A-Level Biology for AQA: Year 1 & 2 Student Book 9781782943143

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AQA

...this CGP book is a brilliant companion throughout the AQA A-Level course! ƒƒ Crystal-clear notes on every topic... That’s right, Years 1 and 2 covered in one book!

A-Level AQA Biology

Biology can really tax your brain, but you’re not on-neurone...

A-Level

Biology Exam Board: AQA

ƒƒ Hundreds of practice questions... Yep, everything from fact recall to exam-style ƒƒ Practical investigations and maths skills... Of course — there’s in-depth advice on both ƒƒ And a free Online Edition... Perfect for studying on your PC, Mac or tablet! When it comes to A-Level Biology, this book makes CGP the natural selection ☻

P.S. L ooking for more A-Level Science? Check out CGP’s superb Student Books for Chemistry and Physics!

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Student Book

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Student Book The Complete A-Level Course for AQA

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A-Level

Biology The Complete Course for AQA Let’s face it, Biology is a tough subject. You’ll need to get to grips with a lot of difficult concepts, and have plenty of practical skills up your lab-coat sleeve. But don’t worry — this brilliant CGP book covers everything you’ll need for both years of the AQA course. It’s packed with clear explanations, exam practice, advice on maths skills and practical investigations... and much more!

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Contents Topic 1 — B: More Biological Molecules

Introduction How to use this book

Practical and Maths Skills

i

1. DNA and RNA

53

2. DNA Replication

57

3. ATP

61

4. Water

63

5. Inorganic Ions

66

Exam-style Questions

68

AL PRACTIC & MATHS SKILLS

1. Planning an Experiment

1

2. Carrying Out an Experiment

4

3. Processing Data

5

4. Presenting Data

9

5. Concluding and Evaluating

15

6. The Practical Endorsement

19

Topics

Topic 2 — A: Cell Structure and Division 1. Eukaryotic Cells and Organelles

70

2. Prokaryotic Cells and Viruses

77

3. Analysis of Cell Components

80

4. Cell Division — Mitosis

86

5. Investigating Mitosis

90

Exam-style Questions

95

Topic 1 — A: Biological Molecules 1. Molecules of Life

21

2. Sugars

23

3. Polysaccharides

26

4. Lipids

29

Topic 2 — B: Cell Membranes

5. Proteins

33

1. Cell Membranes — The Basics

6. Enzymes

37

2. Diffusion

102

7. Factors Affecting Enzyme Activity

40

3. Osmosis

106

8. Enzyme-Controlled Reactions

46

4. Active Transport

110

Exam-style Questions

51

Exam-style Questions

115

97

Topic 2 — C: Cells and The Immune System

Topic 4 — A: DNA, RNA and Protein Synthesis

1. Antigens

117

1. DNA

203

2. The Immune Response

118

2. Genes and Chromosomes

204

3. Immunity and Vaccines

122

3. RNA and Protein Synthesis

206

4. Antigenic Variation

125

4. Transcription and Translation

207

5. Antibodies in Medicine

126

5. The Genetic Code and Nucleic Acids

211

6. Interpreting Data About Vaccines and Antibodies

Exam-style Questions

216

130

7. HIV and Viruses

132

Exam-style Questions

137

Topic 4 — B: Diversity and Selection Topic 3 — A: Exchange and Transport Systems 1. Size and Surface Area

139

2. Gas Exchange

144

3. Gas Exchange in Humans

148

4. The Effects of Lung Disease

152

5. Interpreting Lung Disease Data

156

6. Dissecting Gas Exchange Systems

160

Exam-style Questions

164

1. Meiosis and Genetic Variation

217

2. Mutations

223

3. Genetic Diversity

226

4. Natural Selection

228

5. The Effects of Selection

231

6. Investigating Selection

233

Exam-style Questions

236

Topic 4 — C: Diversity and Classification

Topic 3 — B: More Exchange and Transport Systems

1. Classification of Organisms

238

1. Digestion and Absorption

166

2. Classification Using Courtship Behaviour241

2. Haemoglobin

170

3. Classification Using DNA or Proteins

243

3. The Circulatory System

175

4. The Heart

179

4. Using Gene Technologies to Assess Genetic Diversity

246

5. Cardiovascular Disease

185

5. Investigating Variation

247

6. Transport in Plants — Xylem

191

6. Biodiversity

252

7. Transport in Plants — Phloem

195

7. Agriculture and Biodiversity

255

Exam-style Questions

201

Exam-style Questions

259

Topic 5 — A: Photosynthesis and Respiration

Topic 6 — B: Nervous Coordination 1. Neurones

330

1. Photosynthesis, Respiration and Energy 261

2. Synaptic Transmission

335

2. Photosynthesis and the Light-dependent Reaction

264

3. Muscle Structure

341

3. Photosynthesis and the Light-independent Reaction

4. Muscle Contraction

345

269

Exam-style Questions

351

4. Limiting Factors in Photosynthesis

272

5. Photosynthesis Experiments

276

6. Aerobic and Anaerobic Respiration

280

7. Aerobic Respiration — The Mitochondrial Reactions

283

8. Respiration Experiments

288

Exam-style Questions

293

Topic 5 — B: Energy Transfer and Nutrient Cycles 1. Energy Transfer in Ecosystems

295

2. Farming Practices and Production

299

Topic 6 — C: Homeostasis 1. Homeostasis Basics

354

2. Control of Blood Glucose Concentration

359

3. Diabetes and Blood Glucose Concentration

363

4. The Kidneys

368

5. Controlling Blood Water Potential

371

Exam-style Questions

375

3. Nutrient Cycles in Natural Ecosystems 302 4. Fertilisers and Eutrophication

306

Exam-style Questions

310

Topic 7 — A: Genetics Topic 6 — A: Stimuli and Responses

1. Genetic Terms

378

2. Genetic Diagrams — Simple Monohybrid Crosses

380

1. Survival and Response

312

2. Nervous Communication

314

3. Genetic Diagrams — Multiple Allele and Dihybrid Crosses

383

3. Responses in Plants

317

4. Linkage

386

4. Receptors

320

5. Epistasis

390

5. Control of Heart Rate

324

6. The Chi-Squared Test

393

Exam-style Questions

328

Exam-style Questions

398

Topic 7 — B: Populations and Evolution

Topic 8 — B: Genome Projects and Gene Technologies

1. The Hardy-Weinberg Principle

400

1. Genome Projects

474

2. Variation and Selection

405

2. Making DNA Fragments

476

3. Speciation and Genetic Drift

408

3. Amplifying DNA Fragments

479

Exam-style Questions

414

4. Recombinant DNA Technology

484

5. Gene Therapy

487

6. Gene Probes and Medical Diagnosis

488

7. Genetic Fingerprinting

491

Exam-style Questions

496

Topic 7 — C: Populations in Ecosystems 1. Ecosystems

415

2. Variation in Population Size

418

3. Investigating Populations

423

4. Succession

430

5. Conservation

434

6. Conservation Evidence and Data

437

Exam-style Questions

441

Exam Help

EXAM HELP

1. The Exams

498

2. Command Words

500

3. Time Management

501

Topic 8 — A: Mutations and Gene Expression 1. Mutations

443

2. Mutagenic Agents

446

3. Cancer

447

4. Interpreting Data on Cancer

451

5. Stem Cells

455

6. Stem Cells in Medicine

458

7. Regulation of Transcription and Translation

Reference

461

Answers502

8. Epigenetic Control of Gene Expression 466

Glossary559

9. Evaluating Data on Phenotypes

469

Acknowledgements573

Exam-style Questions

472

Index577

How to use this book Learning Objectives ƒƒ

Learning Objectives: formula to calculate magnification:

These tell you exactly what you need to learn, or be able to do, for the exams. There’s a specification reference at the bottom that links to the AQA specification.

ƒƒ

3. Analysis of Cell Components

ƒ Be able to use this size of image size of real object

ƒ Know the difference between magnification and resolution.

ƒ Know the principles and limitations of optical microscopes and transmission and scanning electron microscopes.

ƒ Appreciate that there was a considerable period of time during which the scientific community distinguished between artefacts and cell organelles.

Maths Skills Examples There’s a range of maths skills that you could be expected to apply in your exams. Examples that show these maths skills in action are marked up like this. There’s also a Practical and Maths Skills section at the front of the book.

ƒ Understand how cell fractionation and ultracentrifugation are used to separate cell components. Specification Reference 3.2.1.3

Exam Tip If you find rearranging formulas hard you can use a formula triangle to help: Size of Image

Exam Tips

Magnific Size of X ation real Object

There are tips throughout the book to help with all sorts of things to do with answering exam questions. 

All you do is put your finger over the one you want and read off the formula. E.g. if you want the size of the real object, you put your finger over that and it leaves behind size of image ÷ magnification.

80

Cancer Mitosis and the cell cycle are controlled by genes. Normally, when cells have divided enough times to make enough new cells, they stop. But if there’s a mutation in a gene that controls cell division, the cells can grow out of control. The cells keep on dividing to make more and more cells, which form a tumour. Cancer is a tumour that invades surrounding tissue.

Cancer treatments Some treatments for cancer are designed to control the rate of cell division in tumour cells by disrupting the cell cycle. This kills the tumour cells. These treatments don’t distinguish tumour cells from normal cells though — they also kill normal body cells that are dividing. However, tumour cells divide much more frequently than normal cells, so the treatments are more likely to kill tumour cells.

Investigating cells, and what's in them, involves digging out your microscope.

Magnification and resolution of microscopes We all know that microscopes produce a magnified image of a sample, but resolution is just as important...

Magnification Magnification is how much bigger the image is than the specimen (the sample you’re looking at). It’s calculated using this formula: size of image size of real object

magnification =

Examples

Maths Skills

Calculating magnification If you have a magnified image that’s 5 mm wide and your specimen is 0.05 mm wide the magnification is: size of image 5 = × 100 = magnification = size of real object 0.05

5 mm

Calculating size of image If your specimen is 0.1 mm wide and the magnification of the microscope is × 20, then the size of the image is: size of image = magnification × size of real object = 20 × 0.1 = 2 mm Calculating size of real object If you have a magnified image that’s 5 mm wide and the magnification is × 50, then the size of the real object (i.e. the size of the specimen you're looking at) is: size of image 5 = 0.1 mm = size of real object = magnification 50 When you’re calculating magnification you need to make sure that all lengths are in the same unit, e.g. all in millimetres. When dealing with microscopes these units can get pretty tiny. The table below shows common units: To convert × 1000 × 1000

Unit

How many millimetres it is:

Millimetre (mm)

1 mm

Micrometre (µm)

0.001 mm

Nanometre (nm)

0.000001 mm

To convert ÷ 1000 ÷ 1000

The table shows that millimetres are three orders of magnitude (103 or 1000 times) bigger than than micrometres, which are three orders of magnitude bigger than nanometres.

Example

Maths Skills

To convert from a smaller unit to a bigger unit you divide by 1000. So to convert 6 micrometres to millimetres you divide 6 by 1000 = 0.006 mm. To go from a bigger unit to a smaller unit you times by 1000.

Topic 2 — A: Cell Structure and Division

Tip: Mutations are changes in the base sequence of an organism’s DNA (see page 223).

Examples

Tip: Cancer is basically uncontrolled cell division.

These are here to help you understand the theory.

Examples Some cell cycle targets of cancer treatments include: G2

M G1

✗

SS

G2

M G1

✗

SS

G1 (cell growth and protein production) Some chemical drugs (chemotherapy) prevent the synthesis of enzymes needed for DNA replication. If these aren’t produced, the cell is unable to enter the synthesis phase (S), disrupting the cell cycle and forcing the cell to kill itself. S phase (DNA replication) Radiation and some drugs damage DNA. At several points in the cell cycle, the DNA in the cell is checked for damage. If severe DNA damage is detected, the cell will kill itself — preventing further tumour growth.

Practice Questions — Application ƒƒ Figure 7: Cancer of the knee — the tumour is sticking out of the leg. Tip: Rapidly dividing cells, like hair cells and cells that line the gut, are often affected by cancer treatments. This can cause side effects like hair loss.

ƒƒ

Practice Question — Application Q1 Methotrexate and vincristine are drugs used to treat cancer. Methotrexate blocks the formation of nucleotides within cells and vincristine prevents the formation of spindle fibres within the nuclei of cells. Which stage of the cell cycle is disrupted by: a) methotrexate b) vincristine?

Practice Questions — Fact Recall Q1 Q2 Q3 Q4 Q5 Q6 Q7

Tip: Don’t let the tricky names of the drugs throw you when answering Q1 here. Just apply your knowledge to the information given in the question.

i

Practice Questions — Fact Recall

What is the cell cycle? Why is mitosis needed? In what stage of the cell cycle does all the DNA unravel? Describe what happens during prophase. Describe what happens during telophase. What is cytokinesis? What is cancer?

Topic 2 — A: Cell Structure and Division

Annoyingly, the examiners expect you to be able to apply your knowledge to new situations — these questions are here to give you plenty of practice at doing this. All the answers are in the back of the book (including any calculation workings).

ƒƒ

ƒƒ 89

There are a lot of facts you need to learn — these questions are here to test whether you know them. All the answers are in the back of the book.

6. Investigating Selection

Testing the effects of antibiotics

REQUIRED PRACTICAL

6

Antibiotics are medicines that are designed to kill bacteria. This makes them a type of antimicrobial substance. You can investigate the effects of different antibiotics on bacterial growth using the following method. The whole investigation must be carried out using aseptic techniques. These are explained on the next page. Read them through before you begin. 1. The bacteria you will use are likely to have been grown in a liquid broth (a mixture of distilled water, bacterial culture and nutrients). 2. Use a sterile pipette to transfer the bacteria from the broth to an agar plate (a Petri dish containing agar jelly). Spread the bacteria over the plate using a sterile plastic spreader. 3. Use sterile forceps to place paper discs soaked with different antibiotics spaced apart on the plate. Various concentrations of antibiotics should be used. You also need to add a negative control disc soaked only in sterile water. 4. Tape a lid onto the Petri dish (without completely sealing it), invert, and incubate the plate at about 25°C for 48 hours. This allows the bacteria to grow, forming a ’lawn’. Anywhere the bacteria can’t grow can be seen as a clear patch in the lawn of bacteria. This is called an inhibition zone. 5. The size of an inhibition zone tells you how well an antibiotic works. The larger the zone, the more the bacteria were inhibited from growing.

Example Figure 1 shows an agar plate after it has been incubated with paper discs soaked in the antibiotics meticillin, tetracycline and streptomycin. 125 mg 250 mg Inhibition zone

Figure 1: An agar plate used to investigate antibiotic resistance.

Required Practicals

Learning Objective:

You can carry out practical investigations into the effects of antimicrobial substances (substances that kill microorganisms, e.g. antibiotics, antiseptics or disinfectants) on microbes. These investigations should show you whether the microbes have evolved resistance to these substances or not.

Meticillin

There are some key practicals that you’ll be expected to do throughout your course. You’ll need to know all about them for the exams. Information about these practicals is marked up with a Required Practical stamp.

ƒ Be able to use aseptic techniques to investigate the effect of antimicrobial substances on microbial growth (Required Practical 6). Specification Reference 3.4.4

Tip: Make sure you carry out a full risk assessment before you carry out this practical. It’s also really important that you understand how to use aseptic techniques properly before you start.

Practical Skills There are some key practical skills that you’ll not only need to use in your Required Practicals, but that you could be tested on in the exams too. These skills are covered in the Practical and Maths Skills section at the front of the book.

Tip: A negative control is not expected to have any effect on the experiment — see page 2 for more.

Tip: Don’t completely seal the Petri dish with tape before incubation — it will prevent oxygen from entering the dish, which may encourage the growth of anaerobic disease-causing bacteria. Don’t open the dish after incubation.

Agar plate Tetracycline Disc soaked in antibiotic

Tips

Streptomycin Negative control (soaked in water)

ƒ ƒ

ƒ ƒ

Lawn of bacteria

The tetracycline discs have no inhibition zones, so the bacteria are resistant to tetracycline up to 250 mg. The streptomycin discs have small inhibition zones, with the zone at 250 mg slightly larger than the one at 125 mg. So streptomycin inhibits the growth of some of the bacteria. The meticillin discs have the largest inhibition zones, so meticillin inhibits the growth of most of the bacteria. The negative control has no inhibition zone, which shows that the other results must be due to the presence of the antibiotics, not the paper disc.

These are here to help you understand the theory. Figure 2: A bacterial culture plate with clear inhibition zones where an antibiotic has stopped the bacteria from growing.

Topic 4 — B: Diversity and Selection

233

Exam-style Questions ƒƒ

ƒƒ

ƒƒ

Practising exam-style questions is really important — you’ll find some at the end of each section. They’re the same style as the ones you’ll get in the real exams — some will test your knowledge and understanding and some will test that you can apply your knowledge. All the answers are in the back of the book, along with a mark scheme to show you how you get the marks.

Exam-style Questions 1

The gills are the gas exchange organ in fish. Figure 1 shows a cross section through a dogfish gill. Figure 1

A

1.1

1.2 1.3

1.4

Exam Help 2

There’s a section at the back of the book stuffed full of things to help with your exams.

Name the structures labelled A in Figure 1 and explain how they increase the efficiency of gas exchange across the gills. (3 marks) Give one other adaptation of the gills for efficient gas exchange. (1 mark) Insects have a tracheal system for exchanging gases with the environment. Describe how oxygen gets into an insect’s respiring cells. (2 marks) Terrestrial insects lose water as a result of gas exchange. Explain two features insects have to reduce unwanted water loss. (2 marks) Figure 2 Figure 2 shows a scanning electron micrograph of alveoli in a healthy human lung (left) and the A effects of emphysema on the alveoli (right). The magnification is x 60.

2.1

Glossary There’s a glossary at the back of the book full of all the definitions you need to know for the exams, plus loads of other useful words.

2.2

2.3

164

Calculate the actual width of the labelled alveolus, A. Give your answer in µm. (2 marks) Describe one difference between the healthy alveoli and the diseased alveoli, and explain what effect this would have on gaseous exchange in the alveoli. (3 marks) Oxygen tents contain a higher percentage of oxygen than normal air. Suggest how being in an oxygen tent might benefit a patient with emphysema. (2 marks)

Topic 3 — A: Exchange and Transport Systems

ii

Published by CGP Editors: Ciara McGlade, Rachael Rogers and Hayley Thompson. Contributors: Sophie Anderson, Gloria Barnett, Jessica Egan, James Foster, Paddy Gannon, Derek Harvey, Liz Masters, Steven Phillips, Adrian Schmit and Anna Fe Williamson.

With thanks to Charlotte Burrows, Katherine Faudemer and Camilla Simson for the proofreading. With thanks to Laura Jakubowski for the copyright research. Clipart from Corel® BATB72 ~ 0920 - 21501 Text, design, layout and original illustrations © Coordination Group Publications Ltd. (CGP) 2016 All rights reserved.

0800 1712 712 • www.cgpbooks.co.uk

AL PRACTIC & MATHS SKILLS

Practical and Maths Skills

1. Planning an Experiment You have to do practical work in class as part of your course. You’ll be asked about it in exams too, so you need to know how to plan the perfect experiment.

Testing a theory Before you start planning an experiment, you need to be clear about what you’re trying to find out. Like all scientists, you should start off by making a prediction or hypothesis — a specific testable statement, based on theory, about what will happen in the experiment. You then need to plan a good experiment that will provide evidence to support the prediction — or help disprove it.

Example Theory — photosynthesis requires light. The rate of photosynthesis is affected by light intensity. Prediction — rate of photosynthesis will increase as light intensity increases. Experiment — measure the rate of photosynthesis at various light intensities.

Exam Tip At least 15% of your A-level Biology marks will come from assessment of Practical Skills in the exams — so you really do need to know this stuff.

Tip: The results of your experiment can’t be used to prove that your theory is right, but can be used as evidence for or against it. There’s more about what results mean on pages 15-18.

Getting good results A good experiment is one that will give results that are: ƒƒ Precise — Precise results don’t vary much from the mean. Precision is reduced by random error (the unpredictable way in which all measurements vary). ƒƒ Repeatable and reproducible — Repeatable means that if the same person repeats the experiment using the same methods and equipment, they will get the same results. Reproducible means that if someone different does the experiment, using a slightly different method or piece of equipment, the results will still be the same. ƒƒ Valid — Valid results answer the original question. To get valid results you need to control all the variables (see below) to make sure you’re only testing the thing you want to. ƒƒ Accurate — Accurate results are really close to the true answer. Human interpretation of a measurement (e.g. determining a colour change) can reduce the accuracy of results. Here are some things you need to consider when designing a good experiment:

1. Variables Variables are quantities that have the potential to change, e.g. temperature, pH. In an experiment you usually change one variable and measure its effect on another variable. ƒƒ The variable that you change is called the independent variable. ƒƒ The variable that you measure is called the dependent variable. All the other variables should be controlled — when you’re investigating a variable you need to keep everything else that could affect it constant. This means you can be sure that only your independent variable is affecting the thing you’re measuring (the dependent variable).

Tip: Precise results are sometimes referred to as reliable results.

Exam Tip Examiners love getting you to comment on experimental design or suggest improvements to methods — e.g. how a method could be improved to make the results more precise.

Practical and Maths Skills

1

Example For an investigation into how light intensity affects rate of photosynthesis: ƒƒ Light intensity is the independent variable. ƒƒ Rate of photosynthesis is the dependent variable. ƒƒ pH, temperature and the time the experiment is left for should all stay the same (and the quantities should be recorded to allow someone else to reproduce the experiment).

Exam Tip If you get an exam question asking why a control is important in a particular experiment, make sure your answer is specific to that experiment (not just about why controls are good in general).

2. Controls Negative controls are used to check that only the independent variable is affecting the dependent variable. Negative controls aren’t expected to have any effect on the experiment.

Example When investigating how light intensity affects the rate of photosynthesis, you should set up a negative control in which the experiment is carried out in the dark. No photosynthesis should happen with this control. Positive controls can also be used. They should show what a positive result of the experiment should look like, to check that it is possible.

Example If you’re testing for the presence of glucose in a solution, you could carry out a Benedict’s test. Before you start the experiment, you could test a solution that you know contains glucose to show what a positive glucose test result looks like. This is your positive control. Tip: In a study with human participants, you should try to keep the variables of all the participants the same, e.g. they should all be the same age, sex, etc.

Tip: When testing a new drug to see if it works, the control group is given a placebo instead of the drug. A placebo is a dummy pill or injection that looks exactly like the real drug, but doesn’t contain the drug. It’s used to make sure that people don’t improve just because they think they’re being treated.

In studies, control groups are used. The subjects in the study are split into two groups — the experimental group and the control group. The control group is treated in exactly the same way as the experimental group, except for the factor you’re investigating.

Example If you were investigating the effect of eating a low sodium (salt) diet on blood pressure, you’d have two groups. One group would be the experimental group and be given a diet low in sodium. The other group would be a control group, who would be given a diet in which sodium wasn’t reduced. This is done so that you can tell that any decrease in blood pressure is due to the low sodium diet and nothing else.

3. Repeats Taking several repeat measurements and calculating the mean can reduce the effect of random error on your experiment, making your results more precise. Doing repeats and getting similar results each time also shows that your data is repeatable. This makes it more likely that the same results could be reproduced by another scientist in an independent experiment.

Example For an investigation into how light intensity affects rate of photosynthesis, the experiment should be repeated at least three times for each light intensity used. A mean result should be calculated for each light intensity (see page 5). Repeating measurements also reduces the likelihood that the results are due to chance — see next page.

2

Practical and Maths Skills

4. Sample Size Sample size is the number of samples in the investigation, e.g. the number of people in a drug trial. As with carrying out repeats, having a large sample size reduces the likelihood that the results are due to chance (e.g. if you get the same result twice it might be because of chance, but if you get it 100 times it’s much more likely that it’s not due to chance).

Tip: Scientists can use statistical tests to figure out if a result is likely to be due to chance or not. See pages 8-9 for more.

Taking accurate measurements When you’re planning an experiment you need to decide what it is you’re going to measure and how often you’re going to take measurements.

Example If you’re investigating the rate of photosynthesis, you could measure the volume of oxygen produced over time or the volume of carbon dioxide used over time. E.g. you could take measurements at 30 or 60 second intervals. Then you need to choose the most appropriate apparatus, equipment and techniques for the experiment. The measuring apparatus you use has to be sensitive enough to measure the changes you’re looking for. For example, if you need to measure small changes in pH, a pH meter (which can measure pH to several decimal places) would be more sensitive than indicator paper. The technique you use has to be the most appropriate one for your experiment. E.g. if you want to investigate plant cells undergoing mitosis, it’s best to prepare a stained squash slide so you see the chromosomes clearly under the microscope (see page 90).

Risk assessments In order to work safely, you need to carry out a risk assessment for your experiment. To do this, you need to identify: All the dangers in the experiment. For example, any hazardous chemicals, microorganisms or naked flames. Who is at risk from these dangers. This could be you and your lab partner, but it could also be anyone who is in the same room or building. What can be done to reduce the risk. You should wear a lab coat and safety goggles as a standard precaution, but you may need to take other safety precautions, such as: ƒƒ Wearing gloves, if your experiment involves substances that are likely to irritate the skin. ƒƒ Carrying out your experiment in a fume cupboard if it involves volatile chemicals (see page 277), and keeping flammable chemicals away from naked flames (e.g. Bunsen burner flames). ƒƒ Using aseptic techniques (page 234) if you are culturing microorganisms.

Figure 1: pH meters can be used to measure small changes in pH.

Figure 2: A scientist wearing eye protection, gloves and a lab coat to protect her while she works.

Ethical issues You also need to consider any ethical issues in your experiment.

Example

If you’re using living animals (e.g. insects) you must treat them with respect. This means handling them carefully and keeping them away from harmful chemicals, extreme heat sources and other things that might cause them physical discomfort.

Practical and Maths Skills

3

2. Carrying Out an Experiment As part of your A-level in Biology, you’re expected to carry out Required Practicals and be familiar with the techniques and apparatus involved in each one (see page 19). You could be asked about the skills you’ve learnt in your exams.

Using the correct apparatus and techniques Examiners could ask you about a whole range of different apparatus and techniques. Make sure you know how to use all the instruments and equipment you’ve come across in class and can carry out all the techniques too. Here are some examples of equipment you should be able to use:

Examples Measuring cylinders and graduated pipettes These have a scale so you can measure specific volumes. Whichever one you use, make sure you read the volume from the bottom of the meniscus (the curved upper surface of the liquid) when it’s at eye level — see Figure 1.

2.0

Water baths Make sure you allow time for water baths to heat up before starting your experiment. Don’t forget that your solutions will need time to get to the same temperature as the water before you start the experiment too. Also, remember to check the temperature of the water bath with a thermometer during the investigation to make sure it doesn’t change.

Figure 1: Measuring volume using the bottom of the meniscus.

Tip: A data logger (or data recorder) is an electronic device that can record data over time using a sensor. They can sometimes be connected to a computer.

Tip: You might need to record your results in a frequency table. These usually have three columns — the first gives the values or names of the different pieces of data, the second shows a mark (tally) for each piece of data, and the third shows the frequency, which you get by adding up the tally marks. E.g. species

4

tally

Data loggers Decide what you are measuring and what type of data logger you will need, e.g. temperature, pH. Connect an external sensor to the data logger if you need to. Decide how often you want the data logger to take readings depending on the length of the process that you are measuring. You should also make sure you know how to do all the Required Practicals described in this book. You should be able to apply the techniques described in them to different contexts. E.g. page 107 describes how to prepare serial dilutions in order to find out the water potential of potato cells. You could also use serial dilutions to prepare solutions of varying substrate concentration in order to investigate the effect of substrate concentration on enzyme activity.

Recording data As you get your results, you need to record them. It’s a good idea to draw a table to record the results of your experiment in. When you draw a table, make sure you include enough rows and columns to record all of the data you need to. You might also need to include a column for processing your data (e.g. working out the mean — see next page). Make sure each column has a heading so you know what’s going to be recorded where. The units should be in the column heading only, not the table itself — see Figure 2. The independent variable should be recorded in the left-hand column and the dependent variable in the right.

frequency

sparrow

7

blue tit

3

blackbird

10

Practical and Maths Skills

heading

Figure 2: Table showing the rate of a reaction at three different concentrations of substrate. row

column

Concentration of substrate / mol dm–3

Rate of reaction / cm3 min-1

0.2

10

0.4

13

0.6

17

units data

If you’re recording your data as decimals, make sure you do it to a consistent number of decimal places. When you’re analysing your results, it makes sense to compare values that have been rounded to the same level of accuracy.

Anomalous results When you look at all the data in your table, you may notice that you have a result that doesn’t seem to fit in with the rest at all. These results are called anomalous results. You should investigate anomalous results — if you can work out what happened (e.g. you measured something totally wrong) you can ignore them when processing your results. However, you can’t just exclude a value just because you don’t like the look of it.

Tip: Doing repeats makes it easier to spot anomalous results.

3. Processing Data

Exam Tip At least 10% of your A-level Biology marks will come from assessment of maths skills in the exams.

Processing data means taking raw data and doing some calculations with it, to make it more useful. This is where your maths skills really come in.

Summarising your data Once you’ve collected all your data, it’s useful to summarise it using a few handy-to-use figures — like the mean and the range.

Mean and range When you’ve done repeats of an experiment you should always calculate a mean (a type of average). To do this add together all the data values and divide by the total number of values in the sample. You might also need to calculate the range (how spread out the data is). To do this find the largest data value and subtract the smallest data value from it. You shouldn’t include anomalous results when calculating the mean or the range. Maths Skills Example Compare the mean absorbance and range of absorbance for test tubes A and B in the table on the right.

Test tube

Tip: Averages and range values have the same units as the data used in the calculation.

Absorbance of solution / absorbance units (AU) Repeat 1

Repeat 2

Repeat 3

A

0.31

0.50

0.52

B

1.2

0.84

1.5

Tip: S.f. stands for ‘significant figures’. You can find out how many significant figures to use when rounding answers on page 7.

To calculate the means: ƒƒ Add up the three data values for A, then divide by three. A: (0.31 + 0.50 + 0.52) ÷ 3 = 1.33 ÷ 3 = 0.44 AU (2 s.f.) ƒƒ

Tip: When people talk about an average, they are usually referring to the mean value.

Do the same for B. B: (1.2 + 0.84 + 1.5) ÷ 3 = 3.54 ÷ 3 = 1.2 AU (2 s.f.) B has the higher mean.

To find the range of results for each test tube, subtract the smallest result from the largest result. A: 0.52 – 0.31 = 0.21 AU B: 1.5 – 0.84 = 0.66 AU A has the smaller range.

Tip: The mean (and other averages, see next page), range and standard deviation are all examples of descriptive statistics. Descriptive statistics simply describe any patterns in the data.

Standard deviation Standard deviation can be more useful than the range because it tells you how values are spread about the mean rather than just the total spread of data. A small standard deviation means the repeated results are all similar and close to the mean, i.e. they are precise.

Tip: There’s more on standard deviation on pages 249-251.

Practical and Maths Skills

5

Median and mode

Tip: If all the values in your data are different, there won’t be a mode at all.

Like the mean, the median and mode are both types of average. To calculate the median, put all your data in numerical order. The median is the middle value in this list. If you have an even number of values, the median is halfway between the middle two values. To calculate the mode, count how many times each value comes up. The mode is the number that appears most often. A set of data might not have a mode — or it might have more than one.

Example

Maths Skills

The heights of bean plants (in millimetres) were measured six weeks after planting. The results were as follows: 112 102 106 120 98 106 80 105 106 110 95 98 Calculate the median and mode of these results. 1. Put the data in numerical order: 80 95 98 98 102 105 106

106

106

110

112

120

2. F ind the middle value (the median): There are 12 values, so the median is between the 6th and 7th numbers. The 6th number is 105 and the 7th is 106, so the median is 105.5 mm. 3. C  ount how many times each value comes up to find the mode: 106 comes up three times. None of the other numbers come up more than twice. So the mode is 106 mm.

Tip: To find the value halfway between two numbers, add the two numbers together and then divide by two. E.g. 105 + 106 = 211, 211 ÷ 2 = 105.5.

Calculating percentages Calculating percentages helps you to compare amounts from samples of different sizes. To give the amount X as a percentage of sample Y, you need to divide X by Y, then multiply by 100.

Example

Maths Skills

In a DNA molecule containing 3000 bases, 900 of the bases are cytosine. What percentage of the bases are cytosine? 1. Divide 900 by 3000: 900 ÷ 3000 = 0.30 2. Multiply by 100: 0.30 × 100 = 30% Exam Tip The examiners just love getting you to calculate percentage changes, including percentage increases and decreases, so make sure you learn this formula.

Calculating percentage change Calculating percentage change helps to quantify how much something has changed, e.g. the percentage change in the growth rate of pea plants when a fertiliser is added. To calculate it you use this equation: Percentage change =

final value – original value × 100 original value

A positive value indicates an increase and a negative value indicates a decrease.

Example Tip: This means that the person’s blood glucose concentration was 55% higher after the meal.

6

Practical and Maths Skills

Maths Skills

A person’s blood glucose concentration before a meal was 4.2 mmol dm–3. Two hours after a meal it was 6.5 mmol dm–3. Calculate the percentage change. 6.5 – 4.2 × 100 = 55% (2 s.f.) Percentage change = 4.2

Percentage change can be either positive or negative, depending on whether the value has gone up or down. However, percentage increase and percentage decrease are both written as positive numbers because the direction of the change has already been taken into account.

Using ratios Ratios can be used to compare lots of different types of quantities. For example, an organism with a surface area to volume ratio of 2 : 1 would theoretically have a surface area twice as large as its volume. Ratios are usually most useful in their simplest (smallest) form. To simplify a ratio, divide each side by the same number. It’s in its simplest form when there’s nothing left you can divide by. To get a ratio of X : Y in the form X : 1, divide both sides by Y.

Examples ƒƒ ƒƒ

Maths Skills

To simplify the ratio 28 : 36, divide both sides by 4. You get 7 : 9. To write the ratio 28 : 36 in the form of X : 1, just divide both sides by 36: 28 ÷ 36 = 0.78 36 ÷ 36 = 1 So the ratio is 0.78 : 1.

Rounding to significant figures The first significant figure of a number is the first digit that isn’t a zero. The second, third and fourth significant figures follow on immediately after the first (even if they’re zeros). When you’re processing your data you may well want to round any really long numbers to a certain number of significant figures.

Example 0.6874976 rounds to 0.69 to 2 s.f. and to 0.687 to 3 s.f. When you’re doing calculations using measurements given to a certain number of significant figures, you should give your answer to the lowest number of significant figures that was used in the calculation.

Example

Tip: If you’re not sure what number to divide by to simplify a ratio, start by trying to divide both sides by a small number, e.g. 2 or 3, then check to see if you can simplify your answer further. E.g. you could simplify 28 : 36 by dividing each side by 2 to get 14 : 18. But you could simplify it further by dividing by 2 again to get 7 : 9. You can’t simplify the ratio any further, so it’s in its simplest form.

Tip: You may also want to round measurements to a certain number of significant figures when you’re recording your data, e.g. if you’re using a data logger that records data to several decimal places.

Maths Skills

For the calculation: 1.2 ÷ 1.85 = 0.648648648... 1.2 is given to 2 significant figures. 1.85 is given to 3 significant figures. So the answer should be given to 2 significant figures. Round the final significant figure (0.648) up to 5: 1.2 ÷ 1.85 = 0.65 (2 s.f.) The lowest number of significant figures in the calculation is used because the fewer digits a measurement has, the less accurate it is. Your answer can only be as accurate as the least accurate measurement in the calculation.

Tip: When rounding a number, if the next digit after the last significant figure you’re using is less than 5 you should round it down, and if it’s 5 or more you should round it up.

Writing numbers in standard form When you’re processing data you might also want to change very big or very small numbers that have lots of zeros into something more manageable — this is called standard form.

Examples 1 000 000 can be written 1 × 106. 0.017 can be written 1.7 × 10-2. To do this you just need to move the decimal point left or right. The number of places the decimal point moves is then represented by a power of 10 — this is positive for big numbers, and negative for numbers smaller than one.

Practical and Maths Skills

7

Example

Tip: When you’re writing a measurement in standard form, make sure you keep the same number of significant figures. E.g. 0.00400 cm3 = 4.00 × 10–3  cm3. This’ll make sure that you don’t lose any accuracy.

Tip: Double check you’ve got it right by doing the multiplication — you should end up with the number you started with. So for this example, you’d check 1.65 × 104 = 16 500.

Maths Skills

To write 16 500 in standard form: 1. Move the decimal point to give the smallest number you can between 1 and 10. 16 500

1.6500

2. Count the number of places the decimal point has moved. The decimal point has moved four places to the left. 3. Write that number as the power of ten. If the decimal point has moved to the left, the power is positive. If the decimal point has moved to the right, the power is negative. 16 500 = 1.65 × 104

Converting between units When processing your data, you need to have all the data in the correct units. Make sure you can convert between common units of time, length and volume.

Examples ÷ 60

seconds

÷ 60

minutes × 60

÷ 1000

÷ 1000

micrometres

nanometres × 1000

centimetres cubed

Tip: One decimetre cubed (1 dm3) is the same as one litre (1 L).

Examples

8

÷ 1000 × 1000

millimetres × 1000

decimetres cubed

÷ 1000 × 1000

metres cubed

Maths Skills

1. 10 cm³ of glucose solution is needed to create a dilution series. What is the volume of glucose solution needed in dm³?

Tip: Make sure your answer makes sense — if you’re converting from a small unit (e.g. cm3) to a larger unit (e.g. dm3) you need to divide the value, so your answer should be smaller than the number you started with.

Tip: Statistical tests are inferential statistics. This means they allow you to ‘infer’ things (draw conclusions) from the data.

hours × 60

There are 1000 cm3 in one dm3, so you need to divide by 1000: 10 cm3 ÷ 1000 = 0.01 dm3 2. The volume of carbon dioxide produced over time was measured in an experiment into aerobic respiration in yeast. The rate of CO2 production was found to be 3.7 cm3 min–1. What is this rate in cm3 s–1? 3.7 cm3 min–1 means 3.7 cm3 carbon dioxide was produced per minute. You want to find out the volume produced per second. There are 60 seconds in one minute, so you need to divide the volume by 60: 3.7 cm3 min–1 ÷ 60 = 0.06 cm3 s–1

Statistical tests Statistical tests are used to analyse data mathematically. You can be more confident in your conclusions (see page 15), if they’re based on results that have been analysed using a statistical test.

Practical and Maths Skills

If you’re planning on analysing your data using a statistical test, you first need to come up with a null hypothesis — this is a special type of hypothesis that states there is no significant difference (or correlation) between the things you’re investigating. You then collect data to try to disprove the null hypothesis before analysing it statistically. There’s more on null hypotheses on pages 257 and 393.

Student’s t-test You can use the Student’s t-test when you have two sets of data that you want to compare. It tests whether there is a significant difference in the means of the two data sets. The value obtained is compared to a critical value, which helps you decide how likely it is that the results or ‘differences in the means’ were due to chance. If the value obtained from the t-test is greater than the critical value at a probability (P value) of 5% or less (≤ 0.05), then you can be 95% confident that the difference is significant and not due to chance. This is called a 95% confidence limit — which is good enough for most biologists to reject the null hypothesis.

Chi-squared test You can use the chi-squared test when you have categorical (grouped) data and you want to know whether your observed results are statistically different from your expected results. You compare your result to a critical value — if it’s larger than the critical value at P = 0.05, you can be 95% certain the difference is significant. There’s more on chi-squared on pages 393-395.

Exam Tip In the exams, you could be asked which statistical test you’d use to analyse some data and to explain why you’d use it, as well as to interpret the test result.

Exam Tip When you’re talking about the results of a statistical test and using the 95% confidence limit, make sure you refer to the probability as less than 0.05 or 5%, not 0.05%.

Tip: If the result of your statistical test is greater than the critical value at a P value of less than 2% (< 0.02), or even 1%, you can be even more confident that the difference is significant.

Correlation coefficient A correlation coefficient allows you to work out the degree to which two sets of data are correlated (see page 15 for more on correlation). It is given as a value between 1 and –1. A value of 1 indicates a strong positive correlation, 0 means there is no correlation and –1 is a strong negative correlation. You can then compare your result to a critical value to find out whether or not the correlation is significant. The Spearman’s rank correlation coefficient is an example of a correlation coefficient. See pages 256-257 for how to use it.

Tip: Make sure you’re familiar with the symbols > (greater than), >> (much greater than), < (less than) and