Biology for CSEC [3 ed.]

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Macmillan Educalion 4 Crinan Street, London, NI 9XW A division o f Macmillan Publishers Limited Compan ies and representatives throughou t Lhe world. www.m acmillan-caribbean .com ISBN 9780-230-43883 -5 Text© Linda Atwaroo-Ali 2014 Design a nd illustration © Macmillan Publishers Limited 2014 The auLhor has assened her rights to be identified as the author/s or this work in accordance wiLh the Copyright, Design and Patents Act 1988. This edition publ ished 20 14 First edition published 2003 All rigbts reserved; no pare of chis publication may be reproduced, stored in a retrieval system, transmitted in any fom1, or by any means, electron ic, m echanical. photocopying, recording, or otherwise, with out the prior wriuen permission or the publishers. Note to Teachers Photocopies may be made, for classroom use. or pages 332-367 without the prior written permission of Macmillan Publishers Limited. 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Contents Series Preface

vii

About this Book

viii

Section A: Living Organisms in the Environment 1

2

3

4

5

2

The Variety of Living Organisms Characteristics of life The major groups of organisms Classification of organisms on the basis of visible characteristics The binomial system Chapter summary Answers To ITQs Examination-style questions

9 10 12 13 13

Ecology and the Impact of Abiotic Factors on Living Organisms Ecology Environmental factors Ecosystem, habitat, population, community Distribution of species Chapter summary Answers To ITOs Examination-style questions

15 15 16 17 21 22 24

Feeding Relationships between Organisms Producers and consumers Herbivores, carnivores and omnivores Predators and prey Food webs Decomposers and detritivores Special relationships Chapter summary Answers To ITQs Examination-style questions Ecosystem, Habitat, Population, Community Trapping the Sun's energy Pyramids of energy Pyramids of numbers Pyramids of biomass Bioaccumulation Chapter summary Answers To ITQs Examination-style questions The Cycling of Nutrients Biogeochemical cycles The carbon cycle The nitrogen cycle Acid rain

Chapter summary Answers To ITOs Examination-style questions

3 3

15

6

7

24

Population Growth, Natural Resources and their Limits Growth of natural populations Resources and their limits Chapter summary Answers To ITQs Examination-style questions The Effects of Human Activity on the Environment Humans and the environment Endangered and vulnerable organisms Other effects of human activity Impact of human activities on marine and wetland environments Impact of increase in greenhouse gases Conservation and restoration of the environment Chapter summary Answers To ITQs Examination-style questions

24 25 26 27 Section B: Life Processes and Disease 28 29 8 Cells Why we need microscopes 30 Plant and animal cells 30 Unicellular microbes 31 33

33 37 37 38 39 9 40 40 40 42

42 43 46 49

50 50 51 52

52 55 60 60 62 63

63 64 66 70 72 72 74 74 75

Cell specialisation in multicellular organisms Movement of substances into and out of cells Chapter summary Answers To ITQs Examination-style questions

78 78 79 80 81 83 87 88 89

Photosynthesis Plants are the food supply for animals Photosynthesis Products of photosynthesis Limiting factors in photosynthesis Etiolation Chapter summary Answers To ITQs Examination-style questions

91 92 96 96 97 98 98 99

91

iii

10 Feeding and Digestion

11

100 16 Excretion, Osmoregulation and Homeostasis

181

Diet A balanced diet Malnutrition Holozoic nutrition Digestion and absorption along the alimentary canal Assimilation Functions of the liver Chapter summary Answers To ITQs Examination-style questions

101 106 107 108 111 116 117 117 118 119

181 182 183 184 189 189 192

Respiration Aerobic respiration Anaerobic respiration Chapter summary Answers To /TQs Examination-style questions

122

12 Gaseous Exchange

122 125 127 128 128

The importance of transport in plants Transport systems of plants Movement of water through a plant Transpiration Adaptations in plants to conserve water Uptake and movement of mineral salts Transport of manufactured food Chapter summary Answers To ITQs Examination-style questions

15 Storage in Plants and Animals Why do organisms store food? Food storage in plants Food storage in animals Chapter summary Answers To ITQs Examination-style questions

iv

The importance of movement in animals Movement in plants The skeleton of humans Chapter summary Answers To ITQs Examination-style questions Irritability Stimulus The sense organs of humans The nervous system The endocrine system Drugs and the effects of drug abuse Chapter summary Answers To /TQs Examination-style questions

142 19 The Eye, the Ear and the Skin

The need for a transport system 142 143 The circulatory system of humans Blood 149 Hypertension 152 The role of blood in defending the body against disease153 Chapter summary 156 Answers To ITQs 157 Examination-style questions 159

14 Transport in Plants

17 Movement

130 18 Irritability, Sensitivity and Coordination

Importance of gaseous exchange in humans 130 Mechanism of gaseous exchange in humans 131 Importance and mechanism of gaseous exchange in plants 135 Characteristics common to gaseous exchange surfaces 135 The effects of smoking 137 Chapter summary 139 Answers To ITQs 140 Examination-style questions 140

13 Transport and Defence in Animals

Metabolism Excretory products in animals Excretory products in plants The human excretory system Osmoregulation Homeostasis Chapter summary Answers To ITQs Examination-style questions

160

The eye How we see Sight defects and their corrections The ear How we hear Balance The skin Temperature regulation in humans Temperature regulation in birds Skin care Chapter summary Answers To ITQs Examination-style questions

160 161 163 166 167 20 Reproduction in Animals 168 Reproduction 168 Reproduction in humans 170 The male reproductive system 170 The female reproductive system 172 Hormones of the gonads Fertilisation 173 Development of the embryo, fetus and placenta 173 Birth 173 The role of contraception 178 HIV/AIDS and other STDs 179 Chapter summary 179 Answers To /TQs 180 Examination-style questions

1~3

194

196 196 197 199 205 205 206

208 209 209 210 211 217 219 222 223 224 225 226 227 230 232 233 234 235 237 239 239 240 240 242

244 244 246 246 247 248 250 250 252 253 254 256 256 258

21

Reproduction in Plants Life cycle of a plant Structure of a flower Pollination Fertilisation and development of seed Dispersal Chapter summary Answers To ITQs Examination-style questions

22 Disease and Humans Health and disease Pathogenic diseases and vectors Social and economic implications of disease Chapter summary Answers To ITQs Examination-style questions

259 24 Meiosis 259 The importance of meiosis 261 The process of meiosis 262 Significance of meiosis 263 Chapter summary 264 Answers To ITQs 267 Examination-style questions 267 25 Heredity and Genetics 269

23 Mitosis Chromosome number The cell cycle Importance of maintaining species chromosome number The process of mitosis Mitosis and asexual reproduction Chapter summary Answers To ITQs Examination-style questions

Genes Examples of genetic effects Pedigree charts Chapter summary Answers To /TQs Examination-style questions

296 297 302 304 306 306 309

Genetic variation Importance of genetic variation DNA testing and forensic science Natural selection Artificial selection Mutation Genetic engineering Chapter summary Answers To ITQs Examination-style questions

310 310 312 313 313 317 319 321 323 324 325

Practical work in Biology School-Based Assessment contents

328 328 332

271 271 273 275 275 276 26 Variation and Evolution 276

Section C: Continuity and Variation 278 278 279

290 290 291 293 294 295 295

281 282 282 287 287 Section D: School-Based Assessment 288 27 School-Based Assessment

Index

368

v

Series Preface, 3rd edition Macmillan's textbooks for the Caribbean Secondary Education Certificate (CSEC) Science subjects have been written by teachers with many years' experience of preparing students for success in their examinations. These revised third editions have been written to align with the new CXC syllabuses (to be first examined in 2015). Additional practical activities have been included to reflect the new emphasis on practical work, and new [eatures (such as group work and discussion activities) will help teachers to cater to a variety of different learning styles within the classroom. These books are specially designed to stimulate learning, whatever the reader's needs. Students starting a topic from scratch may need to be led through the explanation one step at a time, while those with prior knowledge of a topic may need to clarify a detail, or reinforce their understanding. Others may simply need to cl1eck that they understand the material. Each CSEC science syllabus specifies the areas to be used for the School-Based Assessment (SBA). Each book in the series has a section designed to help students with their SBA, by offering advice on how to approach the task, presenting examples of good SBA work or suggesting suitable material to use within it. Teachers are free to photocopy these pages. The CSEC Science series covers everything a student needs pass their CSEC examination, as well as providing a firm foundation for more advanced study at CAPE level. Dr Mike Taylor

-ly becomes inadequat('. even wi1h panting ror air and 1he increased hl'.in r.lte. The muscle cell~ 1hcn respire anaerobia lly. Encrg) h )lill produced when cell) respire anaerobicall y, although h Is a much ~mailer amount for each molecule of glucose. Thi~ means that they c m con1lnue 10 dn work (cuntr.1C1 and rdax).

Anaerobic respiration in bacte

L~

Sometimes bacteria can be found In canned foods or tins, desp1te the lact that the cans and tins are sealed so that no air can enter. How Is this possible?

Some baocria a lso respire anaerobically. LikC' ani acid as a wastl' 1noduct. We make use of 1hb in 1 and cheese (Og urc 1 1.9 ),

lnoculatlon

ClCOtd 10 40°C n a 'statttt' CUii.i'ii of

• nat'mhic fC'\J,l ~1lon

e.g.~toboc:lb~

}:lurosc - -- -- - l actic acid+ energy In musclt- cdl~

.~ \;'-'

Humans respire mosUy aerobically. When do humans respire enaeroblcally?

f9f?Mnt11Uon

n::uoeraa in twge Yd1lt (.CO cc tar lltX)I

Lactic acid is a waste produa of th.ls reaction. It builds up In the muscles and a u ses 1hem to ache (rig ure 11 .6). This is often called fallgue . A her cxcrdsc:, rho bod1 ha> to gel rid of 1hc lact ic acid as quickly as pos

.~

Rgure 1.15 Bidens - Shepherds needle, Spanish needle, The fl owering plants have Beggar-ticks, st1cktight. true fl ow ers and so m ake seeds. They are a lso called angiosperms and are divided into two groups: • the m onocotyledons; • the dicotyledons.

Table L. l shows the distinguishing features of monocoi:yledons and dicotyledons.

l/'-1

(i)

Plants range in size from unicellular to giant. Put these plants in order of size starting from the smallest: fern, mango tree, croton, moss and lettuce. (ii) List five reasons why plants are important.

Feature

Monocotyledons

Dicotyledons

seed

has one cotyledon or seed leaf

has two cotyledons or seed leaves

leaf

has parallel veins

has net-like or branching veins

example

corn (Zea mays)

Hibiscus

Table 1. 1 D1stmgwshmg features of monocotyledons and dicotyledons.

Angiosperms a re th e la rgest group of plants. They include most crop plants, orna men tal pla nts and plants used as he rbs or m edicin al plants. They vary in size fro m the very sm all to gigan tic (over 90 m ta ll) and are ofte n very bea utiful (figu re 1.1 6). They can live in a wide variety of habitats, from deserts to ra inforests. Figure 1.16 Flame tree.

Phyla is the plural of phylum.

Animals (Animalia) The animal kingdom conta ins multicellular, heterotrop hi c orga nisms. Th ey a re grouped in phy la as sh own in figure 1.17. Animalla

Cnidaria

Platyhelminthes

• lh.

Annelida

invertebrates

Nematoda

vertebrates

Figure 1. 17 Animals are placed in phyla. (Tho:;e .>hown mred are described in more detail overleaf.)

7

Living Organisms 1n the Environment

Table I .2 shows examples of each animal phylum.

Phylum

Examples

Cnidaria

jellyfish, sea anemone, coral

Platyhelminthes

flatworms, e.g. tapeworm

Mollusca

slug, snail, mussel, octopus

Annelida

roundworm , earthworm, leech

Arthropoda

insect, spider, lobster, millipede, centipede

Nematoda

roundworms

Chordata

fish, amphibian , reptile, bird, mammal

Table 1.2 Example:s of the animal phyla.

Arthro'pods (Arthropoda)

Figure 1.18 An invertebrate that lives on land , a snail.

Arthropods dominate life on Earth. They include the crustaceans, millipedes, centipedes, arachnids and insects. They all have an exoskeleton (outer skeleton of chitin) and jointed limbs. • The cru stacea n s a re aquatic or live in damp pla ces . They include woodlice, crayfish, crabs, lobsters a nd barnacles. • The arachnids include spide rs, scorpions, mi tes and ticks. They have four pairs of wa lking legs and are mainly terrestrial and carnivorous. • The insects have a distinct head, thorax and abdomen, and three pairs of walking legs. They include locu sts, bees, ants, beetles, aphids and fleas.

Molluscs (Mollusca)

Rgure 1. 19 An invertebrate that lives in water, a sea cucumber.

The molluscs have a soft body which is often covered by a shell. They include con ch, sna ils, slugs, cockles, mussels, octopus, squid, clams and oysters. Figures 1. 18 a nd 1.19 show examples o f mollusccs. Some molluscs like conch and oysters are important to Caribbean people as a source of food and an exotic trea t to loca ls and tourists. Farming of mo ll uscs is pra ctised on some islands as demand exceeds supply from wild pop ula tions. These a nimals are a renewable resource but popula tions can decline rap idly because of over- harvesting from their natural habitat.

Chordates (Chordata) Most ch ordates are also vertebrates because t hey have a vertebral column. The vertebra tes include the fishes (ca rti laginous and bony), amphibians, reptiles, birds and mammals (figure 1.20).

8

1 · The Variety of Living Organisms

frog (ampilibian)

lizard (reptile)

scarlet ibis (bircf)

monkey (mammal)

Figure 120 There are five groups of vertebrates: fish amphibians. reptiles. birds and mammals. Birds (Aves) have the fo llowing ch aracte ristic [ea tures: • front pair o[ limbs modified to form wings; • skin covered with [eath ers; • produce hard-shelled eggs (reproduction);

fish

• are warm-blooded. Mammals (Mammali a) have the followin g chara cteristics: • four limbs; • sk in covered with hair; • most give birth to live you ng; • feed their yo ung with milk made by the mother (suckle); • are warm-blooded.

Q9.:.,

l:tQ8

vv

Name the five groups of vertebrates, giving two examples of each.

Classification of organisms on the basis of visible characteristics

:X Practical activity SBA 1.1 : To observe visible

characteristics of plants and animals. page 333 artificial classification

>

natural classification

>

The simplest way co classify organisms is according to similariti es in their visible ch aracteristics. For example, if we see a number of organisms, we could stan to group them by putting those with w ings together. We can make another group of those with eight legs. We cou ld also put the ha iry ones together. And so on . However, where do we put those th at are both h airy and winged? There are two types of classification, artificia l and natural. Artificial classification is based on easily observed ch a racteristics, like colour, shape or number of legs. This is a convenient and easy method of groupi ng organisms and is designed for a practica l purpose. However, worms and snakes have the sam e sh ape, but snakes have a backbone while worms do not. Natural classification tries to use natural relation ships between organisms using both internal and external characteristics. For example, organisms with backbones are grouped together because th ey all have backbones and many other similarities. Similarities in anatomy, physiology and behaviour may all be considered when grouping organisms in a natural classification. Organisms are grouped by similarities that sh ow descent from shared ancestors. For example, a bird wing and a human arm show descent from a ve rtebrate ancestor. A bird w ing an d an insect wing are derived from different stru ctures.

9

Similarities in DNA (deoxyribonudeic acid) sequ ences are increasingly being relied on to determine ancestry. The more alike the DNA sequences a re for two types of organisms, the recently they diverged from a shared ancestor. Remember that each organism has its own DNA 'fingerprint'. Biologists can now construct new evolution ary tree diagrams that sh ow how existing orga nisms are related to one another using their DNA .

~

IT:Q9 l..)'...J

Classify these organisms according to similarities in their visible characteristics into three groups.

Dichotomous keys A dichotomous key is a tool that enables classification of organisms. It works by asking a series of qu estions in a step-by-step fashion until you are led to the name of the orga nism. Dichotomous mea ns 'divide d into two parts' and a dichotomous key always offers two answers to each question.

Sii:nple example of part of a dichotomous key Does it have wings?

2 3

yes - go to question 2 no - go to question 5 Does it have feathers? yes - it is a bird no - go to question 3 Are the wings brightly coloured? yes - it is a moth or butterfly no - go to qu estion 4

And so on. Dichotomous keys can be used to dassify organisms according to both artificial or n atural criteria, including DNA information where it is available.

The binomial system Carl Linnaeus was a scientist in the 18th cen tury who first grouped organ isms together by a natural classification. Many people had tried grouping o rganisms before, but they ha d all used artificial classification. Linnaeus' classification

10

binomial system

>

Genera is the plural of genus.

made it easier to study organisms, since the enormous variety is organised into closely related groups. Carl Linnaeus also put forward a system for naming each species of organism with a biological name, which is called the binomial system. He did this because organisms may have many common names. For example the plant called shadow benny, bandania and cilantro in Trinidad and Tobago, is called sit weed or spirit weed in Jamaica, and in Martinique and Guadeloupe it is known as bandanie. Each biological name has two parts which are the sa·me in all these countries and all over the world - the biological name for the pla~ t is Eryngium foetidum. Th e first word of this name is the genus name and always starts with a capital letter. If you are writing it several times, the first word may be shortened. Fo r exa mple Eryngiumfoetidum may be abbreviated to E.foetidum. The second word is the species name. Every known species has a place in this classification . It starts with major groups of general features, which are broken down into smaller and smaller groups that get more and more specific. Look at the example of the classification of humans in figure 1.21.

Living organisms Placed In five main groups (kingdoms)

Kingdom

Prokaryotes

Protoctista

Phylum

Fungi

Plantae

Animalia

Annelida

Arthropoda

Chordata

invertebrates

Sub-phylum

Class

Order

Vertebrata (possess a vertebral column)

Reptilia (reptiles)

Aves (birds)

Mammalia (hairy, warm-blooded, suckle young)

Primates (monkeys)

Carnivora

I Family

Hominidae (human-like apes)

Genus

Homo

I

------- I Species

erectus

I

..,.,..

(-II-developed brain)

Agure t .21 The classification of humans.

11

Human beings belong in the kingdom Animalia because we are multicellular and heterotrophic. We belong in the phylum Chordata and the sub-phylum Vertebrata because we have a backbone. We are in the class Mammalia because we have hair, are warm-blooded and suckle our young. We are in the order Primates with all the other monkeys and apes. We belong to the family Hominidae which are the human-like apes. In the past, this family bas included several genera including the genus Homo, grouped by the structure of the skull and teeth . There have also been other species of Homo in th e past, for example Homo erectus. However, that species is separated from the modern Homo sapiens . because they had more body hair and a smaller brain . AJI people today belong to the species H omo sapiens because they all have the same characteristics. Table 1.3 sh ows how the ocelot starts in the sam e large groups as humans but is pla ced in a dillerent group from the level of Order down. It is grouped with all the other kinds of cat. Classification group

Humans

Ocelot

Kingdom

Animalia

Animalia

Phylum

Chordata

Chordata

Sub-phylum

Vertebrata

Vertebrata

Class

Mammalia

Mammalia

Order

Primates

Carnivora

Family

Hominidae

Felidae

Genus

Homo

Leopardus

Species

sapiens

pardalis

Table 1.3 Classification of humans and ocelot.

1Chapter summary • • • • • • • • • • • • • • •

12

-

------

A huge variety of living forms exist on planet Earth . All living organisms show the seven characteristics of life: growth, respiration, irritability, movement, nutrition, excretion and reproduction. Living organisms are grouped into five kingdoms: prokaryotes, protoctists, fungi, plants and animals. · The prokaryotes are bacteria. The protoctists include algae and protozoa. The fungi include yeasts and toadstools. The plants are mostly photosynthetic (make their own food). The animals need to get their food by eating plants or other animals. The phyla of animals are cnidarians, platyhelminths, molluscs, annelids, arthropods, nematodes and chordates. The chordates include fish, amphibian, reptiles, birds and mammals. Each major group or phylum is broken down into smaller groups. Organisms can be classified according to similarities in their visible characteristics. A dichotomous key is a tool for classifying organisms by asking a series of yes/no questions in a step-by-step fashion until you are led to the name of the organism. Each species has a common name and a scientific name. A species is a group of similar organisms that can interbreed.

~

·Jiii

The presence of water, suitable temperature range, the presence of gases in the atmosphere, like oxygen and carbon dioxide. ITQ2 (i) Most plants are able to make their own food in a process called photosynthesis. (ii) A plant moves by growing towards light from the environment. ITQ3 Prokaryotes (bacteria) , protoctists (algae and protozoans), fungi (moulds, yeasts and mushrooms} , plants (mosses, liverworts, ferns, conifers and flowering plants), animals (invertebrates and vertebrates). ITQ4 Microscopic means cannot be seen with the eye without the use of a microscope because they are so small. Unicellular means made up of one cell. A bacterium is a single cell which can carry out all the processes of life. ITQS Algae: Chlorella; protozoan: Amoeba. Both orgarusms have 'true' nuclei; the chromosomes are enclosed in a membrane which is called a nucleus (so they belong to the eukaryotes). (Bacteria differ from this and are prokaryotes.) A difference between Ch/ore/la and Amoeba is that Chlorella has a chloroplast and is able to photosynthesise or make its own food, while Amoeba cannot photosynthesise and must feed on other organisms. ITQ6 Yeast: to make bread. Mushrooms: for food. Moulds: to make the antibiotic penicillin. ITQ7 (i) Moss, lettuce, fern, croton and mango tree. (ii) They produce oxygen which is need by anima ls for respiration. They are a food source. They can be used for medicinal purposes (herbs). They hold topsoil in place. They provide homes for animals. ITQ8 Fish: shark, guppy. Amphibian: frog, toad . Reptile: snake, lizard. Bird: parrot, duck. Mammal: lion, goat. (You may have thought of many other examples.) ITQ9 Two pairs of wings, three pairs of legs, body divided into three parts. ITQ1

Examination-style questions (i) (a) List the characteristics of life. (b) Describe the importance of two of these characteristics. (ii) Explain the difference between: (a) the growth of a crystal and the growth of a plant. (b) the movement of a cloud and the movement of an animal. (iii) Robots have been built that move, detect and respond to various stimuli. (a) In what ways is a robot similar to a human? (b) What are some differences between a robot and a human? 2

(i) Living organisms can be classified into five kingdoms. List these five groups giving a named example of each. (ii) Describe two differences between vertebrates and invertebrates. .. (iii) List the main characteristics of dicotyledons and monocotyledons in order to distinguish between them. (iv) Discuss the importance of microorganisms to humans.

13

(i)

3

Animals can be found almost anywhere on Earth. Describe how: (a) a bird is adapted for flying. (b) a fish is adapted for swimming. (c) a bird is similar to a fish. (d) a bird is different from a fish. (ii) Humans are said to be closely related to chimpanzees. (a) Explain why this is so by comparing visible differences and similarities between humans and chimpanzees. (b) Are there any similarities in their behaviour? Explain fully.

4

(i) List two features common to the organisms shown below. (ii) Using each feature, classify the organisms. List the members of each group. D

B

E

G

H

:;

.

14

'• '

5 cology and the Impact of Abiotic Factors on Living Organisms 0

understand the terms 'ecology', 'ecosystem' and 'environment'

0 0 0

distinguish between abiotic and biotic factors

0

distinguish between population and species

0 0 0

relate the distribution of species to abiotic factors

distinguish between habitat and niche d istinguish between community and population

describe the components of soil understand the advantages and disadvantages of the use of natural and chemical fertilisers

ecology

(

I

)

ecological study

{

biotic factors

environmental

I

abiotic factors

ecosystem

I

r distribution of plants and animals

'

community

r population

I species

I

'

habitat

niche

Ecology Ecology is the study of the relationsh ips of organisms with each other and their environment. Togeth er, all the external con diti ons in which an organism lives con stitute its en vironment.

Environmental factors Environmental factors may be of two kinds: • abiotic or ph ysical factors (non -living); • biotic factors (living) .

15

Abiotic or physical factors edaphic factors >

~

IT:Q1 V'--J

Examine figure 2.1 and its caption. List: (i). two abiotic factors (ii) two biotic factors you can deduce from the images. biotic factors

• Cli matic factors such as light, temperature, rainfall, wind and availability of water. • Edaphic factors (associated with the soil) such as pH, texture, temperature, organic and mineral content. • Aquatic factors such as salinity, wave action and dissolved oxygen. • Topographic factors (associated with physical features of the Ea rth's surface) such as the angle of the slope.

Biotic factors >

(a)

Biotic factors result from the activities of Jiving organisms in the environment. Factors like predation, symbiosis, competition and disease ail involve rhe living elements of the environment. All the relationships that exist between the living organisms, including the feeding relationships (food chains and food webs), camouOage, pollina tion and dispersal make up the biotic part o( the environment. (b)

~

IT:Q2 V'--J

Why is a home aquarium not selfsustaining while a backyard pond might be?

1;mmmu ecos stem >

(C)

Rgure 2. 1 (a) The white-lip anole lizard lives in tropical rainforest and feeds on insects. (b) The bottle-nosed dolphin is a fast-swimming marine mammal that feeds on big-eye scad. The scad swim in big shoals and dart back and forth when attacked to try to confuse the dolphin. (c) The Caribbean flamingo feeds on tiny algae and shrimp which it filters from soda lake water with its specialised bill. Other birds cannot feed in these lakes because soda is caustic.

Ecosystem, habitat, population, community

An ecosystem is a sell-sustaining system of organisms interacting w ith each other and their environment. It is made up of all the plants and animals ~ sharing an environment. It is self-sustained wh en it can take care of itself - no IT:Q3 V'--J human imervenrion is needed to keep it going. Distinguish between (i) habitat and The area in which an organism lives is called its habitat, for example a small niche (ii) population and community. pond, a swamp or a rocky shore. A very small habitat is called a microhabitat, for example the soil at the bottom of a pond, the roots of a mangrove tree, leU3ftJ the crevice of a rock. A niche describes the role an organism plays within the ecosystem. It how the organism lives in its habaitat. population > A population is a group of organisms of the same species which live in a particular habitat. For example, in a pond ecosystem, there may be a community > population of beetles and a population of snails. A community consists of ~ all the populations which live in the same place and interact with each other. IT:Q4 V'--J The community in the pond ecosystem is made up of populations of different Using figure 2.2, describe: (i) a species of organisms, feeding on each other, competing with each other, population (ii) a habitat (iii) a niche (iv) hiding and protecting each oth er and also communicating with each oth er a community. (figure 2.2).

16

Abiotic factors

Biotic factors

Water's edge Water column

Bottom

Water surface

Water column

B ottom

Water's edge

Waterlogged soil

Variable light

Birds - red seal coot

0 2 maybe low

Insects - water strider

Dead/decaying matter Variable sediments

Plants - water lettuce

Microscopic organisms zooplankton, phytoplankton Insects - water boatman, diving beetle Fish - guppy, molly, tilapia, tarpon

Bacteria including blue-greens micro- and macro-algae Shrimp Snails Dragonfly larvae

Flowering plants grasses, sedges Crabs - land crab, fiddler crab Birds - egret, sandpiper

Variable light Variable 0 2 Temperature may vary

Figure 2.2 The biotic and abiotic factors in a pond habitat, including the community and populations of organisms living in the pond habitat.

Distribution of species The distribution of species is related to the physical or abiotic fa ctors of the environment as well as the availability of food or prey. A species is adapted t0 live in its environment. For example, camels are adapted to survive and live in the desert, an extremely harsh environment. Other species simply cannot live there. On ly animals that can tolera te dehydration and survive extremes of temperature can be found there.

Effects of water on distribution

~

IT:QS \./'-I

What are some factors or qualities of water that determine the types of organism that live in water?

Water is an abiotic factor that affects the distribution of species. Organism s like fish and jellyfish that live in water must be able to use oxygen dissolved in water or take their oxygen from the air above the water, like whales. If they do not attach themselves to rocks or bury themselves in the seabed, they must also be adapted to move in water. There are two main kinds of water found on Earth : • fresh water found in lakes, rivers and ponds; • salt water found in the ocea ns and seas.

17

Living Organisms in tha Environment

Fresh water is low in salt and mineral content, but salt water can be very concentrated. Where these two kinds of water meet, such as in estuaries, the waters m ix to give brackish water. Most anima l species are adapted to Live in either fresh water or salt water (figure 2.3). Only a very few that live in estuaries or regularly migrate from sea to river or back again (s uch as salmon and ee ls), can cope with the different conditions. Fish in a marine environment

Fish in a freshwater environment

gills actively excrete salt to the water passing over them

gills actively ab sorb salt from the water passing over them

drinks sea water

t

does not drink fresh water

water constantly enters the organism and collects in a vacuole

small amounts of concentrated urine produced too much salt is a problem and is actively gotten rid of

l large amounts of dilute urine produced

too much water is a problem it is not actively taken in and is actively gotten rid of

Agure 2.3 Adaptations of bony fishes to live in marine or freshwater environments. Freshwater animals, like Amoeba, have mechan isms to get rid of the excess water that enters their body by osmosis (figure 2.4).

~l

vacuole moves to the cell --""-4 membrane

The water is actively expelled from the contractile vacuole of the Amoeba.

Figure 2.4 Amoebae can live 1n fresh water because they are adapted to get rid of the excess water m their bodies.

(a)

Some species do not Live in water, but it still determines their distribution . Toads and frogs Li ve and feed on land but must return to water to reproduce. They are always found near rivers, pon ds and lakes. Others return to water to cool down and are found in or around areas with water. The distribution of planes is also related to water. Plants n eed a constant suppl y of water from the soil. Some actually live in wa ter, like water lilies. Plants that live in areas where water is in shon supply are called xerophytes. They have special fea tures which help reduce transpiration and therefore water loss (figure 2.5). Some of these features are: • redu ction of leaves to fine spikes (e.g. cacti); • the stomata are sunken in grooves and reduced in number (e.g. oleander); • the leaves roll into a cylindrical shape (e.g. marram grass).

(b)

Figure 2.5 (a) Cacti have leaves reduced to spines to reduce transpiration (b) Tile leaves of oleander have stomata sunken in grooves to reduce water loss.

18

-

- __-c- ~ •

Ecology arid tt1e Impact of Abiotio Factors on

Livin~ -011ganisiiis

The change from water to land along the edge of wa ter can create very clear zones of vegetation. Plant species that are more tolerant of having their roots submerged in water for long periods of time, such as the red mangrove, are fo und at the edge of the water. These species are replaced further inland by th ose which can tolerate some su bmersion, such as the black mangrove, and even further inland by those whkh are adapted to cope with on ly a li ttle submersion, such as the white mangrove (fi gure 2.6).

Red mangrove

Black mangrove

White mangrove

pneumatophores that are wider, knobby and less dense

thick stilt roots or prop roots support and spread the weight of the tree in the soft soil

slender aerial roots hang from the trees

nutritive roots absorb nutrients

anchoring roots

Excrete salt through their leaves

An indication of drier, better soil. They have normal root systems.

Excrete salt through glands in the petioles

J

Excrete salt through salt glands

Low tide

SEA (SALl) WATER

Mangrove zonation

Rgure 2.6 Zonation of vegetation along the edge of a mangrove swamp.

19

,; . ~Living Organisms in the Environmer't

_.

Effect of light on distribution

~

ll!Q6 \../V

Light also affects the distribution of plants and animals throughout the Earth. Animals use light mainly to see their prey (figure 2.7). Some use the absence of light to escape predators. Light is vita l to planes because it is needed for photosynthesis. Without light a plant w ill die. Plants are not found in those areas of the Earth without light, like deep caves and deep ocean fl oors. Two aspects of light, its duration and its intensity, are important for the distribution of species, particularly plants . . However, heat is usually associated with high light intensity or bright light, and temperature is also an abiotic factor that affects species distribution.

Name some herbivores that come out at night to feed, hoping to escape their predators. (ii} Predators that hunt at night may use mechanisms other than light to detect their prey. Describe, using examples, two other means apart from light that can be used to detect prey. (i}

Effect of temperature on distribution Temperature also affects the distribution of species. Poikilothermic animal s are particularly affected because their body temperature reflects the temperature of the surroundings. If it is too cold, they cannot generate eno ugh energy to move around to find food or escape predators; if it is too hot, the proteins in their bodies start to break down and they die. Homeotbermic animals, such as mamma ls and birds, may be able to live in a greater range of temperatures but they show adaptations to cope with extremes of temperature. Camels are adapted for desert life. Desert hares have long ears which give off heat to keep the animal cooL but arctic hares have very short ears to reduce heat loss. Other mammals that live in the polar regions, like the polar bear, have thick layers of body fat and fur to keep them warm. Mammals such as whales, walruses and seals, are also able to live in cold polar waters because they have a thick layer of fat, called Rgure 2.7 Chameleon actively hunts its prey. blubber, just beneath the skin. This insulates them Crom the cold: whale blubber can be up to 50 cm thick.

Effect of heavy metals on distribution

micronutrients

20

>

Our environment, and in particular the sea, contains in large or smaller amounts almost every metal known to humans. Life began in the sea and so most living things, through the process of evolution, have acq uired a tolerance for sma ll concentra tions of these metals. Some of them, such as copper, are essential in trace quantities and are called micronut rients. Metals such as copper, mercury and lead (called the heavy metals) are not tolerated in more than trace amounts. In larger conce ntrations they become toxic to animal and plant life, and we think of them as pollutants. These large concentrations often arise as a result of human activities. For example, some slag heaps on the island of Anglesey in the United Kingdom are so rich in copper that nothing, except a few clumps of horsetail grass, will grow on them. Mercury and lead are particularly dangerous to humans. The poisonous effects of mercury have been known since Roman times. By the J 9th century, mercury was widely used for 'silvering' mirrors, and for treating sexually transmitted diseases. Makers of fe lt hats, who used mercury, suffered from various nervous and mental disorders - hence the phrase 'mad as a hatter' . As the chemical industry developed, organic compounds of mercury were discovered. These are even more toxic because they bind to proteins and fats in body cells. The cells of the brain and the nervous system appear to be more

. affected by these compounds and nowadays many mercury compounds which were once commonly used, for example as seed dressings, are prohibited. Lead is hardly less dangerous. Lead compounds damage the brain, particularly in young children, and lead poisoning can ca use to serious mental disorders. Th e three main ways in which lead was released into the environment were from local water pipes, from lead compounds in paint and from additives in petrol. In many countries alJ three are now prohibited. Tolerance to heavy metals like lead, copper, zinc and mercury, is inherited and passed on to offspring. Random mutations can result in some organisms having greater tolerance to heavy metals th an others. Plants may be able to: • trap heavy metals in the cellulose cell walls; • confine the metals to the vacuoles; • excrete the metals back in to the environment. These heavy-metal tolerant plants are rarely found in unpolluted areas as they are less competitive than other plants. They flourish in polluted areas as the heavy metals kill the competing plants. Tolerant plants pass on th~ir tolerance to their offspring.

Effect of soil on distribution ~

Practical activity

SBA 2.2: Water-holding capacity of three

types of soil, page 338

Practical activity SBA 2.3: Percentage of water in a soil

sample, page 339

Practical activity SBA 2.4: Percentage of air in a soil

sample. page 340

Soil supports terrestrial life. For plants, it provides an anchor for roots and is a medium for nutrients. It acts as a ·sponge for water, holding it for absorption by the roots of plants. Plants are able to grow where the soil can provide all their needs. This means that soil type is very important to the distribu tion of plants. Animals depend on plants which depend on soil. Thus soil is also and so important to the distribution of terrestrial animals. It provides shelter for subterranean animals, but more importantly, thousands of microbes exist in soil tl1at replenish the microbes that live in the digestive tracts of herbivores. Humans have adapted to life on land. We build homes on land and depend on agriculture for our food. All crops require special types of soil. The soil sustains all forms of life across the planet. .

,.Chapter summary

I~ II

• Ecology is the study of the relationships of organisms with each other and their environment. • There are two kinds of environmental factors: abiotic and biotic. • Abiotic factors make up the non-living part of the environment. • Biotic factors result from the activities of the living organisms in the environment. • An ecological study involves looking at the biotic and abiotic factors of an area. • Sampling methods include quadrats, line transects and sweep nets. • A habitat is a place or area where an organism lives. • A niche is the role an organism plays within the ecosystem. • A species is a group of organisms that can interbreed and are adapted to live in their environment. • A population is a group of organisms of the same species living in an area. • A community consists of all the populations living in the same area. • The abiotic factors of an environment affect the distribution of the species found there. • Water and light are examples of abiotic factors that affect the distribution of species.

21

Living Organisms Jrr:t11 e~_En_1.1i_r:onn1e_n~,

-

.

_·

__

~

ITQ1 J

Abiotic factors

Biotic factors

The temperature of water.

Feeding relationships, e.g. between the lizard and insects that are its prey.

The amount of light available to the organisms. Behaviour of scad when attached by dolphin.

You may have noted other examples from the pictures. A home aquarium is a limited ecosystem; it doesn't contain the diversity of species that would be found in the naute. A backyard pond is more likely to be a complete ecosystem with all the diversity necessa ry to sustain itself. ITQ3 (i) A habita t is the place where an orga nism li ves. A niche is the role an organism plays in an ecosystem. (ii) population is a group of organisms of one species living together in one habitat. A community is all the populations of all the organisms living together in an ecosystem. ITQ4 (i) A population is a group of organisms, all of the same species living together in one habitat. In this pond there are populations of many different species of fish and plants. (ii) A habitat is the place where an organism lives. The habitat is the pond. (iii) A niche is the role an organism plays in an ecosystem. Each organism in the pond has its own niche. (iv) A commun ity is all the populations Jiving together. This pond community includes the populations of au the plants, fish and other animals found there. ITQS Water may be salt water or fresh water. Salt water makes up the oceans and seas. Fresh water includes the lakes, rivers and ponds. Water can be stagnant or fast-fl owing and all the stages in between. Rocky shores have strong curren ts and wave action . Mangrove swamps have brackish water, which is a mix of salt and fresh. Orga nisms are adapted to live in these different habitats. ITQ6 (i) Examples are fruit-eating bats, and agouti which feed on fruits and seeds; there are many others that you might have thought of. (ii) Snakes have heat sensors found in pits on their face which can determine the presence of other living organisms. Snakes also use their forked tongue to pick up tiny particles left by an organism in the air. The tongue is then pushed into the pits of the mouth, and the snake 'tastes' the organism. Many other organisms use scent to find food. Insect-eating bats use sonar, or sound, to determine exactly what is around them and help them catch prey. ITQ2

Examination-style questions (i)

Explain, using examples, the meaning of the terms: (a) abiotic factor; (b) biotic factor. (ii) Define: (a) environment; (b) habitat; (c) population; (d) community. (iii) Describe, using examples, how abiotic factors of the environment affect the distribution of species. (iv) (a) Amoebae live in fresh and salt water habitats. Describe a major problem of amoebae living in fresh water.

22

(b) Explain how Amoeba is adapted to live in fresh water. An ecological study was conducted in a cocoa estate and the data collected by a student are seen below.

2

Animals caught in the sweep net

Animals seen

Plants seen

spider beetle caterpillar grasshopper other (unidentified)

frog kiskadee lizard worm squirrel dog iguana millipede

grass mango tree cocoa tree unknown shrubs coffee tree pea plant pomerac tree

Quadrat throw

Millipedes

Spider

1

40

4

2

30

0

3

10

0

4

5

0

5

23

6

28

2

7

51

3

8

19

4

9

37

0

10

40

{

(i) Construct a possible food web from the plants and animals recorded. (ii) These organisms interact with each other in a number of ways. Suggest two possible relationships that may exist between the organisms recorded. Using names examples, describe fully each example. (iii) Suggest some sources of error when using sweep nets. (iv) Calculate the population density of the millipede and spider. (v) The area studied was approximately 12 m wide and 20 m long. Calculate the population size for the millipede and spider. (vi) Describe fully how a quadrat can be used to estimate the number of organisms present in an area. (vii) Compare the use of the quadrat for these two organisms, millipede and spiders. Which do you think are the more accurate results? Explain why.

23

Fieeding Relationships betvveen Organisms 0

understand the meaning of the terms producers and consumers in a food chain and relate the position in the food chain to the mode of feeding

0 0 0 0

understand the terms herbivore, carnivore and omnivore

0 0

identify a food chain identify predator/prey relationships construct a food web that includes different trophic levels explain the role of decomposers understand that special relationships ex ist and discuss the advantages and disadvantages of such relationships

food chain first trophic level

second trophic level

third trophic level

fourth trophic level

producer

primary consumer

secondary consumer

tertiary consumer

plants

herbivore

carnivore

carnivore

l

)

symbiosis - relationships between organisms of different species

decomposers

food web - interlinking of food chains

I parasitism commensalism mutualism predator/prey

CHAPTER 9 Phytoplankton are microscopic organisms, like algae and blue-green bacteria that live in the oceans. They are seen in rivers, lakes and puddles of water. They are important since they start food chains in the world 's oceans or seas.

Around deep-ocean hot water vents, there are bacteria wh ich get their n utrients and energy from the wa ter. These bacteria are the food for animals, an d these food ch ains are the only ones we know on Earth which do not depend on the Sun for th eir energy. Life depends on photosynthesis which is carried out by planes (chapter 9). Most animals get their n utrients (their source of energy) either directly or indirectly from p lants. Plants photosynthesise or make food from water and carbon dioxide, using light energy from the Sun to carry out the process. So the Su n is the ultimate source of energy for almost all life on Earth.

Producers and consumers Plants are called producers because they produce or make their own food. They include mosses and green plants on land, and algae, aquatic planes and phytoplankton in water. Organisms that consume the plants or producers, ma in ly the animals, are ca lled consumers (figure 3. 1). Decomposers feed on dead o rganic matter (figure 3.2).

nutrients (humus) made available by decomposers

! producer consumer consumer plant - - - - - - caterpillar - - - - - + small bird

~

1 /

they all die and their bodies are eaten

~i/ decomposers

~turn nutrients to the soil /

Figure 3.2 dead fruit.

producer

Mould (a fungus) feeding on

in the form of humus consumer

etc ..... .

Figure 3. 1 The relat1onsh1p between producers, consumers and decomposers

Herbivores, carnivores and omnivores herbivores

>

Herbivores are organisms that feed only on plants. Examples are some insects (like grasshoppers, locusts, butterflies, bees), some birds (such as seed-eating and fruit-eating species) and some mammals (cows, horses, elephants, giraffes). In water, herbivores may be very large like the manatee or very small like a shrimp. carnivores > Carnivores are organisms that feed only on anima ls. They may hunt and kill other animals for food. Examples include some insects (like the praying mantis), some reptiles (such as snakes), some birds (eagles and hawks) and some mammals (lions, dolphins and leopards). lelulm1•li4-tl Omnivores feed on both plants and animals. Examples are pigs and humans.

Food chains food chain >

A food chain is a simple diagram that shows how the food or nutrients (the energy source) pass from one organism to another. For example:

leaf-+ caterpillar-+ small bird -+ hawk The arrows show the movement of energy along the food chain. The leaf is a part of a green plant that is photosynthesising and is a producer. The caterpillar eats the leaf to get food (energy) to live and is thus a consumer. The sma ll bird and the hawk are also consumers because they are getting their food or energy from eating other organisms. Indirectly, their food comes from the leaf, since the food made by the leaf is first taken into the caterpillar, then into the sma ll bird as it feeds on the caterpillar and fina lly to the hawk. So all the consumers in the food chain ultimately get their food from the producer. We can also describe the food chain in terms of herbivores and carnivores. Herbivores feed on the plants or producers and then the carnivores feed on the herbivores. An omnivore may feed on the producer or herbivore (and even carnivore in some cases). producer -+ herbivore -+ carnivore

(grass)\ (chi:ken) (mongoose) omnivore (h uman )

25

Herbivores can only feed on the producers and are called the primary consumers. Carnivores which feed on herbivores are secondary consumers. Tertiary consumers feed on the secondary consumers and so on. producer

--+

primary consumer

Example: waterweed Producer

trophic level

--+

--+

secondary consumer

tadpoles primary (1°ry) consumer

--+

small fish secondary (2°ry) consuITier

--+

--+

tertiary

--+

consumer

bigger fish tertiary (3°ry) consumer

> Each organisITI in the food chain represents a trophic level. The three food

A food chain is composed of trophic levels.

chains below each consist of four trophic levels. These are examples of terrestrial food chains. Food chain I leaf --+ caterpillar -+ toad -+ snake Food chain II grass --+ grasshopper -+ insect-eating bird

This is an example of an aquatic food chain.

--+

hawk

Food chain ill algae -+ sna il -+ leech -+ fish Table 3.1 Shows how the organisms of these three different food chains can be classified. Food chain I Food chain II Food chain Ill

Type of feeder

Consumer level Trophic level

leaf

producer

producer

first trophic level ·

herbivore

primary consumer

second trophic level

carnivore

secondary consumer

third trophic level

carnivore

tertiary consumer fourth trophic level

grass

t caterpillar

t

t

t insect-eating leech bird

t snake

t

grasshopper snail

t toad

algae

t hawk

t fish

Table 3. 1 Different ways to classify organisms in food chains

CHAPTER 4

All food chains have certain characteristics in common, as seen in table 3.1. The number of trophic levels in a food chain is normally limited to four or five, since the amount of energy being passed on gets smaller and smaller at each level (d1apter 4).

Predators and prey i•li§•F?U•li•-11

i!mlfJd

Animals also show predator/prey relationships. Predators are carnivores that feed on other animals that are called their prey. Predators hunt, capture, kill and eat other anima ls and those that are hunted and eaten are the prey. Food chains therefore include predators. They are the higher order consumers. rosebush -+ aphid -+ ladybird -+ spider -+ insectivorous bird

26

Prey

Predator

aphid

-+

ladybird

ladybird

-+

spider

spider

-+

bird

Table 3.2 Predator/prey relationships in the rosebush food chain.

In this food chain, while the spider is a predator because it kills and eats the ladybird, it is also prey to the insectivorous bird. The food chain shows three predator/prey relationships(table 3 .2) Animals that are prey have evolved to hide and escape predators, using characteristics such as camouflage, mirrticry and speed. Predators, on the other hand, have evolved characteristics to improve their chances of catching prey, like speed, lures and traps. When all these organisms die, decomposers return their nutrients to the' plants through the soil, and the nutrients return to other feeding animals in the food chains.

Food webs A food chain shows one organism feeding on one other organism only, but feeding relationships are more complex than this. One organism may feed on a number of organisms and in turn may be eaten by a number of organisms. The interlinking of a number of food chains is called a food web (figures 3.3 and 3.4) .

•?a7 \../'-)

From the food web shown in figure 3.3: (i) name (a) two herbivores, and (b) two carnivores. (ii) give the name of an organism which is (a) a primary consumer; (b} a secondary consumer; (c) a producer; (d) a tertiary consumer; (e) both a secondary and tertiary consumer. (iii) name (a} two predators, and (b} two prey. (iv) name an organism found in: (a) the first trophic level; (b) the third trophic level.

~

IT:Q2

frog

rat

I butterfly

caterpillar

hibiscus plant

grasshopper

snail

\)

mango tree

grass

Rgure 3.3 A terrestrial food web. warbine

l due'

_

-

cosf rob

\../'-)

Construct a food web seen in a (i} marine habitat (ii) a tree (such as a mango tree).

~

~ water beetle

mayfly nymph / water boatman

water-flea

~

pond weed

l

algae

Figure 3.4 A freshwater (aquatic) food web.

27

Decomposers and detritivores decomposer >

CHAPTER 5

~

IT:Q3 \../'--)

Define the terms 'producer', 'consumer' and 'decomposer' and give two named examples of each.

All living organisms eventually die. Their bodies are composed of complex compounds like carbohydrates, lipids and proteins that they stored when they were alive. T\.vo groups of organisms called the decomposers and detritivores obtain their food or energy from the remains of the dead organisms. As they feed on the dead organisms they cause their decay or decomposition (figure 3.5). They help in the recycling of nutrients (chapter 5) since they return the nutrients trapped in the dead organisms back to the environment. The nutrients become available again to living organisms. Dead organism fungi and bacteria simple substances complex compounds (proteins, lipids, - - - - - - - - - - - - • (carbon dioxide (CO:U. carbohydrates, etc.) compounds of ammonia (NH:i) from the proteins) carbon dioxide (C02) released into the air as the fungi and bacteria respire fungi and bacteria live

f

f

/

in the dead organism

SOIL ammonia is released into the soil and combines with substances in the soil to form ammonium compounds

/

after some time the dead organism is broken down completely by the fungi and bacteria

SOIL

Agure 3.5 A dead organism decays or decomposes as fungi and bacteria feed on it. 1n11 ..11i.9

saprophyte

>

~

IT:Q~ \../'--)

Draw a diagram to show the feeding relationship between a producer, a consumer and a decomposer using examples from your answer to ITQ3.

lullil!mH•i••U

28

Decomposers include bacteria and fungi. They secrete enzymes which break down dead plants and animal material into a substance called humus. Humus enriches and improves the structure of soils in which plants grow and from which they derive nutrients. Imagine the build-up of dead plants and animals on the Earth's surface if there were no decomposers. All the vital chemical elements or nutrients trapped in these dead organisms would not be able to return to living organisms or be recycled. Detritivores also help in the removal and recycling of dead o rganisms by feeding on small fragments of the dead material, which are called detritus. Examples of detritivores include woodlice and earthworms. Saprophyte is the name given to any organism that feeds on dead organic material, so decomposers and detritivores are all saprophytes.

Special relationships The environment supports a host of organisms all living together. But some organisms live in very special relationships with each other. These relationships may be advantageous to all the organisms involved but, sometimes, one organism can cause harm to another. Symbiosis describes any relationship that exists when different species of organisms live together. There are three types of symbiosis: • mutualism; • commensalism; • parasitism.

3 · Feeding Relationsl1ips between Organisrns

Mutual ism In this kind of association,

CHAPTER 5

two organisms of different species Live closely together and both benefit. Here are some examples. • Some sea anemones and hermit crabs - The anemone attaches itself to the shell used by the hermit crab and obtains scraps of food as the crab feeds. The crab gains protection from predators as it is camouflaged by the anemone and protected Figure 3. 6 A hermit crab and sea anemone. from preda tors by the stinging tentacles (figure 3.6). • Leguminous plants and the bacterium Rhizobium (chapter 5) -The bacteria live inside swellings on the roots of the leguminous plants, like peas and beans. These bacteria convert nitrogen gas into ammonia, which is then con vened into arp.ino acids and used by the plants for growth. The plants benefit because they can thrive in a LI types of soil, even soil where nitrate is in short supply. The bacteria also benefit by having a place to live and an e nergy supply which they ger from the plant. • Egret and cow - The egret perches on th e cow's back as it feeds on insects and aradmids, especially ticks that can harm the cow. The egret is obtaining food and the cow benefits by having blood-sucking insects removed from its body.

Commensalism commensalism

~

IT:QS I../'-)

Using named examples, distinguish between mutualism and commensalism.

> Commensalism is a relationship between two species in which one clearly benefits and the other is not harmed. Here are some examples. • Some orchids or ferns on trees - The orchids or fems are small plants that grow high on tbe tree to obtain sunligh t for photosynthesis (figure 3.7). They use the Rgure 3.7 An orchid growing on a tree. tree for support but n ot as a food source. The tree is not harmed, nor does it benefit. • Egret and cow - When the egret walks behind the cow, it feeds o n insects that fl y up as the cow sha kes the grass while it walks. The egret benefits but the cow does not. • Shark and remora - The remora attaches itself to the shark and moves around with it. As the shark feeds, the remora also feeds on scraps of food that are floating around. The remora obtains food wh ile the shark is not harmed, but nor does it benefit.

29

Living Organisms in the Environment

Parasitism l•l§liU-1H51J ectoparasite

>

endoparasite

>

A parasite is an organism wh ich lives and feeds on or ins ide another o rganism , which is called the host. The parasite gains while the host is harmed. • Parasites wh ich live on the outer surface of their hosts are called ectopa rasites. For example, ticks, lice, fleas a nd leeches feed on th e blood of th eir h osts such as dogs, hwnans, cattle and fish (figure 3.8). • Parasites that live within a h ost are called endoparasites . An example in humans is th e o rganism which causes mala ria. A protozoan of the genus P/asmodium enters the human bloodstream through the bite of an infected female Anopheles mosquito. Once in th e body, the parasite multiplies, ca using bo uts of fever, pain, shivering and sweatin g. Millio ns of people die each year from malaria, although anti-malarial drugs like quinine and choroquinine have been developed.

,, Chapter summary Figure 3.8 A leech sucks blood from a human

• • • • • • • • • • • • • •

The Sun is the ultimate source of energy for most life on Earth. Plants make food and are called producers. Animals eat plants or other animals and are called consumers. A diagram which shows the sequence in which organisms feed on each other is called a food chain . A food web shows the interlinking of a number of food chains. Decomposers feed on dead plants and animals. Herbivores feed on plants alone. Carnivores feed on animals alone. Omnivores feed on both plants and animals. Symbiosis describes relationships between two different species. Mutualism describes a relationship where both species benefit. Commensalism is when one species benefits and the other is not harmed but nor does it benefit. In a parasitic relationship, one species benefits at the expense of the other. Predators are carnivores that feed on other animals which are called their prey.

II

II

ITQ1 (i) (a) Aphid, butterfly, hummingbird, beetle, caterpillar, grasshopper or snail. (b) Ladybird, frog, kiskedee, tarantula, rar. snake, mongoose or hawk. (ii) (a) Aphid, butterfly, beetle, hu mm ingbird, caterpilla r, grasshopper or sna il (b) Ladybird, frog, kiskedee, tarantula, rat (c) Hibiscus, mango tree, grass (d) Frog, snake, mo ngoose, hawk (e) Frog, hawk (i ii) (a) Ladybird, frog, kiskedee, tarantula, ra t, snake, mongoose or h awk (b) Snake, frog, ladybird, kiskedee, ta rantula, rat hummingbird, aphid, beetle, caterpillar, grasshopper o r snail (1v) (a) Hibiscus, mango or grass (b) Ladybird, hawk, kiskedee, frog, tarantu la or rat

30

ITQ2 killer whale

'

( penguin

seal

\.

J

fish

'

( krill

zooplankton

\.

J

bachae

phytoplankton

mango tree

Foodweb for a marine habitat

Foodweb for a mango tree

ITQ3 A producer is an organism tha t produces or makes organic food. A plant makes organic food during photosynthesis, so any plant is a producer. Examples are mango tree and hibiscus plant, bu t you may have thought of many others. A consumer is an organism that eats or consumes organic food. Animals cannot make their own food, so any animal is a consumer. Examples are caterpillars and humans. A decomposer is an organism that feeds on dead organic food (dead animals and plants). The food is said to be decaying or rotting as the decomposer feeds on it. Examples are bacteria, fungi. ITQ4 hibiscus plant -+ caterpillar

bacteria/ Mutualism and commensalism are both relationships between two species or partners that are beneficial or good. In mutualism, both partners benefit. In commensalism, one partner benefits while the other, though not benefitting from the relationship, is not harmed in any way. An example of mutua lism is between the pigeon pea plant (leguminous plant) and Rhizobium bacteria that live in swellings of its roots. The pigeon pea plant gets amino acids for growth, and the bacteria obtain shelter and energy. An example of commensalism is seen with sharks and remora fish . The remora fish obtain food an d protection from the shark which benefits nothing from the relationship and is also not harmed. ITQ5

Examination-style questions (i) Construct a food web from the information given in the table. Animal

What it was seen doing

small moth

feeding on nectar of a flower (morning glory)

lizard

feeding on insects

small bird

with a lizard in its beak

spider

feeding on insects trapped in its web

small butterfly

feeding on the nectar of a flower (lxora)

31

(ii) Examine the food web constructed and describe three consequences of the removal of the lizards. (iii) Describe the relationship between: (a) the moth and the morning glory; (b) the spider and the moth. (iv) Name one predator/prey relationship from the food web and describe: { (a) how the predator is adapted to catch its prey; (b) any feature used by the prey to escape the predator.

2

(i)

(ii)

(iii)

(iv) (v)

Using named examples, describe a: (a) parasite relationship; (b) mutualistic relationship. (a) Draw a food chain with four trophic levels. (Use named organisms.) (b) Identify the producer. (c) How does the organism in the fourth trophic level obtain energy from the Sun? (d) Which organism is the primary consumer? Which organism in the food chain is a: (a) herbivore? (b) carnivore? (c) predator? (d) prey? Describe the role of the decomposers in the food chain. Copy the table below and use examples from these food chains to complete it. root -+ earthworm - frog - fox pondweed - mayfly nymph -+ water beetle

Stages of food chain producer primary consumer predator prey herbivore second trophic level third trophic level first trophic level

32

Two examples of organisms, one from each food chain

Eicosystem, Habitat, Population, Community 0 0 0 0 0 0

understand that the Sun is the ultimate source of energy for life on Earth explain why food is the source of energy needed by living organisms understand that respiration is the process by which energy is released from food describe pyramids of energy d escribe pyramids of numbers d escribe pyramids of biomass

food chain plant makes food using light energy from Sun - - - - , some energy passed on

energy lost due to animal eats and obtains ~ ~espiration , in food (chemical energy) - - - - - i urine and faeces

I some energy passed on J animal - - - - - - -

(

)

food - source of energy for all organisms

feeding pyramids: • energy • numbers • biomass

I

importance of photosynthesis to food chains

All living organisms need energy to carry out life processes; for example your body uses energy to grow, move, inhale and eat. The energy that your body is using came from your food. U you made a food web for everything you eat, you would find that all the energy you use was trapped by plants from tbe Sun. Ultimately, all energy for life comes from the Sun.

Trapping the Sun's energy Plants use the Sun's energy to make food during photosynthesis (chapter 9). During photosynthesis carbon dioxide and water are combined to make glucose and oxygen. energy from the Sun

carbon dioxide + water-------+ oxygen + glucose The glucose is then used to make other carbohydrates, lipids and protein s and everything else the plant needs. These become the components of food (chapter 13) for consumers. The term 'food' can thus be used for the term 'energy', beca use energy is released from food.

33

Living Organisms in the Environment

respiration

~

IT:.Q·1 \_.)'...J

Why is the Sun considered to be the ultimate source of energy for all life on Earth?

>

So the energy in the light from the Sun is converted to chemical energy (as glucose and other chemicals) in the plant. The chemical energy (as food) then passes on to consumers as they feed on the plants (figure 4.1). Respiration releases the energy trapped in the food so that it can be used by the organism. Respiration also makes carbon dioxide and water. Food (usually glucose) is ' burnt' during respiration by plants and animals to release energy so that they can carry out all the processes necessary for life. So, not all the energy gained by a plant is passed on to an animal that eats the plant (figure 4.2). Likewise, not all the energy gained by an animal is passed on to a predator (figure 4.3) glucose+ oxygen-+ energy+ carbon dioxide+ water

energy from Sun passed to

Plants · (photosynthesis) make food/chemical energy

l l

energy from plants passed to

Animals (when they feed on plants) During respiration this energy is made available to be used for everyday activities.

Rgure 4. 1 Energy from the Sun is used by plants and by animals.

Rest of energy stored in plant tissues. Passed on to herbivores when they feed on plant

light energy

energy taken in

energy stored

energy lost Some energy changed to heat during respiration, for life processes. Heat lost to the environment

Rgure 4.2 Only some of the energy taken up by a plant can be passed on to a herbivore.

Energy passed on to carnivore when it eats the COW

Rgure 4.3 Only some of the energy that an animal gains through eating can be passed on to a predator.

34

How a plant gains and loses energy

~

IT:Q2 V'-'

What happens to the energy that a plant gains during photosynthesis?

• A plant gains energy wh en it converts light energy to chemical energy during photosynthesis. • It stores some of the energy by changing the glucose it made into other chemicals. • It uses up some of the food during respiration to release energy to grow and carry out other llie processes. Some of the energy that is released is lost as heat energy from the plant.

How an animal gains and loses energy For each animal at each trophic level: • energy is gained as the organ ism feed s; • some of this energy is stored as tissue as the animal grows; • some energy is lost as faeces and urine straight out of the animal's body; • some of the stored energy is relea sed du ring respiration for the organism to stay alive and some of that energy is lost as heat to th e envirom11ent.

~ IT:Q3 V'-'

What happens to the energy that an animal obtains?

Movement of energy through a food chain Energy flow through a food chain or web is related to the movement of food through the chain. Figure 4.4 shows the movement of energy through a food chain.

~

Energy lost as heat due to respiration

Energy lost as heat due to respiration

Energy lost as heat due to respiration

Energy lost as heat due to respiration

l

l

l

l

PLANT

Energy stored in tissue

-

HERBIVORE or PRIMARY CONSUMER Energy stored In tis_sue

!

--·---------, Energy lost in urine and faeces

-

CARNIVORE or SECONDARY CONSUMER

CARNIVORE or TERTIARY CONSUMER

Energy stored in tissue

! Energy lost in urine and faeces

! Energy lost in urine and faeces

Figure 4.4 Movement of energy through a food chain.

~

IT:Q~ V'-'

How is energy transferred through a food chain?

~

IT:QS V'-'

What is the importance of respiration in a food chain?

Figure 4.4 shows that energy is lost at every step in the food chain. This means there is less energy at each level for the animals in tha t level than in the level below. The length of a food chain is limited by the energy loss at each level. There will come a point when there is not enough energy to support another level. There are usually not more than five steps in any food chain. When the plants and animals die, the energy stored in the dead bodies is passed on to the detritivores and decomposers as they feed. They also feed on the urine and faeces made by animals.

35

Living Organisms in the Environment

CHAPTER 5

l•n•1uh-S-» productivity

>

Unlike energy, the elemencs of which organisms are made, such as carbon and nitrogen, are recycled (chapter 5). Energy is not recycled, it moves through and out of the food chains. Energy enters a food chain as light energy from the Sun, and is lost from every trophic level as hear energy ro the environment. Its flow is non-cycl ical, which means that the energy cannot be returned to a living organism. The length of a food chain depends on the energy in the biomass availabJe at each level. Ultimately this depends on how much energy is being trapped by the producers (their productivity). If the whole ecosystem is highly productive, then the food chains will be longer because there w ill be more energy entering at the producer level of the chain. If there is on ly a small amount of energy being trapped by the producers, then they can support only a few trophic levels (figure 4.5) . Ecosystems in eq uatorial regions are generally more prod uctive tha n those in higher latitudes because they get more light (figure 4.6).

fJl!I Productivity of ecosystem is high

plant

fJl!I Prod uctivity of ecosystem is low

plant

fJl!I

energy loss

-

energy loss

fJlt energy loss

-

~

energy loss

-

Figure 4.5 The productivity of the producers in an ecosystem limits the length of food chains that can be supported

(a)

(b)

Figure 4. 6 (al Ecosystem of high productivity. (b) Ecosystem of low product1v1ty

Crop plants are mass-harvested for human consumption. If these plants are eaten directly by humans, a lot more energy can be obtained by the humans than if the plants were fed to other animals and those animals then eaten by h umans (figures 4.7 and 4.8).

36

•

f/11

energy loss

-" energy

~

energy loss

energy loss

energy

energy

~

energy

HUMANS

Figure 4. 7 Efficient use of food chain for energy by humans.

f/11

energy

OTHER ANIMAL

-

HUMANS

Figure 4.8 Inefficient use of food chain; a lot of energy is lost that could be available to humans.

Pyramids of energy pyramid of energy

>

A pyram id of energy is a good way of showing the energy relationships between organisms in different trophic levels. Figure 4. 9 shows the pyramid of energy for a simple food chain . Each block in the pyramid shows the amount of energy available to the next trophic level. Using figure 4.9 as an example, 90 000 units of energy are available to the grasshoppers. The grasshoppers consume that energy as food and lose some of it to the environment as heat during respiration and activity, and some of it as faeces. That leaves only 15 000 units for the insect-eating birds. The birds consume that energy and lose some of it to the environment in faeces and as heat. So only 2000 units are available to the next level, the cats. The cats lose energy to the environment as faeces and as heat, leaving only 100 units of energy in their bodies. This is not enough to support another trophic level, so there are only four trophic levels in this chain.

grass ---+ grasshopper

---+

insect-eating bird

---+

cat

TERTIARY CONSUMER SECONDARY CONSUMER PRIMARY CONSUMER

100 units of energy

2000 units of energy

15 000

r--___.__ , _ ___ - - --- - -

PRODUCER A pyramid - each block gets smaller as you go up

90000 unit s of energy

Pyramid of energy

Figure 4.9 A pyramid of energy shows that less and less energy is available to higher trophic levels in a food chain.

Pyramids of numbers pyramid of numbers >

A pyramid of n u mbers is like a pyramid of energy but shows the numbers of all the organisms at each trophic level of a food chain within a given area . Look at the pyramid in figure 4.10 (overleaf). The pyramid shows that, within the area being studied there were 80 leaves. On these leaves, 8 caterpillars were feeding. Tvvo birds were seen feeding on the caterpillars and one cat ate both birds. Ecosystems usually contain a large number of sma ll organisms and a smaller number of large animals. Predators are usually larger than their prey and must eat a number of them to stay alive.

37

Ll:ving Or-ganisms in the Environment

1O leaves

grasshopper --....___

10 leaves - - - - - - - - - - - grasshopper - - ---...,. _ __,. • bird grasshopper --------------•~

1O leaves

10 leaves - - - - - - - - - - - • grasshopper 101eaves - - - - - - - - - - - • 10 leaves - - - - - - - - - - - •

~ ~

cat

Each cat eats 2 birds a day

10 leaves - - - - - - - - - - - • 101eaves - - - - - - - - - - - • Each grasshopper eats 10 leaves each day

cat bird grasshopper leaves

Figure 4.10 A pyramid of numbers is obtained by counting all the individuals at each trophic level.

With this type of ecological pyramid, no allowance is made for the size of the organism. Each cat and ea ch caterpillar is each counted as one. So sometimes we can see different shapes in pyramids of numbers (figure 4.11 ). One tree may be eaten by many caterpillars, though we could have counted each leaf separately to get a ' normal' pyramid shape. One dog is host to many ticks, and each tick may have several parasites, but in this case each 'predator' is actually smaller than its 'prey'.

Figure 4.11

pyramid of biomass )

hawk

parasites on ticks

small bird

ticks

caterpillar

dog

Some pyramids of numbers are of different shapes.

Pyramids of biomass Instead of estimating the numbers of organisms at each trophic level we can estimate their biomass or dry weight. From this we can construct a pyramid showing the biomass of organisms at a given time in each trophic level. The width of the boxes indicates the relative amounts of biomass present at each trophic level. At the start of the food chain in figure 4.12 is a large biomass of green leaves. The p yramid shows that a large amount of plant material supports a smaller mass of herbivores and an even smaller mass of carnivores. mass of tertiary consumers mass of secondary consumers mass of primary consumers mass of producers

Rgure 4. 12 A pyramid of biomass.

38

Bioaccumulation Pesticides can spread through the environment in a food chain. Pesticides (such as fungicides, herbicides and insecticides) are chemicals that are toxic to some organisms. They work in one of two ways, on contact or once the chemical has entered the organism. A grasshopper feeding on plants sprayed with insecticide will only need to take in a small amount to kill it. But this can harm other animals in the food chain. For example, a bird feeding on the grasshoppers will accumulate in its body all the insecticide that the grasshoppers have ingested. Remember that the bird will eat a large number of grasshoppers every day. So the bird may end up with levels of insecticide high enough to poison it or harm it in some way. A hawk or other predator feeding on the small birds could end up with even higher levels of pesticide in its body, again enough to poison or harm it. This is called bioaccumulation or biological magnification.

DDT (dichlorodiphenyltrichloroethane) DDT provides a well-known example of bioaccumulation (figure 4.13). It is a very effective insecticide that was used in many countries in the 1950s and 1960s to control mosquitoes, which carry malaria, and to control other insect pests. However, DDT is stored in fatty tissue so predators absorb the chemical when they eat prey that contains it. Levels of DDT that accumulate in the bodies of top predators may be enough to kill them or to harm them in other ways. In a study of ospreys (North American birds) adult birds were found to contain 8 million times more DDT than organisms at the bottom of the food chain. These high concentrations did not kill the birds, but caused the females to lay eggs with very thin shells. Many eggs broke and so numbers of these birds dropped rapidly. Since 1972 the use of DDT has been banned in many countries.

DDT accumulates in the top consumers

fish eats herbivore and accumulates DDT

124 ppm DDT 5 ppm DDT

herbivore eats phytoplankton and accumulates DDT

1 ppm DDT Producer 0.0025 ppm DDT

DDT enters phytoplankton

run-off from agricultural land carries a dilute solution of pesticides, e.g. DDT

Figure 4. 13 Pesticides like DDT accumulate In the tissues of each trophic level of a food chain.

39

- - Living Orgar:iisms In :the£nvir_o11rn_~~t

_

r

Chapter summary • Energy from the Sun is used by plants to make food during photosynthesis. • The equation for photosynthesis is: carbon dioxide + water + light energy -+ glucose (food) + oxygen • The energy that is stored in a plant is passed on to other organisms when they feed on the plant. • Respiration releases energy in plants and animals for life and growth. • The equation for respiration is: food (glucose) + oxygen -+ energy + carbon dioxide + water

I •

Most of the energy released in respiration is lost as heat to the environment and cannot be passed on to the next trophic level. • A pyramid of energy shows that less and less energy is passed on to the higher trophic levels of a food chain. • A pyramid of numbers shows.the number of organisms found in each trophic level of a food chain . • If the dry mass of the organisms at each tropic level of a food chain is measured, a pyramid of biomass can be produced.

The energy from the Sun is used by plants or producers to make organic food that is used directly and indirectly by all animals, including humans. Withou t the Sun, plants would die so there would be no food for the animals. They would also die and life, as w e kn ow it, would cease to exist. ITQ2 A plan t stores som e of the energy in its tissues as it grows and uses som e en ergy to stay alive. Some energy is lost as heat en ergy. Som e en ergy is thus lost to the environment and some is kept in th e plant's body. ITQ3 An animal uses some of the energy from respiration to stay alive. Much of the energy is lost to the environment as heat . The animal may use up more energy than a plant sin ce it is more active. It also stores some energy as ch emical energy in its tissues as it grows. ITQ4 Energy is transferred from one trophic or feeding level to another when an organism feeds. En ergy is transferred in the form of food. Th e food is n eeded for respiration which makes energy available to the organism. So energy moves through a food ch ain when the organisms eat. ITQS Some of the ene rgy tha t is released during respiration is lost to the en vironment in the form of heat from the organism. Respiration is important in a food chain because at each level in th e food chain en ergy is lost. Only a proportion of the energy entering one trophic level is stored in the organism 's body and is thus available to the next trophic level. ITQ1

Examination-style questions grass (i)

grasshopper

bi re

Copy the diagram above and, using arrows, annotate it to show the movement of energy into and out of each organism. (ii) What is the importance of the following in a food chain: (a) respiration? (b) photosynthesis?

40

(c} digestion? (iii} How is light energy converted to chemical energy? (iv} Most animals spend a great percentage of their day looking for food. Why must animals eat food? (v) On the TV programme Sesame Street, there is a story about a boy who ate the Sun. What do you think of this story? Give details.

2

(

Food chain A grass --+ cow --+ tick --+ egret Food chain B grass

--+

grasshopper --+ izard

(i}

Construct possible pyramids of numbers for food chains A and B. In each case, discuss the shape of the pyramid. (ii} Construct possible pyramids of energy for the same two food chains, A and B. Discuss the shapes of the pyramids.

cat

bird

grasshopper

I

leaves

I

(iii} Look at the pyramid of energy above. Why do leaves contain the greatest amount of energy? (iv} What happens to the energy that is not passed on to the grasshoppers? (v} What will happen to the cats if all the grasshoppers were killed by the use of insecticide?

41

0 0 0 0 0

explain the carbon cycle understand what is meant by the greenhouse effect and global warming explain the importance of nitrogen to plants and animals explain the nitrogen cycle describe the causes and effects of acid rain

atoms in animals

atoms in plants ,..-

carbon carbon hydrogen hydrogen oxygen oxygen .-+-- nitrogen - - - - - - - - - - - nitrogen _ ......_ others others biogeochemical cycles

atoms in the environment carbon hydrogen oxygen - - - - - - - - - - nitrogen _.i_ _ _ _ _ _ _ _ _., others

( greenhouse effect

) acid rain

leaching

global warming

Biogeochemical cycles Living organisms are made u p of different kinds of atoms. The most common atoms are carbon, hydrogen and oxygen, with nitrogen following closely behind. Smaller amounts of other atoms, such as iron, calcium and sodium, are also found in living organisms. These atoms bond together to form larger structures such as protein, carbohydrates and lipids. These larger structures are then arranged in particular ways to make up all the tissues needed to build a living organism. All living organisms are, in essence, complex structures of organic molecules. If we look at a person, we see skin, hair and nails - it is difficult to imagine that basically we are just atoms of carbon, hydrogen, oxygen and nitrogen.

A carbon atom that was present in Einstein's body could be present in your body right now.

biogeo chemical cycles

>

Remember that carbon is found in carbon dioxide (C0 2) , carbohydrates, lipids and proteins, since carbon is an integral part of those compounds.

As an animal grows from birth to adulthood, the growing tissues come from the food it eats. The animal increases its store of these atoms as it eats and m uscle, bone and all the tissues that make up the organism increase in mass. Then, when the organism dies, the body is broken down or decomposed, and the atoms are released back into the environment. The atoms become pan of the soil as the organism 's decomposed body becomes mixed into the soil. They may then be taken up by plants and built. into the plant's tissues as the plant absorbs them from the soil with water. These plants are then eaten by animals and the atoms thus become part of an . animal once again. The cycling processed by which these essential atoms are released and reused in nature are called biogeochemical cycles. The carbon and nitrogen cycles are examples of such cycles.

The carbon cycle The carbon cycle shows how c;arbon atoms are passed from one organism to another and to their environment as they live, breathe, eat, die and decay. The numbers of the following paragraphs refer to numbers in figures 5.1 and 5.2.

carbon dioxide (CO:z) in the air (0.04%) photosynthesis

organic compounds in green plants

respiration

combustion

respiration eaten by animals

------....i death and decay

death and decay

fossilisation

organic compounds in bacteria and fungi

..........

-..,;.;_~

fossilisation

organic compounds in fossil fuels

~~------------'

Figure 5. 1 The carbon cycle shown 1n d1agrammat1c form Equation for respiration: food (glucose) + oxygen -+ energy + carbon dioxide + water

1

.~

2

What atoms are living organisms made up of? (ii) How do they obtain these components? (iii) What happens to these components after the organism dies? (iv) What is a biogeochemical cycle?

3

\./'-I

(i)

4

5 6

The atmosphere contains about 0.04% carbon dioxide. During photosynthesis, plants use carbon dioxide from the atmosphere to make carbohydrates, proteins and lipids. This is the first source of carbon in living organisms - as a part of the plant's body. Animals then obtain their supply of carbon by eating plants or other animals that have eaten plants. As plants and animals respire, molecules of carbon dioxide are released back into the atmosphere. Waste materials from living organisms (like urine and faeces) and their dead bodies (all organisms die) , are used as food sources by decomposers. Decomposers, like bacteria and fungi, feed on dead organic matter. Carbon atoms then become incorporated into the bodies of the decomposers. Respiration of the decomposers releases carbon dioxide into the atmosphere. In waterlogged soils where oxygen is in short supply, decomposers are not able to break down tissues completely in dead bodies and the remains

43

Livir1g Organisms in lhe Environment

lt•1"1§!1(!tfUflJ

7

accumulate. For example, in the Carboniferous period (about 290 million years ago) huge areas of waterlogged swamps covered many parts of the world. When the swamp plants died, partially decomposed plant material accumulated and eventually turned to coal, a solid fossil fuel. Oil and natural gas are liquid fossil fuels that formed in a similar way from the remains of plants and animals that died in oceans. Fossil fuels contain a large proportion of carbon. The burning of fossil fuels (combustion) releases carbon dioxide into the atmosphere.

l~ndioxide in~the air

photosynthesis

Z combustion

..

gas

coal

decomposers in the soil

respiration of decomposers

Figure 5.2 The carbon cycle in more detail.

~

l'.T:Q2 \..AJ (i) What is the importance of photosynthesis in the carbon cycle? (ii) What is the importance of respiration in the carbon cycle? (iii) What is combustion? (iv) What role do decomposers play in the carbon cycle?

44

And so the cycle continues, carbon dioxide in the atmosphere is taken up by plants, which are eaten by anin1als, and returned to the atmosphere through respiration, decomposition or combustion of fossi l fuels. Note the importance of plants in this cycle. Without plants, the carbon stays in the atmosphere and cannot be reused and incorporated into the bodies of animals. If there were no plants, there wou ld be no animals.

The human effect on the carbon cycle Figure 5.3 shows how the level of carbon dioxide in the air has been rising. The rise in human population has been supported by an increase in manufacturing and other types of industries. Since the Industrial Revolution, humans have been burning fossil fuels to release energy for machines. This has added carbon dioxide to the air at an alarmingly fast rate. The carbon was locked away in the solid or liquid forms of fossil fu el for millions of years. Increased combustion of these fossil fuels increases the carbon dioxide concentration in the air. Increased concentration of carbon dioxide in the atmosphere is associated with the environmental problem known as global warming.

- - -

-

-

-

-

-----

-

-

-

~utrients

5 --The Cycling of

The Industrial Revolution is a term used to describe the time when people started to make and use machines to do a lot of their work. It began about 200 years ago. Machines need energy to make them work, and most of this energy comes from burning fossil fuels. 7

Carbon emission from burning of fossil fuels (billion tonnes)

370

6

360

5

350

4

340

3

330

2

320

Atmospheric carbon dioxide (parts per million)

310 300

0

290 1840

1860

1880

1900

1920

1940

1960

1980

2000

'-~--'--~--'-~-'-~~'--~-'---~-'-~--'-~-----'

~840

1860

1880

1900

Year

1920

1940

1960

1980

2000

Year

Figure 5.3 The levels of carbon dioxide in the atmosphere over the last 160 years.

The greenhouse effect and global warming

greenhouse gases > greenhouse effect

>

global warming

>

When heat from the Sun reaches the Earth's surface much of it bounces straight back into the atmosphere (figure 5.4). Within the Earth's atmosphere there are gases like carbon dioxide and methane that absorb some of the escaping heat and send it back to the Earth's surface, keeping it trapped around the Earth. They act like a greenhouse around the Earth and thus are called greenhou se gases. This is a natural process which helps keep the surface of the Earth warm. Without this natural greenhouse effect, the Earth would be too cold for most of the organisms living on it. A problem arises when the proportions of these gases in the atmosphere increase . They bounce more of the heat back to the Earth's surface. This is called the 'enhanced' greenhouse effect. As a result the temperature of the Earth increases, which is known as global warming.

infrared radiation (heat) radiated back towards space absorbed by 'greenhouse gases'

to space

incoming solar radiation (ultraviolet, visible and infrared)

reradiated into space

atmosphere heated - raising Earth's temperature reflection from clouds

Earth

Figure 5.4 Some solar radiation that reaches the Earth is absorbed by the atmosphere rather than going back out to space.

45

Carbon dioxide concentration in the Earth's atmosphere has increased by about 20 % over the last 100 years. This effect has also been worsened by deforestation. Trees (forests) remove carbon dioxide from the atmosphere during photosynthesis, but large areas of forests are being cut down. It is not proven that higher carbon dioxide levels cause temperature increase, but scientific research suggests that the two may be associated. Some people think tha t global warming might cause the Earth's temperature to rise between 1.5 °C and 4.5 °C by the end of the 2 1st century.

Possible effects of global warming • The polar ice caps may melt which could cause sea levels all over the world to rise significantly. Many millions of people now live in lowland areas and these may be flooded, driving people from their h omes. • Fertile, crop-producing land would be lost by flooding. • The distribution of organisms over the face of the Earth may change as land floods and temperature and rainfall patterns change. • Changes in the amount of land and sea could change weather patterns. This could increase rainfall in some places and increase periods of drought in others. Natural storms like hurricanes and typhoons may be more severe. • Cold countries may become more temperate and fertile.

~ IJ:Q3 \.../'-J

Why are the carbon dioxide levels in the atmosphere rising? (ii) What might be some consequences of this rise?

(i)

We must be very careful not to say that every example of extreme weather is due to global warming. There have always been variations in climate over the years and over centuries. Also, we must be careful not to make unjustified assumptions about fu ture changes. For example, on the island of Svalbard in the Arctic Ocean, one of the glaciers is retreating, but a neighbouring glacier has advanced by more than a mile in seven years. Some sea levels are said to be lower now than in the 18th century - for example mean sea level in the Cook Islands has apparently dropped by about 20 cm in 200 years. Globally, mean sea level is rising at about 3 mm per year. So although global warming is a reality, and many experts attribute this to the enhanced greenhouse effect, we should not be too quick to predict catastrophe.

The nitrogen cycle About 79 % of the air around us is nitrogen gas. This gas is very unreactive - it passes in and ou t of animal's bodies unchanged when they breathe. However, nitrogen is an essential component of biological molecules such as proteins and DNA. Muscle is composed of long strands of protein and DNA is the m olecule in each nucleus of a cell which contains the information about how to build that cell and make it work. Plants manufacture protein by absorbing nitrogen from the soil mostly as nitrate ions. These are combined with carbon, hydrogen and oxygen taken from glucose that was made during photosynthesis. The elements are then arra nged in another way as they combine with the nitrogen, to make the building blocks for proteins and DNA. Remember that glucose is made during photosynthesis and is composed of carbon, hydrogen and oxygen. Animals obtain their nitrogen from the protein in their diet, through eating plants or other animals. The protein they eat is digested, absorbed and reused as needed in the feeding animal. That is, the nitrogen obtained from the protein of a piece of plant material or meat can be used to build growing muscles, make DNA, enzym es and other proteins, and everything else requiring nitrogen. The numbers of the following paragraphs refer to numbers in figure 5.5.

46

-

_

_

_ _ -

-

_

=- -_

5 /ftle

Cycling ~ot, Nutrients

-

nitrogen in the air

nitrogen-fixing bacteria

lightning

denitrifying bacteria · animal protein

plants eaten

nitrogen oxide

in root nodules Rhizobium

and decay

ammonium compounds

plant protein

nitrifying bacteria Nitrosomonas

rain (acid rain)

nitrates absorbed in soil Clostridium

nitrifying bacteria Nitrobacter

nitrates in the soil

nitrites in soil

~1 leaching of the soil

fertilisers

Figure 5.5 The nitrogen cycle shown in diagrammatic form. nitrogen fixation

>

I

2

nitrification

>

3

denitrification

>

4

5

Nitrogen fixation - This occurs in nitrogen-fixing bacteria that convert nitrogen gas in the air to nitrate. Some of these bacteria, like Azotobacter and Clostridium, live in in the soil and convert the nitrogen gas found in the air in the soil to nitrate. Plants cannot absorb nitrogen gas, only substances that contain it, like nitrates. So nitrogen-fixing bacteria thus make nitrogen available to plants in a form they can absorb. Plants use the nitrogen from nitrates in the soil to make proteins and DNA. Other kinds of nitrogen-fixing bacteria, called Rhizobium, live in the roots of legumes (plants of the pea family). There, nitrogen gas is converted to nitrates and used directly inside the plant to make protein. Decay - When plants and animals die, their bodies are decomposed by decomposers to make ammonium compounds in the soil. Animal wastes, like faeces and urine, are also decomposed by bacteria living freely in the soil. Nitrification - The ammonium compounds formed during decay are converted to nitrites and then nitrates. The processes that lead to the formation of nitrates in the soil are called nitrification and are carried out by nitrifying bacteria like Nitrosomonas and Nitrobacter. Plants take up n itrate ions from the soil and make proteins. Denitrification - The nitrogen cycle is completed by denitrifying bacteria. They convert nitrates in the soil back to nitrogen gas. The activities of these bacteria reduce soil fertility, since they take nitrates out of the soil which the plants need to grow well. Lightning - This provides energy to convert a little nitrogen to nitrogen oxides which dissolve in rain to form nitrates.

47

_

.- Living Organisms ,in the E~w!ror~!])~_ml'._ _

6

1@'30!.t•iJ

7

-~ ~-=

_-=

-

-

-

-

~ =_·_ ;;_~-:_ ~c ~~-.

-

Fertilisers - To make crops grow better, we add artificial and natural fertilisers to the soil to increase the levels of nitrates. Leaching - As rain water passes through the soil on its way to the rivers, lakes or seas, it carries with it dissolved nitrates and other soil nutrients. So the nitrates can be washed out of the soil. This is called leaching (figure 5.6).

~

IJ:Q'1 L)'...I

Copy and complete this table. Process in Importance Examples nitrogen cycle of bacteria involved

river

nitrogen fixation

soil water to river takes nutrients with if

decay

Agure 5.6 Diagram showing how n1trates can be leached from soil.

nitrification denitrification

CHAPTER 3

The nitrogen cyde is thus essential to life as nitrogen is a vital component of every living organism (figure 5.7). This biogeod1emical cycle allows nitrogen to be used over and over by living organisms. Nitrogen atoms cannot be created and there is only a certain amounc on Earth. The importance of bacteria should be noted because they are an integral part of this cycle. Nitrifying bacteria can be considered 'good' bacteria, without which living organisms would slowly become extinct. The relationship be tween the plants and the nitrogen-fixing bacteria is an exampl e of mutualism (chapter 3). nitrogen in the air

~

protein 1n animals

nitrogenfixing _bacteria

denrtrifying bacteria

lightning nitrogen fixation

Rhizoblum

nitrates in soil

N1trobacter

t

nitrites in soil

ammonium compounds In soil

Agure 5.7 The nitrogen cycle in more detail.

48

'

5 · The

Cyclin~

of

Nutri~nts

Acid rain Ft;l•li:O.U

Combustion of fossil fuels in industry and from motor vehicles releases acidic gases sud1 as sulfur dioxide and nitrogen dioxide. These gases dissolve in atmospheric water vapour in clouds and later fall as acid rain (figure 5.8). Sulfur dioxide dissolves in atmospheric water to give, eventually, dilute sulfuric acid. Oxides of nitrogen dissolve to form dilute nitric acid.

oxides of sulfur and nitrogen from pollution dissolve in water in the cloud to make acid rain

acid pollutant s from vehicles, power stations and industry

pH of rain

4-5

pH of rain

5-7

Figure 5.8 The formation of acid rain. pH is a measure of how acidic or how alkaline a solution is. ApH of 7 is neutral. A solution with a pH less than this is acidic. If it has a pH above 7, it is alkaline.

The acid clouds may be carried hundreds of miles away from the source of the pollution by air currents. It has been recorded that rain with a pH as low as 4 has fallen over Scandinavia, Germany and Canada. • Acid rain may kill plants and trees. Some forests, like the Black Forest in Germany, have been severely damaged (figure 5.9) . But it has been found that acid rain enhances the growth of pine forests in Scandinavia. • Acid rain also dissolves some compounds of poisonous m etals thus introducin g them into lakes and rivers. This poisons organisms living in the water. Rgure 5.9 These trees have been killed by acid About 400 lakes in Norway are rain. now rendered fishless because of acid rain. • In cities. stone (statues and carvings) and metal structures have been damaged because of erosion due to acid ra in. Governments are trying to reduce acid rain by introducing regulations th at demand th.at industries do not release atmosph eric pollutants. The design of engines for motor vehicles is also important to reduce the amount of pollutant gases that they make.

49

'Y

Chapter summary • Living organisms are built up from single atoms, mostly carbon, hydrogen, oxygen, with some nitrogen, iron, calcium, sodium, sulfur and other elements. • Biogeochemical cycles show how materials are reused in nature. • The carbon cycle shows how carbon passes between the air, soil, plants and animals and back again. • The greenhouse effect is an important natural process, caused by greenhouse gases in the atmosphere that absorb heat energy from the Sun and keep the surface of the Earth warm enough for life as we know it. • Increasing levels of carbon dioxide in the air could lead to global warming which could affect sea levels and weather, with devastating consequences. • The nitrogen cycle shows how nitrogen passes between air, soil, plants and animals and back again. Bacteria are very important in this cycle. • Acid rain forms when acidic gases such as sulfur dioxide and nitrogen dioxide dissolve in atmospheric water vapour. It can be very damaging to life.

(i) A Living organism is composed of different forms of proteins, carbohydrates and lipids. These are made up of atoms of carbon, hydrogen, oxygen, nitrogen and other atoms such as sodium, calcium and iron. (ii) An animal obtains these components when it feeds. Food is organic and contains carbon, hydrogen, oxygen, nitrogen, sodium, calcium and iron, etc. Food is ingested, digested, absorbed into blood and transported to all parts of the body to build tissues. Plants take in simple inorganic molecules, carbon dioxide and water from the atmosphere, and nitrates form the soil to build their tissues. (iii) The large organic molecules in dead bodies are broken down by decomposers and detritivores into their smaller components. Then the components can return to the environment and be used again by other organism s. They are recycled through living tissue in different organisms in food chains. (iv) A biogeochernical cycle is a cycling process by which an atom is released and reused in nature . ITQ2 (i) Photosynthesis is an important part of the carbon cycle because it is the only means by which carbon from the air is taken into an organism. Plants take in carbon dioxide and turn the carbon into glucose and other chemicals in the plant's tissues . When animals eat the plant, the carbon atoms can then become part of the animal's tissues. (ii) Respiration is the means by which carbon atoms get back into the air (a s carbon dioxide) from living organisms. (iii) Combustion is the burning of fu els, a process which uses oxygen. When fuels, such as wood, gas and coal, are burnt, carbon dioxide is produced, thus returning carbon atoms to the air. (iv) Dead plants and animals have carbon molecules, in carbohydrates, proteins and fats, trapped in their bodies. Decomposers feed on the dead bodies and release the carbon to the environment as carbon dioxide when they respire. ITQ3 (i) Carbon dioxide levels in the atmosphere are rising because of the vast amount of combustion of fossil fuels to release energy, especially in industry. An increase in human population leads to a greater demand for energy. Widespread deforestation adds to the problem. (ii) Global warming (or the enhanced greenhouse effect) which could lead to higher temperatures, melting of polar ice caps, flooding and changes in weather patterns. ITQ1

50

ITQ4

Process in nitrogen cycle

Importance

Examples of bacteria involved

Nitrogen fixation

Nitrogen gas is converted to nitrates in the soil and Azotobacter absorbed by plants; or to amino acids in the root Rhizobium nodules and used by the plant to make protein.

Decay

Tissues of plants and animals are broken down and Decay bacteria their components can be reused. They are broken down to ammonium compounds.

Nitrification

Ammonium compounds are converted to a more usable form, nitrates. Nitrates are absorbed by plants and used to make proteins.

Nitrosomonas Nitrobacter

Denitrification

Nitrates are converted back to nitrogen gas in the air.

Denitrifying bacteria

Examination-style questions (i) Using only an annotated diagram, describe the carbon cycle. (ii) In the carbon cycle, carbon 'moves' as it becomes incorporated into the bodies of organisms or is released into the environment during various processes. Copy and complete the table below to show the movement of carbon in the processes listed.

Movement of carbon

Process

From

To

respiration in an animal combustion of coal photosynthesis decomposition (iii) State three ways human activities add carbon to the atmosphere. (iv) State four possible effects of global warming. 2

(i)

Copy and complete the diagram of the nitrogen cycle shown below. nitrogen in the air

c

ammonium compounds

A

nitrate In soil

(ii) (iii) (iv) (v) (vi)

Describe what happens at A, B and C. How are some plants like the garden pea able to survive in soil deficient in nitrates? Describe how nitrates are leached from the soil. Describe an example of symbiosis as seen in the nitrogen cycle. Nitrogen is a vital component of every living organism. Describe its importance.

51

Aopulation Grovvth, Natural Resources and their Limits 0 0 0 0 0

understand that factors affect the growth of natural populations understand why humans are not subject to the same constraints as other organisms describe various resources and their limits understand the advantage and difficulties of recycling manufactured materials consider biodegradable and non-biodegradable materials population growth

(

1

population growth of humans

natural population growth

I

depletion of

i i"'

renewable

resources - ( non-renewable

manufactured materials - - - -· recycle

r biodegradable

' non-biodegradable

11 reuse

I reduce

Growth of natural populations A population is composed of all the members of the same species living together in the same place. Many populations live together as a community occupying the same habitat. A population size may grow or decline depending on condjtions at the time. If food is rearuly available, or there is adequate space, then the population may grow (the number of individuals or members of the species may increase). Consider a population colonising a new habitat in which conditions are initially ideal. l At first, there are few reproducing individuals and population growth rate is slow. 2 Then, since there is an abundant food supply, no competitors, no predators or rusease, population growth rapidly reaches its maximum rate. Birth rate exceeds death rate and the population size doubles at regular intervals. This phase is called the exponential growth phase or log phase. 3 Exponential growth cannot. and does not, go on forever. Eventually the population growth slows down. This is because of various factors in the environment sud1 as lack of food or space, increase in numbers of predators, increased competition or an increase in the incidence of disease.

4

sigmoid growth curve

>

The population growth rate slows down and stops and the population size remains fairly constant.

Figure 6. 1 shows the typical growth curve resulting from steps 1-4 above . It is called a sigmoid growth curve (or S curve). little growth

rapid growth

growth slows down

3

2

no growth, population size is constant

---

4 ''

''

Growth parameter (e.g. number of individuals In the population)

\,- - A population may decline, for example through sudden disease, a serious change in the environment or an increase in predation. (An example of this is humans over-fishing a lake.)

Figure 6. 1 A typical growth curve.

Factors which reduce population size carrying capacity

QSb

>

l:t:Q-1 V'-1 What is meant by 'an environment can carry a certain number of organisms of a population?'

The maximum population size that can be sustained over a period of time by the environment is called the carrying capacity of the en vironment. The environment has enough space, food and whatever is needed to sustain or 'carry' a certain number of individuals. Some individuals die from disease or predation (eaten by p redators). However, the death rate is more or less equal to the birth rate as the population stabilises. Disease and predators help to keep the population size 'in check' or constant or stable. This will continue until there is a major change in the en vironment. For example, a natural disease may develop in a popula tion that could wipe ou t or kill most of the individuals. The popula tion size would then decrease drastically (figure 6.2). population size increases plenty of suitable space few predators able to avoid predators

good food supply good water supply ability to resist disease

suitable abiotic conditions ~--------- (e.g. light, soil type, ideal temperature) population size poor food supply many predators present inability to avoid predators population size decreases

inadequate water supply susceptible to disease

unsuitable abiotic conditions (e.g. extreme temperatures, poor soil)

Figure 6.2 Some of the factors affecting population size.

Alterna tively, another organism may arrive in a habitat. It may be 'fitter' (more able to adapt to small environmental changes), or a better competitor for space and food, or it may reproduce at a faster rate than already existing populations.

53

Living Organisms in the Environment

~ IT:Q2 l..l'V

How is it possible for the planet Earth to carry all of the different kinds of animals and plants known to exist on it?

This organism could ' take over' the habitat as its population size increases causing others to decrease. Such an organism is called an invasive species. Or, a natural disaster cou ld drastically reduce population size as many individuals are killed, damaged or left homeless. For example, a fire blazing through a forest could kill many of the organ isms there.

Growth of the human population

Humans are subject to the same constraints as other organisms. They need space for homes and adequate food fo r th eir families . They are a lso 9 susceptible to many types of disease: 8 hereditary, deficiency, physiological and 7 Assuming an average of 6 pathogenic. Pathogenic diseases can be 2.6 children per woman 5 considered to be predators of humans. 4 At present, it is estimated that there Assuming an average of 3 2.1 children per woman are about 7.2 billion peop le on Earth. 2 The human population growth curve in 1 o ~~~~~~~~~~~~~~~~~~~ figure 6.3 shows that the population is 1950 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050 now doubling abour every 44 years. Human population growth depends Figure 6.3 The human population growth curve 1950- 2050. on th e carrying capacity of th e Earth, or the maximum number of people that the Earth could sustain over a long time. Uni red Nation analysts predict that rhe world population may stabilise at about 12 billion in abo ut 120 years' time. Although humans are subject to the same constra ints as other populations. they have actively worked on overcoming chem. • Space - Humans have developed the equipment to move into and inhabit most places in the world. Forests are cleared, coastal ;-va ters are filled and developed for h ouses, and deseRs are made inhabitable. Some apartment blocks are over JOO storeys h igh, to increase the possible living space. Some people live permanently in boats on rivers and coasts. Humans need space for h omes and also for factories and industries to support their needs (figure 6.4). • Food - Humans practise agriculture, which is the mass production of food. Farming techniques and. more recently, developments in genetic engineering, have increased agricultural and livestock ou tputs . • Disease - Humans are constantly studying diseases, their causes, symptoms, prevention and cures. Prenatal and postnaral care, and well developed immunisation programmes, prevent the death of millions of children. Education about disease. technology to prolong life, development Agure 6.4 Humans can adapt how they of vaccina tions and gen etic engineering help preven t dea th from disease. live in order to create more space. • Predators - Humans invented gunpowder, which gave them an advantage over all other animals, even those much larger and fiercer than themselves. Humans no longer have any effective predator. 12 11 10

Population (billions)

~

IT:Q3 l..l'V

The activities of humans can cause the sizes of populations of other organisms to change. Using examples of animals or plants, discuss how humans can cause these population increases and population decreases.

54

Some environmentalists believe that we need to do something now to curb our population growth because of tl1e way we are exploiting the environment. Imagine ha ving to feed 7.2 billion people every day. They say that food resources may be used up, but there is still an abundance of food in the world. However, agriculrural practices encourage pathogens, pests a nd parasites to fl ourish . Others point o ut that as a population becomes more advanced, popu lation growth slows naturall y. For the moment, disease is still the greatest controlling factor on human morta lity (death rate). Even with all our resea rch and technology, there is still a prevale nce of disease. In developing countries, such as Africa, Cen tral

6 · Population Growth, Natural Resources and their Limits

~

IT:Q~ V'-J

Discuss four ways humans have 'conquered disease'.

America and India, overcrowding and poor living conditions and medical care have led to prevalence of infectious diseases. In more developed countries, like USA, Canada, Japan and the UK, a far smaller percentage of people die from infectious disease. Most deaths in these areas are due to degenerative diseases (those that get worse with time) , some of which are social and self-inflicted in nature. Smoking can cause harm in a number of ways (cancer, brond1itis, asthma). Misusing drugs like alcohol and heroin may lead to the development of physical and mental disease. Eating large quantities of salty and fatty foods puts people at risk of becoming obese and then at risk of obesity-related diseases (diabetes, hypertension, heart disease). People in developed countries generally live longer than those in less-developed countries, and so there is a greater prevalence of diseases related to old age. Population growth is a function of how many individuals that are born survive to adulthood and reproduction. Birth rate and death rate are therefore important controlling factors . In natural populations, these are not usually under the control of the individual, but in humans we have a greatly reduced death rate. We can also do something that very few other species can do: control birth rate. In many developed counh·ies, birth rate has fa ll en to around the same level as the death rate because of the use of contraception, so population numbers in those countries are stabilising. In a few countries, population size is actually falling. But there are still large areas of the world where population size in increasing rapidly.

Resources and their limits ~

IT:QS V'-J

Name two renewable resources and two non-renewable resources.

renewable resource non-renewa e resource >

Figure 6.5 The solar energy panel provides a renewable electricity supply to the house.

Resources are features of the environment that can be used by human society. There are many different types of resource whid1 indude: • mineral resources like bauxite and other metal ores; • soil resources for agriculture; • biotic resources, like fish and plants for food and other purposes; • water; • fuel and other energy resources, like petroleum and natural gas. Resources can also be class ified as renewable or non-renewable. A renewable resource is one which can be reused or quickly replaced. Nonrenewable resources are in limited supply and once they are used up, they are gone forever. • Mineral resources are non-renewable: once they have been removed from the ground they cannot be replaced. • Soil resources remain renewable so long as the soil is cared for properly, but if damaged by pollution or washed away by rainfall, soil is non-renewable. • Biotic resources (including for food and timber for paper) are renewable so long as they are cared for and managed properly. • Water resources are renewable so long as we prevent the water from becoming contaminated with pollution. • Fuel resources based on fossil fuels (e.g. oU, coal and natural gas) are non-renewable; others (e.g. wind, sunlight and water) are renewable (figure 6.5).

Energy resources Some form of energy is used for every form of human activity. In the past, renewable energy sources like wind, water and firewood were used. Today, most energy is derived from fossil fuels, like coal, oil and natural gas.

55

-

-

-.

-

--Liv Jna:.Ol1c-1c:\r.i.i$r'llllai,i_ r:t~tt~ ~E"rn.v11JQl1~fil)lj _-~ ~ ----~~;;t__ - ~---- --

-

·=

_

-

_

_-

_ --~=. · -----=---

At present, the m ost impo rtant commercial energy resources in the Caribbean are oil and natural gas which are fou nd m a inly in and aro und Trinidad an d Tobago (figure 6.6). L liquified natural gas plant R oil refinery

c

cement plant .

• Point Usas

II

oilfield

oil

.coal

gas field •

•

oilfield with gas

natural gas nuclear energy

gas pipeline

firewood and biomass energy hydroelectric, geothermal, etc.

(a)

(b)

Figure 6.6 (a) On-shore and in-shore gas and oil fields of Trinidad; Large oil and gas fields are also found to the north-west and east of Trinidad (b) Percentage share of Trinidad and Tobago energy source.

Local mineral resources

Bauxite from Kwakwani is transported to Everton by barge for processing. Bauxite from ltunl is transported to Linden by rail. Bauxite and alumina from Linden and Everton are exported by ship.

Baux ite is a red clay, an ore from wh ich alumin ium is obta ined. Bauxite is important in th e econ omies of Guyana and Jama ica (figure 6.7). Alumin ium and its alloys are u sed in the m anufacture of a ircra ft, trains, buses, cooking foil an d man y other item s. Ba ux ite mining uses a large area of land. In Jamaica, there are regu lation s which en sure th at: • land rema ins in agricultural use until mining begin s; • after mining, land is restored fo r oth er u ses. When land is cleared for mining, the topsoil is removed and p reserved for later use since it con tains most of the nutrients and organic m a tter. After m ining, the land is smoothed, reshaped and th e topsoil replaced. Fertilisers can be added . The reclaimed land can be used for pasture, h o using o r small-scale farming.

Reducing resource consumption

o

processing plant bauxite bearing area

.& Berbice Deepwater Facility

e

mining area

•

alumina plant (closed)

-

rail transport of Bauxite

-

water transport of Bauxite

Figure 6.7 The bauxite industry of Guyana.

56

By definition, n on -ren ewable resources will eventually run o ut if w e continue to u se them . In order to preserve as much of these resources as p ossible for future gen eration s, people are being tau gh t to: • re use; • reduce; • recycle. This is particularly important in rela tion to discarded m anu factured materials such as paper, glass, metals, plastics and textiles. Human s are th e o nly species to use th ese materials.

biodegradable '>

These manufactured materials are used at home, school offices and factories and after a while they are discarded because they are worn out, used up or no longer needed. The average composition of domestic waste is shown in figure 6.8. When discarded into the environment, some of these materials break down naturally into simpler, usually harmless forms, by the action of microorganisms. These are called biodegradable materials, and include organic material, paper, some textiles and plastics. Non-biodegradable materials, such as metals, glass and other kinds of plastic, cannot be broken down or take a very long time to do so. As a result, these wastes accumulate in the environment, leading to all kinds of pollution of soil and water. The useful life of a fast-food package is often minutes, but it may persist in the environment for years or centuries.

Reuse

H•lui•M•iH

Some types of waste can be reused in the same or other ways. For example, tins and jars can be reused as eontainers, soda bottles may be returned to the company for refilling and organic waste can be made into compost . Compost is used as a natural fertiliser and soil conditioner (figure 6. 9, overleaf).

Reduce Table 6.1 list some manufactured materials together with their sources and uses. By simply reducing the accumulation of these manufactured materials, the amount of discarded waste will be reduced, and less pollution will take place. We can do this by buying only what is needed and choosing products that are not over-packaged.

thers

~

textiles 7 .3%

5.5%

Manufactured material

Source

Uses

paper

pulp from wood

writing, printing, wrapping

glass

molten mixture of soda ash, silica.sand and lime

bottles, windows, spectacles, drinking utensils, containers, ornaments

metals

iron, gold, tin, aluminium

cars, ships, buildings, containers, electrical appliances, cables (and many others)

plastics

petroleum

bottles, bags, containers, kitchen utensils, cases for appliances, fibres

organics 26.7%

paper 19.7%

Table 6. 1 Some manufactured materials, their sources and uses.

Figure 6.8 Pie chart showing the average composition of domestic waste.

57

Living Organisms in the Environment

(a)

(b)

Figure 6.9 Recycling in the Caribbean. (a) Collecting glass for recycling. (bl Collecting plastic for recycling.

Recycle This is the process of collecting materials from the waste stream, separating them by type, remaking them into new products, and marketing and reusing the materials as new products.

Advantages of recycling • Resources will not be used up as quickly, so there will be more for later generations. • Less land is needed for disposal of waste. • Less pollution of soil and water occurs as waste decomposes. • Less toxic waste is generated. • Harm to animals is prevented (for example, plastic bags and glass are a danger to animals). • In many cases, less energy is used to recycle than to make a new product.

Difficulties of recycling Collection of recyclables is rime-consuming. Each household must sort and h ave suitable containers for rubbish and recyclables. The recyclables must be further separated into glass, metal, plastic and so on, and placed in separate containers. A problem is that recycle bins or containers must be provided and placed at strategic p laces that are accessible to all. People must be educated about the importance of separating their garbage and how to separate their garbage. The recyclable materials must then be transported to recycling factories by trucks or vans. Money must be spent on vehicles, maintenance of the vehicles and gas. This uses fuel resources which must be taken into account when trying to judge whether it is worthwhile recycling a material. Tonnes of recyclables are collected weekly and there mu st be vast amounts of storage space. There must also be space for the recycling facto ries and for storage of the new product (figure 6.10) .

58

The waste for recycling is put into a box for collection or taken to a recycling depot

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glass is crushed

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metal is compacted

P.E.T. is flattened, baled

Mater i a l s are repro c essed to make new products in factories

...- - - - - - - - - ·...... ~ ~~

• • • •

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l P.E.T. plastic metal for shipping to pu r chaser s at the r ec y cl i ng l l ant

glass These mater i als are

I

It is separated into different materials

ca ~ ~

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new bottles fibre glass road building materials sewer pipes

• • • • • •

J

newsprint boxboard insulation cat litter roofing felt egg trays

REDUCE WASTE

I

0 11 .....- - •....._ _ _ _ _ _ ___._

• new cans • tin plating

• new Iron and steel

• cars

• insulation • clothing • new P.E.T. containers

• other metal items

SAVE OUR RESOURCES

Figure 6.10 Recycling of household waste.

Ideas for making less waste Shopping and the en vironment

~

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Define these terms associated with preserving and conserving the environment: (i) reuse (ii) reduce (iii) recycle.

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What are some advantages of reusing?

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What are the difficulties of recycling?

• R educe - Buy only what you need. Can you make do with what you have already? Buy long-lasting goods whenever you can afford them - you may save in the long run. Consumer magazines can h elp you make informed choices. • R ecycle - Choose containers that can be recycled (such as cans, glass and recyclable plastic) and recycle them. If you can, buy drinks in returnable bottles. • Avoid d isposables - The 'throw-away convenience' of some products may not be worth the environmental cost. Paper plates and cups, throwaway lighters and razors, and disposable diapers are handy but why not buy the sorts you can use and use again? • Avoid product s that are hazardous t o the e nvironment - Baking soda can be used to scrub out tubs and sink; warm water and vinegar can be used to dean windows; twice-weekly boiling water rinses keep drains open - and if they do clog, use a metal 'snake' or a plunger to unblock them.

Re ducing waste • Save and reuse things like string, gift-wrap, shopping bags. • Give magazines and books to friends, hospitals, doctors' offices. • Help nursery schools; they love to have egg cartons, yogurt containers, toilet paper rolls, apple baskets. • Give old clothes or furniture to charitable organisations such as Goodwill, The Salvation Army, Saint Vincent de Paul.

59

• Cut down on food waste; 20% of food we buy ends up as garbage. Keep track of what you have and use groceries while they are still fresh . • Repair broken toys, appliances, fu rniture instead of bu ying new ones. • Start a compost heap with kitch en and yard waste. You can use things like banana peel, shells, coffee grou nds, leaves, grass clippings. You will reduce your garbage by a third and make a good soil conditioner for your garden . '7

Chapter summary

• A population is composed of all the members of the same species living together in the same place. • The population grows if food is readily available and there is adequate space. • Many factors like disease, predation, competition, availability of food and space keep a population size constant. • A typical growth curve is sigmoid in shape (S-shaped). • The maximum population size .that can be sustained in an area is called the carrying capacity of that area. • Humans, though subject to the same constraints as other organisms, have devised many ways to overcome such constraints and, as a result, human population growth is still increasing. • Humans can make use of many natural resources such as minerals, biotic resources, water, soil and fuel. • Resources can be defined as being renewable and non-renewable. • A renewable resource can be reused. • A non-renewable resource is limited in supply and once used up is no longer available for use. • Oil and natural gas are important resources in Trinidad and Tobago. • In Guyana and Jamaica, bauxite mining is important. • Manufactured materials like paper, plastic, textures and metals are used by humans. These materials can be biodegradable or non-biodegradable. • Biodegradable materials can be broken down and recycled back into the environment. • Non-biodegradable materials accumulate in the environment. • People are being taught to reduce, reuse and recycle these materials. • There are many advantages to recycling. • There are also many difficulties involving in recycling.

ITQ1 An en vironment has a certain amount of space; this will support only a specific amount of organisms, depending on how they use it. It can also provide food for a specific numbe r of organisms. Man y offspring m ay be produ ced, but some will be eaten by predators and some may die of disease. At an y time, the en vironment will make it possible for some to live a nd so the size of the population remains m ore or less constant unless there is a change to th e environment. This constant point is called the carrying capacity for the population in that environment. ITQ2 Earth h as many different environments in which a ctiversity of organism s exists. Animals and plants show adaptations for living in the differen t habitats and changing environments. Natural disasters, diseases and predators help to keep popu lation sizes under control. As long as there is physical space and food for all these organisms, they will survive on Earth.

60

ITQ3 Animals and plants that humans use for food are encouraged to grow, by creating ideal conditions in which they can live, and removing their pests and predators. Humans will cause population decreases by taking too much of a population of plant or animal for food or other purposes, such as over-fishing of whales for oil; dearance of an area of natural animal and plant populations so that the area can be used for human purposes, such as housing, industry or growing food crops; or accidentally by introducing a species from one environment to' another where it outcompetes the natural populations. ITQ4 The invention of vaccines and immunisation against deadly diseases prevents thousands from dying each year. Diseases that prevent couples from having children have been researched and studied and now many couples who would have been childless can have children of their own. Many more babies are born daily because of technology, drugs and healthcare services. The baby and mother are treated and cared for during the pregnancy, at birth and after birth. Many people live longer because of the research and technology applied to the treatment and cure of diseases like pneumonia and cancer. ITQ5 (i) Renewable resources: forest trees and fish. (You may have mentioned others.) (ii) Non-renewable resources: fossil fuels and precious metals (e.g. gold and silver). (You may have mentioned others.) ITQ6 (i) Reuse - find another use for a product which has already beed used, so that there is no waste to pollute the environment. (ii) Reduce - decrease the amount of products being used, so that there is less to pollute the environment. (iii) Recycle - utilise an already-used product in the manufacture of another product that will be used again. This reduces the amount of pollution in the environment. ITQ7 Advantages of reusing include: • less waste is generated; • there is a reduction in the rate at which the resource which makes the product is used, so the resource is not depleted as quickly; • it allows for less pollution of the environment; • fewer organisms will be affected by pollution. ITQ8 The difficulties of recycling include: • it is expensive; • the public may not cooperate, for example in separating their garbage; • waste may not be generated in large enough amounts and rates to keep the recycling plant functioning; • recycling plants cover a large area, so there is often not enough land available for the plant.

61

Examination-style questions (i) List four factors that could lead to an increase in population size. (ii) The graph below shows a typical sigmoid growth curve. Number of individuals in the population

Time

(a) Describe each phase of growth. (b) What factors influence the carrying capacity of the environment? (c) A disease developed that wiped out all the individuals of the population. Copy the graph above and continue the line to show how the population size is affected. Population size

Time

(iii) The growth curve above shows the growth of the population of humans over the last two centuries. (a) How is this curve different from the typical sigmoid growth curve of a population? (b) humans are not subject to the same constraints as other populations. Describe how humans has overcome two factors that keep other populations 'in check'. (c) What are some social and economic consequences of over-population by humans? (d) What are some implications to the environment of human over-population?

62

2

Explain the following terms giving examples of each: (a) renewable resources; (b) non-renewable resources; (c) biodegradable waste. (ii) In an effort to reduce the production of waste, people are being taught to (a) reduce, (b) reuse, and (c) recycle. Explain each of these terms using examples.

3

(i) Describe the impact of agriculture on the environment. (ii) (a) What is compost? (b) Every home should practice backyard composting. What do you think of this statement? What are some advantages to the environment of composting? (iii) Discuss some difficulties of recycling.

(i)

9ihe Effects of Human Activity on the Environment 0 0 0 0 0 0 0

understand that human activities have great effects on the environment describe the origin and effects of air, soil and water pollution discuss deforestation and its consequences on the environment understand the impact of industrialisation understand the impact of human activities on marine and wetland environments discuss current and future trends regarding climate change understand how the environment can be conserved and restored human activities

')

( destruction of the environment

industrialisation

deforestation

pollution

II

climate change

111 soil

marine

I wetland

11 water

I air

Humans and the environment The planet Earth is a beautiful and green place brimming with life-s usta ining water and ideal for life as we know it (figure 7.1). Millions of different species of plants and animals inhabit the Earth, living in balance w ith each other and the various reso urces that they use. One species, Homo sapiens (humans) is able ro dominate life on Earth. Humans have been able to use their intellectual abilities to 'conquer' th e Earth, to exploit all kinds of environments and harness its natural resources for their own use. Humans are very successful, as seen by their great population size; some parts of the world are very densely popu lated by humans (figure 7.2). The human environment is where humans live. Th e whole Earth can be considered as the human environment since humans can be found almost everywhere. Th e Earth itself and other living species, plants and animals, make up the Rgure 7.2 The human population is increasing. environment for humans.

63

CHAPTER 6

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l'.'f:Q·1 L/'--J List three reasons why it is believed that humans are 'successful'.

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Food, disease, predators and space are all factors that control population size and keep populations in check or at a constant number. List three practices of food production that have helped humans to feed billions of people worldwide.

Humans have been able to overcome those factors which naturally keep populations in check, as we saw in the chapter 6. Each year, nearly 100 million people are added to the Earth's population. This increase in population has led to many effects on the environment. • Humans are con stantly looking for new space, and have taken over areas already inhabited by other species. They need space for homes, industries, agriculture and other activities. A5 a result, many other species are losing their living space and are becoming extinct. Changing the use of the lana for human purposes changes its suitability for other species and they die out . . • Humans are constantly producing waste from their various activities and this has led to all kinds of pollution of land, sea and air. • The great increase in human population size, together with the developmen t of global travel and communications, leads to a greater need for factories and industries. The impact of industrialisation includes the generation of more waste and increasing use of natural resources on an ever larger and more dangerous scale. • Although humans do not live in water, their activities have polluted the seas, over-fished marine life, caused oil spills and other disasters, and led to severe stress on marine environments all over the world. • More humans means more carbon dioxide in the atmosphere; many people consider this is leading to global warming. The release of other chemicals is also though t to cause depletion of the ozone layer which protects us from harmful ultraviolet radiation. • Human use and management of crops and food species changes the balance of nature. In trying to control pests, many natural populations may be destroyed. The development of genetic engineering makes these possibilities and the risks of affecting th e environment even greater. ·

Endangered and vulnerable organisms biodiversity >

endemic species /

There is an extraordinary variety of life on Earth. Biodiversity refers to this biological diversity of the millions of microorganisms, plants and animals that inhabit the Earth. Th e Caribbean islands, like most places, have a rich heritage of biodiversity. In particular, th ere are a large number of endemic species which are only found in a single geographic area, maybe just on one island. People are concerned over the rapid decline of the Earth's biodiversity. Extinction can be seen as a natural process - as environments change naturally over time, some organisms will not be as well suited to the environment and will die out. However, human activities have increased the rate of extinction to several hun dred times greater than the natural rate. For example, there are believed to be less than 1000 giant pandas in the wild in China. Since 1974, their bamboo forests h ave died out at an alarming rate. This is part of the natural life cycle of the bamboo, but increased population of the area by humans has made the possibility of the panda becoming extinct much more likely. We now classify organisms that are at risk for extinction as follows: • endangered species are species whose numbers have been reduced to the point that the survival of the species is unlikely; • vulnerable species are those that may become endangered in the near future because their populations are decreasing at alarming rates; • extinct species are no longer known to exist. Much research is being done on endangered species, to find ways to save them from extinction. The International Union for the Conservation of Nature (IUCN) regularly publishes lists of endangered organisms.

64

Reasons for increasing extinction rates The increased rate of extinction is the result of many kinds of human activity: • habitat destruction, including deforestation; • pollution; • introduction of more competitive species; • hunting or cropping more than the population can sustain.

Habitat destruction The manatee is also called sea cow because it grazes on plants that grow on the sea bed.

Habitat destruction is the leading cause of species extinction around the world. For example, all species of gorilla and many of the other apes are endangered because they Uve in areas where human population is increasing. Many large predators, such as tigers, and large herbivores, such as the West Indian manatee, are also enda ngered because they need space that competes with humans's need for space.

Deforestation (a)

Deforestation is of particular concern at present. Large areas of rainforest are being destroyed, to provide hardwood timbers and to create space for roads and mining. These forests are not only important for their contribution to carbon dioxide removal in the carbon cycle, but contain thousands of species that cou ld easily become extinct.

Introducing competitive species

(b)

The introduction of more competitive species has a major e[fecr on endemic island species. For example rats, cats, dogs and mongoose have ca used the decline of many Caribbean species, especia lly reptiles and birds that are killed or lose their eggs through predation by the new species.

Hunting

(C)

Hunting may directly cause the extinction of a species, for example, the Tasmanian Wolf may have been hunted to extinction in Australia by the middle of the last century, and the Dodo is a famous example of extinction through hunting which happened around 1680. The polar bear and black bear are killed for their fur and are under threat of extinction from hunters. In addition, food fish, like populations of tuna, shark and card, are over-fished and are at risk. Hunting may also indirectly affect other species. For example, drift nets are enormous nets used for commercial fishing. The nets trap and kill many creatures apart from fish, especially dolphins, porpoises, sharks and turtles. Today, many animals are at risk in the Caribbean (figure 7.3). These include many birds, like the West Indian Whistling-Duck, the West Indian Flamingo, the Barbuda Warbler and the Cayman Parrot. Of the reptiles, the Hawksbill Turtle, the Leatherback Turtle and the Loggerhead Turtle are all at risk. The American Crocodile, the Cuban Tree Boa and many iguanas are also at risk.

Reducing the risk of extinction Rgure 7.3 Endangered species in the Caribbean. (a) Whistling duck (b) Leatherback turtle (c) Manatee.

There are ways to reduce the risk of extinction of other species. Wildlife parks and reserves set aside land where the activities of humans are strictly controlled. This can help to increase the numbers of endangered species and prevent extinction. Zoos often specialise in looking after particular animals. However, there are many problems in the breeding of anima ls in captivity.

65

__::_Livin9 Orgenisms ir:t t~'l°'

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__

_ _ _ -::-.: _

-

_

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Although much research is being done on maimaining the Earth's biocliversity, we need to be sensitive to the needs of all the species living around us and the effect that we are having on them.

Other effects of human activity Shortage of water Most living organisms must have a daily supply of water to live. Humans often use more water than is strictly necessary and the shortage of fresh water is now a worldwide problem. A number of factors working together have resulted in this water sh ortage problem. • Deforestation - This can remove large areas of densely growing trees, like rainforests. Water is transferred to the atmosphere by evaporation and transpiration by plants. This is an important part of the water cycle as it leads to the formation of rain. Rain falling into rivers, lakes and dams provide our supply of water. Removing large areas of trees can change the pattern of rainfall ail around the area and possibly beyond. • Industrialisation - Industry uses a lot of water most often simply to cool equipment. For example, water ensures the constant functioning of the iron, steel and oil industries. • Pollution - Water pollution results in less water being suitable for drinkjng. Each individual needs water for hygiene and food production. Billions of inclividuals use and waste water every day, though much of this can be recycled. • Depletion of ground water - Much water is trapped in lakes underground after it has seeped through the soil and rocks above. These lakes are known as aquifers and we get a lot of our water from these. It can take many years, even hundreds or thousands of years, for the water to seep into them, so they can be used up faster than they are replerushed by rainfall. Over 90% of the Earth's water is salt water, which we cannot drink but which could be desalted and made available for use by humans. Desalination plants convert salt water of the oceans and seas into drinkable water. However, desalination plants are costly to install. At national level, efforts co reduce pollution (especially of the rivers, lakes and seas) may result in increased water supplies, but also many simple, everyday practices could add up to conserve water and reduce shortages. These include: • turning off taps while brushing teeth; • fixing leaks in water pipes irnmeillately; • recycling-reusing water when possible; • minimising use of water in the garden; • not having over-long showers; • using a bucked instead of a hose to wash cars; • being mindful of how valuable a commodity water is.

Pollution Rgure 7.4 Chemical pollution from an oil refinery.

66

Poilution is the contamination of land, water or air by the discharge of harmful substances (figure 7.4). Humans are constantly polluting the environment. Tables 7.1, 7.2 and 7.3 describe the origin of these pollutants and the effects on the environment and on humans.

Pollutants

Origin

Control of pollutant

non-biodegradable waste

household and industry Harmful substances (e.g. mercury, lead, can accumulate on land plastics, cyanide) and enter air and water, where they can poison living organisms.

Careful removal of toxic materials from waste to be disposed. Replacement with nontoxic alternatives where possible.

Insecticides and herbicides

agriculture (e.g. DDl)

Replacement with less toxic alternatives. Replacement with biological control.

Effects

Accumulate in organisms through food chains, even killing top consumers. May cause mutation. May cause eutrophication in rivers and lakes. May upset the balance of food chains.

Table 7.1 Land pollution. biological control

>

Biological control is the use of living organisms to control pests, often in horticulture and agriculture. Introducing a predator or parasite of a pest can greatly reduce the popula tion size of the pest to the point where the level of damage is economically acceptable. However, great care must be taken to make sure the introduced predator or parasite does not become a pest itself by damaging populations of other organisms in that environment as happened when the cane toad was introduced to Australia. Pollutants

Origin

Effects

Control of pollutant

oil

spilled from tankers or offshore rigs

Forms 'slicks' on the sea which blocks oxygen and light, killing marine life. Stops birds flying and feeding. Ruins beaches. Is toxic to some marine life. Clogs respiratory systems of fish.

Legislation to prevent cleaning of tankers near land, and reduce the risk of loss from rigs. Spills can be localised with floating booms and dispersed with detergent. The dispersed material may still be harmful to marine life.

hot water

power stations

Changes the temperature of the habitat which results in death or migration of marine life, especially death of coral. Some organisms may flourish.

Cool the water before it is released into the environment. (Some of this heat can be used to heat or power local homes.)

organic waste untreated sewage from housing, ships, farms

Bacteria multiply and use up oxygen. Can lead to eutrophication of water. Can cause disease.

Treat all sewage to remove the biological risk before release into the environment.

mineral salts (e.g. phosphate, sulfate and nitrate ions)

Eutrophication as increased nutrients in water encourage algal growth. Light is blocked because the growth is so thick. Algae die resulting in rapid growth of anaerobic bacteria. Oxygen used up so other organisms die.

Use detergents which contain minimal amounts of these substances. Use fertilisers in controlled amounts, only as needed, to reduce leaching into water systems.

detergents, fertilisers in sewage and water from housing and farms.

(continued)

67

Pollutants

Origin

toxic chemicals (e.g. organic mercury acid wastes, heavy-metal products)

industrial plants May be toxic to aquatic organisms. Concentrated in organisms as they move up food chains. May change the behaviour of aquatic organisms.

Effects

Control of pollutant

Screen all wastes from industrial processes to remove toxic materials before release to environment.

Table 7.2 Water pollution. Pollutants

Origin

Effects

carbon dioxide burning fossil Builds up in the atmosphere fuels, increased trapping heat which could lead human to global warming. population (deforestation increases problem) car exhaust

Carbon monoxide combines Control CO by use of more more readily with haemoglobin efficient engines and catalytic than oxygen does. Causes converters. Use lead-free fuel. headaches, unconsciousness, death. Lead may cause mental retardation.

smoke

burning fossil fuels

Combines with fog to form smog. Particles cause respiratory disorders, increase the risk of lung cancer.

Cleaning of waste gases from industrial processes can remove these and prevent their release into the environment.

Dissolves in rain water forming acid rain. Affects plant growth, damages leaves, corrodes surfaces of rocks and buildings, causes the death of fish and plant life. Crop yields may be reduced. Aggravates asthma.

Cleaning ('scrubbing') of waste gases from industrial processes can remove sulfur dioxide and prevent its release into the environment.

Agure 7.5 This vehicle is run on biogas.

l"·@@t•W

biochemical oxygen demand

68

>

For energy, use renewable energy resources. For vehicles, use mass transport where possible, use biogas replacements for fossil fuel.

carbon monoxide (CO), lead

sulfur dioxide burning fossil fuels

Biogas is a fuel made from the fermentation of waste plant products, such as sugar cane after sugar extraction. In Brazil and parts of the US, this is now being used to fuel vehicles instead of gasoline (figure 7.5).

Control of pollutant

Table 7.3 Air pollution.

Sewage treatment Sewage consists of liquid and solid waste from urine and faeces, warer from domestic use and industrial waste. When raw sewage is discharged into rivers and sea, bacte ria in the water decompose the organic matter in the sewage. These aerobic bacteria multiply rapidly and need large amounts of oxygen dissolved in the water. This is called biochem ical oxygen dem and (BOD). The release of nutrients by the bacteria into the water encourages the rapid growth of algae. The algae add oxygen to the environment as they photosynthesise. But they are a problem because they blanket the surface of the water and restrict the entry of oxygen from the atmosphere. They also restrict the entry of light and this causes submerged plants to die thus adding to the organic matter available to the bacteria. When all this takes too much oxygen from the water, other aerobic organisms like

7 · The Effects of Human Activity on the Environment

eutrophication

domestic sewage

>

plants and fish cannot respire and so die. The decomposition of the dead material and sewage by anaerobic bacteria continues. This process, whereby large amounts of nutrients are added to a water system and lead to the death of many of the organisms living in it, is known as eutrophication. To prevent pollution of water, raw sewage must be treated to remove solid waste and other harmful substances, and decomposed aerobically before it is disposed of into the rivers and the sea. Most sewage works use a biological method of sewage treatment (figure 7.6). water drainage from streets

Grit tank Large tanks where grit, stones and other heavy objects sink to the bottom.

Coarse screen Has wire nets to strain out solid litter.

Aeration tank Air is bubbled through the liquor to encourage growth of aerobic bacteria. These bacteria decompose organic waste into harmless substances and carbon dioxide.

industrial sewage

liquid sewage

Primary sedimentation tank Ughter matter such as faeces settles to form sludge.

Sludge digester Sludge undergoes anaerobic treatment to form methane gas (biogas) which is a good fuel. Residual matter is dried and used as a fertiliser.

Biological filter The liquor is sprinkled onto large tanks from a rotating arm. The liquid sewage percolates down t hrough a bed of stones covered with a film of aerobic microbes (bacteria and protozoa) which digest the organic matter in the sewage.

Effluent • Discharged Into the river and sea. • Used to water plants. • Used for flushing toilets.

II

Rgure 7. 6 The sewage treatment process.

Deforestation

Figure 7.7 Forest clearance.

Every week, at least 1 million acres of forest are cleared or degraded worldwide. Although a forest is a renewable resource, removal at that rate is much greater than the rate at which the trees can be replaced. Forests are not being managed and maintained, but are being exploited in a destructive way (figure 7.7). Rainforests are being cleared for farming, timber, mining, largescale cattle ranching, housing and industry. When a small area is cleared, the forest recovers quite quickly, but when large areas are cleared, there are many consequences on the environment. • Soil erosion - Deforested slopes encourage surface run-off during rainfall. When the forest is in place, it intercepts the rain water and lets it trickle slowly to the soil. When the canopy of trees and smaller plants is removed, rain falls directly on the soil causing erosion of the topsoil as it runs off the surface of the land. The soil below is not as fertile, so will

69

- Living Orga"lsms in the E9vironrnent

•

•

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(i) What is deforestation? (ii) Why do people practise deforestation? (iii) What are two consequences of deforestation?

~

•

_ __

-

_

not be able to sustain growth as well as the lost topsoil. The land can be permanently damaged. Soil degradation - If the land is cleared for agriculture, the soil becomes infertile due to the removal of minerals by the crops. Leaching also occurs which removes minerals that would otherwise have been taken in by the trees. Increased flooding -The increased run-off takes silt with it which blocks rivers and causes flooding in low-lying areas. Flash floods also occur because of the increased run-off. Species destruction - The plants removed may become extinct. The habitats of many organisms are destroyed, and the food chains that dependent on those plants will break down Many organisms, plants and animals, may become extinct. Destruction of natural resources - Many crop plants originated as rainforest species (cocoa, banana, rubber). Many forest plants also produce medicinal drugs. Disease-resistant varieties of plants are usually found in the wild. If we destroy these forests we may lose these resources before we have even learned about them.

ll!Q~ V'-'

Look back through this chapter and make a list of the results of industrialisation.

~

ll!Q5 V'-' For each item on your answer to ITQ4, give an example of how humans are attempting to reduce the impact of industrialisation on the environment.

99:>

ll!Q6 V'-'

(i) What is industrialisation? (ii) Discuss one advantage and one disadvantage of industrialisation.

Industrialisation Industrialisation is a sign of human success. The word industry is used to cover all forms of economic activity: primary (farming, fishing, mining and forestry); secondary (manufacturing and construction); tertiary (back-up services such as administration, retailing and transport); and quaternary (high technology and information services). Industry requires a workforce and thus provides jobs for people who can enjoy a 'good ' standard of living. It generates income for the country which can be spent on improving education, healthcare and public utilities for its people. There is, however, a price to be paid for industrialisation. If not managed properly, it can lead to serious environmental consequences (discussed above) including pollution of land, air and water, water shortages and deforestation. Industrialisation is two-sided. On one side, we see successful humans harnessing resources and living a comfortable life. On the other side, we see widespread destruction of the environment. The Earth is our only home and we should take care of it.

Impact of human activities on marine and wetland environments Water is important to us. We love to go to the beach, snorkel, lie by the river, fish and do many other activities which involve wa ter. On the physiological side, over 75% of the human body is water. We cannot exist without water and die faster from dehydration than from starvation. Other living organisms depend on water as much as, or even more than, we do .. Our planet is able to sustain life because of the presence of water. Yet many human activities lead to a hotter and drier Earth, and to pollution and the destruction of aquatic environments. Belwo are examples' of the specific effects of human activity in marine environments in the Caribbean.

70

_ -

.~ IJ'-.)

Humans do not live in water, yet their activities and practices have affected the marine environment. Describe two effects on the marine environment that humans have had, and how they were brought about.

~

IT:Q8 IJ'-.)

Why are coral reefs important to Caribbean people?

~

IT:Q9 IJ'-.)

Discuss two reasons why mangrove swamps should be conserved and protected.

__ 7 · The §t(ects of ir-lll!.mar'1_Activity on

the.~Enviroi1rnent

Some effects of human activities in the Caribbean • Destruction of mangrove swainps - Using the swamps as dumping grounds or for building rice farms or other development, leads to many problems, such as: - nursery grounds for all kinds of fish are affected; - pollution of water with toxic materials, or those wruch cause eutrophication; - pollution of water with human and animal waste can spread diseases like cholera and diarrhoea; - breeding grounds and nesting and roosting grounds for birds are reduced; - the natural habitat of many invertebrates, like oysters and crabs, is destroyed; - money generated from eco-tourism (tourists visiting the area to see the wildlife) is reduced; - money generated from fishing crabs, oysters and other local fish is reduced. • Over-fishing - In the seas of the Carinbbean this is a serious problem. Much money is spent on catching fish and too little on managing and producing them. • Destruction of coral reefs - This occurs by: - collecting coral for sale; - dynamiting for fish; pollution with raw sewage, garbage, industrial and kitchen waste; - pollution with hot water which kills the coral organisms; - pollution with insecticides and fertilisers; - smothering with silt from soil erosion; - smothering with red mud ba uxite waste, dust and cement from construction; - removal to allow the building of harbours, channels, etc.; - damage from anchors; - damage by visitors to the reef. • Damage to wetland environments - this occurs by: - allocation for rice and other farming; - fertilisers and other dangerous chemicals in the water; - n oise pollution from use of heavy rice equipment; - damage by visitors to the wetlands; - pollution from visitors; - hunting of animal s; - noise pollution from hunting; collection of animals for sale; - squatters or development of housing settlements; - over-fishing.

71

The coral reef is a natural resource important to the Caribbean islands and, as such, needs LO be conserved. Damage such as that shown in figure 7.8 is to be avoided if at all possible.

Rgure 7.8 Coral reef smothered by land run-off.

Impact of increase in greenhouse gases Global climate change has already had observable effects on the environment. Effects that scientists have predkted are now occurring. These changes are largely due to an increase in the levels of greenhouse gases; they include: • increase in Earth 's average temperature; • changes in the pattern and amount of rainfall; • reduction in ice and snow cover; • rise in sea level; • increase in the acidity of the oceans.

Conservation and restoration of the environment conservation

>

sustainable

>

Since humans are responsible for widespread destruction of the environment on the Earth, they should take responsibility for the widespread restoration and conservation needed to 'heal' this damage to our planeL. We need to manage the environments that we live in and the resources that we use in a sustainable way. That means using them in a way that they are not damaged and depleted, so they are available for future generations . There are many ways that this can be done.

Reduce pollution • Replace fossil fuels with alternative, non-polluting energy sources like solar energy, wind energy, wave energy, biogas and hydroelectricity. • Treat all sewage. • Use unleaded petrol and catalytic converters on vehicles to reduce all emissions of pollu tants in exhaust gases. • Replace harmful insecticides and herbicides with biological control or biodegradable insecticides.

72

• Use cleaners of all exhaust gases from industry. • Improve effluent release standards of industries to purify, treat and reduce effluent release. • Use recyclable materials at home and in industry; for example, paper and biodegradable plastics rather than plastics, which are not biodegradable. • Ban the disposal of garbage in rivers, swamps and seas.

Conserve natural resources • • • •

Recycle resources such as glass, metal and paper. Use alternative energy sources. Replace renewable resources, such as forests. Manage food species, plants and animals, in a sustainable way by imposing closed seasons and strict restrictions on how heavily populations are cropped. • Limit deforestation to a rate at which forest recovery can be maintained.

Protect endangered species • Ban the killing of species in danger of extinction, such as turtles and the West Indian manatee. • Set up breeding programmes for species in danger of extinction. • Set up National Parks to provide areas for species to live and breed undisturbed by human activity.

Conserve soil • Replant trees immediately after harvesting to prevent run-off when it rains. • Use crop rotation to maintain a balance of soil nutrients; different crops remove different minerals from the soil . • Use natural rather than artificial fertilisers to preserve soil structure, but only as needed so as to prevent eutrophication of nearby water sources. • Use terracing and contour ploughing around hillsides to prevent erosion by soil run-off when it rains. • Prevent over-grazing by animals like cows and sheep that remove plants and their roots which hold the soil when it rains and prevent erosion.

Preserve clean water • Prevent eutrophication by treating sewage properly and using only as much fertiliser as is needed on farm land.

Improve land used in mining • Replace vegetation as soon as possible after mining has finished . • Fill the mined areas with soil and use for farming, housing or industry, so that new areas do not need to be used. Eanh is home not only for humans, but for millions of species of plants and animals. The Earth can successfully 'carry' all these organisms when there is balance in population size and the natural cycles can take place efficiently. It is our responsibility to maintain that balance.

73

ITQ1 (i) They have inhabited most places on Earth and continue to colonise new areas. (ii) They have been able to overcome many natural diseases. (iii) They mass-produce food, and so do not have to search for food each day. ITQ2 (i) Agricu lture - mass cultivation of plants that can be used as food. (ii) Livestock production - mass production of animals (cows, chickens) and their prod ucts, (eggs, milk) that can be used as food. (iii) Use of tech nology to intensify production to increase the yield (e.g. the use of artificial selection) an d, in the future, possibly increasing use of genetic. engineering to speed this up. ITQ3 (i) Deforestation is the removal of a large number of trees from an area. (ii) • For quarrying - to obtain gravel, soil, sand, etc. for building. • To clear land to build homes. • To provide lumber for housing, furniture, etc. • To plant crops or for ranching. (iii) Any of the consequences mentioned on pages 69- 70 wou ld be approrpiate. ITQ4 Your list should include examples for each of the following: pollution of land, air and water, water shortages and deforestation (see tables 7. 1-7.3). ITQ5 Your answer should include examples for each of the fo llowing: control of pollution of land, air and water, water shortages and deforestation (see tables 7. 1-7.3). ITQ6 (i) Ind ustrialisation is th e spread of industries. As a coun try develops, it becomes more industrialised. (ii) As more industries develop, fewer foreign products are imported, so more money is generated by the country and more jobs are available. This is the major advantage of industrialisation. A major disadvantage is that as industrialisation increases, damage to the environment is more likely. ITQ7 (i) Hot water from power stations is poured into the sea, changing the temperature and thus the environment of marine life. Plants and animals may not be able to adapt fast enough and are killed. (ii) Oil spills from tankers form a layer of oil on the surface covering many miles of ocean. This can have disastrous effects on the marine environment as shown in table 7.2. ITQS Coral reefs have many important functions.

74

• As a to urist attraction - A lot of money is generated from to urists who wish to visit the area to see the reefs. • As a source of job opportunities in tourism and conservation . • In preserving bioctiversity - Man y different species live in the reefs. • As a spawning ground - Many species of fi sh breed in coral reef areas, including fish that w e use for food. • In protecting the coastline from erosion - The reefs absorb bursts of wave energy, especially during a storm. • As a sensitive inclicator of global environmental changes - Cora ls are very sen sitive to environrnentaJ conclitions and any change results in a chan ge in the population of corals. ITQ9 Mangrove swamps are spawning grounds for large fish and ensure the continuation of man y fish species. Mangrove swamps support bioctiversity: there are many species that live in that one ecosystem.

Examination-style·questions (i) Humans are constantly looking for and occupying new space. (a) List some reasons why humans needs new space constantly. (b) Suggest three consequences of occupying new space constantly. (c) Discuss the greenhouse effect with particular reference to the over-population of humans. (ii) (a) What is a pollutant? Give some examples. (b) Account for the presence of lead as a pollutant in the atmosphere. (c) How can lead pollution be controlled? (d) Describe the causes and effects of eutrophication. (iii) (a) List four effects of sulfur dioxide pollution. (b) Account for the presence of chlorofluorocarbons in the atmosphere. (c) What are some of the effects of the build-up of these pollutants? 2

(i) (ii) (iii) (iv)

What is deforestation and why is it practised? State some consequences of deforestation. State some consequences of industrialisation. Why are conservation efforts so important?

75

Section B:

Life Processes and Disease

0 0 0 0 0 0

draw diagrams to show the structure of typical plant and animal c ells understand the functions of the cell wall, cell membrane, cytoplasm, mitochondrion, chloroplast, nucleus and vacuole compare plant and animal cells understand that most microbes are unicellular understand why specialisation is important in multicellular organisms understand how some substances move into and out of cells

plant

animal

microbes - bacteria, Amoeba

)

l cell - basic unit of life

1

( microscope

tissue

'

organ

calculating size of cells 6 image seen Is 400 times bigger than the actual specimen

system

--- - -- 1'5 light passes through eyepiece lens,

I

magnification x10

organism

movement into and out of cell

diffusion

osmosis

Why we need microscopes

4 light passes through lens,

r:a.1----'~--- magnification x40

~_....l~;;nr;::,:;;;:~;--- 3 light passes through the slide

(specimen)

2 light passes through the filter and condenser l light reflects on mirror and travels up to the eyepiece

Th e cell is the basic un it of life. A cell cannot be viewed by the naked eye since it is too small. It can on ly be seen w ith a mi croscope. Cells are thus described as being m icroscopic. A microscope is used to produce a magnified image of an object. There are different kinds of microscopes, for example light and e lectron. When looking through the microscope at a piece of tissue, separate cells can be distinguished which would not have been seen with the naked eye. How much you ca n see w ith a microscope depends on h ow powerful its magnification is. A light microscope typically magnifies between l 0 and 400 times real size (figu re 8.1). An electron microscope is more powerful and can magnify tens of thousands of times actual size.

8 ·Cells

Calculating the size of cells ~ IT:Q·1 V'-J

What is the purpose of a microscope?

The actual size of an object in a photograph can easily be calculated from the image and the magnification given. If the length of the object in the ph oto is measured as Z, and the magnification is given as xlOO, that m eans the object is 100 times larger than in real life. So, the actual size of the object is Z-;-100.

Plant and animal cells

Syllabus reference C1.1

--

than the cell contents, there is a greater concentration of water molecules outside than inside . Some water molecules move out of the cell but more move into the cell, so there is a net flow of water into the cell . The cell becomes full of water and is described as being turgid . When a plant cell is put into a solution that is more concentrated (hypertonic) than the cell contents, there are fewer water molecules outside than inside. A few water molecules move into the cell but many more move

a ·Cells

1!@13!elJ

out of it, so there is a net flow of water out of the cell. The cell loses water and is described as being flaccid . Flaccid cell s are easy to distinguish un der the microscope beca use the cell membran e an d contents pull away from the cell wall.

Osmosis in animal cells

~ 11!Q8 vv

An animal cell placed in water will burst. Explain fully why a plant cell will not burst when placed in water.

An anima l cell h as no cell wa ll like a plant cell, so hypotonic and h ypertonic solutions have different effects. In a h ypotonic (dilute) solution there is a net flow of wa ter into che cell. With no strong cell wa ll to prevent th e membrane from stretching too far, it even tually bursts. In a h ypertonic (concentra ted) solution there is a net flow of water out of the cell and the whole cell shrinks (figure 8.17) .

•·

,,

• cell in hypotonic solution • net flow of water into cell • no strong cell wall so cell bursts

• cell in isotonic solution • no net movement of water

• cell in hypertonic solution • net flow of water out of cell • cell loses water and shrinks

Figure B. 17 The effect of different concentrations of solution on an animal cell .

CHAPTER 16 ~

It is important for cells to be protected from large changes in con centration of the solutions aro und them . Animal bodies ha ve complex m echanisms to do this called osmoregulation and homeostasis (ch apter 16).

rChapter summary • The cell is the basic unit of life. • A cell contains smaller parts called organelles. • The nucleus, cell membrane, cytoplasm and mitochondrion are some organelles found in typical plant and animal cells. • Plant cells also contain cell walls, chloroplasts and large central vacuoles. • Most microbes are unicellular.

87

Life Processes and Disease

• • • • • • • •

• •

Cells in multicellular organisms are often specialised for a particular function. A group of specialised cells that have the same function is called a tissue. An organ is a group of different tissues that work together. Organs working together make up a system. Systems coordinate with each other and work together in a living organism. Many substances can move into and out of a cell through the cell membrane which i~ selectively permeable. Diffusion is the movement of a substance from a high concentration to a low concentration. Osmosis is the movement of water across a selectively permeable membrane from a solution where there is a high concentration of water molecules to a solution where the concentration of water molecules is lower. Diffusion and osmosis occur at many places in a living organism. Different concentrations of solution have different effects on plant and animal cells. ,.

II..

ITQ1 A microscope is an instrument used to produce a magnified image of an obj ect . Organisms and objects that cannot be seen by the naked eye may be

visible under a microscope. ITQ2 The measured width of the chloroplast in the photograph is 14 mm (or 14 x 10- 3 m ). The magnifica tion is x5000 . This means that the measured size is 5000 times large r than in reality. So the actual size is (14 7 5000) x 10- 3 m = 0 .0028 x 10- 3 m (or 2.8 x 10-6 m or 2 .8 µm ). ITQ3 (i) Plant a nd a nimal cells have: cell membrane, nucleus, cytoplasm, mitochondria, small vacuoles. (ii) Plant cells have a cell wall*, chloroplasts, large central vacuole. (*Fungal cells and som e bacteria also have cell walls,bu t these have a completely different structure from those in plants.) ITQ4 The plant cell wall has protective an d structural functions. It p rotects the plant by protecting each plant cell from bursting when the plant takes up w ater. It also helps to support stems and leaves of the plant when th e cells are full of wa ter, because plants have no skeleton like many animals. The chloroplast con tains the pigmen t chloroph yll which collects the light energy of the Sun. Chloroplasts are the sites of photosynthesis, so animals do n ot need them . The large plant vacuole is importan t during exchange of wa ter and minerals, and stores various substances including waste products. ITQS (i) The cell membrane is a partially permeable barrier th at controls the passage of substances into and ou t of the cell whereas the cell wall provides support and protection and allows the free passage of wa ter. (ii) Th e mitochondrion is the site of respiration during which en ergy is released from sugar. All cells have mhocd1ondria. The chloroplast is the site of ph otosynth esis where sugar is made. Chloroplasts are found only in plant cells. ITQ6 Cell: e.g. mu scle cell . Tissue: an y group of on e kind of cell w orking together e.g. muscle cells in muscle tissue. Organ: any group of tissues working together e.g. stomach , m ade up of secretory tissue, muscle tissue and other tissues; leaf, made of palisa de tissue, xylem tissue. System : any group of one kind of organs working together e.g. digestive system, made up of stomach, liver, intestines and other organs.

88

8·Cells

ITQ7 A unicellular organism is an organism that h as only one cell. This small organism shows all the characteristics of We and lives an independ ent life. For exa mple, Amoeba and Chlorella . A multicellular organism is made up of man y cells. These cells work together, and the organism is able to show all the characteristics of life. For example, a human and a worm (there are many other examples you co uld have chosen ). ITQ8 A plant cell has a cellulose cell wall around the cell membrane. The wall is strong and cannot stretch. When placed in water, th e cell will take up water, but the cell membrane wilJ n ot burst because th e cellulose cell wall stops it stretching to bursting point. An animal cell does not have a cellulose cell wall and so can stretch to the point w here it bursts.

Examination-style questions 1

(i) The drawing below was constructed by a biology student after viewing a slide under the microscope. The drawing made was magnified 2500 times. What is the actual size of the cell labelled A?

( C) I C) I C) J A

(ii) The figure below shows how a section of a root or stem is mounted for microscopic

investigation.

/

I

Explain why it is necessary to cut a very thin section of the material which is to be observed under the microscope. (iii) (a) Name two types of microscope. (b) Why are cells described as being microscopic? 2

(i) (ii) (iii) (iv) (v)

Make labelled drawings of typical plant and animal cells. Use a table to compare typical plant and animal cells. Give one advantage of being multicellular. Name one difference between a tissue and an organ. Give one named example of: (a) a tissue; (b) an organ to be found in: • •

an animal; a plant.

89

Life Processes and Disease

3

The figure below shows onion rings A, B, C and D before and after immersion in water and a salt solution. onion ring A

0

onion ring before immersion in water

onion ring C

0

onion ring before immersion in salt solution

onion ring B onion ring D

0

onion ring after immersion in water

0

onion ring after immersion in salt solution

(i) Copy and complete the table below to show the measurements of the rings. Onion ring

Outer diameter

Inner diameter

Mean diameter

A B

c D

(ii) (a) Using measurements.from the table, describe what happened to the onion ring placed in: • water; • salt solution. (b) Explain fully the results seen in: • water; • salt solution. (iii) (a) What process is taking place? (b) Give an example of the occurrence of this process in living organisms. (iv) Describe two examples of diffusion as it occurs in living organisms.

90

P. hotosynthesis

0

understand the difference between heterotrophic, autotrophic and saprophytic nutrition

0 0

describe photosynthesis in green plants

0

explain how environmental factors affect the rate of photosynthesis

relate the structure of the leaf of a flowering plant to its function in photosynthesis

photosynthesis autotrophic nutrition inorganic substances converted to organic substances

( heterotrophic nutrition - animals

saprophytic nutrition

leaf structures

limiting factors - light, temperature, carbon dioxide; water

1 conditions

adaptations for photosynthesis

Plants are the food supply for animals Th e relationship between autotrophs, heterotrophs and sproph yres is sh own in figure 9. 1.

AUTOTROPH

HETEROTROPH

'self-feeders' e.g. plants that make their own food during photosynthesis

feed on other organisms e.g. consumers that feed on plants and other animals

SAPROPHYTE feed on dead organic material e.g. decomposers that feed on the dead autotrophs and heterotrophs

/

Figure 9.1 Relationships of autotrophs, heterotrophs and saprophytes.

91

Life Processes and Disease In the study of food chains we saw that plants are producers and are at the

heterotroph

>

QSb

start of almost all food chains. Animals are consumers and feed on the plants or on other animals. Plants do not eat, yet they are full of food. They are rich in carbohydrates, fats and proteins. This is because they are able to manufacture their own food. We call them autotrophs (self-feeders) because they are able to make organic substances (glucose) from simple inorganic substances (carbon dioxide and water). This process is called photosynthesis and requires light from the Sun to provide the energy needed to carry it out. From glucose, the plant makes all the other carbohydrates, fats and proteins it needs. . Consumers feed on the organic substances made by the plants. Consumers are h eterotrophs (other or different feeders). Heterotrophic nutrition is the intake of complex organic substances when animals feed. Autotrophic nutrition is the intake of simple inorganic substances by plants during photosynthesis and must occur before heterotrophic nutrition (figure 9.2). C02

IT:.Q-1 l./'-J

Distinguish between an autotroph and a heterotroph.

~

'

Autotrophic nutrition

...__ _. . ~ I plants take in inorganic H20 substances and make inorganic organic substances substances

Heterotrophic nutrition animals take in organic substances when they feed

Figure 9.2 Autotrophic nutrition must occur before heterotrophic nutrition can occur. Food chains start with plants, then animals feed on the plants.

IT:.Q2 l./'-J

Why must autotrophic nutrition occur before heterotrophic nutrition?

~

IT:.Q3 l./'-J

Why is saprophytic nutrition important?

When plants and animals die, saprophytes feed on the dead bodies which are full of organic substances such as carbohydrates, fats and proteins. Saprophytes are also called decomposers and they are very important to the cycling of these materials back to the earth, from where they are then available to plants again.

Photosynthesis Practical activity SBA 9.2: Is light needed for photosynthesis? page 344

Photosynthesis can be summarised in words or by the simple equation: light

carbon dioxide + water _ _ _ __, glucose + oxygen chlorophyll

photosynthesis equation

>

light

6C0 2 + 6H 2 0

C6 H 12 0 6 + 60 2 chlorophyll

light-dep endent stage ) light-independent stage

>

Chlorophyll is a complex green pigment. At the centre of a chlorophyll molecule is a single atom of magnesium chemicaly bonded to four atom s of nitrogen. Without supplies of nitrogen, a plant cannot make chlorophyll and so cannot photosynthesise successfully. Experiments show that there are two main stages in photosynthesis (figure 9.3), namely: • the light-dependent stage • the light-independent stage

Light-dependent stage Chloroplasts are organelles seen in green plants cells. They contain the green pigment chlorophyll which 'traps' the light energy from the Sun. The energy is used to 'split' water (Hp) into hydrogen and oxygen. The oxygen is a waste product and diffuses out of the leaf.

92

9 · Photosynthesis

Light-independent stage The h ydrogen then combines with carbon dioxide (COJ to make glucose (C6 H 12 0 6 ). This stage of photosynthesis does not n eed light and can h appen when it is dark.

~

3~chlorophyl

, .•diffuses out of the leaf O>

The glucose produced during photosynthesis is used in several ways. • It is broken down during respiration to release energy so the plant can carry out all the processes of life. • It is converted to starch and stored in the leaf to be used in the night when the plant is not photosynthesising. • It is converted to sucrose and transported to other parts of the plant. It can then be converted to other carbohydrates, lipids and proteins and used for growth, or it can be converted to starch and stored, as in potatoes . Oxygen is a waste product of photosynthesis. The cells in the leaf will use some for respiration, but the rest of the oxygen is not needed by the plant. Inside the leaf, photosynthesis is taking place and oxygen is being produced. It is thus at higher concentration inside the leaf than oucside. So oxygen diffu ses out of the leaf through the stomata (figure 9.9).

Limiting factors in photosynthesis Photosynthesis is a chemical reaction, and the rate at which a reaction can happen depends on how fast the chemicals that are reacting can get together. In photosynthesis, a plant requires water, carbon d ioxide and light. If any one of these is in short supply, the rate of the reaction will slow down. For example, a plant may have sufficient carbon dioxide and water, but not enough light for photosynthesis to take place at its maximum rate. Light is then said to be the limiting factor, since the rate of photosynthesis is limited by the amount of light. The reaction will take place at a rate that is limited by the factor which is a t its least favourable value (light, in this example). Water, light and carbon dioxide may all be limiting factors for photosynthesis at different times. The limiting factors which affect photosynthesis are: • temperature; • light intensity; • carbon dioxide concentration; • availability of water.

Temperature

CHAPTER 10

96

The rate of a reaction increases as temperature increases. With heat, the molecules move about and come together faster. Photosynthesis also involves a series of enzyme-catalysed reactions. Enzymes have an optimum temperature or temperature at which they work best (chapter 10), so this w ill also affect the rate of the reaction. Temperature is often the limiting factor on the rate of photosynthesis in cool seasons in temperate regions.

9 · Photosynthesis

Carbon dioxide concentration The concemrarion of carbon dioxide is re latively low in the atmosphere. So carbon dioxide is us ually the limiting fa ctor when cemperature and light levels are high. Commercial growers who grow their crops in large greenhouses often pump in extra carbon diox ide to increa se the rate of photosynthesis in the crops (figure 9.10).

Light intensity The amount of light in the e nvironment varies grea tl y between night and day. Light is usually the limiting fa ctor from dusk until dawn (figu re 9.10). Rate of photosynthesis

rate slows down, some ractor 1s limiting the rate

0.13% C02 at 30 °C -

areater C02 concentration rate increases

0.03%'C02 at 30°C

~

co_

rate of photosynthesis slows down because of concentration - C02 is the hm1t1ng factor, not light

\./'-I

Which factor will most likely be limiting photosynthesis in each of these cases? (i) Middle of the day after plenty of rain in Jamaica. (ii) Cool autumn day in Britain. (iii) Dry season in Australia.

+------ rate or photosynthesis increases as hyht 1ntens1ty

increases - light Is the limiting factor

Light intensity

Rgure 9.10 How light and carbon dioxide may limit the rate of photosynthesis

Availability of water The ava ilability of water varies in the environment. If the soil is dry, wa ter may be the limiring factor on photosynthesis.

Etiolation

mmmm.],p

Cf the plant cannot get sunlight, for example it is shaded by a rock or anoth er plant, it cannot photosynthesis. Without photosynthesis it cannot make food. But this does not mean that it cannot continue to grow. For a short while, it can use some of the food stored w ithin the plant to grow and lengthen. This gives it a chance to get som e leaves into the light a nd so start to photosynth esise again. The form o f growth a plan t shows wh en it is out of light is different from n ormal. All the energy is u sed to make long thin cells, so the stem becomes e lo ngated and thin, and leaves are kept very small. Th e stems and leaves are also pale yellow as n o chJorophyll is made. This fom1 of growth is called etiolation (figure 9.11). If it does not reach light Figure 9. 11 The et1olated plants on the quickJy the plant will run o ut or rood nght have long thm, white stems and small reserves and die. yellow leaves

97

Life Processes and Disease

.. _

-

An autotroph is an organism that is able to make its own food (organic substances) from simple substances (inorganic substances) . A plant is an autotroph - when it photosynthesises it makes glucose from carbon dioxide and water. A heterotroph is an organism that takes in organic food when it feeds. It must have a supply of organic food since it cannot manufacture it for itself. ITQ2 Autotrophs make organic food which is eaten by heterotrophs. Autotrophic nutrition must therefore take place first so that heterotrophs can have something co eat. ITQ3 Saprophytic nutrition is important for the recycling of nutrients in the environment. Nutrients trapped in an organism are made available when that organism dies. Saprophytes can digest cellu lose and lignin and can decompose all plant remains. ITQ4 Some desert plants close their stomata during the day to p revent loss of coo much water from the leaf when it is hot. They open their stomata at night to exchange gases for photosynthesis. (They have a special mechanism which allows them to trap the energy from sunlight during the day and store it. until the stom ata open at nigh t and the energy can be used to make glucose.) ITQ5 Carbon dioxide is in the atmosphere around the leaf and gets to the photosynthesising cell by diffusion. A photosynthesising cell uses carbon dioxide, and so the ca rbon dioxide concentration decreases within the cell. Carbon dioxide diffu ses into the cell from the surrounding air space where its concentration is greater. The carbon dioxide concentration is thus lowered in the air space. Carbon dioxide _from the atmosphere can now diffuse into the air space through the stomata. ITQ6 • The leaves are spread around the stem and lie at right angles to the Sun's rays so that they can intercept as m ud1 light as possible. • Leaves are green because the cells contain chlorophyll. This captures light energy which is needed in photosynthesis. • Xylem vessels in the stem transport water to the leaf. • The leaves are thin and flat so gases can diffuse in and out as quickly as possible. ITQ7 (i) Carbon dioxide (ii) Temperature (iii) Water ITQ1

98

9 · Photosynthesis

Examination-style questions The diagram below shows a transverse section of a leaf as seen under a microscope.

(i) Label the parts A to F. (ii) Which cell is most actively photosynthesising? (iii) (a) Write the equation that summarises the process of photosynthesis. (b) From the equation, identity three factors/conditions necessary for photosynthesis to take place. (c) Describe how two of these factors reach a typical photosynthesising cell. (d) Describe the role of the cell labelled E. 2

(i)

Define:

(a) autotrophic nutrition; (b) heterotrophic nutrition. (c) saprophytic nutrition (ii) Photosynlliesis"ls summarised in one equation, but described as two stages (a) lightdependent, and (b) light-independent. Describe the two stages of photosynthesis. (iii) List five ways a plant is adapted for photosynthesis. 3

The diagram below shows a leaf in its actual size.

(i) (ii) (iii) (iv)

Making a drawing of the leaf. Write a heading for the drawing. Calculate the magnification of your drawing. Label the parts of the leaf.

99

./ understand the importance of minerals in plant nutrition ./

understand the importance of a balanced diet to humans

0

describe food tests for carbohydrates, proteins and fats

./

relate a balanced diet to age, sex and activity of an individual

./ explain the meaning of the term 'malnutrition' ./ describe hypertension and diabetes ./ describe health problems associated with food additives

0

describe the role and structure of teeth

f?

understand the action of enzymes

Z, understand how the alimentary canal of humans works ./) describe what happens to the products of digestion

age

minerals in plants

pregnancy

food additives diet

balanced diet

malnutrition

sex alimentary system

I

activity

ingestion physical digestion - teeth chemical digestion - enzymes

}

I digestion

absorption

villus

I assimilation

egestion

liver

constipation

All organ isms, plan ts and animals, must be supplied with a source of energy fo r metabolism. This energy is used for ma intenance, growth, and repair of their bodies to sustain thei r lives. Plants (autotrophs) are able to make their own food using energy tram the Sun. They take in only very simple inorganic substances like water, carbon d ioxide and also magnesium and nitrate ions. Nitrogen and magnesium are basic components of chlorophyll. Choprophyll allows a plant to grow more rapidly and produce large amounts of succulent green leaves. These minerals also strengthen and support the roots thus enabling p lants to take in more water and nutrients from the soil. Nitrogen is

10 · Feeding and Digestion

also important in the formation of proteins. Animals (heterotrophs) can only obtain energy when they feed on other living organisms made up of complex materials such as carbohydrates, proteins and lipids.

Diet l!l@IJ

balanced diet

>

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~

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vv

Define the terms 'diet' and 'balanced diet'.

To maintain their bodies in good hea lth, anima ls need various materials. These include carbohydrates, proteins, lipids, vitamins and minerals. Animals eat food that contain these materials or nutrients. The term 'diet' is used to describe the quantity and quality of food eaten (i.e. which nutrients and how much of each is present in the food being eaten every day). A balanced diet is a diet which has the quality and proportions of nutrients needed to maintain good health. This includes water and fibre. Water is essential because around 70% of our body mass is water. If we do not get enough water, systems in the body soon stop functioning properly. Fibre, or roughage, is the tough fibres that come from plant material. We cannot digest and absorb them, but they an; essential to the healthy working of the gut. Without enough fibre, we soon suffer from constipation. Eventually this can lead to bowel disease. Some nutrients that are needed are organic and some are inorganic (table 10.1). Organic nutrients

Inorganic nutrients

Carbohydrates Minerals contain carbon (C), hydrogen (H) and oxygen (0) calcium, iron, potassium, sodium, iodine, phosphorus Proteins contain C, H, Oand also nitrogen (N) and small amounts of sulfur (S) Lipids contain C, H and 0 Vitamins contain C, H and Oand other essential elements Table 10. 1 The organic and inorganic nutrients needed by living organisms.

Organic nutrients

Figure 10.1 Three-dimensional ball-andstick model of a glucose molecule. monosaccharide

>

disaccharide

>

These are required in the diet in relatively large amounts (tables 10.2 and 10.3, overleaf). Carboh ydrates are compounds of carbon, hydrogen and oxygen in the radio 1 C: 2 H: 1 0. An example is glucose. Figure 10.l shows a baJJ-and-stick model of a molecule of glucose. It can also exist as a ring formed from five carbon atoms and one oxygen atom. The sixth carbon atom in a - CH 2 0H group is attached to a ring carbon. Compounds with one such ring structure are called monosaccharides . The formula can be shortened to a symbol which can be either or, for diagrams, just • . Glucose and fructose are examples of monosaccharides. Monosaccharides are often called simple sugars. Two monosaccharides can combine to form a disaccharide (figure 10.2, overleaf). This happens in a condensation reaction as a water molecule is removed. Disaccharides can be broken back down to monosaccharides by hydrolysis which is a chemical reaction involving recombination with water.

0

101

Life Processes and Disease

Monosaccharides and most disaccharides reduce Benedict's solution to an orange/red compound. Sucrose is the only common disaccharide which does not react in this way. This provides a distinguishing test for sucrose. Disaccharides are called complex sugars.

H O H HO H I H 20

HO

OH

condensation (water removed)!

HO

monosaccharides

OH

j hydrolysis (water added)

H: OoO :H j H~ O\oO o001 0oO :H condensation!

/

dlsaccharide

hydrolysis

polysaccharide

Figure 10.2 D1saccharide molecules are made when two monosaccharide molecules join together. Polysacchande molecules are made of many monosacchande molecules.

Organic nutrient Major groups Carbohydrate

Protein

Structure

Characteristics

Importance

monosaccharide (e.g. glucose, fructose)

five carbon atoms and an oxygen atom form a ring

called simple sugars small molecules, soluble, sweet taste

major energy source

disaccharide (e.g. maltose, sucrose)

two rings join together

called complex sugars soluble, sweet taste

major energy source

polysaccharide (e.g. starch, cellulose, glycogen)

many rings join together long chains of simple sugar (glucose) joined together

insoluble and do not have a sweet taste

starch is used as the energy store in plant cells and as a food source · for animals cellulose is found in plant cell walls glycogen is used as the energy store in animals cells

a difference in the order of amino acids in the chain results in different protein there are millions of proteins, some soluble e.g. haemoglobin, red pigment in blood), and some insoluble (e.g. keratin, from which hair and nails are made).

used for making new cells, growth and damaged parts of the body antibodies, hormones and enzymes are also proteins

insoluble in water

secondary energy supply after carbohydrates have been used up important for storage (oils in seeds) also function as insulation (fat under skin) especially for animals living in cold regions foods like butter, oils and nuts are rich in lipids

JlD

made up of long chains of amino acids there are about 20 different amino acids they can be arranged in the protein chain in any order /

(

·-o- ~

~

·-~

Upids (fats and oils)

four moelcules (three fatty acids and one glycerol) joined together --

[E ff

glycerol

fatty ac ids

(continued)

102

10 · Feeding and Digestion

Organic nutrient Major groups

Vitamins

Structure

A, B, C, D, Eand K each vitmain has many funcitons

Characteristics

Importance

small amounts needed for good health

ABCDK-

aids vision in dim light asissts in respiraiton keeps tiossues helathy aids absopriton of calcuim aids in blood clotting

Table 10.2 The major organic nutrient groups.

Vitamin

Sources

Functions

Symptoms of deficiency

A

carrots, spinach, egg yolk, cod liver oil, butter

keeps skin and mucous membranes healthy, aids vision in dim light

dry skin, mucous membranes degenerate, poor night vision

B1

liver, rice, cereals, whole wheat flour, yeast

helps in respiration

beriberi - muscles become weak and painful, nervous system affected

B6

leafy vegetables, eggs, liver, fish, kidney

helps in metabolism

depression and irritability

c

citrus fruits, green vegetables

keeps tissues healthy

scurvy - guns bleed, wounds take longer to heal, heart failure

D

egg yolk, dairy products, cod liver oil, also made by the action of sunlight on the skin

controls calcium and phosphorus absorption, important in bone and tooth formation

rickets - growing bones do not calcify, results in 'bow' legs in young children, and 'knock-knee' in older children

Table 10.3 Some vitamins needed by humans for healthy growth.

polysaccharide > NB Both Benedict's and Fehling's solutions contain copper sulfate. Reducing sugars reduce the copper(ll) ions (CU2• ) present in the copper sulfate to insoluble red-brown copper(I) oxide which contains cu• ions and is a precipitate.

Practical activity SBA 10.1: Which food groups are present in a food sample? page 348 Substance to be tested

Many monosaccharides can be joined to fo rm or syn thesise a very large molecule called a p o lysacch aride. Since condensation (deh ydration) reactions are involved in the synthesis of these polymers, these reactions can be called dehydration synthesis. Starch, cellulose and glycogen are examples of polysaccharides. They can form very large molecules.

Food tests Table 10.4 sh ows the standard tests which can be made on a sample of food indicate each of the main food grou ps.

Test

to

Observations 3

Reducing sugars - all monosaccharides (e.g. glucose, fructose) and some disaccharides (e.g. maltose)

(i) Benedict's test: 2 cm of the solution to be tested The initial blue colour of the mixture turns green and 3 is put into a test-tube. 2 cm of Benedict's solution is then yellow and may form a brick-red precipitate. then added. The mixture is shaken and brought gently to the boil. (ii) Fehling's test: 2 cm3 of the solution to be tested is Same as Benedict's test. put into a test-tube. 1 cm3 of Fehling's A is added. 1 cm 3 of Fehling's B is then added. The mixture is shaken and brought gently to the boil.

Non-reducing sugars (e.g. sucrose)

1 cm3 of the solution is put into a test-tube and 1 cm 3 of dilute hydrochloric acid (HCI) is also added. The mixture is bolled for 1 minute. 1 cm 3 of aqueous NaOH (NaOH solution) is added, followed by 2 cm3 of Benedict's solution. The mixture is then shaken and boiled gently.

A red-brown precipitate results as the sucrose is hydrolysed to fructose and glucose by the acid. Fructose and glucose are reducing sugars, so Benedict's test then can be carried out.

Starch

2 cm3 of 1% starch solution is added to a test-tube. A few drops of iodine in potassium iodide (12KI) solution is added.

A blue-black precipitate results.

(continued)

103

Life Processes and Disease

Substance to be tested

Test

Observations

Protein

Biuret test: 2 cm3 of protein solution is put into a testtube, 2 cm3 of 5% potassium hydroxide (KOH) is then added. The mixture is stirred and 2 drops of 1% copper sulfate (CuSOJ is added.

A mauve or purple colour slowly develops.

Fats

Ethanol test: 2 cm 3 of fat solution or oil is put into a A cloudy white suspension can be seen when the water test-tube. 2 cm3-of absolute ethanol is then added. is added. The mixture is shake~ vigorously and 3 cm3 of water is added. Grease spot test: a drop of the sample is dropped onto A permanent translucent spot is seen on the paper. a piece of paper. Table 10.4 Tests for the main food groups.

Inorganic nutrients trace element

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Give three named examples of foods which can be eaten to obtain (i) organic nutrients (ii) inorganic nutrients.

Minerals are inorganic nutrients that are required in small amounts for good health and development. Some are required in only trace (very small) amounts for good health and thus are called trace elements . Table 10.5 shows some mineral elements required by plants and table 10.6 shows some minerals required by humans. Element

One function

Deficiency effects

nitrogen (N) (absorbed as necessary for proteins nitrates)

small yellow leaves and poor growth

magnesium (Mg)

necessary for chlorophyll

leaves yellow between the veins

iron (Fe)

necessary for chlorophyll

new leaves yellow between veins

calcium (Ca)

necessary for cell walls

poor stunted growth, leaves yellow, terminal buds die

potassium (K)

maintains the salt balance in cells yellow/brown edges on leaves, edges wither, plant dies early

sulfur (S)

makes proteins

young leaves small, thin, yellow between green veins

phosphorus (P)

makes some proteins

poor growth, small reddish-brown leaves

Table 10.5 Some elements needed by plants for healthy growth. Mineral

Sources

Functions

calcium

milk, cheese

formation of bones and brittle bones and teeth teeth

iron

red meat, green leafy vegetables

formation of haemoglobin

Symptoms of deficiency

anaemia - tiredness, lack of energy because of a reduction in the number of red blood cells

(continued)

104

Ingredients: sugar, enriched bleach flour (wheat flour, niacin, reduced iron, thiamine mononitrate, riboflavin, folate) food starch-modified, partially hydrogenated soybean and cottonseed oils, leavening (sodium bicarbonate, sodium aluminium phosphate), emulsifier (propylene glycol monoester, monoglyceride, sodium stearoyl lactylate), salt, natural and artificial flavours, citric acid, guar gum, xanthan gum, isolated soy protein, whey. Blueberries: blueberries, water.

Agure 10.3 There are many additives in the ingredients of manufactured food as seen in this list of ingredients for a blueberry muffin mix.

Mineral

Sources

iodine

sea foods, iodised table formation of the salt hormone thyroxin

goitre (adults) - reduced metabolic rate, swelling of the thyroid gland cretinism (children) - physical and mental retardation

phosphorus

meat, fish

brittle bones and teeth

Functions

combine with calcium in the formation of bones and teeth

Symptoms of deficiency

Table 10.6 Some elements needed by humans for healthy growth.

Food additives Many additives a re used in preparing food, for many different reasons (figure 10.3) . Food additives may be natural or artificial. Common natural additives include sugar, corn syrup and pepper. Common artificia l additives are some flavours and sweeteners. The major groups of additives include the following.

Dyes and colourings These are purely cosmetic and rarely add nutritional va lu e. Tartazine is used to give a yellow colour to foods and drinks, for example, orange juice, fish fingers. It does, however, h ave some adverse effects as it is associa ted with: • h yperactivity in children; • a llergic reactions; • adverse efiects on asthmatics.

Preservatives These make food less susceptible to bacterial infection, so food can be kept for longer periods of time in tins. packets, spreads and bottles without spoiling. When food is produced and packaged it may travel thousands of miles, over several months, before it is used. The health of the general population has improved because preservatives reduce the risk of bacterial poisoning. They are perhaps the most easily justified additive, but only make up l % of a ll additives used.

Synthetic flavourings During preparation, food can lose some of its flavour, so these are added to improve or even ch ange the flavour.

Flavour enhancers and sweeteners Saccharin is often used to sweeten prepared foods. Monosodium gl utamate (MSG, Ali-jo-moto, Vet-sin) is a commonly used flavour enhancer found in processed foods including soups, fast foods and Chinese foods. Young children and pregnant and lactating women are advised not to eat foods containing MSG as it may be related to asthma, attention deficit disorder, acute headaches, extreme mood swings, depression and paranoia.

Propellants Carbon dioxide and nitrous oxide may each be used to form an aerosol, forcing food out of contain ers.

105

Life Processes and Disease

Acids These are added to give a sour taste to prepared food.

Firming agents Aluminium sa lts are used to retain crispness; gums increase the thickness of sauces and soups.

A balanced diet 41% staples (a) cereal grains (b) starchy fruits, roots and tubers

21% legumes and nuts

11 % dark green leafy and/or yellow

vegetables

.. ~#

11%

i'if

it/ f

11 % fruits

food from animals

Rgure 10.4 Pie chart showing the relative proportions of foods in a balanced diet.

~

ll!Q3 V'--J Describe a meal which includes all the nutrients necessary for good health.

We ca11 group all the foods available to humans into six food groups. • Staple foods -These include cereal grain (e.g. rice), cornmeal, wheat flour, oats, sta rchy fruits, roots, tubers. • Peas and beans (legumes) - These include red beans, pigeon peas, black eyed peas, broa d beans. • Dark green, leafy vegeables and yellow vegetables - Cabbage, pak choi, lettuce, spinach are leafy examples; pumpkin and carrot are examples of yellow vegetables. • Foods from animals - Fish, poultry, meat, milk, eggs, cheese are all food s from animals. • Fruits - Citrus fruits, bananas, apples are all examples. • Fats - These include oils, butter, margarine and food with a high proportion of fat such as ca kes, biscuits. Figure 10.4 shows th e components of a balanced di et. Each block represents a food group and the size of the block indicates the proportion of the ctiet which. that food group should constitute.

Balanced diet related to age, sex and activity of an individual Nutritional requirements vary with age, sex and activity.

Energy requirement Energy requirements are generaJJ y greater for men (figure 10.5). They usually have more muscle, relatively less fat and weigh more than women. In women, the energy requirements are higher during the last three months of 12 000 - Energy (kcal per day) emae • mae • pregnancy. Extra energy is needed for growth of the fe tus and deposition of fat 10 000 in preparation for breast feeding. This requires extra energy because the energy 8000 needed by the baby for rapid growth in early postnatal life comes from its 6000 mother's milk. Energy requirements for a growing 4000 individual increase up to about the age of 18 years, when the energy requirements 2000 are the greatest. The requirement for energy then decreases as the person 0 1-3 I 7-10 I 15-10 I 23-26 I 31-34 I 39-42 I 47-50 I 60-64 I 75+ gets older. 4-6 11- 14 19--22 27-30 35-38 43-46 51-59 65-74 Physical activity of an individual Age (years) varies with both occupation and leisure. Rgure 10.5 Ener requirements for men and women vary as they get older Some people are mostly sedentary

1

106

10 · Feeding and Digestion

(sitting for much of the time, such as in an office) and others are very active. Energy req uirements for different levels of activity can vary greatly.

Protein requirement Men require more protein than wo men from around the age of 11 years onwa rds. This is when the muscle-to-fat ratio starts to differ because of the development of secondar y sexual characteristics. Women start to store fat in their hips and breasts and men develop more muscle, especia lly on their shoulders and legs. Extra protein is required by women during pregnancy and breastfeeding.

Requirements of minerals and vitamins Mineral intake is especially important during pregnancy and lactation. The mother's diet must con tain sufficient iron, calcium, vitamin C, folic acid, and everything needed to make the baby's tissues inccluding blood, bone and muscle. Extra folic acid may be given to the mother to red uce the risk of spina bifida in the baby.

Malnutrition malnutrition

>

Malnutrition means bad nutrition, and can be applied to under-eating, overea ting and bad eating habits. Malnutrition is the cause of many diseases like deficiency diseases, obesity, hea rt diseases and anorexia. Education on ba lanced di et and good health is very important in preventing the occurrence of many diseases.

Under-eating

Figure 10.6 A child suffering from kwash1orkor and marasmus.

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Starvation is one kind of under-eating and is most often associated with developing countries. It means not eating enough food to supply the energy requirements for daily activHies. Also, not enough protein and vitamins are eaten which are necessary for growth, development, resistance to infecti on and a healthy life. Marasmus and kwashiorkor are common conditions ca used by under-eating (figure 10.6). Some signs and symptoms of marasmus and kwash.iorkor: • very underweight (less than 60 % fo r age); • thin muscles, thin a rms and legs; • redu ced growth may lead to reduced mental development; • reduced resistance to infection; • sometimes swelling of the body tissues with fluid (oedema); • shru nken features giving the face the appearance of an o ld person; • hair becomes thin, sparse and easily removed; • rough skin; • little interest in surrou ndings. Anorexia is another kind of under-eating but is associated with developed countries. It is the voluntary refu sa l to ea r and is most commonly found in teenage girls, though teenage boys can also get this illness. It is as much a psychological illness as a physical one, because the refusa l to eat is based on a poor self-image. The patient continues to think that they are fat even when they are underweight. Recovery requires treatment for the psychological condition as well as an improved diet.

107

Life Processes and Disease

Obesity FMMIMJ

r;rrmmm CHAPTER 13

Obesity results from over-eatin g, especially of fatty foods, a nd a lack of exercise. Excess fat accumulates in the body and body mass increases to well above norma l (figure 10.7). Obese people are predisposed to many diseases like diabetes (see below), hypertension (high blood pressure, chapter 13 ), coronary heart disease, arthritis, cancer and stroke. Over-earing can be prevented by earing sensibly, and engaging in regular aerobic exercise.

Heart diseases and cardiovascular disease

corona

heart disease

>

Some diseases of the heart and cardiovascular system develop slowly after years of living on a diet of fatty foods and not much exercise. Atherosclerosis is a disease of blood vessels . It is a thickening of the inner layers of artery walls, even tu a lly the artery may become blocked. If the affected artery is the coronary artery, the heart muscle is not supplied with food and oxygen and char part of the hean dies. This could result in a heart anack (coronary heart disease). A similar blockage in a blood vessel in the brain results in a stroke. The rough surface of the thickened wall could also encourage formation of a blood clot which may block blood vessels.

Diabetes

Rgure 10.1 A person 1s described as 'obese· if they weigh at least 20% more than the average for someone their height. holozoic nutrition

>

Diabetes is a group of metabolic diseases in which a person has high blood sugar, either because rhe pancreas does not produce enough insulin, or the body cells do not respond to the insulin that is produced. Management of diabetes concentrates on keeping blood sugar levels as close to normal as possible, which can usua ll y be accomplished with diet, exercise and appropriate medication. Obesity, high blood pressure and lack of regu lar exercise accelerate the harmful effects of diabetes.

Holozoic nutrition Mammals, including humans, feed by taking in or ingesting organic food. This particular type of heterotrophic nutrition is termed holozoic nutrition, and includes ingestion, digestion, absorption, assimilation and egestion. • Ingestion - The act of taking in food (into the mouth in humans). • Digestion - The process of breaking down large, complex, insoluble material into sma ll, simple, soluble molecules. The teeth physically break the food into pieces, and enzymes then chemically break down the large molecules into smaller ones. • Absorption -The diffusion of soluble fo od molecules (glucose, amino acids, fatty acids, glycerol, vitamins, minerals and water) into the bloodstream. • Assimilation - When these food molecules are taken from the blood and used by the body cells for respiration, growth and development. • Egestion - The process by w hich the undigested part of rhe good is removed from lhe body. It is also known as defecation.

Digestion The physical action of teeth physical digestion

108

>

Teeth h elp with the physical breakdown or mechanical breakdown of food. This is ca lled physical digestion. The structure of a typical tooth is shown in figure 10.8.

10 · Feeding and Digestion

I

-~---- enamel

(hard material)

crown

~

gum

-----

root

-

-

- - - -- pulp cavity (contains blood vessels and nerve endings) - - - - - cement (holds the tooth in the bone)

- - - - - jaw bone - - - - - - - nerve

Rgure 10.8 A section through a tooth showing the general structure.

Mammals differ from other animals in that they have more than one type of tooth. In humans, there are four kinds and Table 10.7 summarises their structures and functions. Figure 10. 9 shows the position of the different teeth in the mouth. Type incisor

canine

Shape

Function chisel-shaped for cutting 1 root

pointed or dagger-shaped 1 root

cutting food biting off bits of food

grasping and tearing food (well developed in carnivores for tearing flesh)

premolar

flat with cusps or bumps on crush and grind food the fairly broad surface 2 pointed cusps 2 roots

molar

flat, with cusps on the broad surface 4 or 5 cusps 2 or 3 roots

large back teeth to crush and grind food

Table 10.7 The shape and function of human teeth.

mJU:lrffimlJ

Milk t eeth are the first set of teeth in humans. They appear singly or in pairs from the time a child is approximately 3 months old. By age 3 years, most children have about 20 teeth. These begin to fall out when a child is about 7 years old.

109

Life Processes and Disease

permanent teeth

wisdom teeth

>

>

Permanent teeth are the teeth which replace the ones that have fallen out. An additional 12 new teeth also erupt which make up the complete set of permanent teeth by about age 17. Most adults have 8 incisors, 4 canines, 8 premolars and 12 molars. The 4 molars at the end of the jaw are the last set to grow through the gum and are called wisdom teeth.

molars,

Q:ioot-'T--1::>,._,..,r--duct from salivary gland

The u se of fluorides Figure 10.9 There are four types of teeth in humans. The use of fluorides in toothpaste or in water supplies helps to prevent tooth decay in humans. Fluorides are compounds of the element fluorine which improve resistance to tooth decay by hardening the enamel. When permanent teeth are developing in children, the use of fluorides is effective in helping these 'new' teeth resist decay. Compounds of fluorine can act as serious pollutants in the environment. In the production and extraction of aluminium from bauxite, sodium aluminium fluoride (Na 3AIF 3 ) is used to lower the melting temperature of alumina from 2050 °C to 950 °C, so that less energy is used. The exhaust gases from the manufacture of aluminium then contains fluorides. Fluorides seem to affect trees, and on grass they can enter food chains. The teeth and bones of grazing animals are affected badly. Although fluoride provides resistance to tooth decay in humans, an excess in the environment can be harmful. Also fluorides can be dangerous to young children and they should never swallow fl uoridated toothpaste.

The chemical action of enzymes in digestion Despite the action of teeth in breaking down food physically, food must also be chemically broken down. Chemical digestion involves enzymes . Enzymes are organic catalysts, which means they speed up chemical reactions occurring in living cells. During digestion, enzymes speed up the rate at which the large, insoluble food molecules are broken down into small, soluble food molecules.

chemical digestion

Practica] activity SBA 10.2: The action of an enzyme,

enzymes (amylase)

polysaccharides

page 349

disaccharides + monosaccharides

en zymes (protease)

proteins

amino acids

enzymes (lipase)

lipids------+ fatty acids +glycerol

£?.Sb

IT:Q4' V'--1

Define the terms 'physical digestion' and 'chemical digestion'.

110

There are thousands of enzymes bu t all have similar properties. • They are all proteins. • Each enzyme is specific for the type of chemical reaction it speeds up. • They are required in small amounts. • They are inhibited or prevented from working by poisons like cyanide and arsenic.

10 ·Feeding and Digestion

• They work best at a particular temperature called the optimum temperature (Figure 10.10). • They are denatured or destroyed by high temperatures. • They work best at a particular pH, called the optimum pH (Figure 10.11 ).

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The substance that the enzyme breaks down is called the substrate and the substances that are made are known as the products.

Digestion and absorption along the alimentary canal .--

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What is the important product of respiration? What are the waste products of respiration?

r!u:IJ

Respiration or cellular respiration occurs in a series of steps, each of which is catalysed by enzymes. The overall process can be summarised in words or by the respiration equation below: glucose + oxygen -+ energy + carbon dioxide + water C6 H 12 0 6 + 60 2 -+ energy + 6C0 2 + 6H2 0 During aerobic respiration, glucose is broken down completely into carbon dioxide and water. At each step in the breakdown of glucose, energy is released. This is used to con vert a d1emi cal called ade nosine diphosphate (ADP) into adenosine triphospbate (A T P ). Ead1 molecule of ATP acts as a little 'packet' of energy. The energy can be stored and used later when needed. There are m an y advantages of storing and using energy in small packe ts like this. • The energy ca n be released from ATP wherever and whenever it is required by a cell.

123

Life Processes and Disease

• The energy can be released rapidly. • Energy is not wasted. A large amount of energy is released by oxidising one glucose molecule and ma ny ATP molecules are formed. A cell may not require very much energy a t on ce. By storing the energy in small packets in ATP molecu les, the cell can use sm all amounts of energy as required (figure 11.3). • The energy can be used to drive many different chemical reactions rapidly. • Energy can be stored as ATP in one part of a cell and transpo rted and used elsewhere without causing reactions in between . adenosine (

ADP

r"'"--•: energy \..+ATP

c

ATP

ADP oxidation ,. . . . _ _ " energy ( . of ATP glucose 1""'' - - • . energy

,....,_...,. .energy

high energy bond ADP

I

ADP (.

ATP

the energy from the breakdown of glucose is stored in this high energy bond

ATP

ATP is a packet of energy!

Rgure 11.3 The oxidation of glucose results in the formation of many molecules at ATP.

Ene rgy production and utilisation are very effici ently and carefully controll ed by the cell.

Where does aerobic respiration occur? mitochondrion >

~ ll:Qll

Respiration occurs in an organelle called the mitochondrion (figu re 11.4). Mitochondria are present in all cells, animal and plant, and are sometimes referred to as the 'power houses' of the cell. The en ergy stored in ATP (aden osine triphosphate) is released when it is converted back to ADP (adenosine diphosphate).

v......i

Where does respiration occur?

co2 transported to lungs

~

11:Q5 v......i Give three reasons why it is advantageous to store energy in small packets.

0 2 transported to cell

--+- - energy is released during respiration in the mitochondrion

_ _ _...,. energy ('' -"'-) ATP

+

energy used by the cell

+

p ADP

+ +

p phosphate

Figure 11.4 Energy can be released from ATP made during respiration in the mitochondrion.

124

11 · Respiration

Anaerobic respiration anaerobic respiration

>

Respiration can also occur without oxygen and this type of respira tion is called anaerobic r espiration. Both anaerobic and ae robic respiration involve the breakdown of glucose (figure 11.5). However, in anaerobic respiration, it is n ot completely broken down.

(a) Plant and animal cells can respire aerobically.

ID1. > '"""' 0 oxygen

oxygen ~ Q

/

/ glucose

water

I) energy •

"' "

. / water

~

oa

Lactic acid is a waste product of this reaction. It builds up in the muscles and causes them to ache (fi gure 11.6). This is often called fatigue . After exercise, the body has to get rid of the lactic acid as quickly as possible. This is done by using oxygen to change it back to a chemical like glucose so that it can be broken down completely in aerobic respiration. When anaerobic respiration occurs in muscles it is in addition to aerobic respiration and not in place of it. A person continues to 'breathe hard ' or pant for some time after exercise as oxygen is needed to get rid of the lactic acid. The oxygen required to get rid of the lactic acid is called the oxygen debt (figure 11. 7).

~

cell during __ _. used for contraction etc. anaerobic respirat~ energy (smaller amount) - - - - -

IT:Q8 V'-1

Q

What is alcoholic fermentation and what are two of its uses?

~lactic

e.g. muscle cells, during prolonged strenuous exercise

alcoholic fermentation

sugar

fermentation

• sugar from barley seeds • cane sugar or molasses

after some time

1

>

ethanol + carbon dioxide fermentation fermentation

t

series of reactions leading to breakdown to C02 + H20

Figure 11 . 7 The oxygen debt is the oxygen needed to break down the lactic acid formed during exercise.

Anaerobic respiration in yeast

rum

During anaerobic respiration in yeast ethanol and carbon dioxide are produced as waste products. Ethanol is an alcohol and the process is known as alcoholic fermentation . Yeast is very important in the making of alcohol and bread (figure 11.8). The ethanol can be produced in many ways to make a wide range of alcoholic drinks, including beer and wine, which are enjoyed by humans. The production of carbon dioxide is used in bread-making to make dough rise. The carbon dioxide produced by the yeast as it respires accumulates inside the dough in small pockets. The dough is seen to get bigger or rise as the gas expands with warmth. Ethanol is also produced but in small quantities - it evaporates when the bread is baking in the oven.

• dough rises as bubbles of C02 get caught in the dough • baking kills the yeast and evaporates the ethanol

126

acid

beer

• flour and yeast dough, after kneading flour has starch which is broken down to maltose • yeast uses the maltose as a source of sugar and fermentation occurs

Agure 71 .B Uses of fermentation.

I

~O'YQOC

11 · Respiration

~ IT:Q9 \.../'-I

Sometimes bacteria can be found in canned foods or tins, despite the fact that the cans and tins are sealed so that no air can enter. How is this possible?

Anaerobic respiration in bacteria Some bacteria also respire anaerobically. Like animal cells, they make lacti c acid as a waste product. We m ake use of this in th e manufacture of yoghurt and cheese (figure 11.9). milk contains lactose

'

pasteurisation heat treatment (90°C) to kill disease-causing organisms

'

inoculation cooled to 40°C and a 'starter' culture of bacteria added e.g. Lactobaci//us bulgaris

'

fermentation incubated in 1arge vats (40 ° C for about 5 hours)lactose converted to lactic acid producing natural yoghurt

' ' '

cool, add fruits, etc.

package and distribute at 4.5 ° C the bacteria remain alive but no more fermentation occurs at this temperature

store at 2 ° C

Figure 11.9 The manufacture of yoghurt depends on the anaerobic respiration of Lactobacillus bacteria.

I

I

• Anaerobic respiration releases a small amount of energy without the use of oxygen . • Humans usually respire aerobically but their muscle cells can respire anaerobically during prolonged exercise. • Lactic acid is produced during anaerobic respiration in animals and creates an oxygen debt which has to be repaid. • Anaerobic respiration in yeast produces ethanol which is used in the alcohol industry and carbon dioxide which is used in making bread. • Anaerobic respiration in bacteria is used in the making of yoghurt and cheese.

127

Life Processes and Disease

Du ring respiration, the energy from the food eaten by an organism is made available. This en ergy can be used to carry o ut all the processes of life: m ovem ent, growth, reproduction, and so on . ITQ2 Animal cells respire all th e time because th e animal is in constant need of en ergy. Plane cells also respire all the time. During the day, while sunlight is available, plants also photosynthesise, but they never stop respiring. ITQ3 The important product of respiration is energy, which an organism n eeds to carry ou t th e characteristics of life. The waste products of respiration are carbon dioxide and water. ITQ4 Respiration occurs in the mitoch ondria of cells. ITQ5 Energy is released only when necessary; only as mu ch energy as is needed is used; energy is released rapidly when it is needed. ITQ6 Organism s that respire aerobically include humans and birds (there are m an y others). Organisms that respire anaerobicaIJy include yeast and tapeworms inside the intestine. Yeast can also respire aerobically if it has access to oxygen . ITQ7 Human muscle cells use anaerobic respira tion during prolonged exercise, when oxygen cannot be supplied fa st enough for sufficient aerobic respiration to take place. As a result, energy is produced to do the work n ecessar y when exercising, although less en ergy is produced from each glucose molecule than in aerobic respiration. ITQ8 Alcoholic fermentation occurs when yeast respires anaerobically to produce ethanol. This process is important in the bread, beer and win e industries. ITQ9 The bacteria tha t are fo und in cans and tins respire anaerobically. This means they do no t need oxygen to release energy for all their living processes. So the fact that there is no air in the ca n does not affect them ; th ey can live in that environm ent. ITQ1

Examination-style questions {i)

Respiration is described as a characteristic of life. What is the importance of respiration to plants and animals? (ii) Although respiration occurs in a series of steps, it can be summarised in an equation. {a) Write the equation. (b) Describe how energy is made and stored. (c) Discuss three advantages of storing energy in this way. (iii) A form 2 student remarked that she had not eaten any food for breakfast or lunch and that she felt 'weak'. Explain to her why she is feeling weak and why it is important not to skip meals. 2

128

{i) Using a table, outline the differences and similarities between anaerobic and aerobic respiration. {ii) Explain the importance of anaerobic respiration in humans. {iii) Define: {a) oxygen debt; {b) alcoholic fermentation. {iv) Outline the importance of anaerobic respiration in: {a) the bread-making industry; {b) the alcohol industry. (v) Describe how yoghurt is made.

11 ·Respiration

3

Diagrams A and B below show investigations to demonstrate the products of respiration and photosynthesis. A

0

0

0

0

0

0

0

0

0

B

0

~ (i)

Copy the diagrams and, using annotated labels only, complete diagram: (a) A to show how carbon dioxide is produced during respiration; (b) B to show that oxygen is produced during photosynthesis. (ii) The diagrams below are investigations to show that oxygen is used up during respiration. (a) What is the importance of soda lime? (b) How does the investigation show that oxygen is being used up? (c) Calculate the rate at which oxygen is being used up. {d) What would you see if more organisms were put in the flask and what would this indicate about the total amount of respiration? at the start

capillary oil drop tube ----------- -

wiregauze

after 30 minutes

I" '" I"' ""'l""""'l""""'I I

small animals, e.g. woodlice or millipedes

129

G aseous Exchange

0

understand the function and mechanisms of breathing and gaseous exchange in humans

0 0 0 0

understand the function and mechanisms of gaseous exchange in plants identify characteristics common to gaseous exchange surfaces discuss the effects of cigarette smoking in humans understand marijuana addiction gaseous exchange

I

r respiratory system

'

respiratory surface

rcigarette smoking

inhalation

exhalation lungs

humans

leaf

plant

gill

fish

Amoeba

cell membrane

characteristics thin large surface area rich blood supply constantly moving transport medium

~ lltQ·1 V'-..J

(i) What is gaseous exchange? (ii) List two places where it occurs in the human body.

Importance of gaseous exchange in humans Respiring cells need a continuous supply of oxygen. They must also be able to get rid of the carbon ctioxide that is being produced constantly. The blood is the means by which oxygen and carbon dioxide are transported to and from cells. At some point, blood has to pick up oxygen and give off carbon dioxide, that is, exchan ge these two gases. In humans, gaseous exchange takes place in th e lungs (figure 12. 1).

12 ·Gaseous Exchange

mouth, nose air inhaled - has more oxygen than air exhaled blood rich in oxygen flows to the body cells respiring body cell uses oxygen , produces f carbon dioxide

air exhaled has more carbon dioxide than the air inhaled GASEOUS EXCHANGE

oxygen net diffusion } into the blood blood vessels carbon dioxide net in the lungs diffusion out of the blood

____/

~

._...._..._

blood rich in carbon dioxide flows towards the lungs

Figure 12. 1 The role of the lungs in exchanging gases with the environment.

Mechanism of gaseous exchange in humans FWl:Z•l!IJ

The human respiratory system is involved in the exchange of gases in humans. The lungs are very important and are made up of many tiny air spaces or air sacs called alveoli (figure 12.2) .

-1----,F---

-+--r---

rib - l'---internal intercostal muscle

-

trachea opens to mouth and nose

larynx (voice box)

----r

-+----~

Figure 12.2 The human respiratory system.

131

Life Processes and Disease

l!iB9•!¥1J

Air enters the nose and/or mouth and moves down the trachea (windpipe). The trachea is supported by C-shaped rings of cartilage so that it is kept 'open' a t all times. The trachea then divides into two bronchi, the right and left. These a re also supported by rings of cartilage. Each bronchus branches into smaller and sma ller tubes called bronchioles. At the end of each bronchiole are the many tiny sacs called alveoli (figure 12.3). Gaseous exchange occurs in the alveoli.

bronc hiole > leU.letijUll'-11

ring of cartilage, supports the soft tissue of the trachea and keeps the trachea 'open' so that air can pass easily

bronchus branches into smaller and smaller branches

~

1:r.Q2 V'--J

What is the importance of the rings of cartilage in the wall of the trachea?

~

alveolus or air sac found at the end

l:r.Q3 V'--J Describe the passage taken by an oxygen molecule from the air to a capillary in the lungs.

site of gaseous exchange

Figure 12.3 The route taken by air into and out of the lungs.

The walls of th e alveoli are the gaseous exchange surfaces or the respiratory surfaces. The smallest blood vessels, capilJaries, are closely wrapped around each alveolus (figure 12.4). Blood is thus brought to and taken away from each alveolus. Oxygen diffuses across the walls of the alveolus into the capillary and the bl ood in the capilla ry becomes oxygenated. Carbon dioxide diffuses from the capillary into the alveolus and is exhal ed out of the body. The walls of the alveolus and capillary are very thin (onl y one cell across) so that diffusion can occu r readily (figure 12.5). capillary, transporting blood that has little oxygen (deoxygenated blood) to and from the alveolus - blood has high concentration of carbon dioxide

capillary transporting oxygenated blood from lungs blood has low concentration of carbon dioxide

t

deoxygenatecl blood

air

~--"i.---

capillary (the wall is one cell thick)

alveolus (the wall is one cell thick)

section of one alveolus

network of capillaries surrounding alveolus - gases are exchanged here

Figure 12.4 The blood supply to one alveolus.

132

flow of blood in capillary

Figure 12.5 Gaseous exchange between an alveolus and the blood in a capillary.

12 ·Gaseous Exchange

~ l:fQ~ \.../'--)

list the difference between the blood in the capillary coming to, and the blood leaving, the alveolus in figure 12.5.

mm

inspiration )

The continuous exchange of gases in the lungs is extremely important. Body cells can obtain a constant supply of oxygen for respiration and the carbon dioxide that is constantly being produced is exhaled out of the body. Gaseous exchange also occurs at the level of the cells. Here oxygen leaves the blood and diffuses into the cells. Carbon dioxide moves in the opposite direction (figure 12.1). The trachea is Lined with mucus, a slimy substance which traps and holdsr dust and microorganisms. The tra chea is also lined with microscopic hair-like extensions called cilia. These beat in a wave-like manner, moving the mucus containing dust and microorganisms upwards and out of the lungs. Pathogens can enter the lungs with air as it is breathed in. The mucus and cilia afford some protection by trapping and moving them out of the lungs. If an irritating substance like dust is breathed in, this can stimulate a sneeze during which the irritant is ejected out of the lungs. The other parts of the respiratory system, namely the ribs, intercostal musdes and diaphragm, are also involved in gaseous exchange. They help to move air in and out of the lungs. Breathing in is called inspiration and breathing out is called expiration. Table 12.1 compares the constituents of inspired and expir ed air. Table 12.2 (overleaf) compares inspiration with expiration. Constituent gases

Inspired Expired air air

Reason for difference

oxygen

21%

16%

some of the oxygen is used by the cells of the body during respiration

carbon dioxide 0.04%

4%

carbon dioxide is made by the cells and is transported by blood to the lungs

nitrogen

78%

not used

78%

water content variable

usually higher the alveolar surface has a thin film of moisture to aid than when gaseous exchange, and some of this evaporates inspired

temperature

usually higher air is warmed by the body heat while within the body than when inspired

variable

Table 12.1 A comparison of inspired and expired air.

133

Life Processes and Disease

Inspiration

Expiration

• External intercostal muscles contract (internal intercostals relax) and the ribcage is raised. • The muscles of the diaphragm contract and the diaphragm moves downwards.

• Internal intercostal muscles contract (external intercostals relax) and the ribcage is lowered. • The diaphragm muscles relax and the diaphragm moves upwards.

air in

air out ribs move up and out

diaphragm contracts (moves down)

diaphragm recoils upwards

volume decreased

volume Increased

air out

air in

vertebral

sternum moves upwards and forwards

diaphragm contracts

sternum moves downwards and backwards

diaphragm recoils

• These two movements increase the volume of the thorax.

• These two movements decrease the volume of the thorax.

• The pressure inside the thorax is lowered to below atmospheric pressure. This pushes air into the lungs so they expand.

• The pressure inside the thorax increases which squeezed the lungs.

• •Air rushes into the lungs through the mouth/nose and trachea.

• Air is pushed out of the lungs. It passes out through the trachea and the mouth or nose, out of the body.

Table 12.2 A comparison of inspiration and expiration.

134

12 · Gaseous Exchange

Importance and mechanism of exchange in plants

CHAPTERS

X

~

IT:Q5

vv

What is breathing and why is it important? (ii) Which muscles are involved in breathing in humans?

(i)

~

The leaf is the respiratory surface or gaseous exchange surface. There are tiny pores called stomata on the underside of the leaf through which the gases pass. From the air space inside the leaf, the gases diffuse into and out of the plant cell. The gases move down their concen tration gradients (chapter 8). During the day, plants photosynthesise and need carbon dioxide. Oxygen is a waste product and must be removed. Plants respire all the time but, during the day, photosynthesis is also being carried out. More oxygen is made in photosynthesis than is used up in respiration and more carbon dioxide is used than is made. So there is a net flow of oxygen out of the leaf and a net flow of carbon dioxide into it. At night photosynthesis stops because there is no light but respiration continues. Oxygen moves into the leaf and carbon dioxide moves out figure 12.6). Day

IT:Q6

gase~us

Night

vv

Which gases leave and enter a leaf at: (i) 12 noon? (ii) 12 midnight?

• respiration occurs • no light, therefore no photosynthesis

6C02 + 6H20 -+ CsH120 5 + 602 photosynthesis equation

C5H120e + 602 -+ energy + 6H~ + 6C02 respiration equation

Figure 12.6 The net flow of gases diffusing in and out of a leaf during the day and at night is different.

Characteristics common to gaseous exchange surfaces gaseous exchange surface

> Gaseous exchange or respiratory surfa ces are those surfaces where the exchange of oxygen and carbon dioxide occur. These surfaces must have certain characteristics that encourage: • a lot of gaseous exchange to take place; • gaseous exchange to take place quickly; • gaseous exchange to take place continuously. This means that organisms respiring aerobically can get a constant supply of oxygen and remove carbon dioxide. Without oxygen, ce lls die and carbon dioxide, if allowed to accumulate in cells, could poison and kill them.

135

Life Processes and Disease

Adaptations for efficient gaseous exchange Large surface area For gaseous exchange to take place quickly and in large amounts, respira tory surfaces must have a large surface area or a large area over which the exchange of gases can occur (figures 12.7, 12.8 and l2.9). blood brought to the alveolus - it is low in 0 2 and high in C02

blood taken away it is rich in 0 2 and low in C02

thin wall of capillary -

layer of moisture - oxygen dissolves in this moisture and there is always a high concentration of oxygen next to the capillary blood flows constantly

lungs, highly folded to Increase surface area

Figure 12.7 Adaptations of the lungs in humans for efficient gaseous exchange.

stiff gill rakers, which filter out food particles from water as it passes over them; the food particles are then swallowed

bony gill bar, supporting the gill

leaf is thin and flat for large surface area

I

oxygen leaving leaf is 0 2 blown away, leaving a low concentration around the leaves

)

wind

soft, dark red gill lamellae, where gas exchange takes place -

'" 7

g

a to ugh waterproof layer and may also be covered with hairs, fur and feathers. It is impossible for oxygen to diffuse to cells inside the body of a m ammal or other la rge organism. In an y organism larger than a few cells, any substance needed by a cell within the bocy must be speciaDy transported to the cell. A transport system is necessary to get important and needed substances to every single cell and also to transport waste or toxic substan ces away from every cell. Just to stay alive, a m ulticellular organism requ ires a con stant supply of substances like oxygen and glucose to all its cells. When active, the~e substances are required in even greater amounts. Table 13. l shows some of the substan ces which need to be transported in animals. Substance to be transported

Transported from

Transported to

dissolved food (chapter 1O)

ileum where it is absorbed

cells of the body - to be used for respiration, stored, converted to other materials, etc.

nitrogenous waste (chapter 16)

cells where produced

kidneys to be excreted

oxygen (chapter 12)

lungs where it diffuses into the blood

body cells to be used for respiration

carbon dioxide {chapter 11)

body cells where it is produced in respiration

lungs to be excreted

hormones (chapter 18)

endocrine glands where they are produced

organs where their effects are needed

white blood cells including antibodies

marrow of bones where they are produced

where there are infections or invasions by microorganisms

Table 13. 1 Some substances which are transported in animals.

The circulatory system of humans Blood is the m eans by which substances travel to and from cells. These substances dissolve in blood, w hich is mainly water and diffused into th e cells where they are n eeded. The blood is transported around the body in blood vessels. The heart h elps to push blood a ro und the body.This transport system is called a circulatory system. Most substances dissolve in the plasma, but the red blood cells are specialised to transpo rt oxygen . The circula tory system is made up of three parts: • the heart, which is a pump; • th e blood, w hich is the fluid being pumped a nd contains all the ma terials to be transported around the body; • the blood vessels (like pipes) through which blood flows to get to and from the cells, these are the arteries, veins and capillaries.

The structure of the heart The heart pumps blood so tha t it can get around the body. It pushes blood forcibly thus ca using it to be constantly m oving in the blood vessels. The walls of th e hea rt are made of a special type of muscle, ca lled cardiac m uscle.

143

Life Processes and Disease

cardiac muscle )

fliill!ull i@nrmlm

Cardiac muscle contracts and relaxes regu larly and constantly throughout life. It never grows tired. But it may stop working iI it is not supplied with the substances it needs to release energy - oxygen and glucose. These are supplied via the coronary circulation. The mammalian heart is divided into a right side and left side (figure 13. l ). Each side has two parts or chambers: • the atrium, which receives blood; • the ventricle, which pumps blood away. (a)

left pulmonary artery

right pulmonary ~--­

veins

left pulmonary

(b)

_ ____ left pulmonary artery

r~======!=

::;;;;.---left pulmonary vein left atrium 't--- - - semilunar valves

:"r-- - -

---"~--

- tricuspid valve --~~---== vena cava from - - - - - i 4 - lower part of body I

-

bicuspid valve

-tendon, holds the valve in place left ventricle

r

Agure 13.1 The heart: (a) showing its blood supply and (b) in section.

The action of the heart Deoxygenated blood, that is blood coming from the body cells w here some of the oxygen bas been used in respiration, flows into the right atrium through the vena cava . This blood is also rich in carbon dioxide made during respiration in the cells. The blood must now be tran sported to the lungs wh ere it can load up with more oxygen and offload the excess carbon dioxide.

144

13 ·Transport and Defence in Animals

tricuspid valve }

bicuspid valve }

~

l:f:Q3 \_,)'...J

The heart beats continuously for years. How is heart muscle nourished and supplied with oxygen and glucose?

From the right atrium, blood passes through the tricuspid valve into the right ventride. The walls of the ventricle contract and the blood is pushed into the pulmonary artery and travels to the lungs. There, gases are exchanged. Excess carbon dioxide leaves the blood and diffu ses into the lungs, and oxygen moves into the blood from the alveoli. Oxygen-rid1 blood returns from the lungs via the pulmonary veins and flows into the left atrium. It passes through the bicuspid valve and flows into the left ventricle. The thicker muscular walls of the left ventricle contract strongly and blood is pushed forcefully into the aorta and all the way around the body. Blood therefore flows through the heart twice in one circuit of the body (figure 13.2)

deoxygenated blood from head

deoxygenated -+-- 1 -- - - - + blood to the lungs

deoxygenated blood from body

J-- ,oxygenated blood to head and body

oxygenated blood to the body

Figure 13.2 Blood flows through the heart twice in one circulation atrioventricular valves

>

semi-lunar valves

>

Q9;,

l:tQ~ \_,)'...J

Describe the route taken by a red blood cell from the vena cava to the aorta.

Valves prevent the back-flow of blood in the heart. The bicuspid and tricuspid valve, known as the atrioventricular valves, ensure that blood flows in one direction through the heart only. Tendons attached to the walls of the heart hold them in place. When the ventricles contract, blood pushes back on these valves, forcing them shut. So the blood can only move forward into the pulmonary arteries and aorta . Semi-lunar valves are found at the start of the pulmonary artery and aorta . They prevent the back-flow of blood into the ventricles when they relax.

Heartbeat The heart 'beats' when the muscles of the heart contract and relax. The re are three phases to a heart beat. The sound s heard - 'lub dub' - are the sounds made by the valves closing and blood hitting the valves. The 'lub' sound is made during ventricular systole as blood is forced against the closed tricuspid and bicuspid valves. The 'dub' sound is made during ventricular diastole when blood impacts on the d osed semi-lunar valves in the aorta and pulmonary artery. The third stage, diastole, when blood flows into the empty atria and ventricles, makes n o sound (figure 13.3). The rate of heartbeat is controlled by the ' pacemake r', which is found in the mu scle between the ventricles. It has its own na tural rhythm of stimulating

145

Life Processes and Disease

con tractions, which is usually around 70- 80 beats per minute. This can be speeded up by h ormones such as adrenalin, and by activity. from lungs

atria and ventricles relax (diastole)

Diastole - when all the muscles of the heart relax and blood flows into the heart

atria contract (systole)

Atrial systole - the muscles of the atria contract and force blood into the ventricles

~

I

ventricles contract (systole)

1+I

,I ...._

Ventricular systole - the muscles of the ventricles contract and push blood out of the heart

Agure 13. 3 The three phases of a heartbeat.

~

l:t!QS V'-1

List the three main stages of the heartbeat and explain the importance of each.

FUC§W l!hl·lllblt'B

m1eu

Blood vessels Blood flows through blood vessels to get to all parts of the body from the heart and then from the body back to the heart. There are three kinds of blood vessel: • arteries (and arterioles) whlch carry blood away from the heart; • capillaries which are tiny vessels that pass close to all body cells; • veins, (and venules) which carry blood back to the heart. An artery branches into smaller and smaller vessels called arterioles. These branch into even smaller and smaller vessels, until the vessels are very small and the walls are only one cell thick. These tiny vessels are ca lled capillaries. Capillaries flow in between the cells of the organs and the exchange of substances food, oxygen, wastes, etc. takes place at this level. Capillaries then join up to form larger and larger vessels called venules, which then join to form veins which carry blood back to the heart. Fiigures 13.4 ansd 13.5 show the relationships between the three types of blood vessel. Table 13.2 compares arteries, veins and capillaries.

146

13 ·Transport and Defence in Animals

arteriole from artery

Rgure 13. 4 The relationship between arterioles, capillaries and venules.

cells give out waste products cells take in

Deoxygenated blood full of waste products goes to the heart. then to the lungs and gut to collect oxygen and food.

Diffusion occurs across the capillary network. Oxygenated blood full of food and other useful substances goes to the cells.

Figure 13.5 A network of capillaries surround all the cells of tissues Capillaries

Arteries

wall composed of a single layer of cells

fibrous layer muscle and elastic layer

.,

smalllumen ~

Veins fibrous layer

lumen and red blood cells pass in single file

endothelium ___/one cell thick

• thick elastic walls to withstand the hgh pressure of blood and absorb some of the energy of the pulse

• walls one cell across - thin enough for diffusion to take place easily

• thin elastic walls (do not have to withstand high pressure) (continued)

147

Life Processes and Disease

Arteries

Capillaries

Veins

• carry blood away from the heart

• carry blood to the cells of the tissues and organs

• carry blood towards the heart

• blood pressure is high

• blood pressure decreases along the length of the capillaries

• blood at low pressure

• blood flows rapidly in pulses created by • blood flow is smooth and slow contractions of the ventricles (this is the pulse you can feel most easily at your wrist)

• smooth and slow flow - the large lumen offers little resistance

• carry oxygenated blood, except the pulmonary artery

• as it flows through a capillary network • carry deoxygenated blood, except the pulmonary vein the blood loses oxygen to body cells and gains carbon dioxide

• lie deep within the body

• run through the tissues

• lie close to the body surface

• no valves present

• no valves

• valves prevent the back-flow of blood because the 'push' of the heart is not felt here flow of blood

l

blood can flow in one direction only valve open

valve closed

• blood can flow in one direction only

~

ll'.Q6 \...l'-1 Describe two differences between blood leaving an arteriole and blood entering a venule, having passed across a capillary network. lfl•l•f?M

G#•€•1f§kkU coronary arteries

CHAPTER 16

>

Table 13.2 The main differences between arteries, capillaries and veins.

The circulation Blood leaves the left side of the heart at a high pressure and flows through the aorta , the largest artery, to all parts of the body. When the capillaries reach the body cells, the blood gjves up food and oxygen and picks up wastes, such as carbon dioxide and urea. Deoxygenated blood returns to the heart via the veins whid1 collect into a main vein called the vena cava. From the right side of the heart, blood flows to the lungs to be oxygenated, then back to the left side of the heart. This flow is repeated continuously. The tissues of the heart itself are supplied with oxygen by the coronary arteries . In its circulation throughout the body, blood picks up food (such as glucose and amino acids) from the gut, hormones from endocrine glands, and other vital substances. It also drops off waste products to be excreted, like urea and carbon dioxide, at sites where the body an get rid of them. that is the kidneys (chapter 16) and the lungs (figure 13.6).

.~ \...l'-1

Why does the aorta have the thickest walls of all the vessels in the circulatory system?

148

~

(1~8

(i) What is the pulse? (ii) What is the pulse rate?

13 ·Transport and Defence in Animals

----

-

jugular and - ----,;-7.,._...a.. subclavian veins

- carotid artery (to head)

head and arms

subclavian artery (to arms)

1

subclavian vein pulmonary vein

carotid and subclavian arteries

1

-~----,f----3it::....:

pulmonary -+-..___.,,, artery

vein

pulmonary artery--r--..,,, (to lungs)

1--'ilft--

hepatic portal vein (liver) mesenteric artery (to gut)

. -- -----'-

- --

renal artery (to kidney) hepat ic -f-~~

---~E="

arteries

vein

•- - - --

Iliac - 1---- -vein

-hepatic artery

Iliac artery (to feet) Iliac vein

deoxygenated blood

pulmonary circulation -

oxygenated blood -

systemic circulation -

~

deoxygenated blood

11!09

oxygenated blood -

l../'-J

Describe the route taken by a red blood cell from the renal vein to the hepatic vein.

Rgure 13.6 The circulatory system in humans.

Blood

l~O l../'-J

Describe the differences in composition between blood: (i) in the renal artery and the renal vein (ii) in the pulmonary artery and pulmonary vein. blood plasma >

red blood cells

trunk and legs

white blood cells

>

Blood is the medium by which substances or materia ls are transported. It is made up of about: • 55 % blood p lasma; •

45 % blood cells.

The blood plasma is about 90% water and most of the substances which must be transported are dissolved in it. This includes dissolved food (glucose, amino acids, fatty acids and glycerol), carbon dioxide (as the bicarbonate ion), nitrogenous wastes, hormones and mineral salts (as ions such as Na+, K+, Cl+). The blood cells are of two main types, red and white. There are also fragments of cells called platelets. Table 13.3 (overleaf) summarises the structure and function of blood cells.

149

Life Processes and Disease

Blood cell

Function

Red blood cells or erythrocytes • biconcave disc shape (squeezed in from both sides) gives large surface area for diffusion • have no nucleus so only live for 3-4 months • new cells constantly made in the bone marrow and destroyed in the liver and spleen • contain the red pigment haemoglobin which combines with and releases oxygen readily • 1 mm3 of blood contains about 5 million of these cells

contains haemoglobin, a red pigment which contains iron; no nucleus

transport oxygen combined with haemoglobin, from the lungs to tissues where the oxygen is given up readily t

White blood cells or leucocytes • two main types: phagocytes and lymphocytes Phagocytes • move like Amoeba by pseudopodia - can move through the capillary walls to sites of infection • formed in bone marrow

engulf disease-causing organisms at sites of infection

Lymphocytes • produce antibodies • formed in lymph nodes and spleen

produce antibodies that kill pathogens by causing them to clump together, or neutralise their toxins

Platelets or thrombocytes • cell fragments • no nucleus • formed in bone marrow of lone bones

help blood to clot to prevent loss

~

Table 13.3 Structure and function of blood cells

ll':Q·1 1 V"-J

Protection of the body is one of the functions of blood. List two components of blood concerned with protection and explain how each works.

~

ll':Q-12 V"-J

Describe the process that leads to blood clotting after a cut to a blood vessel.

Carriage of oxygen and carbon dioxide in the blood The respirato ry gases, oxygen and ca rbo n dioxide, are transported aroun d the body in the blood. Most of the carbon dioxide is transported in solution in blood plasma as h ydrogen carbonate ions. Oxygen is carried by the molecule haemoglobin, which is found inside red blood cells. Haemoglobin is a protein that is combined with iron - this gives it its red colour. Each m olecule of haemoglobin combines reversibly with up to four molecu les of oxygen. haemoglobin + oxygen

-+

oxyhaemoglobin

The oxygen is readily given up in the body tissues wh ere oxygen levels are low. The body cells can then use th e oxygen for respiration . oxyhaem oglobin

-+

haemoglobin + oxygen

Red blood cells are so full of haemoglobin that there is no space for a nucleus. That is why th ey only survive for 3-4 mon ths, after which th ey are cleaned out of the blood by the liver.

150

13 • Transport and Defence in Animals

Blood clotting HH•M•E®U When the skin is cue and a small blood vessel is broken , a blood clot ronns

haemorrhage

>

to prevent further blood loss (figure 13. 7). A series or reactions cake place at the site of the cut vessel which results in the formation of fibrin, an insoluble fibrous protein which traps blood cells and pl ugs the gap (figure 13.8). The dot a lso prevents the entry of disease-causing organisms. Loss of blood from a vessel is called a haemorrhage, and losing a lot of blood could result in death. In this case, a blood transfusion can be given to replace blood and save the person's life. (a)

disease-causing organisms may enter

no more loss of blood, pathogens have a barrier once more

I

~ssofblood

clot The cut is sealed with a blood clot

A cut vessel (b)

platelets exposed to air In damaged tissue

l

calcium ions vitamin K prothrombin - - - - - - - - - - - thrombin (inactive protein (active) in the blood)

1

fibrinogen ----"--+~ fibrin insoluble fibres (inactive protein that trap red blood in the blood) cells and form a clot

Figure 13. 7 The formation of a blood clot.

Blood groups

Figure 13.8 Red blood cells trapped In fibrin.

Ft.1e1.111 l(§lff i•U§ellJ

During a blood transfusio n, a person is given another person's blood. Early attempts at transfusion worked in some cases, but in many they resulted in death. We now know that for a transfusion to be successful, the two sets of blood must be compatible - able to mix wirht each other without the red cells sticking together.. Th ere are four blood groups, known as A, B, AB and 0. These groups are based on proteins, ca lled antigens, that are present on the surface of red blood cells. For example if an tigen A is present on all the red blood cells of a person, that person is said to have blood group A. There are also antibodies present in the blood plasma. These are associa ted with the antigens. So a person with b lood group A, fo r example, has antigen A (A) on their red cell and and antibody anti-B (b) in their plasma (table l 3.4). During a transfusion it is important to note: • the protein (or antigen) on the red blood cell of the donor; • the type of antibody present in th e plasma of the recipient. If the antibody matches the antigen, the red blood cells stick together and transfusion will not be successfu I.

151

Life Processes and Disease

Table 13.4 shows the success of transfusion for all the blood groups. A tick means that this combination of donor and recipient will make a successfu l transfusion; a cross indicates that this combination will lead to a reaction (potentially fatal) in the recipient. Donor's blood type

Recipient's blood type

~

IT:.Q·1 3 v-..;

State whether these transfusions are possible: • donor AB, recipient O • donor AB, recipient A • donor O, recipient A • donor B, recipient A • donor B, recipient B

A

(b)

B

(a)

AB 0

A

B

AB

0

(A)

(8)

(A) B

none

none

" " " "

.I

.I

(a) (b1

" "

.I

t antibody present

.I

.I .I

.I

+-

antigen present

.I .I

+- universal recipient: blood group AB

t universal donor: blood group 0

Table 13.4 The success or failure of blood transfusions between different blood groups.

Hypertension hypertension >

EmlDJ

~

IT:.Q-1 4 v-..;

(i) What is hypertension? (ii) What factors in a person's life may increase the chances of suffering from hypertension?

152

High blood press ure is when the pressure caused by the blood pushing against the inside walls of the main arteries is high . Persistent high blood pressure is called h yp er tension . Capillaries are tiny blood vessels, with walls that are one cell across. Blood flowing at a high pressure can cause these vessels to burst. If a vessel in the brain burst, then a portion of the brain becomes damaged from a lack of oxygen . This is called a stroke and can resu lt in paralysis or even death. Capillaries in other important organs like the kidneys may burst because of high blood pressure. This could lead to shutdown of the organ (e.g. kidney failure) and can have serious consequences on the body. Hypertension can develop without symptoms or signs and is sometimes called the 'silent killer'. It is linked with a number of factors such as: • high levels of emotional stress; • lack of exercise; • obesity; • tobacco smoking; • high alcohol intake; • high blood cholesterol levels. All these factors are influ enced by lifestyle, and can be controlled by changing lifestyle. A h ea lthy lifestyle, that includes regular exercise, no smoking, low intake of fat, salt and alcohol, can prevent the development of hypertension.

13 · Transport and Defence in Animals

The role of blood in defending the body against disease

physical barrier

>

phagocytes

>

Microorganism s are all around us. These microscopic o rganisms (viru ses, bacteria, etc.) are in the air we breathe, in the food we eat, on everything we tou ch and all over our bod ies. The skin is the body's first Line of defence (figure 13. 9) . It acts as a physical barrier. When there are breaks in this barrier, such as cuts or sores, the body reacts to produce blood clots and a m eshwork of fibrous scar tissue. The opening is thus blocked, which prevents pathogens (microorganisms that can cause disease) from entering the body. Sometimes the white blood cells called phagocytes move out of the blood and to the infected areas. There they engulf the invading microorganisms, killing and removing them from the body before they can cause disease. This is our second line of defence (figure 13. 10). wax in the auditory canal traps dust and other particles

- - - tears contain a mild antiseptic - - - hairs in the nose trap dust and other particles trachea lined with mucus and cilia to move dust and other particles out of the lungs

skin -a physical barrier stomach produces hydrochloric acid which can kill microorganisms

vagina - mucus moves out constantly

break in the skin - a clot forms

Figure 13.9 The skin is our first line of defence. Any openings in the skin have special means of expelling dust which carries many disease-causing organisms.

bacterium within the

~ phagocyte - it is killed and digested

8

G Figure 13. 10 White blood cells (phagocytes) are our second line of defence. They leave the blood and migrate to a site of infection

phagocyte engulfing bacterium phagocyte moves toward site of infection

blood capilliary

153

Life Processes and Disease

Immune response lymphocyte

>

lmJOitimWJ

The phagocytes can cope with any small, non-specific invasion by pathogens. U more dangerous, specific pathogens enter, then an immune response is activated. In this case, another kind of white blood cell, called lymphocytes, recognise the specific pathogen and mobilise other lymphocytes to make antibodies to attack, disarm, destroy and remove these pathogens.

1m1u.14,u Antigens and antibodies

immune response

>

memory lymphocytes

>

natural immunity

>

Anything that is foreign or different and causes antibody formation is called an antigen. This is our third line of defence against disease. When antigens, such as the measles virus, enter the body, lymphocytes recognise them and start to produce specific antibodies on a large scale to destroy the viruses. The immune response is very specific - only the antibodies for that particular antigen are made. To defend the body against disease, antibodies act in a number of ways: • they cause the antigens to dump together resulting in their death and easy removaJ by the phagocytes; • they neutralise toxins produced by the antigens; • they prevent the antigen from entering body cells. Recognition of antigens and production of the specific antibodies against them takes time. During that time, the antigens will have produced symptoms of the disease. Once the antibodies are produced, the antigens or the toxins they produce are destroyed or neutralised and the symptoms disappear. The antibodies then gradually disappear from the blood, but they leave behind speciaJ memory lymphocytes. If the specific antigen invades a second time, the memory lymphocytes immediately recognise them, and rapidly make large amounts of the specific antibody. This time, the antigens are destroyed before symptoms develop, and the person is said to be immune to that disease. This happens naturally and is called natural immunity (figure 13.11) . Antibody concentration in the blood first infection

second infection

slow build-up of antibodies gives pathogens time to cause disease

0

10

20

30

large amount of antibodies made immediately, pathogen destroyed before symptoms develop - person is said to be immune to that disease

1--- -

40

50

60

70

80

90

Time in days

Agure 13. 11 Immunity 1s a rapid large increase of antibodies in the blood

1?ot.s V"...J

We are surrounded by pathogens. How is the body protected from infection?

154

There are two types of naturaJ immunity. • Actively acqu ired immunity - When the body has already experienced an infection by a pathogen or antigen, as described above, the lymphocytes produce large quantities of antibodies to fight the disease before symptoms develop a second time. • P assively acquired immunity - Antibodies can pass across the placenta providing a newborn baby with immunity against diseases that the mother's

13 ·Transport and Defence in Animals

body is immune to. Also, antibodies present in breast milk help to protect the baby against antigens.

Immunisation and the control of communicable diseases vaccination

>

~

IT:Q·1 6 \....)'-/

Explain what is meant by a vaccine.

artificial immunity

>

1~7 \....)'-/

Copy and complete the table. Natural immunity

Artificial immunity

Active Passive

Immunisation provides immunity to communicable diseases. This is achjeved by injecting, or administering orally, small amounts of dead or weakened (attenuated) antigens into the body. This is called vaccination . The body is stimu lated to produce antibodies. One example is the MMR vaccine given at around 2 years of age or younger to protect children against measles, mumps and rubella. DTP vaccines, administered at any age, protect against diphtheria, tetanus and pertussis (whooping cough) Smallpox has been eradjcated because of immunjsation programmes. Vaccines against tuberculosis (TB) and hepatitis B have also been developed, but there are still not vaccines against diseases such as cancers, leprosy, malaria and AIDS, despite much research. The World Health Organization (WHO) Expanded Program of Immunisation (EPI) aims to extend immurusation to children all over the world, especially in developing countries so that d1ildren can be immunised at no cost to their parents. Immunisation is known as artificial immunity. There are two types of artificia l immunity, • Actively acquired - This is by vaccination at a suitable time in the person's life, when they are not infected with the antigen. The vaccine used contains treated antigens which cannot cause the disease, but which can stinrnlate the body to make antibodies. Immunity is obta in ed because if the real antigen should enter the body, antibodies are immediately and rapidly produced to destroy il. This happens before symptoms develop and the person is said to be immune to that disease. • Passively acquired - The vaccine contains ready-made antibodies whid1 provide immediate relief by destroying the antigens. Thls is given when the person has been infected with the antigens and has no previous immunity. The importance of immunisation or vaccination is seen when children are protected from dangerous diseases like polio, measles, mumps, tetanus and whooping cough (figure 13.12). Thls is achieved in a programme of immunisation where often a second, booster injection is given. This stin1ulates a much quicker production of antibodies which is longer lasting and whlch protects the chiJd from the disease for a considerable time.

~

IT:Q·1 8 \....)'-/

Explain the meaning of the term 'immunisation' and give one advantage of immunisation.

(a)

(b)

(C)

Rgure 13. 12 (a) Mumps and (b) chickenpox are common childhood diseases. In some children they can cause long-term damage. (c) Poliomyelitis can cause life-long damage to the body even after the infection has gone.

155

Life Processes and Disease

~

II

Chapter summary • Large multicellular organisms have a small surface area-to-volume ratio. This means that they need transport systems to carry substances to and from cells around the body. • A cell needs nutrients, oxygen and other substances to stay alive. t • Waste products are produced and need to be removed from cells so they do not damage them. • The transport system of humans is composed of a pump (heart) a transport medium (blood) and vessels (blood vessels) through which blood flows. This is the circulatory system. • The structure of the heart is suited to its function as a pump. • Blood passes through the heart twice for each time it circulates the body; after one of these passes through the heart blood goes to the lungs for the exchange of gases. • There are three kinds of blood vessel: arteries, veins and capillaries. • Blood is composed of plasma, blood cells and platelets. • Plasma is mainly water with the substances being transported dissolved in it. • There are two types of blood cell; red blood cells transport oxygen and white blood cells protect the body against pathogens. • Platelets help blood to clot; this is important to prevent blood loss. • For a successful blood transfusion, the donor's and the recipient's blood groups must match because if the antigens and antibodies in their blood react together, the transfusion will not be successful. • Hypertension is persistent high blood pressure, which is dangerous to health. • White blood cells protect the body against pathogens. • Phagocytes can leave the bloodstream, gather at sites of infection and engulf and kill pathogens. • The body has three lines of defence against infection: the skin (and blood clotting), phagocytes and the immune system. • In the immune system, lymphocytes form antibodies which are specific for the pathogen. • After an infection, memory cells remain in the blood, which recognise the pathogen again quickly. A second infection does not result in symptoms of the disease because the production of antibodies is much faster and greater. • A person is immune to a disease if, on infection with the disease, no symptoms develop.

-

156

13 · Transport and Defence in Animals

ITQ1 In the unicellular organism, the surface area to volume ratio is large, which means that there is a lot of surface area for the volume of the organism. Diffusion can occur fast enough across the cell membrane and get to all pans of the cell for all life processes to happen effectively. In a multicellular organism, for each cell to get a supply of oxygen and everything else it needs as fast as it needs it, a transport system is necessary , because the surface area is not large enough in proportion to the volume for diffusion from the external environment to be effective. ITQ2 (i) Oxygen is used in respiration, which in tum provides the body with energy. (ii) Glucose is oxidised during respiration to provide the body w ith energy. (You may h ave chosen other substances.) ITQ3 Heart muscle has its own set of blood vessels, called the coronary arteries and coronary veins. The coronary arteries supply it with glucose and oxygen needed for respiration. ITQ4 vena cava -+ right auricle -+ right ventricle -+ pulmonary artery-+ lungs -+ pulmonary vein -+ left ventricle -+ aorta ITQS Atrial systole - pushes blood from the atria into the ventricles. Ventricular systole - pushes blood out of the heart, so that it can be pumped to the lungs through the pulmonary artery, and through the aorta to all parts of the body. Diastole - allows blood from the body to collect in the atria, before it is forced into the ventricles by contraction of the muscles around the atria. ITQ6 Blood leaving arteriole: • has a lot of oxygen in it as the arteriole carries oxygenated blood to body cells; • is rich in glucose, hormones, water, vitamins, etc. , which will be used by the cells for different purposes . Blood entering venule: • h as less oxygen, as some has been used by the cells in contact with the capillary network; • has less of other substances, as the blood has been depleted of these when passing through the capillary network • has more carbon dioxide and other waste products from cells. ITQ7 The aorta receives blood at the highest pressure from the contraction of the muscles of the left ventricle. As the blood enters the aorta, its thick muscular walls are stretched but do not burst. ITQS (i) Each heartbeat results in a surge of blood, which can be felt as the arterioles stretch to accommodate blood flowing at a high pressure. Each heartbeat results in one pulse. (ii) The pulse rate shows the rate at which the heart is beating because ead1 . heartbeat is felt as one pulse. ITQ9 renal vein -+ vena cava -+ heart -+ lungs -+ heart -+ aorta -+ hepatic artery -+ live r -+ hepatic vein

l

i

mesenreric artery -+ gut -+ hepatic portal vein ITQ 10 (i)

Renal artery

Renal vein

rich in oxygen

little oxygen present

little carbon dioxide present

rich in carbon dioxide

rich in glucose

little glucose present

157

Life Processes and Disease

(ii)

Pulmonary artery

Pulmonary vein

rich in carbon dioxide

little carbon dioxide

little oxygen present

rich in oxygen

ITQ11 Platelets are involved in blood dotting, which prevents entry of pathogens when there is a cut on the skin. White blood cells destroy foreign bodies that might harm the organism. ITQ12 On exposure to air, platelets in the blood, in the presence of calcium ions and vitamin K, cause prothrombin, an inactive blood protein, to be converted to thrombin. The presence of thrombin causes the conversion of fibrinogen, another inactive blood protein, into fibrin. Fibrin is made up of insoluble fibres that trap red blood cells and form a clot. ITQ13

• • • • •

donor AB, recipient 0: no donor AB, recipient A: no donor 0 , recipient A: yes donor B, recipient A: no donor B, recipient B: yes ITQ14 (i) Hypertension is prolonged high blood pressure. (ii) Factors that contribute to the chances of suffering from hypertension are: • a genetic predisposition (having a relative who suffers from the disease ); • smoking; • obesity; • diet that contains many fatty foods; • no exercise; • old age. ITQ15 The body ha s three lines of defence against infection: • the skin is a physical barrier and openings in the skin have special mechanisms, such as blood clotting, to keep pathogens from entering the body; • if pathogens get into the body through a wound, phagocytes migrate to this site and 'eat' the invading microorganisms; • antibodies are produced to seek out antigens (foreign invaders like bacteria) that have entered the body, and destroy them or neutralise their effects. They are completely destroyed at this point. ITQ16 A vaccine is a substance injected into the body. It contains antigens which cause the immune response, or antibodies which protect the body. ITQ17

.

Natural immunity

Artificial immunity

Active

Antigens enter the body naturally and bring about the immune response

Antigens are introduced in a vaccine to the body and the immune response is generated

Passive

Antibodies enter the body naturally Antibodies are introduced in a and protect the body against disease vaccine to the body so the body is protected

ITQ18 Immunisation is a programme of dispensing vaccines for various diseases. The act of introducing the vaccine to a person is also immunisation. One advantage is protection against disease.

158

13 • Transport and Defence in Animals

Examination-style questions The functions of blood include transport of substances and protection against blood loss and infection (i) Describe the transport of a named substance from the site where it is picked up by blood to a named body cell. (ii) Describe how a blood clot forms and how the clot protects the body against blood loss and infection. 2

(i)

Make a labelled drawing of a transverse section of the heart. Use arrows to show the movement of blood through the heart. (ii) Describe the role of the heart in the circulatory system. (iii) Explain why the muscles of the left ventricle are thicker than those of the right. (iv) Explain the effects on the heart if the coronary artery becomes blocked.

3

(i) Compare the structure of an artery with that of a vein. (ii) How does the structure of an artery related to its role as a blood vessel? (iii) Describe how a glucose molecule moves: (a) from the heart to a capillary next to a body cell; (b) from the capillary into the cell. (iv) Compare the composition of blood as it enters and leaves the lungs. A blood vessel in the brain may burst, resulting in a condition called a stroke. Major strokes can result in severe paralysis or even death. Minor stokes may occur without symptoms.

4

(i)

Suggest a likely consequence of the bursting of a blood vessel in the brain and how this could result in paralysis or even death. (ii) Suggest reasons why some strokes occur without symptoms.

159

•

0 0 0 0 0 0 0 0 0

Plants

describe the movement of substances in plants understand the structure of xylem vessels, sieve tubes and companion cells understand how the structure of xylem vessels suits them for their function describe the movement of water through a plant describe the processes involved in transpiration describe the effect of external factors on transpiration discuss adaptations in plants to conserve water understand the function of phloem in the transport of substances in plants explain how the structure of the phloem is suitable for its function

transport in plants vascular bundle

f

\

xylem

phloem

I

I

uptake of water from soil

translocation movement of food

water in soil around root hairs

food storage

across cells of root to xylem

perennating organs

I movement through xylem

I movement through eel lls of leaf } transpiration -

potometer

movement through stomata to air

The importance of transport in plants There is much activity going on inside a plant, but it is difficult to imagine that j ust by looking at one. Th e main activity is photosynthesis. During daylight,

14 · Transport in Plants

all the leaves of a plant are actively photosynthesising and therefore need all the substances necessary to carry out this process. Transport in plants is thus rela ted to photosynthesis as substances are transported to and away from leaves (figure 14. 1).

li;il';' - - - -

water may travel up to 1000 ft against gravity without a pump I

root extends { further out below the soil than the branches and leaves above

Rgure 14.1 Transport In plants.

Photosynthesis is summarised by the equation: light

carbon dioxide + water ----+ glucose + oxygen chlorophyll

• The gases carbon dioxide and oxygen move between the atmosphere and the leaf. The leaves are thin, broad and flat and cover a wide surface area so that diffusion is adequate to transport these gases. • Light rays from the Sun pass into the leaf and get to all the photosynthesising cells. • Chlorophyll is p resent in the leaf cells. • Water must be transported from the soil through the roots to the leaf. Dissolved salts are present in the water. • Some of the manufactured food is transported away from the leaves to be used and/or stored in other pans of the plant.

~

l:fQ1 V'-'

Use a table to show the substances that are transported in a plant. Indicate where the substance is transported from and to, and its importance to the plant. xylem vessels phloem tubes

> >

Transport systems of plants The transport system of plants is much simpler than that seen in animals. There is no pump (heart) or specialised transport medium (blood). Ir is made up of two types of transport vessel: • xylem vessels, which carry water and minerals; • phloem tubes, which carry food materials that the plant has made.

Structure of xylem vessels

llrffilhll

Xylem vessels are long, very narrow, tubes formed from columns of elongated cell s that are joined end to end. The end walls of the cells have disappeared, so a long, open tube is formed . These cells are all dead and con ta in no cytoplasm or nuclei (figure 14.2). The cell walls become thickened with tough lignin . Lignin is very strong and so xylem vessels help to support the plant by keeping them upright. Wood is composed almost entirely of lignified xylem.

1.61

Life Processes and Disease

space (no cytoplasm) where water passes

cB

thick cell wall containing lignin

t

gap where end walls of two cells have been lost

transverse section

longitudinal section

Figure 14.2 Transverse and longitudinal sections through xylem tissue.

Structure of phloem tubes ..1@t:llelff100 sieve tube element companion cell

> >

Phloem tubes are also made up of cells joined end to end. However these end walls do not break down completely, but become perforated with small holes . These perforated end walls are called sieve plates. Each cell, called a sieve tube element, contains living cytoplasm, but no nucleus. The cell wa lls do not contain lignin. Each sieve tube element has a companion cell next to it. The companion cell has a nucleus which probably controls both cells (figure 14.3).

The end walls of the two cells are not completely broken down. This is called a sieve plate and has small holes.

companion cell

I

sieve plate transverse section

'-··

Companion cell containing a nucleus and cytoplasm; found next to a sieve tube element.

sieve plate sieve plate

._#----l'*'i!.,...=--i.;;+-~-fl!"il!-~• ~

-

section cut through phloem

H+- - cytoplasm - seen as strands

cell wall does not contain lignin

·-·

Transverse section of these cells. Companion cells are seen when the section runs through the sieve plate.

longitudinal section

(a)

(b)

Figure 14.3 (a) Transverse and longitudinal sections of a sieve tube element and a companion cell. (b) Longitudinal section through a phloem tube and transverse section across it.

162

14 ·Transport in Plants

Vascular bundles vascular bundles

> Vascular bundles are made up of bundles of xylem vessels and phloem tubes close together. The arrangement of these bundles in roots and shoots is shown in figures 14.4, 14.5 and 14.6. phloem

xylem

xylem is made up of xylem vessels which run the length of the stem, trunk, branch, etc.

a number of xylem vessels lie close together

phloem is made up of phloem tubes which run the length of the stem, trunk, branch, etc.

I I made up of a number of xylem vessels

xylem

phloem ~

vascular bundle

I made up of a number of phloem vessels

Agure 14.4 Transverse section of a root of a dicotyledonous plant.

xylem phloem

J

vascular bundle

Figure 14.5 Transverse section of a stem of a dicotyledonous plant.

Figure 14.6 Diagram relating the transverse section to the longitudinal section of a stem.

Movement of water through a plant ~ ll:Q2 V'-1

How are xylem vessels and phloem tubes arranged in a vascular bundle? (ii) Explain how the structure of the (a) xylem and (b) the phloem are suited to their function.

The movement of water th rough a plant can be broken down into five stages (Figure 14.7). The numbers in the following text relate to numbers in the figure).

(i)

~

ll:Q3 V'-1

(i)

What are the functions of vascular bundles? (ii) Draw a sketch to show how they are arranged in the stem of a dicotyledonous plant.

Figure 14.7 Diagram showing the movement or water through a plant.

163

Life Processes and Disease

1 2 3 4

Practical activity SBA 14.1: The rate of transpiration, page 353 CHAPTER9

X

~

IT:.Q~ V'-....1

Describe the route taken by a water molecule from the soil to the air as it passes through a plant.

5

Absorption of water by the root hair cells. Movement of water across the root cortex Movement of water up the xylem. Movement of water across the leaf cells. Movement of water from the leaves.

to

the xylem.

Evaporation of water from the leaves Stomata are found on the underside of leaves (chapter 9). Just inside the stomata are the leaf cells which contain and are surrounded by water. The concentration of water molecules inside the cells is higher than in the air space, and higher there than outside the leaf. So some of the water evaporates from the cells into the air space and diffuses out of the leaf through the stomata, down the concentration gradient (figure 14.8).

Figure 14.8 Diagram showing how water evaporates from cells around the air space and diffuses out of the stoma. transpiration

All the cells of the leaf obtain water by osmosiS ---+-++from the xylem.

Water evaporates -~~__, from the cells around the air space.

> Transpiration is the loss of water by evaporation from the surface of leaves. This constant loss of water from the leaves creates a 'pull' of water through the plant. During the day, water is also constantly being taken up from the top of the xylem vessels to supply the photosynthesising cells in the leaves. This reduces the pressure at the top of the xylem vessels, and water thus flows up to the top because the pressure below is greater. This constant flow of water through the plant is known as the transpiration stream. The conversion of liquid water into watervapour as it lea ves the cells and enters the air space requires heat. Using Water moves from heat to convert water into water vapour cell to cell by osmosis. helps to cool the plant. Water moves out n.~~+:.._ of the xylem to surrounding cells.

Movement of water within the leaf

As water evaporates from the surfaces of cells near the air spaces, its concentration in those cells Water diffuses out (A in figure 14.9)) is lowered. Its of the plant. -+-__.,• concentration in the adjacent cells (B) is now higher than in the A cells. This Rgure 14.9 Diagram showing the movement of water from the xylem, through the cells of the resu lts in the movement of water from leaf, to the air space and then out of the leaf as water vapour. the B cells to the A cells by osmosis.

164

14 ·Transport in Plants

~ IT:QS

Water is drawn through all the cells of the leaf by osmosis as it moves towards the air space and then out of the leaf.

Describe how water travels from the xylem in a leaf to an air space.

Movement of water up the xylem

\../'-I

lmlet4il•leN m.t¥UeleM

~

IT:Q6 \../'-I

List the processes by which water travels up the xylem.

-

glass tube

-

water moves higher up a narrow tube

Water moves up and through the xylem vessels because of three factors. • Capillarity - When a thin straw is placed in a glass of water, the water rises a little up the straw (figure 14.10). This is due to attraction between r the water molecules and the walls of the straw, which is called adhesion. Water molecules tend to stick together, which is known as cohesion. Thus; water molecules stick together and to surfaces of narrow tubes and the water rises up the tube, which is called capillarity. The narrower the straw, the higher the water will rise. Xylem vessels are extremely narrow and the attraction between the water molecules and the xylem walls is great. • Root pressure - Water constantly moves into root cells by osmosis because the presence of sugars and other dissolved substances in the root means that the water concentration can never be as high in root cells as in the soil. Water absorbed into the plant from the soil creates a pressure in the root xylem. The pressure there is greater than in the leaf xylem because water is being pulled out of the leaf xylem by transpiration. So water moves from the high pressure in the roots up the xylem vessels in the stem to the low pressure in the leaves (figure 14.11 ). • Transpiration pull - The flow in the system is mainly by cohesive forces holding water molecules together and the loss of water by evaporation in the upper areas of a plant creating a tension that 'pulls' water upwards. This is the transpiration pull.

a lot of w ater coming from the roots creates a lower pressure in this region since water is lost at the top xylem, made up of narrow xylem vessels

\' rf+--+--r--H----r- water molecules are attracted to ,_ the walls of the tube

water

Figure 14. 1O Water moves up narrow tubes.

Figure 74. 11

Low pressure at the top, high pressure below and water is pushed up.

Movement of water across the root cortex Water moves between the cortex cells of the roots by osmosis (figure 14.12, overleaf). As water enters the xylem, the cells next to the xylem now have a lower concentration of water. Water then moves into those cells from adjacent cells. These cells now have a lower concentration of water and water flows into them by osmosis. In this way, water moves from the root hair cells to the xylem.

165

Life Processes and Disease

cortex cells

xylem vessels

t

t 1 water moves into the xylem and is pulled up

r--.

~* - ----'2 water moves from these

these cells (B) have-----' more water than cells A and so water moves by osmosis to those cells

cells into the xylem and so these cells have less water than the ones next to them

Agure 14.12 Water moves between all the cells of the root cortex by osmosis.

Absorption of water by the root hair cells ~ l:W:Q7 V'-1

Describe how water travels from the soil to the xylem vessel in the root.

The soil particles are surrounded by a film of water which contains some dissolved salts. Inside the root cells, there are sugars and other dissolved substances at a much higher concentration. So water is continuously moving into the root cells by osmosis (figure 14.13). The root hair cells extend into the surrounding soil and the surface area for absorption is thus increased. soil particle

cortex cells

water moves from the root hair cells to the cortex cells and so water is pulled into the root hairs

xylem vessels

water moves across these cells to the xylem

Agure 14.13 Water moves into the root hair cells.

Transpiration Transpiration is the evaporation of water from a plant. It is important beause: • it pulls water up to the leaves for photosynthesis; • the moving water carries dissolved mineral salts up to the leaves; • the evaporation of water cools the plant.

166

14 ·Transport in Plants

transpiration rate

>

• The rate at which a plant takes up water depends on the rate at which it is lost from the plant, called the transpiration rate. The faster the transpiration rate, the faster the plant takes up water. Environmental factors affect the transpiration rate. • Temperature - With high temperatures, as on a hot day, evaporation occurs rapidly. Transpiration rate increases as temperature increases. • Humidity - With high humidity, the air is almost saturated with water r vapour. So the concentration gradient of water between the air spaces and the outside air is low and the rate of evaporation of water through the stomata is slow. Transpiration decreases as humidity increa ses (figure 14. 14). • Air movement (wind) - In windy conditions, water vapour is carried rapidly away from the leaves and the rate of transpiration is fast. During still conditions, the water vapour remains around the leaves and transpiration is slow. Transpiration increases as wind speed increases (figure 14.15) . • Light intensity - During bright light, the stomata are fully opened. This may be to supply carbon dioxide for photosynthesis. With stomata fully open, the rate of transpiration can be high. With dim light, the stomata almost close and transpiration is slow.

LJ

HO

o/ i?JwH ,O H0

H.,Q .,,,,,.---. ~ H· 0 As water molecules H,O Q < diffuse out, they are taken away by wind currents. 11 0 The concentration of water molecules outside will always be small and so there

~n

::) Agure 14.14 High humidity means that the concentration gradient of water molecules inside compared to outside the plant is low.

0

"'II be oot ootw""' d;Ho•oo.

Figure 14. 15 Windy days result in more rapid transpiration.

When there is very little water in the soil, the stomata almost close. This reduces the rate of transpiration to conserve water. The plant cells become flaccid and the plant wilts as the water lost in transpiration cannot be replaced.

~ IT:Q8 l../'-1

(i) What is transpiration pull? (ii) What is the transpiration stream? (iii) What is transpiration?

xerophytes

water molecules taken away by wind currents

Adaptations in plants to conserve water >

Plants need water for their existence. Transpiration occurs constantly, so a supply of water from the environment is vital. Plants that live in places where water is in short supply are called xerophytes. They show striking adaptations which: • reduce the transpiration rate; • m aximise water uptake from the environment. Other plants are grouped into three categories.

mesophytes

> • Mesophytes are plants that live in areas where water is readily available.

hydrophytes

>

• Hydrophytes are plants that live in very wet, freshwater environments such as ponds, lakes and rivers.

167

Life Processes and Disease haloph es >

• Halophytes are plants Lhat live in water with a high concentration of salt, such as in salt marshes, swa mps or areas close to th e sea. Xerophytes may have any of the following fea tures: • fine spine-like leaves to reduce the number of stomata and so reduce transpiration; • thickened stems or leaves capable of storing la rge amounts of water; • an extensive root system to absorb water quickJy when it rains; • a thick epiderm is covered with a thick waxy cuticle to reduce water loss and reflect light and infra -red radiation (so the plant remains cooler); • the ability to trap carbon dfoxide at n ight so that the stom ata can be closed during the da y; • other features such as sunken stomata, rolled leaves and interlocking hairs. Table 14.1 compares the adaptations of a xerophyte (e.g. cactus) and a h ydrophyte (e.g. water lily) to the environments in which th ey live. Xerophyte

Hydrophyte

Description of environment

Very hot, sunlight intense as few or no Still, fresh water from 15 cm to 2 m clouds, no larger shade trees, soil hot deep and dry

Description of leaves

Small spikes, not green, usually black Broad, flat, green, lie on surface of or grey in colour water, stomata on upper surface

Description of stems

Thick, green, with thick cuticle

Colourless, entirely under water and extending to roots in mud at bottom

Table 14. 1 Comparison of a xerophyte and a hydrophyte

Uptake and movement of mineral salts Mineral salts are absorbed by the root hairs as ions dissolved in soil water. They are taken up using energy because the concentra tion inside the root is much higher than outside. They are then carried throughout the plant in the xylem.

Transport of manufactured food translocation

168

>

The soluble product o f photosynthesis are sugars (ma inl y sucrose) and amino acids. These are transported in the phloem tubes. The transpo rt of organic food through a p lant is called translocation . This manufactured food is transported from the leaves (called the source) to wherever it is needed (called the sink) for respiration or storage.

14 ·Transport in Plants

Phloem and the movement of food Plants rely on pressure gradients to move their phloem sap. The pressure- flow hypothesis, also called the mass fl ow hypothesis, was proposed by Ernst Munch in 1930 to explain h ow the phloem transports food (figure 14. 16). leaf

pholem sieve tube sugar is made during elements photosynthesis ..._..;;;:_..__ _ _ _ _" _ " )- - -

sugar lowers the water potential (increases the concentration) which draws in water and raises the pressure

L - - - L - --

roots water sink

sucrose solution transported from high to low pressure

sucrose enters the roots and lowers the pressure in pholem as water is lost

Figure 14. 16 The pressure-flow hypothesis.

1

IT:Q9

Sugar made during photosynthesis moves into the sieve tubes at the source (leaf) which makes it more concentrated there. 2 Water then moves into the sieve tubes since it is now more concentrated. 3 The uptake of w ater causes the pressure to build up in the sieve tubes at the source (leaf) which pushes the sap down. 4 Unloading of sugar at the sink (other parts of the plant) relieves the pressure since water is also lost at the sink. Sugar is thus translocated from the leaf to th e root and other parts of the plant that are respiring, storing and using the sugar.

How are sugars transported in the phloem?

Evidence that phloem translocates organic food

~ V...J

~

Radioisotopes

V...J

If a plant is supplied with carbon dioxide containing radioactive carbon it

IT:Q·1 0

What is translocation?

will make food containing radioactive carbon. If the source of radioactive carbon is removed then, after a while, the radioactivity is detected only in the phloem tubes. This means that the food which the leaves h ave made is being transported in the phloem .

169

Life Processes and Disease

Ringing The phloem tu bes in a woody stem are j ust underneath the bark. If a ring of bark containing the phloem is removed, sugars accumulate above the ring, resulting in a slightly swollen appearance (figure 14.1 7). food cannot get to lower regions

Figure 14. 17 Removing a nng of bark also removes the phloem.

Using aphids Aphids are insects that feed on plant juices by pushing their mouth pa rts (stylets) into phloem tubes. If the mouthparts of a feeding insect are cu t off, phloem sap keeps flowing out through the stylets (figure 14. 18). The sap can be analysed and shows the presence of organic material.

~

(a)

(b)

l'.tQ·11

Mouthpart cut off and left In the plant.

L.l'--1

Describe three ways in which the food manufactured during photosynthesis is used by a plant.

stylet oozing 'food'

Mouthpart (stylet) of aphid penetrates the phloem. It sucks 'food'; this is how it obtains food for energy. Aphids are parasites of plants.

Figure 14. 18 If the mouthparts of (a) a feeding aphid are cut off (b) they continue to ooze sap fY

-

Chapter summary • • • • • • • • • • •

All materials needed for photosynthesis must be transported to the leaves of a plant. Water is transported from the soil. Carbon dioxide diffuses in through the stomata. The products of photosynthesis, manufactured food and oxygen, must be transported from the leaf. Manufactured food is transported in the phloem to various sites in a plant. Oxygen diffuses out through the stomata. The structure of the xylem is suited to its transport and support functions. The structure of the phloem is suited to its transport function. Transpiration is the evaporation of water from the leaves of a plant. The rate of transpiration is influenced by many external factors. Translocation is the transport of organic food through a plant.

ITQ1

Substance Taken from

Transported to

Importance to plant

Means of transport

carbon dioxide

air surrounding leaves all photosynthesising cells

needed for photosynthesis

diffusion

water

soil - water forms a thin layer around soil particles

needed for many purposes including photosynthesis

xylem

all cells

(continued)

170

14 ·Transport in Plants

Substance Taken from

Transported to

Importance to plant

Means of transport

minerals

soil - present as soluble ions in water

all cells

healthy growth

xylem

organic food (glucose)

leaf cells where it was all cells made

all cells must respire to phloem have energy for giving processes

oxygen

leaf cells

oxygen produced during diffusion photosynthesis and not needed for respiration must be removed

outside the leaf

ITQ2 (i) The xylem vessels are positioned together in the vascu lar bundles in a region where only xylem vessels are found. Similarly, the phloem tubes and their accompanying comparuon cells are positioned together in a part of the vascular bundle where only these structures are found. (ii) (a) Xylem vessels are elongated, tubular and made up of dead cells thus providing a water-proof vessel for the transport of water and absorbed minerals. (b) Phloem is also elongated and tubular but made up of living cells. Energy is thus available for the transport of manufactured food. ITQ3 (i) The vascular bundles consist of xylem vessels and phloem tubes. The xylem vessels transport water containing dissolved minerals from the roots to the leaves of the cell. The phloem tubes transport organic food from the leaves where it is produced to all the cells of the plant. (ii) Your sketch should look like figure 14.5. ITQ4 soil - root hair cell - root cortex cells - xylem - palisade mesophyll - air space - stoma - air ITQ5 Water travels by osmosis out of the xylem and then travels across the cells of the leaf to an air space by osmosis also. The water is moving down a concentration gradient, from a high concentration of water molecules to a lower concentration. ITQ6 Water travels up the xylem by: • root pressure; • transpiration pull; • capillarity, coh esion and adhesion. ITQ7 Water moves by osmosis into the root hair cell from the soil. The water then moves across the cells of the root cortex to the xylem. Th e water is moving along a concentration gradient. As water travels up the xylem, more water moves into the root from the soil . ITQS (i) The flow in the xylem is mainly by cohesive forces holding water molecules together and the loss of water by evaporation in the upper areas of a plant creating a tension that 'pulls' water upwards. This is the transpiration pull. (ii) The water moving up the xylem is the transpiration stream. (iii) Transpiration is the loss of water vapour from the leaves of a plant. ITQ9 Sugar is transported as sucrose which loads into the phloem from the leaf (source). This increases the concentration of the solution in the phloem, which draws in water. This increases the pressure of the solu tion. The pressure is lower at the roots and movement thus occurs from the leaf to the root. At the root (sink), the pressure is lower because the sugar moves into the roots thereby lowering the concentration of the solution, causing water to move our. ITQ10 Translocation is the movement of manufactured food from the leaves to all the cells of a plant.

171

Life Processes and Disease

ITQ11

• It is stored as starch in plant cells. • It is used for respiration to release energy for use by plant cells.# • It is used for the production of fruits and seeds during reprodu ction.

Examination-style questions (i)

State the functions of: (a) phloem; (b) xylem. (ii) Describe how aphids can be used to investigate the function of phloem. (iii) Explain why it is necessary to water many potted plants at least once a day. 2

(i) Define transpiration. (ii) State three environmental factors that affect the rate of transpiration. (iii) Using an annotated diagram only, describe how the photometer can be used to measure the rate of transpiration.

3

(i) Suggest three ways in which transport in plants differs from transport in animals. (ii) Suggest two ways a xylem vessel is similar to an artery. (iii) A plant may be 50 metres in height and does not have a pump to push water to the leaves at the top. Describe how water travels in the xylem, from the soil to the uppermost parts of a plant.

4

Two tubes A and B were set up as shown below. Both tubes were left indoors for 50 minutes and then taken outdoors for another 50 minutes. The tubes were weighed every 1Ominutes. The table shows the results obtained.

, ~·

cotton wool

- J..~

water

Tube A

Time (min.)

0

10

TubeB

20

30

40

50

60

70

80

90 100

Tube A (g)

305 294 285 272 262 250 220 206 184 150 120

Tube B (g)

280 280 280 279 279 279 278 278 278 278 278

(i) (ii) (iii) (iv) (v)

State the processes by which water was lost form (a) tube B, and (b) tube A Explain the role of the cotton wool in the investigation. Plot a graph of the results for both tubes A and B on the same page. Explain fully, the differences seen between tubes A and B. Describe the differences seen for tube A between the first and second parts of the investigation. (vi) Explain fully the differences seen for tube A between the first and second parts of the investigation.

172

Storage in Plants and Animals 0 0

understand the importance of food storage in living organisms identify some products stored and the sites of storage in plants

,/) draw and annotate stages in germinating seeds

ZJ

describe the structure of a dicotyledonous seed

Q) describe the processes taking place within a seed during germination ,/) draw buds from plant storage organs

0

identify some products stored and the sites of storage in animals

food storage

importance

I

I

(

plants

roots

stems

leaves

'

animals

seeds

fruits

liver

fat deposits

develo~ment

1 of em~ provide for periods of scarcity

vegetative reproduction

overcome the need for continuous food intake

Why do organisms store food? Manufactured food (from photosynthesis using the Sun) is the source of chemical energy for aU living organisms. Glucose, which is the chem ical compound made during photosynthesis, is oxid ised to release energy. All living things depend on this energy fo r life processes to take place. Some of this food, however, is stored. Plants and animals store food in their bodies for the same reasons, some of which are listed below: • to overcome the need for contin u ous manufacture; during the night, photosynthesis stops because there is no light; • to overcome the need for continuous food intake; animals cannot eat continuou sly because other activities are also important; • to provide for periods of scarcity, like droughts and famines; • to provide for special functions (muscle cells need their own store of food); • to produce reproductive structures (fruits, seeds and embryos must store food).

Food storage in plants The food made by a plant during photosynthesis may be stored temporarily as starch in the leaves. For longer periods of time, other parts of the plant are used, such as roots, stems, fruits and seeds. Table 15.l (overleaf) lists the importance of the various sites of food storage in a plant.

173

Life Processes and Disease

Site of storage

Importance of storage

Leaves

The cells of the leaf need to respire and there is a store of glucose as starch in starch grains.

cabbage

bokcho Sometimes underground leaves are used to store food.

omon

garlic

leaf, swollen with food

-

bulbs e.g lily Stems

Some stems can become swollen with stored food. Some plants can protect themselves against unfavourable conditions of weather by reserving food in underground stems which will be used to generate new plants.

harsh environmental conditions

-food stored in 'good' conditions plant stores food

plant is still 'alive' underground

good conditions again - new growth seen

(continued)

174

15 · Storage in Plants and Animals

Site of storage

Importance of storage Underground swollen stems are sometimes used to store food. - - new shoot growing from rhizome stem tuber - the tip of an underground stem is swollen with food

~ _ y roots growing

r

rhizome - food stored in an underground stem

from rhizome

' r--

roots

stem tubers, e.g. Potatc. ,

- -bud - grows into new shoot

corm - the base of a vertical stem becomes swollen with food

rms e g eddo dasheen

Above-ground swollen stems are sometimes used to store food .

...._ __,.,__ swollen stem stores food

sug

ne

(continued)

175

Life Processes and Disease

Site of storage

Importance of storage

Roots

Underground swollen roots are sometimes used to store food. root tuber - tip of root swollen with food

root tubers

1:;-

'd r

-

veet potato

tap root - swollen main root

These swollen organs (like underground stems) are called perennating organs. They are filled with food stores built up in the time of good growing conditions. They lie protected in the soil. The food store enables the plant to grow quickly when good conditions arrive again. Fruits

Most fruits are adapted to protect seeds and to help their dispersal. Succulent or juicy fruits store mainly sugars to attract

CHAPTER 21 animals that use the fruits as a food source. The animals help to disperse the seeds that are in the fruit (chapter 21). fleshy edible part of fruit seeds

pumpKJr,

seed

arape

Seeds

Seeds contain a store of food for germination. The cotyledon(s) or endosperm of seeds store starch, protein and lipids. This store is used up during germination as the embryo develops. The seed respires, using the stores to provide energy for growth and development into a seedling. The stores are needed until the seedling develops leaves and can photosynthesise.

Table 15.1 Sites of food storage 1n pants.

176

15 · Storage in Plants and Animals

Germination mu1 ..111m 1m.rmm endosperm >

Germination is the growth of the seed into a seedling. The seed conta ins the embryo which is made up of the plumule (grows into the shoot) and the radide (grows into the root). The parent plant sends the embryo out into the world with a store of food in the cotyledon and/or endosperm. The embryo is protected by a tough testa (figure 15.1) .

~

IT:Q2 L)'....J

(i) What is a perennating organ? (ii) Distinguish between a stem tuber and a root tuber (Iii) What is the importance of the food stored in each of the following - a fruit, a seed, a leaf and an above ground stem?

hilum

- - --+---

micropyle (tiny hole)

micropyle radicle -

(a)

J

embryo

-+-- -- plumule

(b)

Agure 15. 1 A seed (a) in side view, (b) in section.

~

IT:Q3 L)'....J

(i) What is germination? (ii) Describe the differences between epigeal germination and hypogeal germination.

In its inactive and dehydrated state, a seed can stay a seed for a long time. It is said to be dormant. When conditions are favourable, germination begins (figure 15.2). Germination requires three conditions: • water - moves rapidly into the micropyle and to all the cells. Enzymes are activated and starch is broken down to glucose for respiration; • ox ygen - needed for respiration ; • warmth - to provide the optimum temperature for enzymes. The energy demands of a germinating seed are very Wgh. Energy is released from the stored food by respiration and is used for growth of the rad icle and plumule. The radicle grows down into the soil and the plumule grows upward to develop into the shoot above ground. When the first leaves develop, the seedling begins to photosynthesise to make its own food. It continues to grow and develop more leaves and a root system until it is an adult plant ready to produce flowers.

Epigeal g ermination - cotyledons brought above ground cotyledons open, are green and photosynthesise for a while food store used up as grows

Hypogeal g ermination - cotyledons remain below ground

leaves grow and begin to photosynthesise -

leaves grow and photosynthesise

_ plumule

iil (a)

(b)

Figure 15.2 Germination.

177

Life

Prq~esses

a!1d .Diseas~

~

IT:Q't V-...J

Why would a human being die after 10 minutes without oxygen but could go for 5 days without food?

~

IT:QS V-...J

How do humans become obese? Why are wild animals never obese?

Food storage in animals Animal cells also store glucose, bur nor as search. Animals store glucose as glycogen in granules. Cells respire continuously and animals breathe continuously for their supply of oxygen bur they do not feed concinuously for their supply of glucose.

Fats Triglycerides (fats) have a higher proportion of h ydrogen than either carbohydrate or protein. This means fats are a more concencrated source of energy than eicJ1er carbohydrate or prootein. One gram of fat yields twice the amount of energy that a gram of carbohydratecan yield. In mammals, excess fat is laid down for storage under the skin. When we eat excess food, we become obese, 'fat'. Animals that live in cold conditions have a thick layer of fat (blubber) under the skin which serves both as an energy store and provides insulation for the extreme cold (figure 15.3).

Glycogen As blood passes through the liver, the excess glucose (from a meal) is changed to glycogen and stored. The liver is the main storage organ for glycogen. Glycogen is also stored in the muscles where it can be qu ickly accessed for muscle contraction. The liver also stores minerals (iron and potassium) and the vitamins A, D and B12 • After a meal, vitamins, minerals and other nutrients from the food pass from the intestine into the blood. This nutrient-rich blood then passes through the liver where the vitamins and minerals in excess are stored for times when they are lacking in the blood (figure 15.4). hepatic vein

Rgure 15.3 Penguins and whales need extra fat stores to stay warm in polar conditions. bile duct

~

IT:Q6

hepatic portal vein from ileum

Rgure 15.4 Nutrient-rich blood travels directly to the liver from the Intestine.

V-...J

What is the importance of food stored in: (i) a seed (ii) an egg (iii) the liver (iv) a fruit (v) a tap root?

178

Eggs Embryo birds and reptiles (snakes, turtles, alligators, etc.) develop inside a shell from cJ1e time eggs are laid until they hatch (figure 15.5). The egg white is made up of water and a protein called albumen. The yolk contains protein, far and lecithin (a natural emulsifier).

15 ·Storage in Plants and Animals

yolk sac stalk amniotic cavity allantoic cavity chorionic cavity

yolk sac

Figure 15.5 The eggs of birds and reptiles store food for the developing embryo. ~

'I Chapter summary • • • • • • • • • •

Glucose is manufactured by plants during photosynthesis; some of it is stored Plants and animals store food in their bodies Plants store food in their leaves, stems, roots, fruits and seeds Animals store glucose as glycogen for respiration; plants store glucose as starch One gram of fat yields more energy than one gram of carbohydrate Germination is the growth of a seed into a seedling Water, oxygen and warmth are needed for germination In animals, the liver is an important storage organ Glycogen is stored in the liver and muscle of animals Birds and reptiles store food in the eggs for development of the embryo into a hatchling

So that food is available when needed. To prevent shopping for every meal.. To have the choice to plan a healthy and balanced

all living organisms, to susta in life. These metabolic reactions produce a range of waste produces, called excretory products, which muse be eliminated from the organism. The removal of the excretory products is called excretion. Many of these excretory products are toxic and slow down metabolic reactions. If these substances were allowed to accumu late in the body, they could damage and kill body cells. They n eed to be continually removed. Excretion must not be confused with the removal of faeces (defecation.) during egestion in humans (figure 16. l ). Defecation or egestion is the removal of undigested food but excretion is the getting rid of waste products produced by cells during metabolism. Undigested food sin1ply passes through the alin1entary system and is not absorbed into the cells of the body. It passes out of the anus as faeces. Excretion must also not be confused with secretion, which is the release of a substance, such as a hormone, from cells (figure 16.1).

Explain the terms 'metabolism', 'excretion', 'egestion' and 'secretion'. r ,Hf-- - -r---+-- secretion of female homlones into blood by the ovaries

~?""9---1--

excretion of urine containing metabolic waste by the urethra

excretion of urine containing metabolic waste by the urethra secretion of male-;----------t~~ hormones into blood by the testes

Figure 16.1 The difference between egestion, secretion and excretion.

Excretory products in animals Waste products of respiration All cells respire co release energy which is used to do the work necessary to keep the cell , and therefore the organism, alive. During respiration, carbon dioxide is also produced. Carbon dioxide is dangerous to living tissue because it increases the acidity of fluids which can affect other reactions. ln humans, carbon dioxide is transported in the blood to the lungs and removed from

182

16 ·Excretion, Osmoregulation and Homeostasis CHAPTER 12 ) {

CHAPTERS 10, 19

the lungs during exhalation (chapter 12). Other animals have similar ways of removing the carbon dioxide. Some of the energy produced during respiration is converted into heat energy, the accumulation of which increases the temperature of the body. At high temperatures, enzymes can be denatured (chapter 10) which means reactions will stop. Excess heat is lost through the skin (chapter 19). During exercise, when the rate of respiration is increased, the excretory ' products of respiration are being produced at a faster rate. Breathing rate increases to get rid of the excess carbon dioxide and sweating occurs to get rid . of the excess heat faster.

Waste products from red blood cells A red blood cell has a short life span of about three month s. After this time, it is destroyed in the liver or spleen. The red blood cell is packed with haemoglobin, a protein pigment which transports oxygen. The excess protein portion is broken down into excess amino acids and reused by the body. The iron is extracted, stored, and may be reused later. The remainder is broken down into bile pigments and is later excreted by way of bile into the gut and out with faeces.

Waste products of protein metabolism Proteins are essential in the diet. However, proteins contain nitrogen and the breakdown of protein that is not needed by the body produces nitrogenous waste which is converted to urea (figure 16.2). Urea is removed by the kidneys during the production of urine (discussed later in this chapter).

Excretory products in plants Waste products of photosynthesis

CHAPTER9

X

Plants photosynthesise or manufacture food from inorganic compounds. This food can then be used by the plant to make energy. During photosynthesis, oxygen is produced as a waste product. It is lost from the leaves of the plant through the stomata (chapter 9). Water is also a product of photosynthesis, but this is either needed by the cells or lost from them in transpiration. liver - the amino acid groups (NH2) of the excess amino acids are converted to ammonia (NH3, highly toxic) and urea (non-toxic, soluble)

absorption of amino acids into blood kidneys filter urea out of blood

urine, containing urea collects in bladder Blood rich in amino acids is taken to the heart via the liver and distributed to all parts of the body. The amino acids are used during metabolism for growth and repair of body organs.

Excretion of urea

Figure 16.2 Ingestion of protein leads to the excretion of urea.

183

Life Processes and Disease

Other plant wastes

~

IT:Q2

vv

Draw up a table to show how the following excretory products are produced and where they are excreted. In animals: carbon dioxide, heat, nitrogenous compounds; in plants: oxygen.

Plants al so produce some nitrogenous wastes which are converted into insoluble substances. Calcium oxalate is another insoluble waste product. These wastes are stored in leaves, bark, flowers, fruits and seeds. When the plant sheds these structures, the excretory products are removed. These products can be poisonous to the plant but may be useful to humans as dyes, oils, perfumes and for medicinal purposes. These waste products include tannins, resins, ' latexes, nicotine, caffeine, morphine and gums. Some waste products are stored permanently by the plant, such as in the old xylem (hard wood) ·

The human excretory system The kidney The human excretory system includes a pair of bean -shaped organs, the l3Gl114'IJ kidneys, which are positioned in the lower back region behind the intestines (figure 16.3 ). The kidneys are the major excretory and osmoregulatory organs osmoregulation > of mammals. Osmoregulation is the control of the amount of water in the blood. Since the blood is constantly in close contact with all cells of the body, this means that the kidneys control the amount of water in the body.

·' < - - ---'If - - -

left kidney (slightly higher)

~l!llt---t---'----t--

renal artery takes blood with wastes to the kidney

.JEL'----~--+--

- t-,-=-=;,.,.-o---;sphincter muscle controls the release of urine ++~~+~~:.:..,ai~;_~ r~,~E;~~~~tfrom the bladder

renal vein takes cleansed blood away

bladder - sac which stores urine temporarily urethra tube leading to the environment

Figure 16.3 The excretory system of humans.

The renal artery brings blood with nitrogenous and other waste products to the kidneys to be cleansed. After passing through the kidneys, this cleansed blood returns to the heart via the renal vein, while the nitrogenous and other iilileIW wastes flow down through the ureter as urine to the bladder to be stored. The Hr:DAM• bladder stores urine temporarily before it is released into the environment via lili§Jhli§M the urethra . Sphincter m uscles control the release of urine from the bladder. Sense cells in the bladder walls are stimulated when the bladder fills, triggering the desire to relax the sphincter muscles and contract the walls of the bladder. When this happens, urine flows out of the bladder through the urethra. Trying to hold back the release of urine requires conscious tightening of the sphincter muscles which can be uncomfortable. Babies are not usually ~ IT:Q3 capable of controlling this muscle before the age of 2-3 years. vv Describe the function of each of the When a person is said to be suffering from a 'weak bladder', that person following: kidney, bladder, renal vein, has to urinate frequently. In this case, it is really the sphincter muscJes that are urethra, the bladder sphincter muscle. weak and the bladder does not hold as much urine as it normally stores.

184

16 ·Excretion, Osmoregulation and Homeostasis

ii1¥J•h!i•hiJ

11@00

Figure 16.4 illustrates a longitudinal section through a kidney. The three regions seen are the cortex, medulla and pelvis. Each kidney is made up of thousands of tiny structures called nephrons. Each nephron spans the cortex and medulla, the two outer regions. The pelvis, the innermost region, collects urine from the collecting ducts. The nephrons all end at collecting ducts so that urine, as it forms, flows through these collecting ducts and then out into the pelvis. The urine then flows to the bladder through the ureter. collecting duct Into which urine from a number of nephrons flow renal vein glomerulus Bowman's_. _- capsule

renal artery

cortex - made up of Bowman's capsules and convoluted tubules of all the nephrons

_..,..___ pelvis - collects urine from all the collecting ducts

medulla - contains - ......---loops of Henle and collecting ducts which open into the pelvis

(a)

ureter

(b)

Figure 16.4 (a) False-colour X-ray showing blood supply to kidney. (b) A simplified diagram of a longitudinal section through a kidney, showing the position of the nephrons. Bowman's capsule > glomerulus >

The nephron and urine production

Bowman's capsule

arteriole from renal artery cortex

venule to renal vein

-

J,.

collecting duct

The main regions of the human nephron are shown in figure 16.5. It is basically made up of a cup-shaped structure called a Bowman's capsule and a long tubule with clearly defined regions. These are called the proximal convoluted tubule, the loop of Henle, and distal convoluted tubul e and the collecting duct. Each has a very important role in the formation of urine. A mass of capillaries, called a glomerulus, is enclosed by the Bowman's capsule. The blood supply to the glomerulus comes from the renal artery which brings blood carrying nitrogenou s and other waste products to be cleansed.

Figure 16.5 Detailed structure of a nephron.

185

Life Processes and Disease

Pressure filtration

IOifEl'fll

The afferent arteriole whlch comes to the capsule has a bigger diameter than the efferent arteriole leaving it. As a result, pressure builds up in the capillaries of the glomerulus. As blood flows under this high pressure, the smaller components of the blood plasma are pushed out into the surrounding cup-like Bowman's capsule. This becomes the filtrate whlch contains water, glucose, amino acids, vitamins, hormones, salts and urea, which are some of the small compon ents of blood. Large molecules, such as plasma proteins, and blood cells (erythrocytes and leucocytes) remain in the blood. The arteriole leaving the · capsule continues to flow through a network of capillaries whlch surrounds the rest of the nephron as shown in figure 16.6. The filtrate flows into the proximal convoluted tubule (figure 16.7). High blood pressure can cause the capillaries of the glomerulus to burst thus destroying the nephron which is the basic unit of the kidney. This can lead to kidney failure. efferent arteriole has

Bowman's capsule capillaries

pressure builds up in the glomerulus

filtrate minus glucose\ \ \ moves down to loop of Henle

the~

Figure 16.6 Bowman's capsule.

Rgure 16.7 The proximal convoluted tubule reabsorbs glucose from the filtrate.

Selective reabsorption proximal convoluted tubule >

186

Selective reabsorption is the reabsorption of a substance in preference to others that are present. This occurs in the region of the nephron called the proximal convoluted tubule. Glucose is a small molecule, so it is a component of the filtrate as it moves through the proximal convoluted tubule. Here it is reabsorbed into the plasma of the capillaries that are wrapped around the tubules. Glucose is not a waste product - it is needed by the body because it is a source of energy. It is reabsorbed from the filtrate, which continues on into the loop of Henle. A person who has diabetes mellitus has glucose in the blood at such a high level that it exceeds that which the kidneys can reabsorb and so glucose is excreted in the urine. This condition is also known as sugar diabetes. The urine of non-illabetics does not contain glucose since all is reabsorbed back into the blood. The urine of a person with illabetes tests positively for reducing sugar (glucose).

16 · Excretion, Osmoregulation and Homeostasis

Reabsorption of water loop of Henle

>

The filtrate now flows through the loop of Henle where water is reabsorbed into the blood capillaries. The longer the loop of Henle, the more water is reabsorbed. The filtrate continues to the distal convoluted tubule. The kangaroo rat, a rodent which lives in deserts, has a very long loop of Henle. Most of the water in the filtrate is thus reabsorbed and conserved by the anima l. It is so good at this that the rat rarely has to drink water.

Selective reabsorption distal convoluted tubule collecting duct ,, CHAPTER18

X

As the filtrate moves through the distal convoluted tubule and collecting duct, reabsorption of salts and water occurs (figures 16.8 and 16.9). This reabsorption, however, is controlled by h ormones and depends on the concentration of solutes in the blood (chapter 18).

Urine

~ l:S:Q~ l.../'-J

Describe the route taken by a red blood cell from the renal artery to the renal vein.

~

l:S:QS l.../'-J Describe the route taken by the urea molecule as it travels from the renal artery to the external environment (in urine).

The filtrate is now called urine, and contains the water, salts and urea that are not needed by the body. The urine flows to the pelvis of the kidney from the thousands of collecting ducts. It then travels, via the ureter, to the bladder to be stored before urination. filtrate from

filtrate with less water

""-....

r+

blood leaves with more water

filtrate, water and salts reabsorbed according to the needs of the body filtrate now called urine flows to the peMs then to the bladder

Figure 16.8 Water is reabsorbed from the filtrate in the loop of Henle.

Figure 16.9 Reabsorption of salts and more water occurs in the distal convoluted tubule and collecting duct.

Table 16. 1 compares the composition of the renal artery with the renal vein and sh ows the effect of the kidneys on 'cleansing' the blood. Renal artery

Renal vein

• contains more water

• contains less water because some is lost with the urine

• contains a high concentration • contains little or no urea because all ls filtered and lost as of urea urine • salt concentration is higher Table 16. 1 Composition of blood in the renal artery and renal vein.

• salt concentration is lower

• more oxygen and less carbon • more carbon dioxide and less oxygen because kidney cells dioxide respire to stay alive and do their work

187

Life Processes and Disease

l.A-J

Both the renal artery and the renal vein contain red blood cells and blood proteins since these are coo large to be filtered out into the Bowman's capsule. All glucose is reabsorbed apart from that used by the kidney cells for respiration. The glucose content is thus lower in the renal vein than in the renal artery.

~

Kidney failure and kidney transplants

~

IT:Q6

Describe the route taken by a glucose molecule as it travels from the renal artery to the renal vein. IT:Q7 l.A-J

Explain these terms: 'pressure filtration', 'filtrate', 'selective reabsorption', 'urine'.

kidney dialysis

>

Kidneys sometimes fail as a result of damage or infection. If one kidney is lost. . the remaining one can undertake the work necessary to remove metabolic waste and keep the body healthy. However, loss of both kidneys is fataJ if not treated. It is possible to transplant a kidney from one person (the donor) into the patient (the recipient). The tissue of both persons, the donor and recipient, must match closely since the body rejects anything that it 'perceives' to be foreign or not itself. A person suffering from kidney failure must have regular treatment on a kidney machine which carries out dialysis (figure 16.10). Dialysis must take place for many hours (up to 10 hours) every few days, to ensure the removaJ of wastes and prevent the build-up of toxic compounds that could poison and kilJ the body cells. Dialysis is a method of separating particles of different size in blood by passing the blood through a tube made from a selectively permeable membrane. This tube is surrounded by a dialysis fluid that has the same concentration as normal blood. Any substance in excess in the blood, such as urea and salts, will diffuse out. Dialysis fluid leaving the machine will therefore be rich in saJts and body wastes like urea. (b) heparin - prevents blood clotting

dialysis fluid out

-

dialysis fluid in

(a) to heart (cleansed blood)

-..........~..,, cleansed blood returned to patient

Figure 16.10 (a) A dialysis machine can do the job of the kidneys after kidney failure. (b) How the dialysis machine works.

188

16 ·Excretion, Osmoregulation and Homeostasis

Osmoregulation osmoregulation

>

~

11~Q8 l/V

(i) Define osmoregulation. (ii) Reabsorption of water occurs in the loop of Henle, and in the distal convoluted tubule and collecting duct. How is reabsorption of water different in the two areas?

The kidneys have a second important function - osmoregulation . They regulate the concentration of body fluids. The amount of water and salts found in the blood is never constant. Daily activities such as sweating and eating cause the concentration to vary. The kidneys regulate the concentration of blood by controlling the amount of water and salts that are reabsorbed into the capillaries during selective reabsorption in the distal convoluted tubules and ' collecting ducts. During its normal circulation, blood passes through the hypothalamus in the brain. The h ypothalamus monitors the concentration of the blood and if the blood is too concentrated - for example, from excessive sweating, ingesting large amounts of salt or drinking too little water - the hypothalamus sends a message to the pituitary gland. The pituitary gland is situated next to the hypothalamus; when it receives the message, it secretes more antidiuretic hormone (ADH) into the blood. ADH stimulates the walls of the distal convoluted tubules and collecting ducts to reabsorb most of the water from the filtrate. As a result, small amounts of concentrated urine are produced (figure 16. 11 ). Hypothalamus detects solute concentration in blood hypothalamus If too high, sends message to pituitary to secrete more ADH

ADH travels in blood to kidneys

More ADH makes distal convoluted tubules and collecting ducts more permeable to water - more water reabsorbed from filtrate

If too low, sends message to pituitary to secrete less ADH

ADH travels in blood to kidneys

Less ADH makes distal convoluted tubules and collecting ducts less permeable to water - less water reabsorbed from filtrate

kidneys

Small amounts of concentrated urine produced

Large amounts of dilute urine produced

Figure 16. 11 The concentration of urine 1s controlled by the secretion of ADH by the pituitary If the hypothalamus detects that the blood is too dilute - possibly due to drinking large volumes of water, little sweating or low salt intake - less ADH is released and little water is reabsorbed. In this case, large amounts of dilute urine are produced.

Homeostasis homeostasis

>

Homeostasis is used to describe all the mechanisms by which a constant internal environment is ma intained. While the external environment outside the body may change, the internal environment inside the body must remain

189.

Life Processes and Disease

fairly constant otherwise all the reactions needed in living cells may be disrupted . The body must detect any deviation from the normal and make the necessary adjustments to return it to its normal condition as quickly as possible. The temperature within the body and the composition of tissue fluid which bathes the body cells must remain as steady as possible for the chemical reactions that occur within these cells to proceed normally (figure 16.1 2). substances to be taken away enter the capilliary b lood flows

body cell

blood from an arterio le

Figure 16.12 Body cells surrounded by tissue fluid and capillaries.

Tissue fluid must: • be within a small range of pH (acidity); • contain enough glucose for respiration and activity; • contain enough oxygen for respiration; • not contain high levels of carbon dioxide; • not contain high levels of nitrogenous wastes; • contain enough, but not too much, water; • be within a small range of temperature; • be specific in many other ways for body cells to function normally.

~

IT:Q9

vv

Define homeostasis and explain why it is important.

Excretion and osmoregulation are examples of homeostasis. A build-up of waste products could damage and even kill cells. Here are some examples of how. • Carbon dioxide causes the pH of the blood and tissue fluid to be lowered, which then affects the rate at which chemical reactions can occur within cells. • Nitrogenous wastes are toxic to cells so they must be cleared from the blood quickly. • Too low a temperature makes chemical reactions too slow, and too high a temperature denatures proteins, including enzymes. • In extreme amounts, water causes body cells to malfunction.

Feedback The body can detect changes in these factors in the blood and has mechanisms to bring the levels back to a normal range. These mechanisms are called feedback mechanisms because a change in the internal environment causes a correction to happen which feeds back to the conditions in the internal

190

16 ·Excretion, Osmoregulation and Homeostasis

negative feedback >

~

environment. Such mechanisms are used to keep the internal environment constant. ll the internal environment is disturbed, the disturbance sets in motion a sequence of events which tends to restore the system to its original state . This is called negative feedback because it removes the effect of the change. Examples of negative feedback can be seen in figures 16.1 3, 16.14, 16.15, 16. 16 and 16.17.

r::=

IT:Q-1 0 V'-1

In the regulation of carbon dioxide in the blood, when does an increase in carbon dioxide concentration come about, and how is the concentration brought back down to a normal level?

too moch - - - - - - • corrective mechanism

normal level

normal level

Rgure 16. 13 A typical feedback mechanism. body fluids too concentrated

high levels of ADH

kidneys reabsorb most water from the filtrate

• excessive sweating • excessive salt intake • low water intake

small amounts of concentrated urine produced correct concentration of body fluids

correct concentration of body fluids

• little sweating • low salt intake • large water intake

large amount of dilute urine produced body fluids too dilute

low levels of ADH

kidneys do not reabsorb much water from the filtrate

Rgure 16. 14 Osmoregulation: control of concentration or blood plasma and body fluids. too much carbon dioxide in the blood

_ _ _ _ _ _ _ _ _.,.

increases breathing rate - - - - - - - -...... carbon dioxide lost more rapidly from lungs

e.g. exercise

normal level of carbon dioxide

normal level of carbon dioxide

carbon dioxide lost less rapidly from lungs too little carbon dioxide in the blood

breathing rate reduced

Figure 16. 15 Control of the amount of carbon dioxide in the body.

191

Life Processes and Disease

• skin produces sweat dilate ,__ _ _ _ _ _..,. body temperature _ _ _ _ _ _ _... •• skin hairscapillaries lie flat ~ rises above 37 °c • respiration slows • panting occurs • fever • exercise • hot environment

body temperature drops

normal body temperature (37 °C)

normal body temperature (37 C) .

°

body temperature rises • cold environment

body temperature

.....,,_ _ _ _ _ _..,.drops below 37 °c

• • • •

no sweat produced skin capillaries constrict hairs become erect -------~ shivering occurs

Figure 16.16 Control of body temp~rature.

~------+

high glucose level - - - - - - - -... (150 mg/100 cm3)

pancreas secretes insulin • liver converts glucose to glycogen and fat • cells absorb glucose

such as after a meal

correct amount of glucose in the blood (90 mg/100 cm3

correct amount of glucose in the blood (90 mg/100 cm3

• liver converts glycogen, fat and protein to glucose • cells absorb less g lucose

such as fasting low glucose level _ _ _ _ _ _ _ _... (70 mg/100 cm3)

pancreas secretes little insulin

Rgure 16.17 Control of blood glucose.

I

192

16 ·Excretion, Osmoregulation and Homeostasis

• Homeostasis is the term used to describe all the mechanisms by which a constant internal environment is maintained. • Feedback mechanisms are used during homeostasis. • Feedback mechanisms are used to control blood glucose levels, water, body temperature and the amount of carbon dioxide in the blood.

.... ITQ1 • Metabolism is all the activities of a cell. These require certain substrates and produce many useful products as well as some waste. The term 'metabolism' encompasses all these reactions at any given time in a cell. • Excretion is the process by which cells and the organisms get rid of metabolic waste. • Egestion is the process by which undigested food in the alimentary canal is leaves the body through the anus. • Secretion is the process by which a chemical, such as a hormone, leaves a gland; for example, the salivary gland secretes saliva into the mouth. ITQ2

How produced

Where excreted

carbon dioxide

during respiration

from the lungs

heat

during respiration

through the skin

nitrogenous compounds

from breakdown of protein

from the kidneys

during photosynthesis

through the stomata

Excretory product In animals

In plants oxygen

The kidney is the organ of excretion of metabolic waste and excess water from th e body. • The bladder stores urine temporarily before excretion. • The renal vein takes cleansed blood away from the kidneys . • The urethra is the tube through which urine passes from the bladder to the outside environment. • The bladder sphincter muscle controls the release of urine from the body. ITQ4 renal artery -+ afferent arteriole -+ glomerulus -+ efferent arteriole -+ renal capillary -+ renal vein ITQS renal artery -+ afferent arteriole -+ glomerulus -+ Bowman's capsule -+proximal convoluted tubule -+ loop of Henle -+ distal convoluted tubule -+ collecting duct -+ pelvis -+ ureter -+ bladder -+ urethra ITQ6 renal artery -+ afferent arteriole -+ glomerulus -+ Bowman's capsule -+ proximal convoluted tubule -+ renal capillary -+ renal vein ITQ7 • Pressure filtration is the filtration of the smaller components of blood into the Bowman's capsule. It occurs because of pressure that builds up as blood flows from a wider vessel into a smaller vessel. • The filtrate consists of the smaller compon ents of blood (urea, water, salt, glucose, etc.) that are filtered into the Bowman's capsule and move along the tubes of the nephron. • Selective reabsorption is the reabsorption of a substance in preference to others that are present. Glucose is selectively reabsorbed back into the blood while other components of the filtrate continue along the nephron . • Urine is the substance that collects in the bladder. It is m ade up of water, salts and urea. ITQ3 •

193

Life Processes and Disease

ITQ8 (i)

Omsoregulation is the maintainance of constant osmotic conditions

in the body. The regulation of the water content and solute concentration of

body fluids is important for cells to work efficiently (ii)

Loop of Henle

Distal convoluted tubule and collecting duct

re~bsorption

reabsorption of water is controlled by antidiuretic hormone (ADH)

of water is automatic

the longer the loop of Henle, the more water is reabsorbed

water reabsorbed according to needs of body

Homeostasis is the maintenance of a constant internal environment. CeJls need a constant environment in which to function efficiently. Homeostasis describes all the mechanisms that come into play to keep the internal environment constant. For example, enzymes need a specific temperature and pH to function efficiently. ITQ10 During exercise, respiration increases and so does the concentration of carbon dioxide because it is a waste product of respiration. Carbon dioxide is transported to the lungs to be excreted. When there are increased amounts of carbon dioxide in the blood, the heart beats faster, allowing blood to flow faster to the lungs and therefore more carbon dioxide is excreted. The carbon dioxide concentration is thus brought back to the normal level in the blood . ITQ9

Examination-style questions (i)

Define: (a)

excretion

(b) osmoregulation.

(ii) Using an annotated diagram only, describe how urine is formed in a nephron. (iii) Describe how and why the volume and composition of urine changes: (a) after strenuous exercise; 2

(i)

(b)

after drinking large volumes of water.

List three excretory products, besides nitrogenous waste, produced by animals.

(ii) List two excretory products produced by plants.

(iii) (a) Describe the production of nitrogenous waste in humans. (b) Describe the excretion of nitrogenous waste in humans. 3

(i)

(a) Label the parts A, B, C, D, E, F and Gin the figure below. (b) In each case state its role in excretion. (c) List four differences between A and B.

f!

i------+---

+++---

-

A

-+-- B

1------+--- C

r._- - -I + - --

194

-

-

---+- F --+-

G

16 ·Excretion, Osmoregulation and Homeostasis

(ii) The kidneys are very important organs involved in the removal of toxic substances which, if allowed to accumulate in the body, could be fatal. (a) The body offers some physical protection of its internal organs. How are the kidneys protected? (b) Suggest two ways the kidneys may be damaged. (iii) Describe how a dialysis machine works to cleanse blood during kidney failure. 4

(i) Define homeostasis. (ii) The figure below shows some body cells and their supply of blood. (a) List some differences between blood at A and B. (b) Name the process occurring at C. (c) Name the process occurring at D. (d) Explain fully how the cell labelled Eis supplied with oxygen. (e) Name the substance found in F. (f) State three properties of F. (g) State the importance of the properties listed in (g) above. (h) Describe the mechanism by which one of these properties is regulated.

195

ovement

0 0 0 0 0 0 0

understand the importance of movement in animals understand growth movement in plants understand how external factors affect plant movement describe the structure and function of the skeleton of humans describe the mechanism of movement in a limb of humans describe the long bones of a fore and hind limb describe the cervical, thoracic and lumbar vertebrae

movement

I

r

r

plants

skeleton

auxin

___,__

,,

'

limbs (

I

muscle -

tendons

'

vertebral column+ skull

l

joints

I

( hinge

'

animals

ligaments

'

r phototropism

' geotropism

vertebrae • cervical • thoracic • lumbar

ball-and-socket

The importance of movement in animals Most animals have to look actively for food and, to do this, they must move from one place to another. Movement from one place to another is called locomotion and it involves the expenditure of energy. There are a number of reasons why animals move from one place to another and these include: • to find food; • to escape predators; • to find a mate; • to disperse offspring; • to reduce competition; • to avoid danger; • to avoid waste products; • to avoid extreme environmental conditions. Animals move in many ways, which include flying, swimming, walking, running and gliding. Each animal is adapted to move in its own special way. For example, humans are adapted to walk or run from place to place.

17 · Movement

Movement in plants Movement in plants can be demonstrated by a germinating seedling. When a seed has germinated, it will grow into a seedling if all the conditions for germination are met. Movement is seen when it grows. The shoot grows towards a light source and the roots always grow downwards towards gravity into the soil. This is related to nutrition in the plant as plants need light and water for photosynthesis. Movement in plants is thus usually growth movement. and when we study movement in plants we look at factors that lii•l•ll•juiJ affect their growtJ1. Growth movements are called tropisms. A few plants show another kind of movement apart from growth movement. The sensitive plant (Mimosa pudica), for example, can fold its leaves when touched (figure 17. l). Some plants like the hibiscus can fold their petals at night. Insectivorous plants like the Venus flytrap can catch small insects by moving a part of its body, and the pods of pigeon pea can curl and split when dry to disperse the seeds, as a part of reproduction.

Figure 17. 1 Plant movements. (a) Mimosa pudica 'wilts' when touched. (b) The trap of a Venus flytrap closes when an insect touches the sens1t1ve hairs on the surface.

Growth movement in plants The most important plant movements are tropisms or growth movements. Growth in response to the stimu lus of light is called phototropism , and geotropism is growth in response to gravity. Growth in plants is controlled by l:Uf3hQ the hormone, auxin. Auxin is made in the tips of roots and shoots which are the growing parts of the plant. It diffuses to the region just behind the tip and Practical activities there it causes growth (figure 17.2). Light and gravity are examples of external factors (factors in the SBA 17.1 : Does gravity affect plant growth? page 354 environment) that affect growth in plants. The shoots of plants respond to light SBA 17.2: The growth of a radicle, by gi·owing towards it. When a shoot is Lit from one side, a uxin breaks down page 355 on the Light side and accumulates on the shaded side. This results in more SBA 17.3: Does light affect plant growth on the shaded side so the shoot bends towards the light (figure 17.3, growth? page 356 overleaf) . In a shoot whid1 is not upright, gravity causes the auxin to collect on the lower side. This has the same effect as before, rn make the shoot grow faster on that side, so it bends away from gravity. phototropism geotropism

> >

more auxin produced which diffuses down

Rgure 17.2 A shoot grows because of the hormone auxin.

197

Life Processes and Disease

more auxin accumulated on the shaded side

__

growth;;~ .. •:.·/(· .

..... ' '-"'-"'

.,

·") ' "

(

light

light

less growth

shoot grows towards the light shoot is horizontal

less

grow~;~!:))

·~ore

auxins accumulate on the lower side due to gravity

growth

shoot grows upwards

~ IT:Q3

Figure 17.3 Shoots always grow towards light and upwards or against gravity.

Agar blocks were placed under cut tips of shoots. The blocks were then placed on growing shoots as seen. How will each shoot grow?

However in roots, concentrations of auxins slow down growth. As in the shoot, auxin accumulates on the lower side because the gravity, but in a root the upper side will grow faster because it is less affected by the auxin. No matter how a root is placed in the soil, it will always grow downwards (figure 17.4).

l.../'-J

gravity

TI I!

gravity

l

l

l agar block

root placed horizontally

·.:.

·:·~

growing tip

l

l ~ gro~

more growth

less

·:'-..,

root grows downwards

'(.

roots always grow downwards or towards gravity

Figure 17.4 Roots always grow down

Simple investigations can show the effects of light and gravity on germinating seedlings as shown in figure 17.5. They use agar blocks containing auxin because the hormone can easily diffuse through the agar. :·., shoot tip cut

and placed on an

agar absorbs

! : ~'i -------_.theauxin

..~ shoot tip cut off cut shoots are used in investigation

agar block with auxin placed on a cut shoot - shoot will grow

/~ agar block with auxin placed to one side of a cut shoot

agar block with no auxin results in no growth

Figure 17.5 Investigations of growth in seedlings. Agar blocks which absorb auxin are used.

198

17 · Movement

Uses of plant hormones

· 1;rm;mmm

Pesticides are poisonous chemicals which kill pests. Some plants, especially weeds, are described as being pests and herbicides are used to kill them or remove them from the environment. Synthetic plant hormones, for example auxin, can be used as a herbicide. When present in excessive amounts, much more than produced naturally, auxin can disrupt plant growth and so kill plants. 2,4-D and 2,3, 5-T are examples of selective herbicides which kill broad-leaved plants. They stimulate auxin production in the plants. The weed killer causes dicotyledons to grow so fast that they cannot sustain their own growth and they die. Some herbicides work because they are translocated throughout a plan t. They are called systemic herbicides. They are translocated from the leaves, where they were applied to the roots where they interfere with root function. Because the root is killed, the whole plant dies.

The skeleton of humans endoskeleton >

exoskeleton )

axial skeleton

>

appendicular skeleton >

The skeleton of h umans is an endoskeleton , which means that it is inside the body. All vertebrates have the same arrangement of endoskeleton, with the bones inside and the muscles and other body tissues surrounding it. Some invertebrates also have an endoskeleton, such as squid and octopus, but many have an exoskeleton where the hard part is on the o utside. For example, insects have a jointed exoskeleton made of chitin, and many molluscs, like dams, have a hard calcified shell. Exoskeletons have an advantage in that they can protect the whole of the body, but they also limit the size to which the organism can grow. The body of h umans is held upright by a skeleton which is made of bones arranged as seen in figure 17.6 (overeaf). The human skeleton can be divided into two parts: the axial skeleton, which is the skull and vertebral column with the rib cage, and the appen dicula r skeleton, which includes all the other bones, the fore and hind limbs, and the pelvic and pectoral girdles.

Functions of the skeleton in humans

~

l'.T:Q'4 L.-1'-I Name the bones found in the lower limb, from the pelvic girdle to the toes.

~

l'.T:Q5 L.-1'-I What is the importance of the blood vessels and the marrow in a long bone?

• Protection of organs - The skull protects the brain, the vertebral column protects the spinal cord and the ribs protect the lungs, heart and much of the liver. Bones surround these delicate organs, forming cup-like structures or tube-like structures in which the organs are housed. • Support of the body - Humans are supported upright more than most mammals and can stand on two feet. The skeleton acts Like a fra me supporting the soft body parts. The limbs are separated by the width of the girdles and this helps to keep the body stable. • Movement - The skeleton is made up of a number of bones joined together. Muscles, and other tissues such as tendons, can cause movement of a single bone. The coordinated movement of many bones results in walking, running and all the movement seen in a human. • Manufactur e of red and white blood ce lls - These are made in the bone marrow of th e pelvis, ribs, sternum and leg bones.

Structure of a long bone The long bones are the femur, tibia and fibula of the hind limb and the humerus, ulna and radius of the fore limb. The structure of the long bone is shown in figure 17.7 (overleaf).

199

Life Processes and Disease

skull cranium -

+---

shoulder girdle (pectoral girdle} ~----clavicl e

thorax

upper limb -+--

- - - humerus

metacarpals phalanges

lower limb -+--

-

femur

-+H--tibia r + - - - fibula

the axial skeleton (skull and vertebral column) is coloured

D

tarsals """M'•:i:--- metatarsals

phalanges

Figure 17. 6 The human skeleton.

The vertebral column In humans, the vertebral column extends from the neck to tailbone or coccyx

~

rf:Q6 vv Describe two functions of the vertebral column.

200

(figure 17.8). It is made up of 33 bones called vertebrae. All vertebrae have the same basic structure (figure 17.9). There are 7 neck or cervical vertebrae, the first of which are the atlas and axis. The cervical vertebrae are followed by 12 thoracic vertebrae, then 5 lumbar vertebrae. The sacrum follows the lumber vertebrae and is made of several vertebrae fused together. Finally, the tail vertebrae are fused to form the coccyx.

17 · Movement

neural spine

~ ,,.,,,.... in

anterior facet

cervical

epiphysis

J....- cartilage spongy bone (contains red marrow)

Structure of a typical vertebra thoracic vertebrae (12)

-

-compact bone marrow cavity

shaft

spinal cord runs through the neural canal

blood vessel lumbar vertebrae (5) three vertebrae interlock to form part of the vertebral column sacral vertebrae (fused)

epiphysis

coccyx_o_r _'ta _i_I' - - - - -

Rgure 17.8 The vertebral column in humans.

Figure 17.7 The structure of a long bone.

v

~C centrum

transverse process

~ertebrarterial canal (two small holes in vertebra, one on either side)

Cervical vertebra - has two small holes apart from the large neural canal neuraI canaI

\If

(}___neural spine (long)

transverse process (short) - facet - -

't- neural spine (short)

neural canal ~ ' facet

~

6

centrum big and - - well developed

~

rib

_/

-...-

J

\

.,J-~

centrum ~ Thoracic vertebra - articulate with ribs as well as other vertebrae

~ transverse process (long)

)

Lumbar vertebra - has large centrum and long transverse processes

Figure 17.9 The cervical, thoracic and lumbar vertebrae in humans.

201

Cervical vertebrae • • • •

large neural canal because these vertebrae are closest to brain; vertebraterial canals present; shore neural spine; short transverse processes.

Thoracic vertebrae • neural canal smaller than cervical because further from brain; • very long neural spine for attachment of back muscles; • short transverse processes to accommodate rib bones on either side.

Lumbar vertebrae • • • •

centrum big and well developed to support weight of body; neural canal small; long, wide neural spine; long transverse processes for muscle attacl1ment.

Table 17. l summarises the functions of the various surfaces and projections of ead1 vertebra. Part of vertebra

Function

neural canal

protects the spinal cord

neural spine

muscle attachment

transverse process

muscle attachment

facet

articulates with facets of adjacent vertebrae and allows slight movement

centrum

central rigid body of vertebra, discs of cartilage separate adjacent vertebrae

Table 17.1 The functions of the different parts of a human vertebra.

Movement in a limb of humans Practical activity SBA 17.4: Compare the movements of

tour animals, page 357

spongy bone compact bone

articular cartilage - functions as a

Movement in a limb is brought about by many tissues, such as muscles, tendons, ligaments and bones, all working together. Bones are able to move because of the presence of joints in the skeleton. A typical joint is seen in figure 17. 10. Bones are attached to each other by ligaments. They cannot move on their own. Muscles are seen around the bones and move the bones when they shorten (contract) and lengthen (relax). This is sh own in the simplified diagram in figure 17.1 1.

muscle

ligament

synovial membrane contract ligament

synovial fluid - lubricates joint reducing friction during movement

Figure 17. 1O A typical joint.

202

contract

+-bone moved to the left

Figure 17. 11 Bones are moved by muscles.

-

bone moved to the right

antagonistic muscles

>

l(ijet•l•hll

The muscles of the arm move the bones of the arm to flex or extend the arm in the same way as seen in figure 17.12. The bones are attached to each other by ligaments and attached to muscles by tendons. They have special names (triceps and biceps) and contract or relax to move the bones. All the bones of the body need muscles to help them move. Imagine the coordination of contraction and relaxation of muscles needed to cup the fingers around a , bottle, and then move the bottle to the lips to cake a drink of water. Movement is brought about by the contraction of antagonistic muscles . Antagonistic muscles are pairs of muscles that always work together: when one is contracting, the other is relaxing. They move many bones of the human skeleton . In the joint of the upper arm, the triceps and biceps are antagonistic muscles. They are attached to the bones by tendons which are non-elastic. A muscle shortens when it contracts and is lengthened when it relaxes. Movement of the bone is brought about when the muscles pull on the bones (figure 17.12). Flexing the arm

Extending the arm

/\~

tendons, attach r muscle to bone

~

biceps muscle (contracts) (flexor muscle) triceps muscle (contracts) (extensor muscle)

triceps muscle (relaxes)

ulna

biceps muscle (relaxes) ,.

arm bends or flexes arm extends

Figure 17.12 Flexing and extending the arm.

When the biceps contracts (and triceps relax), it pulls the bones of the lower

flexor muscle

> arm upwards so the arm bends or flexes. The biceps is called a flexor muscle.

extensor muscle >

When the triceps contracts (and biceps relaxes), it pulls th e bones of the lower arm so that the arm straightens or extends. The triceps is called an extensor muscle.

Types of joint There a re three types of joint: • immovable joint; • partially movable joint; • movable joint. E!UliiM

gliding joint

>

l•M•llt.lleiH

Immovable joints are also called sutures. The bones are fu sed together allowing no movem ent. Examples are joints of the cranium and pelvic girdle. Partially movable joints allow some movement. Examples of joints between the tarsals (ankle) and carpals (wrist). The bon es ca n slide over each other producing the movements seen in the wrist and ankle. These are also called gliding joints. A partially movable joint also exists between the atlas and axis at the top of the neck allowing some m ovement of the head in relation to the spine (e.g. nodding or shaking). This is called a pivot joint.

203

synovial joint

>

Moveable joints are also called synovialjoints. Synovial fluid in these joints reduces friction allowing free movement of the bones. There are two types of synovial joint (figure 17.13). • Hinge joint - Allows movement in one plane; for example, elbow, knee and finger joints. Bones of a hinge joint are capable of carrying heavy loads. • Ball-and-socket joint - Allows movement in all planes; for example, the shoulder and hip joints. Hinge joint

Ball-and-socket Joint

one plane

- - s o cket

• movement is restricted to one p lane • may be dislocated • e.g. elbow, knee, fingers

• movement is allowed in three planes • easily dislocated • e.g. pelvic girdle (hip), shoulder

Figure 17. 13 The hinge and ball-and-socket joints.

~

~

l:F:Q1'

l:F:Q9

(i) What is a joint? (ii) What is the importance of joints?

Put these in the order they occur when extending the arm: (a) the arm is pulled down. (b) biceps muscle relax. (c) ligaments stretch arm. (d) muscles pull on radius and ulna. (e) triceps muscle contract.

\../'-I

~

l:F:Q8 \../'-I Name the structures found around a typical joint, giving a reason why each is important.

\../'-I

~

l:F:Q·1 0 \../'-I

(i) What kind of joints are seen in the fingers? (ii) What is the advantage of each finger having a number of hinge joints, rather than one hinge joint?

204

17 · Movement

fl'

-

Chapter summary • Movement is a characteristic of life. • There are a number of reasons why animals move. • Some plants can move some of their parts, but all plants show growth movement or tropisms. • During growth, plants respond to light (phototropism) and gravity (geotropism). • The tips of the growing parts of a plant produce auxin, the hormone responsible for growth in plants. • The skeleton of humans has many functions, one of which is movement. • The skeleton forms a framework inside the body of humans and is made up of the axial and appendicular skeletons. • The axial skeleton consists of the cranium and the vertebral column. • The vertebral column is made up of many bones called vertebrae and includes the cervical, thoracic and lumbar vertebrae. • The appendicular skeleton consists of the limbs and rib cage. • The skeleton is made up of many bones joined together; movement is seen at these joints. • Movement is brought about by muscles, tendons and ligaments at these joints. • There are many kinds of joint: immovable, partially movable and movable joints.

Any two of the examples from the bullet list on page 000 is suitable. A plant has to move (grow) towards the ligh t because light is necessary for photosynthesis. Some plants are able to close special leaves that trap small insects. These plants need to acquire their protein from insects, because they live in nitratedeficient soil.

ITQ1 ITQ2

ITQ3

Pelvic girdle, femur, tibia and fibula, tarsals, metatarsals, phalanges. The blood vessels bring nutrients and oxygen to the bone, since it is alive and must respire. Also, these vessels take away waste produced by the bone. The marrow cavity is important for the production of red blood cells. Red blood cells are constantly produced in the bone marrow. ITQ6 The vertebral column holds the body upright and protects the spinal cord. ITQ7 (i) A joint is where two bones m eet. It is lubricated to reduce friction when the two bones move. (ii) Joints are important for movement. All movement takes place because muscles contract and move the appropriate bones. The bones move from where they are joined to another bone. Without joints, no movem ent would not be possible (not movement of the entire body, nor movement of a part of the body). ITQ4 ITQS

205

~;.; :.- .... L~f~! ~.recesses

and DiseC!_se ·

·

..

..

ITQ8

Structure

Importance

ligament

joins bone to bone, and can stretch as the bones move away from and towards each other during movement

muscle

can contract or lengthen - because it is attached to bone, it can pull on (extend) the bone, so muscles bring about movement (

tendon

joins muscle to bone, is non-elastic, so the effect of the muscle contraction or· relaxation can be applied to the bone

synovial fluid

fluid found in the joint which helps to reduce friction when bones move with respect to one another.

ITQ9 1 - b and e (antagonistic muscles); 2 - d; 3 - c; 4 - a ITQ10 (i) The fingers have hinge joints. (ii) Having a number of hinge joints allows fingers to be curled around an

object.

Examination-style questions (i) List the main functions of a vertebrate skeleton. (ii) Make a labelled drawing of: (a) a typical vertebra; (b) a vertical section of a typical long bone. (iii) The human skeleton, as is typical of mammalian skeletons, can be divided into two components or parts. Name these parts and the bones included.

206

2

(i) Suggest some differences between movement in plants and animals. (ii) Define: (a) phototropism; (b) geotropism. (iii) Explain fully how plants respond to light. (iv) How do plants growth substances differ from animal hormones? (V) (a) Since the late 1980s, scientists have been conducting experiments on the effects of space travel on seed germination. Why do you think there is an interest in such studies? (b) Experiments conducted on seeds in space yielded growing plants, but these 'extra-terrestrial' plants did not grow straight, they grew in all directions. Explain what might have caused this to happen. (c) Suggest ways of producing 'straight plants in space.

3

(i) Make a labelled drawing of a typical joint. (ii) The exoskeleton of an insect lies outside the muscles that are attached to it. Like the joints of the endoskeleton, the joints of an exoskeleton, provide an excellent means of locomotion. The diagrams I and II below show joints seen in an insect and humans.

I - insect's limb extensor muscle

II-human limb

A

E

c (a) (b) (c) (d)

Name the parts A, B, C, D, Eand F. Using a diagram, show how the insect can flex or bend its leg. Using a diagram, show how the human can bend the arm. What name is given to muscles that work together to move a limb? Give examples of these muscles as seen in the insect's limb and human's limb. (iii) Describe the state of these muscles when: (a) the insect's limb is extended or straightened. (b) the human's limb is extended or straightened.

207

l·nritabi Iity, Sensitivity and Coordination 0 0 0 0 0 0 0 0 0 0

define the terms 'stimulus' and 'response' describe responses of green plant and invertebrates to stimuli understand why responses to stimuli are important for survival of organisms define the terms 'receptor' and 'effector' identify the main sense organs and the stimuli to which they respond describe the main sense organs describe the nervous system describe the endocrine system explain a simple reflex action distinguish between a cranial and spinal reflex

0

describe the functions of the main regions of the brain

0

discuss the physiological, social and economic effects of drug abuse

sense organs eye ear nose tongue skin

receptor

stimulus

survival of organism

'

nervous(system spinal(chord

response

'

brain

effector

motor sensory relay synapse

neurone

movement towards or away

:

Irritability, Sensitivity and Coordination

Irritability CHAPTER 1

1n chapter 1, we found that irritability is one of the seven d1aracteristics of

living things. It means that living organisms can respond to changes in their internal environment and the world around them. These responses usually increase their chances of survival. Animals and plants react to changes in the environment, not only drastic climate changes, but also simple everyday changes. For example, a snake looking for food will move toward the scent of a rat, and the shoots of a seedling will grow towards Ugbt.

Stimulus Eihuii!ilO-fl li%l•r•lli'--J#I

A stimulus is a change in the environment that an organism reacts or responds to. It could be light, temperature, a texture, a chemical in the air or moisture, a response is the d1ange in the organism brought about by the stimulus (figures 18.1 and 18.2). The response to stirnuU is important for the survival of organi sms.

Response of animals Stimulus

Figure 18.2 This male moth has large antennae that can sense just a few molecules of a chemical attractant that a female several miles away has released.

Q5b

IT:Q·1 l...A.J (i) Define irritability. (ii) Why is it important for the survival of an animal?

Molecules from the rat are in the air. The snake detects these as it 'tastes' the air with Its forked tongue.

Response

The snake moves towards the rat. It is delicious food and important to the survival of the snake.

Figure 18.1 A snake responds to the stimulus of food.

Table 18.1 shows some examples of stirn ulL the responses and the importance to the organism of responding in this way. Stimulus

Possible response

Importance to organism of response

chemical from an organism

move towards organism

organism may be a potential mate or potential prey

moisture in soil

move towards moist areas

prevent desiccation or dying, especially for organisms without a waterproof outer covering

light

move from light to darker areas

escape from predators since it is harder to be seen in darker areas

cold temperatures

move away from cold temperature

organism cannot survive In cold temperatures, body not adapted

Table 18.1 Responses to some stimuli and the importance of those responses.

209

---

.

Life Processes and Disease

Response of green plants In chapter 17, we saw that seedlings respond to unilateral (one-directional)

CHAPTER 17

stimuli of light and gravity. The roots grew in the direction of gravity and the shoots grew towards light. Plants need water and minerals from the soil, so the roots must grow down into the soil to reach them. Green plants, including seedlings, also need light for photosynthesis. It is therefore important for the survival of green plants to grow towards light (figure 18.3). A plant in a room will grow towards the window where there is sunlight. A seed may be taken into a cave by a bird or bat. It may germinate and then the seedling will grow towards light and out of the ca ve's entrance or any other opening. Otherwise, the plant will die for lack of food in the darkness. Invertebrates, like millipedes, earthworms and wood li ce, need certain conditions to survive. They respond to variations in light intensity, temperature and moisture. The investigation illustrated in figure 18.4 shows that these invertebrates respond by moving towards a cooler temperature, moist soil and away from bright light. These responses ensure that they do not dehydrate and are hidden from predators, that is, the responses help to ensure their survival.

It germinates and begins to grow towards the light at the cave's entrance. It could die in the darkness.

seed taken into a cave by a bird or bat

It reaches the entrance where there is light. It can now photosynthesise efficiently and will survive.

Figure 18.3 A plant responds to the stimulus of light.

Ilght

dry soil 10 organisms, e.g. woodlice, are placed in the apparatus

After a while they all move towards the dark, moist areas.

Figure 18.4 Many small invertebrates respond to the stimuli of light and moisture.

~

ll'!Q2 V-...J The senses of some animals are said to be better developed than in a human. Give two examples of animals like this, and explain the importance of the sense to the animal.

Unlike most humans, anima ls in the wild have to find food every day and maybe avoid being food for another organism. They have to be very aware of stimuli coming from their environment and be able to make the appropriate response. More often than not, these everyday changes in the environment are a matter of life or death.

The sense organs of humans Humans have five senses: hearing, sight, smeU, ta ste and tou ch. In humans,

sense organs

> the main sense organs are the eyes, ears, nose, tongue and skin. A group of sen se cells and other tissues fo rm a sense organ. • Eye - At the back of the eye is the retina which is a layer of sensory cells that respond to light. Impulses are sent from these ceUs to the brain by the optic nerve so that d1anges in shape, colour, brightness and distance are detected.

210

:

Irritability, Sensitivity and Coordination

• Ears - Sensitive hairs in the inner ear respond to vibrations in the air (sound waves). Impulses are sem from these hairs to the brain by the auditory nerve so that changes in the quality, tone, pitch and loudness are detected. • Nose - As air flows into the nose during breathing, chemica l molecules in it touch sensitive hairs. These send messages to the brain so that changes in scent are detected. • Tongu e - Groups of receptor cells, called taste buds, respond to chemicals in the food (figure 18.5 ). Different parts of the tongue are sensitive to different flavours like salt, sweet, bitter and sour. These send m essages to the brain so that changes in flavour of the food are detected. • Skin - This is the largest organ of the body. Nerves ending as sensory cells are scattered throughout the skin. These are sensitive to pain, touch, change in temperature, light pressure and heavy pressure. They send in1pulses to the brain so that it can detect what has been touched.

Figure 18.5 Taste buds on a human tongue.

The nervous system 1.r411g.ur41 Practical atctivity SBA 18.1 : Touch receptors in skin, page 359

The ne rvous system is made up of neurones or nerve cells. Neurones transmit electrical impulses to and from the brain. The nervous system is made up of: • the central nervous system (CNS) whkh consists of the brain and spinal cord; • the peripheral nervous system (PNS) whid1 consists of all the nerves outside the central nervous system (figure 18.6).

central nervous system (CNS)

cranial nerves (from brain) peripheral nervous system (PNS) ~r++- spinal nerves (from spinal cord)

Rgure 18. 6 The nervous system.

sensory neurone motor neurone relay neurone

> > >

The peripheral nervous system forms a vast communication network linking the reception of the stimuli to a response. Receptors receive stimuli from the environment and responses are brought about by effectors. Sensory neurones conduct impulses from receptors to the central n ervous system. Motor neurones conduct in1pulses from the centra l. nervous system to the effectors. Intermediate or relay neurones link sen sory and motor neurones. They are found in the central nervous system (figure 18.7, overleaf).

211

Life Processes and Disease

dendron - carries impulses towards the cell body

intermediate or relay neurone

motor neurone

sensory neurone

node of Ranvier

Figure 18. 7 Motor, relay and sensory neurones.

.,____

-----

stimulus

sensory neurone

I

receptor

IITT-O~L relay neurone /

-

motor neurone

effector brings about response

Figure 18.8 The structures of the sensory, relay and motor neurones can be related to this typical nervous pathway.

~

IT:Q3 V'-1

Describe the nervous system of humans. (ii) How do you respond to a stimulus? (i)

212

Figure 18.8 is a typical pathway, from the s~ulu s touching the receptor to the effector bringing about a response. The numbers in the following paragraphs refer to figure 18.9. 1 The stimulus is, say, a hot object touching a pain receptor in the skin of the hand. 2 A signal travels along the sensory neurone to the central nervous system (CNS). 3 In the CNS, a relay neurone carries the signal through the brain. 4 The rela y neurone passes the signal to the motor neurone. 5 The signal travels along the motor neurone to the effector (biceps muscle) which responds (contracts). 6 The hand is moved away from the hot object.

.

18 ·Irritability, Sensitivity and Coordination

contraction of the biceps muscle moves hand away from hot object

axon

dendron

-

CNS spinal cord to brain

stimulus e.g. hot object

motor neurone conducts nervous impulses from CNS to the effector

sensory neurone conducts nervous impulses from receptor to CNS

Rgure 78.9 A typical pathway of receptor to effector.

~

l'.fQ~ V'-'

Describe a typical nervous pathway.

The nervous system is adapted to carry messages quickly between specific locations in the body, so that quick responses can be made. Sometimes the effector may be a gland. Endocrine glands are found throughout the body and they regulate a wide range of activities, including heart rate, metabolism and reproduction. Together, the nervous system and endocrine system co-ordinate all of the body's activities.

The synapse E'AhM"1¥il

Signals travel along neurones as electrical impulses, which are very fast. However, there are millions of neurones in your body, and where the ends of two neurones meet there is a small gap called a synapse (Figure 18.10). Electrical impulses cannot cross th ese gaps, so they are converted to a chemical signal in order to cross the synapse. As they reach the other neurone, they are converted back into electrical impulses so that they can con tinue quickly on their way. A

sensory neurone impulse arrives in sensory neurone cell body mltochondrlon

sensory

A~neurone synapse

Figure 18.10 A synapse.

- - next nerve cell message is transferred to another nerve cell

B

213

Life Processes and Disease

~

l:T:QS V'-1

What is the receptor, the effector and response of an animal when it sees and moves towards a mate?

Table 18.2 describes some receptors, effectors and responses in humans. Stimulus

Receptor

Effector

Response

object moving towards retina of the eye the face receives and sends a message to the brain

muscles of the neck

head turns away so the object cannot hit the face

very hot object which is nerve endings in about to be picked up the skin sensitive to temperature send a message to the brain

muscles of the arm

hand pulls away from. the hot object

chemicals from food (smell) reach the nose

chemoreceptors in the salivary gland nose send a message to the brain

saliva secreted and body prepares to digest food

Table 18.2 Receptors, effectors and their responses in humans.

~

l:T:Q6 V'-1

(i)

What is a synapse?

All activity involves the coordination of the brain, spinal cord, sensory and motor neurones. Stimuli are constantly being received, sent to the brain where they are analysed and appropriate responses sent back (figure 18. 11).

(ii) Describe what happens at a

synapse.

1 Receptors in the skin (and face) receive information and messages are sent to the brain.

@ Brain receives and interprets

'-._. '

/

message: appropriate response determined.

/

·!

/

-1 I\ Examples

3 Messages are sent from the brain to the appropriate effector.

Jl

Touched on shoulder nervous Person turns around Receptors in skin of shoulder ~ system ~ Effectors are muscles needed to turn the body Your name is called Receptors in ears

Sees a friend Receptors in eyes

nervous

system ~

nervous ~

system

Person stands up Effectors are muscles needed to stand up (legs) Person walks towards friend Effectors are muscles needed to walk

Rgure 18. 11 Every day, millions of messages are received by the brain and appropriate responses made. This Involves the coordination of sensory neurones, CNS and motor neurones.

214

~~~.l!!'itabil.!_ty, Sensitivity and Coordination

Reflex actions reflex action

> A reflex action is a rapid and automatic response to a stimulus. It does not

Practical activity SBA 18.2: Two reflex actions, page 360

~

IT:Q7 V-...J

What is the reflex action?

require conscious control (you do not think about doing it). Examples of reflex actions are the knee jerk, sneezing, the pupil reflex and blinking. The pathway between the receptor and effector is called the reflex arc. There are two kinds of reflex: 1 • spinal reflexes are nerve impulses that pass through the spinal cord and do not go to the brain (e.g. the knee jerk response, figures 18. 12 and 18.13 ); • cranial reflexes are reflexes in the head region (e.g. blinking and the response of the pupil in the eye to light, figure 18.12).

(a) Simple (flow) diagram of the knee jerk reflex (spinal)

(b) Simple (flow) diagram of the pupil reflex (cranial)

bright light stimulus stimulus received by pressure receptors at base of knee

.....I

sensory nerve from receptors in eye

sensory nerve to spinal cord

---~

r----~

motor nerve to leg muscles (effector) pupil gets smaller in response to bright light to protect the retina

motor nerve to muscles in iris

Rgure 18. 12 Simple diagrams of a spinal reflex and a cranial reflex.

2 stretch receptors detect

impulse causes the leg muscles to contract. pulling the foot forwards

the pressure on the tendons ::::~~~===f=.==~

9

sensory neurone 4 Impulse comes to the CNS but does not go to the b rain

knee cap

femur

1 st imulus - pressure on tendons ---i- ---r- tibia

7 foot pulled forwards - res ponse

Figure 18.13 Detailed description of the knee jerk spinal reflex.

215

Life Processes and Disease

The brain

~ IT:QS \./'-I

A person who suffered brain damage is now unable to see. Explain how this could happen. (ii} What consequences would result from damage to the cerebellum of the brain? (i}

The brain is the most important part of the nervous system. It enables humans to 'think' or 'reason', a skill which is supposedly lacking in most animals. The brain has grey maner on the outside and white matter on the inside. It is surrounded by tough membranes, called meninges, and cerebrospinal fluid which cushion it from knocks. It is also surrounded by the bones of the skull. Your brain is very well protected (figure 18.14) . Humans can perform compl ex mental and physical activities co-ordinated by different areas of the brain (figure 18.15). They receive stimuli from the environment and the brain brings about the appropriate response.

(a)

(b)

meninges - the membranes covering the brain and spinal cord

~--

left cerebral hemisphere

bone

cerebrospinal fluid

LM-....._,,....,__~

h

~

IT:Q-11 V'-1

(i) What are hormones? (ii) Name four different hormones produced in humans.

Most, but not all, endocrine glands work under the influence of a single master gland - the pitu itary, which is situated beneath the brain. The hypoth a lamu s is situated close to the pituitary. While the h ypothalamus is not an endocrine gland, it regulates the secretion of some of the pituitary gland hormones. If the pituitary is thought of as the master gland of the endocrine system, then the hypothalamus can be tho ught of as the manager. The hormones produced by the pituitary and their effects are shown in Table 18.3. Hormone

Functions

pituitary growth hormone

stimulates growth of the entire body: too much causes gigantism; too little causes dwarfism

antidiuretic hormone (ADH)

stimulates the kidneys to reabsorb more water from filtrate when the blood plasma becomes too concentrated

other hormones: e.g. follicle stimulating stimulate other glands such as the hormone (FSH), luteinising hormone (LH), thyroid ovaries,thyroid, and testes into activity stimulating hormone (TSH) Table 18 3 Functions of the hormones produced by the pituitary.

Drugs and the effects of drug abuse li!il[eiJ

A drug is any substance or chemical which alters the body's action, or interferes with some aspect of the body's metabolism. It affects chemical reactions in the body and ultimately, has effects on the brain. A drug can be administered to the body in many ways: • by injection; • orally; • applied to the skin; • inhaled.

Medicinal use of drugs (prescription drugs) Doctors assess the need for these drugs very carefully, since many have side-effects. However, some people abuse steroids, diet pills, tranquillisers and antibiotics for personal 'miracles', ignoring and sometimes ignorant of the harmful effects. You should take care to read all instructions on any medicines that you use, or are prescribed by your doctor to make sure you are aware of the risks and side-effects (figure 18.20).

J

•

f f.dl.t-J d P ror" ""'*

Wh en th e pupil is sm all (in bright Ligh t) the eye has a grea te r depth of focus . The sh a pe of the lens does not n eed to chan ge quite so much to sw itch from viewing a distant obj ect to viewing a n ea re r o ne. In dim light w hen the pupil is wide, the depth of focus is less and the lens m ust change m ore.

229

Life Processes and Disease

This is sometimes noticed if a person is slightly Jong-sighted or slightl y short-sighted. ln bright Light the y will see objects clearly which in dim ligh t appear a little blurred. Also, as a person ages, their power of accommodation gets less and the range of distances over wbich they can see sharp images is reduced. This is much more noticeable in dim light than in bright light.

The retina

mm H·leI:tJI l(.l@fil

The retina is a photosensitive layer at the back of the eye. It is made up or two types of phororeceptor ca lled rods and cones (figure 19.11) . The rods are sensitive to light and dark onJy; the y do not react to colour. They function best in low light intensities such as when it is getting dark. This is why we see on ly in black and whjte at night. The rods are located around the sides or the retina away from the fovea . Rods are desensitised by bright light, whid1 explains why yo u cannot see clearJ y if yo u move from a bright area to a dim or dark o ne. After a few minutes though , the rods recover th eiT sensitivity and yo u can see more dearly aga in. retina

B -

-+--

ID-CC~

-

-fovea (cones only)

retina magnified

A blind spot -~~~

optic nerve - messages from all the photoreceptors go to the brain

Figure 19.11 The retina is made up of rods and cones (light-sensitive cells). Light falling on the cells causes nerve impulses which travel to the brain via the optic nerve.

~

IT:QS V'-1 Describe how we see.

liJnml#l•i•iH

The cones are sensitive to colour and function best in high light intensities. They are located mostly around the centre of the retina. Tbe fovea is composed entirely of cones and is at the centre of the retina. Light focused on the fovea produce a clear well -defined image in tbe brightest colour. Th e point of exit o[ the optic nerve from th e eye is called the blind spot because it lacks photoreceptors and is in sensitive to light. Light fa lling on thls spot does not ca use a response in the nerve, so you are 'bli nd' at th.is point.

Sight defects and their corrections A sight defect is caused by any condition that prevents proper focu sing of Light on the retina . A faulty focusing mechanism may be ca used by a number of fa ctors, such as th e shape of the eyeba ll or hard ening of the lens. Some common sight defects a re lo ng-sightedness, near-sightedness, cata ract and glaucoma.

230

19 · The Eye , the Ear and the Skin

Long-sightedness hypermetro ia

>

Long-sightedness, o r hypermetropia , is ca used by the eyeba ll being too short from rront to back, or the lens being too fla t. As a resu lt, li gh t from distant objects can focus on the retina, but light from nea r objects is focused behind the retina. So disra11 t objecrs are seen m ore clearl y than near ones. The condition can be corrected by wear ing con vex o r con ve rging lens (fi gure l 9. 12).

light rays from near object

- - - -tt:JCTb~::::=:::'.~

focus of light rays from near object before correction - behind retina

converging lens bends light rays inwards before entering the eye

Figure 19.12 Long-sightedness and its correction.

Near-sightedness

~

Nea r-sighted ness (or sl1o n -sigbted11ess), or myopia , is ca used by the eyeball being too long from front to back, or the le ns being too cu rved. As a res ult, lig ht rays fro m a distant o bj ect a re bent m ore tha n necessary a nd focus in frolll o f th e retina. However, light rays from near o bjects focus 0~1 the re tina . So near objects are seen more cl ea rl y than d istant o nes. Wea ring concave o r diverging · len s h e lps the person to see far objects clearly (figure L9.l3).

IT:Q6 V'-'

What kind of lens is needed to correct (i) long-sightedness (ii) nearsightedness?

focus of light rays from distant object after correction - on retina

light rays from distant object

diverging lens bends light rays outwards before entering the eye

'-1

focus of light rays fro m distant object before correction - in front of retina

~ "\.. ' \

Figure 19.13 Near-sightedness and its correction.

Astigmatism This is ca used by the surface o f Lhe lens or corn ea being curved irreg ula rly. Specia ll y shaped lenses, whid1 balance o ut these irreg ula rid es, need to be worn to provide a clear image o n tbe re tina .

Cataract

Figure 19. 74 A cataract reduces the light entering the eye.

This occu rs when the Lens becomes opaqu e a nd light cannot pass th ro ug h, so the person is una bl e to see (figure 19.14) . The le ns ca n be removed during surgery. Adjustments t:O vision ca n be made with approp ri ate specrad es or conta ct lenses, so that the person ca n see clearly again. Alternatively, tbe lens ca n be replaced w ith an intra ocu lar le n s.

231

Life Processes and Disease

Glaucoma

W-,

IT:Q7 V'-'

(i) What are defects of the eye? (ii) Name two defects of the eye and

explain what causes them.

This occu rs w he n there is a build-u p o f pressure in Lhe aqueous h umou r. This increased pressure inside Lhe eyebaU ca n da mage the optic ne rve. The sufferer expe ri e nces pa in ful and infla m ed e yes, and a ha lo is seen a ro und objects. Visio n is p oor, and the su ffe re r may experie nce sightless a reas io the fiel d of visio n. It is associa ted w itb an increase in age bu t may develop a t a n y time fro m in [an cy on . t The ris k factors for gla ucoma a re age, h ered ity, myopia, and general diseas.e such as a su o ke . In its earl y stages, glau coma can be effecti vely treated with m edicatio n, li ke e ye drops a nd o ral m edi catio n . Tf left un treated, it ca n cause vision loss o r blindness. ln its late r stages, su rge ry ma y be necessary to ease the pressure in the eyeba lls. Gla uco ma is the most common ca use o f bli ndness. Damage to th e optic ne rve is irreve rsible .

The ear Structure of the human ear The ma mma lian ear per[orms two fu nctions: •

hea ring;

•

ba la nce.

It is d ivided into three regio ns: the o u te r ear, the m iddle ear a nd the in ne r ear. Figure 19. l 5 sho ws tbe str uctu re o f the hum a n ear. pinna

bones of skull

semicircular canals

vestibular apparatus

stapes ossicles

[

incus

--~

malleus

tympanum (eardrum)

outer ear

middle ear

Figure 19. 15 Structure of the human ear.

232

inner ear

19 ·The Eye, the Ear and the Skin

How we hear A n o ise is se t o f vibrari ons o r so und

waves in th e a ir. Th e so und wa ves reach

l•liehfiH th e ea r a nd rh e pinna (tbe o u te r ear ) directs the m into the a ud itory ca na l. Th e l•1'i'·U'9!4'1J

13-13E@IJ

la@l+@QIJ

so un d waves travel down th e ear ca nal to the ea rdrum. The ea rdrum vib ra tes w h e n hi t by the so und waves. This ca uses rhe ea r ossicles, o r ear bo n es, in the middle ea r w vibra te. Th e inne r ea r is filled w ith flu id . The vibra tio ns a t th e , ova l w indow start up press ure waves i11 the flui d o r th e coch lea (fi gures 19. 16 a nd 19 .17 ). The iJ1ne r ear is made up o f two pa ns, th e cochlea a nd th e vestibu la r a ppa ra tus. Th e cochlea is a lo ng, coiled, three-cha mbe red tu be w hi ch is res pon sible [o r o ur sen se oJ h earing. The i11ne r ea r is fill ed w ith fluid. Th e vibra tion s at rh e ova l window start up press u re waves in th e fluid o f th e cochlea. Th e cochlea conrains receptor s ca lled hair cells which vibra te in respo nse to the press ure waves in the cochl ea r fl u id. Nerve impul ses a re gen e ra red which pass a lon g th e aucfaor y n erve to th e brain and we h ea r. Th e vib ra tio ns th e n pa ss a way lO the roun d wi ndow and we are read y to h ea r aga in.

middle ear

outer ear

vibration is amplified - -

inner ear (fluid-filled) membrane covering oval window

sensory cell stimulated impulse sent to the brain

-------~~---- impulse taken

to the brain

-+

- j~

sound wave membrane covering round window (absorbs the waves and prepares the fluid to detect new waves)

cochlea

Figure 19. 16 Sound waves are vibrations that travel through air to the outer ear. They are amplified as they pass through ossicles of the middle ear and then converted to pressure waves in the cochlea. m anic membrane

>

The eardrum

tympanic membrane or eardrum kept taut by equal pressure on both sides

inner ear outer ear pressure fluid

air

\

ressure

~

\ \

Figure 19. 17 The three bones (ossicles) of the middle ear.

Eustachian tube from the throat controls the pressure of the air in the middle ear

1

The ea rdrum is a thin membran e which is puU ed ta ut a nd separa tes th e o u te r a nd middle pa rt o[ the ear (fi gure 19. 18). lt is a lso called th e tympanic membrane. Th e vi bra ti o ns in the sound waves a re con ve rted to m ovem ent w h e n th ey ' hit' th e eardrum a nd are amplifi e d as they pass thro ugh the tb ree ea r bo n es. Pressure o n both sides o f th e ty rnpan ic me mbra n e must be eq ua l so tha t it stays stra ight and ra ut, and sound m essages ca n be passed o n effi cie n tly. We som etimes feel o ur ea rs 'pop', such as w he n fl yin g in an ae ropla n e. This ha ppens as rhe rympa ni c m e m bra n e m oves back ifl to positio n w hen th e press ure o n both sides equa li ses (Figure 19.1 9, overl ea f).

~

IT:Q8 V'--J

When we hear, what is the role of (i) the pinna (ii) the ear bones (iii) the cochlea?

Rgure 19. 18 The eardrum separates two air-filled regions of the ear.

233

Life Processes and Disease m iddle ear

Equal pressure on the eardrum whilst airplane on the ground.

sound waves

As the airplane g oes up, the atmospheric pressure is lower. The pressure in the middle ear is now greater and the eardrum 'bends'. Hearing is distorted.

The p ressure may equalise (naturally, or by chewing gum) and the eardrum returns to the normal position. It 'pops' as it does so.

Agure 19. 19 The eardrum can 'bend' if the pressures on either side are unequal.

Balance vest ibular apparatu s >

sem icirc ular canals )

The vestibu lar apparatus is responsible for o ur sense of ba lance and in formation about the position and movement o f our body. Th e vestibu lar apparatu s is made up of: • th e semicircu lar canals w hich detect movement o f th e head;

mmam ..1uaa11m •

Elule!iilrn

the utricle and saccu le (ea r sac) wh ich detect the position of th e head .

Receptors inside these stru ctu res are the ha ir cells tha r deflect o n mo vement. This causes a n impu lse co be sent to the brain. The semicircuJar ca na ls are at right-an gles co each other, in th e three pla nes, so that any movement of the head, and th erefore the body, is detecteo. At the base of each sem ici rcular cana l is a sweUing ca lled a n ampu lla . Figure 19.20 sh o ws how the am.p ulla works. movement of the body

/

cupula displaced by _ __ movement of endolymph

Figure 19.20 Movement of the body moves fluid in the ampullae in the opposite direction. The brain gets impulses from all three ampullae and interprets the messages as movement.

234

vestibular nerve to the brain, which interprets the message

-

--

relative movement of endolymph because of movement of the body

19 ·The Eye, the Ear and the Skin

~

l'.'f:Q9 \.../"-' Even with the eyes closed, the brain can detect movement of the body, that is, the direction and the relative speed of the movement. How is this possible?

Tbe ea r sac is posirio n ed below the sem i-circu la r ca na ls. lnfo rmation a bo m tbe position of rh e h ead and therefo re the body is de tected by receptor cells a nd impulses are sent to th e bra in. Th e utricle responds Lo verrica l movemems o f the bead and th e saccul e responds LO la tera l o r sid eways m ovem e m of rhe head. Fig ures 19.2 1 a nd 19.22 illustrate h ow they wo rk.

lean forward ball pulls on sensory hairs

Figure 19.21 Movement of the head vertically, pulls on sensory hairs in the utricle. Impulses are sent to the brain which are interpreted as movement of the head.

Rgure 19.22 The saccule responds to lateral or sideways movement of the head.

The skin World te mpera LU re varies fro m -58 °c in th e col d polar regio ns LO around 30 °C in tropica l ra info rest a nd over 60 °C in h o t deserts. Some a nim als are adapted to live in extrem ely cold enviro n m e nts w hile others ex ist whe re e n vironme nta l te mpe ra tu res can exceed 60 °C. Despite the te m perature o f th e e n vironme nt, th e bod y te mpera ture o f a huma n is a lways abo ut 37 °C (fig ure 19.23).

Figure 19.23 The body temperature of both boys is about 37 °C, even though one lives in an extremely hot environment and the other in an extremely cold environment

Temperature control homeotherm

>

oikilotherm

>

In the an imal kingdo m , birds and mamma ls a re a ble to mai nta in a fa irly constam body te m perarnre. They are described as being homeotherm ic (or, less co rrectl y, as warm -b looded). Th is fa irly constan t body temperature is ma i;1rained using ph ysiologica l m echanisms o r processes w hich occur wit hin th e body, fo r exa mple respiratio n w hi ch gen e rates heat, a nd consrrictio n o f blood vessels which reduces b lood flow to the skin a nd the re fore h ea t loss. All invertebra tes, fish, amp h ibia ns a nd repti les are unable to regula te the ir body Le mperarure by ph ysio logica l means. They are described as poi kilothermic (or, less correctl y, as cold-blooded) . They re ly o n heat derived from th e e nvi ro nm e nt ro kee p the body warm. Conrrol o[ bod y te mpera ture

235

Life Processes and Disease

is ach ie ved by beha vio ura l mecharusms, fo r example moving ro a cool place under a rock a nd basking in sunsh ioe. The body tempera ture or poiki lo t11erms usuall y depe nds on their e nvironment (figure 19.24).

~

l:tQil 0

Noon - lizard hides from the Sun

Morning - lizard warms up

l/'-)

Define (i) homeothermy (ii) poikilothermy, giving examples of each.

Its body temperature was low from the night. so It basks in the Sun to increase its body temperature.

Its body temperature could rise too high if exposed to the Sun so it hides under a rock.

Rgure 19.24 The body temperature of a lizard (poikilotherm) varies with the environment.

~

l:tQ· 11 l/'-) Why are humans able to live in extremely hot and extremely cold environmental conditions, unlike some animals?

Th e gra ph Lo figure I 9.25 compares the body te mpera tures of a hu man and a lizard for 24 hours of a day. The change in body tempe rature of the liza rd may be m o re than 10 °C, while th e change in body tempe rature o r a human is less than 2 °C. The bo dy temperature o r the lizard may drop Lo abo ut 5 °C a t n igh t when the re is no solar heat, and be raised to a bout 20 °C in the hea t of the day.

liemperat ure 1

50

air temperature varies from morning to noon to evening

-·· ·-- -·i----······ ····· -· · ·--:r-· '°C)

40 ····- -· - --~-··-·-·--·-· ~--------

-------

· - -· - ·

1

- -- - - - · -

----·--1

so ----~

i I

20

----1--7''-----=4--=:--- t - -Y-"' -

12 noon

human temperature

-------1- - stays about the same

18

I

lizard comes out of hiding to bask or move around looking for food lizard in a cool place - its body temperature is lower than air temperature

24 midnight

Rgure 19.25 How the body temperature of a lizard and a human. and the air temperature may vary in one day.

CHAPTER 16

236

~

A body te mpe rature o f about 37 °C is idea l fo r chemi ca l reactions to ra ke place as maoy en zymatic reactions have an optim um temperature o f aro und 37 °C. These reactions are impo rtant Lo sustain life; they occu r continuo usly in eve ry cell. The regu la tio n a nd maioteoan ce o f cons taot condition s wi th in an o rganism is called ho meostasis (chapter 16). Therefore, keeping the te mperatu re of tlle tissue fluid surround ing cells fairl y constant is an example of ho meostasis. The temperat ure r ange o n Earth is ve ry w ide and va ries with lalitude (from the poles ro the equator) : con ditions range from extrem e cold in the po lar regio ns to extreme beat in the tropics (figure 19.26). Mosq u itoes and fli es {invertebrates) are poikilo the rmic and infest the tropics w he re the e n vironmental temperature (28-3 1 °C) is idea l fo r them to live a nd nourish. M a n y are vectors o f disease, and so tb e tropics team w ith disease-ca rrying and disease-ca using organ.ism s. Diseases li ke ch o le ra, malaria, de ngue fever a nd yello w fe ver are m onitored con sta ntly in order to try to keep them unde r

19 ·The Eye, the Ear and the Skin

~

ll!Q-1 2 V'-1

Why are people who live on and around the equator more likely to suffer from certain diseases, such as malaria and dengue fever?

control. Poikilo therms a re restricted polar N to ce rtain a reas in th e wo rld because th ey beco me sluggish a nd e ven rorally inactive in low temperatures. Low te mpera tures slow down e n zymatic reactions. Mamma ls a re able to ma intain th eir bod y tempe rature close to o p tim um despite changes in th e environm e nt. They ca n re main acti ve da y a n d night, summ e r a nd winter, a nd can inhabit or li ve in a ny part of the wo rld . Howeve r, the y req uire more food . Mainta ining s region a bod y te mperature diffe rent from the environme n t requires a lot oJ e ne rgy. A Figure 19.26 The surface temperature of m o use, ror example, eats about its own the Earth varies with latitude. bod y mass of food per day w h ereas a cockroach can go for days without a mea l. All a nimals wi ll die in te mpe rature extrem es.

Temperature regulation in humans Practical activity SBA 19.1: Heat flow from a warm object, page 361

Meta bo lic reactions (especia lly in 1he Uver ) ge ne rate h eat and this h eat is transported by b lood thro u gh o ut the body to keep it warm at 37 °C. Som e heat is lost to the environmenr through the skin . The loss o f this gene ra ted hea t is regu lated a nd controll ed; for example, in a cold e n vironme nt, less is lost and mo re is conserved. Regu la tion of body temperature is contro lled by the h ypotha lam us of th e brain. The organ wh ich brings abo u t the changes if n ecessa ry, to conserve or lose h eat, is the skin (fi gure 19.27). Temperature receptors in th e skin receive the stim u lus o f ch a nging external te mpe rature (fig ure 19.28, overlea f) . T hey send impulses to the hypotha lam us, which mon ito rs these stim uli as well as interna l body temperature. If body temperature is chan ging, the hypotha la mus responds by sending impul ses to effectors in the skin to bring a bo ut the responses sh own in table 19.2 (overleaf) .

sweat pore

epidermis[

cornified layer (old skin cells)

~I ·

I-

~

Malp ighian layer (pigmented} - j p igment protects tower layers from damage by ultraviolet rays in sunlight

- - .F------19-

~~-

dermis

hair erector muscle sebaceous gland produces oil that coats and protects hair capillary network

-+--

hair follicle

·'----"-...n..:'"'._..__...,,,.

_ fatty layer below skin

i.,;.---1--

.__-...~_.__.,.,

sweat g land

Figure 19.27 A section of human skin.

237

Life Processes and Disease

Heat gain Sun

Heat toss convection of heat by wind

- - • • • atmosphere warmed

evaporation of water from body surface

,__,_____ __

reflected sunlight

/ ~

~"'of""

to cooler parts of the environment

radiation of heat from warmer parts of the environment

heat lost in urine and faeces

soil conduction lo the cooler ground close to water

conduction from the warmer ground heated by the Sun

Figure 19.28 The skin of a mammal is important tor temperature regulation.

To conserve heat

To lose heat • Sweating increases evaporation of the sweat removes heat from the body • Vasodilation occurs - capillaries in the dermis dilate so blood flow through skin increases, heat is lost from the blood • Hair erector muscles relax hairs lie flat so moving air can get closer to skin and remove heat

sweat heat is carried away ./" as sweat evaporates

•

Sweating decreases less blood now close to the skin

sweat gland

\ t I a lot of b lood flow close to the skin heat is lost from the blood

• Vasoconstriction occurs capillaries of the dermis constrict so blood flow to skin decreases, heat is retained in blood vessels deeper in the body • Hair erector muscle contract hairs stand up trapping a layer of warm air next to the skin (insulation)

layer o f warm air trapped which keeps body warm

hair erector muscles contract

--.~_..,___ arterioles

dilate

heat can be easily lost from the skin

hair erector muscles relax

Table 19.2 Responses in the skin of a mammal that help it to conserve or lose heat.

238

19 ·The Eye, the Ear and the Skin

Name the main organ of temperature regulation in humans. Describe two ways it is adapted to perform this function.

Humans can gene rate an excessive amo unt o r hea t du ri ng exercise o r increased activi ty. To maimain a consrant temperature we have 10 lose th is excess heat. Temperawre reg ul ation is physio logica l in huma ns si nce we are a mamma l. However, we may cha nge our behaviour to h elp the process. If yo u are is hot because o f stre nuo us exerci se, yo u co u ld: • re move some clothing;

~ IT:Q-1 4

• •

bave a co ld dr ink; move to a coo le r place;

•

stop activity.

~

IT:Q-1 3 \J'-1

\J'-1

What changes occur to control body temperature in the body of a human who is running a race?

Humans do n ot have a Ll1ick laye r o[ hair an d, if the e nvirornn ent is very cold, th ey d o not ha ve effective insu latio n . Heat is genera ted by th e live r, bur this may not be e no ugh to kee p body temperatu re at the ri ght level. M uscles sta rt LO shiver involu ntarily, to genera te more heat. Huma ns get 'goose b u mps' as the ha ir erector muscles contract. However, we are conside red Lo be 'na ked ' o r hai rless and can on ly tra p a thin laye r of warm air arou nd the skin . We can b e lp by: • putting on thick cl o thing; • having hot d rinks; • moving to a warmer place; •

m oving a ro und ro generate more heat.

Temperature regulation in birds The e ffect o[ e rector muscles is m ost marked in birds. In cold weather, the mu scles contact, as in hum ans, and th e birds' feathe rs sta nd o ut fro m U1e skin . (fig u re 19.29). This tra ps a great deal of air next to th e skin. wh ich acts as a good ins ula ro r. Tt a lso srops air fl ow over th e skin, vvhi ch redu ces loss o [ beat by convecti on .

Skin care O ne o f tb e most imponam way tO take ca re o f the skin is to protect it from the Sun. Ultravio let rays o f the Sun ca n ca use w rin kles, age spots and increase rbe risk o [ cance r. To protect skin fro m th e Su n : • use sunscree n; • seek sh ade; • wear protective cloth ing.

Figure 19.29 A bird can insulate itself from the worst of the cold by fluffing up its feathers.

Smokin g m ay damage co llage n and e lasrin, the fi bres tha t give skin it"s e lasticit:y and stren gth. So, a good skin ca re regim e incl udes no t smoking. Daily cl eansing a nd shaving can ta ke a tall on th e skin so strong soaps shou ld be avoided and a moi sturiser used. A healthy diet and ma naged stress p ro m ote yo unger loo king and hea lthy skin .

239

Life Processes and D isease

r

Chapter summary • • • • • • • • • • • • •

The main sense organs in humans are the tongue, nose, skin, ear and eye. The eye enables us to see variations in colour, shape, size, brightness and distance. We see when light enters the eye. Light is refracted as it passes through the cornea and lens. The iris controls the amount of light entering the eye. The lens controls refraction of light for near and far objects - this is called accommodation. Anything that prevents proper focusing of light on the retina is a sight defect. Astigmatism occurs when the surface of the lens or cornea is irregular. A cataract occurs when the lens become opaque and light cannot pass through. A build-up of pressure in the aqueous humour results in glaucoma. The ear is a sense organ that enables us to hear sounds from the environment. The ear detects sound waves from the environment. The ear is made up of three parts: the outer ear, middle ear and inner ear.

• • • • • •

Sound waves reach the cochlea from which impulses are sent to the brain. The ear is also involved with balance. The semicircular canals detect movement. The utricle and saccule in the ear sac detect the position of the head. Surface temperatures on the Earth vary greatly. Animals can be grouped as poikilotherms and homeotherms depending on their ability to control body temperature. • In humans, the skin is an organ of temperature regulation, meaning that skin care is important.

ITQ1 (i) Ln humans, the eyes are posirioned o n the uppe r front side of the face. The human skull bas a pair of bo les ca lled eye sockets wh ich 'cradl e' th e eyes. In this position, th e eyes obta in som e protection and the optic n e rve can easily connect w ith the b rain . Human eyes a re used mainJ y fo r mo vem e nt and to focus o n the task at ha nd . A zeb ra's eyes are positioned on eithe r side of its head wh ich greatly increases the a nimal's field o( visio n so it ca n spot predators easi ly. H uman s do no t need to be on th e con stant lookout for predato rs. (ii) A zebra is a la rge h erbi vo re and is prey to many la rge cats sucb as lio ns and cheeta hs. Having eyes o n the sides o f its head allow it to h ave an a lmost comple te view of its surro u ndings at any time, even while it is grazing and feeding. The e nables the anima l to be on th e lookou t for preda tors and aware of any m ovem e nt in its surroundings. Carn ivores, on the o the r hand , need to focus o n their p rey. Th e ir eyes are p ositio ned in front o [ their faces. This makes it possible for them to judge the distance between the m selves and their prey. ITQ2 To aid in movement (avoid obstacles, note di stances. etc.); to aid eatiJ1 g (finding food, ingestion, etc.); to focus o n an y task (reading. cookin g). Other a nswe rs are possible. ITQ3 Conj un cti va -+cornea -+ [pupil] -+aqueous hum o u r-+ lens-+ vi treous hu mo ur -+ retina ITQ4 Accommodation descri bes the adjustm em or th e pupil a nd the le ns to allow a person to see objects at diffe rent distances. ITQ5 Light rays from an object ente r the eye. a nd a message is sent to the brain, which interprets tl1 e message. The light rays pass th rough maJl y structures

240

f ·

19 ·The Eye, the Ear and the Skin

in the eye, each performing an importanr function. The lighL rays from an object mu st focus or meet at a point on tbe retina, from w here the opti c nerve sends a message to the brain. The cornea, aqueous humour and lens are important because they bend the ligh Lrays to focus on the retina. The cornea and aqueous hu mour bend ligh t automatica ll y, but the Lens can control the degree of ben ding. The pupils a re ' holes' in the eyes, the size of which ca n be adjusted to aJJow controlled amo u nts o f ligh t to e nter the eye. The deg ree of refraction is adjusted by the lens and th e rays focus on the retina . At the retina, ligh t-sensi ti ve cells { send messages to the brain, which interprets rhe message as sigh t. ITQ6 (i) Converging or convex len s. (ii) Diverging or concave lens. ITQ7 (i) A de fect oJ the eye is th e malfunction ing of a n y o n e part of the eye so thaL good vision is preve nted. (ii) A cataract occurs when the lens becom e L1a rd e ned a nd ca nnot adjust to focus light ra ys from o bjects ar varying distances. Astigmatism is a defect which occurs when tb.e cornea does not have a smooth curve; the rays are not refracted evenly as they enter the e ye. There are othe r de fects. ITQ8 (i) The pitrna traps the so und waves and directs them into the auditory ca n.al. (ii) The ear bones ampli fy the sou n d waves afrer they have passed th rough the ou te r ea r on their way LO the inner ear. (ii i) The sound waves ca use pressure waves in the .flu id of Lhe coch lea. Depend ing o n the pressu re of the wave, specific hair cells in the cochlea are stim ulated a nd specific messages are sent to th e brain. The brain interprets these m essages as sounds th at we hear. Th e coch lea is responsi ble for o ur sense of hearing. ITQ9 The ears are a lso concerned w ith ba lance, so any m ovement of the body can be de tected by the ears. The sem i-circular ca nals in the ear are filled · w ith fluid . Any m ovement is detected b y this Ouid. At the base o f tbe semicircular ca nal are s tructures called ampullae. The fluid in the ampu llae moves in the opposite direction to the bod y's movement, pulling on sensory ha ir cells as it does so. Messages are sent to the brain from the sensory cells and are iJJterpreted as movement. ITQ10 (i) Hom eotherm y is the abili ty of an orga n ism LO comrol its body te m pera tu re and keep it at a ce rtain val ue; for example, h um ans ma intain their body tempera ture at aro und 37 °C (birds ha ve a slightly higher body tempe ra ture around 39 °C). (ii) Poikilotbermy describes the inability of an organism to co ntrol its bod y temperature. Th e o rga nism 's body temperature varies w ith the e n vironmenra l te mperature; for exa mple, the body tempera tures o f repLiles and fish vary with the ir e nvironmenta l temperature. ITQ11 Humans can live in extremely hot a nd extremely cold e n viro nments because th ey be lp mainta in their bod y tempera ture ar a constanL va lue by some beh.avio LLral processes. For examp le, they can wear clothes to su it the ir needs, live in buildings which protect them and whi ch may be cooled or heated . Al so, huma ns do not ha ve to go in sea rch of food every day in extreme te rn pera w res. ITQ12 Organisms lik e bacteria and viruses that cause disease can su rvive in any tempe ra ture, but the vectors that carry the pa thogen from host ro host are usua ll y insects (like mosq uiros and flies) wh ich a re poikilotherm ic. These no u rish in the stea dy wa rm temperawres around the equator. ITQ13 The skin is th e major o rgan of tempe ra ture regula tion . l Lis adapted to sui t thi s fu n ction in several ways: • it co nta ins a layer o f [at, which acts as ins ulalio n; • it can co ntrol the fl ow o f blood into the many capillary networks close to the skin;

241

Life Processes and Disease

• it has hai rs wh ich can be ra ised o r lowered to increase o r reduce air now next to the skin; • it conta ins sweat glands which produce swea t tha t evaporates to cool the sur[ace o( the body. (Any two o[ tha above) ITQ14• Sweat is produced by the sweat glands. • Erector muscles rela x, causing the hai rs to li e fl at againsLthe ski n . • Hea r is lost from the body as the water prod uced in sweat uses the h eat from the body to evapo ra te. • B lood vessels n ea r the skin open wider, allowing blood LO fl ow through the many capillaries close to Lhe skin. As a result, hea t is brough t close to th e surface o f the body, and ca n be lost by radiation, conductio n, or eva poration of swear.

Examination-style questions (i)

Make a diagram of the eye as seen in a vertical section. Label these parts in each case stating its function: (e) retina (a) iris (b) cornea (n sclera (g) choroid (c) pupil (d) lens (ii) (a) What is the shape of the lens when the eye is focused on a near object? (b) Describe fully the mechanism that changes the shape of the lens when focusing on a near object. (iii) (a) Which structure controls the size of the pupil? (b) Using annotated diagrams only, explain how the size of the pupil is controlled. (iv) Suggest why, on first entering a dimly lit room , it is difficult to see objects clearly, but that they gradually become more clearly visible.

242 - - - ---- ·

2

(i)

A sight defect is caused by any condition that prevents proper focusing of light on the retina. Describe fully these common sight defects: (a) long-sightedness; (b) short-sightedness; (c) glaucoma. (ii) Using annotated diagrams only, describe how these eye defects are corrected: (a) short-sightedness; (b) long-sightedness.

3

(i) The diagram below shows the structure of the human ear. Copy the diagram and label the parts listed below and in each case state its function.

19 ·The Eye, the Ear and the Skin

(a) (b) (c) (d)

pinna tympani um ear ossicles vestibular apparatus

(e) cochlea (n auditory nerve (ii) Describe how the ear functions as an organ of balance when (a) the position of the head changes; (b) the body moves. 4

(i)

Explain the meaning of the following terms: (a) homeotherm ; (b) poikilotherm. (ii) The graph below shows the variations in temperature during the course of one day for a human, a lizard and the air. Copy the graph and label, appropriately, the three lines in the graph. · 50 40 - -- --------- - -..:::::::.::.: :.:~~·:,._____ __ _ ··.. . :· 30 .

.....

Temperature (OC)

..

.. .

20 10

.· .· 6

12 noon

18

24

midnight

(iii) Explain fully, the changes seen in the body temperature of the human. (iv) Explain the changes seen in the air temperature. (v) Suggest what the lizard might be doing and where it might be found during these times: (a) 6.00 a.m.

(b) 12.00 noon (c) 6.00 p.m. (d) 12.00 midnight 5

(i)

Draw a diagram of a section of human skin and include the following labels: (a) epidermis (b) dermis (c) receptor (d) capillary network (e) sweat gland (n sweat pore (g) hair follicle (h) hair erector muscle (ii) Describe fully the possible behavioural activities and physiological mechanisms that enable the human body to lose excess heat and maintain a fairly constant temperature.

243 -

-

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-

- ---

-

-

-

-

-

-

•

0 0 0 0 0 0 0 0 0 0

distinguish between sexual and asexual reproduction describe the structure and function of the human male reproductive system describe the structure and function of the human female reproductive system describe the structure and function of the ovum and spermatozoon understand the menstrual cycle understand fertilisation understand the development of the embryo in humans understand the role and methods of contraception discuss the advangates and disadvantages of contraception discuss the transmission and control of AIDS and gonorrhoea

reproduction

I

t

""sexual

asexual

advantages and disadvantages

reproduction in humans

contraception

Reproduction I

Reprod uctio n is a characteris tic o f We. Every living thing mu st di e and, a ltho ugh ind ividua ls die, a s pecies w ill continu e as lo n g as its me mbe rs a re able to live lo ng eno ugh l"O reprodu ce. n the m embers o f a species die be l'ore the y ca n reprod u ce the n tha t species is in danger o f becoming extinct. Reprodu ctio n is th e re fo re im porta nt fo r a species to conrinu e ro exist, to be ab le to colon ise new ha bi ta ts and r.o s m vive ch a nging e n viro nrne llla l conditio ns. Th er-e a re two ma in types o [ reprod u ction : • asexual; •

sexual.

Asexual reproduction (one parent) Asex ua l rep rodu cti o n hap pe n s w he n o n e indi vidua l p rodu ces o Hspri ng w itho u t ferti lisatio n . Th.is invo lves cell division by m itos is o nl y (cha pter 23). Th ese o ffs p rin g a re geneticall y ide mica l to each oth e r a nd tO th e ir parent. ft is described as bei ng con se rvative a nd, in esse nce, clon es a re p roduced.

Advantages of asexual reproduction •

No time o r e n e rgy is wasted seeking a mare.

•

Large numbers o f o ffspring ca n be prod u ced .

20 · Reproduction in Animals

•

Offspring ca n be prod u ced continu ously and therefore q u ickly.

•

O([spring can make good use of fa vo urab le e n vironrnem al co nd irions.

• lf the pare nt is of 's uperior' quality, all th e offspring wi ll also be of ' superior' q ua lity.

Disadvantages of asexual reproduction

Figure 20. 1 An aphid producing live young

•

Uthe e nvironme nt is cha ng ing, the offspring may fi nd it d ifficu lt

•

If th e parenr is o f ' poor' q ua li ty, th e offspring w il l also be o nl y o f that ' poo r.' quality.

•

Over-crowding a n d competition rnay occu r as o ffs pri ng colo nise th e same a rea as tbe pare nt.

LO

surviv e.

CHAPTER 23

There a re severa l rypes of asex u a l reproduction : • vegetative propaga ti on (ch apte r 2 3);

CHAPTER 23

•

cl on ing (chapte r 2 3) (figure 20. l );

•

binary fissio n see n in unice llular orga nisms like bacteria and protozoans such as Amoeba (Figure 20.2) .

C!J- 0 one parent

identical offspring

Figure 20.2 Asexual reproduction in Amoeba.

Sexual reproduction (two parents)

CHAPTER 24

Sexua l reproductio n involves two pare nrs prod ucing specia l re prod ucti ve cells o r ga metes. This happens as a result o f meiosis (chapte r 24). fusion of rh e gametes produ ces o ffspri ng tha t are di[(e renL fro m ea ch o ther and from both parents.

Advantages of sexual reproduction •

Generic va riabi lity o f the species is increased .

• The species is thus m o re li kely to be able ro adapt to a ch ang iJ1g e ovironme nc. • The species ma y be a ble to colo n ise new a reas successfull y.

• lI the pa rents are bo th of poor qu a lity, the o [fsp ring may be o f be tter quali ty.

~

IT:Q-1 \../'-' Draw up a table to show the differences between asexual and sexual reproduction.

Disadvantages of sexual reproduction •

A lo t o ( lime a nd e n ergy is spent se eki ng a mate.

•

O ffspring a re no t produ ced continuously a n d therefore not very q uickly.

~

•

Few o ffs pri ng may be p roduced (as in hu m a n s).

\..)'V

•

Even if th e pare nts a re o f good qua lit y, rhe o[(sp ring can be o f poo r q ua liry.

IT:Q2

What is the importance of reproduction?

245 - - --- - - -

Life Processes and Disease ,

-- • •

CHAPTER 24

Practical activity SBA 20.1 : Observing the reproductive

cells of a mammal, page 362

..,,.,...,

spermatozoon >

_

-·

Reproduction in humans ln humans there are two sexes: ma le (ma n) and female (woma n ). Each sex produces gametes o r reprodu ctive cells, by meiosis (chapter 24) . In ma les the gam etes are ca lled spermatozoa, and in females, ova. The sing ular or spermatozoa is spermatozoon , and the singular of ova is ov u m (figu re 20.3). ~ Testes make spermatozoa

Ovaries make eggs

~ IT:Q3

/I

ovary

\/"-/

What type of reproduction do humans show? (ii) Describe two advantages of this type of reproduction. (i)

primary follicle, secretes

/~-- -:~r~:~s~:elops

( ~-@;, . ', .

~

- + ' . ----@ \

\f ]J "'" .,_ - -

-· - -

~ ~~orpus V '-

...... ~ v-v-

~:::::-

luteum

-

k

,.:::;--seminiferous tubule

p rogesterone

mature ovar~:~! "\. Graafian follicle ~ ·

• • • • • • •

-

- tail for swimming

spermatozoon

• one produced per month

• millions produced continuously

• live for about 3 - 4 days after release from the ovary (ovulation)

• live for about 2 to 3 days in the body of the female after release (ejaculation)

• moved along the oviduct by the beating of cilia; cannot move on its own

• can swim actively using its tail, secretions from the seminal vesicles and prostate gland help its movement

Figure 20.3 Details of the ovum and spermatozoon.

The male reproductive system ltffiHFll The plural of testis is testes.

l@11M•*I 246

The visible parts of the ma le reproductive syste m are the penis and scrotum (figure 20.4) . The scrorw11 con tains a pa ir o f testes. Each testis is composed or coiled tubes ca ll ed semini(erous tubules, inside of which spe rmatozoa (or sperms) are fo rmed. After fo rm atio n, th e sperms are stored in the epidid ymis. During sex ual inte rco urse, the sperms are moved o ut o f the epididym is and pass thro ugh th e vas deferens on the wa y to the penis. Fluid is made in the prostate gland a nd semina l vesicles which mixes with the sperms to make semen . This se me n, conta ining 200-500 mi llio n sperms, is ejaculated o r released from the erect penis during mati11g or cop ula tion.

·

20 · Reproduction in Animals

(a)

ureter sperm duct spermatic cord _ (sperm duct and blood vessels)

_

seminal vesicle

_,______._

·~

prostate gland

(b)

erectile tissue: blood sinuses that can fill with blood from the artery at the base or the penis urethra

~-- testis

foreskin glans----"'--""~

~---:;;£..--- scrotum

Figure 20.4 The human male reproductive system (a) in section, (b) seen from the front (front section).

~

The female reproductive system

IT:Q'4 V'-1 Describe the route taken by a spermatozoon from its site of production to ejaculation.

me ns truation

~

>

The fema le reprodu cti ve system is positioned in the pelvic region (figure 20.5). The re are two ovaries, each usua ll y releasing a single ov um (or egg) every otber monch into the fuDJlel o f the oviduct. Th e ovu m is moved along rhe oviduct, or Fa llopia n tube, by the beating o f ci lia w hich line the rube. ff sperms a re not p resent, tbe o the ov u m moves down the u terus and out of the body during menstruation . Each month the wa ll o r lining o f the ute ru s is b uilt up in prepa ration for a fertilised ovum. The lining is shed if fe rti lisatio n does no t take place.

IT:QS V'-1

How does an ovum travel along the oviduct?

(a)

Figure 20.5 The human female reproductive system (a) in section, (b) seen from the front (front section)

247

Life Processes and Disease·;; : - .

_ -

_·

Hormones of the gonads

l'itl'!il'ftim

seconda

sexual c ha racteristics )

Th e gonads, testes in males and ovaries in fema les, also secrete hormones that influence growth a nd deve lopme nt. Even before birth, while still in the ute rus, rhe testes of a boy produce the hormooe testosterone w hid1 in fluen ces sex ua l development and causes th e ma le sex organs to deve lop. At puberty, the ovaries in girls secrete the hormone oestrogen. Boys, ·~ at this time, make larger amounts of testosterone from the testes. These ho rmones are secreted in respon se to signa ls from the pituita ry which is able. to determin e that further develo pment into a man or a woman must begin. Th is sta rts between the ages of 10 and 14, as boys and gi rls begin to develop the ph ysica l features that distinguish male from fema le. These district physica l and emotiona l features, or chara cteristics, are called secon da r y sexu a l c h a r acteristics (table 20. l ).

Males

Females

• enlargement of reproductive orgars. e.g. penis, testes, etc.

• enlargement of reproductive organs and breasts

• ejaculation is possible

• menstruation starts

• increased muscle development

• broadening of the hips for child-bearing

• growth of pubic and underarm hair

• growth of pubic and underarm hair

• extra growth of hair on face and chest • deepening of the voice Table 20.1

~ IT:Q6 V"-J

What are the secondary sex characteristics? (ii) When do they arise? (iii) Why are they important?

~

IT:Q7 V"-J

How many ova are produced by a normal adult female in a year? (ii) How many spermatozoa are produced by a normal adult male in a year? (i)

~

IT:Q8 V"-J

When in the menstrual cycle is the likelihood of fertilisation of an ovum at its lowest?

~

IT:Q9 V"-J

Why is the uterine lining built up every month, only to be shed during each monthly period?

Secondary sexual characteristics of males and females.

These be havio ura l and physica l changes are associa ted with co urtship, mating a nd parenta l concerns. More importantly, these ho rmones also resu lt in the release of the gam e tes. At puberty, gi rls begin ro menstruate, a sign that th e menstrual cycle has begun . Female ga metes or ova are released and can be ferti lised by spermatozoa as boys also begi n to ejacu late o r release ma le ga metes in lO the environment. A d1 il d grows and develops into a sexua l indi vidu al with easi ly recognisa bl e (ea tures tha t are a ttractive lO a potentia l partner, th us ensu ring reproduction a nd co ntinuatio n o r the species (figure 20.6) . Prod uction o f yo ung is a natura l d1a racte ri stic of li fe a nd th ese ho rmo nes produced by the gonads a re impo rtant, no t o nl y for growth and development of an organism into a sexual being, but a lso fo r th ose attractive forces n ecessary fo r the contin uation of Rgure 20.6 Typical physical characteristics of adult the species. human female and male.

248 - - -- - --

- --

- -- -- - - - -- - -- - -- - - - - - -- -

- -

- -

-

-

20 · Reproduction in Animals

The menstrual cycle me ns trua l c c le > me no pa use >

l~O V'-1

When does ovulation occur in the menstrual cycle, and which hormone is responsible for ovulation?

~

IT:Q-1 1 V'-1

What is the importance of the corpus luteum? (ii) Why is progesterone called the pregnancy hormone?

(i)

Figure 20.7 The human menstrual cycle

On reaching puberty (around L2 years o ld ), a human fema le w ill Sta n to re lease ova from he r ovaries: this is known as ovulation. Ovulation is one part o[ her monrhly menstrual cycle, which sLarts at p ube rt y and continues un Lil menopause (around the age o [ 45- 50 yea rs). Each cycle laSLS for approx ima tely 28 days. The events o f the cycle a re controll ed by ho rmo n es which e n sure that, if the ovum is fe rtilised, the ute ru s is rea dy LO receive it. ~ Th e cycl e starts w ith m e nstrua tion (Lhe sh edding of t he uterus lin in g) whid~ lasts for abo ut 5 da ys (fig ure 20.7). After a few days th e uterus lining starts LO build back up agam - by day 14 o f r..he cycle it has th ickened considera bl y and has a n increa sed blood suppl y. This is ca ll ed the follic ul a r pha se and is control led by the h ormone oestrogen . The events a re syn chronised so that on e ov um is n ow fu ll y developed in a Graafi a n fo llicle in the ova ry and o vulation takes place (the ovu lato ry phase) . The peak lr1 oestrogen le ve l ca uses ovu lation. After ovu lation, th e Graa£i an follicle develops into th e corpus lute um . The hormone progestero ne is secreted by the corpu s lute um and is responsible for ma intaining th e built-up u teru s lining. This is the lucea l phase of the cycle. U rhe o vum is no t fe rti lised by a sperm, it passes th roug h the uterus and vagina during menstruatio n . The co rpus lute um degenerates and th e le vel of progesterone dec reases. This ca uses the built-up uterus lining to start to disintegrate and peel away fro m the ute rus wal l. It passes ou t o f the vagina in menstruation or the m o nthl y period. And Lhe cycle stares aga in .

Events in the ovary during a cycle ovulation

ovarian follicle growing

@

'~JJ·

~

®

no fertilisation

corpus lu teum

secretes oeslrogen

~,-•

~ degenerates

produces progesterone

release of ovum (ovulalion)

Hormone levels during a cycle a peak in oestrogen - results in ovulation

progesterone level rises. stays high if fertilisation occurs J• • • • • •• • • •

.... .. ,.··

.·..

........ .. ....···•··•••••••·•·••·••·····•···· ....... ' ... ·· Events in the uterus during a c ycle

menstruation shedding of lhe uterine walls

walls built up as oestrogen level rises

-+

start or another cycle

I

no fertilisation w alls shed fertilisation - walls stay built up

2

3

4

5

6

7

8

g

10

11

12

13

14

15

16

17

18

1g

20

21

22

23

24

25

26

27

28

Days

249

Life Processes and Disease

Ferti Iisation pregnancy > gestation period

>

co ulation fertilisation

> >

~

l:tQ12 vv What do you understand by the terms (i) courtship behaviour (ii) copulation?

~

l:tQ-1 3 vv (i) What is fertilisation and where does it occur? (ii) Describe the route taken by a sperm after ejaculation in the vagina until it fertilises an ovum.

implantation

Tf fe rrilisa tio n takes place and the zygoce successfull y implants itse lf inLO th e bu ilt -up uterus li ning the fe male is sa id to be pregnant. The ute ru s lining musr now sta y bui lt-up LO nouris h th e embryo, so the re is no mo re m e nstrua tion (' pe riods'). This lasts for rhe e n tire pregnancy (o r gestation period ) w hich is usually 9 months in humans. Th is m ea ns that the wo mar 's p roges tero ne level must remain high w ma inta in the bu il t- up ute ru s lining. Also oesrroge n leve ls must remain low so that no more ovulat ion can ta ke place. Tb.e pregnan t woman may experience ' morn ing sic kn ess' (nausea) for the first rbree months o r so as she gets used to th e high le vel o f progesterone and its effects on he r body. Mating, for huma ns (and other mamma ls) , is us ua lly preceded by courtship behaviour. Co urtship establishes a bond betwee n rhe pa rt:ners tha t may keep the m LOgethe r while the young are bro ught up . A m ale and fema le are attracted LO each ocher a nd a s uccessfu l courtship leads to copulation a nd fertilisation . The act o [ copu latio n , o r mating, b ri ngs the gam e tes close LOgether. The penis becomes erect during sex ual aro usa l as the erecti le tissu e fill s w ith b lood. In the fe male, sexua l aro usal res ul ts in the lubrication o f the vagina . The pen is is then inserted into the vagina, bringing the ga m etes close r togethe r. The spe rms a re usua ll y eja cu lated just below the ce rvix, a nd the n 'swim ' across the u te r us and up the ovidu ct. Close LO 500 million spe rms are re leased, bu t only o n e w ill fuse with the ovum. This is fertili sation.

Development of the embryo, fetus and placenta

>

Tbe nucle i o f the spe rm a nd fe rti li sed egg fu se ro form 1be zygote (figure 20 .8). Th e zygo te di vides as it m oves slowly to th e ute rus. After seve ral h ours, it is a ba ll of cells ca lle d a n embryo, a11d on reaching the uterus, it sinks into the th ick spon gy lin ing (fig ure 20.9). This is ca lled implantation . He re it obta ins protectio n and nutri ern s unri l th e pla cen ta develo ps.

more mi tosis -"--------'~

oviduct

occurs stage

stage fertilisation (fusion of the nuclei of ovum and spermatozoon)

uterus

implantation - ball of cells becomes attached to the uterine wall

Figure 20.8 A human sperm fertilising an ovum.

( ~--ovary

Rgure 20.9 Events that occur in the oviduct leading to implantation.

250

20 · Reproduction in Animals

•m••~'*' F1€iB§eiFH

~

The e mbryo deve lo ps tissues and organs and by 8 weeks it is clearly human. It is now a fet u s (figure 20 .10). As the embryo grows, it develops a p lacenta whid1 con nects i1 very closely w ith the wall o ( cbe uterus (figu re 20.1 1).

IT:Q·1 4

vv

Describe implantation and explain its importance.

Figure 20. 1O The embryo develops into a fetus and lives for nine months in the uterus.

motlier's blood] diffusion occurs sending nutrients to fetus' blood and waste products to lhe mother's blood fetus' blood

! l

to fetus

mother's blood

from fetus uterus wall

space filled with mother's blood

vessels umbilical cord

! l

Figure 20. 11

Structure of the placenta.

251

Life Processes and Disease

The structure of the placenta is shown in figure 20. l I . Th e placenta ha s various [un ctio ns w hi ch include the fo ll owing. • Il al lows exchange of materials between the mother and the fetu s, by bringin g tbeir blood sys te ms very close togetJ1er w ithout the two bloods mixing. Oxygen , water, amino acids, glu cose and essential minera ls diffu se thro ugh the placenta to the blood of the fetus. Carbon diox id e, urea and other wastes diffu se from the fetus into the mo ther's blood. ~

• It protects the embryo by pre venting many pathogens and chemicals from crossing the placenta. Howeve r, there are some exceptio ns, lik e the German measles virus, the HIV virus, nicotine, alcohol and heroin , wh ich are all able to cross the placenta. • It protects th e fems and the mother since it a llows their two blood systems to ope rate at different pressures (the m othe r's body needs a highe r pressure to get blood round a larger syste m) . •

lt produces the hormones iJnportant for a successful pregnancy.

Effects of drug abuse in pregnancy Nutrients diffuse from the m o ther's blood to the placenta, and the n trave l to tbe fetu s during gestation . Harmful substances ma y also diHu se across to th e fetus if they are present in th e m othe r's bloodstream . • Carbon monoxide and nicotine from cigarette smoke - Problems associated w ith ciga rette smoking incl ude prematu re birtl1, red uced birth weight and the risk of mi scarriage . • AJcohol - There a re seri o us conseq ue nces of alcohol abuse during pregnancy. Alcoho l crosses tl1e placenta easil y and ca uses symptoms in the baby includi ng poor m enta l dvelopm ent, small h ead and brain size, h yperactivity, poor concentra tion and reduced growth ra te.

1~5 \....)'-./

The placenta is described as the lungs, kidneys and alimentary canal of the embryo. Why is it so described?

• Drugs like heroin and cocaine - Babies may become addi cted to these drugs whi le inside the m other's wom b. • Pharmaceutical products -These a re carefu ll y tested for harmful e ffects, but it is a wise preca uti on not to use any drugs (even for headach es or nausea) d uring pregn an cy unless prescribed by a doctor.

Birth The fetus is surro unded by a stron g m embrane called the a mnio n . Insid e is amnioti c fluid

> a liquid called amniotic fluid wrucb helps to keep a consrant environment

l•J#limiOt•hi 1m;1m1 lftl•I•liill

£6b

IT:Q·1 6 \....)'-./

Give a brief explanation of (i) gestation (ii) parturition (iii) prenatal care (iv) postnatal care.

252

aro und the [ems. The amniotic fluid a lso helps to sup port and protect the fetus from ba rm. A[ter 40 weeks in the m other's ute rus (a lso ca ll ed th e womb), the baby is sent o ur .into tbe world . Parturition is the act o f giving birth and is controlled by hormones. The hormone oxytocin ca uses contractio ns of th e ute rus w hich ca n be very pa inful. This is known as 'labour' o r ' labo ur pa ins'. During th ese powerful contractio ns, tb e amn iotic sac bu rsts. The amn io tic fluid po urs o ur o f the uterus and the baby is the n pushed o ur. The umbi lica l cord is cut, sepa ra ting the baby from its mother. After a few minutes, the placenta sepa rates fro m the uterus wall a nd passes out of the body. Th is is sometim es ca lle d th e a fte r-birth. Prenatal (a ntenata l) ca re ensures good .hea lth of the baby a nd m m her during the pregnancy. The m other should, for example, eat a balanced d iet, should not smoke o r drink a lco hol a nd should avo id dru gs. Postna ta l ca re describes care of tbe child Cro m birth to teens. fr in volves the physica l, e motio na l and me ntal ca re essential for hea lthy growth and deve lopme nt.

20 · Reproduction in Animals

Breast-feeding l•li•IF:tiJIGI limGmiiiluU

Mamma ls suckle their yo ung. After birth of a baby, milk is produced by th e breasts o r mam mary glands as a result of th e effects of m a n y horm o nes, in particu lar, prolactin. The firs t secretio n of th e breast is called colostrum. It is rich in antibod ies and protects th e n ew-born from som e pathogens it may e11counte r on th e first days of its life o ut of the ute rus. Hum an breast milk conta ins the appropriate prop ortio ns of sugar, fat and { protein sui table for a yo ung human ba by. If she is breastfeedjn g, the mother's . ilie t sho uld be rich in foods that w ill provide th e e nergy, p ro te ins, vitarruns and min erals n ecessa ry for hea lth y growth and develop me nt of t he infa nt. 'Formula' milk, w hich is ofte n bottle-fed co infams, at.tem pts to recreate Lhis balan ce. It consists o f dri ed m ilk made to a special fo rmula and is mixed w ith water and fed to the baby in a bo ttle.

The role of contraception The wo rl d's popula tion is do ubli ng every 44 yea rs or so. It may soon be di tficult to su ppl y all the needs of a ll of its people. A solution to t he ove r-popula tion pro blem lies in contracep tio n (a lso known as birth control). Table 20.2 summa rises some commo n methods o f contraception and figure 20.12 (overlea f) shows the si.tes of action of some contracep tive methods.

Method

How it works

Effectiveness

Advantages

Disadvantages

sterilisation

male (vasectomy) - the vas deferens are cut and tied off

100%

no drugs or artificial device used, irreversible no further costs

100%

very reliable if taken as prescribed

possible nausea, breast tenderness, and water retention leading to an increase in weight; may increase risk of cervical cancer, but decreases risk of breast cancer

female (tubal ligation) - the oviducts are cut and tied off contraceptive pill contains progesterone which prevents fertilisation, some also contain oestrogen which prevents ovulation

intra-uterine device (IUD) (loop, coil)

device inserted into the womb by a 99-100% doctor - prevents implantation

reliable

possible menstrual discomfort

spermicide

cream, jelly or foam inserted in vagina before copulation

not reliable alone

simple to use

may reduce the sensitivity of the penis

mechanical barriers

male (condom) - sheath of latex unrolled onto the erect penis

reliable especially available for use by all men and may reduce the sensitivity of the when used with women, and the condom gives penis spermicide some protection against sexually transmitted diseases

female (diaphragm, cap) - domeshaped sheet of thin rubber inserted over the cervix before copulation rhythm method

refraining from sexual intercourse during those times in menstrual cycle when fertilisation is likely

not very reliable

no devices or drugs used

not really reliable because women can have irregular menstrual cycles (continued)

253

Life Processes and Disease

Method

How it works

Effectiveness

Advantages

Disadvantages

injectable hormone

prevents release of ova and thickens the mucus in a woman's cervix

very reliable

no need to remember medication, no device used

injection must be repeated by a doctor every 13 weeks

abstinence

no traditional sexual intercourse (i.e. almost 100% penis I sperm entering vagina)

protects against sexually transmitted diseases if there is no transfer of fluids

Table 20.2 Some methods of contraception, their effectiveness, advantages and disadvantaqes.

1?at.7

In the female

\...)'..J

Intra-uterine device (IUD) injection/implant (contraceptive pill)

Match these forms of contraceptive with mode of action A or B: • contraceptive pill • IUD • spermicide • condom • rhythm method • tubal ligation • vasectomy Mode of action: A prevents implantation B prevents fertilisation.

barrier techniques • • • •

--\--1

In the male

diaphragm (cap) contraceptive sponge spermicides female condom

Agure 20. 12 The sites of action of different contraception methods.

HIV/AIDS and other STDs

liJlB

opportunistic infections

254

>

STDs are sexua lly rransmitted diseases; th is mea ns they a re diseases th at are transferred fro m o ne person to a nother during sex ua l inte rcourse. AIDS (acqu ired immune defi cie ncy syndrome) is tho ught to have origi nated in Central Africa and bas a lrea dy killed over 3 mi lli on people worldwide. AIDS is ca used by the human immunodeficiency virus (HIV ), wh ich can onJ y survive in body flu ids. HIV can be transferred in other ways as well as by sexual intercou rse beca use is transmi tted wben t be blood or semen of a n infected person m ixes with th e body fluids of another person . This can ha ppen during sexua l interco urse, blood transfusion or when sha ring a hypodermic needle. An infected pregnant woman can also pass HJV to h er ba by through the p lacenta or by la ter breast-feeding. Close contact between people with ope n wounds has also been known to pass on the viru s. Infection w ith HIV weakens the body's natura l defence syste m (th e immune system) because the virus attacks particular white blood cells, ca lled T-Lymphocytes (figure 20 .13). This means t he body is vulnerable to other infection s (known as opp o r tunistic infectio n s) like common viral, bacterial and [un gal infections. Table 20.3 compa res two STDs: HIV/AID S a nd gono rrhoea .

20 · Reproduction in Animals

Figure 20. 13 False-colour scanning electron micrograph of a T lymphocyte white blood cell infected with HIV

Disease

Causative agent

Symptoms

Control

AIDS (acquired immune deficiency syndrome)

Virus (HIV)

• Persistent cough, fever. Skin rashes, swollen lymph glands, diarrhoea, wasting away of body, weakness. • Secondary (opportunistic) infections - pneumonia, tuberculosis (TB), candidiasis (fungal), cancers.

• Keep to one sexual partner (or to partners who have been safely screened for STDs) • Do not inject drugs • Use condom during sex • Education about the spread I prevention of disease • A vaccine is being sought

Gonorrhoea Bacterium

• Yellowish discharge from urethra, • Keep to one sexual partner pain when urinating. Often (or to partners who have been not noticed in females. If left safely screened for STDs) untreated, causes inflammation • Treatment by anti-biotics, e.g. of Fallopian tubes and sperm penicillin, streptomycin ducts leading to sterility. • No known vaccine • Arthritis, weakened heart, blindness.

Table 20.3 Information on AIDS and gonorrhoea.

~

IT:Q-1 8 V"-J

What kind of disease is an STD and why is it so called?

Prevalence (%) by WHO region 0 Western pacilic: 0.1 (0.1-0.11 0 Eastern Meditteranean: [0.1-0.31 0 South-East Asia: 0.3 (0.2-0.41 Europe: 0.4 (0.4-0.51 • Americas: 0.5 (0.4-0.61 • Africa: 4.6 [4.4-4.81

' 0

)

Global prevalence: 0.8% [0.7-0.8]

Figure 20. 14 Estimate of the numbers of people (15-49 years) living with HIV (2011) .

Social and economic implications of STDs especially HIV/AIDS • Th e cosLo f rrea ri n g and carin g for chose a fl'ecLed is h igh , especia ll y in co un tries whe re a high perce nrage o ( Lhe pop u la tion is infected . • Th ere is a reduCLio n in the wor kfo rce a n d loss o f va lua ble working h ours. • The family o f a n in fected person suffers e m otio n a lly a nd fin an cia lly. • M illi on s o f do llars a re s pe n t w o rldwide o n resea rch for a possible cure fo r HJ V infection . • People w ith AIDS .(in cl udi ng childre n ) ma y be scorned and a lie nated fro m so cie ty. • STDs a re easily s pread by sexu a l inte rco u rse. • M illio n s o f child re n worldw id e are living w ith the e ffecLs o f HIV /AID S; man y a re orph a n s.

255

Life Processes and D isease

r Chapter summary • Reproduction is necessary for the propagation of life on Earth. • Asexual reproduction involves only one parent and no fertilisation. The offspring are genetically identical to the parent and each other. • Sexual reproduction involves two parents and fertilisation. The offspring are differen\ from each other and their parents. • Variation in offspring resulting from sexual reproduction is important when there are changes in the environment. • In humans there are two sexes: female and male. • The female gamete is the ovum and the male gamete is the spermatozoon. • The menstrual cycle starts at puberty and is usually a 28-day cycle in human females. • Ovulation, the build-up of the uterine walls and menstruation are processes which are part of the menstrual cycle. They are controlled by the hormones oestrogen and progesterone. • If fertilisation of the female gamete by the male gamete occurs in the oviduct, a zygote is formed. • The zygote implants itself in the wall of the uterus. • The developing embryo is protected by the amniotic fluid and is nourished by the developing placenta. • Drug and alcohol abuse are very harmful to a developing fetus. • Contraception methods prevent pregnancy from occurring.

ITQ1

Asexual reproduction

Sexual reproduction

single parent involved

two parents involved

offspring identical to parent

offspring different from parents

offspring identical to each other O.e. no variation offspring different from each other (i.e. variation between individuals) is seen) less likely to survive a changing environment (none may be able to survive because no variation in offspring)

more likely to survive a changing environment (some offspring may be able to survive as a result of variation in offspring)

evolution of the species less likely (only through evolution of the species can occur more readily mutation because of variation type of cell division is only mitosis

type of cell division involves meiosis

ITQ2 Reproduction is the prod uction of offspring and it ensures the continu atio n of the species. lf m ost individuals in a po pula tio n die be fore th ey reprodu ce, the n that popu la tion cou ld beco me extin ct. ITQ3 (i) Sexua l reproduction. (ii) Any of th e adva ntages mentioned on page 000 could be me ntion ed. ITQ4 testes - epididymis - sperm du ct - ureth ra ITQS An o vum is pushed a long the oviduct o n release fro m the ovary. Tt is 's ucked ' in ro the oviduct and is pushed along by a curre nt produced by the

256

20 · Reproduction in Animals

beating o f the cilia that Line the ovid uct. Also, contractions of the oviduct walls he lp to move the ov um aJong. ITQ6 (i) Secondary sexual characteristics are those special [eatures that make a male orga n ism look different Erom a fema le organism (e.g. broad hips, deep voice). (ii) Th ey start a t pu be rty. (iii) Th ey are im porta nt for a ttraction to the opposite sex and courtship. ITQ7 (i) A femaJe usuaJ ly produ ces o ne ovum a month, that is a to ta l of ~ twelve ova in a year. . (ii} A male ca n produce over 1 mrnio n spermatozoa in one ejaculation. There is no set rate at w hich he ejacu la tes, as it depends on how often he h as sex ual intercourse or e ngages in some so rt of sexua l activity. A m a le can produce bi1lions o f spermatozoa in a yea r. ITQ8 An average menstr ual cycle is take n to be abou t 28 days. During the first 10 days, no ovum is presen t to be fertilised. Spermarozoa can live inside the fema le for 2- 3 days after ejacu lation, so inte rcourse 2- 3 days before ovulation may res ul t in fertilisation. The ovum may live for 3-4 days, so sexual intercou rse up to 5 days a fter ovulation may result in fertil isation. Ovulation usua lly occurs around day 14. So, in an average cycle, intercourse is least likely to resu lt in fertilisation during days 1-10, and 20- 28. ITQ9 Every m onth , ovulation occurs, so that fertilisation can occur. Thus, every mon th, the ute ru s has to be prepa red for implantation. If implantatio n does n ot occu r, that m onth's lining is shed . ITQ10 Ovu lation occurs in the m id dJe of the cycle, around day 14 in a 28 -day cycle. Oestrogen is the hormone responsible for ovulation. 1t is secreted by the Graa fi an fo ll icle as it develops in th e ovary. When the oestrogen concentration in rhe blood reach es a certain level, ovula tion occurs. ITQ11 (i) The co rp us lu te u m produces the hormone progesterone, whid1 ma in ta ins the lini ng of the uterus fo r a few days a fter ovu latio n . This prepares the body for imp lanta tion, if fertilisation occurs. (ii) Progesterone is ca lled the pregnancy hormone because its level stays h igh during pregnancy. This h ormone causes the uterine lining co stay thick and rich w ith blood vessels, so that the developing offspring can obtain the nutrients it needs. ITQ12 (i) Courtship behavio u r is used to attract a mate and, hopefu ll y, results in m a ting and p roduction of offspring. Tc includes special body movemem s, ca lls a nd da n ces. (ii) Copulation is th e sex act, th e insertio n o f the pen is into the vagi na. O n ejacula tion, sperma tozoa are r eleased at the base o f the cervix . Copu lation res ults in th e transfe r o f m ale gametes to the female w here fertilisation with the fem ale gam ete is possible. ITQ13 (i) Fertilisatio n is th e fu sion of the ma le nucle us, carried by the sp e rmatozoo n, w ith the female n ucleus that is in the ovum. l r occurs in tbe ovid u ct or Fallopian tube. (ii) vagina-+ cervix ...... u teru s ...... ovid uct ...... ovum ITQ14 The zygote or fertilised egg travels down the oviduct to the uterus. Ir implants itself in the wa ll of the tJ1ickened uterus. A placenta then develops from the emb ryo a nd bei ngs to obtain nutrients and oxygen from the mother's blood. ITQ15 The placenta is th e site o f exchan ge of materials berween mother and fe tus. By diffusion across the p lacenta, the fetus obta ins oxygen and nutrients an d gets rid of its waste products. These are the fu nctions that the lungs, kidneys and a lin1entary canal w ill ca rry ou t after birth. ITQ16 (i) Gestation is the pe riod o f develop ment from impla nta tion to binh. In hu mans it is a bo ut nin e m o nths. (ii) Partu ritio n is birth . I t is the expulsion of the baby from the uterus.

257

Life Processes and Disease

(iii) Prenata l care describes rhe care of pregnant woman takes during pregnancy to ensure the birth of a h ea lr11y baby. It includes a proper diet and abstinence from drugs and alcohol. (i v) The newly born baby is tora ll y helpless and dependent on irs mother to sa tisfy all its needs. Postnatal care is care of the baby after it is born. ITQ17 An l.UD has mode of acrion A; al l the other forms of contraception have mode of action B. ITQ 18 An STD is a sexuall y rransmined disease. Tb is means it can be passed on by sexual Lnrercourse.

Examination-style questions (i) Make a labelled drawing of the human female reproductive system. (ii) Indicate on your drawing with: (a) an X, where fertilisation normally occurs; (b) a Y, where spermatozoa are deposited during copulation; (c) a Z, where implantation can occur. (iii) List three advantages of sexual reproduction. (iv) Is it possible for a woman to have 30 children? Explain fully. (v) Suggest reasons why you think it is disadvantageous to have many children. (vi) List four methods of contraception. 2

(i)

Define the following terms: (a) implantation; (b) fertilisation; (c) gestation period; (d) contraception; (e) asexual reproduction.

(ii) Illustrate, using large, clearly labelled diagrams, to show the differences in size, shape and activity of male and female gametes. (iii) Give full and accurate accounts of how: (a) the zygote develops and moves to be implanted, from the time right after fertilisation to implantation; (b) the embryo is nourished and protected as it develops in the uterus: (c) the baby is nourished and protected right after birth. 3

The events of the menstrual cycle are divided into three phases: the follicular, ovulatory and the luteal. (i)

Copy and complete the table below to show the activities in the uterus and ovary during these phases.

Events that occur in the ovary

Events that occur in the uterus

Follicular phase Ovulatory phase Luteal phase (ii) In human females, the menstrual cycle lasts approximately 28 days. What significant events happen during these parts of the cycle? (a) days 0- 5 (b) days 5-1 O (c) days 13- 15 (d) days 15-25

(iii) Describe and explain the changes that take place in the menstrual cycle after fertilisation.

258

Reproduction Plants 0 0 0

•

understand the life cycle of a plant describe the structure of a flower and relate the structures to their functions understand the differences between wind-pollinatd and insect-pollinated flowers

0

understand fertilisation in a flowering plant and the development of fruit and seed

0 0

describe the structure of a fr.uit and adaptations for dispersal understand why dispersal is necessary and how it can be brought about

plant

I flower

pollen grain male gamete

ovule - female gamete

~,~~~~~~~~~--...--------------------' cross self

}-

pollination

fertilisation

development of seed/fruit

wind water

dispersal animal germination

exploding

new plant

Life cycle of a plant Reprod uction is important for the conlinuatio n o f life. Tr is the process by wh id1 new orga nism s are produced. Flowering plants reproduce sexua ll y (fusion of m a le and fema le ga m etes). Sexual reproduction in hu mans in volves two sexes: the mal e produces the ma le ga mete, a nd rbe fema le prod uces

259

Life Processes and Disease

the female gamete. However, in plants, the reproductive organ, which is the flower, usuall y produces both male and female gametes (figure 21.1). Sexual reproduction in animals involves two sexes: male

female

reproductive organ produces the male gamete

reproductive organ produces the female gamete

Sexual reproduction in flowering plants usually involves one flower that produces both male and female gametes.

Figure 21.1 Parents in sexual reproduction of animals and plants.

The life cycle of a typical flowering plant is seen in figure 21.2.

~

IT:Q-1 v'-' Why is the flower described as a reproductive organ?

pollination followed by fertilisation occurs

~

fruit containing seed

...............

~

IT:Q2 V'-'

Put these in the correct sequence of the plant life cycle, starting with (a): (a) development of flowers (b) germination (c) fertilisation (d) dispersal of seeds (e) pollination (f) formation of seeds (g) growth of plant

seeds are dispersed

.\

•

•••

,,

Figure 21.2 Life cycle of a typical flowering plant.

The plant grows until it is mature and produces flowers. These flowers are

I elelllijfilitelelJ the organs of reproduction. After pollination (bringing the gametes closer together) and fertilisation (fusion of the gametes), fruits are formed which contain seeds. The seed conta ins the embryos or developing plants which are usua lly dispersed. Dispersal ta.ke them to new places where the seeds germinate, if possible, into seed li ngs or young plants . The seedling then grows and mature into ao adult plant and the cyde repeats itself.

260

21 · Reproduction in Plants

Structure of a flower pollen grain

>

Flowers are the reproductive organs of a plant. This m eans tha t the fl owers, regardless of the colour, size or sbape, produce and contain the ga me tes or sex cells. The fema le game te is the ovule and the ma le gamete is the pollen grain. A flowe r is structured to protect, house and bring togethe r the ma le and female gametes. , A typica l flower has fi ve main parts. Th e n umbers in the paragraphs below refer to figure 21. 3 stigma I'

1

Gynaecium -+ carpels (pistils) -+ style \,

ovary -+ ovu le (female gamete); ./'

2 3 4

5

filamen t

Androecium -+ stamens ..,.. a nther -+ pollen grains (male gametes); Corolla --+ petals; Calyx -+ sepals; Receptacle .

e

-

petals form the corolla - -- - - - - often brightly coloured and scented

gynaec ium is composed of:

ca~el

androecium is composed of: anther

l sti: : : _ __

L

- - - filament

ovary - --

J

stamen

the anther c ontains pollen grains (male gametes)

(several carpels fused together are called a pistil)

sepal - part of the calyx which protects the flower in the bud stage

nectary contains nectar

rec eptacle flower stalk

Rgure 21 .3 Parts of a flower.

~

Ta ble 2 1. 1 sh ows th e importance of th efive ma in fl ower pa rts.

IT:Q3 V'-1

Label the parts of the flower below.

Part of flower Importance of function

A

B

c

gynaecium

produces and contains the female gamete

androecium

produces and contains the male gamete

corolla

attracts pollinators, such as insects, to the flower

calyx

protects the flower in the bud stage

receptacle

holdings the flower and then the fruit/seed

Table 21.1 The roles of different parts of a flower.

261

Life Processes and Disease

Pollination self-pollination cross-pollination

> >

Pol lination is tbe transfer of the pollen gra in from the amher to the stig ma or othe r flowe rs o f the sa me species (figure 21.4). Self-pollination is th e trans fe r of po llen to the sa me flower o r flowers on the sa me plant. Cross-pollination is the transfer of poll en to flowers on another plant of tbe sa m e species. Most plants undergo cross- polli nati o n, w h ich increases the va riety in the o ffspring \ p rodu ced. Seli-po!Jination gene ra ll y happe ns w he n cross-polli natio n cam1ot be ach ieved .

flower

cross-pollination ----- - plants of the same species - -

Agure 21.4 Self-pollination and cross-pollination.

Figure 21.5 Flowers are pollinated by different agents, such as insects and birds.

~

IT:.Q~

\./'-'

(i) What is pollination? (ii) Why are agents of pollination necessary? (iii) Name three agents of pollination.

~

IT:.QS

In plants, tb e mal e and fema le ga me tes (po llen gra ins a nd ovules) are bro ught cl oser togethe r usuall y by w ind o r by a nima ls, most co mmonl y insecrs and birds. Tbe planr is dependent on these agents to help bring their ga m etes toge ther. Plowers o f these plants ha ve evolved over mill io ns o r years into orga n s that are high ly specialised to tl1e type of po llin atin g agenr (figure 21.5). For example, if insects are to transfer pollen, then they musr be suffici e n tly attracted tO the flowe rs to approach them . This is achieved by insect-po lJ inared flowers h aving nectar, a sugary liquid which is a food so urce fo r insects. Tl1e insects mu st go tbe flowe rs fo r food. Flowers a lso attract insects with brig l1L colo u rs and strong scents. And so, a n in sect visiting fl ower afte r flo we r as it feeds, picks up the pollen gra ins (male gam etes) from o ne fl ower and tra nsfers them to the fem a le gam et e of other flowers . A wind -pollinated fl ower h as a diffe re nt type o f fl ower. These fl owers ca n be inconspicuous and sm alJ beca use they do n o t need w a m act insecrs o r birds. They a re specia Lised i_n a way that allows rh eir pollen to be p icked up by wind curre nrs. Insect-po linated fl owers an d wind-pollinared fl owe rs are compared in fi g ure 21.6 and ta ble 2 1.2.

colourful petal

\./'-'

Define (i) cross-pollination (ii) selfpollination.

r-~~1---

stamen inside flower stamen hangs out of flower

Figure 21.6 Flowers are adapted for either insect-pollination or wind-pollination.

262

Reproduction in Plants

Q6t.,

Insect-pollinated flower

Wind-pollinated flower

Examples

Pride of Barbados, pea (Crotalaria)

corn (Zea mays), grass, sugar cane

Flower

large and brightly coloured

small and inconspicuous

Petal

large, brightly coloured scented nectaries small, green or brown in colour, no scent at the base of petal and no nectaries

Stamen

short, with anthers firmly attached inside long filaments, with anthers that hang the flower outside the flower

Stigma

sticky and situated inside the flower

large, branched and feathery

Pollen grain

large, sticky or spiky- small quantities produced

small, smooth and light - large quantities are produced

ITQ6 \..?V

How is the flower below pollinated? Give reasons to support your answer.

'

~

ITQ7 v'-.J

Describe what happens in the flower after pollination, leading to successful fertilisation. Table 21.2 Comparison of insect-pollinated and wind-pollinated flowers.

fiower

Fertilisation and development of seed

After fertilisation, the ovules develop into seeds and the ovary into a fruit.

-+--~--

style -

2 pollen tube grows down t11e style to the ovary

! The fruit grows more as the petals begin to drop off.

pollen tube enters micropyle to reach female nucleus

Figure 21.7 The male nucleus is brought close to the female nucleus for their fusion (fertilisation). The fruit containing the seeds ~ continues to mature.

Figure 21 .8 Development of a fruit.

Afrer fertilisa tion, the ovu le d evelops into a seed contain in g th e em bryo. The ova ry grows into th e frui t as the pe ta ls shrivel a nd drop o ff. Th e stigma, style and stame rts a lso d rop off. Th e sepa ls may rema in (figure 21.8).

263 - - - - - -- -

-

Life Processes and Disease

The structure of the fru it and seed of a dicory ledonous plant are related to rbe structure of the fl ower. A fruit, whici1. contains one or mo re seeds, deve lops from the ovary. Its shape and the position of the seeds in it relate directly to the shape of the ovary and the posi tion of the ovu les imide (table 21.3 ). Ovary with ovule(s)

Fruit with seed(s)

long ovary containing four ovules in a row

long pod-like fruit containing four seeds in a row

ovary containing one ovule

oval-shaped fruit containing one seed

round ovary with rows of ovules

a

'i'

round-shaped fruit containing rows of seeds.

Table 21.3 After fertilisation, a fruit develops which relates to the shape of the ovary and the number and position of the ovules.

Dispersal Practical activity SBA 21.1: Dispersal of fruits, page 363

Practical activity SBA 21.2: Seeds and food storage, page 364

The fruits conta ining the seeds a re 6rmly attached to Lhe pla nt as the y develop, grow and mature. When mature, o r ripe, they a re dispersed o r sent away from the parent plant. Most plants depend on the he lp of agents Uke w ind, water and anima ls to di sperse their seeds. Each fruit is th us highly specia lised in structure, size, shape and compositio n fo r its type of dispersa l. Spreading th e seeds away from the parent p lant is im porranr ro: • pre ve nt overcrowding and th e refore competition for light, space, water and mine rals; • a ll ow colonisa tio n of new areas o r habitats.

Dispersal by animals ~

IT:Q8 V"-'

Why does a fruit have two scars?

The wa ll o.f the fruit is ca ll ed the perica rp and may be composed of three layers - the epica rp, mesocarp and endocarp. In some fruits, these layers are fl esh y and succulent and a nimals are attracted to th em for food (fig ure 2 J .9).

-

colourful scented fruit

7---

endocarp sweet, soft (succulent)

succulent/fleshy inside

Rgure 21.9 A succulent fruit, like a passion fruit or orange, is colourful and scented.

264

~ production

hooksonthefruit

in Plants

Fruits li ke .mangoes, romaroes, oranges, and wate rm e lons conta in srored food, a nd a re colo urful and seemed to attract animals. The fruir. may be green, unscen ted and in conspicuous w hen young but, as it ripens and is read y for dispersa l, it develops bright colo urs li ke red and orange, and becomes scented, so that animals a re attracted LO the plant for food. As they eat the fru its, they may move away and so disperse the seeds. If the seeds are large, they a re spa t out or discarded in a new place away f rom the parent plant. 1f the seeds are , small, th ey may be swa llowed and then egested (figure 21.10). Some, like to mato seeds, ca n pass tbrougb the dlgestive system u n harmed.

b ird then flies away

seeds in the faeces of the bird fall far away from the parent plant

fruit hooks onto animal's fur or clothing as they pass through bird eats fruit swallowing the seed

Figure 21.10 Berries are succulent fruits. They can be taken far away from the parent plant by birds.

Figure 21.11 Some fruits have hook-like structures.

Fru its ca n also be dispersed by animals in a different way. These ki11ds of fruit do nor. anract a n ima ls for food because they a re dry. They have hooks or hairs or spikes, and become attached to the an ima l instead (figu re 2 1. 11 ). When the an imal is wa lking tl1ro ugh the env ironment, these fruits, li ke sweetheart and burr grass, stick or book on to the anima l's legs or body and ge l dispe rsed as the animal moves away from the parent plant.

Dispersal by water Fruits d ispersed by water rnusr be buoyant so that they can fl oa t away, for examp le, coconu t trees a re usua ll y found on coastlines. Cocon u ts can be taken by ocean currents to otl1er coasts, islands or countries. The ep icarp is waterproof and the mesocarp fibrous and ligh t - adaptations fo r dispersa l by water (figure 2 1.1 2). the mesocarp is fibrous and traps air which makes the large fruit buoyant

coconut oats on water and is taken away from the parent plant

coconut sprouting on a new beach

Figure 21.12 Some fruits such as coconuts are dispersed by water.

Dispersal by wind Some fruits a re ca rried by the gentlest of w ind currents a nd so mu st be li ght and particu lar adaptations (fig u re 2 1.1 3, overleaf). Dan delion and silk corron

265

Life Processes and Disease

~

seeds ha ve radiaring rhrea ds that form a parac.hure; mahogany seeds h ave w ing- li ke stru clllres w hich a llo w them to be carried away fro m the ir pa re nt.

IT:Q9 V'-1

wing-like structure

What is dispersal?

The fruit spins in the wind and can be taken far away

1--- -

parachute shape can take fruit far away

Figure 21. 13 Some fruits are adapted to 'fly' in wind currents.

Dispersal by explosive devices W h en expl osive fr uits dry, th ey split and curl su ddenly to nick o ul Lhe seeds (fi gure 2 l.1.4). These are fr u its with pods like ga rden pea (Crotalaria), thorn apple and Pride o f Barbados. This is al so ca lled se lf-di spersa l o r m ech an ical dispe rsa l. ln th is case, the h e lp o f som e oth e r agen t is n ot needed - th e drying out o f the pod ca uses it to spl it a lo n g its line o r wea kness a lo ng th e side.

~

IT:Q·10 V'-1

Draw a table with named examples of plants which show each of the following methods of dispersal: animals, water, wind, self-explosive. Describe how their seeds are adapted for dispersal in this way and make a simple sketch in each case.

266

Figure 27.14 Some fruits such as thorn apple (Datura stramonium) 'explode' and release their seeds The pod splits when the walls curl back as they dry out. The seeds are flicked out.

21 · Reproduction in Plants

lf the seeds eventuall y I.and o n fertile soil, away from the parent, the y have a good chance o f germina ting into a seed ling wh ich can then grow into new plant. Life continues and the cycle continues.

'

ITQ1 Tbe fl ower produces the rep rod u ctive cells or ga meces which, on fu sion, prod uce oCfspring. ITQ2 a, e, c, f, d, b, g ITQ3

A petal Banther ~~~.........,..~~-

C ovule

ITQ4 (i) Po lli nation is the transfer of polle n grains from th e a nthe r to a stigma o f a flower o f the same s pecies. (ii) Po lle n gra ins cannot move by themselves. They requ ire agents to move them from th e anther to th e stigma. (iii) Insects (e.g. bee); birds (e.g. hummingbird) ; wind. ITQS (i) Cross-pollination is the transfer o[ pollen grains from the anther o f o ne pla nt to the stigma o f another plant o r the sa m e speci es. (ii ) SeH-pollinatio n is the tra nsfe r o f pollen g rains from the anther o [ one fl ower to the stigm a o f the sa m e flo wer, or to OLher flowers on the same p lan t.

267

Life Processes and Disease

ITQ6 Th is flower is insect-po ll inated because: • it is brightly colo ured; • th e stig ma is inside the fl ower; • it has large peta ls; • the anthe rs a re [ound inside the fl ower. ITQ7 During pollina tion, th e pollen gra ins land on tJ1e stigm a or a fl owe r. U th e polle n a nd th e stigma a re o f th e same species, th e pollen grain th en , germin ates and develops a lo ng polle n tube w hich grows down in side the style. The pollen tube conta ins two m ale nuclei an d it contin ues to grow un til it reach es the ova ry and th en the ovules. It grows th roug h the micropyle an d e nters the ov ule. A male n ucle us in rh e pollen tu be the n fuses w ith the [ema le n ucl eu s in tJ1e ovul e. This is fe rtilisation ITQS Whe n a frui t develo ps from an ova ry, it has a sca r wher e it was atta d 1ed to the rece ptacle or stem a nd a scar wh ich used to be the style. ITQ9 Dispersal is the p rocess which describes how the offspring of a pla n t m ove away from the parent plant. Neither the fru its containing the o ffspri ng n or the seed (offspring) ca n m ove from place to place, so they depend o n age nts of dispersa l li ke water, w ind and animals. ITQ10

Fruit

Agent of dispersal

Adaptations for dispersal in this way

Cherry

Animals

The fruit is brightly coloured and juicy. Animals are attracted to it for food, and they disperse the seeds when they move away from the parent plant with the fruit.

Coconut

Water

The fruit is dull and very large; it is also buoyant and light. It can stay afloat for long periods of time.

Purple petria

Wind

Wing-like structures are present. Seeds are light and small and can be picked up and carried by wind currents for miles.

Thorn apple

Self-explosive

It is dull-coloured and has four parts. They lose water and become harder and harder, placing strain where they join together. Eventually the parts 'burst' and scatter the seeds which were Inside the fruit.

268 -- --~--

- - - - -- -

21 · Reproduction in Plants

Examination-style questions (i)

Define: (c) germination (a) pollination (d) dispersal (b) fertilisation (ii) Examine the diagrams of the two fruits I and II below and describe fully how dispersal occurs in each.

(iii) List two advantages of dispersal. (iv) List three conditions necessary for germination. 2

(i) Name the parts of the flower in the diagrams I and II below.

H

269

Life Processes and Disease

(ii) State some differences between the stamens of the wind-pollinated flower and insect-pollinated flower. (iii) List two characteristics of the pollen grains of insect-pollinated plants. (iv) What is the main advantage of cross-pollination? (v) Pollination can be described as an example of symbiosis. Describe fully the relationship between bees and flowers. 3

(i)

Copy and complete the diagram below which shows the reproductive cycle of a flowering plant. adult plant with flowers

( I

I

..•.

·-

fertilisation

dispersal

(fusion of gametes)

t.____ _ __

-!

(ii) Name A, B, C, D, E, F and G in the diagram below.

:L=--

GI

-

- A

l-+--8

c F

E

(iii) Distinguish between pollination and fertilisation. (iv) Pollen grains from many different species may land on a stigma. However, the seeds produced belong only to the same species as the flower. Explain why this happens.

270

~D) isease

and Humans

0

underst and what is meant by pathogenic, deficiency, hereditary and physiological diseases

0

distinguish among the methods used to treat and control the four main groups of diseases

0 0

understand the role of vectors in the transmission of disease

0

understand the importance of knowing the life history of a vector in relstion to control understand the social and economic implications of disease in plants and animals

disease

r deficiency

r vector

pathogenic

I

I

} MOS

control of disease

hereditary

'

role of blood immunity

physiological

'

social and economic implications

drugs • alcohol • caffeine • cocaine • heroin

life cycle

'

( natural

artificial

vaccination

Health and disease Hea lth h as been de fined a s 'co mplete ph ysica l, m e nta l a nd social we ll -being'. l t is m ore t han just rhe absence of disea se; it incl udes th e me nra l a nd socia l d imension s o [ li fe. A disease is a condiri.0n in w hic h th e h ea lth o f an o rgan ism is impaired. Note that a proper diet a nd adequate exercise a re im po rta nt to good hea lth. Th ey h elp to preve m the onse t of, a nd even he lp to treat, diseases. Eati ng foods Lh.a t make up a ba lanced diet increases the body's resista n ce to in fection. A p rog ramme o f exercise stre n gthens a ll the o rga n systems and leads to overa ll good hea lth - physical, me nta l and socia l.

Types and control of disease Diseases can be divided imo fo ur main types - pathogenic, deficiency, h e redhary and physio logica l.

271

Life Processes and Disease

Table 22. L distingu ish es between th ese types of disease, describes one exa mple of each type and discusses the m ethods used to treat and control these Lypes of diseases.

Type of disease

Named example

cause

Symptoms

Pathogenic Caused by parasitic organisms (pathogens) like viruses, bacteria, fungi, protozoa and worms. Examples: malaria, TB, cholera, influenza

Influenza

Virus (pathogen) invades Headache, sore throat, the body by contact muscular pains, fever with infected person. It is airborne or dropletborne.

Deficiency Caused by a shortage of a nutrient (e.g. vitamin, mineral) in diet. Examples: kwashiorkor, night-blindness, irondeficiency anaemia

Iron-deficiency Deficiency of iron anaemia causes a reduction in the number of red blood cells which reduces the oxygen-carrying capacity of the blood. This is because iron is an integral part of the structure of haemoglobin in red blood cells.

Hereditary Caused by genes passed on from one generation to the next. Examples: haemophilia, cystic fibrosis, sickle cell anaemia

Sickle cell anaemia

Physiological Caused by a malfunction of body's organ. Examples: asthma, hypertension, diabetes, glaucoma, stroke

Diabetes

Treatment

Control

Rest and treatment for the symptoms. Vaccine for specific strains of the virus.

Prevent overcrowding and exposure to the virus. Prevent droplet infection through coughs, sneezes, etc.

Weakness, fatigue, shortness of breath, increased heartbeat, pale appearance

Eat iron-rich foods (e.g. red meat, green leafy vegetables). Take iron tablets.

Education about a balanced diet, food groups, etc.

Gene for the disease is passed to the offspring. The gene causes the red blood cells to be sickle shaped which reduces oxygencarrying ability.

Weakness, tiredness, weight loss, May lead to kidney failure, heart failure

Avoid situations where Genetic counselling oxygen supply is reduced. No treatment or cure available.

Inability of the islet of Langerhans to produce insulin. Body cells are unable to absorb glucose which stays in the blood.

Tiredness, continual thirst, weight loss, increased urination, coma

Insulin injection/tablet Low carbohydrate diet, exercise.

Education on the importance of diet and exercise.

Table 22.1 Some diseases in humans.

CHAPTER 13

~

IT:Q·1 V'-.1

What do you understand by the terms: (i) pathogenic disease (ii) hereditary disease (iii) physiological disease (iv) deficiency disease?

272

Som e djseases are more common ly found in certain pa n s of the wo rld than in oth e rs. For example, in developing countries a grea ter proportion of dea Lbs occur as a result of infectio us diseases, like den gue fever, ch ole ra a nd tube rculosis. In developed co um ries, a smaller proportion of people die from infect ious ruseases. and more death s are d ue to p hysiologica l diseases like ca nce r and hea n disease. These kjnds o f disease are infl uenced by facto rs such as diet, life style, genetic prerusposition and exposure to ha rmful cond itions. For example, hypertension (chapter L3) results from a stressful li fe, filled wi th worry, a nger, nervo us fatigue, n o rest or relaxation, and u nh ea lth y ea ti ng habits (e.g. too mu ch fatty, sa lty fast-food) . Hyperte n sion can to some extent be controlle d by good diet and exercise. This difference in distributio n is refe rred to as th e globa l distributi on of disease. It often reflects th e wea lth and sta ndards ol medica l care in the diffe rent coun tries. Th us, the occu rren ce o f disease in deve loping countries is often iofl uenced by facto rs such as overcrowrung, lack of clea n water, lack of preventative medkin es and lack of proper nutrition.

22 · Disease and Humans

Pathogenic diseases and vectors l•fi\hr•lo!4•*J

A pathogen , or disease -causing organism, Lives on or inside an organism, the

host, causing it to be diseased o r sick. Pathogens ca n move from one host to a nother, or inrect another organ ism in a number of wa ys including by: • water; • food ; • airbo rne droplets; • direct contact; • du st pa rticles; • contact w ith fa eces; • anima ls (ma inly insects), ca lJ ed vectors.

Vectors l@AMIJ

Vectors spread disease- by carryi ng the pathogen from host to host. Examples o f vectors in cl ude fl ies, mosqu itoes a nd rats (figu res 22. 1,22. 2 and 22.3). Table 22 .2 gives sosme examples ..

Vector

Examples of disease(s) spread

mosquitoes

yellow fever, malaria, dengue fever,

flies

gastroenteritis

rat flea

plague

rat

leptospirosis

Table 22.2 Some vectors and the disease they spread. vector 'bites' infected host and picks up the pathogen vector with the pathogen

l

-- -

~~ ~ "

/

--

vector 'bites' new host, transferring the pathogen

infected host

Figure 22.1

new host. infected by a vector

Mosquitoes are vectors for malaria and many other diseases in humans.

~

l:F:Q2 V'-1

Figure 22.2 Flies feed on the food we then eat and so spread disease.

(i) What is a vector? (ii) Discuss why a fly can be considered to be a vector.

273

Life Processes and Disease

Controlling mosquitoes If the vector can be controlled, Lheo th e spread o f tile d isease will also be comrolled, since Lhe chance o[ being in contact wiLh Lhe vector and so getting Lhe infeaio n w ill be red uced. Th us, it is important to study th e venor's life cycle ro find ou t how to prevent them laying eggs, or how to prevent their development into adulrs, or how to destroy the adults. A good example of this is the atLempt to control the mosq uitoes rhar act as vectors (o r malaria (figure 22.3). t spiracles for breathing

/

larva lives in water

~ egg laid on water

pupa lives in water

/ Agure 22.3 The life cycle of a mosquito. Tbe erad ica tion o ( a d isease sprea d by m osquitoes wo u ld be poss ible if a concened effort were made by the genera l public in the following areas. • Drai n stagnan t w ater around t h e ho m e a nd workplaces - Th is wou ld drastica ll y reduce th e number o( places for female mosqu itoes to la y eggs as well as reduce the numbe r o( eggs and pupae s urviving to deveJop into adu lts .

• Spread a thi n layer of oil ove r w ater which must be ke pt - This would preveoc larvae a nd pupae in th e water from breathing and so kill th em . •

Kill t he adu lts with insecticide.

• Use mosqui t o nets - TIJ.js would reduce the possibility of being bine n when mosquitoes are a round . • Keep the area a round the l1o use clear of bush where adu lt mosquitoes rest. In tile sa me way, knowing where flies lay their eggs, how they develop a nd whar Lhey feed o n, can help to reduce the number of flies and thus reduce the incidence of diseases spread by flies .

Pathogens

~111fll

274

Pathogens are usua ll y m icroscopic organisms (li ke viruses, bacteria and protozoans) that Live in Lhe blood and tissues of their host. Some are larger, li ke fungi and worms, which are easie r to get rid of and control. Some pathogens spread by direct comact o r dose in teraction between Lhe infected host and a new hosr. These include the sex ually trans mitted diseases (STDs) like herpes, AIDS, gonorrhoea and syphilis. AJDS is of most impo rtance beca use it has reached epidemic statu s in the world and can lead co

22 · Disease and Humans

dea th because it compromises the body's immune system leaving the pa tient defenceless against secondary opportunistic infections. Expensive drug regimes can prolo ng rhe life of someone li ving w ith HIV, but there is no means of elimating the virus from th e body and no cu re for AIDS. Herpes is also incurable but not fa tal. The other STDs can be trea ted and conrrolled if diagnosed ea rly.

Social and economic implications of disease The loss of life and loss of wo rking hours to disease a re important socia l and eco nomic fa ctors. Trearmems for pathogenic diseases sud1 a s AlDS a nd degenerative disea ses su ch as cancer place increasin g demand s on health services. Lifestyl e diseases related to smoking, lack of exercise and over-eating are increasingly important economica lly in develo ped countries, again beca use of the cost of treatment and their effects o n social and econo mic life.

Implications for humans of disease in plants and animals Human s a re also a ffected eco no mically by the hea lth o f the crops and animal stocks grown for food. Loss of li vestock (cows, pigs, chi cke ns, e tc. ) and agricultural crops (rice, wheat, porntoes, etc.) due to disease can have se rio u s economic impli cation s. A disease can grea tly reduce o r w ipe o ut the livestock o r food crop oJ any area in a short space o f time; [or exa mple, m ea ly bu g in festation in the Caribbean, and foot-and-mo uth disease in Europe. This results in loss of income for the farmers and reduction in food ava ilability. Food, in the form of livestock and ag ricultural prod uce, moves a ll over the world io ships a nd a irpla nes o n a daily basis. Disease control is therdore ve ry difficult. Quarantine procedures at porrs and ai rports help but do n ot prevent the sp read of diseases. Many pathogen s a re microorga ni sms so are not seen; man y can ex ist as spores for long periods of time.

Chapter summary • A healthy person is physically, socially and mentally well. • A disease impairs good health. • There are four main classes of disease: pathogenic, deficiency, hereditary and physiological diseases. • A pathogenic disease is caused by parasitic (and often microscopic) organisms like viruses, bacteria, fungi , protozoans and worms. • A deficiency disease results when there is a deficiency of a nutrient in the diet. • A hereditary disease is passed on by genes from a person to their offspring . • A physiological disease is caused by a malfunction of an organ in the body.

,

II

• A vector transports pathogens from one host to another. • Vectors are usually insects, such as mosquitoes and flies. • Understanding the life cycle of a vector can help to control or eradicate a disease spread by that vector. • The social , environmental and economic implications of disease include loss of life, loss of working hours, loss of money. Disease in crops and livestock can lead to famine. Research into cures for disease is expensive.

_,.... 275

Life Processes and Disease

ITQ1 (i) Pathogen ic disease - symptoms of disease a re seen because of the presen ce of another organ ism (a pathogen ) in th e body. (i i) Hereditar y disease - sym ptoms of di sease seen beca use of the presence of a 'disease-carrying' gene which was passed to an o rga nism from its pare nts. (iii) Physiologica l di sease - symptom s o f disease seen because an orga n o r part of the body is not working. (iv) Deficiency disease - sympto ms of disease seen w he n a nutrient o r nutrients are la cking in the dlet of the orga nism. ITQ2 (i) A vector ca rries a pa thogen from host to bost. It is able to pi ck l:IP the pathogen in or on its body w hen it feeds and then transfe rs the pathogen when it m oves to another host. (ii) Flies pick up microorganisms wben they feed. They feed o n any orga nic matter, especially dead a nd rotting o rganic ma trer. The ir bod ies are hairy a nd can easil y ca rry pa thoge ns. They a lso regurgitate or vomit previous food whe n they eat. If they land to Jeed o n any substance th at is go ing to be food or drink to another anima l, they can pass th e pathogen to a new host. Flies are thus con sidered to be vectors.

Examination-style questions (i)

(ii) (iii) (iv) (v) 2

Explain what is meant by the following types of disease and give one example of each: (a) hereditary; (c) deficiency; (b) physiological; (d) infectious. Describe the causes and symptoms of: (a) gonorrhoea (b) diabetes. Explain how diseases like malaria and sickle-cell anaemia are spread and describe the importance of these diseases worldwide. Describe the social and economic implications of AIDS. List some ways the spread of AIDS can be prevented and controlled. Describe and compare the global pattern of distribution of yellow fever and coronary heart disease.

(i) Describe how phagocytes protect the body against infection. {ii) List four ways the skin and openings on the skin are adapted to control the entry of pathogens into the bo"dy. (iii) (a) Distinguish between active natural immunity and active artificial immunity. (b) Give one example of artificial passive immunity and one example of natural passive immunity. (iv) It is estimated that a human can synthesise 1Omillion different types of antibodies. Describe three ways antibodies defend the body against disease. (v) Copy and complete the table.

Drug

Two effects on the body

Social or economic implications

Alcohol Cocaine Caffeine 3

276

(i)

Explain the meaning of the term 'drug'. Using named examples discuss the use and abuse of drugs. (ii) Describe the immediate and long-term consequences of alcohol consumption. (iii) Discuss the social consequences of excessive alcohol use with particular reference to drink driving, aggressive behaviour, family breakdown and petty crime.

Section C:

Continuity and Variation

0 0 0 0 0

understand the importance of maintaining species chromosome number describe the process of mitosis understand the role of mitosis in growth explain the role of mitosis in asexual reproduction explain why asexual reproduction gives rise to genetically identical offspring

cell cell division

( meiosis

'\ mitosis

( prophase metaphase anaphase telophase

'\ cloning of animals

tissue culture in plants

asexual reproduction

growth

Dolly

Chromosome number ,.

chromosome number >

Ch ro mosomes a re presen t in th e nudei of ce lls. They com a in geneti c info rmacion in the form o f genes. Each species bas a speci fi c nu mber of chro m osomes in its body cell - this is call ed th e chromosome number for th a t species (fi gure 23 .J a n d table 23. J ).

~

ll:Qjl ~ (i) What is meant by the 'chromosome _number' of an organism? What is the chromosome number for (a) humans and (b) an onion?

Figure 23. 1 Each species has its own chromosome number.

23 ·Mitosis

Species

Chromosome number

onion

16

tomato

24

locust

24

corn

40

mouse

40

human

46

potato

48

Th e ch ro m osom e n umbe r for hu mans is 4 6. This mea ns rha t in every bod y cell o [ every hu man rh e re a re 46 ch rom o somes. A chromosom e is m ade up o f gen es. Wh ile th e ch rom osom e o u m be r is th e same fo r all h uma ns, the combina tion of genes is diffe re nt (figure 2 3.2 ). The 46 ch ro m osomes a re diffe re nL fo r e ver y hu m a n except id e n tical twin s.

Table 23. 1 Chromosome numbers for seven different species

~

IT:Q2 \./'-I

(i)

List three differences between the people in figure 23.2. (ii) List three similarities. (iii) Why can these differences be seen?

There are 46 chromosomes in each of his She also has 46 chromosomes in each of her body cells, but the combination of genes 1s cells but with her own combination of genes on special to this individual. This produces outward the 46 chromosomes. Each individual is unique characteristics that are special to this individual. and special. Figure 23.2 Members of the same species have the same chromosome number, but the combination of genes is different for each.

The eel I cycle RRl!ii1fft:IJ [jjlU.~·U•'IJ

IGl@•hfiM:tJ

The cell cycle is the sequ e n ce o f e ve nts rha r occurs be tween the s1a n of o ne cell d ivisio n (mitosis) a nd th e Starr o r th e n ext (fi gure 23. 3) . Th e lo ngesr e ve nt in rh e cell cycle is ca ll ed interphase du ring which the cell grovvs a nd carries o u t its fu n ctions. A L the e nd of inte rphase ce ll division begins. M itosis is di vided into fo u r stages: prophase, m e rap hase, a naph ase and relo phase. At th e end o f telo phase, l\ Tissue culture is a form of vegetative propagation used

to make large numbers of identical plants (figure 23.10). Like binary fi ssion, it also results from mitosis. Using tissue culture propagation or cloning, whole plants can be made from very sma ll pieces cut from the parent plant. This depends on the fact that the majority of plant cells have the potential to form a whole plant.

A very small piece of tissue is taken from this plant.

The tissue is cultured on a sterile nutrient medium. The piece of tissue is made up of a number of identical cells.

nutrient medium

/ The cells divide by mitosis to form a callus - a ball of cells.

Rgure 23. 1O Tissue culture in plants.

284

The callus is stimulated to develop into a plantlet.

Plantlet transferred to soil. This is genetically identical (clone) to the original plant. Many clones can be made from one plant.

23 ·Mitosis

Advantages of tissue culture

~

IT:Q7 V'-1

(i) Define the term 'tissue culture'? (ii) Describe how tissue culture is used to generate many identical plants.

• Large numbers of identical plants can be produced relatively quickly from 'superior' individuals. This can make them much cheaper. • Tissue culture can be used to propagate plant species which do not develop naturally through sexual reproduction easily, such as orchids. The propagation of orchids for sale on a massive scale is now possible.

Disadvantage of tissue culture • Variety within a plant species is being replaced with similarity because it is cheaper. This is risky because if that one kind becomes susceptible to a particular disease or pest, the whole crop may be lost.

Cloning of animals identical twins

~

IT:Q8 V'-1

(i)

Explain what is meant by 'cloning'? (ii) Describe a natural occurrence of cloning?

> A clone is an exact copy of an organism.

Identical twins are, in essence, clones, since after the first cell division of the zygote, two identica l cells are formed. These two identical cells somehow separate from each other and then grow and develop into separate beings that are identical to each other (figure 23. 11 ). The environment confers subtle differences as they grow and develop. one organism

~ --+

w zygote divides into two

~ ---+

two-cell stage

w separation of the two cells occurs resulting in two identical cells

each divides and develops separately

•

another organism ~ ·twin')

they develop in their mother's womb and identical twins are born

Figure 23. 11 The development of identical twins.

CHAPTER 26

Scientists can now easily separate the first four cells of a zygote and use these to create clones of the organism (figure 23.12, overleaf). This is practised mainly in the livestock and dairy industries. It is financia ll y advantageous to make clones of a 'superio r' animal, such as one which produces large amounts of a high-quality milk or high -protein mea t. It is also used to 'copy' individuals which have been genetically engineered (chapter 26). An example here would be Hvestock with genes to produce human hormones in the animal's milk. Cloning may be used to produce an animal with some specia l characteristic (such as speed in a racehorse) as that could be financially beneficial. Clones need a surrogate mother in which to develop. This is a female who is not the genetic mother but in whose womb the fertilised ovum is implanted so that it can develop as a fetus until birth.

285

Continuity and Variation

- - - - - - - - - - - - - - - - - - organism growth and development in mother's womb zygote

2-cell stage

4-cell stage

_ __. organism

• 2-cell stage

4-cell stage each of the four ' cells are separated ~

clones (exact copies)

..

~0 zygote of a 'superior' organism Clones would all have the same 'superior' characteristics.

_ __,. organism

_ __,. organism

(!) - - - organism

Each cell then continues to grow and develop into an organism. They are exact copies of each other. Each can be implanted in a female's womb (surrogate mother).

Rgure 23. 12 Cloning of fertilised eggs.

~ ll'!Q9 \...l'-1

Describe one way that scientists can make copies or clones of a 'superior' animal?

A second way to create a clone is to ta ke the nucleus of a body cell from the 'superior' individual and use it to replace the nucleus of an unfertilised ovum. The cell can be made to divide as it would have done if it was a fertilised ovum and implanted in the womb of a surrogate mother, but all the cells it makes now have the chromosomes from the 'superior' animal. The first example of this kind of cloning was the sheep called Dolly(figure 23.13). normal development

how Dolly was cloned

(9\ ~

unfertilised egg

unfertilised egg

/

1 ~ 1 ~~om 00

.

(!)

egg fertilised byE f sperm forms M a zygote

II" •

1 starts to divide

II'

1

@

@

1

l

/

The nucleus is removed and replaces the nucleus of the ovum. Development continues in the surrogate mother. The dividing nucleus contains Dolly's chromosomes. A surrogate mother is one in which the embryo is implanted and she •oam~· • b•by th•t ;, oot "'"

Surrogate mother gives birth to Dolly who is genetically identical (a clone) with the original sheep.

~ ll'!Q·1 0 \...l'-1

How was Dolly cloned? Rgure 23. 13 How Dolly the sheep was cloned.

286

a sheep

23 ·Mitosis

Advantages of animal cloning CHAPTER24 ~

• Superior traits can be passed on to offspring without the risk of losing them through genetic exchange during meiosis (chapter 24). • The use of surrogate m others means that more 'superior' offspring can be created than could be carried by just the genetic mother.

Disadvantages of animal cloning • The effects of using a body cell, as in crea ting Dolly, are still being studied: It is possible that using what is in effect an 'old ' nucle us may cause problems . in the cloned individual. • The technique used to crea te Dolly could be used to clone humans. Many countries now have legislation to prevent this because it is considered unethical. For example, it might be done for purely selfish reasons.

Chapter summary • Each species has its own chromosome number, that is the number of chromosomes found in each nucleus in the cells of the individual. • The chromosome number of humans is 46. This means that there are 46 chromosomes in every nucleus in every body cell of a human. • Although each individual of a species has the same chromosome number, the combination of genes in the chromosomes varies and so members of a species differ. • Cells divide by mitosis to produce two genetically identical cells. • Mitosis is important for growth, repair and asexual reproduction . • Mitosis is divided into four stages: prophase, metaphase, anaphase and telophase. • During replication each chromosome makes an exact copy of itself. This occurs in interphase, just before prophase. · • These two copies of a chromosome are called chromatids - they lie side by side and are joined at the centromere. • During prophase, the chromosomes become shorter and fatter and are easily stained and seen. • During metaphase, the chromosomes line up along the middle of the cell. • During anaphase, the chromatids are pulled apart to opposite sides of the cell. • During telophase, two identical nuclei are formed. • In cytokinesis, a new cell membrane develops to divide the cell in two identical cells. • Binary fission in Amoeba is an example of cloning. • Cloning is the production of identical copies of an individual. • Animals and plants can be cloned using several techniques. • Tissue culture is one way of cloning plants.

The chromosome number is the number of chromosomes found in a typical cell of an individual of the species. It is a fixed and specific number to each species.

ITQ1 (i)

(ii) (a) 46 (b) 16 ITQ2 (i) The eyebrows are shaped differently and of different thickness.

The size and shape of their lips are different . The shapes of their faces are dillerent. (There are other differences that you may have seen.)

287

on 1nu1ty and Variation

(ii) Both individuals have two eyes. In both individuals, the nose is in the middle of the face.

The position of both their lips is the sa me. (There may be other similarities that yo u have mentioned, relating to characteristics that are general to being of the same species.) (iii) Differences can be seen because, altho ugh they both have 46 chromosomes in each cell, the composition of the chromosomes is different. Their genes code for different characteristics. • ITQ3 (i) Mitosis is cell division that res ults in the formation of two identiq1J daughter cells. (ii) Mitosis is important for growth and repair. ITQ4 A species has special characteristics that separate it from other species. The number of chromosomes is very important. A change in chromosome number may change the species-specific characteristics. ITQS 1 C, 2 B, 3 A. ITQ6 Amoeba divides by mitosis producing gen etically identical offspring. Over the years, when it divided, identical offspring were produced, so that one seen today wou ld be genetica lJy iden tica l to one which existed 100 years ago. ITQ7 (i) Tissue culture uses a piece of tissue from a 'parent' plant to make many plants that are identical to the parent plant. (ii) A piece of tissue is taken from a parent plant. It is placed in a medium conta ining nutrients and growth hormones . It is kept under sterile conditions to prevent microorganisms from entering the medium. In the nutrient medium, the piece of tissue divides rapidly by mitosis, forming a structure called a callus, which is a ball of cells. The callus may be divided and placed in many jars containing the nutrient medium. Each piece develops into a plantlet which is cared for carefully. The many plantlets are all identical to the parent plant. ITQ8 (i) Cloning means making exact copie.s. An exact copy of an individual is m ade when it is cloned. (ii) Cloning may occur naturally in the formation of identical twins. After the zygote is formed it divides into two cells. Usually the two cells stay stuck together and continue to divide to make one individual. In identical twins, these first two cells separate and develop into individual organisms. They are genetically identical. ITQ9 Scientists allow a zygote of a 'superior' animal to divide naturally twice, producing four identical nuclei. These are then separated and implanted in the uterus of other animals (surrogate mothers) and allowed to develop . Four clones of the superior animal are thus made. ITQ10 Dolly was cloned by extracting the nucleus from one of the cells of a ewe. This nucleus contained all the information needed for the forma tion of Dolly. The nucleus of an ovum was also removed and replaced with Dolly's nucleus. The ovum containing Dolly's nucleus was made to implant in a surrogate mother and it developed into an individual which was Dolly.

Examination-style questions (i) List the stages of mitosis. (ii) Explain fully the importance of interphase just before mitosis begins. (iii) Explain the meaning of the term 'diploid'. (iv) (a) Label the parts A to Gin the diagram on the next page. (b) Identify each stage. (c) Describe what happens in each stage of mitosis.

288

23 ·Mitosis

2

(i)

(ii) (iii)

(iv) (v)

(vi)

Explain the following terms: (a) asexual reproduction ; (b) binary fission. List two advantages of asexual reproduction. One major disadvantage of asexual reproduction is that the offspring vary only rarely. Many species use only asexual reproduction but their offspring are not all clones. Suggest how variation comes about in these asexually reproducing species. What is a clone? Suggest an argument: (a) for animal cloning; (b) against human cloning. Give a brief description of tissue culture. Discuss some advantages and disadvantages of the use of tissue culture in agriculture.

289

/

understand the importance of halving of the chromosome number in the formation of gametes

0 0

describe the process of meiosis

f7

understand the role of meiosis in the transmission of inheritable genetic characteristics

distinguish between mitosis and meiosis

cell

I division

( mitosis

'

meiosis

r meiosis I and II

'

variation

evolution

The importance of meiosis Body cells divide for growth and repair, and it is important that the new cells are identical to the existing o nes. This is the significance of mitosis. However, cells of th e reprod uctive o rgans must also divide, but in this case, to form the gametes or reprod uctive cells. Two gametes, one from the ma le and one from the female, fuse to form the zygote which develops into the new organ ism. These gametes must therefore concain half the chromosome number of d1romosomes. If they did not, the new organism would have twice the species chromosome number. Meiosis is the cell division which occurs only in the reproductive organs during gamete formation, and results in the formation of cells containing half the number of chromosomes as the parent cell. Half the number of chromosomes is the haploid o r n n umber. For example, a h uman body cell has 46 chromosomes. When body cells divide by mitosis for growth and repair, cells containing 46 chromosomes (d iplo id or 2n number) are always produced. However, cells of the reproductive organs must divide by meiosis to make gametes. The gametes must contain 23 chromosomes (haploid or n number) so that, after fu sio n with another gamete, the original number of 46 d1romosomes is restored (figure 24. l ).

24 · Meiosis

diploid (2n)

~

l:T:.Q·1 l../V

mitosis diploid (2n) _ _.,. diploid (2n)

Where does meiosis occur (i) in females (ii) in males?

~

l:T:.Q2 l../V

list the differences between a diploid cell and a haploid cell. Give an example of where each can be found in the human body.

~

diploid (2n) ( . : \ reproductive cell ~ of female

~

meiosy / \' haploid (n) @

@

haploid (n)

~

GAMETES

meiosis fertilisation

Figure 24.1 The importance of meiosis in maintaining the chromosome number.

haplok:t (n)

/I \"'eiosis ~

~ ~

0 0 diploid (2n)

reproductive cell of male

GAMETES fertilisation or fusion of gametes to form a diploid zygote

diploid v~ I (2n)

+ The process of meiosis

develops into an organism with 46 chromosomes like its parents

Meiosis ensures that: • each da ughter cell has the haploid number of chrom osomes so that the diploid number can be restored after fertilisation; • each daughter cell has a different combination of genes which leads to variation among th e offspring.

homologous pair >

A human cell has 46 chromosomes: 23 came from the m other and 23 came from th e father. Each chrom osome from the set from the mother pairs up with a corresponding chromosome from the fa ther. These are called homologous pairs. The ch romosome in hom ologous pairs in humans are the same size an d shape apart from the sex chrom osome (figure 24.2). nuclear membrane a homologous pair

centrioles

Figure 24.2 The homologous pairs of chromosomes in a cell with four chromosomes

two chromosomes of maternal origin

homologous chromosomes (a homologous pair) similar chromosomes, one from the mother, one from the father

291

Continuity and Variation

~

crossing over

IT:Q3 V'-J

Why is meiosis important in gamete cells?

~

IT:Q'I V'-J

Explain the terms (i) homologous pairs (ii) chromatid.

>

At the beginning of meiosis, each chromosome forms two chroma tids joined by a centromere, as in mitosis. The homologous chromosomes then come together, so there are now four chromatids dose together. Genetic information is exchanged randomly between the chromatids. This is known as crossing over. In metaphase I, the homologous chromosomes align randomly across the equator of the cell, and then the members of homologous pairs separate and move to opposite sides of the cell. The cell then splits to form two cells. The · division repeats with the chromosomes again lining up randomly along the equator of the cell, only the second time around the chromatids separate, resulting in four daughter cells, each with different genetic information (figure 24.3 and table 24.1).

Prophase II • centrioles migrate to opposite sides of the cells

lnterphase • replication of all four chromosomes occurs • cell with diploid number

l Prophase I • homologous chromosomes come together (bivalent) • pieces of chromatlds are exchanged (crossing over)

.~X~ . ~ :~

bivalent

Met aphase II • chromosomes line up along the equator

Met aphase I • bivalents line up along the equator

CHROMATIDS SEPARATE! Anaphase I • bivalents separate • chromosomes move to opposite sides

CHROMOSOMES SEPARATE!

Telophase II • nuclear membranes form around each set of chromosomes

Telophase I • two cells are formed each with the haploid number

.

•

Figure 24.3 Meiosis of a cell with four chromosomes.

292

Anaphase II • chromatids move away from each other

Four cells formed, each with the haploid number of chromosomes, and are different from each other.

24 · Meiosis

Mitosis

Meiosis

occurs in body cells or somatic cells

either occurs in reproductive cells only or occurs in formation of gametes only

number of chromosomes remains the same in the daughter cells

number of chromosomes is halved in the daughter cells

daughter cells are identical to parent cells and each other

daughter cells are genetically different to parent cell and each other

two daughter cells are formed

four daughter cells are formed

homologous chromosomes do not come together

homologous chromosomes come together

no exchange of genetic material between chromosomes

exchange of genetic material between chromosomes

Table 24. 1 The differences between m1tos1s and me1os1s

Variation of gametes A single human male can prod uce over 100 million spermatozoa or male gametes in one ejaculation. These gametes are all different. This variation of the gametes comes about when the cell divides by meiosis. Variation results from the following processes. • Crossing over between homologous pairs of chromosomes in the early stages of meiosis is random. There are no limits to how this happens. Every homologous pair of ch romosomes exchanges genetic material differently. Imagine the various ways a cell yvith 23 homologous pairs can exchange genetic material. • During metaphase I. the pairs of chromosomes align themselves long the equator of the cell randomly. Imagine the various ways 23 pairs of chromosomes can be aligned along the equator. The pattern of alignment determines which chromosomes are grouped together. • During metaph ase II, the chromosomes (now formed of two chromatids) align randomly along the equator of the cell. This also determines how the chromosomes are grouped in the gamete.

Significance of meiosis

Figure 24.4 These people all belong to the same family and so share some of the same genes.

At the en d of meiosis, four genetica ll y different cells are produced from each original cell. This means that the gametes from each individual are all different. When these fuse with gametes from another individual, there will be even more variation in the genetic information of the offspring (figure 24.4). The gametes carry genetic information from the parents . When they fuse to form an offspring, genetic information is transmitted from the parents to the offspring. The offspring are all differenr from each o ther, since the gametes are all different. They are also different from their parents, though some characteristics will clearly come from the mother and some from the father. Some features may appear that are unlike either parent.

293

Continuity and Variation

Conditions in the environment are not constant. They may change, sometimes abruptly. The survival of a species depends on the ability of the individuals in that species to adapt to changes in the environment. When there is variation among offspring, some will be able to withstand the changes of the environment and survive to reproduce. The surviva l of the species is thus ensured.

~

IT:Q5 \.,..)'....J

The daughter cells of meiosis are all different from each other. List three ways in which this variation is brought about.

Darwin's theory of evolution Darwin's theory of evolution

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Variation obtained from meiosis ensures that the gametes are all different. Give one advantage and one disadvantage of this variation.

>

Darwin's theory of evolution through natural selection is based on the fact that among the variety of offspring produced, some will be better able to withstand changes in living conditions than others. That is, some are better adapted or 'fitter' to survive in the struggle for existence. These offspring will then produce offspring that are similar (not identical ) to themselves, passing on the advantageou s characteristics. Through these gradual changes, over many generations, the evolution of new species is possible.

• During meiosis, homologous chromosomes come together and crossing over occurs, whereby genetic information is exchanged. • Each gamete of the millions produced is unique and so each organism produced by their fusion is unique. • Inheritable genetic characteristics are transmitted from the parents to the offspring by the gametes. • The resulting offspring are all different to or vary from each other and to their parents. • This variation can be important if the environment changed, as those organism better adapted will survive. • This variation can lead to evolution.

294

24 ·Meiosis

ITQ1 (i) In females, meiosis occurs in the ovaries. (ii) In males, meiosis occurs in the testes. ITQ2

Haploid cell

Diploid cell

half the number of chromosomes in the nucleus the full number of chromosomes in the nucleus found only as gametes in the reproductive organs - ovaries and testes

found all over the body

occur as individual cells as gametes, some are able to move (e.g. sperm)

most are fixed and occur together, forming tissues

ITQ3 Meiosis is important for the formation of haploid gametes so that, when two gametes fu se during fertilisation, a diploid zygote with the original number of chromosomes is obtained. ITQ4 (i) The 46 chromosomes of a human cell are made up of 23 homologous, or corresponding, pairs. One chromosome of each homologous pair came from the father and one from the mother. (ii) In the early stages of cell division each chromosome replicates to form two identical copies of itself that are joined by a centromere. Each copy is called a chromatid. ITQ5 • Crossing over - the exchange of genetic information between chromosomes. • Random alignment of the homologous pairs of chromosomes along the equator before separation of the chromosomes. • Random alignment of the chromosomes along the equator before separation of the chromatids. ITQ6 One advantage is that all the offspring have different characteristics, so some may be able to survive a change in an environmental condition. The propagation of the species is more likely to be ensured. One disadvantage is that all the organisms may be different from the parents and not as adapted to the environment as the parents. All the offspring may die easily.

Examination-style questions

2

(i) Explain these terms and state and importance of each: (a) mitosis; (b) meiosis. (ii) List four differences between mitosis and meiosis. (iii) Explain the following terms, giving an example of each: (a) diploid number; (b) chromosome number. (iv) Explain the importance of crossing over which occurs during meiosis. Explain the importance of meiosis in making evolution possible.

295

0

understand the terms gene, allele, dominant, recessive, genotype and phenotype

0 0 0

explain the meaning of the terms codominance, homozygous and heterozygous

0

predict the results of crosses involving one pair of alleles

use a genetic diagram to explain the inheritance of a single pair of characters explain the inheritance of traits using sickle cell anaemia and albinism

/) understand the inheritance of sex in humans

0

understand crosses involving sex-linked characters

variation

continuous

discontinuous

l

)

genes on chromosomes/DNA

I phenotype -

genotype -

alleles

~

dominant recessive

back cross incomplete dominance co-dominance inheritance of characters

blood groups pedigree charts sickle cell anaemia sex determination -

sex-linked characters

The Earth is h om e to billions of organ ism s, eve ry on e of w hich is un iq ue. Millio ns of species can be found on the land, and in the wa te r and air of the Earth 's surface. Different species m ay differ greatly from each other and may be easy to distinguish . For example, birds differ grea tly from fish . However, the membe rs of rhe sa me species ma y differ in only sm all ways . Th ese dilferen ces a re the result of the genotype a nd the enviro nment. The gen otype of organism is its generic m ake-up. Th e envi ron m ent is the

25 · Heredity and Genetics

su rrounding of the organism. Identical twins have the same genetic make-up but their environments are different (such as the food they eat, th eir activities, relationships and experiences) and so subtle differences develop between them (figure 25. 1).

Genes

Figure 25. 7 The differences between identical twins are due to the environment. as they have the same genes

homologous Generic information is passed o n chromosomes from parents to offspring in the / chromosomes. Chromosomes occur in pa irs in body cells. In a human body cell, there are 23 pairs of chromosomes: 23 individual chromosomes are paternal (from the father) and 23 are maternal (from the mother). Pairs are caUed homologous chromosomes (figure 25.2 ). Each chromosome is made up of genes, or units of inherita nce. These control specifi c characteristics in the homologous organism. Each chromosome of a chromosomes homologo us pair carries the same set of genes, therefore each body cell has two Figure 25.2 A diploid cell h:t.. chromosomes has two pairs of homologous copies o f each gene. However a gene that are the same as each other, or two chromosomes. alleles for a gene that are different. If the alleles of a gene are the same, we say the organism is homozygous for that gene o r character. If the alleles are different, the o rgan isms is sa id to be heterozygous for that gene o r character (figure 25.3 ). uu

homozygous

>

heterozygous

>

gene for eye colour gene for hair colour

gene for hair texture

Chromosomes exist in homologous pairs the genes are the same but the form the gene can take may be different. These are called alleles.

gene for shape of nose brown gene for size of lip gene for length of finger gene for length of arm

gene for eye colour gene for hair colour gene for hair texture

A chromosome is made up of genes. This is a very simplified diagram of a chromosome.

In the gene for hair colour, there are many alleles for hair colour, producing many different hair colours. In this case the two alleles are for red and brown. The alleles present determine what the individual will look like: the outward charactenstics. Alleles exist for every feature of every organism and each organism has its own combination of alleles which make it unique.

Figure 25.3 Homologous chromosomes.

297

Continuity and Variation

Dominance If the alleles are different, one may mask the expression of the other. The

dominant allele recessive allele

> one that is expressed (visible in the organism) is called the dominant allele, > and the one that is masked is the recessive a llele . We use capital and lowercase letters to represent the different alleles. For example, in the gene for hair colour, B represent the allele for black hair, and b represents the allele for red hair. Black hair, B is dominant to red hair, b; and red hair, b, is recessive to black hair, B. The dominant allele is expressed in the homozygous (BB ) or heterozygous (Bb) genotype, whereas the recessive allele is expressed only in the homozygous (bb) genotype (figure 25 .4). homologous chromosomes

'"" foe Mic "'"""

l

allele for black hair symbol B (dominant)

genotype is the genetic make-up, the alleles

allele for red hair symbol b (recessive)

B and b

phenotype is the outward characteristic - - - black hair homozygous genotype has same alleles, e.g. BB

bb heterozygous g enotype has different alleles, e.g. Bb

Figure 25.4 The allele for black hair, B, will mask the expression of the allele for red hair, b. The heterozygous individual will have black hair phenotype )

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The composition of genes, or genetic make-up, within the cells of an organism is its genotype. The phenotyp e is the observable characteristics of the organism. These observable characteristics are the result of the genotype and the environment interacting (table 25.1). genotype

phenotype

BB (homozygous)

black hair

Bb (heterozygous)

black hair: B is dominant to b

bb (homozygous)

red hair

Table 25.1

Phenotype 1s determined by the genotype.

11'.Q-1 \../'-'

Define the following: (i) chromosome (ii) gene (iii) allele.

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l'.fQ2 \../'-' Define the following: (i) genotype (ii) phenotype.

298

Genetic diagrams A genetic diagram shows the cross between two genotypes. It shows the phenotypes and genotypes of the parents and the possible genotypes and phenotypes of the offspring. For example, a cross between a homozygous dominant genotype (BB) and homozygous recessive genotype (bb) is shown in figure 22.5.

25 · Heredity and Genetics

phenotype of parents

black hair

x

red hair

genotype

BB

x

bb

I

I

0

gametes

offspring genotype

~ Bb /

offspring phenotype

black hair

segregation

8

All offspring heterozygous with black hair. Probability of a red-haired offspring is 0% Ratio 4 black hair : 0 red hair

Figure 25.5 A genetic cross of a homozygous black-haired parent and a red-haired parent.

Note in the diagrams that the offspring from a single cross are ca lled the F 1 (which means first filial) generation. We then use the term 'F2 generation' to refer to offspring of a cross between individuals of the F 1 generation. If both parents are heterozygous, the cross is as shown in figure 25.6. phenotype of parents

black hair

genotype

Bb

x

black hair

Bb

gametes segregation offspring genotype offspring phenotype

BB

Bb

Bb

bb

black black red black hair hair hair hair (homozygous) (heterozygous) (heterozygous) (homozygous) Ratio of 1 red hair : 3 black hair

~

25% probability of an offspring having red hair

IT:Q3 lJ'-1

Albinism (absence of pigmentation) in humans is caused by a recessive gene which is transmitted in a normal fashion. A phenotypically normal (nonalbino) couple have four children: the first three are normal and the fourth is albino. (i) What can you say about the genotype of the parents? (ii) What is the possibility that their next child will be albino? (iii) One of the normal children eventually marries a normal woman. What predictions can be made of their first child? (iv) The albino child eventually marries a normal woman. What predictions can be made of their first child? Where there are several possibilities, state them all.

Figure 25.6 A genetic cross showing how two black-haired parents can have a red-haired child.

A cross can also be represented in another way. A cross between a heterozygous parent and a homozygous recessive parent can be drawn as in figure 25.7. x

phenotype of parents

black hair

genotype

Bb

bb

~

~

gametes

0

8

red hair

8

8

gametes

Bb Bb ® B black hair black hair ---- ----··- -------·- ·-- ---- bb

red hair

...

bb red hair

Ratio 1 black hair : 1 red hair 1, 2 offspring are heterozygous with black hair 1t2 offsprlng are homozygous with red hair Chances of an offspring having red hair is 50%

Figure 25.7 A genetic cross using a table.

299

Continuity and Variation

Test cross or back cross Homozygous dominant and heterozygous individuals have the same phenotype - you cannot teU which is which just by looking at them . A test cross (back cross) is used to determine the genotype of individuals which have the same phenotype. In a test cross, the individual is crossed with a homozygous recessive and the offspring examined, as shown in figure 25 .8.

~

l:F:Q'I \./'-I A breed of dogs has long hair dominant over short hair. A long-haired bitch was first mated with a short-haired dog and produced three long-haired and three short-haired puppies. Her second mating, with a long-haired dog, produced a litter with all the puppies long-haired. Use the symbol L to represent the allele for long hair and I to represent the allele for short hair. (i) What was the genotype of the long-haired bitch? (ii) How could it be determined which of the long-haired puppies of the second mating were homozygous?

phenotype of Individual

----+

black hair

possible genotypes - - - - - BB

and

Bb

Individual is crossed with bb and offspring examined.

gametes

0

®

·0 --

0

Bb

®

Bb

Bb

0

gametes

'0

Bb

-

I

0

Bb

Bb

bb

bb

1-

If there are red-haired individuals in the offspring, then the genotype is Bb.

If all the offspring are black-haired, then the genotype is BB.

Rgure 25.B A test cross.

Incomplete dominance incomplete dominance

>

QSb

l:F:QS \./'-I What offspring will you expect, and in what proportions, if two pink-flowered plants are crossed?

Flower colour in some plants, such as Impatiens, shows incomplete dominance of the alleles. This means that there is a blending or combination of expression of both alleles in the h eterozygous condition. If allele CR produces red flowers {genotype CRR) and the allele cw produces white flowers (genotype cww), then the genotype CRW produces pink flowers (figure 25.9). A blending of red and white will produce pink.

99:,

l:F:Q6 \./'-I The figure shows the result from a cross between a red-flowered plant and a white-flowered plant, and what happens when the offspring produced are crossed with red-flowered plants.

• A

x B

x

• •

' D

300

white

red

c RR

x

cWW

I

offspring genotype

0 a11 c Rw

offspring phenotype

F

Using the alleles CR and CW, give the genotypes of the plants labelled A-F. (ii) If plant D had been white, what would the result of the cross between C and D have been?

(i)

genotype

gametes

l

E

phenotype of parents

Figure 25.9 Incomplete dominance as seen in Impatiens.

all pink

25 · Heredity and Genetics

Co-dominance co-dominance

>

In co-dominance, there is expression of both alleles in the heterozygous

genotype. In this case, the gen otype cRw produces fl owers that have patches of red and white colour. There is n o blending, each allele is expressed as shown in figure 25. 10.

phenotype of parents

red

white

cRR

genotype

c ww

x

I

I

0

gametes

0 au c Rw

offspring genotype

offspring phenotype

all red and white

Figure 25.1O Co-dominance in Impatiens.

Genotype

Phenotype

Another example of co -dominance is found in ABO blood groups in humans. Your blood group is controlled by three different alleles: JA, 18 and 1°. IA and 18 are equally or co-dominant to each other and both are dominant to 1°. Only two alleles can be present in any cell, one on each homologous chromosom e that carries the gen e for blood group. This gives four possible phenotypes for blood group (table 25.2).

IAA

blood group A

!AO

blood group A

!BB

blood group B

IBO

blood group B

IAB

blood group AB

Worked example

100

blood group 0

1

Table 25.2 Genotypes and phenotypes of blood group in humans.

~

What are the possible blood groups of children whose parents are blood group A (h eterozygou s) and B (h omozygous)? The h eterozygo us gen otype fo r blood group A is JA0 . The h om ozygou s gen otype for blood group B is 18 8 •

l:tQ7

genotype of parents

What offspring will you expect, and in what proportion, if two Ft generation plants from figure 25.10 were crossed?

gametes

V"-1

~ l:tQS V"-1

A baby has blood type B, his mother had blood type A. His paternal grandfather has blood type A and his paternal grandmother has blood type B. Determine {i) the genotype of the baby, and {ii) the possible genotypes of the baby's father.

offspring genotypes

! AO

~~ 1AB

1BB

x

1AB

100

JBO

Possible blood groups of children are AB and B.

301

Continuity and Variation

Worked example 2

What if both parents had heterozygous genotypes? The heterozygous genotype for blood group A is JA 0 . The heterozygous genotype for blood group B is 18°. X

genotype of parents

JAO

gametes gametes

®

@

®

JAB

1BO

@

1AO

·--

100

Possible blood groups of children are A, B, AB and 0.

Examples of genetic effects Sex determination Practical activity SBA 25.1: How the sex of an offspring is

determined, page 365

Of the 23 pairs of chromosomes in any human cell, one pair determines the sex of the organism. There are two sex chromosomes, X and Y. The genotype XX is female and the genotype XY is male in humans (figure 25.11).

® ®

human cell

I

All the cells of a female have two X-shaped chromosomes nucleus has 23 pairs of homologous chromosomes

one pair determines the sex of the individual

Figure 25. 11

All the cells of a male have one X-shaped chromosome and one Y-shaped chromosome. The Y chromosome is an X chromosome with a missing piece.

How sex is determined in humans.

Figure 25.12 sh ows the inheritance of sex in humans. Each time a couple has a child, there is a 50% possibility it could be a boy and a 50% possibility it could be a girl. x

male

phenotype of parents

female

genotype

gametes

offspring genotype offspring phenotype

female

female

male

Ratio 1 male : 1 female

Figure 25. 12 How sex is inherited in humans.

302

male

25 · Heredity and Genetics

Sex-linked characteristics The sex chromosomes also carry genes other than those which determine sex. The characteristic of those genes are said to be sex-linked, and they are carried on the X chromosome.

Haemophilia or bleeder's disease sex-linked characteristics haemophilia >

Sex-linked characteristics include haemophilia and colour-blindness. Table 25.3 shows sex-linkage in haemophilia. Genotype

Phenotype

XHXH

female, normal clotting of blood female, normal clotting of blood; she is a carrier since she carries the recessive allele but it is not expressed.

XhXh

female, a haemophiliac

XHY

male, normal clotting of blood

Table 25.3 The genotypes and phenotypes in haemophilia.

The dominant allele, H , causes blood to clot normally. The recessive allele, h , causes haemophilia, a condition in which blood does not clot and any small cut will bleed for a long time. The inheritance of haemophilia is shown in figure 25.13.

~

IT:Q9 V'-J

(i)

What is mean by the term 'sex linkage'? (ii) A normal man marred a normal woman and all the female offspring were normal, but half of the male offspring were colourblind and the other half were normal. How do you account for this?

Worked example 3

A carrier female marred a normal male. What is the possibility of their having a haemophiliac child? carrier female

normal female

normal male

x

normal male

normal female (carrier)

haemophiliac male

Figure 25. 13 Haemophilia inheritance

The mother transfers the haemophiliac gene to her son. There is a 25 % possibility of having a haemophiliac son .

Other genetic disorders Sickle cell anaemia sickle cell anaemia > CHAPTER 26

In sickle cell anaemia (chapter 26), the red blood cell can take a sickle shape

instead of the normal biconcave shape. Allele HbNproduces normal red blood cells and the allele Hb5 produces sickle-shaped red blood cells; the possible genotypes and phenotypes are shown in table 25.4 (overleaf). The inheritance of sickle cell anaemia is shown in figure 25 .14 (overleaf).

303

Continuity and Variation

Genotype

Phenotype

HbNN

all red blood cells are normal, the person is normal

Hb55

all red blood cells take the sickle shape, the person has sickle cell anaemia

HbNs

30-40% of the red blood cells are sickle shaped, the person has sickle cell trait

Table 25.4 Genotypes and phenotypes in sickle cell anaemia.

Worked example 4

If two people with the sickle cell trait were to marry, what are the possible genotypes and phenotypes of their offspring? parental phenotypes

x

trait

trait

parental genotypes

gametes

offspring genotype offspring photype

normal

trait

trait

sickle cell anaemia

Figure 25.14 Sickle cell anaemia inheritance.

The possibility of having a child who suffers sickle cell anaemia is 25%. The possibility of having a normal child is 25%. Ratio is 1 normal : 2 rraH : I anaemia .

Pedigree charts A pedigree chart shows the occurrence of a particular characteristic in a family tree (figure 2 5. l 5). The chart can be used to show the possible genotypes of individuals in the chart, which can be important in genetic counselling.

304

25 · Heredity and Genetics

2

•

female with black hair

II

male with black hair

female with red hair

3

4

5

6

7

8

9

10

12

13

14

15

16

17

18

II

male with red hair

II 11

The allele for black hair B is dominant to the allele for red hair b. What are the genotypes of all the individuals? The males and females with red hair would have to be bb.

I

( bb 3

B? 4

B?

6 bb

1

(

~b r ~?

bb 3

(

I bb

l=

I

1

I

I

To have a bb offspring, individual 2 would have to be Bb. All their black-haired children are Bb.

B? 9

bb 7

Bb 2

Bb 6

I I

µ-;

:·r~

8

bb

bb Bb

To have a bb offspring, individual 5 would have to be Bb.

bb

Figure 25. 15 A pedigree chart showing the inheritance of hair colour in members of a family.

~

l~Q·10

L-"-1

The family tree below shows how coat colour in mice is passed on from generation to generation. both parents are homozygous for coat colour p

F,

5

11

e

D Q

brown coat colour white coat colour

305

Continuity and Variation

(i) (ii) (iii) (iv) (v) (vi)

Explain the term 'homozygous'. What do the symbols P, F1 and F2 stand for? Which generation of mice is heterozygous for coat colour? What is the percentage of brown and white mice in the F2 generation? Which allele for coat colour is recessive? What might be the percentage of white coat mice if breeding pairs were set up between: (a) 1 and 4 (b) 1 and 5?

, Chapter summary • • • • •

II

• • • • • • • • • • ~

Each chromosome is made up of genes, or units of inheritance. An allele is the form of gene taken. The genetic make-up of an organism is its genotype. The observable characteristics of an organism make up its phenotype. If the alleles of a gene are the same on the homologous chromosomes, the genotype is described as being homozygous. If the alleles of the gene are different, the genotype is described as being heterozygous. In a heterozygous individual, the allele which is expressed in the phenotype is described as being dominant. Recessive alleles are only expressed in the phenotype if present in the homozygous form. Their effect is masked by the presence of a dominant allele. A test cross is used to determine the genotype of an individual. A genetic diagram shows the cross between two organisms for a characteristic. Incomplete dominance occurs when no one allele is completely dominant over the other. As a result, the expressions of the alleles blend. Equally dominant alleles are described as being codominant. Both alleles are expressed in the phenotype. The sex of an organism is determined by the sex chromosomes. In humans XX codes for female and XY codes for male. Characteristics carried on the X chromosome are said to be sex-linked. Examples are haemophilia and colour-blindness in humans. A pedigree chart shows the occurrence of particular characteristics in a family tree.

~ ~

ITQ1 (i) Ch romosome - In humans there are 46 chrom osomes in sid e the nucleus of each cell . Each chromosom e is a separa te stru cture which came from a parent and is made up of a stand of DNA or deoxyribonucleic acid. Each chromosome has its homologous partne r which came from the other parent . (ii) A gen e is a sm all pa rt of a chromosome . It has the code to make a specific protein which may lead to a specific ph ysical characteristic. (iii) An allele is the form a gene can tak e. It is the actual code of the gene. ITQ2 (i) The genotype of an organism is th e total com bina tion of all th e alleles that make up tha t organism . (ii) Th e phenotype describes the specific physical characteristics that can be seen and result from the genotype and the effect of the environment. ITQ3 Using N for the dominant allele of normal skin colouring, and n for the recessive allele of albino colouring. Parents a re n ormal; children are 3 normal : l albino.

306

25 · Heredity and Genetics

(i) The albino child's genotype is nn. The parents' genotypes can only be Nn and Nn (heterozygous) since they are both normal and the albino child must get a n allele from each parent.

N

n

N

NN

N

Nn

Nn nn

(ii) The probability of the next child, or any child of these parents, being albino is 1 in 4 or 25%. Whenever this couple have a child, the probability of having an albino child will be 25 % . It would be quite possible for them to have 4 albino children and no normal children. (iii) The no rmal child can have either of two genotypes, Nn or NN. The normal woman that he marries could also have either of the same two genotypes. There are therefore three possible crosses: • NN x NN offspring all normal • NN x Nn offspring all normal • Nn x Nn offspring 3 norma l : 1 albino First child has a 75% to 100% of being normal. (iv) The albino child (nn) marries a normal woman (Nn or NN). Two possible crosses are: • nn x NN offspring are all normal • nn x Nn offspring 1 normal : 1 albino Their first child has a 50% to 100% chance of being normal. ITQ4 (i) To produce short-haired puppies, the long-haired bitch must have the genotype LI. The short-haired puppies, being 11. got one of the I alleles from their mother. (ii) The long-haired puppies from the second mating can have two possible genotypes, LI (heterozygous) or U (homozygous). To determine which is homozygous, the breeder must do a test cross, that is cross each puppy (when mature) with a short-haired dog and examine the offspring of each mating. If there is at least one sh ort-haired puppy in the litter, then the genotype is LI. If all the o ffspring are long-haired, then the genotype is LL. ITQS Crossing two pink- fl owered plants (CRw) would give the following result: offspring 1 CRR (red ) : 2 cRw (pink) : 1 cww (white)

CR

ITQ6 (i)

cw

A: CRR, B: cww, C: cRw, D: CRR, E: cRw, F: CRR

(ii) Genotypes of offspring: CRW and CWW. Phenotypes of offspring: pink and white. ITQ7 The F 1 generation have the genotype cRw. If two of these are crossed, the offspring woul d be produced as follows:

CR

cw

This would give a proportion of phenotypes of I red (CRR) : 2 red and white (CRw) : 1 white (cww).

307

Continuity and Variation

ITQ8

~

grandparents

~

\/ m":"

\/

/

~

parents

d" A

B

rath"

babyB

Mother has blood group A, possible genotypes are JAA or JA 0 . The baby has blood group B, possible genotypes are 18 8 or 180 . The baby must get one allele from the mother, but has no JA allele. So the mother's genotype must be JA0 and the baby's genotype must be 18°. (ii) The father must have given the baby the other allele, 18. The father's mother had blood group B and so had genotype 188 or 180, and the father's father had blood group A and so had genotype JAA or JA0 . The father got the 18 allele from his mother and could have inherited either the JA or the 1° allele from his father. The possible genotypes for the baby's father are JAB or 180 . ITQ9 (i) Characteristics that are carried on the X sex chromosome are described as being sex-linked. (i)

normal man

x

normal female

xNy parents

gametes

/

y

offspring

X"Y normal female

normal female

female offspring all normal

normal male

colour-blind . male

male offspring about half normal and the other half colour-blind

The gene for colour-blindness is recessive and sex-linked. The normal female parent is heterozygous XNX" and passes the recessive allele to some of her sons. ITQ10 (i) Homozygous describes the genotype with two similar alleles, such as BB or bb. (ii) P stands for parents (genotypes and phenotypes). F 1 is the first (filial) generation, and F2 is the second (filial) generation. (iii) The F 1 generation. (iv) 75 % are brown and 25% are white. (v) The allele for white coat colour is recessive. (vi) (a) Probability of cross Bb x Bb producing white mice is 25%. (b) Probability of cross Bb x bb producing white mice is 50%. (ii)

308

25 · Heredity and Genetics

Examination-style questions (i)

(ii) (iii) (iv)

(v)

2

3

(vi) (i) (ii) (iii) (i) (ii)

(iv) (v) 4

(i)

(ii)

Define the following: (a) homozygous; (b) heterozygous; (c) dominant; (d) recessive; (e) allele. Distinguish between genotype and phenotype. Explain how mutation contributes to variation. Cystic fibrosis is an inherited condition caused by a single recessive allele. The normal gene is dominant and masks the recessive allele. What is the probability of two healthy people being parents to a child born with cystic fibrosis? Show all the working and use a genetic diagram. (a) Explain what is meant when a person is said to be a 'carrier' for cystic fibrosis. (b) What is the probability of two healthy people (where one is a 'carrier') being parents to a child born with cystic fibrosis? Why are most lethal genes (genes which cause mortality) recessive? Distinguish between continuous and discontinuous variation. List all the possible genotypes of a person belonging to blood group B. A woman with blood group A has a child who is also of blood group A. What are all the possible genotypes of the father? Explain fully, using genetic diagrams. Explain, using genetic diagrams, how sex is determined in humans. Relating to the probability of having a boy or girl in part (i), suggest why: (a) at birth there are about 106 boy babies for every 100 girl babies; (b) at puberty the proportions of males and females are about equal; (c) In old age, females outnumber males. Define sex linkage and describe how haemophilia is inherited. What is the probability of a haemophilic father and a mother carrying the allele for haemophilia having a haemophilic daughter? In a test cross, the genotype of an organism showing the dominant characteristic (which can be homozygous or heterozygous) is determined. Using the symbols T for tall plant and t for short plant, show the results of a test cross on a tall plant which turned out to be homozygous for height. Draw a genetic diagram to illustrate your answer. Construct a pedigree chart of the following information.

Female

Male

parents

brown hair

red hair

children

one - red hair

two - both brown hair

grandchildren (from (:laughter who married brown-haired man)

one - red hair

one - brown hair

(iii) Explain the following using an example for each: (a) co-dominance; (b) incomplete dominance.

309

0 0 0 0 0

distinguish between genetic and environmental variation understand why genetic variation is important distinguish between continuous and discontinuous variation define a species describe how new spec ies are formed

0

understand the process of natural selection in evolution

0 0 0 0

distinguish between natural and artificial selection understand the causes and effects of mutation understand what is meant by genetic engineering and how it can be used discuss the advantages and disadvantages of genetic engineeering

discontinuous variation } genotype

environment

l.._____......_____J

continuous variation ( phenotype

(

variation in phenotype I

artificial selection

(

selective breeding

mutation

'

natural selection

'

genetic engineering

( sickle cell anaemia

'

Down's syndrome

selection pressure

evolution

species

Genetic variation Ead1 organism is unique. This uniqueness is a result of genetic differences and influences of the environment, and is expressed in the phen otype. Each organism is born with its own genetic make-up inhe rited from its parents. The genetic make-up of every organism is different except for clones. Genetic variation is inherited and the differences may be small or large (fi gure 26. l).

~

IT:Q·1 l....l'V What is genetic variation?

26 · Variation and Evolution

Figure 26 1 Variation is seen among and between species.

Variation due to the environment environmental variation

>

The environment also plays a very important role in determining the phenotype of an organism. The variation seen because of the influence of tbe enviromnent is not inherited but occurs because of differences in the surroundings of the organism. For example, ten genetically identical plants grown from cuttings taken from a single parent should be identical in appearance since the genes for all the characteristics are identical. Suppose these plants are divided into two groups of five, and each group is grown in rwo very different environments: • good soil, watered regularly; • poor soil, not watered, After a while, the phenotypes of the two groups will vary greatly. Variation in appearance will be seen be tween plants th at have the same genetic make-up and therefore should look the same (figure 26.2).

cutting grown in rich soil, watered regularly

~

genetically identical plants in different environments (soil and water)

~

IT:Q2 \.../'-'

.

cutting from the same plant grown In poor soil, watered little

1

(i)

What is the phenotype of an organism? (ii) How is the phenotype determined?

~

IT:Q3 \.../'-'

List three differences which may be seen in the phenotypes of identical twins even though they have identical genotypes.

Figure 26.2 The environment plays a very important role in determining the phenotype of an organism Genetically identical plans are very different if grown in different environments.

Genetically identical twins, as they grow and develop, acqu ire subtle differences. These differences occur becau se their e nvironments are different, even if they live in the same house . They eat different foods at different times and in different amounts. Their da ily activities and inte rests are different, and their intera ctions with people, even their parents, are different. The d ifferences in their 'en vironments' may be subtle, but enough to produce differences in their physical appearance. Identical twins also have different fingerprints.

311

Continuity and Variation

Importance of genetic variation Genetic variation among a species ensures survival of that species if the environment changed drastically. This can be seen in the following example. A population of wolves living the wild vary with respect to body hair length. A few have very long hair (5-6 cm) and a few have very short body hair (1-2 cm) . Most have medium hair length (3-4 cm), which is well suited to the temperature of the environment (figure 26. 3).

Practical activity SBA 26.1 : Continuous variation, page 366

_ _____.,__ most individuals in the population of wolves will have body hair length 3-4 cm

Number of individuals in the population of wolves

I

I

2

3

4

5

6

7

8

Body hair length (cm)

Figure 26.3 Graph showing how boay hair length varies in a population of wolves.

~

Suppose the temperature changed drastically, say it got much colder, then the wolves with short hair would be more likely to die, but those with long hair would be more likely to live. Because of the variation which existed naturally, the wolves with very long body hair would be able to survive the cold temperatures better than those with short body hair. Those with long body hair, therefore, would be more likely to reproduce. Surviva l of the species is thus ensured because of natural variation. Figure 26.4 shows what would happen to wolf body hair length in such circumstances.

IT:Q't 1.../'--1

What do you think would happen to the population of wolves shown in figure 26.3 if the environmental temperature got warmer?

-+-- --

Number of individuals in the population of wolves

Figure 26.4 Graph showing a change in the occurrence of body hair length as the environment changed.

continuous variation discontinuous variation

> >

2

3

4

5

6

most individuals now have body hair length 5-6 cm to survive the colder environment

7

8

Body hair length (cm)

Variation is thus the result of the genetic make-up and the influence of the environment. There are two types of variation: • continuous variation; • discontinuou s variation.

are slight and merge or grade into each other to produce a smooth bell-shaped curve (Figure 26.5): for example, height in humans, human foot length, human skin colour, leaf size and pod size in legumes, and body hair length in wolves. In continu ous variation, the differences

Number of 15-yr olds in class

Figure 26.5 The height of a class of 15 year-olds shows contiuous variation.

312

-

145

150

most individuals in the population -\--- - are between 160 and 170 cm

155 160 165 170 175 Height of individuals In cm

180

185

26 · Variation and Evolution

In discontinuous variation, the differences are separate and clear cut; they do not merge or grade into each other (figure 26.6). Examples are tongue-rolling in humans, blood groups in humans and horns in cattle.

Number of individuals

Figure 26.6 Graph showing discontinuous variation in human blood groups.

t

A

B

AB

0

Blood groups

DNA testing and forensic science

Agure 26. 7 Every individual has a unique DNA pattern.

DNA testing or 'genetic fingerprinting' is a technique pioneered by Dr. Alec Jeffreys. He found short DNA sequences from the non -coding part of the DNA that were repeated several times and were uruque to each individual. Dr. Jeffreys developed a genetic probe to look for these sequences and was able to use electrophoresis and autoradiography to produce a DNA image (figure 26. 7). Each dark band in the autoradiograph shows as area w here the DNA probe attached to a similar sequence in the subject's genome. Each person's genetic fingerprint is unique - this means that each individual can be identified by their DNA, perhaps from a strand of hair or a scrape of skin. This application can be used in pa ternity and maternity tests and in as forensic evidence in rape and murder trials.

Natural selection natural selection

>

Practical activity SBA 26.2: Natural selection. page 367

Charles Darwin, an English naturalist, first spoke about natural selection. He observed organisms that lived on the Ga lapagos Islands in the Pacific Ocean. From his observations, Darwin concluded that within a population, although many offspring are produced, m any individuals do not survive becuase they: • compete for limited food and resources; • try to avoid predators; • struggle to avoid disease; • try to tolerate changes in the environment.

There is a constant struggle for existence, and those individuals that are best adapted to their environment have an advantage. That is, th ey are more likely to survive and produce offspring. Their offspring will inherit the advantageous characteristics and the population will remain well adapted to its habitat. Selection by the environment is known as natural selection. It favours those that have the best adaptations for the.environment in w hich they live (figure 26 .8). These organisms Nature 'selected ' birds with strong beaks. Birds with pointed beaks were able are said to have a selective advantage. They were able to crack the hard shell to feed on flying insects. Nature also Sometimes we describe these individuals of nuts and feed on the nuts. 'selected' these birds because of as being the fittest for the environment. They thus obtained food and lived to the shape of their beaks. reproduce; produce offspring with strong b eaks. Because of this, natural selection has become known as 'survival of the fittest'. Figure 26.B Some individuals are better adapted to the environment. selective advantage

>

313

Continuity and Variation

Natural selection and evolution Natural section provides the mechanism for one species co change into another. The change is very slow and is called evolution as one species evolves into another.

Long necks of giraffes

Figure 26.9 Long necks are an advantage when feeding from tall trees. antibiotic resistance

>

insecticide resistance

>

~

IT:Q5 V'-J

How do bacteria evolve to acquire resistance to antibiotics?

The long n eck of the giraffe is thoug ht to have evolved when food was in short supply and only rhe tallest inc:Uviduals co uld reach en o ugh food to survive. The 'tallness' genes were passed on ro the n ext generation so they were, on average, tal ler than their parent genera tion . As selection for lon g necks continued, the gira ffes which produced most offspring were the ta llest inc:Uviduals. After many gen erations of selection, the long-necked species of giraffe have evolved (figure 26.9).

Antibiotic resistance in bacteria Antibiotic resistance of bacteria is a serious problem . When antibiotics are used o n a p opulation of bacteria, any bacteria with genes to resist the drug will s urvive and most of the rest will be killed. The resistant bacteria then multiply, prod ucing populations of antibiotic-resistant bacte ria. Because of the w idespread use of antibiotics like penicillin, many kinds of bacteria a re n ow resistant to these drugs. Insects that are resistant to insecticides have evolved in much the same way as antibiotic-resista nt bacteria, due to the w idespread use of insecticides (fig ure 26.10). For exa mple, many populations of mosq uitoes are now resistant to DDT which was widely used last centu ry to attempt to co ntrol them and the spread o f ma laria.

R

original gene pool

A

selection +-pressure

All the genes present in this population of mosquitoes make up the gene pool. The resistance gene R is also present. There are some individuals with resistance to insecticide.

The application of insecticide to the population of mosquitoes puts a pressure on the gene pool (selection pressure). Those individuals with the resistance gene are seen to be fitter, i.e. able to survive. Most of the others die. (Some may survive because they avoided the insecticide.) The mosquitoes with the resistance gene live to reproduce, passing the resistance gene to their offspring.

gene pool after the use of insecticide

Most individuals are now resistant to the insecticide. The population is said to have developed resistance to the insecticide. A stronger dose or a new insecticide must now be used.

Figure 26. 10 New populations evolve under pressure

Dark form of the peppered moth

camouflage

314

>

The peppered moth (Biston betularia) is found in many parts of England (figure 26. 11). Ir was origina lly found as a pale fo rm, well con cea led by camouflage on the lichen-covered trees on which they rested. Predators fo und the moth difficult to spot. Then in the ea rl y 19th to mid -20th century, pollution in indu strial a reas, blackened the trunks of the trees witb soot. The

26 · Variation and Evolution

pale m oths were n ow easily seen by predators, but the rare black form was better camouflaged. So the frequency (numbers) of the dark form increased as they were better suited to the environment. ln the unpolluted areas, the pale form still predominated. Today, as pollution from industry gets less, the pale form is again becoming more common than the dark form. (a)

(b)

dark form p redominates

light form predominates selection pressure Ondustrialisation)

Rgure 26. 11

(a) Pale and dark forms of Biston betularia moth. (b) Populations change with a change in the environment.

Geographical isolation and speciation

~

IT:Q6 V'-J

Describe briefly the following terms: (i) natural selection (ii) selective advantage (iii) survival of the fittest (iv) evolution.

A popu lation that is geographically isolated from another may experience different environmental conditions and so evolve differently due Lo natural selection. Over time, the isolated population would become more and more different from the origina l popu lation to fill a new and different ecological niche. During his visit to the Galapagos Islands, Darwin observed several different species of finch (now called Darwin's finch es). This group of islands is in the Pacific Ocean, about 600 miles from the South American mainland. Darwin concluded that the islands must have been colonised by a few individuals from a species of finch found on the mainland. These individuals then evolved independently to fill the different ecological niches on each island. Today 14 different species o f finch are found on the Galapagos Islands, differing greatly in size and other features, including beak size and shape (figure 26. 12).

warbler finch beak long and thin. feeds on insects

vegetarian tree finch feeds on buds, leaves and fruit

Galapagos Islands

Rgure 26. 12 Four of the 14 species of finches that Darwin observed on the Galapagos Islands.

probable common ancestor. likely to have been a seed-and insect-feeder

woodpecker finch often holds a small twig and uses it asa tool

cactus finch beak long and slightly curved

Mainland

315

Continuity and Variation

The islan ds of the Caribbean are a group of volcanic islands in the Atlantic Ocean. This arc of islands sweeps north and west from t he South American mainland and is ca lled the West Indies. It h as been suggested that the Anolis lizards of the West Indies have evolved in much the same way as Darwin's find1es. A few individuals may have drifted (for example on a log) along a water current from South America and reached the banks of Grenada (th e most likely arrival point for a rafting colonist). The original species may still exist in Guyana, Venezuela or north-western Brazil. The colonisa tion of other close islands followed, such as the islands of the southe rn Lesser Antilles, including St Vincent, St Lucia, Martiniqu e, Barbados, and La Blanquilla and Bonaire far to the west of Lhe main chain. Each population would have been subjected to dillerent environmental conditions. The vegetation, in sect population, air te mperature and weather patterns all diifer from island to island. By natural selection, each p opulation would have evolved inde pendently adapting to each new ecological niche (figure 26.13). Over time, the popu la tions would have become different from each other, evolving into nine different species (table 26 .1 and figure 26.14). Locality

Species

Martinique

A. roquet

Barbados

A. extremis

Grenada

A. aeneus A. richardii

St Vincent

A. trinitatus A. griseus

St Lucia

A. luciae

La Blanquilla

A. blanqillenus

Bonaire

A. bonairensis

Martinique

St Lucia

(,

St Vincent

Barbados

La Blanquilla

Bonaore

Naturally occurring current which brought the Anolos lizard to Grenada

.

Table 26. 1 Species of Ano/is lizards in the Lesser Antilles.

South

American

Ma i nland

G U YAN A

Rgure 26. 13 Possible colonisation sequence of the Ano/is lizards of the southern part of the Lesser Antilles. (a)

(b)

(C)

Figure 26 14 Ano/is lizards of the Lesser Antilles. (a) Ano/is roquet, (b) Ano/is trmitatis, (c) Ano/is mtens tandae- a rare species found in Peru. It is also suggested that two independent landings on St. Vincent separated

by sufficient time could have resu lted in a second colonisation. Two reproductively isolated species are found there, the giant Ano/is griseus and the smaller A. trinitatus.

316

26 · Variation and Evolution

Ecological speciation and behavioural speciation

predators present

Ecological speciation is the evolution of barriers to gene flow resu lting from ecologically based rnvergent selection. For example, there are two populations of the Bahamas mosquitofish (Gambusia hubbsi), one has larger and more powerful caudal (tail) region than the other. The fish with the larger tail regions are in environments where there are predatory fish that will feed on the mosquitofish. The mosquito fish with the smaller tail regions live in areas without predators. Modern research suggests that the more powerful tail regions are more powerful swinuners than the mosquito fish with the sma ller tail regions. The bigger fish can therefore escape from their predators more easily. Speciation is resulting because each fish chooses the same type to mate with (figure 26.15). Behavioral speciation is seen when species engage in distinct courtship and mating rituals. For example, the birds called blackcaps from Germany. These birds generally fl y to Spain or north Africa to overwinter but some have adapted to 'backyard bird feeding' areas in the UK where there is a ready supply of food waiting for them. The change in migration behaviour has led to a shift in mate availability and populations are becoming increasingly reproductively isolated as they choose birds from the sa me population to mate with (figure 26.16).

no predator

~on

I ''

' '''

' fish here do not interbreed with fish from region A

some interbreeding may occur between fish from region A and fish from region B

Figure 26. 15 Ecological spec1at1on

backyard feeding in the UK means food Is readily available for birds in winter

Artificial selection

Rgure 26.16 Behavioural spec1ation.

artificial selection

>

Artificial selection is the process by which plants and anima ls used by humans in agriculture, horticulture, transport, companionship and leisure have been obtained from wild organisms. In natural selection, nature selects the fittest individuals but, in artificial selection, humans select individuals with characteristics they see as useful. Only those individuals selected by humans are allowed to produce offspring.

317

Continuity and Variation

selective breeding

>

Due to the constant removal of those with unwanted features and breeding of only chosen individuals, the genetic composition of the population changes. This is called selective breeding and continues today by a combination of inbreeding (between closely related individuals) and outbreeding (between geneticall y distinct individuals). The aim of artificial selection is to produce animals and plants with characteristics humans find desirable. These include: • high yield; • improved quality; • reduced production costs; • faster growth rates; • greater resistance to disease . Drugs like growth hormones and steroids are sometimes used LO enhance or quicken growth and development in animals that are used for food, such as poultry. These can have negative effects on humans and increase the risk of populations of antibiotic-resistant bacteria evolving. A comparison of natural and artificial selection is made in figure 26. 17.

NATURAL SELECTION

ARTIFICIAL SELECTION

gene pool of a population selection pressure

selection pressure

The environment may change, e.g. get hotter, colder, drier, etc. Those that can survive in the 'new' environment live to reproduce, passing on those genes.

Humans allow some individuals to live to reproduce, passing on those genes advantageous. e.g. greater yield, speed (horses), size, etc.

New population

New population

• • • •

• may not be able to survive in the wild • Is advantageous to humans • may have lost other advantageous genes

able to survive in the wild is a natural part of the environment may not be advantageous to humans retains most genes

Rgure 26. 17 Similarities and differences between natural and artificial selection.

Examples of artificial selection in the Caribbean Very productive (meat and milk) breeds of cattle have been developed in temperate countries, such as the UK and the US. Beef cattle, like Hereford and Angus, and dairy cattle, like Friesian and Jersey, do not thrive well in tropical conditions because: • they suffer from heat stress; • tropical grasses are generally less nutritious than temperate species; • diseases like tick fever, foot rot and mastitis are serious problems that these breeds suffer from.

318

26 · Variation and Evolution

Thus, cattle fa rmers in the Caribbean have developed new breeds. • In Jamaica, cross-breeding Indian and European breeds with local Creole cattle has led to beef and dairy herds such as the Jamaica Red Poll and the Jamaica Hope. These can cope with heat stress and poor pasture, and are disease- resistant, while producing much more milk and mea t than traditional Caribbean breeds. • In Trinidad, a new breed, the Buffalypso, has been selectively bred from the water buffalo brought from India in 1903 to pull cars and help in ploughing. The matu re animal produces high grade meat, which is marketed as beef. The ca lves are sold for breeding to va rio us countries, including Guyana, Cuba and other Latin American countries, and the US .

Captive breeding Plants and anima ls are kept b y scientists in order to introduce particular genes into the population. The genome is improved by manipulated crossing of parent organisms or breeding. For example, corn origina.lly grown from wild seeds would gradually change as breeding introduced into the population genes for resistance to disease, large cobs and gra ins rich in nutrients. Similarly, animals can be kept in breeding programmes to maintain and improve the genome. Captive breeding programmes a re important for p reventing extinction and for improving on the diversity of the population of the organism concerned.

Mutation lu!mmt.leU A mutation is a change in the amount or number of chromosomes, or a change in the structure of the ch romosome or DNA of an organism . It results in a change in the genotype of an organism. An example of a gene mutation is sickle cell anaemia, and an example of a chromosome mutation which d1anges the number of chromosomes is Down's syndrome. Mutation occurs randomly - you ca nnot predict exactly where or w hen a change will happen. The ca uses of mutation are: • exposu re to high-energy electromagnetic radiation like X-rays, ultraviolet light and gamma (g) rays; • exposure to certain chemicals like m ustard gas, caffeine, formaldeh yde, colchicine, tar in tobacco, a n increasing number of drugs, food preserva tives and pesticides. mutagenic

>

Any substance or process that increases the frequency of mu tation is described as mutagenic. If a mutation happens in a body cell, it w ill not be inherited or passed on to offspring and is lost when the organism dies. However, if it occurs in a gam ete cell, it can be inherited. This ca n add variation to the population. The offspring of sexual reproduction show variation naturally, because of crossing over and random alignm ent of the chromosomes on the equator of the cell, before anaphase. A muta tion which can be inhe rited, can add new variation. The change in the chromosome because of the mutation is n ew information. It may resu lt in an advantageous or disadvantageous chara cteristic in th e organism. Most major mutations a re disadvantageous.

319

Continuity and Variation

Sickle cell anaemia Sickle cell anaemia is a good example of how a mutation of a part of a chromosome can have drastic effects. It also shows the role o f natural selection in control lin g the occurrence of mutated genes. ln sickle cell anaemia, the gene or part of the chromosome that determines the shape of the haemoglobin in red blood celJs has mutated or changed. This new form of a llele of this gene causes the red blood cell to take a sickle shape instead of the n orma l biconcave disc shape (figure 26. 18). The sickle-shaped red blood cell cannot transport oxygen efficiently which makes it a disadvantageous characteristic. However, the presence of sickle-shaped red blood cells in the body makes the person far less susceptible to infection by the malarial parasite than a person withou t sickle-sh aped red blood cells. Thi s is an advantageous characteristic since malaria is a leading cause of death in areas where it occurs. Since every person carries two alleles for th is gene, one on each homologous chromosome, there a re three possible genotypes: • homozygous for normal haemoglobin; • heterozygo us with one allele for normal a nd one allele for sickle cell haemoglobin; • homozygous for sickle cell haemoglobin.

Figure 26. 18 Sickle and normal red blood cells.

sickle cell disease

>

sickle cell trait

>

Those people who are homozygous for sickle cell have sickle cell disease. They experience severe pain in the joints, anaemia, kidney failure, poor growth and development, are prone to infections and a re likely to die young. In those who are heterozygous, only about half the red blood cells change to sickle shape. These people are unaffected by the condition except at low oxygen concentrations, such as when flying in an ai rplane or goi ng to high a ltitudes. This conditions is known as sick le cell trait. The sickle cell gene was selected for in those regions of the world where malaria is seen (parts of Africa, the Middle East, Tndia and sou thern Europe). People h ere who are heterozygous for the gene are at a selective advantage, as they are less likely to die from malaria tha n those who do not have the sickle cell allele, and less likely to die than those who have two sickle cell alleles. By natu ral selection, the gene continues to be passed on to offspring, since these people survive malaria. However, the sickle cell alle le is at a selective disadvantage in areas where rhere is no malaria. People who originally came from malarial areas. such as Africa, bur now live in areas where there is n o malaria , such as America, still carry the allele. Abou t l in 400 black people in America have sickle cell anaemia. and the disease causes about 100 000 deaths per year worldwide.

Down's syndrome Down's syndrome is a change in the number of chromosomes in a cell. It occurs in all races and a correlation wi th the age of the mother is seen.

Inciden ce of th e disease rises with the mother's age, especia lly after 40 yea rs. Th.is may be due to the fact that a woman is born with all her eggs and they age with he r. Men, on the other hand, constantly produce new sperms. The cells of a person with Down 's syndrome a ll have 47 ch romosomes instead of 46. People with the cond ition show typical facial features (fl at and rounded). Other symptoms include: • learning difficulties; • • • •

320

sh ort stature; heart defects; increased risk of infection; intestinal problems.

26 · Variation and Evolution

People with Down's syndrome are generally very friendly and cheerful, and greatly enjoy music.

Genetic engineering

genetic engineering >

~

rr.01 \J'V

Describe brieflythe following terms: (i) genetic engineering (ii) a transgenic organism.

Biotechnology is the science which involves the harnessing and exploitation of biological processes, systems and organisms (particularly microorganisms) iJ;l manufacturing industries. The most powerful tool available to biotechnologists is genetic engineering. The benefits of genetic engineering include the development of high-performance food crops that grow quickly with less use of fertiliser. This could ease the pressure on food supplies from the growing human population. Another important area of development is diseaseresistance in crop plants, which would reduce the need for use of pesticides. An organism that has genes added to it from another species by genetic engineering is known as a transgenic organism. Some examples of genetic engineering in food production include: • resistance to pathogenic fungi in maize and potato; • resistance to insect pests in many crop plants; • increased growth rates in salmon and chicken; • production of meat with less fat in pork and beef animals;

bact~mjO ~J

pl~d I"'°"'" y DNA

found in bacteria) plasmid cut

using enzymes

v."'P./ human DNA (insulin gene)

human DNA inserted into bacterial plasmid and joined back up again

bacterial DNA

l

@~ @""~"""'

/@

~ ~ DNA extracted from

• •

human pancreatic cells

recombinant DNA - DNA from two different species

• •

plasmid (recombinant DNA) introduced into a bacterium

As bacteria multiply, the genes are expressed to make their different parts. The insulin gene which was inserted will also be expressed.

• production of higher quality dairy products (e.g. milk with more protein);

The plasmids are mass produced as the bacteria multiply. The insulin gene is also being mass produced and insulin is produced when the gene is expressed.

human insulin is separated and purified

•

•

•

increase in the proportion of protein in seeds such as soya; long shelf-life of fruits such as tomato and bananas; tastier and more nutritious foods like tomato; increase in size, and therefore in yield, of many crop plants and cattle and dairy animals; production and subtropical crops so they are able to grow in temperate climates (e.g. sugar cane and millet); production of cows and sheep from temperate areas so that they can grow well in tropica I regions; grain crops that can fix atmospheric nitrogen (e.g. wheat and maize).

Human insulin is now manufactured in bacteria as a result of genetic engineering (figure 26.19). Insulin was previously obtained from cows or pigs and caused many side-effects in people with diabetes who needed it. It is now produced by inserting the human gene that codes for insulin into bacteria and allowing them to

Agure 26.19 Using genetic engineering to make insulin from bacteria.

321

Continuity and Variation

grow and multiply. As they do so, they produce insulin. The insulin is then separated, purified and packaged. Production of human insulin rhis way is now a large-scale enterprise and rhe product is used by thousands of people w ith diabetes. Genetic engineering is also being used to help treat some hereditary diseases in humans. Cystic fibrosis is a disease which affects around one in every 2500 babies. It is caused by a recessive allele which makes the mucus in the lungs thick and sticky. Bacteria get trapped in the mucus and cause infections whlch can lead to early death. Traditionally, the on ly treatment fo r cystic fibrosis is daily physiotherapy to clear the mucus in the lungs. Current research is studying trearment using a viral vector to transmit the normal a llele inro the lungs. ti the vector is taken up by the cells, they wou ld than be able to make normal mucus. Treatment wo uld have to be continuous because the ce lls lining the lungs a re shed frequenrly and replaced with new ones.

Implications of genetic engineering Are there risks to human health? Some people argue thar there may be long-term risks from genetic engineering. There is much discussion on the effects of genetically modified organisms (GMOs) used in food production. An example is bovine somarotrophln (BST). This hormone is produced artificially by bacteria and injected into cows to stimulate growth and increase milk production. The hea lth o f humans drinking the milk or eating the meat appears to be unaffected by the hormone. But are there long-term effects on human health? As yet, we do not know. Should genetically mod ified food be labelled as such ? What would you p refer?

Feeding the world Many crop plants are modified for disease resistance and increased yield. Plants can be engin eered to incorporate the characters of a number of different species (e.g. starchy potatoes with bera-caroteoe from green vegetables and vitamins from citrus fruirs). Millions of people are starving in the world - why not use such foods to ease the problem of starvation? Scientists have a lso been able to insert two genes from daffodil and one gene from a bacterium into rice so thar it can now contain vitamin A and its precursor beta-carotene. This gives the rice a yellow colour - hence it is w idely known as golden rice. It is hoped that it will help to combat malnutrition in less developed nations, especially those where a lack of beta-carotene in the diet leads to blindness.

Economic and diversity problems

Figure 26.20 Genetically modified soya bean could replace conventional crops.

322

There is also fear that small farmers would no longer find it economical to culti va te loca l varieties of crop plants when they have to compete w irh imported, economicall y superior varieties (figure 26.20). Cou ld this lead co a serious loss of genetic diversity among cultivated crop plants? This would make the dwindling genetic diversity problem worse. Are there dangers in relying on just a few varieties of crop plants? A new strain of disease could then wipe out a major crop. Gene banks o f many varieties of seeds and plants have been set up in many countries to conserve diversity, for example cocoa seeds and planes are stored in Trinidad . Some people fear that the genetically engineered trait cou ld get transferred into wild relatives of A engineered crop plant (it has been shown that pollen from crops such as o il-seed rape can spread for at least I 00 m from the GM plants). Might this produce pest species w hich cou ld spread uncontrollably

26 · Variation and Evolution

and eliminate other plants, upsetting th e ecological balance? What are the implications of genetically engineered tra its transferring into other species?

Treating disease Advances in genetic engineering will undo ubtedly eventually lead to the control of genetic diseases, su ch as cystic fibrosis, by replacing defective gen es with healthy ones. This could be wonderful for those livin g with theg'e conditions. There are many advantages of this kind of technology. Might this be taken further? Wou ld genes for low intelligence by replaced by those for · high er intelligence? Would this be good or bad? Are some human characters superior to others? Who would decide?

~ r:roa l/'V

Describe two benefits and two hazards of genetic engineering.

The future Should humans be allowed to genetically manipulate animals and plants to serve the needs of humans rath er than the environment as a whole? Is exploitation of living organisms, whether for commercial gain or to reduce suffering, the height of misuse of the environment, or is it another example of human s's triumph over adversity?

• The theory of natural selection is based on genetic variation among a population. It is selection of the fittest organisms by nature. • In artificial selection, humans select individuals that are allowed to reproduce and produce offspring. We select characteristics advantageous to us like high yield and reduced production costs. • Mutation can occur that change the genotype of an organism . • A mutation may be a change in the structure of the chromosome, such as sickle cell anaemia. • A mutation may change the number of chromosomes in a cell, as in Down's syndrome. • Genetic engineering is the deliberate changing of the genotype of an organism by humans. • The production of human insulin by bacteria is an example of genetic engineering; • There is much discussion around the possible advantages and disadvantages of genetic engineering.

323

Continuity and Variation

Each organism has its own genotype which is different from every other genotype (except for identical twins and individuals produced by asexual reproduction). Genetic variation is variation in the genotype that helps to determine differences in the phenotype. Genetic variation explains wh y every organism is unique. ITQ2 (i) The phen otype is the physical appearance of an organism. It describes all its physical characteristics. (ii) An organism develops its physical characteristics from a combination of its genotype and its e nvironment. The genotype confers on the organ ism the possibility of developing certain characteristics. The environ ment guides the development of these characteristics. ITQ3 Height - one may be taller; complexion - one may be darker; body size - one may be fa tter (there are many other examples). ITQ4 If the en vi ronmental temperature got warmer, all might survive, but the ones with the shorter hair length would be at an advantage. The wolves with long hair stand a chance of over-heating because of the insulation provided by the thick coat of hair. They are at a disadvantage. The wolves with the advantage for that new environment would be selected by nature (i.e. they would be more able to Live and reproduce). Eventually a population of shorthaired wolves would be seen . ITQ5 The use of an antibiotic on a population of bacteria results in an increase in occurrence or frequency of those with the gene that gives resistance to that antibiotic. Over time, and with constant use of many different antibiotics, a population of bacteria could evolve that is resistant to many different antibiotics. ITQ6 (i) Natural selection is a theory first put forward by Charles Darwin. He explained how the environment could select for characteristics in a population showing variation. He concluded that new species could come into being by slow and gradual changes, called evolution, as a result of the process of natural selection. (ii) A characteristic that suits an organism to its environment has selective advantage because organism with that characteristic stands a better drnnce of surviving and reproducing than those which do not have it. (iii) The process of natural selection is also known as 'survival of the fittest' because nature selects those individuals best 'fitted ' (adapted) to the environment. (iv) Evolution describes the change which takes place in a species over time and which leads to the formation of a new species. ITQ7 (i) Genetic engineering is the technology in which genes from o ne organism are transferred to another organism, often a different species. (ii) A transgenic organism is one which has had gene (s) transferred to it from another species. The transgenic organism s can live and reproduce normally although it has been changed. ITQ8 Any of the benefits and hazards mentioned in the text could be mentioned, or you might have researched some more. This new area of knowledge is constantly changing and new developments are frequently reported in the media.

·ITQ1

324

26 · Variation and Evolution

Examination-style questions 1

(i)

Explain these terms: (a) evolution; (b) mutation; (c) artificial selection; (d) selection pressure. (ii) Explain what is meant by 'selective advantage; using antibiotic resistance as an example. (iii) Describe, using an example, how the environment may affect the phenotype. (iv) Explain, using the sickle cell gene, how mutation may affect the phenotype.

2

(i)

Describe four examples of artificial selection and the characteristics that are being selected for by humans. · (ii) Explain, using examples, how environmental factors like temperature, act as forces of natural selection. (iii) Using a table, list five differences between natural and artificial selection. (iv) If two offspring (not identical) are brought up in different environments, suggest why there may be difference in the development of the following characteristics: (a) body weight; (b) intelligence. Compare this with two identical offspring, brought up in the same environment.

3

(i)

The aim of artificial selection is to produce animals and plants with characteristics desirable to humans. Suggest four characteristics of animals and plants that may be chosen. (ii) The peppered moth exists as two main types, a pale form and a dark form. (a) What is the importance of the colour of the moth? (b) What effect did industrialisation and the production of pollution have on both forms? (c) Why do you think heavy-metal tolerant plants are rare in unpolluted areas?

4

(i)

Outline the general process of genetic engineering.

(ii) Give two uses of genetic engineering in (a) agriculture, and (b) medicine. (iii) Discuss the possible risks of genetic engineering. How can these risks be reduced? (iv) Many people are against the practice of genetic engineering. Suggest some reasons for this.

325

Section D:

School-Based Assessment

Practical work in Biology The present CSEC Biology syllabu s (201 3) makes clear that assessed practical work - the School-Based Assessment (SBA) - is an integral part of a student's studies. This aspect of the course gives the chance to personalise the curriculum to meet students' particular needs and assess the development of h is or her skills.

Specified topics In biology, assessment in at least 18 exercises (spread across 7 specified topics)

is needed to satisfy the CXC requirements. The specified topics are: 1. Ecological study 2. Movement at molecular level (diffusion, osmosis) 3. Photosynthesis/respira tion 4. Food tests 5. Germination 6. Nutrition and diseases 7. Genetics This chapter indudes outlines of 31 activities (in addition to those mentioned in the syllabus itself), which are suitable, after proper development, for use in SBA practical investigations. There are both qualitative and quantitative investigations. The chapter comains at least one practical exercise associated with each of the specified content areas, as well as other topics encountered in this course. Sufficient detail is given to make possible the practical conduct of each experiment, and each one can be developed to illu strate material in the text. Each gives students the opportunity to develop their experimental and reasoning skills and also their ability to present results in the clear, appropriate way detailed in the syllabus.

Assessment of skills If you are doing an experiment in class as part of your week's work, your

teacher may have done som e or all of the planning for you, collected alJ the ma terials that you need, and give you instructions how to do the work, or even a written worksheet to fo llow. But you still have to show that you can follow the instructions, do the experiment and present your resu lts well. The skills which will be tested are:

Experimental skills (XIS) • • • •

Manipulation and measurement (M/M) Observation, recording and reporting (O/R/R) Planning and designing (P/D) Drawing (D)

27 • School-Based Assessment

• Use of knowledge (UK) • Analysis and interpretation (A/I)

A three-step approach in preparing for your SBA When planning and presenting your project, your SBA has three parts: l . Planning and designing the experiment 2. Doing the laboratory work 3. Presenting a lab report

1

Planning and designing the experiment

In any experiment, you are trying to find an answer: it might be a relationship or a value. You will need to devise and follow a logical series of steps to find out that information. Therefore, whatever your hypothesis, you must have a plan. In some cases, the proposed activities already in this section contain an outline plan. However, you would still need to design your experiment bearing in mind the equipment and facilities ava ilable in your lab. In other activities, you are given a problem to solve, and here you would have to Plan and Design your investigation from the beginning. In this latter case that your work would most likely be assessed under Planning and Designing [PD] . Part of your planning is specifying, in detail, how to carry out your experiment. You need to plan the experiment in such specific detail that someon e else reading your design would be able to do exactly the san1e as you did and get the same results. Think about: • What apparatus will you need? (e.g. 'a 250 cm 3 beaker' - n ot just 'a beaker') • What ch emicals will you need? (e.g. '3 or 4 potassium manganate(VII) crystals abou t 2 mm long' ) • What will you do? (e.g. 'stir the mixture gently with a sturdy plastic drinking straw just before taking each temperature' - not just 'stir the mixture and take the temperature') • What could go wrong? • What are some possible hazards? What safety precautions should be taken? You will n eed to put your plan in writing. It is always a good thing to have your teacher, as well as colleagues, check your plan before attempting to carry ou t your investigation. Often, no matter how carefully you have planned an experiment, it doesn't go as you thought it would . However, it has still told you something - an experiment never 'fails'. If it didn't produce the effect you n eeded, find out why. If the experiment 'worked' but didn't give the result you expected, then you've found ou t som ething new.

2

Doing the laboratory work

Here you carry out your plan. Always consult with your teacher if you are not sure exactly how to use the equipment, and how to use it safely. CSEC expects you to have practised using equipment before being formally assessed in its use.

3

Presenting a lab report

Labs should be written up using the following format. The questions included in each activity are a guide to the content of the discussion .

329

School-Based Assessment

Writing in the correct tense Remember that your research is already finished. Use the past tense when talking about the experiment: 'The objective of the experiment was ...' or 'The mixture was added to the beaker.' Your report, the theory you are testing and your equipment still exist; therefore, these get the present tense: 'The purpose of this report is .. .'

Date and title The title should be brief and describe the main point of the experiment or investigation.

Aim Keeping it simple and achievable, describe the purpose of the experiment.

Discussion Background information to relate the experiment to something you have learnt or seen, or to a problem you have faced.

Hypothesis Should be clear and be in the form of a question that you want to find the answer to, for example: 'Does vitamin C in orange juice oxidise over time when exposed to the air?' (From the data you obtain from your experiment you should be able to say if the hypothesis has been either supported or not in your conclusion.)

Procedure Use the past tense (see box above). In a bulleted list or in separate paragraphs, state, in order, what you did. Include clear, quantifiable detail (e.g. quantities stated and apparatus specified). Your teacher will suggest that you use one of these two forms of words: 'I washed a 250 cmJ beaker.' or 'A 250 cmJ beaker was washed.'

if you have written a good account then someone else, having read it, should be able to repeat the experiment exactly as you did it without any other help.

Diagram Draw the apparatus as neatly as you can.

Results • Table - title stated, neatly drawn with accurate data (times, volumes, masses, colours .. .) and proper units for quantities. • Graph (if necessary) - title stated, axes labelled with proper units, points accurately plotted. Use a line or a bar graph as required. Remember that you should choose the correct type ofgraph for the data you are presenting. • Include any calculations you used. Explain the results in detail, using values found in your results. Answer the questions given in paragraph style.

Limitations Explain any condition or factor that is out ofyour control and affects the results obtained.

Conclusion One short paragraph to summarise results. Make it related to the aim. Review your data and state your opinions and arguments of what the results show, for example, 'as graph 3 shows, there is a marked difference between group A and group B which allows the conclusion that .. . ' .

330

27 · School-Based Assessment

State ifyour hypothesis has been supported or not. If the data you have obtained is not sufficient to support or reject the hypothesis, state why and say what further work could be done that would allow you to draw a stronger conclusion. If you are undertaking a complete project (which will be the case in the second year when you carry out your investigation) then more will be expected of you, and you can see from page 45 of the syllabus how marks for the project will be awarded. Your report will need to be more comprehensive than for a class experiment. It will be assessed for Planning and Design and for Analysis and Interpretation. Planning and Design has twice the marks of Analysis and · Interpretation.

Safety first There are several sources of danger that you need to address as you develop your SBA activities. There are dangers to yourself, your colleagues, the equipment and even the school building itself. Here are a few safety symbols concerning situations you should bear in mind.

Electrical hazard

Hot surface

Laser light

No food and drink allowed

Biohazard

Wear safety goggles

Radioactive

No pointed objects allowed

Corrosive substance Flammable materials

Toxic materials

•

331

School-Based Assessment

School-Based Assessment contents

332

Photocopiable

1.1 To observe visible characteristics of plants and animals

333

2.1 A simple ecological study

334

2.2 To compare the water-holding capacity of three types of soil

338

2.3 To estimate the percentage of water in a soil sample

339

2.4 To estimate the percentage of air in a soil sample

340

8.1 To observe diffusion in a solution

34~

8.2 To observe some effects of osmosis

342

9.1 To investigate the presence of starch in a green leaf 9.2 To see if light is needed for photosynthesis

343 344

9.3 To see if chlorophyll is needed for photosynthesis

345

9.4 To see if carbon dioxide is needed for photosynthesis

346

9.5 To see whether oxygen is produced during photosynthesis

347

10.1 To investigate the action of an enzyme

348

10.2 To investigate which food groups are present in a food sample

349

11 .1 To discover whether carbon dioxide is produced during respiration

350

11.2 To observe whether heat is produced during respiration

351

11.3 To discover whether oxygen is used up during respiration

352

14.1 To investigate the rate of transpiration using a photometer

353

17.1 To discover how gravity can affect plant growth

354

17.2 To investigate the growth of a radicle

355

17.3 To discover how light can affect plant growth

356

17.4 To compare the movement of four animals

357

18.1 To find whether the skin of the back of the hand, the palm or the back of the neck contains the most touch receptors

359

18.2 To investigate two reflex reactions

360

19.1 To investigate heat flow from a warm object

361

20.1 Observing the reproductive cells of a mammal

362

21 .1 Dispersal of fruits

363

21 .2 Seeds and food storage

364

25.1 To investigate how the sex of an offspring is determined

365

26.1 To investigate continuous variation

366

26.2 To investigate natural selection

367

© Linda Atwaroo-Ali 2014. Design and illustration © Macmillan Publishers Limited 2014.

27 · School-Based Assessment

1.1 To observe visible characteristics of animals and plants Chapter 1 The Variety of Living Organisms ' ) (

Syllabus skills: O/R/R Procedure: animals 1. Visit a backyard garden, a nearby cocoa estate, a nature centre, foothills of forest (anywhere a range of organisms can be seen). 2. Copy the table below into your lab book and observe five animals (include three insects). Describe what each animal was seen doing e.g. sucking nectar from a flower, sitting on the bark of a plant. Make a simple drawing of each animal.

Animal

3. 4. 5. 6.

What it was seen doing

Simple drawing

For the three insects, list visible characteristics that they share. Name the phylum and class they belong to. List two ways one insect is different to the other two. Draw a simple classification table to include the five animals.

Procedure: plants 1. Visit a backyard garden, a nearby cocoa estate, a nature centre, foothills of forest (anywhere a range of organisms can be seen). 2. Copy the table below into your lab book and observe five plants. Make a simple drawing of a leaf from each plant.

Plant

Drawing of a leaf (show parallel or branched veins)

3. List all the dicotyledonous and monocotyledonous leaves. 4. Choose two leaves and list three differences you observe. 5. Choose three leaves and list similarities you observe.

© Linda Atwaroo-Ali 201 4. Design and illustration © Macmillan Publishers Limited 201 4. Photocopiable

333

School-Based Assessment

2.1 A simple ecological study Chapter 2 Ecology and the Impact of Abiotic Factors on Living Organisms

Syllabus skills: O/R/R; M/M The area to be studied should be small, such as a tree, a small pond, a small area in the foothills, a small area in a cocoa estate or a small garden. The aim is to study the biotic and abiotic factors of the area. The area being studied can be marked by funning string and the area calculated.

The biotic factors A list of all the animals and plants seen in the area should be made. This can be done by walking quietly and slowly through the area {if it is on land) A representative sample of any study area and observing the organisms. Organisms may be found in and on the soil, can be taken . under leaf litter and stones, on the stems and leaves of small plants, flying in the area, on and under the bark of a tree, on the branches of a tree or just visiting the area for a short time. Food chains and a simple food web can then be constructed using the organisms (plants and animals) on the list. Interrelationships between the organisms may be noted as examples of the parasitism , commensalism and mutualism. Other interrelationships like competition (for light, space, etc.), camouflage, pollination and protection should also be noted. A reader of the study should have a good idea of the organisms seen there and what they are doing. An ecological study may also involve collecting data about the abundance and distribution of organisms. The population size of an organism in the area may be difficult to obtain since it means counting every individual in the area. However, the population density may be calculated from a smaller area, as the number of organisms present per square meter (m 3) . Then the population size of the whole area can be calcu lated if the area is known. To do this, representative samples of the area must be taken. These are usually chosen at random to avoid bias. Sampling methods include line transects, belt transects and the use of quadrats and sweeping nets. The most appropriate sampling method for a particular study depends on the area being studied.

Sampling methods Quad rats These can be used if the area is fairly uniform and flat. A quad rat is a square frame (meal, plastic or wooden) of a know n area, usually 0.25 m2 or 1 m 2 . It is placed randomly at several places within the study area and the number of individuals counted. This method is suitable for plants and slow-moving animals like millipedes and some insets. The results can be tabulated as shown . Quad rat

Number of individuals*

7 2

14

3

3

4

23

5

5

*number of individuals of one population e.g. millipedes or nutgrass. The mean number of individuals per quadrat is then calculated and used to find the population density or population size of the species counted in the whole study area.

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For example: 7 + 14 + 3 + 23 + 5 - 52 =

5

-

5

10 . 4

There are on average 10.4 individuals in every quadrat. If the 1 m 2 quadrat was used then: population density = 10.4 individuals/m2 If the size of the area being studied is known, for example 25.6 m 2 , then the popu lation size can be calculated: If in 1 m 2 there are 10.4 individuals, then in 25.6 m 2 there are: 10.4 x 25.6

=266.24 individuals

So the population size for the area studied is 266 individuals (m illipedes or nutgrass or whatever was being estimated).

Line transects A line transect is a better sampling method if one type of habitat changes into another or the area is sloping, such as a rocky or swampy shore. A string is pulled in a straight line across the area being studied . All the animals (slow-moving) and plants actually touching the line are considered to be a representative sample of the animals and plants there. Measuring the height of the line at regular points can describe the slope of t he area. A

B

c

D

E

A transect line

Position along line

Distance between soil and string

Description of soil (water present)

Plant and animals observed

A B

c D

E Worksheet for the transect line

Transect line across the edge of a pond, and a recording sheet.

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Sweep nets These are used for sampling insects, especially flying insects. At randomly set places within the area a net is swept through the plants (like grass) a fixed number of times and the individuals caught are counted. The mean represents a sample of the insects found t here. Sweep nets can be used with quadrats or line transects.

The abiotic factors The distribution and abundance of organisms relate to abiotic factors. Commonly measured and described factors are temperature, pH, light intensity and wind. The soil is a very important abiotic factor since it directly influences the distribution of plants and therefore the animals that feed on them.

Temperature Temperature can be measured using a thermometer. The temperature range over a period of time like a day may be more important than a reading at any particular moment. Standard maxim um/ minimum thermometers can be used.

pH pH is a measure of the alkalinity or acidity. To determine the pH of the soil, about 1 cm 3 of soil can be missed with 10 cm3 of distilled water. After shaking, the mixture is allowed to settle and the pH determined with the use of universal indicator or pH paper.

Using various sampling techniques in an ecological study. (a) Pond dipping. (b) Collet1ng insects in a net.

14 13

12 pH strip

11

10

This one is pH 6

9

8 7

A pH strip

placed into the solution being tested, then compared to these standards 1n order to determine the pH of the solution.

Light intensity Light can be measured at any time using light meters (like those used by photographers). However, it is the light received over a long period of time that affects plant growth.

(a)

card

,.......,1-- pin upon which vane 1s pivoted tunnel

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card vane

wooden

arm

Wind Wind speed and direction affect those animals and plants exposed to the elements of nature. Wind speed is

~-+-- scale

- supporting pole upon which wooden arm pivots

I \

~

wind direction

Two simple wind gauges.

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27 · School-Based Assessment

measured by an instrument called an anemometer and a simple wind vane can measure the direction. Simple, but effective gauges can be improvised which may not give the exact speed values but give comparative readings .

Water flow Water speed can be determined by measuring the time taken by a floating object to travel a measured distance of the stream or river. The speed per hour can then be determined.

'-

Soil Factors of the soil which affect plant growth include the pH, water content, air content, humus content, water-holding capacity and soil type (composition and distribution of inorganic soil particles). Investigations to measure the water content, air content and water-holding capacity follow.

Humus content A sample of soil is heated at 100 °C (to remove all the water) and weighed to give weight X. The dry sample is then heated again until red hot; this mean that the humus is burnt off. It is then reweighed to give weight Y. The percentage of humus in the soil

=xx Y =100%

Soil type The distribution and composition of the rock particles can be determined using the sedimentation test. A sample of the soil is taken and mixed with excess water in a measuring cylinder. The mixture is shaken vigorously and left to settle. The largest and heaviest particles will settle first, the smallest last, the particles will settle in layers. The thickness of each layer can be recorded to indicate soil type. :S:::~~~~--- bits of twigs, leaves, etc.

·: · ·· .: : : : ·: : ..·: / .: :-..·:-.+-- -- very small particles (clay)

.

·:.:· ~ :' ·~ ·:..::. ·: ·.:

.~.=:::-. ~·:.:·: ~: :':::. :.: ·..:...:-:· .··.... ....·::.·:... . . . ·. ·.· ·.·. ·: ... .·... .:..·. .·.·..:... •

· •

.+--

--

• • : ••• : •: •• : • •- 1 - -- -

....... ·......·

=+---

-

-

small particles (silt)

large particles (sand) stones and gravel

Results of a sedimentation test.

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2.2 To compare the water-holding capacity of three types of SOil

Chapter 2 Ecology and the Impact of Abiotic Factors on Living Organisms

X:

Syllabus skills: O/R/R; M/M

Procedure You need samples of a sandy soi l, a clay soil and a loamy soil.

measuring cylinder

1. Set up three sets of apparatus as shown in the diagram. Use 100 g samples of soils A, B and C.

2. Draw up a table like the one shown below. 3. Pour 100 cm3 of tap water through each sample. 4. Wait until no more water is passing through the samples. (This may take some time!) Record the volume of water which has passed through each.

Soil sample A Amount of water drained through

Soil sample B

Soil sample C

(cm~

Amount of water retained in soil (cm~

Questions 1. Through w hich soil did the water flow (i) most quickly (ii) most slowly? 2. Which soil retained (i) least water (ii) most water? 3. From this data, which do you think is the sandy soi l? Explain your reasoning. 4. From this data, which do you think is the clay soil? Explain your reasoning.

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2.3 To estimate the percentage of water in a soil sample Chapter 2 Ecology and the Impact of Abiotic Factors on Living Organisms } (

Syllabus skills: M/M Share the samples A, Band C from investigation 2.2 among class members.

Procedure 1. Weigh a suitable sized sample of soil. 2. Heat the sample of soil in a dish until it seems dry. Do not heat the soil strongly enough to decompose organic matter - a temperature of about 90 °C is ideal. 3. Let the soil cool and reweigh it. 4. Reheat the soil for several minutes. 5. Repeat steps 3 and 4 until there is no further loss of weight. 6. Calculate the percentage of water in the soil from the formula: % = mass of wet soil - m ass _of dry soil x 100 mass of wet soil

Questions 1. Which type of soil contained the highest percentage of water? (Your answer may be different from season to season!) 2. Explain the necessity for step 3 in the experiment.

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2.4 To estimate the percentage of air in .a soil sample Chapter 2 Ecology and the Impact of Abiotic Factors on Living Organisms

X

Syllabus skills: O/R/R; M/M

Procedure 1. Choose a small tin with a volume of about 200 cm 3 • Punch several holes in the base. 2. Press the tin down into the soil which you are going to test. (Take care! Some tins have very sharp edges.) A

tin ,,---., tin of soil collected, the holes at the bottom are plugged with plasticine

tin pressed into the soil

3. 4. 5. 6.

Plug the holes in the base of the tin with plasticine. Pull out the tin without losing any of the soil inside it. Add 300 cm 3 of water to a large (1000 cm3 or larger) measuring cylinder. Pour the soil from the tin into the water in the cylinder, swirl or stir the mixture and allow it to settle. Note the new volume. Call this X cm3 . B

15001400 1300 1200 1100 1000 900

eoo 700

volume of soil 3

and 300 cm y of water and tin full of water

{

: 400

:

J

J

100

volume of the tin

{ volume of soil X and 300 cm3 of water

7. Fill the tin with water to the brim and pour the water into the cylinder. Again note the new volume. Call this Y cm3 .

Calculation Volume of tin

=Y -

X cm 3• This is the volume of (soil + air).

X = 300 +total volume of the tin - volume of air in the soil (the air is lost as bubbles) X = 300 + (Y - X) - volume of air. Therefore volume of air = 300 + Y - 2X.

% of air = 300 +CY - 2 Xl x 100 (Y - X)

Questions What is the importance of air in the soil?

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8 . 1 To observe diffusion in a solution

Chapter 8 Cells

Syllabus skills: O/R/R ; M/M

Data Potassium manganite(Vll) is soluble in water giving an intensely purple solution.

Procedure 1. On a sheet of white paper draw five circles all with the same centre. Make their radii 1, 2, 3, 4 and 5 cm.

2. Place a large beaker over the circles and fill it to three-quarters with water. Put the beaker aside, out of direct sunlight, for five minutes so that the water can become quite still. 3. Choose a single crystal of potassium manganite(Vll) (potassium permanganate) and drop it through the water so that it lands near the centre of the rings you have drawn.

crystals placed

4 . Time how long it takes for the pool of dark purple solution to spread out through each of the rings. Put your results in a table.

Questions 1. Why was it important to keep the beaker of water out of the sunlight? 2. Why did the colour move through the water? 3. What is t he mean speed of diffusion of the purple coloration through the water? 4. In a vacuum the coloured particles would move very quickly. Why did they move so much more slowly in your solution?

Extension It is not easy to get the potassium manganite(Vll) crystal to fall where you want it. Can you devise a better way of placing it in the water in the beaker? Remember that the water must remain still.

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8.2 To observe some effects of osmosis

Chapter 8 Cells

X

Syllabus skills: A/I; O/R/R; M/M

Procedure 1. Cut two potato strips roughly 1 cm square and 3 cm long.

2. Measure the length of each as accurately as you can. 3. Rub the potato strips between your fingers to assess their texture. 4. Put each potato strip into a petri dish. Cover one with clean water and the other with a strong solution of sodium chloride (common salt).

t

t

_........__ ___,,....___ _ _.....__,

wat~r petn dish

potato strip

5. 6. 7. 8.

Leave the potato strips in their dishes for 15 minutes. Remove the potato strips, dry them, and measure the length of each as accurately as you can. Note the texture of the potato strips. Record your observations in a table like the one below.

First texture

Final texture

First length

Final length

Change in length %±

water salt solution

Questions 1. In terms of the cells forming the potato strips, why have the lengths of the strips changed in the way they have? 2. Do the changes in texture of the strips fit in with your explanation? Explain. 3. The cells of the potato contained water to start with . Why did more water move one way than the other across each cell wall?

Extension Design an experiment to investigate the effect of using different concentrations of sodium chloride to surround the potato strips. What result would you expect to find in your experiment?

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9.1 To investigate the presence of starch in a green leaf Chapter 9 Photosynthesis

Syllabus skills: M/M

Caution: ethanol is flammable. Do not heat the tube directly with a Bunsen flame

Data Starch reacts with iodine to give a blue-black coloration.

Procedure 1. Take a small fresh green leaf from a suitable dicotyledonous plant. 2. Dip the leaf into boiling water for about 10 seconds. 3. Put the leaf into a test-tube no more than one-third full of ethanol (alcohol). 4. Place the test-tube into the beaker of boiling water. 5 . When the leaf appears colourless, remove the leaf and rinse it in water. 6. Lay the leaf in a petri dish and pour a little iodine solution over it. Leave the leaf for several minutes. 7 . Pour the iodine solution back into the beaker provided. 8. Rinse t he leaf in water. Observe t he colour of t he leaf.

Questions 1. What effect did the boiling water have on the leaf? 2. What happened when the leaf was boiled in alcohol? What did the alcohol remove from the leaf? 3. Why was the leaf then rinsed in water? 4. What was the colour of the leaf at the end of the experiment? 5. What do you conclude about the original green leaf?

The leaf is dipped in bolling water for about 10 seconds

l

beaker The leaf is placed in a test tube of alcohol that is in boiling water. NB alcohol is very inflammable and must not be heated directly over a bunsen flame.

test tube alcohol leaf boiling water

l The leaf is dipped in water

The leaf is placed in a petri dish and covered with iodine solution. Iodine turns blueblack in the presence of starch.

@

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9.2 To see if light is needed for photosynthesis Chapter 9 Photosynthesis

Syllabus skills: A/I; O/R/R; M/M

Procedure 1. Choose a small potted plant (such as Impatiens or a geranium). De-starch the whole plant by putting it in darkness for at least 24 hours.

2. Cover a part of one leaf on the plant with foi l or black polythene held in place with paper-clips. (Leave the leaf on the plant.) 3. Put the plant in the sunshine for at least 3 hours. 4. Remove the test leaf, remove the covering and at once test the leaf for starch. 5. Make a drawing to show your results.

Questions 1. Why was the plant de-starched? 2. What was used as a control in the experiment? 3. Which part of the leaf contained starch before the foil cover was added? 4. Which part of the leaf contained starch at the end of the experiment? 5. What do you conclude from your results? Explain fully why you reach this conclusion .

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9.3 To see if chlorophyll is needed for photosynthesis Chapter 9 Photosynthesis

Syllabus skills: A/I; O/R/R; M/M

Procedure 1. Choose a small potted plant with strongly variegated leaves. Some portions of the leaves should be as nearly white as possible. 2. De-starch the whole plant but putting it in darkness for at least 24 hours. variegated leaf

chlorophyll - - + -absent chlorophyll present

3. 4. 5. 6.

Choose a boldly marked leaf and make a careful drawing of it to show the green and white areas. Place the plant in the sunshine for at least three hours. Carry out a starch test on the leaf that you sketched. Make a drawing of the leaf showing the brown and the blue-black areas.

Questions 1. Why is a variegated leaf used for this experiment? 2. Why was a drawing of the leaf made before the experiment began? 3. What do the results of the starch test show? 4. Is there anything in common between the blue-black areas of the starch test and the green areas of t he original leaf? 5 . What conclusions can you draw from your experiment?

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9.4 To see if carbon dioxide is needed for photosynthesis Chapter 9 Photosynthesis

X

Syllabus skills: A/I ; O/R/R ; M/M

1 - --

-

bell jar -

-----