon: for example, the idea of blending where it was believed that organisms arose from a mixing of their parents where parental features were ‘averaged out’. It is also fruitful to provide a quick review of arguments for and against species being fixed entities. The concept of the gene is a complex one and something which is not easy to pin down. However, there are useful ways of building up the idea of the gene that students can then use when they have to think about how genes move between generations and their roles in protein production. Increasingly, the term ‘genome’ is used for the genetic material inside the nucleus. This means that the term ‘gene’ can be restricted to those sections of DNA coding for proteins. This is discussed more fully in Section 8.4.

A teaching sequence Developing a coherent teaching sequence is essential for a complex, multi-faceted topic such as variation and inheritance. An engaging way to start students thinking about inheritance is to play the game of matching baby photographs to students. Students could bring in physical or digital photographs, which can be collated and numbered on one sheet of A3 paper, and the activity run as a quiz. It is important to note the need for sensitivity surrounding students having access to photographs and so the activity is best run as an optional one. It can, of course, be fun for the teacher to include their own baby photograph. The activity prompts conversations about why we look the way that we do and how we can explain how specific features might be observed throughout several generations in a family. This second point is useful for challenging the idea of ‘blending’. You can look like a mixture of

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8.1   Inheritance and genes

both parents, with your ‘mother’s eyes’ and your ‘father’s nose’, for example. It is often more complex than this, of course. You can be the average of the heights of your two parents and sometimes certain features seem to ‘skip’ a generation. Even so, inheritance patterns are often discrete and predictable, and are caused by units. These ‘units’ are what we call genes and what the father of modern genetics, Gregor Mendel, termed ‘factors’; see the section on ‘Mendel and single gene inheritance’ later in the chapter.

Scientific literacy When considering genes, it is important that teachers focus on simple definitions and explanations so that students can start to build up a model of both their structures and functions. A useful way to build up these ideas is to define genes as: ➜ being

made of DNA ➜ having a specific sequence of DNA bases which provides a code for building specific proteins ➜ being found on chromosomes in a specific place or locus ➜ being found as different versions called alleles which have slightly different DNA base sequences. Getting students to engage with these ideas is a very useful starting point because it helps them recognise genes as a set of discrete instructions which make things happen in living things. This is especially important because research shows that students struggle to conceptualise genes as entities, despite careful teaching (Venville and Donovan, 2008). One of the problems is that teachers do not give students the opportunity to build mental models of the hierarchy of connections between DNA, chromosomes and genes, before they consider how genes work. Physical models can be a useful way of conceptualising the relationships. Everyday items like modelling clay, pipe cleaners and coloured counters can represent DNA, chromosomes and genes. Students should be encouraged to say how their models fit with biological structure and how their models are not like the biological structure. Often the models represent only one feature of the structure. A token representing a gene, for example, shows that the gene is a single discrete factor, but it says nothing about the relationship of that gene to the DNA molecule. Evaluation of models is always a good activity in helping students understand the usefulness and limitations of such models in relation to the biology structure or concept.

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8 Variation

What are genes?

KEY ACTIVITY

Students sometimes think that genes are only found in ‘complex’ living things and that there is a clear relationship between complexity of an organism and the number of genes and chromosomes it has.

Comparing chromosome number in different organisms A useful way of introducing the idea of all living things having genes is to ask students to research the number of chromosomes in a list of organisms which you will provide. This type of activity works best when students make predictions before they carry out research, providing reasons for their predictions. It also opens up discussion about whether viruses are living organisms, and the presence of genes in living things as providing evidence for the genetic relationships between living things. Students might also be surprised by the results, and this can open up discussion about how humans view themselves as being genetically related to other species. Table 8.1  Chromosome number in some common organisms Organism

Does this organism contain genes?

Does this organism contain chromosomes and, if so, how many?

lion

yes

38

Euglena

yes

45

banana

yes

33

measles virus

yes

 0

Salmonella

yes

 1

yeast

yes

16

Where do we find genes? Having explored the idea of genes as being universal in the living world, it makes sense to consider where genes are found: in eukaryotic organisms this is mainly inside the nucleus. Drawing a scale bar and placing the names of structures in their relevant positions can support students in getting some appreciation of scale and the relationship between the cell and genes. A suitable hierarchy is: ➜ the cell ➜ the nucleus ➜ chromosome

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8.2   The discovery of DNA ➜ DNA

molecule of DNA (gene).

➜ section

This type of activity can be developed by asking students to produce a diagram of the cell and the nucleus and then imagining the order of hierarchy of size inside the nucleus to reveal the size and relationship between chromosomes, DNA and genes as sections of DNA, such as Figure 8.1. cell DNA nucleus genome

gene chromosome

A G

T A

G T

T

C

C A

T

Figure 8.1  Diagram showing the hierarchical organisation of the cell, nucleus, chromosome, DNA and gene

This is also an opportunity to encourage students to think about how DNA is packed tightly into the nucleus. Taking a metre length of cotton and asking students to organise it into a structure that would fit inside a very small space is a good way of encouraging students to visualise the coiling of DNA into chromosomes. A demonstration of the teacher coiling up a long piece of rope has a similar effect. Observation of prepared microscope slides of various stages of mitosis is also helpful in thinking about the relationship between the relative sizes of DNA molecules and chromosomes.

8.2 The discovery of DNA Students enjoy listening to stories in science and it can help them to both make sense of complex scientific ideas and see science as an endeavour which involves the development of ideas, and appreciate the impact of different personalities involved in scientific discovery.

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Science in context The story of the elucidation of the structure of DNA provides an excellent opportunity for students to learn something about the history of science. The story of how the structure of DNA was determined is well known and well documented and opens up discussion about how scientists collaborate and the ‘jigsaw’ nature of scientific discoveries. It forces students to consider science as a profession and not some romantic image that they may have about scientists searching for truths in an altruistic way. It also questions the role and position that women have played in science and how history remembers the victors of scientific discovery. Exploring these ideas with students is always useful. The resource section at the end of the chapter contains a DNA story task which would support this kind of activity. The key aspects of the story of the discovery of DNA are detailed below. ➜ Scientists

had been working on a model of the structure of DNA for a long time. ➜ Different groups of scientists had different ideas about the structure of DNA, despite having access to the same evidence. ➜ In the early 1950s, Francis Crick and James Watson, working at the University of Cambridge, were interested in the heritability of the molecule, and this drove their desire to work out its structure. ➜ Maurice Wilkins and Rosalind Franklin, working at King’s College London, were working on the crystal structure of DNA and were interested in its shape. ➜ Watson was friends with Wilkins and visited his laboratory to discuss their ideas about the structure of DNA. During these discussions, Wilkins showed Watson a photograph that Franklin and her colleague Raymond Gosling had taken which revealed specific details about the structure of DNA. Using this information, Crick and Watson were able to complete their model. ➜ Crick and Watson published their model of DNA in the scientific journal Nature in 1953. ➜ Controversy surrounded how Crick and Watson gained access to Franklin and Gosling’s work, including the famous photograph and unpublished research papers. ➜ In 1962, Crick, Watson and Wilkins were awarded the Nobel Prize in Physiology or Medicine for their ground-breaking work. ➜ Franklin died in 1958 and was unable to receive a Nobel Prize for her work.

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8.3   Variation within and between species

8.3 Variation within and between species Students have a good sense that variation exists within species, and certainly within their own species. Students are ‘natural taxonomists’ in the sense that they readily recognise that living things are organised into groups. However, they lack much understanding about the explanation for these differences, and this is where thoughtful introduction about variation with and between species can be very powerful. Having explored the discovery of the structure of DNA, students can be reminded that DNA is found in all living things. This can then be expanded to consider the question that if all living things contain DNA, what accounts for the differences within and between species? This is at the heart of variation and is a useful inquiry question to form the basis for subsequent teaching and learning. Understanding that living things show variation that allows humans to classify them into groups with shared characteristics requires knowledge from different areas of biology: taxonomy and classification, and the genetics of variations. The former is dealt with in Chapter 10; the latter is addressed here. Once students have understood that genetic information is coded within DNA and that it can be passed on to successive generations, they can consider that differences in genes explain variation between species. It makes sense to start thinking about this idea by exploring differences between species. Some of these differences are obvious (such as fur of many mammals versus feathers of birds); others are much more subtle. If students have a good understanding of the relationship between genes and characteristics, they can appreciate that many differences between individuals of different species will be influenced by genes. This then naturally leads on to a consideration of how variation within a species can be explained. Although students sometimes struggle with understanding the notion of alternative versions of genes, often by conflating genes and alleles (see below), they are well placed to explain that the genes within the same species must be able to function differently if we are to be able to explain differences in characteristics such as eye colour, for example.

Differences between species A species is defined as a group of organisms that are able to reproduce with each other successfully to produce fertile offspring. The reason that there are distinct species is because they do not mix their genes, and are therefore genetically isolated.

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KEY ACTIVITIES

8 Variation

Comparing the genomes of different species To begin to explore this idea, students could be asked to research the percentage similarity between the genomes of closely and less closely related species. Completing a table such as Table 8.2 will reveal to students that more closely related species share more DNA in common. Table 8.2  Exploring the percentage of DNA that various organisms share with humans Organism

Percentage of DNA shared with humans

chimpanzee

99

cat

90

dog

84

cow

80

mouse

73

zebrafish

73

platypus

68

banana

60

chicken

60

fruit fly

60

yeast

26

mustard

15

Students could also be asked to make predictions about the percentage of DNA each organism shares with humans; they might be surprised by the data. This can then open out into a discussion about evolutionary relationships and why analysis of genes might be a more appropriate method of building a tree of life than observable characteristics. The Tree of Life website (see Resources) has a useful tool for exploring these patterns.

Versions of genes Having considered that there are different genes in different species, students can be introduced to the idea that there are differences between the versions of genes within a species; these different versions are called alleles. Understanding that genes have different alleles can be challenging for students, with research showing that they find it hard to make links between how the alternative version of a particular gene might actually differ in

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8.3   Variation within and between species

structure and how this gives rise to the cell or organism appearing different. Understanding alleles as alternative versions of genes is really important, as this helps explain variation within a species and is therefore central to the topic of variation. A useful way to introduce students to this idea is to collect data from the class about variation that they exhibit which is caused by genes. While humans in general have multiple genes associated with a characteristic, there are a few that can be looked at in terms of single gene traits. Relevant characteristics could include eye colour, hair colour, attachment of earlobes, straightness of thumbs and the presence of a widow’s peak hairline. Asking students to sort these features into categories can be an interesting way of opening up discussion about the binary nature of the way that genes sometimes work (also see below about how this discussion extends into thinking about how multiple genes and the environment affect the way that people’s characteristics vary.

Variation within a species Having collected data on variation within a class for characteristics that are controlled by genes, students are well placed to explore how these types of data can be represented and investigate some of the different types of variation exhibited within species. Many characteristics are determined by a combination of genes and the environment. Take, for example, height in humans: there are genes which control the length of bones and muscles and the structure of tendons and other body tissue. However, inheriting alleles that build tall bodies is not enough for an individual to achieve their full, potential height. Ideas about diet, exercise, absence of disease and general health are also important in determining the final height that an individual reaches (see page 202). Students will be familiar with the idea that they look different from one another and can be encouraged to make a list of characteristics which are straightforward to measure which they think are controlled by both genes and the environment (for example, height, hand span, personality, skill at playing a musical instrument). They can also be asked to make predictions on the extent to which genes or the environment influence such characteristics. The aspect of this type of activity that students will find most challenging is how to collect and present these data. Students find it hard to visualise how numbers can be organised in ways that allow them to make meaning from these numbers. There are two ways of approaching this problem as a teacher: the first is through direct instruction and the second is through exploration. Research does not support either approach as being better but the latter has been shown to help students make more meaning in their learning (Wong, 2017, or Boohan and Needham, 2016).

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KEY ACTIVITY

8 Variation

Investigating variation An exploration approach would involve first getting students to collect data about themselves (or by providing them with secondary data). The data should cover characteristics which are traditionally thought to be controlled by genes alone (such as eye colour and earlobe shape) and characteristics where both genes and the environment have influence (such as height and personality). In principle, no characteristic can really be entirely controlled by genes or the environment. All characteristics, to a greater or lesser extent, are produced by interactions between genes and the environment (see the section on ‘How the genome interacts with the environment’ later in this chapter). In practice, some characteristics are so stable across a wide range of environments that any environmental effects can be ignored, and they appear to be entirely controlled by genes. This is particularly true for the characteristics studied by the early geneticists (see the section on ‘Mendel and single gene inheritance’). It is important, especially in the later stages of the course, that students understand that the production of most characteristics is dependent on many different interactions between the genome (the complete set of genes in an organism) and the environment.

Maths Students can be asked to organise the data in as many ways as they can. This will open discussion about the appropriateness of charts and graphs and how the choice of data presentation and manipulation affects the meaning attached to it. This will help students to realise that they should pay attention to the frequency of particular characteristics. For those controlled by genes alone this is straightforward; individuals displaying the different characteristics can be counted and plotted in bar charts. These types of data are described as being discontinuous because they fall into discrete categories (for example, blue, brown, green eye colour). It is more challenging to organise data where characteristics show a range of values; this is called continuous variation and includes height. Here students should be guided to produce histograms. In biology, simple histograms are required; they should be direct counts and with equal-sized categories. Histograms of this type are easily interpreted by students and, for the higher-attaining students, this can lead to conversations about distribution patterns. Whichever approaches are taken when teaching students about constructing and using charts and graphs, it is important to allow enough time for this to be taught explicitly. See Boohan and Needham (2016) for further ideas about teaching mathematical skills in the science classroom.

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Cross-disciplinary A discussion with colleagues in both the mathematics and geography departments will be useful to discover when these types of chart are taught and the approaches that are taken. It is likely that students will find it hard to construct histograms if your approaches to teaching about them, and your use of them, are significantly different to other subjects. The two most common differences are that in mathematics, students might produce histograms with unequal-sized categories and in geography they might be required to complete frequency histograms.

8.4 The genome During this century, the term ‘genome’ has become popularised, and it is likely that students will have an awareness of its meaning, although there is a real possibility of misconceptions. The term ‘genome’ was first used in 1920 to describe the genetic information in a haploid set of chromosomes, such as those found in a human sperm or egg. Thus, an individual would have ‘paternal’ and ‘maternal’ genomes, inherited from the father and mother. From its inception, the term combined the idea of the physical material of chromosomes with the idea of genetic information. These days, the term is used to describe all of the genetic information an organism has, while retaining the idea that it can also describe the complete set of chromosomes in a cell. In humans, only about 1% of the genetic information in a genome are ‘genes’ coding for the amino acids in proteins, while much of the remaining 99% regulates when and how the protein-coding regions are used. Thus, when talking in general about an organism’s chromosomes or genetic information, it is probably better to use the term ‘genome’. It can be introduced in the early stages of the course, when students observe the nuclei of their cheek cells, by saying that the nucleus contains the genome. Although most of the genome is found in the nuclei of eukaryotic cells, small amounts are also found in mitochondria and in the chloroplasts of some plant cells. In bacteria, the cytoplasm contains the genome, which consists of a single large circular loop of DNA and, sometimes, smaller loops of DNA called plasmids.

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Scientific literacy Strictly, the term ‘gene’ ought to be limited to specifically named regions of DNA that code for proteins, and the term ‘genotype’ to a description of the factors in a Mendelian analysis (see the section on ‘Mendel and single gene inheritance’ later in the chapter). Encouraging the use of the term ‘genome’ would also prepare students for a future in which genomics will increasingly be used in medical diagnoses. It is possible to extract the genomes from the nuclei of plant cells (such as onion, strawberries, kiwi or bananas) as DNA, using only salt, detergent and ethanol, and this can be a key part of education for students in the lower part of secondary education.

How the genome interacts with the environment The majority of the genome of eukaryotic cells resides within the nucleus of a cell, which has a double membrane with pores connecting the genome with the rest of the cell. It is a common misconception that the nucleus is the ‘brain’ of the cell or that genes ‘control’ cell activity. Contemporary biologists no longer think in such a way and genes are now seen as being one component of a wider functioning genome. There are continual two-way interactions between the cell and its nucleus, involving chemicals that act as messengers. The proteins that a cell makes can be determined by the chemical environment in and around the cell. The process illustrated in Figure 8.2 continues as long as the levels of glucose in the blood stay high, stopping only when the level of glucose returns to normal: an example of negative feedback. The ‘gene for insulin production’ is the DNA sequence containing the information needed to allow the ribosome to build the insulin molecule, but many other parts of the genome are also involved in the regulation of the gene and the production of ribosomes, the cell membrane and the signalling mechanisms. Protein production is an interaction between the genome and the physio-chemical environment in and around the cell. This is what biologist Conrad Waddington called the ‘epigenetic landscape’, a useful metaphor for describing these metabolic pathways. At its simplest, the prefix ‘epi-‘ means ‘above’ or ‘around’ and refers to the actions of chemical messengers that increase or decrease the production of protein by the genome. In advanced studies, students will encounter more specific examples of the term, but this general definition will provide a sound introduction for future elaboration.

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Beta cells in the islets regions of the pancreas produce the protein insulin only when the concentration of glucose in the blood rises above normal.

This is detected by the surface membrane of a beta call, which triggers a sequence of chemical changes within the cell.

This leads to a DNA sequence on chromosome 11 of the genome being copied into mRNA.

This mRNA leaves the nucleus and enters a ribosome in the cytoplasm.

The information in the mRNA molecule is used to assemble amino acids to form the protein insulin, which is exported from the cell into the blood.

Figure 8.2  A flow chart illustrating the process by which insulin is produced

Mendel and single gene inheritance Traditionally, genetics is the study of variation: how variation arises and how it is transmitted from parents to offspring. It has largely grown from the pioneering work of Austrian monk Gregor Mendel. Human genomics is a new science that investigates the effects of variation of individual DNA bases on the expression of characteristics in different people. It is also built on the principles established by Mendel. Mendel is properly considered to be the father of genetics and the grandfather of genomics. This section considers these principles, which are essential to a full understanding of genetics and genomics. First, Mendel was a careful experimentalist. His breeding experiments with pea plants selected seven contrasting characters which were inherited independently; he ensured that his parental generations were pure breeding (they only produced offspring of the parental type when self-pollinated). He also grew large numbers of offspring in his experiments (28 000 in total 193

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between 1856 and 1863). He, and his assistants, counted and recorded the characteristics of the plants in the first and second generations. Without this careful and systematic approach to his study, the experimental data would have been meaningless. Secondly, Mendel built a theoretical model to ‘explain’ his data. This was a tool for thinking with, and it is still used as a basis for thinking about genetics and genomics. It has a specialist vocabulary, which can be bewildering for students when they first encounter it. The observable characteristic being studied is called the ‘phenotype’. The term ‘genotype’ describes the particular alleles of a gene that are inherited from parents to offspring affecting the characteristic. We will follow the inheritance of a single gene for stem length with two alleles: A (tall; the dominant characteristic) and a (short; the recessive characteristic). Mendel called these alleles ‘factors’. Mendel’s principles are described below: ➜ The

parents of Mendel’s crosses were chosen to have pairs of contrasting factors, AA and aa. The factors in each parent were the same. Later geneticists called these factors ‘homozygous’ alleles. ➜ This parental cross is AA (female parent) with aa (male parent). ➜ Each gamete produced by these parents contains only one of their factors. ➜ Gametes combine at random at fertilisation. An offspring receives one factor from the female parent (A, via the female gamete) and one factor from the male parent (a, via the male gamete). ➜ Mendel called the offspring of the parental cross the filial generations, abbreviated to F. Thus, the F1 generation contains the offspring of the parental cross and the F2 generation contains the second generation. ➜ The parents in the F1 generation will have different factors, because they have inherited A and a and have the genotype Aa. Later geneticists called these factors ‘heterozygous’ alleles. ➜ The gametes of an F1 organism will contain only one of the two factors, which will be produced in equal proportions ( 1 A and  1 a). Each male 2 2 gamete has an equal chance of fertilising any female gamete. The English geneticist Punnett (the world’s first professor of genetics, at Cambridge University) expressed these relationships in terms of a square, which now bears his name (Table 8.3). The frequencies of the genotypes in the F2 generation are obtained by: ➜ multiplying

together the frequencies of the female and male gametes for each possible fertilisation ➜ adding together the frequencies of the heterozygotes, Aa 1 ➜ giving a total F2 genotype frequency of 1AA :  Aa : 1aa. 4

2

4

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8.4   The genome Table 8.3  A Punnett square showing genotype frequencies for an F2 generation for two alleles, A and a Female gametes Male gametes 1 2A 1a 2

1A 2 1AA 4 1Aa 4

1a 2 1Aa 4 1aa 4

The phenotypes of the F2 generation show that the recessive character (short stem) is hidden if there is a dominant factor present. Since AA and Aa both contain an ‘A’ factor, they express the tall-stem phenotype. Only aa expresses the recessive short-stem phenotype. This means that the F2 phenotypic frequencies are three tall stems : one short stem. Mendel’s legacies are a rigorous experimental approach and a new way of thinking, which was largely confirmed by later biologists. Mendel’s contrasting characters became identified with genes and his ‘factors’ became alleles of those genes, which were shown to be located on chromosomes at identical (homologous) positions. Pairs of chromosomes are segregated at random during meiosis. This led to the firm conception of characteristics being ‘controlled’ by genes, neatly contained, like ‘beads’ on a chromosome necklace. This conception is being challenged by genomics and is now regarded as an oversimplification. Mendel’s stem length character is a good example of this oversimplification. In peas, the difference between the tall and short stem phenotypes is associated with a single base change from G to A in the allele of the LE gene on chromosome 4. This leads to a change in the amino acid sequence (alanine to threonine) of the active site of an enzyme, which disrupts its normal function. This change prevents the synthesis of gibberellin, a major plant hormone that leads to the growth of stems. Thus, homozygotes for the mutant allele have short stems. The recessive alleles exert such a large effect by disrupting the normal growth of the plant. Many single genes with major effects often work by disrupting the normal processes of metabolism. Breeding experiments, like Mendel’s, are designed to discover the genetic differences between the parents for a character. In the case of stem length, a DNA base difference acted as the factor which was inherited from parents through to the F2 generation. The genetic similarities between the parents, which are considerable, could not be studied in this way. Thus, it is a misconception to say that stem length is controlled by a single gene, since the whole of the genome is involved in the production of both tall and short stems.

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Science in context Mendel’s model of inheritance can be explored using card games. The cards are the ‘alleles’ of genes in gametes that can be shuffled and combined in pairs. This represents the random process of fertilisation. Genotype cards can be linked with the appropriate phenotype card. Students can repeat this numerous times to produce values for the genotype frequencies for the next generation. Variation in living material can be represented by growing genetic seeds of parent, F1 and F2 generations (such as tomatoes or rapid-cycling brassicas) and using model organisms such as vestigial-winged and long-winged fruit flies. These can all be purchased from commercial suppliers, such as Philip Harris, SAPS or Blades Biological Ltd, in the UK.

8.5 Sex determination in humans Punnett squares can show how chromosomes can be transmitted from parents to offspring. In the following example, the factors being inherited are the mammalian sex chromosomes, X and Y. Females have XX chromosomes and males have XY chromosomes. The Punnett square in Table 8.4 shows how the balance of sexes is maintained at 1 : 1 in each generation. Females produce eggs containing an X chromosome; males produce two kinds of sperm, 1 2

carry the X chromosome and 1 carry the Y chromosome. The frequency of 2

the offspring of the cross being either biologically male or biologically female is obtained by: ➜ multiplying ➜ adding

the individual gamete frequencies together together the XX frequencies and the XY frequencies.

Maths Punnett squares are a good opportunity to cover both fractions and percentages with students, as well as probability.

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8.5   Sex determination in humans Table 8.4  A Punnett square showing how to calculate the frequencies of biological males and biological females in the next generation. Female eggs

Male sperm 1 2

1 2

1 2

X

1 4XX

1 4XX

1 XY 4

1 XY 4

X

X

1 2     Y

 

The probability of a biologically female child being born is 1, the same as 2 the probability of producing a biologically male child. Statistically speaking, each fertilisation is an independent event, so the probability of producing a sequence of two female children is the probability of producing each child independently, multiplied together, i.e. 1 × 1 = 1. 2

2

4

In humans, the sex chromosomes, X and Y, are very different in appearance. The X chromosome is larger, with about 800 protein-coding genes, while the Y chromosome only has about 70. The Y chromosome contains the gene SRY, which triggers male sexual development. The protein produced from the information on SRY is used to activate the genomes of the cells that will develop into testes, to produce the hormone testosterone, about 6–8 weeks after conception. This topic should be taught with sensitivity. A person’s biological sex (being male or female) is not necessarily the same as the gender that a person perceives themselves to be. The presence of a Y chromosome does not always stimulate the production of male genitalia and secondary sexual characteristics. Gender is a complex character, which is partially socially constructed.

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You may want to refer to your school policies to ensure you understand how your school requires you to discuss issues of gender.

Science in context Genetic cards can be developed to allow students to experience the randomness of the fertilisation of the gametes. This time the cards show X or Y chromosomes, rather than alleles. Some diseases, such as haemophilia, result from inherited recessive alleles on the X chromosome (Xh). These alleles have different patterns of inheritance from the tall- and short-stem plant example in the previous section. Because male children only have one X chromosome, a recessive allele on that chromosome will always be expressed (XhY). This means there is a greater frequency of haemophilia in male children than female children. Students can be encouraged to consider why haemophiliac female children are uncommon.

Science in context The symptoms of haemophilia are caused by the failure to produce a factor essential for blood clotting (factor VIII). Queen Victoria was heterozygous for the haemophilia allele (XhXH) and one of her sons (Leopold) died aged 30 from bleeding after a fall. It is known that the haemophilia allele spread through her descendants into the royal families of Spain, Germany and Russia. The impact of haemophilia on the Romanovs, the last generation of Russian czars, is well documented and could make a fascinating extension activity for students.

8.6 Genomics and medicine Genomics reveals the DNA base sequence of our genomes. At present, genetic tests only sample parts of our genomes but, as laboratory costs fall, whole genome sequences will become routinely available in a few years’ time. Genome sequencing allows scientists to develop tests for diseases that have a genetic basis. Cystic fibrosis, for example, is caused by DNA base changes (mutations) in the CFTR gene, which prevent the CFTR protein from transporting chloride across cell membranes. The most common CF allele is DeltaF508, which deletes a single amino acid, phenylalanine, from the CFTR protein, preventing its successful production. People will develop cystic fibrosis if they have two faulty

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copies of the gene, because these CF alleles are recessive. A genetic test for the DeltaF508 allele will show whether a person has copies of the recessive allele. There are three different combinations of alleles for the CF DeltaF508 allele, which are shown in Table 8.5. Table 8.5  Three possible combinations of alleles for cystic fibrosis; C = normal allele, c = CF DeltaF508 allele CF alleles

Combinations of alleles

Effects of alleles on the Can transmit CF allele to phenotype the next generation?

CC

homozygous dominant

no symptoms of CF

no

Cc

heterozygous

no symptoms of CF

yes

cc

homozygous recessive

symptoms of CF

yes

Scientific literacy The terms ‘homozygous’ and ‘heterozygous’ refer to combinations of alleles. Strictly, it is a misconception to apply the terms to organisms, although it is sometimes done informally in conversation. We ought to discourage students from writing ‘she is a homozygote’ or ‘the heterozygous mice’, unless they refer explicitly to the combination of alleles under consideration. There are over 1500 different mutations in the CF gene known across the world, and a test for DeltaF508 will detect only 70% of the potential cases of CF. This can lead to a false-negative result, where a person can be given a negative result for DeltaF508, but still have CF, because of the presence of two other CF alleles. Likewise, some tests can give false-positive results, where an allele is reported as being present when further, more rigorous, testing shows that it is absent. Recent reports suggest that some consumer DNA testing companies are reporting high levels of false-positive results, which is concerning medical professionals. Furthermore, the presence of homozygous alleles associated with disease does not mean that the disease will necessarily develop. The APOE gene is associated with the development of Alzheimer’s disease, and tests for its alleles are becoming increasingly available. One of its alleles is called E4. A person homozygous for two E4 alleles has about a four-fold increased risk of developing the disease compared to other people. In addition to the E4 alleles, certain lifestyle factors are also needed to trigger the condition,

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such as high cholesterol levels and blood pressure. The risk of developing Alzheimer’s disease can be significantly reduced by healthy diet and exercise.

Careers Health professionals are increasingly using genetic tests as a basis for diagnosis. They are looking to see how differences in DNA bases can make us susceptible to diseases, such as cancer, and to adverse reactions to medicines and recreational drugs. Testing of genomes is undertaken by teams of people with a range of different expertise. Technicians undertaking the laboratory work are supported by statisticians, software engineers, psychologists, doctors, nurses, counsellors and administrators. The field is changing rapidly and could make an interesting career for many students.

Science in context This is the emergence of the era of personalised medicine, and teachers will need to be sensitive to the backgrounds of the students in their classes. What is academic for some will be deeply personal for others. Even so, students should be given the opportunity to discuss and debate the ethics of these controversial issues. How do people respond to the results of genetic testing? What is it like to receive falsepositive or false-negative outcomes? These can be followed up in news reports (such as on the BBC News website).

8.7 Polygenic inheritance Most characteristics are produced by the combined effects of many different genes interacting with the environment. Each individual gene has a small effect on the characteristic and the effects of hundreds of genes might combine together. This is called polygenic inheritance. Polygenic characteristics, such as height in humans, show continuous variation in a population, as shown in Figure 8.3.

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8.7   Polygenic inheritance 50

Number of people

40

30

20

10

0 150

165

180

195

Height/cm

Figure 8.3  Height in humans is a characteristic controlled by many genes, resulting in continuous variation.

The heights of people in the population can be any value from the minimum to the maximum. With large sample sizes, the frequency distribution becomes close to a symmetrical ‘bell-shaped’ curve, called a normal distribution. Other examples of normal distributions for human variation include body mass, intelligence and blood pressure.

Maths This is a good opportunity to discuss with students data analysis in relation to distribution and how frequency charts and graphs can be used to represent these data. As described above, human height is a good teachable example of a continuous characteristic. It is easy to collect frequency data from a class to show continuous variation. Comparisons of the DNA bases in human genomes suggest that there are about 700 DNA base-pair differences (called ‘genetic variants’) between people that affect height, distributed across the whole genome. Most of these genetic variants have individually small effects, typically changing height by less than a millimetre. A few genetic variants seem to have a larger effect on the final height, each adding up to a centimetre.

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Many of the genetic variants detected for height will be within protein-coding genes, as we would expect, but some are not. They may be in the parts of the genome that help the protein-coding regions to function correctly. This suggests that scientists have yet to understand fully the influence of the whole genome on a character like height. Even so, the genetic variants discovered so far account for less than 30% of the differences in height between people; other analyses suggest that up to 75% of the differences between people can be explained by the wider genome. Current genome analyses only detect differences in DNA bases between people. There must be many DNA bases that are the same in everyone, that make a contribution to human height, but which cannot be detected by current genome analyses. We also know from other studies that about 25% of the differences in height are caused by environmental differences (such as nutrition, exercise and healthcare) between people. In countries with malnourished diets, the differences can be even larger. One way to explore this in the classroom is to consider the average height of male skeletons from different periods of history (Table 8.6). This is largely based on the research of Steckel in 2004 (see References section, paper published in Science Daily). Table 8.6  Average male heights for Europeans during different periods of history Time period

Average male height/cm

9500bc

166

10th century

173

17th century

167

21st century

176

It is interesting to note that Stone Age hunter-gatherers were not much shorter than humans living before the modern era. Assuming the genomes are more or less unchanged across the ages, the fluctuations in height are thought to be due to differences in the environment; in particular, the changes in climate affecting agricultural productivity and food availability. The increase in average height in the twenty-first century is thought to be due to a significant increase in the quality and amount of food available.

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8.8   The importance of genetic variation in populations

8.8 The importance of genetic variation in populations The human genome contains about 3.2 billion pairs of nucleotide bases and about 20 000 protein-coding genes. The pairs of bases are mostly the same across everyone, and only about 0.6% of our genome varies from person to person. That is a difference of about 20 000 000 base pairs between two unrelated individuals. The figure is lower between family members. In total there are currently about 350 million genetic variants known in human genomes, although this figure is likely to rise as more genomes are sequenced. The similarities are associated with those structures and processes that keep us alive and are maintained by natural selection. Mutations to DNA bases linked to vital life processes are likely to be harmful and are selected against. The differences are important, too. They form the genetic basis of the variation we see between organisms, which enables natural selection to occur and evolution to proceed. Populations of blue tits in England, for example, are evolving slightly longer bills, which enable them to feed more easily from bird feeders. Evolution is selecting from the genetic variation naturally present in the population to respond to changes in the environment, as discussed in Chapter 9. The similarities in the structure of DNA and the genetic code across all living organisms suggest that all life on Earth is descended from a single common ancestor, thought to have lived some 3.8 billion years ago. Scientists think that it contained about 355 protein-coding genes, which are the originators of the genes of today’s organisms. All of the different forms of life that have existed since then have evolved by natural selection acting on genomes, through mutation, and subsequent modification. The universal genetic code across all living organisms means that a human protein-coding gene can be inserted into another organism, which would then be able to produce a novel protein. This is the fundamental basis of much of contemporary biotechnology. One common misconception is that genes ‘belong’ to their donor organisms, so that introducing a human gene into a bacterium makes it more human. This is incorrect; the similarity of our genome with other organisms is significant (human genomes share a 96% similarity with chimpanzees and 60% similarity with bananas), and we share many genes with these organisms.

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8.9 Resources Websites General websites This website provides a range of teaching resources focused on genomics: www.genome.gov/about-genomics/teaching-tools The OneZoom tree of life explorer allows students to explore the phylogenetic relationships between different living things: www.onezoom.org

Websites about DNA This activity on the STEM Learning website allows students to make jewellery while learning about the base-pairing rules in DNA and the coding nature of the molecule. From the STEM homepage, input the search term ‘DNA sequence bracelets’: www.stem.org.uk This is a paper-based activity where students make models of DNA. It provides opportunities to discuss the advantages and limitations of scientific models: www.yourgenome.org/activities/origami-dna An activity where students extract DNA, and its associated proteins, from fruits: www.genome.gov/Pages/Education/Modules/StrawberryExtractionInstructions. pdf Another modelling activity which explores the base-pairing rules of DNA: www. yourgenome.org/activities/yummy-gummy-dna This is a useful website for students to use when researching the discovery and importance of DNA: https://profiles.nlm.nih.gov/ Background information about DNA and its discovery: bbsrc.ukri.org/ documents/fullbooklet-pdf These activities support role play of the central characters in the DNA story: www.thinkib.net/files/biology/files/activity%20worksheets/genetics/NOS_ biologist-masks-activity--Rosalind-Franklin.pdf and www.thinkib.net/biology/ blog/18601/a-new-way-to-role-play

Websites about inheritance This website contains materials that Mendel wrote, including his important research paper: www.esp.org/foundations/genetics/classical/gm-65.pdf These two websites explore inheritance through building a baby dragon: concord.org/teaching-genetics/dragons and https://serendipstudio.org (search ‘dragon genetics’ from the homepage)

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This story-based card game from Philip Harris explores inheritance in humans. Search ‘gene-packs b8r06974’ from the homepage: www.philipharris.co.uk Genetic seeds of parent, F1 and F2 generations (such as tomatoes or rapidcycling brassicas) can be purchased from SAPS (search ‘rapid cycling brassica kits’ from the SAPS homepage, www.saps.org.uk), Philip Harris Ltd or Blade’s Biological Ltd.

Other interesting websites www.centreofthecell.org www.genome.gov/ www.rsb.org.uk/get-involved/biology-for-all (search ‘genetics and DNA’) www.scienceinschool.org www.wellcomegenomecampus.org www.yourgenome.org

References Boohan, R. and Needham, R. (2016) The Language of Mathematics in Science. Hatfield: The Association for Science Education. Ingram, N. R. (2019) Genetics for tomorrow’s world. School Science Review, 101 (375), 26–30. Ingram, N. R. (2020) Introducing the epigenetic landscape into middle years biology teaching. School Science Review, 101 (377), 32–36. Pearson, H. (2006) What is a gene? Nature, 441 (25th May), 399–401. Science Daily (2004) Men from early Middle Ages were nearly as tall as modern people. Available at: www.sciencedaily.com/releases/2004/09/040902090552.htm Venville, G. and Donovan, J. (2008) How pupils use a model for abstract concepts in genetics. Journal of Biological Education, 43 (1), 6–14. Wong, V. (2017) Variation in graphing practices between mathematics and science: Implications for science teaching. School Science Review, 98 (365), 109–115.

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Further reading Albright, R. N. (2014) The Double Helix Structure of DNA: James Watson, Francis Crick, Maurice Wilkins, and Rosalind Franklin (Revolutionary Discoveries of Scientific Pioneers). New York: Rosen Classroom. Boerwinkel, D. J., Yarden, A. and Waarlo, A. J. (2017) Reaching a consensus on the definition of genetic literacy that is required from a twenty-firstcentury citizen. Science & Education, 26 (10), 1087–1114. Knippels, M. C. and Waarlo, A. (2018) Development, uptake, and wider applicability of the yo-yo strategy in biology education research: A reappraisal. Education Sciences, 8 (3), 129. Schultz, M., Cannon, Z. and Cannon, K. (2009) The Stuff of Life: A Graphic Guide to Genetics and DNA. New York: Hill and Wang. Watson, J. (2003) DNA: The Secret of Life. New York: Alfred A. Knopf.

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9 Evolution

Alistair Moore and Chris Graham

Topic overview Evolution by natural selection is not only a big idea of biology education, it is a unifying concept in the biological sciences. As the Russian geneticist and evolutionary biologist Theodosius Dobzhansky famously asserted in 1973, ‘nothing in biology makes sense except in the light of evolution’. Understanding the theory of evolution by natural selection enriches our understanding of the vast array of forms and behaviours that we see in nature – including the seemingly miraculous – as adaptations that exist and proliferate because they enable living creatures to survive, reproduce and thrive. It helps to persuade us that the structure and function of every biological entity and system at every level of organisation, from biological molecules to ecosystems, has been powerfully honed by selection over countless generations, resulting in the incredible biodiversity we see today. It enhances our ability to explain the connectedness, similarities and differences between species, and challenges our perspective on our place in nature. Crucially, it helps us understand crises of our time, including the rise of antibiotic-resistant bacteria and the disappearance of species unable to adapt quickly enough to survive rapid climate change. Teaching and learning about evolution need not be a daunting prospect; it is a chance to inspire students with the tremendous explanatory power of science. This chapter starts by discussing the teaching and learning of evolution by natural selection. It then discusses how the topic of adaptation can be used to reinforce and assess students’ understanding of evolution by natural selection and the language they use in their explanations. It also discusses how the teaching of the evolution of adaptations can be used to help students develop their scientific thinking skills.

9.1 Evolution by natural selection Prior knowledge and experience At the beginning of secondary education at about age 11–12, students are likely to have a wide variety of ideas about evolution. While some of these may be derived from earlier formal teaching, others will have come from everyday life – from sources as varied as museums, natural history television programmes, family members, religious teachings, and extraordinary tales of mutation and evolution in comic books and movies. Some students may be feeling understandably conflicted, as not all of these sources will offer accounts that are consistent with one another or with the scientific 207

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explanations presented in biology lessons. Many students will be familiar with fossils, ‘survival of the fittest’ and perhaps even the name Darwin, but may not appreciate their relevance to the scientific explanation for the evolution of species. From prior learning, students should know that: ➜ there

is a great diversity of organisms with many similarities and differences between them ➜ organisms reproduce to produce offspring that are similar, but not identical ➜ organisms have features (adaptations) that enable them to live in habitats that provide for their needs ➜ fossils provide information about organisms that lived a long time ago, including many that are now extinct. Some teaching programmes specify that students should be taught explicitly about evolution before their secondary schooling, but the breadth and depth of treatment is likely to vary.

A teaching sequence Put simply, the characteristics of all species change (evolve) over time, and the theory of evolution by natural selection is a widely accepted scientific explanation for this. Secondary biology education should aim to develop students’ appreciation of natural selection as a scientific explanation. Students should be provided with opportunities to explore evidence from which the theory was developed, including: differences between fossils and extant species; the effects that selective breeding has had on the characteristics of particular plants and animals; and differences between populations living in different conditions. Exploring the scientific explanation for evolution is an excellent opportunity to explore the work of scientists, including Charles Darwin and Alfred Russel Wallace, and appreciate how scientific explanations are developed and modified using evidence. From their education at primary level, and from everyday experiences, students at age 11 should appreciate that there is a vast variety of living things on Earth, including many species that are now extinct. They may also have been introduced to the idea that the characteristics of species change over time, and that this is called evolution. Secondary education should draw together and develop the key ideas that biologists use to explain evolution, including variation (see Chapter 8), competition, fitness and natural selection. These ideas will enable students to develop their understanding of classification and speciation (see Chapter 10).

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At first, the key ideas can be explored at trait (phenotype) level; by age 16, more able students should be able to explain variation and natural selection at the genetic level, including the idea that alleles associated with advantageous traits become more common in populations over generations due to natural selection. Students who progress to further study of biology after age 16 may begin to develop their understanding of population genetics and the effects of selection, gene flow and genetic drift on allelic frequencies. They may study different types of selection (stabilising, directional and disruptive), speciation (such as allopatric and sympatric), co-evolution and symbiosis. They may begin to explore the complexities of heritable variation using ideas from genomics and epigenetics. It is difficult to find consensus within the science education research literature on the best sequence for developing students’ understanding of evolution by natural selection, but one possible sequence is outlined below.

Fossils: evidence that species change over time The characteristics of all species change over time (or, more precisely, over generations); a process called evolution. This is an uncontroversial fact for which there is ample evidence, including fossils that can be examined during lessons in the lab and in the field.

Science in context

KEY ACTIVITIES

Many students are keenly interested in and knowledgeable about fossils in general and about dinosaurs in particular, and thus fossils can be used as a starting point for learning about evolution (Borgerding and Raven, 2018; Hunter et al., 2018).

Investigating fossils Allow students to examine fossils (ideally real, but also models and pictures) that include examples of some organisms very similar to and some very different from extant species. Fossils illustrating differences between modern horses and their evolutionary ancestors are a well-used example of a sequence of change over time, from smaller, multi-toed animals with low-crowned teeth to larger, single-toed animals with high-crowned teeth (see Figure 9.1). As with most depictions of evolution, this should be presented with some caveats: the evolution of the horse was not a linear march through distinct stages to a final, perfect form – it was branched and tree-like, with many co-existing species and evolutionary dead-ends. Also well known are the fossils of Archaeopteryx – bird-like dinosaurs that had feathers and broad wings like a bird and also sharp teeth, three-fingered claws and a long, bony tail like a dinosaur. Archaeopteryx are transitional species that provide evidence that birds evolved from dinosaurs. 209

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Skull

Recent

Forelimb Hindlimb

Teeth top view

Height (cm)

side view

Equus

150

Pliocene

Pliohippus

125

Miocene

Merychippus

100

Oligocene

Mesohippus

60

Palaeocene

Eohippus

28

Pleistocene

Eocene

hypothetical ancestor with five toes on each foot and monkey-like teeth

Figure 9.1  Selected fossil evidence showing differences between the modern horse and its evolutionary ancestors

It can be difficult to incorporate practical work into teaching about evolution, but working with fossils provides an opportunity for students to develop their skills in scientific observation, recording and classification. It also provides an opportunity to develop and challenge their thinking on the nature of evidence and how much confidence we can have in explanations based on incomplete evidence. The fossil record is notoriously incomplete, partly because relatively few organisms were preserved as fossils, and fossils are often incomplete representations of the original organisms. For example, fossils of fewer than ten Stegosaurus have ever been found, the most complete of which (nicknamed Sophie) now resides in the Natural History Museum in London. All of our knowledge of Stegosaurus comes from these few individuals, which may or may not have been typical representatives of the genus.

A sense of scale: geological time Learning about fossils and evolution requires an appreciation of the timescales involved, but these can be so vast that students can struggle to comprehend them. Students can find it difficult to appreciate the absolute 210

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ages of fossils; one study found that when students aged 10–11 were asked to estimate when dinosaurs lived, their answers ranged from 1000 to ‘millions’ of years ago (Trend, 1998).

KEY ACTIVITY

Students may also be under the impression that all the dinosaurs that they are aware of lived at the same time. In fact, the T. rex is closer in time to us in our modern day lives than it is to the Stegosaurus, which became extinct nearly 80 million years before the emergence of T. rex.

The ‘year of life’ timeline A ‘year of life’ timeline could be constructed (see Table 9.1) to model the history of life on Earth compressed into a single year. Students could undertake this activity through group discussion, attempting to reach a consensus on where to put each event on the timeline; listening in to the group conversations will give insights into students’ thinking. Table 9.1  Key events for a ‘year of life’ timeline (all dates approximate, based on reported estimates) Event

Years ago

Position in imaginary year

the Earth is formed

4.5 billion

1st January

first living (unicellular) organisms

3.8 billion

25th February

the oldest fossils ever found are formed

3.5 billion

21st March

first cells with a nucleus

2.1 billion

14th July

first land plants

850 million

24th October

first mammals

200 million

15th December

extinction of the dinosaurs

66 million

22nd December

first modern humans (Homo sapiens)

350 thousand

31st December, 11:19 p.m.

present day

0

31st December, midnight

Note: A billion is defined here using the ‘short-scale’ definition of one thousand million (109).

Maths If students struggle to conceptualise a billion, a simple challenge to their thinking may be helpful: ask them to estimate a thousand seconds, a million seconds and a billion seconds in other units of time. A thousand seconds is just under 17 minutes; a million seconds is 11.5 days; a billion seconds is almost 32 years.

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Selective breeding: evidence that selection can cause species to change over time Selectively bred plants and animals are essential for human food security, but also played an important part in the development of the theory of evolution by natural selection. Selective breeding has produced varieties of plants and breeds of animals that are quite different from their wild ancestors. This was noted by Charles Darwin in the first chapter of On the Origin of Species, in which he discusses the ability of ‘artificial selection’ (as he called it) to cause changes in species over generations. This is part of the evidence that led him to theorise that a process of selection in nature could also cause changes in species, and could have created new species.

Science in context Ask students whether they have ever seen a wild cow – the answer is no! Domesticated cows that could be put to work and farmed for meat and milk were selectively bred from wild cattle called aurochs, horned herbivores about the size of a bison. Aurochs became extinct in 1627, replaced entirely by the selectively bred cows and bulls we recognise today.

Darwin and others: developing a scientific explanation for evolution A simplified account of the development of the theory of evolution by natural selection can be used to illustrate how scientists can develop a scientific explanation by: ➜ asking

a question about the natural world observations and collecting evidence ➜ suggesting an explanation to account for the evidence ➜ taking account of other scientists’ work, and sharing their suggested explanation ➜ modifying the explanation as new evidence becomes available. ➜ making

In the nineteenth century, Charles Darwin, Alfred Russel Wallace and many other naturalists before them had wondered if there was a scientific explanation for how the vast diversity of species arose. Darwin and Wallace collected specimens and made observations of species living in different places, Darwin famously on a round-the-world trip aboard the HMS Beagle, and Wallace notably in Singapore and Indonesia. They collected evidence of differences between fossils and living examples of organisms, of differences between individuals within populations, and of adaptations that helped related but isolated species to survive in their particular environments (for example, the Galápagos tortoise). 212

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Science in context The Galápagos tortoises are native to seven of the Galápagos Islands. On humid islands with highlands, the tortoises are larger, with domed shells and short necks. On others, with dry lowlands, the tortoises are smaller, with ‘saddleback’ shells and long necks. Darwin and Wallace, working independently, drew together some key ideas to explain their observations. These ideas became the backbone of the theory of evolution by natural selection: ➜ Variation:

there are differences between members of a species, some of which can be inherited. ➜ Competition (or ‘the struggle for existence’): over-production of offspring means organisms compete with one another for limited resources, including mates and sources of nutrients and energy. ➜ Fitness and natural selection: some individuals have features that give them a better chance of surviving to reproduce, especially when environmental (biotic and abiotic) conditions change, and these individuals are more likely to pass on heritable features to their offspring. Darwin and Wallace’s ideas were shared with the scientific community in the 1850s, and Darwin’s book On the Origin of Species was published in 1859. Originally, the theory of evolution by natural selection could not explain what caused variation or how it was inherited. Since then, the theory has been modified and improved by the work of many scientists to include ideas about inheritance, DNA, genes and genomes. The story of Darwin and Wallace provides historical insight, can increase engagement and can help to develop students’ appreciation of how scientific explanations have been developed. Rich, open-access online collections of information related to Darwin and Wallace are available, including many of their original notes, drawings and letters (see the websites section at the end of the chapter). Many excellent suggestions for ways to use Darwin in lessons are presented in a collection of essays from the Charles Darwin Trust entitled Darwin-Inspired Learning (2015).

Consolidation of the key ideas: variation, competition, fitness and natural selection An important learning objective of teaching about evolution in secondary science is to develop students’ understanding of the key ideas that biologists use to explain evolution, including: variation, competition, fitness and natural selection. At first, it may be appropriate to explore these ideas only at trait (phenotype) level, as Darwin did, to help students develop a secure understanding of the concepts and how their interactions lead to evolution. 213

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9 Evolution

Modelling natural selection A simple activity can be used to model the key natural selection ideas. Red and green cocktail sticks are used to model a population of insects in which there is variation in colour. A square of grass (around 15 m × 15 m) is marked out to model a habitat. The cocktail sticks are randomly distributed within the square. A group of students act as ‘birds’ feeding on the red and green ‘insects’, competing to collect as many ‘insects’ as possible in 30 s. From the results, students should be able to draw out the ideas that the green ‘insects’ have a selective advantage in this environment and that the ‘bird’ predators vary in their ability to detect prey (red/green colour blindness is particularly disadvantageous). Ask students to think about possible outcomes for the insect and bird populations over a longer period of time. When students are secure in their understanding of the key ideas of variation, competition, fitness and natural selection, they can begin to appreciate and explore some of the implications of them, including that: ➜ advantageous

features become more common in subsequent generations due to natural selection ➜ populations of the same species may evolve differently, especially if they are isolated and/or exposed to different conditions, and thus new species with different adaptations may evolve from existing species ➜ if we imagine pressing the ‘rewind’ button on evolution, we might see that all species evolved from earlier simpler forms, and that different species can share a common ancestor. Once students have learned something about the structure and function of the genome, it may be appropriate to consider the key ideas at the genetic level, including random mutation as a cause of heritable variation, how the natural selection of advantageous traits leads to the passing on of alleles associated with these traits to subsequent generations, and thus that these alleles will become more common in a population over a number of generations.

Controversy? The scientific explanation and students’ worldviews Evolution is sometimes regarded as a controversial topic in teaching. Evolution itself is not controversial; fossils and other evidence show that the characteristics of species change over generations. What has been subject to controversy is the theory of evolution by natural selection – a scientific explanation for the observed changes in species, and for how the vast variety of species arose. The theory is strongly supported by evidence and is widely accepted by scientists. However, bones of contention in the wider discourse have included the perception that the theory implies the lack of a ‘Creator’, the timescales involved (and implications for the age of the Earth), and the

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idea that humans evolved from simpler (or ‘lower’) forms. In a 2017 poll conducted by YouGov, 64% of surveyed adults in the UK (n=2129) found it very easy, easy or somewhat easy to accept evolutionary accounts of the origin of species (including humans). Among those adults who identified as religious or spiritual, the figure was 53%. Learning about natural selection as a scientific explanation for evolution need not conflict with religious beliefs; for example, the theory does not make any claims about how life started – rather, it explains the development and diversity of life after it arose. But for some students, particularly those with literalist religious beliefs, learning about evolution in biology lessons may leave them feeling conflicted. It has been suggested that evolution should be treated as a sensitive issue rather than a controversial one (Reiss, 2019a), and that students’ ‘worldviews’ – complex collections of concepts that help us to understand as many elements of our experiences as possible, and which can contain incompatible ideas – should be accommodated respectfully.

Tip Rejecting a student’s religious views or forcing them to choose between science and religion is not likely to help them understand the scientific explanation for evolution, but biology lessons can help all students to appreciate how this explanation was developed from evidence and why the great majority of scientists (including many with religious beliefs) therefore see it as robust.

Cross-disciplinary Teaching strategies for evolution should be discussed and agreed within your school’s science department, and possibly also with the religious studies department. In some schools, strategies may have to be approved by the senior leadership team.

Further activities Modern examples of natural selection in action can be used to engage students, and to consolidate and check their understanding. Useful examples are: ➜ colour

change in populations of the peppered moth (Biston betularia) as a consequence of air pollution during the Industrial Revolution ➜ the emergence of antibiotic-resistant bacteria (such as MRSA) ➜ increasing beak length in great tits (Parus major) as a result of bird-feeder usage in the UK ➜ the discovery of a bacterium (Ideonella sakaiensis) that survives by digesting man-made plastic. 215

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All of these are examples of changes in the frequency of a trait within populations of a species, in which human activity indirectly caused a selection pressure. Diagnostic questions and activities can be used to probe students’ thinking about evolution, for example at the start of teaching the topic to reveal preconceptions from everyday life, and during teaching to provide evidence of learning and of misunderstandings. The evidence they provide can be used formatively to decide how best to help students progress towards understanding the scientific explanation for evolution. Learning about evolution by natural selection is rife with misunderstandings, which could be the subject of diagnostic questions. Newall (2015) groups these into four broad categories as follows. 1 Teleology and anthropomorphism: it is common for students – and grownups! – to think and use language that implies that adaptations arise by design, intention or in order to fulfil a need (teleology), and that evolving organisms are clever or ‘want’ to adapt (anthropomorphism). Richard Dawkins described natural selection as a ‘blind watchmaker’ in his book of the same name, because evolution does not plan in advance but can build functional structures of incredible complexity. See Section 9.2 (Adaptation) for a fuller discussion of teleology and anthropomorphism. 2 Lamarckism: this is belief in the inheritance of acquired characteristics, whereby evolution proceeds because organisms pass on characteristics they have acquired through use or disuse during their lifetime. Such ideas are wrong, and examples such as a weightlifter’s muscles or even something as simple as a scar can be used to probe and challenge students’ Lamarckian ideas. A similar and common misunderstanding is that changes within an individual’s lifetime constitute evolution (they do not; evolution refers to changes in populations over generations). 3 Terminology: terms such as ‘evolution’, ‘fitness’ and ‘mutation’ are used in everyday life in ways that do not reflect the scientific usage. The phrase ‘survival of the fittest’ is often used without (or in order to avoid having to demonstrate) understanding of the mechanisms involved. Many students incorrectly think ‘fittest’ refers to the most athletic or strongest individuals rather than to the best adapted. 4 Views about the status of humans: students may incorrectly think that evolution has finished, that the present form of a species (particularly humans) is the final or perfect form, or that humans are exempt from evolution by natural selection. The notions that life on Earth evolved to support humanity and that humans therefore have the right to abuse ecosystems are dangerous misconceptions.

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Scientific literacy The phrase ‘the theory of evolution by natural selection’ is often abbreviated to ‘the theory of evolution’, incorrectly implying that evolution itself is theoretical. The scientific use and meaning of the term ‘theory’ is commonly misunderstood in classrooms (Williams, 2013). In everyday use, ‘a theory’ often refers to something that is theoretical in the sense that it is unproven or untested. A scientific theory is an explanation that applies to a large number of situations or examples, and which has been tested and evidenced by collecting data.

9.2 Adaptation Prior knowledge and experience From earlier formal teaching, students will be familiar with the idea that animals and plants have features which allow them to do certain things well. For instance, students may have investigated survival in cold climates or may have compared the teeth of carnivores and herbivores. Students are often encouraged to ask questions about ‘why’ something is the way it is, or asked to make predictions before investigating something, such as heat transfer and insulation. Some students will bring knowledge, sometimes a considerable amount of it, from their experiences outside of school. Most often these come from trips to zoos or aquaria, reading books or watching nature programmes. Some of these students have a particular interest in one group of organisms (such as dinosaurs). These experiences and interests provide students with many examples of adaptations. However, the thinking behind them (particularly in terms of how they came about) and the language used to describe them is likely to need moving to something more scientifically appropriate.

A teaching sequence When this topic is first introduced it is worth starting with students sharing their examples of adaptations. These can be used to differentiate between an individual organism adapting (for example, dilation of pupils in dim light) and adaptations that have evolved. How adaptations evolve can then be taught (see the previous section of this chapter). Students can then practise using the precise terminology to describe the evolution of adaptations with some of their own examples. Further examples of adaptations can be provided to enthuse students about the amazing things that life does. Students can practise explaining each 217

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example, allowing assessment of, and feedback on, their understanding of how adaptations evolve and their use of appropriate terminology. Once students are adept at using the appropriate terminology, this topic provides a great opportunity to help students develop their questioning and thinking skills. While this topic, when taught as a topic, tends to focus on adaptations for specific habitats (for example, camouflage or mimicry, carnivorous plants, Arctic vs desert animals, hydrophytes vs xerophytes), the concept of adaptation runs throughout biology education (and research). Many students thoroughly enjoy exploring ideas of how something evolved and what adaptive value it has. This enthusiasm can be exploited to enrich the teaching and learning in any topic (such as enzymes, DNA, organelles, mass transport systems, gas exchange) rather than reserving that way of thinking exclusively for the topic of ‘Adaptation’. The relationship between the environment and adaptations is an interesting one. The biotic and abiotic factors in the environment provide selection pressures which, by the process of natural selection, result in adaptations in species. This process occurs over a long time period. On a shorter timescale, the adaptations species have (including their ability to disperse) determine where successful populations of them are likely to be found. This idea is at the heart of ecology. Students often come to adaptations first from an ecological angle. Then they learn how adaptations are generated by natural selection acting on variation within a species. Later, students learn to describe this variation and the process of natural selection in terms of genetics. As well as developing students’ understanding of adaptation, there is the opportunity in this topic to develop students’ questioning, logical thinking and investigative skills. In that order, students can develop their ability to think like a research scientist: ask a question, hypothesise one or more logical answers, come up with a way to investigate which (if any) answer seems to be correct. Having come to adaptations from an ecological angle, students are already likely to have thoughts on what an adaptation allows a species to do. However, there are many other questions that can be asked, such as: ➜ How

might that adaptation have evolved? Do we have any evidence from the fossil record? ➜ Why do some species in that environment have that adaptation and others do not? ➜ Why might two different species with similar adaptations have differences in the details of the adaptation? Through encouraging this line of thinking at a ‘habitat’ level, students can be led towards questioning other biological topics, particularly those that 218

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require an understanding of ‘structure and function’ along similar lines. For example, they could explore exchange surfaces as adaptations or consider why transport systems evolve and the similarities and differences between transport systems in animals and plants. This could link with a consideration of surface area : volume ratios in Chapters 2 and 4. Following on from that, students can be encouraged to question biology at a much more profound level: ➜ Why

is life cellular? do some cells have a nucleus and others do not? ➜ What are the advantages of being multicellular? ➜ What challenges does multicellularity impose on a species and what adaptations have evolved because of these selection pressures? ➜ Why

The skills students develop through these lines of enquiry are likely to be transferable. They are useful wherever situations require an inquisitive mind, a reluctance to take things at face value, an ability to suggest explanations and an ability to suggest ways to find out which explanation is correct. On top of this, students with this approach are likely to increase their enthusiasm for biology as more and more of life’s elegant solutions to its problems are studied.

What adaptations do students know already?

KEY ACTIVITY

This topic could start with a collation of examples of adaptations from the class by using an open task that is broad enough to include things that are not adaptations as well as eliciting plenty of adaptations.

Thinking about adaptations Ask students to do something similar to the following: ‘List things that animals or plants have, or things that they do, that make them good at living in different places and good at doing the different things they do’. Doing this in pairs, or small groups, with mini-whiteboards encourages discussion and commitment to writing something down to share initial ideas.

Science in context To ensure that students think broadly, it helps to give a few examples at the start. These should be accessible to students considering their prior knowledge. Examples could include the following: l people

visiting high altitudes for several weeks adapt to the lower oxygen content in the air by producing more red blood cells l the leaves of some plants have ‘hairs’ which reduce water loss by evaporation l plants grown in nutrient-poor soils invest a higher proportion of their resources in root growth than individuals of the same species grown in nutrient-rich soils

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l plants

adapt to limited water availability by closing stomata entering a dark room, human eyes take a few minutes to adapt to the lower light intensity l dolphins have a streamlined shape which reduces the effort needed to swim l chameleons can quickly change the colouration and patterns on their skin to blend in with their background. l on

A quick way to collate the class responses involves all students standing up and the teacher going round the class comparing answers, noting similar ideas and novel answers. Students cross off ones that have already been said from their own list and sit down when no novel answers remain on their whiteboard. Only students who remain standing are asked again. The teacher could collate the responses by compiling two lists (those that are adaptations and those that are not) without telling students the reasoning behind the division. Then students discuss the reason for having two lists as well as common features of members of each list. This activity exposes the difference between adaptations and individuals adapting to different environments. The characteristic of being able to adapt to different environments is an adaptation in itself and so statements about ‘adapting’ can sometimes be interpreted as either adaptation or adapting. If this causes disagreements between students then the activity is likely to have done its job!

Teleology and anthropomorphism One of the trickiest aspects of talking and writing about adaptations is the use of language. It is very easy to discuss adaptations in a way that can be misleading about how they came about. Inadvertently using phrases such as ‘The coat of the mountain hare turns white in winter in order for the animal to be camouflaged in snow-covered country’, ‘The leaf of a plant is designed to bring about efficient gaseous exchange’ or ‘The walls of the alveoli need to be thin so that diffusion of gases is efficient’ can give students the impression that there is a sense of purpose involved, a deliberate intention to make something to achieve an objective. Indeed, this is often how the word ‘adapted’ is used in everyday language; for example, ‘An old chimney pot can be adapted for use as a plant container’. When statements indicating intent are used about aspects of biology, they are said to be teleological. Many teachers, and indeed scientists, use phrases like these as a shorthand way to link structure to function when discussing adaptations; however, caution is needed when students are first learning about the mechanism by which adaptations evolve. Another language tool that teachers, scientists and particularly reporters of scientific discoveries use is anthropomorphism. That is where human emotions, needs or competencies are attributed to other living things such as ‘the mountain hare knows that winter is approaching and grows a white coat’ or ‘a virus tricks the cell into copying its genetic code’. This language is used to engage people with the biology by making it easier to relate to what is going 220

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on. However, as with teleology, caution is needed when using this terminology with students. It may well be best to tackle these language pitfalls head on. Providing students with a variety of phrases to critique and then to rewrite using scientifically accurate language (in other words, without using teleology or anthropomorphism) would help students avoid misinterpretation when they inevitably read or hear less scientifically precise language. Using the mountain hare (Lepus timidus) example, three statements to critique, followed by a more scientifically appropriate version, are given below: ➜ ‘The

coat of the mountain hare turns white in winter in order for the animal to be camouflaged in snow-covered country.’ (Teleological: it might give the impression that the hare does it purposefully.) ➜ ‘The mountain hare knows that winter is approaching and grows a white coat.’ (Both teleological and anthropomorphic as it might be taken as meaning that the hare is consciously aware of what is to come and makes a purposeful decision in response to that knowledge.) ➜ ‘Mountain hares change the colour of their coat to adapt to the changing background colour of the landscape when it snows.’ (Due to the use of the word ‘adapt’, this statement’s focus is the individual adapting rather than on the adaptation (the ability to change coat colour) that has evolved in this species. The statement is also teleological.) ➜ ‘Mountain hares that have inherited a tendency to grow a white coat at the approach of winter (triggered by a change in day length) are less likely to be eaten and so leave more descendants, leading to the spread of the characteristic through the population.’ (This statement is more scientifically appropriate and is what students should be aiming to be able to do. It is, admittedly, pretty long-winded, which is why statements similar to the first one are often used instead.) Sometimes activities are purposefully teleological such as ‘Design an animal that lives in … and feeds on … Say why you gave it the features you did’. The activity is good for linking structure to function but to address the teleological issues of the activity, the students could be asked to suggest how each feature could have evolved. It is important to get students to practise using the appropriate language when describing adaptations and suggesting how they might have evolved. This also provides a wonderful opportunity to enthuse students about the bizarre and incredible adaptations that exist as well as opening their minds to the diversity of life beyond the often mammal-centric view of students. Some suggestions are: ➜ caterpillars

(for example, Hemeroplanes triptolemus) that look like snakes releasing volatile organic compounds to attract carnivorous insects that eat the herbivores that eat the plant (for example, Brassica spp.)

➜ plants

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sensitive plant (Mimosa pudica) folding up its leaves when touched ➜ earwigs (Forficula auricularia) showing parental care ➜ bombardier beetles (for example, Brachinus spp.) producing a hot noxious chemical spray from their abdomen when disturbed ➜ acacia trees (some Acacia spp.) producing ethene when eaten that is detected by other parts of the plant, and even other plants, triggering them to produce toxins in their leaves ➜ extra-floral nectaries attracting ants in cherry trees (some Prunus spp.) and cherry laurel (Prunus laurocerasus).

Developing thinking skills through learning about adaptations Investigating adaptations is inherent in all biological research. Scientists are constantly asking ‘why’ something is the way it is, and the answer lies in understanding the adaptive value of the thing being studied (alongside evolutionary constraints and trade-offs). Encouraging students to constantly ask ‘why’ is a valuable tool in engaging them with biology, as well as helping them to develop a deeper understanding and better memory recall of the information being asked about. It is important to phrase questions in a precise way to elicit the thought processes wanted from students, and ‘why’ questions have two alternative (and equally correct) ways of being answered unless additional clarification is used. Sometimes, though, it may be of value to leave a ‘why’ question open ended to see if students can be agile in their thinking and come up with different answers. The two alternative answers to ‘why’ questions are termed ‘proximate’ and ‘ultimate’. ➜ Proximate

explanations are the immediate reason why something happens or is the way it is. (For example, the mountain hare’s fur turns white because the days are getting shorter; red blood cells don’t use the oxygen they are carrying because they don’t have mitochondria that use the oxygen in aerobic respiration.) ➜ Ultimate explanations are the adaptive reason why a characteristic has evolved. (For example, the mountain hare’s fur turns white as it maintains camouflage in the winter months and so fewer hares with this adaptation are eaten by predators; not using the oxygen in transit means that more oxygen can be delivered by each red blood cell, thereby providing a more efficient transport system.) When trying to explain the survival value of an adaptation and how it came to evolve, it is worth considering the thought process as a reverse engineering of what natural selection has produced. The environment has imposed a selection pressure that has acted on variation within a species, over many generations, to produce an adaptation. To understand the evolution of an adaptation we can see the adaptation, we know the mechanism which created it (natural selection), but we have to try to discover the selection pressure that 222

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led to its evolution. Students can be encouraged to come up with their own suggestions for the survival value of an adaptation and how it has evolved but also to suggest ways to investigate whether their hypothesis is correct. Exposing students to different ways of exploring the evolution of adaptations and then providing students with enough information to have a go for themselves can help students develop their logical thinking and investigative skills. Scientists use careful observation and experiments to see whether the survival value hypothesised stands up to scrutiny, or for comparisons of the characteristics of different species to generate a logical argument for a particular selection pressure. Richard Dawkins (2009) describes an excellent example of using careful observation and experiment in his book, The Greatest Show on Earth. This is the work of John Endler on colouration in male guppies and how bright colouration attracts females but when predation is high, camouflaged colouration evolves. The logical thought process involved in suggesting a selection pressure by comparing different species is wonderfully described by Steven Vogel (1992) in his book Vital Circuits: On Pumps, Pipes and the Workings of Circulatory Systems. He compares birds and mammals with alligators and insects to explore the evolution of a closed, double circulatory system in birds and mammals, concluding that gas transport is the most demanding circulatory function and that this is particularly demanding in ‘warm-blooded’ animals. Another approach to investigating adaptation is to look at a particular selection pressure and to consider the range of impacts this has on the evolution of adaptations across all life. As an example of this way of thinking Matt Wilkinson (2016), in Restless Creatures, explores the impact that the selective advantage which locomotion provides has had on a wide range of organisms and organ systems. Along these lines, students could be asked about how the selection pressures involved in the colonisation of the land may have led to certain adaptations in both plants and animals. (They could think about the need for support, a different form of locomotion, dehydration and reproduction, among others.) Another important feature of adaptations, and one that again provides the opportunity to develop thinking skills, is that evolution does not act on a blank canvas. It can only act on variation present in a population. So, if a common selection pressure acts on different groups of organisms, adaptations that perform the same function can evolve in different ways. The ability to fly is clearly a selective advantage in different groups of organisms and wings have evolved several times (birds, bats, pterosaurs, insects) but they all look different. Students could be asked to explain why. The evolution of one particular adaptation cannot be taken in isolation. Being even better at one thing might mean being poor at another. Trees, for instance, could invest a lot more in strengthening trunks and anchorage by the root system and be able to survive much stronger winds than they do. However, 223

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investment there means that there is less to invest in producing seeds and so, given that extremely strong winds are rare, there is a selective advantage to be strong, but not too strong. Students could be asked to explain why certain adaptations have not gone to great extremes or do not exist; this could be due to some trade-off occurring or because sufficient variation does not exist in the population for natural selection to act on.

Adaptations across all of biology Beyond the lessons used specifically to teach about adaptations, taking advantage of discussing adaptation across the whole of biology is useful to help students embed the thinking and questioning skills mentioned previously, as well as to enthuse students about biology. Table 9.2 contains some ideas that could be used to stimulate discussion among students, or to enrich the teaching of a topic. The use of these ideas within their own topic helps to demonstrate the importance of evolution in understanding biological ideas. Table 9.2  Ideas for stimulating discussion in other topics Topic area

Discussion points What is the selective advantage of having a cell-surface membrane? The answer here lies in the value of compartmentalisation and being able to keep hold of water-soluble molecules that have been made.

Cells and organelles

Why do some cells have a nucleus and others don’t? Possibly as a tool to separate transcription from translation needed because eukaryotes have introns that need cutting out before translation whereas prokaryotes do not. Nick Lane discusses the evolution of cell-surface membranes and the nucleus in The Vital Question (2015).

Enzymes

DNA

Gas exchange

Organ systems

Enzymes that have iron and sulfur ions as part of their active site and are universal to all life may have evolved as a way of life taking the catalytic activity of the geology in which it first evolved with it as it ‘escaped’ its rocky confines (see Lane, 2015). Why is genetic information stored as DNA and not RNA? This may be due to the stability of DNA compared to RNA and so linked to the different roles of DNA and RNA. Why does the genetic code match particular codons with particular amino acids? Nick Lane discusses the genetic code in Life Ascending (2009). How could lungs in terrestrial vertebrates first evolve? Students could suggest selection pressures as well as suggesting variation that could be acted on (such as differences in vascularisation of parts of the alimentary canal). Why don’t land-living mammals have gills? This should highlight the idea that the structures of the gill that provide a large surface area (gill filaments and lamellae) in water clump together in air and so don’t allow sufficient gas exchange. What problems were generated by the evolution of multicellularity and what adaptations evolved as a consequence? This can be asked about both plants and animals and covers transport systems, gas exchange, cell signalling and much more.

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Topic area

Discussion points

Ecology

If an organism is the best adapted to its habitat, why isn’t it found dominating that habitat everywhere? This could be about the limits and randomness of dispersal as well as, in plants, the idea that young plants cannot usually outcompete older plants, even when of a different species.

Evolution

How can new adaptations evolve if an organism already needs to possess adaptations to be able to be successful in a particular habitat? This can be answered by the idea that habitats change (in both their biotic and abiotic factors). Another consideration is that where a niche is vacant (for example, the first colonisation of land by plants) an organism does not have to be very well adapted to the habitat just so long as it can survive to reproduce successfully. Adaptations to that habitat can then evolve by natural selection. This highlights an important point that being well adapted is a relative thing: organisms need to be well adapted enough to survive (and reproduce) considering other species that are present (such as pathogens, predators, parasites and competitors) and abiotic factors. Natural selection then selects the ‘best’ of whatever variation exists within a species even if no individuals are ‘brilliant’.

Photosynthesis

Why aren’t plants black? Why don’t they use the green light as well as the other colours? One possible answer to this is problems with overheating if too much light energy is absorbed and that light availability may not be the limiting factor for growth anyway.

Diet and digestion

Why haven’t we evolved to digest cellulose? Maybe there hasn’t been the appropriate variation in humans that would have enabled this adaptation to evolve.

9.3 Resources General resources ➜ Fossils

and other preserved specimens, to enable students to explore adaptations and the similarities and differences between species. ➜ Identification guides and other fieldwork apparatus, to enable students to investigate adaptations in local habitats. ➜ Clips from natural history television programmes, to illustrate the incredible variation and adaptation of life on Earth; examples include Planet Earth, Blue Planet, Life in the Undergrowth and The Private Life of Plants.

Websites A Stegosaurus brought to life (Natural History Museum). The full story of how Sophie the Stegosaurus was reconstructed from the most complete fossil remains ever discovered, including videos in which Sir David Attenborough explains what the fossil skeleton can tell us about how Sophie moved: www. nhm.ac.uk/discover/stegosaurus-brought-to-life.html Best Evidence Science Teaching (BEST): free diagnostic questions and activities to help build students’ understanding, developed from research evidence: www.stem.org.uk/best-evidence-science-teaching 225

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Darwin and Wallace Online: rich online collections of information related to Charles Darwin and Alfred Russel Wallace, including many of their original notes, drawings and letters: http://darwin-online.org.uk and http://wallace-online.org/ The Fittest: a simulator in which stick creatures of various shapes compete in a race. After each round, the one that made it the furthest gets to reproduce, and its offspring race again. Over time, mutations and natural selection lead to faster and faster creatures: https://testtubegames.com/fittest.html Tree of Life: an interactive tool to explore the evolutionary links between living things with Sir David Attenborough (Wellcome Trust): www.stem.org.uk/ elibrary/resource/30498

References Borgerding, L. A. and Raven, S. (2018) Children’s ideas about fossils and foundational concepts related to fossils. Science Education, 102 (2), 414–439. Dawkins, R. (2009) The Greatest Show on Earth: The Evidence for Evolution. London: Bantam Press. Dobzhansky, T. (1973) Nothing in biology makes sense except in the light of evolution. American Biology Teacher, 35 (3), 125–129. Hunter, J. C., et al. (2018) Capitalizing on pre-existing student engagement with fossils: a gateway to generate student interest, participation, and learning. Education, 139 (1), 19–37. Newall, E. (2015) Routes to conceptual change in teaching and learning about evolution: experiences with students aged between 11 and 16 years. In: Boulter, C. J., Reiss, M. J. and Sanders, D. L. (eds) (2015) Darwin-Inspired Learning. Rotterdam, The Netherlands: Sense. Reiss, M. J. (2019a) Evolution education: treating evolution as a sensitive rather than a controversial issue. Ethics and Education, 14 (3), 351–366. Trend, R. (1998) An investigation into understanding of geological time among 10- and 11-year-old children. International Journal of Science Education, 20, 973–988. Vogel, S. (1992) Vital Circuits: On Pumps, Pipes, and the Workings of Circulatory Systems. New York: Oxford University Press. Williams, J. D. (2013) “It’s just a theory”: trainee science teachers’ misunderstandings of key scientific terminology. Evolution: Education and Outreach, 6 (12), 1–9.

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Further reading Boulter, C. J., Reiss, M. J. and Sanders, D. L. (eds) (2015) Darwin-Inspired Learning. Rotterdam, The Netherlands: Sense. Darwin, C. and Costa, J. T. (2011) The Annotated Origin: A Facsimile of the First Edition of On the Origin of Species. Cambridge, Massachusetts: Harvard University Press. (The first edition of 1859 complemented by an accessible guide and additional insights from a working field biologist and evolutionary theorist.) Dawkins, R. (1986) The Blind Watchmaker. Harlow: Longman. Jones, S. (1999) Almost Like A Whale: The Origin of Species Updated. London: Doubleday. (A modern re-telling of the Origin of Species, with up-to-date evidence and genetic perspectives.) Lane, N. (2009) Life Ascending: The Ten Great Inventions of Evolution. London: Profile Books. Lane, N. (2015) The Vital Question: Why is Life the way it is? London: Profile Books. Poole, M. (1995) Beliefs and Values in Science Education. Buckingham: Open University Press. (See Chapter 7, ‘Darwin in context’.) Reiss, M. J. (2019b) Evolution: as a religious professor of science education, we need to rethink how we teach it [Online]. The Conversation. Available at: http:// theconversation.com/evolution-as-a-religious-professor-of-science-educationwe-need-to-rethink-how-we-teach-it-118311 Vogel, S. (2013) The Life of a Leaf. Chicago: University of Chicago Press. Wilkinson, M. (2016) Restless Creatures: The Story of Life in Ten Movements. London: Icon Books.

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10

Biodiversity Marcus Grace and David Slingsby

Topic overview Biodiversity is one of those words that everyone has heard of and knows is somehow important, but which most people do not quite understand. Any attempt to define it as, for example, ‘the variety of life both within and between species’ is somehow inadequate. But once someone has seen, either in person or on film, the abundance of life on a coral reef, in a rainforest or in a sample of clean stream water from a nature reserve, they are more likely to feel a sense of awe at the wonders of nature and the incredible biodiversity that can be found on this planet. Then the science becomes exciting because it helps to make sense of it in the context of how ecosystems work. Many secondary students will have already looked at a variety of organisms in primary school so it will be important to build on this and carry it further.

Science in context Biodiversity is virtually everywhere on the Earth’s surface. It is literally on our doorstep: there might be microscopic creatures such as tardigrades living in small clumps of moss; and of course, whether we like it or not, there are a large variety of organisms living in and on our own bodies. Students may be fascinated to know that recent estimates suggest that we have three times as many non-human cells as human cells (Sender et al., 2016). The greatest biodiversity is probably found in tropical rainforests. It has been estimated that tropical forests comprise only 6% of the world’s surface area and yet contain between one-half to three-quarters of the Earth’s species of plants and animals. Rainforests are being destroyed by human activity, particularly through logging and large-scale agriculture. Rates of species extinction are rising and so biodiversity is decreasing. There are probably many species which will become extinct before anyone discovers them.

Science in context Ask students how many species of animals and plants there are on the Earth today. The short answer is that we don’t know. The number of species that have been recognised has been estimated at between 1.5 and 1.8 million (Rainforest Conservation Fund, 2019). Since new species keep being discovered, the total is likely to be much more than this, with estimates ranging from 10 to 30 million. A lot anyway!

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Prior knowledge and experience Although students may not arrive at secondary school knowing the term ‘biodiversity’, they are likely to know the word ‘habitat’ and should be able to identify some common animals and plants from various local habitats. They will have observed how some of these depend on each other as sources of food and shelter and may be able to construct some simple food chains (such as grass → cow → human). They will have used basic keys to sort and classify plants, animals and micro-organisms into broad groups based on observable characteristics, and they should be able to appreciate that the environment can change and that this can endanger some organisms while benefiting others. At secondary school, students develop a grasp of the importance of maintaining biodiversity and the use of gene banks to preserve hereditary material. While students generally have a good understanding of biodiversity as being the variety of species, they often have considerable difficulty explaining why protecting biodiversity is important.

A teaching sequence As discussed in Chapter 1, one of Wynne Harlen’s ‘big ideas of science’ is that ‘The diversity of organisms, living and extinct, is the result of evolution’. Life continues to adapt and change due to the process of evolution through natural selection (see Chapter 9), and the complexities of biodiversity soon become apparent when we ask why it is important and how we can set about measuring it. A suggested way of progressing understanding at secondary school is as follows: ➜ 11–14-year-olds

might collect data about biodiversity in one or more habitats using a variety of appropriate ecological methods. This would be accompanied by class discussions about why biodiversity is important to the populations themselves and to people and society. At this level, the term ‘biodiversity’ could refer to the number of different species in a particular area: what older students might refer to as ‘species richness’ (see below). ➜ 14–16-year-olds might explore biodiversity in one or more habitats using a more statistical approach and could also begin to consider the relationship between biodiversity, ecosystem stability, feeding the world’s human population and climate change. At this level, more able students could be introduced to the idea that the biodiversity of a particular area is also called species diversity and is a combination of two measurements: species richness and species evenness (see page 248).

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10.1 What is biodiversity and why is it important? In general terms the word ‘biodiversity’ refers to the variety or number of species within a particular area, habitat, ecosystem or the whole biosphere. However, the definition can be more complex than this and can include the diversity of genes within species. Variation in species due to environmental factors may not strictly constitute biodiversity unless the plasticity – the capacity to respond to environmental factors in certain ways – has itself a genetic basis. Some animal species, such as chameleons and octopuses, for example, can change colour in response to different coloured backgrounds. Changing colour against a green, brown, yellow or red background is a purely environmental response, but the ability to respond in this way is the product of natural selection.

How did all these species appear?: speciation arising through natural or artificial selection As with human populations, groups of individuals of the same species in a particular area are generally referred to as ‘populations’, and populations can evolve to become distinct species through a process called speciation. The term ‘species’ is actually hard to define because the ongoing evolutionary processes cause continual changes within populations. A familiar and traditional definition of a species is a group of organisms in which individuals can sexually reproduce to produce fertile offspring. Following on from this, it was generally thought that sexual reproduction between individuals from different species could only result in hybrid offspring which are sterile (for example, a mule is the sterile offspring of a male donkey and a female horse). However, recent advances in DNA analysis have shown that hybrid speciation is actually quite common in both plants and animals, where two closely related species can reproduce to create a new species which is fertile. To thwart the definition further, some species only reproduce asexually. But despite these problems with definitions, species can usually be readily identified, and scientists generally try to work with the concept of species as a recognised ‘unit of biodiversity’. This is a topic where just how much detail is appropriate for a particular class is, of course, something for the sensitivity and judgement of the teacher. Older students should understand the steps that gives rise to new species. Given enough time (which may be thousands or millions of years), due to random

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genetic mutation and environmental pressures, a population may change to the extent that it becomes a new species and is unable to reproduce successfully with individuals from the original species. However, it is worth noting that speciation can also be induced by artificial selection: crop plants and domestic animals can differ from their wild ancestors to the extent that they can no longer interbreed. There are two main types of speciation: allopatric and sympatric. In allopatric speciation, a population is divided into two isolated populations by geographical or ecological barriers: for example through continents drifting apart, mountains rising or rivers changing course; or though habitat fragmentation; or just by being separated by vast distances. This is exemplified by the finches on the Galápagos Islands, made famous by Darwin (and hence known as ‘Darwin’s finches’) who noticed that they differed from one island to another and figured that they had become separate species as a consequence of their isolation from each other. Sympatric speciation is the formation of new species which remain in the same geographical location as the original species.

Science in context Students could consider, for an example of sympatric speciation, the apple maggot fly (Rhagoletis pomonella) in North America which has different populations that feed on different fruits. The original species used to feed on hawthorn, but a new and distinct population emerged in the nineteenth century when apple trees (a non-native species) were introduced. Now, there are populations that only feed on apples and populations that only feed on hawthorn, and they do not interbreed. Over time they have also developed other noticeable differences, such as the time of year at which they mature, and these behavioural barriers will separate the populations, thus further driving the process of speciation.

The importance of biodiversity The United Nations designated 2011–20 as the UN Decade on Biodiversity (United Nations, 2011), so it must be important. But why? It is thought that high levels of biodiversity help to make ecosystems more stable. The loss of species may threaten this stability and could lead to the collapse of the ecosystem. A useful analogy is to imagine an aeroplane in flight which has rivets holding on the wings. If one rivet pops out it probably won’t make any difference, and neither will the loss of two or three, but as you lose more and more rivets, there will come a time when the wing will fall off.

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Science in context Students might be interested to learn about an example of near ecosystem collapse which occurred with the cod fisheries in the North Sea in the 1990s. This near collapse was partly due to overfishing the cod itself, but the story is more complex, involving the North Sea food web, and required considerable scientific research to unravel. At the same time there was a decline in seal populations and in the populations of certain sea birds in Shetland, particularly puffins, kittiwakes and guillemots. Most of the food chains in the North Sea depended on small fish from a number of species, collectively known as sand eels. Overfishing of sand eels, mainly to supply salmon farms and pig farms in Denmark, was part of the problem (Jónasdóttir et al., 2010), but sand eels feed on the plankton Calanus, which itself feeds on microscopic algae. In the 1980s the Calanus was mainly of the species Calanus finmarchicus. This survived the winter in the deep water beyond the edge of the continental shelf and emerged into the shallow water of, for example, the Dogger Bank, as the sea warmed up in spring. Global warming caused this emergence into the shallows to happen earlier but before the phytoplanktonic algae – the primary producers on which the whole ecosystem’s energy input depended – had started multiplying in spring. As global warming increased, Calanus finmarchicus tended to move north (from the warmer south) but as it did so, another species of Calanus moved in from the south replacing it (Maar et al., 2013). This is an example of the importance of biodiversity in maintaining the stability of an ecosystem. The presence of more than one species of Calanus species, each with slightly different ecological requirements, helped to avoid collapse of the North Sea ecosystem. Some conservationists take the point of view that all species have a right to live and that biodiversity therefore has an intrinsic value of its own. In the past, scientific research relating to biodiversity conservation tended to concentrate on documenting the richness (and loss) of genetic, species and habitat biodiversity. However, international efforts have recently adopted a different approach by focusing on ‘ecosystem services’ (Raffaelli, 2017), which refers to the role biodiversity and natural systems play in supporting human health and wellbeing. Humans use different species in different ways (for food, medicine, clothing, firewood, and for constructing the buildings where we live and work, etc.), but we also value species and biodiversity for a wide range of social, economic, cultural and aesthetic purposes. Let’s look more closely at some of these and consider their implications: ➜ The

value of biodiversity for food. Three-quarters of the global food supply depends on just 12 crop species and five livestock species (Bioversity International, 2017). Some of our most popular foods are single varieties with

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limited genetic diversity and grown over large monocultural areas, and they are prone to devastating attacks from pests and diseases which could wipe them out completely. This means there is a strong argument for conserving a wide range of wild species just in case they might one day help protect our sources of food. ➜ The

value of biodiversity for medicine. At least 80% of people around the world depend on medicines derived from plants, animals and microorganisms (Chivian and Bernstein, 2008). Many vital drugs have been isolated from specific organisms, especially plants. Some are very common, like the foxglove (Digitalis purpurea), which provides medicines for treating heart conditions; others are more rare, such as the Madagascar periwinkle (Catharanthus roseus) which originally only grew in Madagascar and has been discovered to contain chemicals now used to treat cancer. But medicinal chemicals are not only extracted from plants: the antibiotics streptomycin and neomycin come from tropical soil fungi, and the venom of particular snake species has been used to treat a range of conditions including blood pressure and Type II diabetes. New medicines are continuously being discovered. The richer the diversity of life, the greater the opportunity for medical discoveries.

➜ Aesthetic

and cultural values. It is said that ‘variety is the spice of life’, and it is human nature to seek out and enjoy a diversity of shapes, colours, textures and sounds which ultimately enhance the quality of our lives. This is demonstrated by the large numbers of people who visit zoos, museums and botanic gardens, the popularity of wildlife and gardening programmes, and the increasing membership of wildlife organisations. Nature tourism is now one of the fastest-growing leisure activities among wealthier people. Wild species have inspired songs and poetry and are often used as symbols of a country’s heritage, as with the bald eagle in the USA and the kiwi in New Zealand. We do have a tendency to prefer colourful flowers and pretty, furry, intelligent animals; perhaps as biology teachers we should also be extolling the virtues of the less-loved organisms such as snakes, spiders, moths and slime moulds!

Careers Several careers require an understanding of biodiversity and its conservation. The GreenJobs website advertises job vacancies for such roles as biodiversity conservation officers, biodiversity science researchers, biodiversity managers, biodiversity monitoring experts and biodiversity policy experts. There are also an increasing number of biodiversity-related careers requiring technology skills, particularly in the field of biological monitoring.

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Science in context Students may be fascinated to know that 99% of all exported bananas are of a single variety – the Cavendish variety. In recent years, vast banana plantations have been seriously attacked by a particular strain of fungus. Researchers are therefore looking at other closely related wild varieties of bananas (and there are many) which might be resistant to the fungus, so that this genetic trait can be transferred to the Cavendish variety through cross-breeding or genetic engineering to create tasty but resistant bananas (Reynolds, 2018). Additionally, there may exist strains able to confer an ability to cope with less hospitable conditions such as areas of high salinity or extremes of temperature.

Why are some places more biodiverse than others? The biodiversity of an area can, of course, increase by animals flying, walking or swimming in, and the seeds of some plants are blown from elsewhere by the wind, or brought in by animals. Not all of these species will necessarily become part of the ecosystem. Another, more subtle, increase in biodiversity results from evolution (see Chapter 9). The reason tropical rainforests are so much more biodiverse than UK woodlands is that in most cases the rainforests have been there for many thousands or even millions of years, so they have had the chance to develop stability by accumulating more and more species and sub-species, both by invasion from other ecosystems and by evolution of species that were already there. In the UK, however, there were ice ages that destroyed all existing forests, leaving just some tundra in what is now southern England. The last ice age ended around 12 000 years ago and forest re-established itself around 10 000 years ago. At first there was conifer forest, but in much of England and Wales and lowland Scotland this was replaced by broadleaved species such as oak. From between 6000 and 5000 years ago, our Neolithic ancestors, some of whom built Stonehenge and similar developments in Orkney, began to carry out agricultural practices, which involved chopping down forests. By the reign of Queen Elizabeth I there wasn’t enough wood available for London from local coppiced woodland, so the citizens were already importing coal from Newcastle to use as fuel (until the queen banned it because she didn’t like the atmospheric pollution; the first recorded example of legislation against air pollution). Most of the UK forests which developed after the last ice age were destroyed by the twentieth century and had to be re-planted after the First World War (when the Forestry Commission was founded in 1919). Given all these factors, it is not surprising that the UK’s woodland and forests cannot even begin to compete with tropical rainforests for biodiversity! 234

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However, in the UK there are other examples of areas of relatively high biodiversity, such as chalk and limestone grassland. These have often been used by humans for sheep grazing for, in some cases, several thousand years, and the grazing has prevented the land from regenerating naturally into woodland. These thin soils have only short grass but can sustain an interesting variety of small flowering plants, giving relatively high biodiversity at least partly due to human activity over several thousand years. But even such biodiversity is recent and limited when compared to that of a tropical rainforest.

What are the causes of loss of biodiversity? Since life began, there have been at least five major mass extinctions. These are thought to have been caused by various natural phenomena such as global warming, global cooling, gamma-ray bursts, volcanism, asteroids and anoxic events (when parts of the ocean become depleted of oxygen). However, the current high level of extinction which has escalated over the last hundred years is generally agreed to be caused by human activity, and is estimated to be at least 100 to 1000 times higher than the natural background rate. Experts calculate that up to 0.1% of all species are becoming extinct each year (WWF; see the Resources section at the end of the chapter). We are familiar with the demise of certain prominent species, such as the dodo, the passenger pigeon and the Tasmanian tiger, and the extreme current threat to mountain gorillas and black rhinos. However, most of today’s seriously endangered species are unfamiliar to us, often being small and easily overlooked. According to the American biologist E. O. Wilson, who developed the concept of biodiversity and is sometimes called ‘the father of biodiversity’, the five main causes of species extinction and impact on biodiversity can be summarised using the acronym HIPPO. These are: Habitat destruction, Invasive species, Pollution, Population (over-population) of humans and Overharvesting by hunting and fishing. Climate change is, of course, directly connected to some of these human activities. The issue of invasive species is an interesting biodiversity conundrum as it raises questions about which species are more important. The issue is perhaps fairly clear-cut when one considers, for example, the accidental introduction of rats to remote islands from visiting ships. Rats spread diseases and eat the eggs and young of birds and mammals. In the Galápagos Islands, rats have decimated populations of the famous endemic giant tortoises, so there is general agreement that the rats should be exterminated. However, what about more endearing species such as hedgehogs (Erinaceus europaeus)? We hear that we have lost over a third of Britain’s hedgehogs since 2000. They are gardeners’ friends, eating slugs and other garden pests. However, hedgehogs which were introduced onto some islands off the 235

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northwest of Scotland to eat slugs escaped from gardens and discovered a new source of food – the eggs and chicks of ground-nesting birds. It is thought that hedgehogs are responsible for up to half of the breeding failures of these birds (Scottish Natural Heritage; see the Resources section at the end of the chapter), and a consequent hedgehog culling programme has been extremely controversial. When Amazon rainforest is felled to provide grassland for feeding cattle there is a massive loss of biodiversity, and also loss of the vast nutrient pools stored in the biomass rather than in the soil. A lot of modern agriculture also tends to lead to the loss of biodiversity in the long term, leaving a reliance on smaller and smaller numbers of species, subspecies and varieties. An enormous modern field of wheat has very low biodiversity. This field is just for the grain from the wheat. The farmer sprays herbicide to get rid of other species of plant (which might be regarded as ‘weeds’) and insecticide to kill insects (wheat is pollinated by wind rather than insects). There will always be the odd field mouse and a few ‘weeds’ but this procedure is tending towards getting as near as possible to only one species (a monoculture) – the lowest biodiversity you can get – to maximise productivity. Images of a large expanse of ripe wheat with a combine harvester at work are easily found on the internet and can be useful to illustrate discussion of modern, ‘high-tech’ arable farming. Students could compare these with images of high biodiversity, including ancient hay meadows such as those in Swaledale in the Yorkshire Dales.

How are we responding to loss of biodiversity? Many countries around the world are responding to the serious threat to the planet’s biodiversity. The UN Biodiversity Conference meets every two years to agree on international decisions relating to biodiversity protection. At a national level, many countries have set up national parks which afford some protection for the species living there. In the UK there have also been many regional responses. As mentioned above, farmers tend to grow only one kind of crop at a time so the biodiversity on their land is very low. Some farmers in the UK now receive subsidies from the government to allow the margins of their fields to grow wild, raising the biodiversity by providing a habitat for wild birds and carnivorous insects, which have been shown to reduce pest insects on the crops. Such margins also increase the number of insects which provide pollination. Maintaining and reviving some old methods of farming is also benefiting biodiversity. There are some large partnerships between central government departments, local councils, businesses and nature conservation charities which are working on large-scale habitat restoration projects. An example is the Great

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 What is biodiversity and why is it important?

Fen project in Cambridgeshire which aims to create a 3700-hectare wildliferich wetland area. (That’s the size of over 5000 football pitches!) It is, however, worth noting that not all conservation programmes are primarily to maximise biodiversity. Some focus on protecting particular rare species, such as the Chinese government’s giant panda conservation project. Giant pandas depend almost exclusively on bamboo shoots and leaves, so a biodiverse habitat is not necessary for the survival of this species.

Careers There are many jobs to be found working for government conservation bodies (for example, Natural England, Scottish Natural Heritage, Cyfoeth Naturiol Cymru (Wales), Environment and Heritage Service (Northern Ireland)). Careers working as environmental consultants (such as for the Environment Agency) and as wardens for national parks or nature reserves (including the RSPB or Wildlife Trust reserves) may also be of interest to students. A recent proliferation of ‘rewilding’ projects has been aimed at restoring biodiversity. This involves large-scale habitat or wilderness restoration, particularly to reconnect fragmented habitats (for example, constructing hedgerows as wildlife corridors between woods), and the reintroduction of predators and ‘keystone’ species.

Science in context Students may be familiar with the recent reintroduction to the UK of Eurasian beavers (Castor fiber), which became extinct in Britain 400 years ago due to over-hunting. Beavers are keystone species because their habit of felling trees creates vital habitats for many other wetland species. Small populations have now been successfully reintroduced in Argyll and in Devon (Rewilding Britain; see the Resources section at the end of the chapter). The re-establishment of beavers has received widespread public support, unlike the suggested reintroduction of wolves! It is understandable why most people in Scotland would not like to see the reintroduction of wolves. However in the past, wolves kept down the numbers of deer, preventing them from destroying woodland habitats and other forms of biodiversity. Not reintroducing wolves means other (artificial) means of controlling numbers of wild herbivores, such as culling, may need to be introduced to maintain ecosystem stability. Some biologists are of the opinion that all species should be protected in the wild in their natural habitats, but others argue that when the numbers of a species get so low that it is on the verge of extinction, it may be better to preserve the small numbers that remain in captivity.

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Zoos around the world play a very important role in protecting biodiversity by co-operating together; individual animals are exchanged to ensure that breeding programmes for endangered species avoid inbreeding and maximise genetic variation (Zoological Society of London; see the Resources section at the end of the chapter). Several well-known species, such as the California condor and Przewalski’s horse, have only avoided extinction due to such breeding programmes. Similarly, botanical gardens have a global network and are home to about a third of all known plant species. Many such gardens have herbaria where dried plant specimens can be studied and compared for identification and classification purposes. Also of great significance are seedbanks where seeds are stored in safety (at –20 °C) in case the species become endangered in the wild. The Millennium Seed Bank at the Royal Botanic Gardens in London (Kew Gardens) has the largest and most diverse collection of seeds in the world, currently housing over 2.3 billion seeds from over 40 000 different species (Royal Botanic Gardens; see the Resources section at the end of the chapter). Natural history museums also make a significant contribution to supporting the protection of biodiversity by collecting, identifying and describing plants and animals and communicating that information to the public. They document species and maintain reference collections for use in future research and they are developing new digital and molecular tools for a better understanding of biodiversity (Natural History Museum; see the Resources section at the end of the chapter).

Careers Biodiversity knowledge is very important for those wanting to pursue jobs at a zoo, botanical garden or museum.

10.2 Classification of living organisms The familiar system of classifying living things into groups based on their structure and characteristics was first developed by Carl Linnaeus, an eighteenth century Swedish scientist. This classification of living organisms according to their natural relationships is referred to scientifically as ‘taxonomy’, but this term is rarely used at secondary school level. Younger students will probably only need to describe and explain how organisms are classified into broad groups using common observable characteristics. They should be given practice using simple ready-made identification keys; an effective way of appreciating the value of classification is for students to construct their own keys using visible characteristics and constructively criticise each other’s designs. Once they look closely, students will discover

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many differences; the task is not as easy as they might imagine (see useful websites at the end of the chapter on keys and classification: SAPS, 2019a; STEM Learning, 2019). An important point to consider is that there is variation within species (either genetically or environmentally induced) and that keys have to be based on consistent features. The importance of variation within species should be recognised because it protects populations against environment changes. A classic example of this is the peppered moth (Biston betularia), which exists as a speckled whitish-grey (‘peppered’) variety, well camouflaged on lichens growing on tree trunks, and a black (melanic) variety which is better camouflaged on dark tree trunks covered in soot from atmospheric pollution. Using a mark– release–recapture sampling method, English geneticist Bernard Kettlewell famously compared peppered moth populations in urban woods heavily subjected to atmospheric pollution and pollution-free woods in the countryside. He found that the less conspicuous varieties (light ones in the countryside and melanic ones in the city) were not so readily spotted by predatory birds and therefore had a better chance of surviving and reproducing. There is an interesting footnote to the peppered moth story. In the Shetland Islands in the most northern part of the UK (students might recognise the name of the principal town Lerwick from the very top of the TV weather map), there are a lot of the melanic moths, and the percentage melanism increases as you go from Sumburgh at the south end to Unst, the northernmost island. There is very little, if any, air pollution in Shetland so being melanic there is unlikely to offer helpful camouflage. It therefore seems likely that there is another advantage in being melanic in the islands that is even more important than being camouflaged against predation by birds. As a dark-coloured surface absorbs more heat from the environment than a light one, this is thought to be an advantage for survival in a cold place like the Shetland Islands where the peppered moth is at the northern edge of its range. Perhaps global warming will mean that being melanic in Unst may become less of an advantage to a peppered moth. If so, this might mean the incidence of melanism may be reduced in favour of more effective camouflage. There is no evidence of this – at least not yet! Older students will learn that scientists initially devised a way of classifying organisms based on observable characteristics, and that there is a hierarchical system of groups, starting with the five kingdoms (animals, plants, fungi, protists and prokaryotes), each being subdivided into increasingly smaller and more specialised groups through: phylum, class, order, family, genus and species. Each organism is named using the binomial system (literally, the ‘two-name’ naming system), comprising the name of the genus (with an initial capital letter) and the name of the species, such as Homo sapiens. Scientific names of species are written in Latin, which helps people around the world know which particular organism is being discussed. For example, the European magpie is Pica pica

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and the Australian magpie is Gymnorhina tibicen. They are both called magpies by local people, and they do (superficially) resemble each other (they are both black and white), but they don’t even belong to the same family of birds, so they are not closely related. With further advances in technology enabling us to look at organisms in much more detail (using internal biochemical features and DNA sequences), we can now use a more evidence-based approach to classification, with some unexpected discoveries. For example, scientists have recently discovered through whole genome analysis that pigeons and flamingos are surprisingly closely related (Jarvis et al., 2014). One notable post-Linnaean biological classification system based on chemical analysis is the three-domain system devised by the American microbiologist Carl Woese and his colleagues. This divides organisms into three ‘domains’: Archaea (single-celled prokaryotes which often live in extreme environments), Bacteria (true bacteria) and Eukaryota (having cells with a nucleus, including protists, fungi, plants and animals). Evolutionary trees draw on current classification data for living organisms and fossil data for extinct organisms to show how organisms are related (see Chapter 9). DNA sequencing of different species is now enabling scientists to create more precise evolutionary trees, which can locate points in evolutionary history where speciation might have occurred.

10.3 Active exploration of biodiversity with students Online exploration of ecosystem biodiversity There are a variety of ways in which 11–14-year-old students could explore this key topic. As a means of introducing students to the concept of biodiversity around the world, they could work in small groups to carry out an online search to find estimates of the number of species in: ➜ a

rainforest temperate broadleaved forest ➜ a conifer forest ➜ tundra ➜ hot desert ➜ a coral reef. ➜ a

Estimates will vary since there are, for example, deserts and rainforests in many parts of the world and they are not all the same. Students could investigate several examples, pool their data and get an idea of the range 240

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of biodiversity both within and between the different types of ecosystem. The ensuing small group and/or whole class discussion should raise questions about whether some species are more important than others, both to the ecosystem itself and to people. Of course, there is no definitive answer to such questions and students need to understand that it will ultimately depend on different people’s points of view. The teacher could introduce questions relating to the relative value of different species, such as what would happen if a particular species were not there.

Pitfall trapping

KEY ACTIVITY

This introduces the idea of systematic data collection appropriate to a particular component of an ecosystem; in this case soil invertebrates.

Investigating biodiversity using pitfall traps Pitfall traps are very easy to set up and can enable students to collect quantitative data and discover that there are a lot more species of soil invertebrates than they thought!

opening is level with the ground

tile to keep the rain and predators out

an empty container (such as a tin can or plastic yoghurt pot)

Figure 10.1  Diagram of a pitfall trap

This also provides an opportunity for students to work as a team. At one level, pitfall traps can be used simply to find out what is there. A grid of (say) 25 traps might be set up by a class in a woodland and then another 25 in a different habitat, such as an open field. The class might have predicted that there would be more woodlice in the wood than in the field because there are more dead leaves for them to eat and the study 241

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should enable this prediction to be tested. For a more quantitative survey, try the mark– release–recapture method to estimate population sizes in different habitats and/or at different times of the year.

A note on the safe and ethical use of organisms Check that students do not have allergies to invertebrates. Always return the invertebrates to the habitat adjacent to the pitfall traps. Empty the traps as often as possible and remember that some invertebrates eat others; for them the pitfall trap is a very welcome banqueting opportunity! When you have finished, remember to remove the empty containers and dispose of them appropriately.

Monitoring water quality: an application of knowledge of biodiversity Monitoring the biodiversity of water from rivers, streams and ponds can be used to assess water quality. In this case, the diversity of invertebrates provides an indication of the amount of oxygen in the water. The more species present, including particular ‘indicator’ species, the more oxygenated the water is and the healthier it is considered to be. The presence and abundance of an indicator species indicates the environmental conditions and can therefore be used as a proxy to determine the health of an ecosystem.

Careers There are many jobs working for water companies in the field ensuring that streams and lakes are not polluted and monitoring output from sewage treatment plants. Using a sampling net, water is gently scooped up from the pond, river or stream and emptied into a shallow tray and the species of organisms are identified. Very high species density, including species with a high oxygen demand such as the larvae of the mayfly (a species of Ephemeroptera) and caddis fly (a species of Trichoptera), indicate that the water is well oxygenated and said to be of ‘high water quality’. This is what you might find in clear mountain streams: the water is almost certainly fit to drink (although we don’t recommend it just in case you’ve misidentified the caddis fly larvae!). However, if the water has low biodiversity with few species, including the sludge worm (Tubifex tubifex), there is almost certainly something wrong. Perhaps there is sewage leaking into the water upstream or a dead sheep rotting in it. If the water has quite a few species, but no mayfly or caddis fly, the water quality is not good, but is better than if there are just sludge worms present (see Table 10.1).

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10.3   Active exploration of biodiversity with students Table 10.1  Indicator species used as a measure of water quality Water quality

Indicator species

very high

larvae of mayflies, stone flies, caddis flies and freshwater shrimp

fairly good

greater water boatman, water snails (especially Jenkins’ spire shell, ramshorn pea shell)

moderate/rather poor

alderfly, lesser water boatman, water cricket and leeches

poor

sludge worm, chironomid midge larvae, cranefly larvae

Professional water quality inspectors use this method as the first step in the water monitoring process. The water quality might fluctuate each day so the water might appear clean on the day of testing, but the presence of sludge worms and few other species will indicate frequently low oxygen supply. This is because sludge worms are one of the few freshwater species which can tolerate low oxygen most of the time. The presence of sludge worms in stream water, and very few other forms of life, suggests that even if it seems clean when you collected a sample, much of the time there is low oxygen supply. The most likely explanation is that it is subjected to pollution frequently even if it seems clean when you examine it. If a water quality inspector found sludge worms and relatively few other species in a stream, he or she would be suspicious and would collect samples to make further investigation back at the laboratory.

Using quadrats to relate biodiversity to environmental conditions A quadrat is usually a metal or wooden frame used for counting individual organisms or species by placing it on the ground. It is generally placed on low-lying vegetation on the ground or could be used for looking at limpets, barnacles or seaweed on a rocky seashore. Technically, it is a sample area defined by a quadrat frame but, in practice, most people refer to a quadrat frame as the quadrat. (A quadrat is usually a square frame 50 cm × 50 cm, which can sometimes be subdivided into 25 10 cm × 10 cm squares for greater accuracy of measurements.) Note that it is not necessary for the teacher or the student to know the names of all species. Sometimes it is just as useful to group the organisms into categories. However, there are now fairly accurate apps which can assist with identification, such as Seek (iNaturalist; see the Resources section at the end of the chapter).

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10 Biodiversity

Using quadrats to investigate biodiversity in an area In general, you cannot possibly look at every square centimetre of a study area, so a quadrat is a method of obtaining a representative sample to gain an estimate of the whole area, or of two areas if you are comparing their biodiversity. You could do this with perhaps ten randomly placed quadrats in one habitat and another ten in the other habitat. There are a couple of things to remember about ‘randomly placing’ quadrats. Firstly, this is actually very difficult to achieve; students are often tempted to place the quadrat over an interesting looking plant, which is far from random. To avoid this, one approach is to mark off the site with two long tape measures set at right angles to create ‘axes’. Prior to the fieldwork, generate and print out a list of random numbers (easily sourced from the internet). Students working in pairs can then use these random numbers as distances along the axes to create random co-ordinates at which to place their quadrats. Secondly, some students are prone to throwing the quadrats as a means of being random! This is, of course, a dangerous practice and one to be avoided.

KEY ACTIVITIES

Quadrats can also be used to show gradual change in biodiversity as you pass from one habitat to another by creating a belt transect. This is a row of quadrats placed at regular distances (say, one metre apart) along a straight line. If this is the first experience of quadrats the class has encountered then you, as the teacher, need to keep the exercise simple with a clear purpose. Examples could include: ‘How does the biodiversity of plant species change as you go from an open meadow into the dark shade of a wood?’ or ‘How does plant biodiversity change from the long grass across a grass path trampled by people or animals?’.

Comparing biodiversity in two defined habitats You can compare the biodiversity of two different areas by recording the number of species present in a quadrat placed in each area. This activity is more authentic and meaningful to students if you investigate the ecological and social history of the two areas beforehand. The data below, collected by a class of 13-year-olds, came from two areas of grassland in a park. At first the class thought it was boring and ‘all grass’ but after ten minutes became interested as they realised that the two areas were surprisingly different.  

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Table 10.2  Plant species recorded in two areas of grassland Species diversity Species

Grassland area 1

Grassland area 2

creeping bent grass

✓

✓

red fescue

✓

✓

sheep’s fescue

✓

daisy

✓

✓

dandelion

✓

✓

slender speedwell

✓

eyebright

✓

germander speedwell

✓

autumn hawkbit

✓

shepherd’s purse

✓

species diversity

4

10

Someone soon observed, ‘some of these dandelions are hairy and some aren’t’. Well spotted! The hairy ones are not dandelions, they are autumn hawkbit. Someone else said, ‘hey, there are two sorts of the blue things’. Yes, there are two species of speedwell (slender speedwell and germander speedwell). In the end the students were surprised how the two sites differed in species diversity. Check out the common names of species you find. They often have some fascinating (sometimes rather humorous) names, which often reveal a story behind them, such as old medicinal uses or peculiar features (for example, the seed pods of shepherd’s purse look like miniature purses and the seeds inside look like tiny coins). These aspects make the activity more interesting for students. The students concluded that grassland area 2 had greater species diversity, or greater biodiversity. The class then discussed why there was a difference in biodiversity between the two closely adjacent areas. The clued-up teacher was able to reveal that this site had a long history as grassland; before it became a public park, it had been part of the grounds of a big house. In the past, grassland area 2 had been an orchard and the grassland was several hundred years old. Grassland area 1, however, had been turned into a croquet lawn in the nineteenth century and then into a cricket pitch. In other words, it had become virtually a grass monoculture and it was difficult for the seeds of new species to grow in the dense matted roots of the grass.

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Using quadrats: what to record Students aged 14–16 years old can build on exploratory biodiversity ideas by looking for trends in data. The commonest measurement to make is the abundance of each species and this is usually measured in one of three ways. (See also the useful websites from SAPS (2019b and c) in the Resources section at the end of the chapter.) 1 Density: this means the number of individuals of each species per unit area. You might use this for counting barnacles on a rocky sea shore as you progress along a transect from the low- to the high-water mark in order to find which part of the shore a particular species of barnacle normally occurs in. You can also measure the density of plants, such as daisies or dandelions, but it doesn’t work very well where it is difficult to count individual plants, such as clump-forming buttercups or for species of grasses or mosses. 2 Frequency: this is a better way to measure abundance. It means the probability of each species occurring in a randomly placed quadrat. Get students to record the presence or absence of each species in (say) ten quadrats. To calculate the percentage frequency for each species, add up the number of times each species is present out of the ten quadrats, then multiply that by 10 to get the % frequency. 3 Percentage cover: here you don’t count individual plants but you record the percentage of quadrat covered by each species. There are two ways of doing this: cover estimate or use of a point quadrat (frame). l Cover estimate: here you make a subjective assessment (by eye) of the proportion of the quadrat covered by each species. It helps to imagine that all the plants of a particular species are moved into one corner of the quadrat (Figure 10.2). It is easier if the quadrats are divided into smaller units, but another approach might be for students to put a grid overlay on their mobile phone camera view, then make the quadrat fill the frame and use that to help them estimate the cover. l Point quadrat: instead of a square frame lying on the ground, a point quadrat frame stands upright and has a row of ten evenly-spaced holes through which a knitting needle can be inserted and allowed to fall (Figure 10.3). Each species hit by the point of the needle is recorded in a tally. The needle may hit more than one species – say a blade of meadow grass and then a dandelion – in which case both are recorded and the final total may therefore exceed 100%, especially in long grass. If the needle does not hit a plant then ‘bare ground’ is recorded. If you are using a point quadrat alongside a transect (marked by a measuring tape), the point quadrat frame is placed at right angles to the tape and recorded at regular intervals.

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Maths There are plenty of obvious links that can be made to calculations within sampling activities. It is a good opportunity to confirm that students understand how to work out percentages. 25% of quadrat area

species A

imagine that all the examples of species A are in one corner of the quadrat

cover of species A is estimated to be around 15%

Figure 10.2  Estimating % species cover by eye 50 cm 5 cm

hole for knitting needle

knitting needle

metal spike to secure frame to the ground

screw

wooden frame

20 cm

2.5 cm

2cm

one needle scoring more than one hit

30 m tape acting as a transect metal spike to secure frame to the ground

Figure 10.3  Estimating % cover using a point quadrat

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Species diversity (biodiversity) A more accurate way of calculating the diversity of plant species in an area is to make two measurements: species richness and species evenness. Species richness is the number of species in a quadrat, but this is actually a rather crude measurement since it does not take into account the relative importance of each species very well. This idea of the relative abundance of each species in the area is called species evenness. Say you had a quadrat (Quadrat A) that was 99% creeping bent grass but with a single very small daisy plant, a single very small eyebright and a single very small speedwell giving a total of 4 for species richness. There’s a lot of one species but only a trace of the other three. Quadrat B, on the other hand, was 25% creeping bent grass, 25% red fescue, 25% sheep’s fescue and 25% moss. This quadrat would also score a total of 4 for species richness, but as it has similar amounts of all four species, it has greater species evenness and therefore a higher species diversity than Quadrat A. So, species diversity can be measured more accurately by combining the species richness and the species evenness, and this is often calculated using the Simpson’s Diversity Index. This index was introduced by Edward Simpson, a British statistician and code-breaker during the Second World War. For a worked example of the Simpson’s Diversity Index see the Offwell Woodland and Wildlife Trust website (see the Resources section at the end of the chapter).

10.4 Deciding where to study biodiversity When organising fieldwork it is important that you choose a good and interesting example which offers a good story. Whenever you take students outside to conduct fieldwork you will, of course, need to consult, and if necessary adjust, an appropriate risk assessment. If it is really not possible to leave the school grounds, you might want to explore the site for areas which offer the greatest biodiversity (such as mixed hedges or patches of unmown grass), and if these do not already exist, see if they can be created! Even if you are at an urban school with no ‘green’ areas on the premises, there are always some opportunities; you just have to be creative. Remember: biodiversity is everywhere. For example, you could make very small quadrats (10 cm × 10 cm) from wire, or by cutting out a piece of transparent plastic, and place them on tree trunks or walls to study the distributions of lichens and mosses in different places. Every site will be different, so the teacher needs to choose it carefully and ensure that it offers: ➜ species

that students will find interesting to identify interesting pattern which emerges without having to do too many quadrats ➜ an interesting, and ideally an unexpected, conclusion for the final discussion, such as the discovery mentioned above that the grassland used to be a Victorian croquet lawn. See if you can find old maps showing what the site used to look like. ➜ an

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10.5   Resources

10.5 Resources General Field Studies Council. The FSC has a large selection of fold-out identification charts, including a Field Guide to Freshwater Life: www.field-studies-council.org/product-category/publications Geopacks. Fieldwork equipment aimed at geography teachers, but very useful for outdoor science too: www.geopacks.com/collections/fieldwork-equipment

Websites BEEP (BioEthics Education Project) Working with discussion. Available at: www. beep.ac.uk/content/484.0.html Great Fen: www.greatfen.org.uk/ GreenJobs. A resource publicising biodiversity jobs: www.greenjobs.co.uk/biodiversity-jobs.cms.asp iNaturalist Seek app: www.inaturalist.org/pages/seek_app National Trust. ‘What is biodiversity?’: www.nationaltrust.org.uk/features/whatis-biodiversity Natural History Museum: www.nhm.ac.uk/our-science/our-work/biodiversity.html Offwell Woodland and Wildlife Trust: www.countrysideinfo.co.uk/simpsons.htm Rewilding Britain. Beaver: www.rewildingbritain.org.uk/rewilding/ reintroductions/beaver Rewilding Europe. ‘What is rewilding?’: https://rewildingeurope.com/what-isrewilding/ Royal Botanic Gardens, Kew. Millennium Seed Bank: www.kew.org/science/collections-and-resources SAPS (Science & Plants for Schools) is an organisation providing many useful resources and guides for measuring organisms in the field: www.saps.org.uk/ a Making and using keys: www.saps.org.uk/attachments/article/560/ SAPS%20Grouping%20&%20classification%20-%20PartE.pdf b Measuring abundance and random sampling: search ‘Ecology Practical 1’ from the SAPS homepage c Further information about working with quadrats: search ‘questions about quadrats’ from the SAPS homepage 249

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Scottish Natural Heritage: www.nature.scot STEM Learning: search ‘classification’ from the STEM Learning homepage: www.stem.org.uk/resources/elibrary United Nations Decade on Biodiversity: www.cbd.int/doc/strategic-plan/UN-Decade-Biodiversity.pdf WWF: https://wwf.panda.org/our_work/our_focus/biodiversity/biodiversity/ Zoological Society of London: www.zsl.org/education/how-breeding-programmes-work

References Bioversity International (2017) Mainstreaming Agrobiodiversity in Sustainable Food Systems: Scientific Foundations for an Agrobiodiversity Index. Rome: Bioversity International. Chivian, E. and Bernstein, A. (eds) (2008) Sustaining Life: How Human Health Depends on Biodiversity. Oxford: Oxford University Press. Jónasdóttir, S. H. and Koski, M. (2010) Biological processes in the North Sea: comparison of Calanus helgolandicus and Calanus finmarchicus vertical distribution and production. Journal of Plankton Research, 33, 85–103. Maar, M., Møller, E. F., Gürkan, Z., Jonasdottir, S. and Neilson, T. G. (2013) Sensitivity of Calanus spp. copepods to environmental changes in the North Sea using life-stage structured models. Progress in Oceanography, 3, 24–37. Pauly, D., Christensen, V., Dalsgaard, J., Froese, R. and Torres, F. (1998) Fishing down marine food webs. Science, 279, 860–863. Raffaelli, D. (2017) What is Biodiversity? NERC Planet Earth. Spring/Summer. p.1. Reynolds, M. (2018) The banana is dying. The race is on to reinvent it before it’s too late: www.wired.co.uk/article/cavendish-banana-extinction-gene-editing Sender, R., Fuchs, S. and Milo, R. (2016) Revised estimates for the number of human and bacteria cells in the body. PLOS Biology, 14 (8), August.

Further reading Bielo, D. (2019) How Biodiversity Keeps Earth Alive. Scientific American: www. scientificamerican.com/article/how-biodiversity-keeps-earth-alive Sethi, S. (2015) Bread, Wine, Chocolate: The Slow Loss of Foods We Love. San Francisco: HarperOne. Wilson, E .O. (1999) The Diversity of Life. New York: W. W. Norton Company.

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11

The environment Melissa Glackin and Steve Tilling

Topic overview The environment is the natural home for biology teaching. It features prominently – directly or indirectly – in all areas of the biology curriculum, and often provides the context and connections through which students transfer biology education to their everyday lives. Furthermore, the environment offers excellent opportunities to collect ‘authentic’ biological data, potentially using a variety of collecting and recording devices and providing many valid reasons for developing mathematical skills. The fact that the environment is allencompassing provides many advantages for biology teachers. However, the sheer scale and complexity of the environment does present challenges when developing a structured approach to teaching. Therefore, in this chapter we borrow a helpful framework from the field of Environmental Education to consider related statutory curriculum and specification requirements. The framework approaches the environment from three overlapping perspectives: biology education about the environment, biology education for (the preservation, conservation and sustainable use of) the environment, and biology education in the environment, as well as teaching sequences formed by combinations of each. It is always important to link your teaching to planned outcomes, and Table 11.1 summarises some of the potential differences between the three approaches. We suggest that all have a place in the biology classroom. Table 11.1  A comparison of education about, for and in the environment. Although the table illustrates differing emphases between three approaches, all will overlap and complement each other. Education about

Education for

Education in

approach purpose

subject and skill acquisition

advocacy and activism

direct/experimental experience

view of knowledge

gaining knowledge and skills; gaining activist knowledge informing metacognition and commitment; enquiry and deeper understanding into issues; contextual

gaining embodied knowledge through contact with the (often novel) surroundings

role of teacher

developing students’ knowledge

stimulating and provoking; guiding

providing and facilitating the fieldwork experience

role of student

developing knowledge and understanding (caution: this can become ‘passive’)

developing skills of activism and advocacy

becoming physically active learners, with all senses engaged

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Five teaching sequences using the about/for/in framework form the bulk of this chapter: ➜ understanding

biodiversity change ➜ food security ➜ air and water quality ➜ health. ➜ climate

However, before embarking on these it is useful to consider some of the broader strengths of environment teaching and the knowledge and experience that your students may bring to the classroom.

Environment and biology education Opportunities to link to Wynne Harlen’s big ideas of science (Harlen, 2010, 2015), met in Chapter 1, are easily identifiable. In particular, environment teaching in biology offers excellent opportunities to showcase how: ➜ scientific

explanations, theories and models are those that best fit what is known at a particular time ➜ the knowledge produced by science is used in some technologies to create products to serve human ends ➜ applications of science often have ethical, social, economic and political implications. This final ‘big idea’ is particularly pertinent in that there are an abundance of ethical dimensions associated with ‘environment’ teaching, including – but not limited to – selecting sources of environmental information and data (about); managing (pre-)conceived ideas and convictions and ensuring appropriate exposure to a range of illustrative examples (for). Relatedly, you also need to consider how to ensure best access to fieldwork and data (in the environment) that includes appropriate opportunities for all your students. Environmental education offers biology teaching an exciting and inspiring host of activists (such as Vandana Shiva), some of whom could be identified as contemporary peers (such as Greta Thunberg), scientists (such as Rachel Carson), writers (including Robert MacFarlane) and TV personalities (such as David Attenborough) to enrich and enliven lessons. Furthermore, students will hear frequently about environmental organisations in the media, such as the Intergovernmental Panel on Climate Change (IPCC), Greenpeace and the Wildlife Trusts, which can be hooked in, explained and, importantly, used to highlight the range of biology-related careers and roles offered in such organisations.

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Traditionally, secondary biology fieldwork is most often linked to biodiversity and ecology. The most common techniques and equipment are covered in Chapter 10. However, the teaching sequences described later in the chapter illustrate possible alternative routes to the integration of biology teaching and the environment. Table 11.2 offers other aspects of biology teaching that you might like to consider linking to the environment. Table 11.2  Examples of biology topics with potential links to environment teaching Topic

Link to opportunity for environment teaching

cells

learning to observe ‘scientifically’ – applying and developing these skills outside the classroom learning to use magnifying lenses, microscopes, binoculars and classification keys

exchange

looking at leaves as exchange surfaces; students can go out and collect different types, shapes and sizes of leaves

communication and control

learning about the control of flowering and fruiting (plant hormones), related to food sustainability and food security (resources might include the website for Compassion in World Farming, given at the end of the chapter)

Prior knowledge and experience Students entering secondary science will have studied life cycles and reproduction in animals and plants, and made scientific observations outside their classrooms through enquiry-led lessons or half-day sessions (sometimes in school gardens or Forest Schools). There might have been an emphasis on animal compared to plant biology, but controversial aspects of human relationships with, and impacts on, the environment are unlikely to have been covered in depth in science teaching (although they may have featured in cross-curricular themes or whole-school assemblies in some primary schools). Teaching for the environment is an area of teaching which can provide rich opportunities in secondary biology, as illustrated in the teaching sequences below. Many of your students will be ready for this and some may already have been involved in ‘taking action’ on behalf of the environment in school, perhaps through recycling initiatives, fundraising and letter writing, particularly if they have come from primary schools which have adopted ‘green’ initiatives. In the UK, these could include Green Flag awards or Forest Schools (see the websites in the section on ‘Websites for whole-school environmental projects and awards’ at the end of the chapter). Some students may also be involved through home or groups outside school. This prior experience can be highly positive and motivating, but occasionally you will encounter students who will be ‘weighed’ down and fearful for the planet’s future.

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Teaching in the environment Why do fieldwork? Practicals are an essential part of biology education. Fieldwork – practicals carried out in the ‘outdoor classroom’ – add an extra dimension to work in the laboratory and classroom. Lambert and Reiss (2015) summarise these as enabling students to: ➜ gain

a deeper awareness and understanding of the challenges facing biologists when investigating in ‘real world’ settings which can often provide ‘messy’ and complex contexts ➜ understand the uniqueness of places and habitats, even those close to home ➜ appreciate the importance of statistics, variability, probability and the need to be cautious in drawing conclusions and making predictions ➜ experience the unfamiliar even in familiar locations, thus opening minds and stimulating curiosity ➜ appreciate the value of working co-operatively. Teaching in the field will also deepen your knowledge of your students and their potential. When setting up fieldwork you may face some challenges, but the good news is that most can be tackled successfully. Table 11.3 sets out common barriers and lists tried-and-tested solutions. Table 11.3  Fieldwork challenges and possible solutions Challenge

Practical solution General: ask your school educational trip co-ordinator for advice on local issues and national policies. Useful guidance from the Department of Education in England is available on their website (detailed at the end of the chapter).

Health and safety issues

On-site fieldwork: Physical Education colleagues will have risk assessments which will contain lots of useful site-specific information. Off-site fieldwork: out-of-school organisations in the UK may carry accreditation, such as the Learning Outside the Classroom Quality Badge (see details at the end of the chapter); these organisations will have exemplar risk assessments adaptable for your own students’ needs. Fieldwork can be completed in the school grounds or local sites close to school.

Lack of time: impact on teaching timetable and other classes

Repeat visits are easy to organise. Remain outside for the entirety of the lesson reducing the time lost travelling to and from the classroom. Subjects, including art, English, history and geography, can combine to create an off-site day which includes the teaching of these subjects.

Lack of teaching staff and cover staff

Fieldwork can be completed in the school grounds or local sites close to school. Incorporate technicians and other support staff.

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Practical solution

Challenge Cost of resources and equipment

Student behaviour

Field equipment can often be made in schools at little expense. Sites will be available within walking distance of all secondary schools if transport is unaffordable. Establish outside routines similar to those used in the classroom (such as a regular position where you stand, setting up timings, clearly indicated physical boundaries). Take a ‘grab and go’ sack with extra equipment including pencils, paper, clipboards, timers, rulers, sitters/something to sit on, poster paper.

Where to teach through fieldwork Every secondary school will have potential fieldwork sites within walking distance. Fieldwork in the most demanding locations – for example in heavily built-up inner city locations (see Glackin et al., 2006, 2015) – can provide great potential for developing a new curiosity and pride in local students while providing opportunities to make links between biology and their everyday lives. Many Wildlife Trusts, local records centres and the Ordnance Survey host online maps, often searchable through locations and postcodes, which will enable you to find potential fieldwork sites including parks and open spaces, streams and ponds. Some will provide additional support and resources (see Resources section at the end of the chapter for links).

How to teach through fieldwork The most critical component in creating effective fieldwork is to consider and plan why you are doing it. Table 11.4 provides a checklist of things to think about if you are considering fieldwork within walking distance. It might look daunting at first, but don’t worry ... it gets easier with practice and there will be experienced colleagues in school, or knowledgeable staff and potential helpers at the chosen site, who will be able to support you. Table 11.4  Fieldwork: things to do and consider (adapted from various sources, including a British Ecological Society (BES) ‘Enhancing Fieldwork’ workshop, 2018) Have clear aims, objectives and learning outcomes (recognising that there are opportunities beyond meeting narrow curriculum requirements). Consult colleagues and check school protocols for off-site visits. Before the fieldwork

Make site visit(s) and find the right field site (allowing for adequate accessibility, wellbeing, supervision and observation). Carry out a health and safety audit, including a risk assessment. Gain permissions (including from school managers and visit co-ordinators, site owners/ managers, parents/guardians).

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Plan timings, travel and logistics. Build a library of supporting resources (many will be free online). Prepare students, accompanying staff and other helpers (assigning roles and responsibilities; allocating students to groups; providing clothing lists). Inform other colleagues in school, particularly if timetables are being disrupted. Prepare fieldwork methods, field equipment and supporting resources (including a ‘grab and go’ sack which contains identification guides, equipment, maps, recording sheets, first aid kit, etc.). Consider value for money and gather financial support (if needed). Make contingency plans, allowing for flexibility and time to adapt to unforeseen situations. Consider student wellbeing (including warmth, hunger, sun, rain). Be flexible: change plans as directed by the students’ capabilities. Allow time for students to plan, reflect and develop hypotheses. Encourage team-working and avoid students working alone. During the fieldwork

Provide informal feedback during activities. Provide time to make sense of ‘accidental’ discoveries and observations (this can be the most inspiring and motivational aspect of the field experience – link it to the fact that many science discoveries can be serendipitous). Give students ownership of their research. Set reflective tasks.

Following the fieldwork

Build on your work in the field in follow-up teaching: connect to other topics throughout the curriculum (for example, use photos from the session). Ask your students for feedback. Reflect yourself and amend plans for next time.

Computer-based and virtual reality fieldwork Virtual reality (VR) resources enable students to be immersed in a range of habitats while still in the classroom. Research shows that these are most effective as a complement to physical fieldwork, either in preparation for a field visit or as a follow-up (Argles, Minocha and Burden, 2015). Currently, the majority of these virtual resources rely on students using desktop computers, tablets or individual headsets to ‘explore’ 360 ° 2D panoramas (photographed on 360 ° cameras), sometimes led by a teacher who is able to control the inputs, provide on-screen prompts and monitor where individual students are in the virtual landscape (such as Google Expeditions; details provided at the end of the chapter).

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Technology use More recent programmes are based in a 3D environment which enables students to walk or fly through real landscapes, making observations and sharing experiences with other students (represented by avatars) (see Open University’s Virtual Skiddaw, detailed at the end of the chapter). Developers are now offering teachers the ability to create their own field trips, including selecting virtual equipment and sampling sites (such as Fieldscapes, see Resources section). Google Street View and Google Earth (website details provided at the end of the chapter) are being used increasingly by biologists and environmental scientists to measure geospatial changes in habitats and wildlife conservation, but can also be used to prepare for field visits (in highlighting features of interest or risks, for example) or to compare with, or illustrate, other locations of interest. Both applications are now free and offer opportunities to develop your own lessons and plans.

Teaching about the environment Why teach explicitly about the environment? The biology curriculum requires teaching linked to places, habitats, ecosystems and associated environmental ‘issues’ (deforestation, conservation, pollution, etc) and related scientific skills. It is tempting to treat these as siloes – tackling each topic separately, using discrete facts, figures and skills for each of them. However, the most effective teaching through and about the environment is multi-disciplinary and inter-connected and recognises that the environment is affected directly and indirectly by the actions of people working both individually and collectively, with social, cultural, economic and technological influences throughout. ‘Ecosystem services’ is a relatively new unifying framework which highlights the benefits (the ‘services’) of healthy environments, including social, cultural, spiritual and aesthetic aspects which are frequently overlooked in science teaching. This framework is useful for reviewing the environmental connections that can be made in teaching and is described in more detail in Chapter 10, Biodiversity. However, it is also controversial with some people who say that attaching ‘values’ to environmental and ecosystem services means that they can become commodities which can be traded away, possibly to destruction. This can be developed as a class-wide debate.

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Cross-disciplinary Throughout this chapter we encourage a multi-disciplinary approach to environment teaching. A recurring criticism of secondary education – including in science teaching – throughout the past 100 years has been failure to break down curriculum barriers. The environment provides exciting opportunities to achieve this. There are many online examples of cross-curricular themes in the various guises of environmental education, education for sustainable development and global citizenship education (see Resources). Talking to colleagues in other departments about environment teaching will bring mutual benefits.

Scientific literacy Teaching about the environment can also highlight the importance of students developing a scientific ‘attitude’, stressing the need for objectivity, accuracy, precision, repeatability and reproducibility when considering complex environmental issues which may appear to have no obvious solutions or consensus. Students can gain an understanding through this teaching that science is rarely static, and that its methods and theories develop as new evidence and ideas emerge and earlier explanations are modified. Effective environment teaching (and associated statistics) can also highlight why environmental scientists may deal in ‘probability’ rather than ‘certainty’, and why they often recommend actions based on the ‘precautionary principle’ when facing environmental risks. Teaching about environmental topics and issues can build key competencies which will help students to become more effective biological scientists in the following areas (after Wiek et al., 2011): ➜ Systems

thinking: understanding the importance of cycles and feedback loops (including impacts and influences from social, cultural and economic domains), and the consequences of actions by people, including ourselves. ➜ Anticipatory thinking: having a more informed picture of what the future will look like, with or without interventions by scientists. ➜ Normative competence: identifying and evaluating environmental situations and states, their associated boundaries of risk and developing precautionary responses. ➜ Strategic competence: becoming sufficiently informed (as biologists) to design and suggest ideas for resolving environmental issues. ➜ Interpersonal competence: gaining interpersonal skills in communicating, negotiating and collaborating, and recognising that science progresses as an open community. 258

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Careers There are some obvious careers linked to the environment, including those associated most closely with ecology and biodiversity (see Chapter 10, Biodiversity). In addition, the broad range of skills and competencies illustrated in this chapter are very transferable to careers which are public-facing, collaborative and/or policyoriented. These include, for example, becoming: translational ecologists, working with a range of stakeholders and decision makers in sociological, ecological and political contexts to tackle an environmental problem; working for environment charities and bodies, including campaigning groups; or pursuing careers in planning, engineering and architecture where the environment now features prominently in training and career development.

Scientific literacy Teaching linked to the environment will often generate student debate – sometimes heated and passionate. This energy can be used positively in structured activities. We have suggested several in the text. For example: using auditing tools to consider the scientific value of online information; developing argumentation skills; encouraging groupwork; and encouraging local activities linked to everyday lives. Together, these activities will help greatly in developing these key competencies in your students. It will also prepare them for higher education and careers.

Bringing teaching up to date The growth of initiatives such as Creative Commons and Open Government Licences (where authors or research funders give other people the right to share, use and build upon their work) has opened up online access to high-quality scientific research. Online databases may be linked to topics throughout the biology curriculum.

Maths Filtering, and refining, raw data from these online databases to suit your purposes can be time-consuming, but upper secondary students can be involved in the process, possibly in preparation for their own fieldwork activities or as practice for mathematical and statistical skills. Doing this, using environmental data – particularly in its rawest form – will help to develop analytical and evaluative skills. Students can refine and present their results (individually and/or collaboratively), draw conclusions and make their own predictions. They can identify the strengths and weaknesses of their own interpretations, evaluate the possibilities of error and suggest future questions and areas for research by biological scientists. 259

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Citizen science projects often adopt simple methodologies which can be replicated in schools. The disadvantage is that many projects are short-lived and their online data may be lost over time. The Open Air Laboratories (OPAL) project, however, is an established, UK-based, citizen science initiative which has involved over one million people, including students from 4000+ schools, contributing to seven national environmental surveys. OPAL’s resources, which include survey advice for group leaders and teachers, methodology and identification sheets, can still be downloaded and results are available online for local and national comparisons (see Resources section for information).

Dealing with bias, inaccuracy and prejudice Environmental information, published through print, TV or online media, needs to be treated with caution. Most of us struggle to tell the difference between ‘real’ and ‘fake’ news, and between objective and sponsored information. So, how do you impose the quality control which guarantees that the information you and your students are sharing is objective, rigorous and representative of best-informed scientific opinion? It is difficult, but colleagues, particularly in social sciences and humanities departments (such as teachers involved in digital literacy, citizenship, personal, social or health education), may recommend resources and auditing tools which can be used by you and your students to detect bias, and also to ensure their own safety when using online resources.

Science in context Providing contrasting data and viewpoints will help to develop your students’ critical, anticipatory and strategic thinking, and support their decision-making.

Teaching for the environment Debates and concerns about environmental issues – for example linked to climate change, conservation, species extinction and pollution – have grown through a rapidly proliferating media in recent years. As a result, students may already have an interest in, or be involved in, activities linked to these issues. Older students may have knowledge of the United Nation’s 17 Sustainable Development Goals (see Resources section). These provide a strong foundation for exploring how evidence from biological science can add a deeper understanding and rigour, including where misconceptions may have arisen previously. This critical role for biology is recognised in non-statutory curricula such as Environmental Education, Education for Sustainable Development and Global Citizenship. In short, biology teaching can enable students to become more committed and informed beneficiaries and custodians for the natural environment.

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11.1   Climate change

However, teaching for the environment can be highly complex (covering many aspects of biology and other subjects), is often contested, and may appear to lack any solutions. Argumentation, that is learning to construct a scientific argument built on evidence, is a useful framework for students to learn to support them in taking their own stances on environmental issues and related activism. Details of what argumentation is, alongside how it can be used, are provided at the end of the chapter. We recommend that this framework is used to develop teaching resources that enable students to ‘argue’ about environmental issues where information from a range of perspectives – scientific, economic, political, ethical – are integrated into the teaching sequence.

Science in context Students can feel that environmental issues have no solution. So, introducing examples where communities and organisations have come together to respond to local challenges can be inspiring; for example, a community clearance of a stretch of a canal path – making it a more pleasant place – and turning part of a park into a meadowland. Such examples can also highlight the roles that both technology and changes in lifestyles will play in future-proofing our planet. Some schools (particularly at primary and junior level) may already participate in whole-school environmental projects and awards that your biology teaching could be involved in (or lead on!) (including Eco-Schools and Green Flag Awards in the UK (see Resources)). Practical Action offers some inspiring examples of community responses and some excellent teaching resources (details at end of chapter).

Teaching sequences: unifying the environment In the introduction we suggested five topic areas where all three approaches to environmental education might be interweaved. Below, we take each topic in turn to illustrate possible teaching sequences where biology teaching can be about, in and for the environment.

11.1 Climate change How are animals and plants adapted to climate, and how could they be affected by future climate change? Global warming is rarely out of the news but teaching about it presents challenges for a biology teacher. All of the issues we raised in the introductory 261

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sections – keeping up to date, unravelling complexity, dealing with perceptions and misconceptions, allowing for bias and subjectivity, facing ethical dilemmas – can be encountered. But there are some basic facts that are indisputable and should form the core of the teaching around which various activities can be developed. A reputable scientific source for information about climate change is The Royal Society, and some evidence they point to – for example, poleward shifts of temperature-sensitive species of fish, mammals and insects – is directly relevant to biology teaching (see Resources section at the end of the chapter). When teaching students aged 11–14, you could highlight the relationship between global climate zones and their natural communities. In simple terms, the major climatic zones are: ➜ polar

and mountains (very cold and dry all year) temperate (cold winters and mild summers) ➜ warm temperate (mild winters, dry hot summers) ➜ arid or desert (dry, hot all year) ➜ tropical (hot and wet all year). ➜ cool

Biologists (including explorers such as Alexander von Humboldt over 200 years ago) have long recognised that these zones are associated with recognisable communities of animals and plants (now named biomes) which are adapted to their local climates (Odum, 1971).

KEY ACTIVITY

With students aged 11–14, it is important for them to experience the effects of climate change and global warming because they probably have little understanding of what changes of even a few degrees Celsius mean for the environment.

Using a model of the Earth to investigate temperature differences In thinking broadly about the potential effects that climate change will have on the flora and fauna across the planet, make a flat model of the Earth by covering a piece of cardboard with white (polar regions), blue (sea) and dark green (land) modelling clay. Leave the map out in the Sun for 10 minutes (on a sunny day). Invite students to feel the difference in temperature between the white and dark green areas; the darker areas should feel much warmer than the white poles. This is due to the colour of the area of land affecting the ability to absorb energy. Whiter regions (like the poles and mountains) reflect light, while darker regions (like tropical rainforest) absorb energy and become warmer.

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KEY ACTIVITY

11.1   Climate change

Invite students to investigate how the weather in one climate zone has had an effect on the way life has evolved. For example, in colder areas: cold-blooded animals (poikilotherms) such as snakes, snails and insects may have darker patches on their bodies to help absorb heat; trees may have thicker and darker bark and shed leaves in the winter to protect from frosts and ice; and warm blooded animals (homeotherms) such as polar bears may have thicker fur, be mostly active during the day and hibernate in winter. In hotter areas: plants and animals may be adapted to retain water (such as cacti and camels), only be active at night or be very pale to reflect sunlight. It is important here to stress that these adaptations have come about through natural selection over a long period of time, where some variation in those animals or plants has made them more successful in surviving, mating and producing the next generation (see Chapter 9).

Further activities Take students outside to observe and sketch several plants. Ask them to research a plant’s life cycle and how it is adapted for the UK seasons. Focused on the plant, ask them to consider the possible impacts of future climate change. What stresses might this plant be under if the temperature rises by a few degrees Celsius? For data and lesson plans related to weather and climate change see the Metlink website (details provided at the end of the chapter).

Enhancement ideas 1 In considering ‘adaptations’, debate how roofs and buildings in urban areas could be adapted to lower excessive heat inside buildings in future summers. One suggestion is that roofs should be painted white or with special paint to reflect some of the Sun’s rays. Another idea is to plant ‘green’ roofs and walls with heat- and drought-tolerant plants to absorb sunshine and also to reduce heat loss in the winter. Ask your students to compare the relative merits of both approaches, and to research the plants which are being used and how they are adapted for this purpose. The website of the Royal Horticultural Society (detailed at the end of the chapter) provides some good information on green roofs. 2 Burning fossil fuels to generate energy for our homes and schools is one of the major causes of climate change. Older students might use the data in Table 11.5 and complete four simple steps in a fieldwork activity to make this link with our own lifestyles: l Select some local trees and measure their stem size at chest height. l Convert the stem sizes into an estimate of the wood biomass. l Calculate the amount of energy that could be produced for heating and hot water through wood-burning boilers in local homes and schools. l Compare this with the actual energy used in schools to work out how many students that tree could support if used as wood fuel for a year.

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This activity can be carried out at any site, including using urban trees lining pavements and in local parklands. It provides a biology-oriented entry into discussion about options for renewable energy generation (including wind, solar, hydro and nuclear, as well as biofuels), its potential long-term landscape impacts and our own levels of energy use (see Tilling, 2007, for further data and explanation). Biomass and energy transfer will link with earlier chapters (such as Chapter 3, Energy and materials).

Maths Cost of energy calculations can be introduced (and linked to physics) in discussions with students about this topic. Table 11.5  Converting trees into energy (after Tilling, 2007) Girth/cm

Volume/m3

Mass/kg

How many secondary school pupils Energy content/GJ could be supplied with annual heating and hot water by this tree?

22

0.01

   3.3

  0.0

  0.0

31

0.07

   44.9

  0.5

  0.1

41

0.15

  101.0

  1.1

  0.3

50

0.26

  171.6

  1.8

  0.5

60

0.39

  257.4

  2.7

  0.7

69

0.54

  356.4

  3.8

  1.0

79

0.72

  475.2

  5.1

  1.3

91

0.98

  646.8

  6.9

  1.8

100

1.21

  798.6

  8.5

  2.2

110

1.46

  963.6

10.3

  2.6

119

1.73

1141.8

12.2

  3.1

129

2.02

1333.2

14.2

  3.6

138

2.33

1537.8

16.4

  4.2

151

2.79

1841.4

19.6

  5.0

160

3.15

2079.0

22.1

  5.7

170

3.54

2336.4

24.9

  6.4

179

3.95

2607.0

27.8

  7.1

188

4.39

2897.4

30.9

  7.9

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11.1   Climate change

Girth/cm

Volume/m3

Mass/kg

How many secondary school pupils Energy content/GJ could be supplied with annual heating and hot water by this tree?

201

5.00

3300.0

35.1

  9.0

210

5.48

3616.8

38.5

  9.9

220

5.99

3953.4

42.1

10.8

229

6.52

4303.2

45.8

11.7

239

7.07

4666.2

49.7

12.7

248

7.64

5042.4

53.7

13.7

261

8.44

5570.4

59.3

15.2

270

  9.07

  5986.2

  63.8

16.3

279

  9.72

  6415.2

  68.3

17.5

289

10.40

  6864.0

  73.1

18.7

298

11.10

  7326.0

  78.0

20.0

311

12.00

  7920.0

  84.3

21.6

320

12.80

  8448.0

  90.0

23.0

330

13.50

  8910.0

  94.9

24.3

339

14.30

  9438.0

100.5

25.7

349

15.10

  9966.0

106.1

27.1

361

16.30

10758.0

114.6

29.3

371

17.10

11286.0

120.2

30.7

3 Planners are promoting the development of more green spaces in future towns and cities as a protection against further climate change. Students could use local biology fieldwork to investigate some of the reasons why, which include: temperature, wind, rainfall and flooding. l Temperature: trees and plants can help to reduce temperatures in built-up areas. Using data loggers, your students can compare average temperatures in built and natural areas.

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Technology use Throughout this chapter we have emphasised that environment teaching need not be dependent on expensive devices and equipment. However, teaching using new technology may help to engage and motivate more students. Many science departments will have access to data loggers that could be used by students to measure such things as temperature and wind strength with appropriate sensors. Smartphones and tablets can now be equipped with similar sensors and 360° cameras will enable 2D ‘virtual’ immersions in your fieldwork locations. The advantage of using these aids is that data can be downloaded to spreadsheets; data handling and geospatial applications (such as GIS) can provide a very swift, visual summary of fieldwork activity. This is a rapidly evolving area, but if your science department lacks these resources, geography colleagues may have access to information and equipment to support fieldwork. l Wind: trees

and hedgerows can reduce wind speeds and associated turbulence by as much as 85%. This will become more important if the frequency of extreme storm events increases as predicted. Wind strength on either side of a natural barrier can be measured easily (or demonstrated) using distance and direction travelled by soapy air bubbles (details of how to make good bubbles can be found on the Home Science Tools website given at the end of the chapter). l Rainfall and flooding: more extreme rainstorms are expected in the future and planted areas can reduce the chance of surface flooding. Your students could measure and compare likely heavy rain drainage from different areas by placing an upright metal tube or tin can (opened at both ends) on contrasting substrates in natural and more impacted areas (such as grassland, weedy areas, flower borders, tree undercover, gravel and tarmac paths). Pour in a measured amount of water and record the time it takes to drain away. The results can be used to assess the effectiveness of different land uses in your study area for reducing potential flooding. These could be superimposed on a hand-drawn map.

Technology use In this activity (and many of the fieldwork exercises described here), students could use a geographical information system (GIS) application to map results (see details at the ESRI UK website at the end of the chapter).

Cross-disciplinary GIS skills are in high demand in a variety of careers. If your science department is not using GIS, ask your geography colleagues. The government website on river and sea levels in England (see details in the Resources section) can be used by teachers to look at data across England. Many other countries have a similar service.

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11.2   Understanding biodiversity

Science in context These in the environment investigations can include a for focus by encouraging your students to consider what future changes and development priorities, including a role for natural areas, they would like to see in the area around school to lessen the impacts of future climate change. The loss of school playing fields is a very topical link in many areas (which could also be linked to the Environment and Health sequence below). See the Resources section at the end of the book for a link to the government website which discusses the disposal of school land. One of the outcomes of these activities should be an awareness among your students that positive changes in the natural environment can be made, and they are encouraged to become part of those changes through lifestyle and career choices.

11.2 Understanding biodiversity

KEY ACTIVITY

How are organisms interdependent within an ecosystem? Investigating trophic levels To revisit or introduce 11–14-year-old students to trophic (feeding) levels and the fact that they can be observed everywhere, bring students outside the classroom and place them into groups of four. Give each group a large white tray with a card identifying one category from: primary producer, primary consumer, secondary consumer, tertiary consumer. Give groups five minutes to collect specimens, or evidence of specimens, that fit in their category. On return, the trays should be placed in order of trophic level. Ask students to explain why they chose particular specimens. Highlight to students how the trays are interrelated and how a greater number of species are located lower down the levels (for example at primary producer level). Invite students to consider how and why their specimens might vary depending on the time of day/year and type of habitat. Here, students have linked their knowledge about the environment while learning in the environment.

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Enhancement ideas Invite students to consider which management practices are being used in their current location and how these might have influenced the range of specimens they collected. Then ask what actions would be needed to improve species richness and how they could help in achieving this. For example, school grounds might currently have limited green space, and students might return to the classroom to compose an email to the headteacher/governors asking them to consider how this might be rectified. In taking a for approach, students should be invited to draw on their skills of argumentation.

KEY ACTIVITY

Food webs

Food webs To introduce students to food webs, distribute a set of A4 laminated cards with a picture of a different species from one habitat on each card. Each student should receive a different organism and they should research it before the lesson (how does it obtain its nutrients, what eats it, where does it live, etc.). The organisms should range between the trophic levels and be representative of different kingdoms. For example, a woodland habitat might include oak, sycamore, ash, grass, owl, blue tit, field mouse, vole, spider, aphid, mushroom, moss or lichen. You could choose a local habitat such as a local graveyard, a recreational ground or a waterway/reservoir. The cards should have string attached that allow students to place them around their necks. Either in or outside the classroom, invite students to stand in a large circle around the student representing the Sun. To represent energy, give a ball of string to the ‘Sun’ and invite this person to hold onto one end of the string but pass on (transfer) the energy to an organism at the appropriate trophic level (that is, a primary producer). The organism representing a primary producer should then hold onto the string and pass the ball of string to an organism at the next trophic level (which they would be eaten by). This ‘chain’ continues to the apex/ top predators. Repeat this process with 5–6 balls of string; on each occasion, the Sun’s energy should land on a different primary producer. At the end, several organisms will find that they are feeding across multiple food chains; in other words, some students will be interacting with more than one ball of string. Figure 11.1 illustrates a food web activity like this.

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Figure 11.1  A food web activity

The activity with older students could introduce issues/threats that the chosen habitat might encounter. For example, for a woodland habitat, ash dieback disease might affect all the ash trees. The student representing ash would sit down and those linked via the string would consider the consequences this would have on the species they represent. A discussion on ash dieback (a fungal disease in ash trees that causes leaf loss, crown dieback and bark lesions), including why it has increased in the UK and how it has benefited from temperature rises, could then follow. To increase the immediacy of the activity complete it in the habitat being studied.

Managing the environment positively To introduce students to the positive management influences that humans can have on the environment – for example, by encouraging natural ecological recovery – share with students the video ‘how wolves change rivers’ (see Resources section). The video introduces ecological terms and illustrates linkages between non-living (abiotic) and living (biotic) inputs, showing how working for the environment (rather than against it) can result in positive outcomes experienced in our own lifetimes. While the video is impressive and convincing, it provides an opportunity for your students to discuss its scientific content and validity using the auditing tools recommended in the ‘Dealing with bias’ section on page 260 of this chapter.

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Enhancement ideas Earlier in the chapter we introduced the Ecosystem Services framework (page 257). We mentioned that it was controversial, with some people worried that giving values to ecosystems and their services will reduce them to tradeable commodities which can be overlooked or destroyed. They also argue that some ‘services’ of the environment are impossible to value: cultural, spiritual and aesthetic aspects, for example. The Resources section at the end of the chapter gives some interesting weblinks. You could invite older students (14–16-year-olds) to debate the merits of the Ecosystem Service framework, setting the context through the two videos listed at the end of the chapter which consider Ecosystem Services at global and local scales (see links to the website of the Food and Agricultural Organization of the UN and the website of GRID Arendal).

11.3 Food security How are plant reproduction and farming important in human food security? Global food security is under increasing threat. There are many videos on YouTube which illustrate this idea and the challenges that we face (see Resources section at the end of the chapter). The organisation Global Food Security suggests that around 24–35% of food from school lunches ends up in the bin. In light of this food wastage, ask students to consider: the number of trees felled for food production, the one billion hungry people in the world, and the amount of irrigation water used to grow the food. Invite younger students to complete a ‘food observation diary’ for three days, during which they list: food waste (in other words, food not consumed); the location of the wastage (school, home, other place); the food type (for example, orange, meat, bread); the amount wasted (whole fruit, fruit peel, one bottle, etc.). They can also record the reason(s) why the food was wasted (such as, past use-by date) and suggest how this waste could be prevented (for example, bring in packed lunch).

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11.3   Food security

Enhancement ideas The Royal Society of Biology has produced some excellent resources to support the teaching of food waste, food production and climate change, highlighting the links between biology and geography curricula (see Resources section at the end of the chapter). Fieldwork for all ages of students could include visits to farms. Biological topics that might be incorporated include: selective breeding, pest and weed control, habitat conservation and creation, carbon and nutrient cycling, soil fertility and erosion, growing renewable crops, the role of micro-organisms in fermenting carbohydrates in hay and silage production, composting, and bio-gas generation. The organisations below can help you get in touch with farms in your area, including in or near to innercity areas. All of the details for these organisations are given at the end of the chapter. l Soil Association l Natural

England’s Educational Access for Schools l Linking Environment and Farming (LEAF) l Countryside Classroom. l Farms

Fieldwork in school grounds could include investigating different pollination strategies. Insect counts on flowers can highlight plant adaptations (including colour and structure) and the differences between insect- and wind-pollinated plants. The critical importance of pollinators, and the impact of habitat loss, intensive agriculture and climate change, are all strongly linked to secondary biology curricula. Pollination counts have been the focus of several national citizen science projects, some supported by online resources (for example, Pollination (details given in Resources section)). Online results databases provide opportunities for local and national comparisons with your results. (The OPAL Data Explorer page in the Resources section provides some data for analysis.) A school garden (which can be as small as a table top) can become an outdoor laboratory, allowing your students to investigate the role of nutrient cycling, fertilisers, pollination, decomposition, plant adaptations, colonisation (by weeds, micro-organisms and invertebrates) and succession. The Royal Horticultural Society provides those tending school gardens with advice and helpful resources, including some specifically for 11–19 science students (see Resources section). Growing Schools – a previous government project – also supported schools to develop gardens and plots. The teaching resources, many of which are primaryoriented but can be adapted for secondary teaching, are still available on the Countryside Classroom website (details at the end of the chapter).

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11.4 Air and water quality How have humans positively and negatively influenced the quality of the environment? Global concerns about the air we breathe and the quality of the water we drink and bathe in have been around for many hundreds of years. However, over that time the main causes of pollution in most countries have changed.

Science in context In the UK, industrial and coal-burning sources were the main causes of poor air quality in the 1950s, but road transport has become the main cause today. While industrial, human sewage and agricultural inputs into water have been important historically, and remain so today, students may be aware from the media that other newly designed chemicals, plastics and nanoparticles are growing threats.

KEY ACTIVITY

Our knowledge about the levels, trends and consequences of air and water quality on our wellbeing, and the future health of local and global environments, has increased hugely in recent years. There are many excellent resources available to support teaching of these topics (such as the Environment Agency’s State of the Environment reports, see Resources below).

Planetary Boundaries To enable older students to consider the interrelationship between the numerous biological/environmental processes and the role humans play, and to get a sense of the health of the planet, the following activity introduces the concept of Planetary Boundaries. Planetary Boundaries, as defined by a cross-discipline team working in the Stockholm Resilience Centre, are ‘safe operating spaces for humanity’ based on scientific evidence as set out by nine Earth-system processes. Distribute information cards related to the nine processes: climate change, biodiversity loss, ocean acidification, land use, freshwater, ozone depletion, chemical pollution, atmospheric aerosols and biogeochemical (phosphorus and nitrogen) cycles. Either inside or outside the classroom, introduce the list and ask how each might be measured (for example in Planetary Boundaries, biodiversity loss is measured by species extinction rates per annum). Taking a rope, leaving one end where the students are standing, stretch the other end a total of five steps away from them. State that within the space reached by the extended rope the Earth-system processes are safely operating, but beyond

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that rope a boundary is reached and it is no longer safe. Using the information on the sheets, students will illustrate how their environmental process has been degraded from between 1800 (pre-industrial, baseline time) to today. Table 11.6 indicates four important time periods. Students should be invited to move forward the number of steps specified and explain what might have been occurring during the period or what the result of the position is on the planet. While the Planetary Boundary model is not definitive, it offers a model to conceptualise and consider the processes individually and together within the Earth’s limits. Table 11.6  Degradation of nine processes measured in ‘steps’ 1950s

1970s

1990s

Present

climate change

2.5 steps

1.5 steps

2.5 steps

2 steps forward

biodiversity loss

no baseline data

start walking and keep walking 10 steps

ocean acidification

no baseline data

3.5 steps

land use

3.5 steps

freshwater

1 step

shuffle forward

shuffle forward

shuffle forward

0.5 steps

0.5 steps

3 steps

back 0.5 steps

remain stationary

nitrogen flow (2 steps)

nitrogen flow (8.5 steps)

nitrogen flow (6 steps)

nitrogen flow (1 step)

phosphorus flow (1 small step)

phosphorus flow (1 step)

phosphorus flow (1 step)

phosphorus flow (1 step)

ozone depletion

biogeochemical

chemical pollution

No baseline data. Current lack of information about the interactions within the environment means that a threshold can’t yet be calculated. More research is required.

atmospheric aerosols

No baseline data. Current lack of information about the interactions within the environment means that a threshold can’t yet be calculated. More research is required.

Local air and water pollution can be measured through simple fieldwork activities measuring the presence or absence of ‘bio-indicators’: plants and animals with varying sensitivities to pollution.

Air Historically, one of the most common forms of air pollution in the UK was caused by burning coal which increased the acidity of rain and prevented lichens, trees and freshwater animals from growing naturally. The impact of acid ‘rain’ has decreased in recent decades in the UK (unlike those countries where coal-burning power stations and factories are still common). However,

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lichens are slow growing and transects on rocks (including gravestones) and trees (on bark) from inner cities outwards into rural areas show an increase in lichen numbers and diversity, including an increase in ‘bushy’ lichens which are still missing from built-up areas. The FSC publishes a series of fold-out charts to support lichen identification (see the Resources section at the end of the chapter). Rural areas can also suffer from air pollution. For example, gaseous nitrogen pollution caused by farm fertilisers and vehicle fuels affects lichens and they have been used as bioindicators in citizen science surveys such as OPAL’s air quality project, which has downloadable resources and survey data. Trees help to trap smoke and dust particles by ‘air washing’: filtering particles on leaves or causing them to drop to the ground in the still, moist air surrounding the canopies. One hectare of beech wood is able to capture about four tons of dust per year from the atmosphere. A very simple method for measuring particulate pollution is to stick sellotape to leaf surfaces, then reapply it to white paper and estimate particles per cm2 using a micro-quadrat or graticule (see description of quadrats, Chapter 10, page 243). Carrying out a transect (see Chapter 10) away from a main road or other possible pollution sources can illustrate local variations in air pollution.

Water Certain types of freshwater invertebrates are used for rapid assessment of water quality in streams and rivers worldwide, including by water authorities in the UK. The methodology and resources are described earlier in this book (see Chapter 10, pages 242–243).

11.5 Health How does our use of the environment affect human health and wellbeing? There is a strong link between healthy people and a healthy environment. Public health authorities and medical practitioners worldwide are now adopting ‘green prescriptions’ for the general public (see the PDF document by NHS Forest in the Resources section). Doctors are encouraging visits to and physical activity in outdoor environments such as parks and woodlands, away from noise and air pollution, particularly to help lose excessive weight, relieve stress and anxiety, and help remedy other conditions such as asthma. These trends provide opportunities for you to link areas of the environment with human health and wellbeing (biomedicine). Most of the environment teaching and fieldwork activities described in this chapter can be linked to human health. For example, questions could include: 274

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11.6   Resources ➜ Biodiversity:

is the current extent and quality of ‘green’ space suitable for meeting the needs of green prescriptions? ➜ Climate change: how could personal features of the individual, such as age and health, affect their sensitivity to future climate impacts? ➜ Food security: what are the advantages and disadvantages of changing farming practices, including the current and projected growth in vegetarian diets, on human health and wellbeing? ➜ Air and water quality: how do airborne particulates affect plants and animals (including humans) and how can people and communities remedy any harmful impacts? An introduction for older students to the precautionary principle provides an opportunity to compare similarities in approaches to human health (medicine) and environmental health (environmental science). The precautionary principle applies the same standard as the ancient Hippocratic Oath which is still used by medical doctors: namely in the absence of scientific evidence, but where there is a possibility of risk, the overriding priority is ‘to do no harm’. Health-related fieldwork activities could include: mapping and surveying the distribution of local green space and making comparisons between different areas (on hand-drawn maps or using GIS, for example, see page 266); measuring respiration and monitoring heart rates while exercising outside; comparing recreational use of natural areas across different generations; measuring dust and soot particles on natural and experimental surfaces (see above); surveying changing diets in peer groups and families.

11.6 Resources Websites Websites related to identification Field Studies Council fold-out charts: www.field-studies-council.org/ publications.aspx i-Spot online community to help you with identification: www.ispotnature.org/ OPAL citizen science charts: www.opalexplorenature.org/identification

Websites to help in auditing online and printed resources (for example, for bias, and personal security) Department for Education online safety information: https://assets.publishing. service.gov.uk/government/uploads/system/uploads/attachment_data/ file/811796/Teaching_online_safety_in_school.pdf

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Websites related to environmental citizen science (OPAL) OPAL’s air quality project has downloadable data and resources: www.imperial.ac.uk/opal/data-download/

Websites for organisations and training providers linked to environment education, inside and outside the classroom Council for Learning Outside the Classroom (umbrella body for teaching outside the classroom): www.lotc.org.uk/ The Department of Education in England provides useful guidance on educational visits and school trips: www.gov.uk/government/publications/ health-and-safety-on-educational-visits The Field Studies Council is a UK charity providing field courses and teacher training (some sponsored, or with bursaries), specialising in secondary biology and geography: www.field-studies-council.org/contact.aspx Learning Outside the Classroom Quality Badge: out-of-school organisations in the UK may carry accreditation, such as the badge detailed at this site: www. lotc.org.uk/lotc-accreditations/lotc-quality-badge National Association for Environmental Education (NAEE): http://naee.org.uk/ Oxfam (Education for Global Citizenship): www.oxfam.org.uk/education/ resources/education-for-global-citizenship-a-guide-for-schools STEM library, hosting over 25 000 searchable and downloadable resources, including many linked to biology and environment teaching in the UK and overseas: www.stem.org.uk/resources

Websites for whole-school environmental projects and awards Eco-Schools (hosted by Tidy Britain Group): practical school-based examples of environmental projects based around ten themes (including biodiversity, healthy living, waste and litter): www.eco-schools.org.uk/secondaryfe-pathway/ten-topics/ Forest Schools: www.forestschoolassociation.org/what-is-forest-school John Muir Trust: a charity which sponsors an award scheme linked to environmental projects, including in schools: www.johnmuirtrust.org/john-muiraward/get-involved Practical Action: some inspiring examples of community responses and some excellent teaching resources. Use the search term ‘inspiring schools’ from this web page: https://practicalaction.org/our-work

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11.6   Resources

Websites providing environmental quality reports Global Environment Outlook: Healthy Planet, Healthy People (2016). A 700+ page global report published by the United Nations packed with authoritative charts and information: https://content.yudu.com/web/2y3n2/0A2y3n3/GEO6/ html/index.html?origin=reader State of Nature (2019): a UK report, based on volunteer data, published by leading conservation organisations. Search ‘state of nature reporting’ from the RSPB homepage: www.rspb.org.uk The state of the environment: air quality (2018). A summary scientific report published by the Environment Agency (England): https://assets.publishing. service.gov.uk/government/uploads/system/uploads/attachment_data/ file/729820/State_of_the_environment_air_quality_report.pdf The state of the environment: water quality (2018). A summary scientific report published by the Environment Agency (England): https://assets.publishing. service.gov.uk/government/uploads/system/uploads/attachment_data/ file/709493/State_of_the_environment_water_quality_report.pdf

Websites to locate local fieldwork sites Ordnance Survey application which helps to locate ‘green’ areas, including nearby parks and open spaces: https://getoutside.ordnancesurvey.co.uk/ greenspaces/

Websites to support online, VR and 3D field trips Fieldscapes – an application to create your own VR field trips: https://live.fieldscapesvr.com/ Google Earth and Google Expeditions: search relevant term from the homepage: www.google.co.uk Google Street View: www.google.com/streetview/ Virtual Skiddaw – an Open University VR earth science field trip to the Lake District. Search ‘Virtual Skiddaw’ from the OU home page: https://learn5.open. ac.uk/

Websites to support teaching of learning sequences YouTube has a useful clip about the science of climate change. From the YouTube homepage (www.youtube.com) input the search term ‘what’s the big deal with a few degrees?’.

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Websites to support fieldwork Countryside Classroom: www.countrysideclassroom.org.uk/ Farms for Schools: www.farmsforschools.org.uk/ Linking Environment and Farming (LEAF): https://education.leafuk.org/ Natural England’s Educational Access: http://cwr.naturalengland.org.uk/ educational-access.aspx Soil Association: www.soilassociation.org/ Thinking Beyond the Classroom project – aimed to develop science teachers’ skills in outdoor teaching. From the homepage of the Primary Science Teaching Trust (www.pstt-cpd.org.uk) input the search term ‘thinking beyond the classroom’.

Website related to teaching about food security and food sustainability Compassion in World Farming: www.ciwf.org.uk/education/downloads/science

Websites of relevant national organisations The Royal Horticultural Society – this website provides information about the benefits of ‘green roofs’: www.rhs.org.uk/advice/profile?PID=289 and supplies resources for those working in school gardens: https://schoolgardening.rhs. org.uk/about-us The Royal Society – an important source for information about climate change. From the homepage (https://royalsociety.org) input the search term ‘climate change evidence and causes’. The following link takes you to a PDF about climate change. It is a joint document between The Royal Society and the US National Academy of Sciences: https://royalsociety. org/-/media/Royal_Society_Content/policy/projects/climate-evidencecauses/climate-change-evidence-causes.pdf The Royal Society of Biology website has resources to support the teaching of food waste, food production and climate change, highlighting the links between biology and geography curricula. Input the search term ‘food and food security’ into this web page: www.rsb.org.uk/get-involved/biology-for-all The website of NHS Forest encourages public health authorities and medical practitioners to adopt ‘green prescriptions’ for the general public: https:// nhsforest.org/sites/default/files/Prescribing%20Green%20Space-3.pdf

Websites of global organisations UN’s Sustainable Development Goals: https://sustainabledevelopment. un.org/?menu=1300

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11.6   Resources

The Food and Agricultural Organization (part of the UN). From the homepage (www.fao.org) use the search term ‘biodiversity, genetic resources and ecosystem services’.

Website related to the skill of ‘argumentation’ Details of what argumentation is, alongside how it can be used, are here: www. pstt-cpd.org.uk/ext/cpd/argumentation/unit3.php

Website providing data about weather and climate change Metlink, a website from the Royal Meteorological Society, providing data and lesson plans related to weather and climate change: www.metlink.org/

Other website resources The Home Science Tools website can be found at: www.homescience​tools. com/article/how-to-make-super-bubbles-science-project Information about geographical information system (GIS) applications can be found at: https://schools.esriuk.com/ This governmental website provides information about river and sea levels in England and any areas at risk of flooding: https://flood-warning-information.service.gov.uk/warnings This Department of Education link discusses decisions relating to the disposal of school land: www.gov.uk/government/publications/school-landdecisions-about-disposals YouTube clip of ‘how wolves change rivers’: https://youtu.be/ysa5OBhXz-Q This website by GRID Arendal covers more about ecosystem services: www. grida.no/resources/8434 YouTube clip from the organisation Global Food Security about the challenges we face: https://youtu.be/0emw7IkFdK8 The Polli-nation website details pollination counts: http://polli-nation.co.uk/ Online results databases of pollination counts provide opportunities for local and national comparisons with your results: www.imperial.ac.uk/media/ imperial-college/research-centres-and-groups/opal/PolliNation_Survey.xlsx Growing Schools was an organisation that supported schools to develop gardens and plots. The teaching resources are still available on the Countryside Classroom website: www.countrysideclassroom.org.uk/

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References Argles, T., Minocha, S. and Burden, D. (2015) Virtual field teaching has evolved: Benefits of a 3D gaming environment. Geology Today, 31 (6), 222–226. Dobson, F. (2015) Urban Lichens 1. FSC Occasional Publication 98. Shrewsbury: Field Studies Council. Glackin, M. A., Leigh, S., Jonusas, G. and Mercer, J. (2015) The oak processionary moth: from London parks into biology classrooms. School Science Review, 97 (358), 79–84. Glackin, M., Jones, M. and Norman, S. (2006) What happened to the holly leaf miner? Studying real food chains. School Science Review, 87 (320), 91–98. Harlen, W. (ed.) (2010) Principles and Big Ideas of Science Education. Hatfield: The Association for Science Education. Available at: www.ase.org.uk/bigideas Harlen, W. (ed.) (2015) Working with Big Ideas of Science Education. Trieste: InterAcademy Partnership. Available at: www.ase.org.uk/bigideas Lambert, D. and Reiss, M.J. (2015) The place of fieldwork in science qualifications. School Science Review, 97 (359), 89–96. Odum, E. P. (1971) The Fundamentals of Ecology. Philadelphia: W. B. Saunders. Tilling, S. (2007) Outdoor science. Linking trees with energy. School Science Review, 89 (327), 11–15. Wiek, A., Withycombe, L. and Redman, C.L. (2011) Key competencies in sustainability: A reference framework for academic program development. Sustainability Science, 6, 203–218.

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12

Microbiology and biotechnology John Schollar and Jenny Byrne

Topic overview Micro-organisms are extremely important in our everyday lives. They are essential for mineral recycling, through the processes of decomposition and decay, and without them, our countryside would be piled high with dead organisms. While many micro-organisms are pathogenic, and are responsible for a number of notable diseases affecting humans, that is only a small part of their story. Scientists have managed to harness the power of microbes, using them successfully in the fields of food and beverages, antibiotics and molecular biology. Finding out what microbes are, what they do, what they look like and where they are found is fundamental work accessible to students of all ages and capabilities.

Careers Microbiology both enables understanding of cells and is also an applied science, helping agriculture, health and medicine, maintenance of the environment, as well as the biotechnology industry. Practical work is the best way to engage pupils in the topic but the main concern that teachers have in teaching microbiology is about health and safety, and this is often used as a reason for not doing practicals. However, as long as the correct procedures are followed, there is no reason why students should not be given many practical activities to do in order to learn about microbes. This chapter sets out a range of age-appropriate activities under specific headings, but these are by no means comprehensive. Useful resources and activities can be found by following the resource links given at the end of the chapter. Microbiology is also full of fascinating stories that can ‘hook’ students into the topic by engaging, informing and educating them. Media reports and web searches are excellent sources to illustrate aspects of microbial science and the social implications of microbiology that can lead to discussion and debates in the classroom. See the Resources section at the end of the chapter for links to more information about these topics:

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12 Microbiology and biotechnology ➜ Theodor

Escherichia discovered a bacterium, named after him (Escherichia coli (E. coli)) that has proved very important for molecular biology research. The emergence of pathogenic strains (such as E. coli 0157) gave the bacterium some notoriety, but the K-12 strains used in schools are nonpathogenic. ➜ The isolation and identification of a new variant strain of the Hanta virus is interesting not just for the science but for its name: Sin nombre (without name). The virus would normally have been called ‘Four Corners’ after the place where it was found, but that name would have upset the local Native American population. ➜ Ebola virus can have a devastating effect on populations. It is easily transmitted by coming into contact with body fluids from an infected person and has a very high mortality rate of 83–90%. Outbreaks receive a lot of interest from the world’s media. ➜ A triple vaccination for measles, mumps and rubella is given routinely to babies and pre-school children to protect them in the UK. In 1998 the safety of the MMR vaccination was questioned, significantly reducing its uptake by parents. Although this evidence was subsequently discredited, child vaccinations are still low and this has implications for child health.

Science in context Important figures in the history of microbiology, for students to find out more about, include Angelina Fanny Hesse, Barbara McClintock, Marjory Stephenson, Van Leeuwenhoek, Louis Pasteur, John Snow, Alexander Fleming, Paul Erlich and Robert Koch.

12.1 What are microbes? Microbes (also known as ‘micro-organisms’) are a very diverse group of tiny organisms. Normally too small to be seen without magnification, they are found in nearly all environments, including water, air and soil. The human body is also home to huge numbers of microbes on the body surface and millions in the gut.

The major groups of micro-organisms ➜ Algae:

photosynthetic organisms found in most aquatic environments (rivers, ponds, lakes and the sea) as well as terrestrial habitats such as the surface of trees. ➜ Archaea: a domain of unicellular prokaryotic organisms which have no nucleus. They used to be classified as bacteria but are now recognised as a separate major domain of life (see Chapter 10). Found in hostile environments such as hot springs, ocean vents and alkaline or acidic water. 282

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12.1   What are microbes? ➜ Bacteria:

the majority of bacteria are beneficial and of immense value to humans but a small number are pathogens and are important because of the diseases they cause. Bacteria are prokaryotic, unicellular and have no cell nucleus or other membrane-bound organelles. ➜ Cyanobacteria: once known as blue-green algae, but now known to be prokaryotes and classified as bacteria. ➜ Fungi: include single-celled organisms, such as yeasts, as well as multicellular filamentous branching organisms, the moulds. They are found in abundance in soils and the air. Many are important in the production of foods, beverages and antibiotics; some are responsible for diseases and spoilage of food. ➜ Protozoa: found in a variety of habitats, such as soil, ponds, lakes, rivers and the sea. They are important in sewage treatment and are involved in many food webs. Very few diseases are caused by protozoa but there are two important exceptions: toxoplasmosis and malaria. ➜ Viruses: have no cells of their own and develop only in the cells of host organisms such as animals, plants and bacteria. This means that they are not living organisms in the strict sense (see Chapter 2).

Reasons for teaching about microbes Microbiology should be given a prominent place in the biology curriculum and can add to students’ scientific literacy. Ideas to start thinking about why microbes are important in so many areas of biology include the fact that microbes: ➜ are

socially, economically and medically important: antibiotics, food preservation, cleaning the environment, recycling of resources ➜ are central to research in genetic engineering and modern molecular biology, including major developments in the biotechnology industries ➜ play a key role in personal, public and domestic hygiene ➜ are important in food production from chocolate to cheese ➜ cause diseases and harm: food poisoning, illness, sepsis ➜ often need to be controlled for a healthy, efficient, ill-free life ➜ are essential in cyclical changes (such as the carbon cycle and nitrogen cycle) ➜ help animals, such as herbivores, to digest their food; microbes also colonise our guts and are necessary for a healthy life ➜ can be responsible for the excess greenhouse gases released into the environment by herbivores. Reasons that micro-organisms interest and motivate students include that they: ➜ can grow rapidly and produce visible growth on natural and laboratory-

made substrates (from ‘invisible’ cells to colonies on an agar plate) be beneficial or harmful to humans – but most are beneficial

➜ can

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12 Microbiology and biotechnology ➜ can

have a profound effect on inanimate and large multicellular structures, including humans, other animals, plants and buildings, even though they are of microscopic size ➜ had ancestors that were the first inhabitants of Earth ➜ have colonised nearly every environment from the depths of the oceans to hot springs ➜ include a wide variety of different groups of organisms and exhibit a diversity of properties and activities ➜ have fascinating facts and stories associated with their identification and influence on the environment. Finally, studying microbes can be used to demonstrate a wide range of principles that connect numerous topics from across biology specifications, including: ➜ population

dynamics and the S-shaped growth curve ➜ photosynthesis and respiration ➜ genetics and molecular biology ➜ biodeterioration, spoilage ➜ disease ➜ enzyme activity ➜ sulfur, nitrogen and carbon cycles ➜ the characteristics of living organisms: microbes can move, respire, grow, reproduce, react to stimuli and feed.

Science in context In addition to all the many reasons given above for studying microbes, they are also extremely useful for demonstrating to students a wide range of biological processes that place microbial science in real-world contexts: l production

of industrial chemicals, including citric acid, acetic acid, amino acids of fuels and solvents, such as hydrogen and methane l industrial catalysts (enzymes), including amylases, cellulases, lipases, proteases and pectinases l healthcare products, such as vaccines and antibodies l food and beverages, including cheese, yoghurt, bread, beer, wine and mycoprotein l waste treatment, for example sewage treatment, refuse breakdown, pollution control l oil and metal recovery, including microbial-enhanced oil recovery and metal bioleaching. l production

Prior knowledge and experience All students will have had experience of microbes in their daily life, from childhood diseases to eating cheese or yoghurt, but everyday experiences 284

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can lead to common misconceptions and difficulties in understanding (Byrne et al., 2009; Byrne, 2011). These include some of the following: ➜ Students

may believe that microbes are not living. Investigating microbial functions helps students understand that, like other living things, microbes demonstrate a wide range of biological functions. There is also scope for discussion about how viruses are not thought of as living organisms since they harness the organelles of their host organism. ➜ Use of the term ‘bugs’ can lead to associations with invertebrates such as certain arthropods; work on classification should help with this misconception. ➜ Terms such as ‘bacteria’ and ‘virus’ are used without distinction and can lead to confusion when antibiotics are not prescribed for viral illnesses, such as colds and flu. The winter flu jab and antivirals, such as those for swine flu and bird flu, can form discussion points as to how they differ from antibiotics. ➜ Students may believe that all microbes are pathogenic or harmful. Exploring the applications of microbes for human ends is helpful. ➜ Microbes are only found in dirty or unhygienic places. Practical activities to illustrate the variety of locations of microbes can help to counter these ideas. (See CLEAPSS advisory notes on suitable locations to sample.) The relative size of microbes can also be difficult for students to comprehend: bacteria are about one-tenth the size of a human cell. This means they are the approximate size of a cat or small dog, compared to adult human size. Microbes exist in very large numbers. Students will surely be fascinated to know that recent studies have suggested that we contain more bacterial cells than human cells. Microbes reproduce by binary fission, where one cell splits into two cells. It is different from mitosis because the nuclear material is not enclosed in a nucleus; students should be clear on the difference. However, the nuclear material replicates and then divides as the cell forms two new cells. Simple demonstrations on binary fission can help students appreciate the rate at which some microbes can multiply; if a bacterium divides every 10 minutes, then we have 64 cells at the end of the first hour and 4096 after the second hour. A colony of bacteria grown on an agar plate may have as many as a billion cells. It is not possible to identify bacteria just by the shape of the colony that grows on the agar plate. Microscopic examination and Gram’s staining narrows identification but biochemical tests are needed. Aseptic techniques are needed to ensure that contamination is kept to a minimum when growing colonies of microbes. Developing good aseptic techniques is important for the health and safety of the student and others, as well as the success of their practical work. Although not encouraging contamination, poor/failed technique can be a valuable source of discussion 285

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and a learning experience. It is important that you familiarise yourself with the CLEAPSS advice on the different microbial techniques. This chapter will suggest a number of activities to address many common misconceptions and aid better understanding. Practical work is particularly important but discussions, research, cross-disciplinary and cross-curricular links (for example with mathematics), and out-of-school visits that put the science in context are also valuable ways to aid learning. It is also a good idea to find out what students already know about microbes before planning any work. Drawings and concept maps are useful ways to discover students’ ideas (Byrne and Grace, 2010).

A teaching sequence There are various teaching sequences detailed in this chapter which fall under the following broad categories: what microbes look like; microbial locations; what microbes need to grow well; microbes and pandemics; microbes and genetic engineering; microbes and biotechnology; and handling microbes successfully and safely.

12.2 What do microbes look like?

KEY ACTIVITY

Microbes are generally very small and cannot be seen unaided, but the growth of micro-organisms and the effects of their growth can be seen easily on natural materials and in Universal bottles or Petri dishes. Organisms can be obtained from a reputable supplier (pure culture) or natural materials/the immediate environment (mixed culture). It should be noted that culturing in liquid broths is always a little more risky because of possible breakages and spills, and the formation of aerosols and possible contamination.

Observing microbes A good way of getting students to understand about the size, shape and structure of microbes is to observe them and draw what they can see. When using a microscope it may be necessary to help students to locate areas of the slide to observe organisms clearly (see Chapter 2). Start with algae and/or yeast, because these are the biggest microbes. Isolated cells or groups of algae can be drawn; groups of two or four provide evidence of asexual reproduction where organisms successively divide into two (see Chapter 2). Students should be asked to explain what is happening, including for groups of three (one of a pair has divided while the other has not, or both have divided and one has separated).

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With yeast, students see mostly individual cells and, occasionally, budding. Some yeast cultures exhibit clumping (when the cells complete fermentation and shut down their enzyme activity and drop to the bottom of the vessel), making single cells difficult to see. Moulds will often show spores and hyphae that make up the mycelium. When examining moulds, care must be taken to prevent the release of contaminating spores into the air.

Technology use A more powerful microscope (with an oil-immersion lens and perhaps with a computer-linked camera) is needed for students to see the shapes of bacteria. Bacterial smears can be made in advance. It should be noted that it takes a little more time to set the microscope up successfully to find bacterial cells.

A teaching sequence Microbial gardens and cultures provide an initial perspective on variety. Using a hand lens on cultures in a Petri dish can reveal interesting information about colonies of some of the major groups of microbes, including shape, colour, translucency and surface structure.

Viewing microbial colonies: suggested activities Microbial gardens can be used as an introduction to this subject for 12–14-year-olds. Students can observe the growth of organisms involved in natural spoilage by putting substrates (moist bread, fruit, vegetables) into a container such as a jam jar with a loosely fitting lid that cannot fall off but allows gases to escape. Meat and meat products should not be used. In later years (14–16), students can be introduced to cultures, for example, microbial culturing on agar plates. Petri dishes allow the observation of a variety of different organisms from a particular environment, including air, water and soil. High-risk areas like toilets and changing rooms should not be used. Samples from fingers (‘finger dabs’), hair from clean areas like the scalp, or vegetable food samples can also be used, but not animal products. Students at this age can be shown pure cultures, which can be sub-cultured onto a suitable medium in Petri dishes to show a collection of different microbes. They can also be grown on slopes in Universal bottles or test tubes.

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It should be noted that test tubes will need to be capped with either a metal/ plastic cap or a cotton wool plug. Test tubes are more fragile than Universals and more care is needed in their handling. Another investigation for this age group involves looking at the macroscopic differences between fungi and bacteria and identifying microbes. This investigation enables students to recognise the differences between fungal and bacterial colonies. Using a hand lens or a plate microscope, it is possible to see colonies clearly through the lid of the Petri dish and much information can be obtained this way. For example, students may easily see the mycelium and spore containers (sporangia) in fungal colonies, although more detailed investigations are needed to say definitely which colonies are bacterial and which are fungal. Students could make further deductions using the information in Table 12.1. Table 12.1 Identifying fungal and bacterial colonies Fungal colonies

Bacterial colonies

colony size

extensive; fills whole plate

smaller; discrete units

colony profile

tall

flat

colony appearance

surface often dull

surface often shiny

colony texture

like ‘cotton wool’

like a drop of liquid

colony colour

often grey/white

grey/white but also yellow, pink, red

growth medium

acid medium (pH 3.5–5.5) best for selecting fungi

medium close to neutral (pH 7.0) is best for selecting bacteria

Science in context If citrus fruit is forgotten and left in a fridge for a long time, you can often see fungi growing on the fruit but very few bacteria. Explain to students that since many citrus fruits have a low pH, this favours the growth of fungi rather than bacteria. A low-power microscope (×5 or ×10 objective lens) is the best way to introduce 12–14-year-old students to this topic. The largest micro-organisms, algae, protozoa and moulds, can be viewed easily with most microscopes found in schools and colleges. ➜ Algae

found in ponds, such as Scenedesmus quadricauda, may be studied. (In many schools it may be used for photosynthesis investigations.) An alternative is Pleurococcus, found on trees.

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12.2   What do microbes look like? ➜ Fungi from food in sealed Petri dishes, such as thin slices of blue cheese or

decaying fruit, are visible under the microscope. Alternatively, pure cultures of fungi can be obtained and, for convenience, slides of the mould could be made up before the lesson. ➜ Yeast cells are visible under the high-power objective (total magnification 10 × 40 = 400). ➜ A drop of water from a well-kept hay infusion can provide a very varied and interesting diversity of micro-organisms (algae and protozoans) with opportunities to see living microbes, many of them moving. Care must be taken with the handling of the hay infusion and subsequent disposal of slides into a disinfectant solution. Hands must be washed after such an investigation. As students become more competent, you could consider using an oilimmersion lens, which is suitable for 14–16-year-olds. To view bacterial cells, the objective oil-immersion lens and eyepiece give a total magnification of ×1000. Skills to stain cells on a microscope slide are needed. The lens is lowered into a drop of immersion oil which has been placed on a stained specimen. A coverslip is not used. Slides to show morphological characteristics can be made from bacteria from live cultures of yoghurt or dried yoghurt cultures. It is also possible to purchase single (pure) cultures from a reputable supplier that are either stained with a single stain or Gram’s staining method.

Bacterial shapes Bacteria can be grouped into three basic shapes: 1 bacilli bacteria (rod-shaped): bacillus; streptobacillus; coccobacillus 2 cocci bacteria (spherical or oval): cocci, diplococci, streptococci; tetrad; sarcina; staphylococci 3 curved bacteria: vibrios (comma); spirilla (spiral); spirochaetes (corkscrew).

Maths Practical demonstrations can help students understand the relative sizes of different microbes and the numbers in a colony. These activities can be linked with work in mathematics.

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KEY ACTIVITIES

Size and numbers of microbes: suggested activities Exploring size A nice introduction for students is to use familiar objects to model relative sizes, for example a cricket ball (human cheek cell, 65 μm), a broad bean seed (a yeast cell, 3–4 μm) and onion seeds (the flu virus, 80–100 nm). You can also cut tape to the size of different microbes using the scale of 10 metres of tape equal to the cross section of a human hair (0.1 mm) and place the various ‘microbes’ in order on the ‘hair’.

Exploring numbers Small objects, such as coloured counters, can be used to demonstrate binary fission and pose questions such as: l If

the counter were a bacterium, how many would there be at the end of the lesson if they could double every 3 minutes? l If a patient were infected with 100 bacterial cells on Friday evening which can divide every 1.5 hours, then how many would there be by Tuesday morning at 11.00? l What might limit the growth of the bacteria?

12.3 Microbial locations A teaching sequence Students tend to be fascinated to find out about the myriad of places where microbes can be found. Start with a class discussion about some places where students know of microbes and then add some more interesting examples when student ideas have been exhausted. Practical activities involving students looking at microbes from locations such as the air, water, soil and even human body surfaces can be very motivating.

Where are microbes found? Microbes are found everywhere! They can inhabit all the ecological environments found on Earth. Microbes can tolerate environmental conditions that other forms of life cannot: high temperatures, low temperatures, high pressure, long periods of drought and a wide range of pH conditions. For example: ➜ microbial

thermophiles: organisms that survive in hot conditions like geysers where the temperature can be close to boiling

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12.4   What do microbes need to grow well?

microbes: organisms that are able to survive in cold temperatures that would kill or disable other organisms ➜ deep ocean microbes: organisms that are ‘fuelling’ a deep sea ecosystem ➜ microbes in unusual environments: bacteria in jet fuel; bacteria in car oil filler caps.

KEY ACTIVITIES

➜ Arctic

Where can we find microbes? The following list gives brief details about some activities that can be carried out by students in the lab to find and view microbes. l Air: expose

agar in Petri dishes (plates) to the laboratory and outside for varying lengths of time (30 seconds to 1 hour). Students can then estimate how many microbes settle from the air onto a Petri dish in 1 hour by counting colonies that grow. l Water: inoculate agar plates with water from different sources (tap, river, pond, bottled). Estimate of numbers can be made per cm3 or litre by counting colonies that grow. l Soil: make up soil sample solutions (they often grow better on diluted agar plates). Students can then estimate how many microbes live in 1 g of soil. l Humans: students could compare the number of microbes in finger dabs from washed and unwashed hands. By estimating how many were on the pad of one finger, they could calculate the number of microbes on the hand. l Plants: students could compare number of microbes found on the upper and lower surfaces of a leaf. Are there more on the upper surface? If so, students could think about why this might be. l Water: the book Algae: A practical resource for secondary schools by James Redfern has a collection of five pupil investigations (further details given at the end of the chapter). In all the above investigations, students must be sure to seal the plates with three or four small pieces of tape when setting up. After incubation, it will also be necessary for agar plates to be sealed around their circumference just before students examine them. Unexposed plates as ‘controls’ will be needed and these can be used to explain aseptic techniques, ideally accompanied by a teacher demonstration of how to pour an agar plate aseptically. Microbiology techniques for pouring plates, inoculating plates, taping plus labelling plates and incubating plates can be found at the websites of the Microbiology Society and MiSAC (the Microbiology in Schools Advisory Committee) (full details are provided at the end of the chapter).

12.4 What do microbes need to grow well? Most microbes will grow well at an ambient classroom temperature and students can investigate the type of medium and other factors, such as pH and temperature, that can affect their growth. 291

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A teaching sequence This part of the topic is best covered with a series of practical activities on the conditions needed for microbes to grow and thrive. Younger students may enjoy re-enacting Pasteur’s original experiments. Simple experiments using yeast can then be carried out to determine under which conditions of temperature, pH and substrate type yeast grows best. Older and more experienced students could investigate the pasteurisation of milk. Investigations on conditions needed for microbes to grow will then lead naturally on to discussion about how the food and drink industries prepare and preserve our food to prevent spoilage by microbes.

KEY ACTIVITY

Investigating conditions needed for growth of microbes Investigating Pasteur It can be a really useful activity for 12–14-year-old students to re-enact a copy of Pasteur’s experiment. Figure 12.3 shows the results that should be obtained by the experimental set-up.

Test tube 1: broth; open tube

Test tube 2: sterile broth; capped tube

Test tube 3: sterile broth; open tube

Test tube 4: sterile broth; empty air lock

Figure 12.3 The expected results from the recreation of Pasteur’s experiment l The

cloudiness of the nutrient broth is an indicator of the extent of bacterial growth. 1 and 3 should turn cloudy due to bacterial contamination from direct sedimentation of spores from the atmosphere.

l Tubes

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l Tube

2 should remain clear because microbes are destroyed by boiling and ‘spontaneous generation’ of living organisms does not occur. l Tube 4 replicates Pasteur’s ‘swan-necked’ flasks (a test tube is used fitted with an air lock used in home brewing). l When the broths are observed by students, take great care to prevent breakages and spills, which would cause aerosol contamination in the laboratory air. l At the end of the investigation all samples will need to be destroyed by autoclaving.

Science in context You should ensure students have at least a passing knowledge of Louis Pasteur, the man who led the fight against germs (thus disproving a theory held since Aristotle!).

Later investigations suitable for 14–16-year-olds can move onto topics such as food preservation. For example, investigating the effect of different preservatives and temperatures on foodstuffs (peas). The degree of spoilage is estimated by viewing the turbidity of the suspension. Colorimetry could also be used to obtain turbidity readings. When carrying out this investigation, frozen peas should be used since they present minimal risk and are easy to count out; ensuring the same number of peas allows fair comparisons between different conditions. More detail on this can be found on page 30 of the 2016 PDF resource Practical microbiology for secondary schools (Preserving food), published by the Microbiology Society (full details of which are given at the end of the chapter). As an investigation, pasteurisation is usually only suitable for 16–18-year-olds, but you could consider using it as extension for particularly able students. Milk contains many nutrients, and that makes it an ideal medium for microbial growth. Milk is pasteurised to avoid contamination by unpleasant pathogens. Different milk samples (pasteurised and UHT) can be used to investigate the number of organisms present in the milk sample. Resazurin dye changes colour with microbial growth to indicate microbial activity. More detail on this can be found on page 36 of Practical microbiology for secondary schools (Microbes and milk).

Further activities ➜ Prove

it!: a simple investigation for 12–14-year-olds exploring the effect of temperature, substrate (flour type) and additives like vitamin C and enzymes on microbial (yeast) growth. The yeast produces carbon dioxide and causes the dough to rise when it grows.

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12 Microbiology and biotechnology ➜ Will

it grow; is it dead?: students can investigate the effect of chilling and/ or freezing food on microbes. Are they killed or do they start to grow again when removed from a fridge or freezer? ➜ Fungal growth: growth (of fungi) can be measured by colony diameter over a few days but the fungus Neurospora crassa grows so quickly, the diameter of the colony can be measured over a single school day (if it is inoculated the night before). ➜ Loss of carbon dioxide: this slightly more advanced investigation for 14–16-year-olds looks at measuring fermentation of yeast and investigating rates of activity. A flask is placed on a balance and the loss of carbon dioxide can be measured over 4 or 5 days. Graphs can be produced from the data. A more open-ended investigation of any of these activities is possible by asking students to consider and explore the different variables inherent in each activity.

Microbial growth and food preservation A range of factors influence microbial growth and these are used to our advantage in food production and preservation. Discussing such matters helps put the science in context and makes cross-disciplinary links with other topics. Table 12.2 summarises some common methods used to preserve our food. Table 12.2 Microbial growth and food preservation Method

How microbial growth is affected

Example

ultra-heat treated (UHT) (135 °C for 2 seconds)

all microbes and spores killed

UHT long-life milk

sterilised by boiling (100–115 °C for 15 minutes)

all microbes killed

sterilised milk

pasteurised (70 °C for about 15 seconds)

many microbes killed

pasteurised milk

canning (boiling and canning to exclude oxygen)

all microbes and spores killed

tinned beans

freeze drying

no water for microbial growth

instant mashed potato

preserving by adding salt

microbes plasmolysed

fish

preserving by adding sugar

water moves out of cells

jam

pickling

pH too acidic for microbes to grow

pickled onions

fridge (4 °C)

microbial growth slowed down

yoghurt

freezer (–18 °C or lower)

microbial growth stopped

frozen vegetables

sterilisation by irradiation (cobalt-60 source)

microbes killed by gamma radiation

salmon; shellfish

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12.5   Microbes and hygiene

12.5 Microbes and hygiene A teaching sequence Students are typically highly motivated by work that relates to bodily functions and fluids. The effectiveness of hand washing and various claims that manufacturers make about their products can be easily tested by designing their own investigations or following a protocol. A simple handwashing practical for younger students can be carried out with finger dabs on agar plates to see if microbes are present on a washed hand compared to an unwashed hand. Older students can also explore handwashing products. You could use finger dabs on agar plates to see what grows after different hand treatments (unwashed, soap, hand sanitiser, etc.). Alternatively, use plates that have been inoculated with cultures of safe microbes. Place filter paper discs impregnated with domestic toiletries on the culture and record the size of each halo. Useful references for these activities can be found on the internet. CLEAPSS sheet PP051 investigates environmental swabbing and finger dabs. The Practical Biology website (a joint project by the Nuffield Foundation, the Royal Society of Biology and CLEAPSS) has an investigation titled ‘How good is your toilet paper?’ and there is information on the NHS website about the best way to wash hands.

Hygiene and food preparation Despite greater public awareness about the need for food hygiene, outbreaks of food poisoning still occur. Students should be made aware of the basic rules for safe food handling and storage. To introduce students to this topic you could get them to research the advice given by the government and other agencies with regard to food hygiene and safe preparation. Students aged 14–16 years can be tasked to investigate different packaged foods to work out how long they can be stored and what ‘sell by’ and ‘use by’ dates mean. These investigations can be linked to work on food preservation (covered in an earlier section). They could also be asked to find out why eggs are a success story, why government advice changed in 2017 and the value of the red ‘British Lion Mark’.

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Useful references for these activities can be found at the end of the chapter, and include the website of The Food Standards Agency, Chilled Education (resources from the chilled food industry) and BUPA.

Clean water and microbes Students’ views and thoughts about ‘clean water’ can be explored by considering some of the following questions: ➜ What

is meant by ‘clean’ water? water from a spring, a well or a tap contain microbes? ➜ Do all parts of the world have clean drinking water? ➜ What are the implications of not having clean drinking water? ➜ How was the spread of cholera halted in London in the nineteenth century? ➜ What advice for consumers on the importance of ‘clean’ water would you give? ➜ Does

Science in context Cholera outbreaks killed thousands of Londoners in the eighteenth and nineteenth centuries. After the 1848–49 cholera outbreak in London, John Snow decided to systematically track down the cause of the disease in London. He suspected it was a waterborne contamination, not airborne as previously thought, and so he methodically mapped incidences of cholera, combining this information with data about which water companies households bought their water from. He proved that those who bought water from companies that drew water from the most contaminated parts of the Thames were most likely to suffer from the disease. John Snow doggedly continued with his work until a particularly brutal outbreak in the area of Broad Street in 1859 led him to draw his now famous spot map of cholera incidences and enabled him to identify a single water pump as the source of the contamination. Once closed off, the incidence rate of cholera fell away. The presence of bacteria in water can be shown by inoculating agar plates with water samples from different sources, such as from the environment or bottled water sold in shops, and investigating for the presence of microbes. Students can find out about the principles of the first stages of water treatment and purification by filtering muddy water samples; this activity can be done as a problem-solving activity. Chemical purification of filtered samples can be demonstrated.

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12.6   Microbes and digestion

Science in context Students could research the United Nations Sustainable Development Goal 6: Ensure access to water and sanitation for all.

12.6 Microbes and digestion Cross-disciplinary This topic lends itself to connections with other topics in biology and with geography, including the cycling of matter, digestive processes and global warming.

A teaching sequence Students aged 12–14 years old can look at the role microbes play in digestion for herbivorous mammals, such as cows and sheep. The digestion of plant material by symbiotic microbes results in the production of methane, a greenhouse gas that is released into the environment. Students can research and report back to the class for discussion arguments around meat production and greenhouse gases. Students can also look at the role microbes play in the breakdown of waste material produced by animals and plants (in the nitrogen cycle). They can observe how microbes break down, or ‘digest’, plant material by looking at compost heaps or piles of lawn clippings. More open-ended investigations that allow students to explore the factors that aid decomposition are possible. Moving into later years, this can be expanded to look at how microbes are important in the digestive processes of humans. Students can research the microbiome of humans and its importance in human health. 14–16-year-olds could also investigate the action of microbial cellulases on filter paper and/or the action of microbial enzymes on substrates (for example, the breakdown of starch by microbes). More activities like this can be found on the document Practical Microbiology for Secondary Schools, published by the Microbiology Society and available to download as a PDF from their website (details found at the end of the chapter).

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12.7 Microbes and disease A teaching sequence Previous sections have introduced students to the range of different microbes, including some that cause diseases in humans, and this leads naturally to a deeper understanding of pathogens and the development of vaccines and, for older students, the functioning of the immune system.

Microbes and pandemics Headlines and news stories about ‘bugs’ that affect humans intrigue students, providing an excellent starting point for work on microbes and health, putting the science in context. Stories also provide a context for understanding fundamental microbiological principles and the ways in which microbes affect the lives of all living things. In late 2019 and early 2020 a new pathogenic virus spread around the world. It was identified as a coronavirus (SARS-CoV-2), a virus similar to one identified in 2013 commonly known as SARS (severe acute respiratory syndrome-related coronavirus). Remarkably, vaccines against the virus were brought to market by the end of 2020, leading to hopes that the worst of the pandemic might be over within another twelve months. Why did this particular coronavirus spread so easily and why was it so infectious? Droplets, as airborne particles from coughing and sneezing or personal contact, transmitted the virus from person to person. The virus could also remain active for several days on surfaces and objects such as door handles, so it was readily passed from human to human. The crown-shaped virus has spike proteins that allow it to bind to membranes of human cells very efficiently – ten times more efficiently than other similar coronaviruses – causing a respiratory disease that affects oxygen diffusion in the alveoli. Worldwide, coronavirus changed the way that governments and the public behaved, resulting in governments imposing unprecedented restrictions. People were told to stay in their homes wherever possible (in ‘lockdown’), working from home and avoiding contact with people from other households, in an effort to hinder the spread of the virus, for which there were no known drugs or vaccines. Experts from different disciplines across the world – scientists, engineers and medical professionals – worked together in a common cause to control the virus and care for people infected with it. Scientists worked on producing

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reliable tests to detect the virus, producing a safe vaccine and looking for antivirals and drugs that might control the spread and infection of the virus. Engineers produced large quantities of medical equipment, such as respirators and ventilators to assist the breathing of infected patients, as well personal protective equipment (PPE) to protect medical professionals and those in contact with the vulnerable.

Science in context The internet played a major role in the 2020 coronavirus pandemic, allowing scientists to exchange data and publish scientific papers while allowing the public to communicate with friends and relatives while they were in lockdown. Unfortunately, it also propagated bogus and inaccurate information about the virus and how to cure it that was dangerous and harmful if followed. Students might be interested to look at some of the many websites, with excellent scientific information from epidemiologists, immunologists and virologists, that were created about the virus and the pandemic. The Microbiology Society produces a collection of resources (fact files) on different microbes and disease, which are a valuable source of information. MiSAC have a collection of MiSACmatters Anniversary Articles on a wide range of micro-organisms and their activities, including many covering aspects of health. Details of both of these resources are provided at the end of the chapter.

Antibiotics and antibodies Students should understand that the use of antibiotics has greatly reduced deaths from bacterial infections but that overuse and inappropriate use of antibiotics have resulted in bacterial strains that are able to tolerate antibiotics (antibiotic-resistant). This is a topic that deserves serious consideration by students and can help to increase scientific literacy. The Longitude Prize has a very interesting and valuable page on their website entitled ‘10 most dangerous antibiotic-resistant bacteria (details at the end of the chapter).

A teaching sequence Students could start by doing some library research to find out the causes of different types of infection, such as viral, bacterial and fungal, and how these are treated.

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They can then move on to investigate microbial sensitivity to antibiotics. Discs impregnated with an antibiotic can be used to test the sensitivity of different microbes to the antibiotic. The discs can be placed on lawn cultures and the presence of halos, indicating where colonies are prevented from growing, can be noted and measured. Another useful investigation into the effectiveness of antibiotics can be carried out using ‘Mast’ antibiotic susceptibility discs. Tests can be set up to show how a specific bacterium should be treated with a specific antibiotic. Students should understand that not all antibiotics kill all bacteria. Open-ended investigations are also possible by asking appropriate questions such as, ‘Which microbes are killed by streptomycin?’. More information on practical investigations relating to antibiotic action can be found at the end of the chapter.

12.8 Microbes and genetic engineering What are microbial model organisms? A model organism is a species that has been widely studied, usually because it is easy to maintain and grow in a laboratory and has particular experimental advantages. Model organisms are used to obtain information about other species, such as humans, where it is difficult to study them directly. Some commonly used model organisms are detailed below. ➜ Streptomyces

coelicolor: notable for its production of pharmaceutically useful compounds, such as anti-tumour agents, immunosuppressants and over two-thirds of all natural antibiotics. ➜ Bacillus subtilis: when stressed, B. subtilis transforms itself into a spore and enters a dormant state. This bacterium is found in soil and the gastrointestinal tract of ruminants and humans. ➜ Azotobacter vinelandii: a nitrogen-fixing soil organism that has been studied for over 100 years. It was the experimental organism of choice for many investigators during the emergence of biochemistry and is a model for biochemical and genetic studies of biological nitrogen fixation (the conversion of N2 into NH3 by a nitrogenase enzyme). ➜ Escherichia coli: the majority of strains are harmless but some, such as E. coli 0157, can cause serious food poisoning and severe infections. Commonly found in the lower intestine of warm-blooded organisms, E.

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coli forms part of the normal range of microbes found in humans. Much of our understanding of the fundamental concepts of molecular biology has resulted from studies on E. coli. The first K-12 strain of E. coli was isolated in 1922 but after many generations of laboratory cultivation it has undergone significant changes, making it a poor coloniser of the mammalian gut. The resulting K-12 strains are particularly safe for use in school practical work. ➜ Saccharomyces cerevisiae: one of the simplest eukaryotic model organisms. It is important in common biochemical pathway studies in organisms including humans. It also has important applications in the food and biotechnology industries.

Working with GMOs: health and safety in schools and colleges in the UK Anyone carrying out work with GMOs must do so only on premises that have been registered with the relevant authority. There is, however, practical work that can be done in schools if it is ‘self-cloning’. (Procedures with recommended strains of E. coli and DNA sequences are exempt from the Contained Use regulations.) To ensure good microbiology practice and exemption from the regulation, the bacteria that students produce must be autoclaved at the end of the investigation. It is important for teachers to keep up to date with safety. It would be useful to look at the ASE’s Topics in Safety (Topic 16 Working with DNA) when teaching this content (see end of chapter for details).

A teaching sequence Students can investigate the way in which bacteria acquire antibiotic resistance through conjugation (a natural process in bacteria). More details can be found on the website of the National Centre for Biotechnology Education (University of Reading), given at the end of the chapter. Older or more advanced students can look at investigating ‘self-cloning’. Transformation kits are available for use in schools; they produce transformed micro-organisms that are green due to a GFP (green fluorescent protein), red due to a RFP (red fluorescent protein) or blue due to X-Gal breakdown by an enzyme (b-galactosidase). A list of suppliers selling transformation kits is given in the Resources section at the end of the chapter.

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12.9 Microbes and biotechnology This topic provides opportunities for studying science in context as well as cross-disciplinary work, such as genetics.

Science in context Ancient civilisations employed biotechnology to increase soil fertility and produce food and drink. These traditional biotechnologies still play a vital role in our modern lives and students could investigate them through simple practical work or out-ofschool visits. They include: l the

production of beer and wine of food like sauerkraut, soy sauce and vinegar l modification of raw products (milk to cheeses, lassi, yoghurt, kefir) l production of clean drinking water and sewage treatment of waste water. l fermentation

A teaching sequence A sequence could start by exploring the importance of microbes in industrial processes. This is best introduced by starting with familiar products. It could then move on to a discussion about microbial by-products. Microbes are important in a range of industrial processes because of the by-product the microbe produces. Some of these products and processes can be investigated by older students (aged between 14 and 18). They include: ➜ the

production of enzymes by microbes – amylase production of bacterial enzymes used in industry (pectinase, proteases, lipases, amylases, lactase, invertase, etc.) ➜ the production of organic chemicals by microbes – citric acid, glycerol ➜ the production of antibiotics and antibodies from microbes ➜ bioremediation – breakdown of cellulose by a bacterium (Cellulomonas spp.) ➜ production of foods by microbes – tofu, tempeh, miso, fermented fish and meat. ➜ investigations

Technology use ICT, such as sensors linked to a computer or data loggers, can be used by students to show the rate of microbial activity. Investigations could include: l making

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Useful resources, such as a collection of suggestions for practical investigations in biotechnology, information on biotechnology or desk-top activities, are detailed at the end of the chapter.

Science in context The success stories for biotechnology are many and varied, but the following list gives a flavour of the diversity of biotechnology. Students could investigate the topic and then produce a report or presentation on the production of: l mycoprotein

– Quorn a novel food fuels – gasohol l drugs and medicines – antibiotics and medicinal proteins l enzymes for the detergent industry l vaccines and antibodies. l alcohol-rich

Modern molecular biology has been employed to create genetically modified microbes that produce the following important products. Researching the topic to find out more about the science behind the organisms and the product produced is suitable for 14–16-year-olds or older students: ➜ the

production of human insulin (Humulin) in E. coli production of human growth hormone in E. coli ➜ the connection in cheese manufacture between the enzyme Fromase and the microbe Rhizomucor miehei ➜ pest-resistant crops, Bt protein and the bacterium Bacillus thuringiensis ➜ the production of golden rice and beta-carotene (Narcissus and the bacterium Erwinia uredovora) ➜ herbicide-resistant crops and the use of ‘Roundup’. ➜ the

The development of genetically modified organisms (GMOs) has been the subject of some considerable debate in many countries over the last 30 years. The principles of genetic modification are easy to understand if students have an understanding of cell structure and DNA. The topic is better suited to students of 14 years and upwards, but physical models and practicals that extract DNA from plant material would help younger students gain a better understanding. The advantages of including work on topics such as genetically modified organisms are that it provides an opportunity for students to experience different points of view and people’s personal beliefs as well as examine the science behind the different views. Topics to study could include: selective breeding, genetic modification and cloning of animals (Dolly the sheep) and plants, and gene therapy. Some useful resources on the topic of microbes, biotechnology and genetic modification are given in the Resources section at the end of the chapter. 303

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12.10 Handling microbes successfully and safely Increasing the success of microbial investigations Early planning is essential for obtaining good results and to ensure cultures are growing well for use in the classroom. Two months before you plan to teach the topic, order micro-organisms from a reputable supplier and check that all equipment and resources are available, including loops, bottles, etc. Three weeks before, liaise with technicians; plates and media can be made up in advance of the lesson. Either keep in a fridge upside down or in a cupboard, in their original Petri dish bags. They must be kept in the dark and taped down to reduce drying out of the medium and the possibility of condensation forming in the plates. (Lid condensation falling onto the medium can make the plate surface too wet to use.) If contaminants are growing on the medium do not use the agar plate. If the culture has come from a supplier or as a stock culture in the school, it will need to be subcultured by the technician or teacher to ensure the culture is active for the classroom. The subculture will need to be subcultured (transfer of culture from one agar plate to new agar plate/s) to have enough for a class practical. Good aseptic techniques need to be understood and adhered to at all times by technicians, teachers and students.

Increasing the safety of microbial investigations CLEAPSS have a comprehensive and informative collection of documents, for CLEAPSS members, on safe handling of microbes. These are available from the website, detailed in the Resource section which follows.

Equipment A list of equipment for safe microbiology practical work can be found in Basic Practical Microbiology: A Manual published by the Microbiology Society (2016). Details of this publication are given at the end of the chapter.

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Risk assessment Microbiology often raises different risks from many other biology topics but none that are insurmountable with good laboratory practice. Students can be involved in developing their own safety rules; being able to assess risk and work safely is an important part of learning. Useful guidance on this can be found in Basic Practical Microbiology: A Manual (Microbiology Society, 2016), which provides information on a suggested risk assessment strategy. Further guidance is provided in the ASE’s publication Topics in Safety (see Topic 15: Microbiology). Full details are given at the end of the chapter.

Aseptic technique The purpose of aseptic technique is to prevent contamination of the environment and worker by the culture being handled and also contamination of the culture by micro-organisms from the environment.

A summary of aseptic techniques ➜ Work

near a Bunsen burner so that airborne organisms which might contaminate the work are carried away by the up-draught. ➜ Flame the neck of glass bottles and tubes to ensure an up-draught to help prevent possible contaminants entering (do not heat too strongly, as rapid cooling may draw air and contaminants into the vessel). ➜ Open culture and sterile equipment for the shortest possible time to help prevent the chance of contamination. ➜ Correctly flame loops to sterilise them before and after use; this helps to prevent contamination. ➜ All equipment and media to be used for microbiology should be autoclaved or sterilised chemically before and after use. Heat and moisture (steam in an autoclave) at 121 °C is used routinely in schools and colleges for sterilisation.

A summary of good laboratory practice for microbiology ➜ Wear

protective clothing. ➜ Clean hands before and always wash hands after working with microorganisms. ➜ Wipe surfaces down with a suitable disinfectant such as Virkon or Biocleanse.

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cuts and scratches with a waterproof plaster. eat or store food or drink in the laboratory. ➜ No hand-to-mouth activities: sucking pens or pencils, licking labels or mouth pipetting. ➜ Treat all cultures as possible pathogens. ➜ Do not remove cultures from the laboratory. ➜ Do not obtain cultures from potential sources of pathogens. ➜ Do not incubate cultures at 37 ºC; 30 ºC is the normal upper limit. (Note: there are exceptions for yoghurt and specific molecular biology organisms.) ➜ Do not grow cultures under anaerobic conditions which can favour some human pathogens. (Note: an exception is the fermentation of yeast, which may need to be anaerobic.) ➜ Obtain cultures from reputable suppliers not from a pathology laboratory. ➜ Never

12.11 Resources A book of this sort cannot contain everything. Many readers will want to use it in conjunction with a good student textbook. In addition, the following organisations and resources are recommended as valuable sources of information and advice.

Websites Websites with information and advice on safe handling of microbes CLEAPSS: one of the foremost resources for advice about practical school science activities, including a wealth of information about safety: http:// science.cleapss.org.uk. (Helpline: https://science.cleapss.org.uk/helpline/; telephone: 01895 251496) ASE (Association for Science Education): www.ase.org.uk/resources/healthand-safety-resources (general enquiries: [email protected]; telephone: 01707 283000) MiSAC, the Microbiology in Schools Advisory Committee, formed in 1969 to promote the teaching of microbiology in schools and colleges: www.misac.org. uk (contact: [email protected]) SSERC (Scottish Schools Education Research Centre): www.sserc.org.uk ([email protected]; telephone: 01383 626070)

Websites with information and advice from learned societies The British Mycological Society: www.britmycolsoc.org.uk/education

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Microbiology Society: https://microbiologysociety.org. Find materials for schools by searching ‘educational resources’ in the Members’ Outreach and Resources tab. Society for Applied Microbiology: https://sfam.org.uk; (telephone: 0207 685 2596)

Websites with information and advice from other organisations NCBE, the National Centre for Biotechnology Education at Reading University: www.ncbe.reading.ac.uk (telephone: 0118 987 3743) National STEM Learning Centre: www.stem.org.uk (telephone: 01904 328300) Science & Plants for Schools (SAPS): www.saps.org.uk (email: saps@cam. ac.uk; telephone: 01223 748455) Nuffield Foundation: www.nuffieldfoundation.org The Royal Society of Biology: www.rsb.org.uk The Wellcome Trust: www.wellcome.ac.uk/

Websites of relevant journals Catalyst: https://catalyst-magazine.org Journal of Biological Education: www.tandfonline.com/toc/rjbe20/current School Science Review: www.ase.org.uk/journals/school-science-review Science Teacher Education: www.ase.org.uk/journals/science-teachereducation

Websites with advice on careers in microbiology Advice on careers in this field of biology can be found by carrying out searches on the following websites: Microbiology Society: https://microbiologysociety.org The National Careers Service, a government run job website: https:// nationalcareers.service.gov.uk/job-profiles/microbiologist UCAS: www.ucas.com

Websites with information on practical microbiology techniques The Microbiology Society has published a resource entitled Basic Practical Microbiology: A Manual which can be downloaded free from their website. Search the title of the publication from the homepage: https:// microbiologysociety.org

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YouTube hosts a number of interesting and informative videos from the Microbiology Society. Find them by searching from the homepage: www. youtube.com The Practical Biology website has an interesting activity on hygiene entitled ‘how good is your toilet paper’: https://pbiol.rsb.org.uk/health-and-disease/ hygiene MiSAC, the Microbiology in Schools Advisory Committee offers advice on practical techniques on its website: www.misac.org.uk The National Centre for Biotechnology Education (University of Reading) details a practical investigation of bacterial conjugation, using E. coli: www.ncbe. reading.ac.uk/PRACTICALS/PDF/Antibiotic1.3_UK_eng.pdf Kits for investigations involving bacterial transformation can be found via the following suppliers: ➜ The

NCBE: www.ncbe.reading.ac.uk/MATERIALS/Microbiology/ transformation.html ➜ Philip Harris, search ‘transformation of E. coli’ from the homepage: www. philipharris.co.uk ➜ SciChem, search ‘Bio-Rad pGLO bacterial transformation kit’ from the homepage: https://education.scichem.com ➜ Breckland Scientific: www.brecklandscientific.co.uk/BIO-900-200-p/bio-900200.htm

Websites related to food hygiene The Food Standards Agency have produced teaching resources for catering students going into the food industry. There is also information and some ideas that teachers may find valuable in the design of their lessons: www.food.gov. uk/business-guidance/safer-food-better-business-teaching-resources-forcolleges The website Chilled Education, supported by the chilled food industry, has many resources for students of differing ages and many interesting and thought-provoking ideas: www.chillededucation.org/food-teacher/foodteacher-lesson-plans The BUPA website has plenty of information on food hygiene: www.bupa.co.uk/ health-information/nutrition-diet/food-safety

Websites related to microbes and digestion Microbes and Climate Change, a well-illustrated fact file from the Microbiology Society, has a section on ruminants and methane production. It can be accessed by searching ‘microbes and climate change’ from the homepage of the society: https://microbiologysociety.org

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A two-page article on the human microbiome – Gut microbes: we are not alone – can be found at: http://misac.org.uk/article-downloads/20. Rolhion-2019.pdf

Websites related to microbes and disease The Microbiology Society produces a collection of resources (fact files) on different microbes and disease which are a valuable source of information: https://microbiologysociety.org Topics include: ➜ Malaria

– a global challenge Wars ➜ Pandemic H1N1 ‘swine flu’ ➜ HIV and AIDS ➜ Influenza: A seasonal disease ➜ Tuberculosis – can the spread be halted? ➜ Cholera: Death by diarrhea. ➜ Cold

MiSAC have a collection of MiSACmatters Anniversary Articles on a wide range of micro-organisms and their activities, including many covering aspects of health: http://misac.org.uk/health.html

Websites related to the action of antibiotics A resource from the Microbiology Society, Basic Practical Microbiology: A Manual, contains a wealth of information about practical activities involving microbes. See page 21 ‘Testing sensitivity to antimicrobial substances’. Search ‘basic practical microbiology: a manual’ from the society’s homepage: https://microbiologysociety.org Another useful resource from the Microbiology Society, Antibiotic Resistance: A Challenge for the 21st Century, is an excellent booklet that explains the discovery of antibiotics, what they are, how they work and how resistance develops. It also offers suggestions for alternatives and ways to slow the spread of resistance. It can be accessed by searching ‘antibiotic resistance a challenge for the 21st century’ from the homepage: https://microbiologysociety.org The Practical Biology website has information about a practical activity relating to anti-microbial action. It can be accessed by searching ‘investigating anti-microbial action’ from the homepage: https://pbiol.rsb.org.uk This PDF document from the National Centre for Biotechnology Education (NCBE) at Reading University details the action of streptomycin on different microbial cultures: www.ncbe.reading.ac.uk/materials/Microbiology/PDF/ streptomycin.pdf

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Websites relating to microbes, biotechnology and genetic modification A PDF of the document Practical Microbiology for Secondary Schools can be downloaded from the website of the Microbiology Society. From the homepage (https://microbiologysociety.org/) input the search term ‘practical microbiology for secondary schools’. This document contains 21 investigations suitable for secondary school science lessons. The ABPI (Association of the British Pharmaceutical Industry) website has a wealth of resources for schools including a number on the topic of biotechnology and genetic manipulation (search ‘genetic engineering’ from the homepage): www.abpischools.org.uk The Biotechnology and Biological Sciences Research Council (BBSRC) has a number of downloadable resources on the topic: https://bbsrc.ukri.org/ engagement/schools/ The Royal Society has produced a report on genetically modified plants that is useful with students. Search ‘GM plants questions and answers’ from the homepage: https://royalsociety.org/ Finally, a resource for the connoisseur may be of interest in ‘GM Science Update’ (2016): https://assets.publishing.service.gov.uk/government/uploads/system/ uploads/attachment_data/file/292174/cst-14-634a-gm-science-update.pdf

Other interesting internet resources This article gives more information about the isolation and identification of a new variant strain of the Hanta virus, the Sin nombre virus: wwwnc.cdc.gov/ eid/article/5/5/99-0512_article Facts about Ebola virus from the WHO: www.who.int/news-room/fact-sheets/ detail/ebola-virus-disease The NHS website gives information for the public about the MMR vaccine for measles, mumps and rubella: www.nhs.uk/conditions/vaccinations/mmrvaccine Notable female microbiologists you’ve probably never heard of: https://blog. oup.com/2019/03/notable-female-microbiologists/ Using a laser pointer lens and a smartphone as a microscope: www.misac. org.uk/article-downloads/31.Schollar.pdf e-bug, a comprehensive collection of fun activities and teaching resources: www.e-bug.eu MiSAC, the Microbiology in Schools Advisory Committee, offers a number of PDF documents giving some historical perspectives to the topic:

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12.11   Resources ➜ Pasteur

and Lister through the microscope: www.misac.org.uk/articledownloads/28.Grainger-Pasteur.pdf ➜ Bacterial endospores – their roles in resolving the spontaneous generation controversy and understanding cell development: www.misac.org.uk/articledownloads/29.Grainger-Endospores.pdf The Longitude Prize has a page on their website entitled ‘10 most dangerous antibiotic-resistant bacteria’: https://longitudeprize.org/blog-post/10-mostdangerous-antibiotic-resistant-bacteria

References Byrne, J., Grace, M. and Hanley, P. (2009) Children’s anthropomorphic and anthropocentric ideas about micro-organisms: Do they affect learning? Journal of Biological Education, 44 (1), 37–43. Byrne, J. and Grace, M. (2010) Using a concept mapping tool with a photograph association technique (CoMPAT) to elicit children’s ideas about microbial activity. International Journal of Science Education, 32 (4), 479–500. Byrne, J. (2011) Models of micro-organisms: Children’s knowledge and understanding of micro-organisms from 7 to 14 years-old. International Journal of Science Education, 33 (14), 1927–1961. Jones, G., Gardner, G., Lee, T., Poland, K. and Robert, S. (2013) The impact of microbiology instruction on students’ perceptions of risks related to microbial illness. International Journal of Science Education, Part B, 3 (3), 199–213. Leach, J., Driver, R., Scott, P. and Wood-Robinson, C. (1996) Children’s ideas about ecology 2: ideas found in children aged 5–16 about the cycling of matter. International Journal of Science Education, 18 (1), 19–34. Redfern, J., Burdass, D. and Verran, J. (2015) Developing microbiological learning materials for schools: best practice. FEMS Microbiology Letters, 362 (6), 1–7. Simonneaux, L. (2000) A study of pupils’ conceptions and reasoning in connection with ‘microbes’, as a contribution to research in biotechnology education. International Journal of Science Education, 22 (6), 619–644.

Further reading ASE Health and Safety Group (2018) Topics in Safety (3rd edition). Hatfield: The Association for Science Education. Available at: www.ase.org.uk/ resources/topics-in-safety (Topic 15 Microbiology, Topic 16 Working with DNA, Topic 20 Working with enzymes)

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ASE (2006) Safeguards in the School laboratory (11th edition). Hatfield: The Association for Science Education. Hogg, S. (2013) Essential Microbiology (2nd edition). New Jersey: Wiley-Blackwell. Parker, N. et al. (eds) (2017) Microbiology. Washington, DC: ASM Press. (Available online at: https://openstax.org/details/books/microbiology) Postgate, J. (2008) Microbes and Man (4th edition). Cambridge: Cambridge University Press. Redfern, J. et al. (2012) Algae: A Practical Resource for Secondary Schools. London: Microbiology Society (previously the Society of General Microbiology). Stearns, J. C., Surette, M. G. and Kaiser, J. C. (2019) Microbiology for Dummies. New Jersey: John Wiley & Sons.

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Index

adaptation  217–25 adolescence  169 adrenal gland  141 adrenaline  141 aerobic respiration  46, 52–3 agriculture  68, 70, 109, 228 air quality  272–4 algae  282, 288, 291 allopatric speciation  231 ammonia  36 anaerobic respiration  46–7, 49, 53–5 anatomy, misconceptions  94, 169–71 animations  15 anthropomorphism  216, 220–2 antibiotics  299–300 antibodies  299–300 anticipatory thinking  258 archaea  282 arteries  118–20 artificial insemination (AI)  174 artificial selection  231 aseptic technique  305 assessment, summative  4 asthma  87–9 ATP (adenosine triphosphate)  49–50, 81 Attenborough, D.  252 bacteria  283, 289–90, 296 balance  139–40 beavers  237 bias  7, 260, 269 big ideas  4–6, 44, 252 binary fission  37 biodiversity  5, 228–48, 267–9, 272–3 conservation  232–8 defining  230 fieldwork  248 habitats  229, 244–5, 267–8 importance of  231–3 loss  235–8 overview  228 practical work  241–7, 267–9 prior knowledge  229 quadrats  243–7 restoring  236–7 role of zoos  238 speciation  230–1 teaching sequence  229 trophic levels  267 water  242–3

biological reasoning  8 biologists  6 biology big ideas  4–6, 44, 73, 252 in context  8, 14–15 defining  2 ethics  9 history of  3 key concepts  2–3 biomass  67–8, 264 biotechnology  302–3 blood  114, 121–3 giving blood  115 see also circulatory system body temperature see thermoregulation botanical gardens  238 breathing  78–88 calorimetry  48 cancerous growth  159 capillaries  120–1 carbohydrates  35, 46, 49, 60 carbon cycle  67 carbon dioxide  3, 22–3, 36, 46–8, 53–5, 57–8, 63, 65–6, 78–9, 93, 123 carcinogens  159 cardiac cycle  118 careers  42, 47, 51, 68, 109, 114, 123, 147, 158, 176, 200, 233, 237, 242, 259, 281 Carson, R.  252 cells and adaptation  224 division  36–41, 159 drawing  31–2 and environment  253 eukaryotic  17, 37, 54 as fundamental unit  2 gametes  38–41 organelles  17, 54 overview  17–18 practical work  33 prokaryotic  17, 37, 54 somatic  38 structure  32–3 studying  30–4 teaching sequence  18 three-dimensional models  32–3 cell theory  17–18, 54 characteristics of living organisms

(COLO)  18–24 chemical communication  141 chemistry, of living organisms  34–6 chlorophyll  56–7, 59 choking  83 cholera  296 chromatography  59 chromosomes  37–41, 184–5 circulatory system  114–24 see also transport systems classification of living organisms  238–40 skills  13 climate change  261–7 cloning  158–9, 177 collaborative technologies  15 colorimetry  293 combustion  48, 51 compost  70 concept cartoons  11 constructing understanding  9–11 contexts  8, 14–15 contraception  173–4 controlling variables  7 control systems  127 chemical control  141 disturbances  151–2 and environment  253 feedback loops  130 homeostasis  147–51 hormones  141–7 nervous system  132–3 practical work  139–40, 150 prior knowledge  127 senses  134–40 stimulus-response chain  130–1, 141, 148 teaching sequence  127–8 thermoregulation  129–30, 148–50 co-ordination  127 coronavirus  298–9 cortisol  141 COVID-19 impact of exercise  90 impact of smoking  88–9 spread of  298–9 Crick, F.  3, 186 cyanobacteria  283 cystic fibrosis  198–9 cytoplasm  17, 35, 54

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Index Darwin, C.  3, 208, 212–13, 231 data logging  15, 62 da Vinci, L.  58 Dawkins, R.  216, 223 decomposers  69 decomposition  69–70 detritivores  69 diagrams, human cheek cells  32 dialysis  94–5 dietetics  51 diffusion  73–7, 82, 98–100, 102–3 digestion  48, 225, 297 digital technologies  15 dinosaurs  210–11 diploid number  38 diseases  152, 282, 296, 298–9 dissection  94, 116–17, 136 DNA  3 DNA (deoxyribonucleic acid)  17, 37, 56, 183, 185–8, 192–3, 198–9, 201–3, 224 Dobzhansky, T.  207 drugs  151–2 ears  138–40 ecological pyramids  68–70 ecosystems  5, 67–70, 225, 228, 231–2, 270 see also biodiversity egestion  93 emphysema  102 endangered species  235 Endler, J.  223 endocrine glands  141–2 energy calorimetry  48 dissipation  2 ecosystems  67–70 flow  51, 67–70 food as source of  51–2 law of conservation of  2 overview  44–5 in physics  45–6 storage  49 stores and transfers model  45 teaching sequence  56, 67 transfers  51–3, 264–5 transformations model  45–6 energy drinks  45 environment air quality  272–4 climate change  261–7 education perspectives  251, 257–61

fieldwork  254–7, 265–6, 270–1 food security  270–1 and health  274–5 in the media  260 overview  251 practical work  262–4, 272–3 prior knowledge  253 water quality  272–4 see also biodiversity enzymes  34, 224 Escherichia, T.  282 ethanol  47, 54 ethical issues  9, 117, 176–7, 242 eukaryotic cells  17, 37, 54 evolution  3 adaptation  217–25 artificial selection  231 controversy  214–15 fossils  209–10 geological time  210–11 misconceptions  216–17 natural selection  207–17, 230–1 overview  207–8 practical work  214 prior knowledge  207–8, 217 selective breeding  212–14 teaching sequence  208–9, 217–19 timeline  211 exchange diffusion  73–7, 82, 98–100, 102–3 and environment  253 gas exchange  78–88, 224 kidneys  93–5 misconceptions  79–80 overview  73–4 physical principles  75–6 practical work  76–8, 85–6, 89–90 pressure changes  80 prior knowledge  74–5, 79 surface area  77, 100–1 teaching sequence  75, 79 volume ratio  77, 100–1 see also excretion excretion  21, 34–6, 92–5 physical principles  92–3 prior knowledge  92 teaching sequence  92 see also exchange exercise  89–91, 123 extinction  235 eyes  135–7 fake news  260 fats  49

fermentation  54 fertility, control  172–4 fieldwork  7, 13, 20, 248, 252, 254–7, 265–6, 271 fitness  90 flooding  266 folded membranes  101 see also surface area food absorption  47–8 chains  67–70, 229, 232 digestion  48 ecosystems  67–70 labels  51 misconceptions  67 preparation  295–6 preservation  293–4 security  270–1 as source of energy  51–2 waste  270–1 webs  268–9 forces  127 Forest Schools  253 fossils  209–10 Franklin, R.  3, 186 fungi  48, 283, 289 Galápagos tortoise  212–13 gametes  38–41, 195–6 gas exchange  78–88, 224 gas sensors  62 gender  197–8 genes  182–200 genetically modified organisms (GMOs)  301 genetic engineering  300 genome  191–6, 198–9, 203, 214 genotype  194 geographical information system (GIS)  266 geological time  210–11 germinating seeds  52–3, 64, 161–2, 163 global warming  232, 261–2 glucagon  141 glucose  46–7, 65, 123, 141 glycosis  47 Gosling, R.  186 gravity  37 group work  12 habitats  229, 244–5, 267–8 haemophilia  198 handwashing  295 Harlen, W.  4–6, 44, 73, 229, 252

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Index Harlen progression  5–6 Harvey, W.  120 hay infusion  289 hearing  138–9 heart  116–18 heart rate  89, 123 hedgehogs  235–6 height  201–2 heliotropism  58 heredity  2 histograms  190 historical reasoning  8 history, of biology  3 homeostasis  3, 23, 34, 94, 147–51 Hooke, R.  25, 99 hormones animals  141–4 plants  144–7 human cheek cells  31–2 hybrid species  230 hydrogencarbonate indicator  21–2, 53, 66 hydroponics  56–7 hygiene  29, 69, 295–6 hypotheses  8 ice ages  234 Ingenhousz, J.  56 inheritance  182–3 insectivorous plants  70 insulin  141 intensive farming  68 International Space Station  37 interpersonal competence  258 invasive species  235–6 inverse square law  62–3 investigation  13–14 see also practical work in vitro fertilisation (IVF)  174 iodine solution  29, 35 Kettlewell, B.  239 kidneys  93–5 knowledge organisers  13 Krebs cycle  47 labelling  12–13 lactic acid  46–7, 53–4 Lamarckism  216 law of conservation of energy  2 law of conservation of mass  2 leaf litter  69 life cycles  20 light absorption  59

intensity  62–4 see also photosynthesis limewater  85 living organisms characteristics of living organisms (COLO)  18–24 chemistry  34–6 observing  19–20 lungs  80–1, 84 see also breathing lux meter  63 lymphatic system  123–4 MacFarlane, R.  252 magnesium  56 marine organisms  20 masturbation  170 matching  12 material recycling  69 mathematics  7, 120, 161, 190–1 calorimetry  48 energy calculations  264–5 histograms  190 large numbers  211 micrometry  34 organising data  190, 259 matter circulation  2 law of conservation of mass  2 medicines  151–2, 233 meiosis  37–41, 156 Mendel, G.  3, 183, 193–5 menstruation  172–3 metabolic pathways  34–6, 46 methane  47 methylene blue solution  29 microbes see micro-organisms microbiology algae  282, 288, 291 antibiotics  299–300 antibodies  299–300 aseptic technique  305 bacteria  283, 289–90, 296 biotechnology  302–3 clean water  296 digestion  297 and disease  282, 296, 298–9 food preservation  293–4 genetically modified organisms (GMOs)  301 genetic engineering  300 and hygiene  295–6 microbial colonies  287–8 microbial growth  291–4

microbial locations  290–1 micro-organisms  281–4, 286, 304 misconceptions  284–6 model organisms  300–1 overview  281–2 Pasteur’s experiment  292–3 practical work  286–8, 290–4, 304–6 prior knowledge  284–5 teaching sequence  286–7, 292, 295, 297–8, 299–303 micrometry  34 micro-organisms  281–4, 286, 304 microscopes  24–30, 287–8 rules for using  26–7 studying cells  30–4 temporary slides  28 misconceptions  10 anatomy  94, 169–71 evolution  216–17 food chains and webs  67 gas exchange  79–80 microbiology  284–6 photosynthesis  55–6 reproduction  155 respiration  46 transport systems  102, 115 see also prior knowledge mitochondria  17, 54–5 mitosis  36–9, 156, 158–9 mobile phones  15 model organisms  300–1 models breathing  80–1 cell structure  32–3 chromosomes  38 climate change  262 complex molecules  35 eyes  136–7 natural selection  214 osmosis  105 Planetary Boundaries  272–3 reproductive systems  171 seeds  164 motivation  24 multiple choice questions  10 muscles  55 ATP (adenosine triphosphate)  49– 50 natural selection  207–17, 230–1 Needham, R.  45–6 nervous system  132–3 see also control systems

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Index neurons  132–3 nitrogen  56 normal distribution  201 normative competence  258 nucleic acids  35 nutrient cycles  70 nutrition  21, 51 see also food observation skills  13–14, 20, 111 oestrogen  142 onions  30–1 Open Air Laboratories (OPAL)  260 organelles  17, 54 organ transplants  95 osmosis  102–3 overfishing  232 oxidation  46–9, 51 see also respiration oxygen  36, 46, 50, 52, 78–9, 93, 123 photosynthesis  61–3 pancreas  141 pandemics  298–9 Pasteur, L.  292–3 pasteurisation  292–3 phenotype  194–5 pheromones  144 phloem  112–13 photosynthesis  21–3, 36, 44, 47–8, 51, 79 and adaptation  225 chlorophyll  56–7, 59 equation  57–8 gas sensors  62 leaves  58, 63 light absorption  59 light intensity  62–4 limiting factors  64 measuring rate of  61 misconceptions  55–6 plant growth  56–8 practical work  60–3, 66 prior knowledge  55–6 and respiration  65–6 phototropisms  144–5 pH sensor  54 pitfall traps  69, 241–2 pituitary gland  142 Planetary Boundary model  272–3 plants cell study  30–1 excretion  93 gas exchange  82–3 germinating seeds  52–3, 64

glucose  65 growth  56–8, 145 hormones  144–7 hydroponics  56–7 leaves  58, 63 life cycle  20–1 nutrition  56, 65 reproduction  160–4, 178 response to stimulus  21 root hairs  111–12 stomata  58, 109–10 transport systems  97, 102–13 see also photosynthesis pollutants  86–7 pollution  272–4 polygenic inheritance  200–1 pond water  29, 69, 99, 242 pondweed  31, 53, 61, 63, 66 populations  230 potometer  110–11 practical work  6–7, 10–11, 13–14, 209–10 biodiversity  241–7, 267–9 breathing rate  89–90 control systems  139–40, 150 dissection  94, 116–17, 136 environment  262–4, 272–3 ethics  117, 242 evolution  209–10, 214 exchange  76–8 fieldwork  7, 13, 20, 248, 252, 254–7, 265–6, 271 gas exchange  85–6 health and safety  78, 86, 121, 254 investigating photosynthesis  60–3, 66 investigating respiration  22–3, 52–3 investigating specialised cells  33 microbiology  286–8, 290–4, 304–6 observing living organisms  19–20 plant reproduction  160 risk assessment  305 transport systems  101, 103–9 variation  190 predictions  10–11, 12 pregnancy  175–7 prejudice  260 Priestley, J.  56–7 prior knowledge  10, 18 biodiversity  229 control systems  127 environment  253

evolution  207–8, 217 exchange  74, 79 excretion  92 microbiology  284–5 photosynthesis  55–6 reproduction  161, 167–8 respiration  50 transport systems  98, 102, 114 variation  182 probability  8 problem-solving skills  13 processing text  13 progesterone  142 prokaryotic cells  17, 37, 54 proteins  35 protozoa  283 puberty  169 Punnett squares  194–7 pyramids of numbers  69–70 quadrats  243–7 rainfall  266 rainforests  228, 236 random movement  76 rapid-cycling brassicas  21, 146 reading  12–13 reasoning  8 receptors  134–5 reductionism  45 reflexes  132–3 refutation tasks  10 religion, and evolution  214–15 reproduction  2, 21 animals  164–6, 178 asexual  157–61, 165, 177–8, 230 cloning  158–9, 177 fertility  172–4 humans  166–77 hybrid species  230 male and female systems  169–72 menstruation  172–3 misconceptions  155 overview  155–6 plants  160–4, 178 practical work  160 pregnancy  175–7 prior knowledge  161, 167–8 sexual  161 teaching sequence  162, 168–9, 175–6 respiration  11, 21–3, 36, 45, 78, 81, 93 aerobic  46, 52–3 anaerobic  46–7, 49, 53–5

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Index and cell theory  54 misconceptions  46 as oxidation reaction  46–9, 51 and photosynthesis  65–6 practical work  22–3, 52–3 prior knowledge  50 teaching sequence  50 respirometers  52 rewilding  237 rigor mortis  50 risk assessment  305 RNA (ribonucleic acid)  17 Ross, K.  46 salinity gradient  106 Saussure, N.  3 Sayre, A.  3 scaffolding  9 Schleiden, M.  17 Schwann, T.  17 scientific language  11–12, 25, 35, 45, 150–1, 199, 220–2, 239–40 selective breeding  212–14 senses  134–40 sensitivity  21 sequencing  12 sex determination  196–8 sex education  156, 166–7, 172–3 see also reproduction sex organs  142 sexually transmitted infections (STIs)  174–5 Shiva, V.  252 Simpson, E.  248 Simpson’s Diversity Index  248 smell  140 smoking  86–9, 90 Snow, J.  296 social constructivism  9 somatic cells  38 speciation  230–1 Stegosaurus  210–11 stem cells  37 stick insects  20 stimuli, responses to  21

stimulus-response chain  130–1, 141, 148 stomata  58, 109–10 strategic competence  258 subjectivity  7 sucrose  48 summative assessment  4 surface area  77, 100–1 swallowing  132, 133 sympatric speciation  231 systems thinking  258 taste  140 taxonomy see classification teleology  216, 220–2 testosterone  142 thermal insulation  49 thermoregulation  129–30, 148–50 Thunberg, G.  252 thyroid gland  141 thyroxine  141 tissue culture  158 touch  140 translocation  112 transpiration  108–12 transport systems and adaptation  224 animals  97 circulatory system  114–24 features of  101 humans  97, 114–24 misconceptions  102, 115 multicellular organisms  100–1 overview  97 plants  97, 102–13 practical work  101, 103–9 prior knowledge  98, 102, 114 purpose  97 teaching sequence  98, 102, 114 translocation  112 transpiration  108–12 unicellular organisms  99–100 vascular tissue  107–8 trophic levels  267 tropical rainforests  228, 236

tropisms  144 unicellular organisms  6, 33, 99–100 van Helmont, J.B.  3, 56–7 variables  7 variation diseases  198–200 genes  182–4, 182–200 height  201–2 inheritance  182–3 overview  181–2 polygenic inheritance  200–1 in populations  203 practical work  190 prior knowledge  182 sex determination  196–8 teaching sequence  182–3 within species  186–96, 239 vascular tissue  107–8 vegetarianism  67 veins  118–20 ventilation  83 Virchow, R.  2, 17 virtual reality (VR)  256–7 viruses  282–3 vitamins  147 Vogel, S.  223 volume ratio  77, 100–1 Wallace, A.R.  3, 208, 212–13 water biodiversity  242–3 clean  296 quality  242–3, 272–4 supply  109 Watson, J.  3, 186 Wilkins, M.  186 Wilkinson, M.  223 Wilson, E.O.  235 wind  266 wormery  70 writing  13 yeast  54, 289 zoos  238 zygotes  38

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