macmillan cxc science series biology...macmillan cxc science series biology linda atwaroo-ali...

Macmi l l an CXC Sc ience Ser ies

B io logyLinda Atwaroo-Ali

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Macmillan EducationBetween Towns Road, Oxford OX4 3PP

A division of Macmillan Publishers LimitedCompanies and representatives throughout the world

www.macmillan-caribbean.com

ISBN-13 978-0-333-80368-4ISBN-10 0-333-80368-X

Text © Linda Atwaroo-Ali 2003Design and illustration © Macmillan Publishers Limited 2003

First published 2003

All rights reserved; no part of this publication may be reproduced, stored in a retrieval system, transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publishers.

Designed by Jim Weaver DesignIllustrated by Raymond Turvey (Turvey Books Ltd)Cover design by Gary Fielder, AC DesignCover photographs:Front cover and title page: Corbis royalty free (leaf), Digital Vision (frog, dolphin),Photodisk (flower, DNA), Science Photo Library (heart)

The authors and publishers would like to thank the following for permission toreproduce their material:Figure 24.5 Global Distribution of HIV/AIDS from UNAIDS/WHO Report (1996).(Courtesy: United Nations Office & Information Centre, London).Figure 4.3 Carbon Dioxide Concentrations from the Whitehouse Initiative on GlobalClimate Change. (Courtesy: The White House, Washington, DC).Figure 27.3 The Human Population Growth Curve, AD 1750–2000. (Courtesy: WorldResources Institute, Washington, DC).

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Series preface ix

About this book x

Section A: Living organisms in the environment

1 The variety of living organisms 1

Characteristics of life 2The major groups of organisms 2Classification of organisms on the basis of visible characteristics 7The binomial system 8Species 10

Summary 10Answers to ITQs 11Examination-style questions 12

2 Feeding relationships between organisms 14

Producers and consumers 15Decomposers and detritivores 15Herbivores, carnivores and omnivores 16Food chains 16Predators and prey 17Food webs 17Special relationships 18


3 Energy flow within a food chain or food web 22

Trapping the Sun’s energy 22Movement of energy through a food chain 24Pyramids of energy 26Pyramid of numbers 26Pyramids of biomass 27


4 The cycling of nutrients 30

Biogeochemical cycles 30The carbon cycle 31The human effect on the carbon cycle 32The greenhouse effect and global warming 33The nitrogen cycle 34Acid rain 37


iii

C o n t e n t s

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Section B: Life processes

5 Cells 40

Plant and animal cells 41Specialisation in multicellular organisms 42Movement of substances into and out of cells 44


6 Photosynthesis 52

Plants are the food supply for animals 52Photosynthesis 53Products of photosynthesis 56Limiting factors in photosynthesis 57Etiolation 58


7 Feeding and digestion 61

Diet 61Organic nutrients 62Food tests 64Inorganic nutrients 65Food additives 66A balanced diet 67Malnutrition 68Utilisation of food in Man 69Digestion 70Digestion and absorption in the alimentary canal 72Assimilation 78Functions of the liver 78


8 Respiration 83

Aerobic respiration 83Anaerobic respiration 85


9 Gaseous exchange 91

Importance of gaseous exchange in Man 91Mechanism of gaseous exchange in Man 92Importance and mechanism of gaseous exchange in plants 94Characteristics common to gaseous exchange surfaces 96The effects of cigarette smoking 98


10 Transport in animals 101

The need for a transport system 101The circulatory system in Man 102Blood vessels 104Blood 108

iv

Contents

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Blood groups 109Hypertension 110


11 Transport in plants 114

The importance of transport in plants 114Transport systems of plants 115Movement of water through a plant 117Adaptations in plants to conserve water 121Uptake and movement of mineral salts 122Transport of manufactured food 122Food storage 122


12 Excretion, osmoregulation and homeostasis 127

Excretory products in animals 128Excretory products in plants 128The human excretory system 129Osmoregulation 134Homeostasis 135


13 Movement 141

The importance of movement in animals 141Movement in plants 142The skeleton of Man 144


14 Sensitivity and coordination 152

Stimulus 152The sense organs of Man 154The nervous system 155Reflex actions 158The brain 159Autonomic nervous system 160


15 The eye and the ear 163

Structure of the human eye 163How we see 164Sight defects and their corrections 168Structure of the human ear 170How we hear 170The ear drum 171Balance 172


v

Contents

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16 Temperature control in animals 176

Temperature control 177Temperature regulation in humans 178Temperature regulation in birds 180


17 Growth and development 183

The growth of an organism 183Growth and development in plants 184Growth and development in animals 186


18 Reproduction in animals 196

Reproduction 196Reproduction in Man 197The role of contraception 204


19 Reproduction in plants 209

Life cycle of a plant 209Structure of a flower 211Pollination 212Fertilisation 214Dispersal 215Germination 217


Section C: Continuity and variation

20 Mitosis 222

Chromosome number 222The cell cycle 223Importance for maintaining species chromosome number 225Mitosis 225Mitosis and asexual reproduction 226


21 Meiosis 233

The importance of meiosis 233The process of meiosis 234Significance of meiosis 236


22 Heredity and genetics 239

Variation 240Genes 240

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Contents

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Genetic diagrams 242Test cross or back cross 243Incomplete dominance 243Co-dominance 244Examples of genetic effects 246Pedigree charts 248


23 Variation and evolution 253

Genetic variation 253Importance of genetic variation 255Natural selection 255Artificial selection 260Mutation 262Genetic engineering 263


Section D: Disease and Man

24 Disease and Man 269

Health and disease 269Pathogenic diseases and vectors 271AIDS and other STDs 273The role of blood in defending the body against disease 274Immuisation and the control of communicable diseases 276Drugs 278Social and economic implications of drug abuse 281Social and economic implications of disease 281


Section E: Environment and Man

25 Soil 285

The components of soil 285Soil erosion 289Fertilisers 290


26 Ecological factors and their effects on distribution 294

Ecology 294Ecosystem, habitat, population, community 295Distribution of species 296


27 Natural resources and their limits 303

Growth of natural populations 303Growth of the human population 305

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Contents

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Resources and their limits 306Reducing resource consumption 308


28 The effects of Man's activities on the environment 313

Endangered organisms 314Water shortages 316Pollution 317Deforestation 320The impact of industrialisation 321Impact of Man's activities on marine environments 321Conservation and restoration of the environment 322


29 Practical work in Biology 326

Index 360

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Contents

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ix

S e r i e s p r e f a c e

Teachers throughout the region emphasised that inclusion of SBA materialwould be essential to these books. Each CXC (C-SEC) science syllabus speci-fies the areas in which an SBA exercise is expected. Accordingly, at least oneexercise is included for each area.

The books will also provide a firm foundation for more advanced study appro-priate to the Caribbean Advanced Proficiency Examinations [CAPE].

Dr Mike Taylor,Series Editor

This new series of textbooks for Caribbean Examinations Council [CXC] GeneralProficiency examinations has been developed and written by teachers withmany years’ experience of preparing students for CXC (C-SEC) examinations inCaribbean schools.

A textbook is used in different ways at different times.

© Readers may be starting a topic from scratch, and need to be led through alogical explanation one step at a time.

© Students with a working knowledge of a topic may need to clarify a detail, orreinforce their understanding. Or, they may simply need to believe that theydo have a good grasp of the material being studied.

The specially created format is the same for all of the books in the series.

© Diagrams and pictures are placed on the page in such a way that that theycan be consulted as the reader wishes but interrupt the text as little as possible.

© Short-answer questions (called In-Text Questions even though they are notplaced in the main body of the page) allow the student to test his or her graspof the topic. A student who can answer an ITQ gains confidence; a studentwho cannot knows to go back over the topic and try again.

© The first use of any important technical term is highlighted in the margin column to make subsequent revision easier.

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About th i s book

© This book isn’t just words on a page. Here are some important features. Eachwill help you, in its own way, if you take advantage of it.

© There are TWO COLUMNS.The bigger column has the text and some really large diagrams; you canread straight down it without interruption.The smaller column has other diagrams which the text mentions. Look atthem carefully as you need them. You may find that a few seconds lookingat a diagram is worth a few minutes reading.

© The first time that an important NEW WORD occurs, it is repeated in the smaller column. If you want to check what a word means you can find it quickly.

© There are QUESTIONS called ITQs. These are ‘In-Text Questions’. When youhave read the nearby big-column paragraph, try to answer the question, inyour head or on paper, just as you wish. If you can, you’re on the road tounderstanding. If you can’t, just go back and read that bit again. Answers areat the end of each Chapter, so you can tell how good your answer was.

© Some possible SBA EXERCISES are included. They are printed in a separateChapter on paper with tinted edges They have outline instructions and ques-tions to answer. Don’t copy them! Use them as models for designing yourown work.

© There is a detailed INDEX. Don’t be afraid to use it to find what you want.

© At the end of each Chapter there are some EXAMINATION-STYLE QUESTIONS. Your teacher will suggest how you can use them.

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The planet Earth, the third planet from the Sun, has all the conditions necessaryto support life as we know it. Our planet is positioned at such a distance from theSun that living organisms can survive in the range of temperatures on its surface(although it is a fairly wide range). The presence of water in all its forms (solid,liquid and gas), and the combination of gases which make up the atmosphere(including nitrogen, oxygen and carbon dioxide), are all conditions that are essen-tial to life on Earth.

A huge variety of living forms exist on the planet Earth. They can inhabit mostof the Earth’s surface, land, air and water. They show an enormous range in sizeand complexity – from the microscopic, which cannot be seen by the naked eyeand are as simple as one cell, to giant-like whales which must live in water sincethey are too heavy to support themselves and move on land.

ITQ1

List three characteristics of the planet Earththat enables it to sustain life.

The var ie ty o f l i v ing organ ismsú understand why there exists a range of living organisms on Earth;ú list and define the characteristics of life;ú describe the major groups of organisms;ú understand how a classification system is used to group all living

organisms;ú observe and classify living organisms according to visible

similarities and differences;ú understand the meaning of the term ‘species’.

By the end ofthis chapter

you should beable to:

1

range of living organisms

ProkaryotaProtoctista

FungiPlantaeAnimalia

characteristicsof life

classified accordingto common features

growthrespiration irritability

movementnutritionexcretion

reproduction

KingdomPhylumClassOrderFamilyGenus Species

species – caninterbreed

with each other

breeds varieties

races

Concept map

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Characteristics of lifeBiology is the study of life and how living things stay alive. All living organisms,microscopic to gigantic, possess certain characteristics. These are the characteris-tics of life that distinguish living things from non-living things.

There are seven of these characteristics.1 Growth – Living organisms increase in mass, size and numbers.2 Respiration – The energy released during respiration is needed to carry out

all life processes.3 Irritability – Living organisms can respond to changes in their internal envi-

ronment and the world around them. These responses usually increase theirchances of survival.

4 Movement – Most living organisms can move. Plants show growth move-ments. Most animals can move from place to place to find food or a mate.

5 Nutrition – All living organisms need food which is used as a source of energy.Plants make their food during photosynthesis. Animals get their food by eatingplants or other animals.

6 Excretion – All living things make waste products during metabolism. Thesemust be removed from the body.

7 Reproduction – This is the production of new organisms.Living organisms are able to carry out all these processes on Earth. Most organ-isms are adapted to live on land or in water, more or less close to sea level. Somesurvive in ‘extreme’ places such as:• in hot sulphur springs where chemical conditions are toxic to most living things;• in extreme cold, such as at the North and South Pole;• in deep parts of the ocean where no light can reach, such as the Marianas

Trench;• in the upper atmosphere;• in extremely hot deserts, such as the Gobi desert;• inside other living organisms.Wherever they live, as long as they are able to carry out the processes of life livingorganisms survive and produce offspring. Most places on Earth can support life.

The major groups of organismsAll organisms used to be classified or placed in two kingdoms or main groups –animals and plants, depending on whether they get their food from other organ-isms or make their own food. However, living things are more diverse than thisand a classification system of five kingdoms is now used. These kingdoms are theProkaryotes, Protoctists, Fungi, Plants and Animals.

Viruses do not fit into this classification. They are the smallest organisms,though it is difficult to think of them as living because they can only ‘live’ inside

2

1 · The variety of living organisms

characteristics of life ©

ITQ2

Animals and plants are able to carry outcertain processes which distinguish them fromnon-living things. Describe briefly how a plant(i) feeds, (ii) responds.

The kingdoms have scientific names that areslightly different from their common names:• Prokaryota;• Eukaryota;• Protoctista;• Fungi;• Plantae;• Animalia.

Living organisms

Prokaryotes(chromosomesnot enclosedin a nucleus)

Eukaryotes(chromosomes enclosed

in a nucleus)

Viruses

Protoctists Fungi Plantae Animalia

multicellularunicellular

T

Figure 1.1 Living organisms are placed in five major kingdoms (shown incolour).

virus ©

ITQ3

What are the five major groups of life-forms ororganisms?

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another living cell. They also do not have a true cellular structure like other organ-isms.

Billions of viruses ‘exist’ around us and it is only when they enter the cells ofan organism that they show some of the characteristics of life. There they canreproduce and grow in numbers.

Viruses have a great impact on life on Earth, since they can live inside everytype of living organism, from bacteria to plants and animals. It is believed that theyhave changed the course of human history because of diseases like smallpox,measles and now AIDS.

ProkaryotesThe prokaryotes are organisms that are commonly called bacteria. They occupymany environments such as soil, dust, water, air, in or on animals and plants.Some are found in hot springs where temperatures may be higher than 78°C.Some can survive freezing in ice. Some have been found in deep cracks in theocean floor, at very high pressures and temperatures of 360°C. They can be foundin every part of the living world.

They are the most ancient group of organisms. They are also the smallestorganisms that have a cellular structure. Many exist as single cells, others arefound in groups (figure 1.4). Their cells have a much simpler structure than thoseof the eukaryotes (figure 1.5).

Prokaryotes are vital to all other organisms since they cause decay of deadplant and animal material which releases nutrients back into the environment.They are essential to the nitrogen cycle. They are also important to humansbecause they cause disease (such as cholera and TB – chapter 24) and are used inbiotechnology (for example in insulin production – chapter 23).

ProtoctistsMost protoctists are unicellular, that is made of one cell. This cell shows all thecharacteristics of life. Algae and protozoa are two kinds of protoctists.• Algae live in both marine and fresh water, and some live on land where the

surface is damp. They make their own food by photosynthesis. Some live as single cells, others are found in groups or colonies. A few, such as the seaweeds,

3


Viruses that attack humans

Influenza virusHIV or

human immunodeficiency virus

Viruses that attack bacteria are called bacteriophages or simply phages.

Phage 2bacteriophage

phage DNA

Phage DNA is injected into the bacteriumwhere it makes copies of itself (20-1000)

which are released to infect further bacteria. surface of bacterium

Figure 1.2 The structure of some viruses.

Figure 1.3 Escherichia coli is a rod-shaped bacterium which is part of thenormal gut ‘flora’ of humans andother vertebrates.

Figure 1.4 Anabaena is a bacteriumwhere the cells stick together in longchains.

cytoplasm

cell membrane

cell wall

strand of DNA

Figure 1.5 Structure of a typicalbacterium, e.g. Escherichia coli. Thechromosomes are not enclosed in anucleus and there is little structure inthe cytoplasm.

ITQ4

Bacteria are described as being microscopicand unicellular organisms. What do theseterms mean?

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can grow extremely large. These have structures like stems, roots and leaves, butthey are much simpler than true plants. Rapid growth (blooms) of algae canform scums on the surface of ponds, lakes and rivers, turning them green.

• Protozoa are unicellular and feed on other organisms (heterotrophically). Theyare found in all environments, especially in water, and examples includeAmoeba and Paramecium. They are important to humans because diseases suchas malaria and sleeping sickness are caused by protozoan parasites.

Malaria infects millions of people each year and it is estimated that 2.7 millionpeople worldwide die from this disease each year.

FungiFungi range in size from unicellular yeasts to large toadstools. Some are used byhumans for medicinal and dietary purposes. They are heterotrophic organisms(see chapter 7) and obtain their food from the environment. However, they do nottake in large particles of food that need to be broken down. They digest their foodoutside the body using enzymes which make it soluble. Then they absorb thefood. So they are usually found living in or on their food, which can be a dead orliving organism.

Fungi reproduce by producing spores asexually or sexually. These are dispersedby the wind to new environments. Common fungi are:• moulds;• yeasts;• mushrooms and toadstools.

4


Figure 1.6 Amoeba proteus (× 200).

cytoplasm

contractile vacuole

food vacuoles

nucleus cell membrane

pseudopodia

Figure 1.7 The structure of Amoeba.

cytoplasmlight-sensitive

spot

chloroplast

nucleus

starch storage

flagella

T

Figure 1.8 A photosynthetic alga.Note the presence of the chloroplast,where photosynthesis takes place.

Figure 1.10 Penicillin spores aremade in sexual reproduction.

ITQ5

Using one named example of each, describeone similarity and one difference betweenalgae and protozoans.

insoluble food

solublefood

enzymes

vesicles releaseenzyme and

food is digested

hypha offungus

spore body

myceliumabsorbed into fungus

Figure 1.9 The hyphae of fungi extend into their food. Digestion occurs outsidethe body.

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Importance of fungi to Man• Important in the making of the antibiotic penicillin.• Essential to many fermentation processes, such as bread, wine, beer and other

alcoholic beverages.• Used to make a range of chemical products, such as anaesthetics, birth control

pills and meat tenderiser.• Moulds and rusts are fungi that are important in damaging growing crops.• Cause of spoilage of food.• Used to make food, such as sufu of East Asia.

Plants (Plantae)The plant kingdom includes mosses, liverworts, ferns, conifers and floweringplants. Almost all plants are photosynthetic.

Some plants can be used as medicines. Bidens is a weed which has a smalldaisy-like flower. The leaves and flowers are steeped and used to ‘cool the blood’(prickly heat) and to relieve a sick stomach. Sometimes it is given to children tocure worms.

Flowering plantsThe flowering plants have true flowers and so make seeds. They are also calledangiosperms and are divided into two groups:• the monocotyledons;• the dicotyledons.

Table 1.1 shows the distinguishing features of monocotyledons and dicotyle-dons.

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Figure 1.11 Yeast cells bud to makenew cells in asexual reproduction.

Figure 1.12 Mushrooms are thespore bodies of some fungi.

Table 1.1 Distinguishing features of monocotyledons and dicotyledons.

Figure 1.13 Bananas Figure 1.14 MangrovesFigure 1.15 Bidens – Shepherds needle,Spanish needle, Beggar-ticks, sticktight.

ITQ6

Name three kinds of fungi and a possible useof each.

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

ITQ7

(i) Plants range in size from unicellular togiant. Put these plants in order of sizestarting with the smallest. Fern, mangotree, croton, moss and lettuce.

(ii) List five reasons why plants are important.

angiosperms ©

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Angiosperms are the largest group of plants. They include most crop plants,ornamental plants and plants used as herbs or medicinal plants. They vary in sizefrom the very small to gigantic (over 90m tall). They can live in a wide variety ofhabitats, from deserts to rain-forests.

Animals (Animalia)The animal kingdom contains multicellular, heterotrophic organisms. They aregrouped into six phyla which are shown in figure 1.17.

Table 1.2 shows examples of each animal phylum.

Arthropods (Arthropoda)Arthropods dominate life on Earth. They include the crustaceans, millipedes, centipedes, arachnids and insects. They all have an exoskeleton (outer skeleton ofchitin) and jointed limbs.• The crustaceans are aquatic or live in damp places. They include woodlice,

crayfish, crabs, lobsters and barnacles.• The arachnids include spiders, scorpions, mites and ticks. They have four pairs

of walking legs and are mainly terrestrial and carnivorous.• The insects have a distinct head, thorax and abdomen, and three pairs of

walking legs. They include locusts, bees, ants, beetles, aphids and fleas.

Molluscs (Mollusca)The molluscs have a soft body which is often covered by a shell. They includeconch, snails, slugs, cockles, mussels, octopus, squid, clams and oysters.

Some molluscs like conch and oysters are important to Caribbean people as asource of food and an exotic treat to locals and tourists. Farming of molluscs ispracticed on some islands as demand exceeds supply from wild populations. Theseanimals are a renewable resource but populations can decline rapidly because ofover-harvesting from their natural habitat.

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Figure 1.16 Flame tree

Table 1.2 Examples of the animal phyla.

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

Chordata fish, amphibian, reptile, bird, mammal

Phyla is the plural of phylum.

Animalia

Cnidaria Platyhelminthes Mollusca Annelida Arthropoda Chordata

invertebrates vertebrates

T

Figure 1.17 Animals are placed in six phyla. (Those shown in colour aredescribed in more detail below.)

Figure 1.18 An invertebrate thatlives on land, a snail.

Figure 1.19 An invertebrate thatlives in water, a sea cucumber.

phylum ©

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Chordates (Chordata)Most chordates are also vertebrates because they have a vertebral column. Thevertebrates include the fishes (cartilaginous and bony), amphibians, reptiles, birdsand mammals.

Birds (Aves) have the following characteristic features:• front pair of limbs modified to form wings;• skin covered with feathers;• produce eggs (reproduction);• are warm-blooded.

Mammals (Mammalia) have the following characteristics:• four limbs;• skin covered with hair;• most give birth to live young;• feed their young with milk made by the mother (suckle);• are warm-blooded.

Classification of organisms on the basis ofvisible characteristicsThe simplest way to classify organisms is according to similarities in their visiblecharacteristics. For example, if we see a number of organisms, we could start togroup them by putting those with wings together. We can make another group ofthose with eight legs. We could also put the hairy ones together. And so on.However, where do we put those that are both hairy and winged?

There are two types of classification, artificial and natural. Artificial classifica-tion is based on easily observed characteristics, like colour, shape or number oflegs. This is a convenient and easy method of grouping organisms and is designed

7


Figure 1.20 There arefive groups of vertebrates:fish, amphibians, reptiles,birds and mammals. birds monkey (mammal)lizard (reptile)

fish frog (amphibian)

ITQ8

Name the five groups of vertebrates, givingtwo examples of each.

artificial classification ©

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for a practical purpose. However, worms and snakes have the same shape, butsnakes have a backbone while worms do not.

Natural classification tries to use natural relationships between organismsusing both internal and external characteristics. For example, organisms withbackbones are grouped together because they all have backbones and many othersimilarities. Similarities in anatomy, physiology and behaviour may all be consid-ered when grouping organisms in a natural classification.

The binomial systemCarl Linnaeus was a scientist in the eighteenth century who first grouped organ-isms together by a natural classification. Many people had tried grouping organ-isms before, but they had all used artificial classifications. Linnaeus’ classificationmade it easier to study organisms, since the enormous variety is organised intoclosely related groups.

Carl Linnaeus also put forward a system for naming each species of organismwith a biological name, which is called the binomial system. He did this becauseorganisms may have many common names. For example the plant called shadowbenny, bandania and calantro in Trinidad and Tobago is called sit weed or spiritweed in Jamaica, and in Martinique and Guadeloupe is known as bandanie. Eachbiological name has two parts which are the same in all these countries and allover the world – the biological name for the plant is Erynzium soetidum. The firstword of this name always starts with a capital letter. If you are writing it severaltimes, the first word may be shortened. For example Erynzium soetidum may beabbreviated to E. soetidum.

Every known species has a place in this classification. It starts with majorgroups of general features, which are broken down into smaller and smallergroups that get more and more specific. Look at the example of the classificationof Man in figure 1.21.

We belong in the kingdom Animalia because we are multicellular and heterotrophic. We belong in the phylum Chordata and the sub-phylum Vertebratabecause we have a backbone. We are in the class Mammalia because we have hair,are warm-blooded and suckle our young. We are then grouped in the orderPrimates with all the other monkeys and apes. We belong to the family Hominidaewhich are the human-like apes. In the past this family has 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 the past, for example Homo erectus.However that species is separated from the modern Homo sapiens because they

8


ITQ9

Classify these organisms according tosimilarities in their visible characteristics intothree groups.

binomial system ©

natural classification ©

Genera is the plural of genus.

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had more body hair and a smaller brain. All people today belong to the speciesHomo sapiens because they all have the same characteristics.

Table 1.3 shows how the ocelot starts in the same large groups as Man but isplaced in a different group from the level of Order down. It is grouped with all theother kinds of cat.

9


Living organismsPlaced in five main groups (kingdoms)

Kingdom

Phylum

Sub-phylum

Class

Order

Family

Genus

Species

Prokaryotes Protoctista Fungi Plantae Animalia

Annelida Arthropoda

invertebrates

Chordata

Vertebrata(possess a vertebral column)

Reptilia(reptiles)

Aves(birds)

Mammalia(hairy, warm-blooded, suckle young)

Carnivora Primates(monkeys)

Hominidae(human-like apes)

erectus(well-developed brain)

Figure 1.21 The classification of Man.

Table 1.3 Classification of Man and ocelot.

Classification group Man 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

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SpeciesA species is defined as ‘a group of individuals of common ancestry that closelyresemble each other and are normally capable of interbreeding to produce fertileoffspring’. This means that they have similar characteristics and are capable ofmating with each other.

The process by which species are formed is called speciation and this occurswhen groups of a population become isolated in some way. For example someindividuals of a species may get separated from the rest such as by an ocean,mountain or desert. This is known as geographical isolation.

They may have to feed on different materials, and may be subjected to differ-ent environmental conditions compared with the rest of species. After a while,they may evolve to look and behave very differently from the rest of the species.If they came back together again they may not be able to reproduce with them.This is called reproductive isolation. The two groups may be physically able toreproduce, but have different courting behaviours and a completely different mating season. They are thus never able to mate with each other and produce offspring. So they will be considered different species.

Sometimes, closely related species interbreed to produce infertile offspring. Across between a horse and a donkey produces a mule, which is infertile and cannot make other mules. The horse and donkey are separate species, thoughclosely related.

Each species has its own special structural, behavioural and ecological charac-teristics.

Within a species though, individuals may look very different, but can inter-breed.• In animals, these are called breeds. For example, there are many different breeds

of dogs, which vary in size, shape, hairiness and so on. However, despite differ-ences in appearance, they can interbreed and all belong to the same species.

• In plants these variations of the species are called varieties. For example, thereare many different varieties of corn. The different varieties of plants differ inmany ways but pollination between them is possible.

• In humans, there are physical differences in colour, hair type and body struc-ture. The different groups are called races. They can still interbreed so all theraces belong to the species Homo sapiens.

Þ A huge variety of living forms exist on planet Earth.Þ All living organisms show the seven characteristics of life.Þ The characteristics are growth, respiration, irritability, movement, nutrition,

excretion and reproduction.Þ Living orgamisms are grouped into five kingdoms: prokaryotes, protoctists,

fungi, plants and animals.Þ The prokaryotes are bacteria.Þ The protoctists include algae and protozoa.Þ The fungi include yeasts and toadstools.Þ The plants are mostly photosynthetic (make their own food).Þ The animals need to get their food by eating plants or other animals. Þ The six phyla of animals are cnidarians, platyhelminths, molluscs, annelids,

arthropods and chordates.Þ The chordates include fish, amphibia, reptiles, birds and mammals.Þ Each major group or phylum is broken down into smaller groups.Þ Organisms can be classified according to similarities in their visible

characteristics.

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species ©

speciation ©

geographical isolation ©

reproductive isolation ©

breeds ©

varieties ©

races ©

ITQ10

Describe a way in which a group of organismscan be divided into two separate groups thatare incapable of mating with each other(species).

Summary

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Þ Each species has a common name and a scientific name.Þ A species is a group of similar organisms that can interbreed.Þ In animals a species can be sub-divided into breeds.Þ Variations in a species of plants are called varieties.Þ In humans the different groups of Homo sapiens are races.

ITQ1 The presence of water, suitable temperature range, the presence of gasesin the atmosphere, like oxygen and carbon dioxide.

ITQ2 (i) Most plants are able to make their own food in a process calledphotosynthesis.

(ii) A plant responds by growing towards light from the environment.

ITQ3 Prokaryotes (bacteria), protoctists (algae and protozoans), fungi (moulds,yeasts and mushrooms), plants (mosses, liverworts, ferns, conifers andflowering plants), animals (invertebrates and vertebrates).

ITQ4 Microscopic means cannot be seen with the eyes without the use of amicroscope because they are so small. Unicellular means made up of onecell. A bacterium is a single cell which can carry out all the processes of life.

ITQ5 Algae: Chlorella; protozoan: Amoeba. Both organisms 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 areprokaryotes.)

A difference between Chlorella and Amoeba is that Chlorella has achloroplast and is able to photosynthesise or make its own food, whileAmoeba cannot photosynthesise and must feed on other organisms.

ITQ6 Yeast: to make bread. Mushrooms: for food. Moulds: to make theantibiotic penicillin.

ITQ7 (i) Moss, lettuce, fern, croton and mango tree.(ii) To produce oxygen which is needed by animals for respiration.

To be used as a food source.For medicinal purposes (herbs).To hold topsoil in place.To 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 otherexamples.)

ITQ9 Two pairs of wings, three pairs of legs, body divided into three parts.

ITQ10 Members of the group could move to another area and develop adifferent mating season, i.e. a different time of the year whenreproduction occurs. If the two groups were to meet again they wouldnot be able to mate with each other because each group has its ownmating season which it responds to. It may be physically possible for thetwo groups to mate with each other, but they are incapable of matingwith each other because they mate at different times.

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Answers to ITQs

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Examination-stylequestions

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1 (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 fivegroups 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 micro-organisms to Man.

3 (i) The evolution of life on Earth is believed to include a move fromliving in water where life started, to living on land.(a) In what ways are conditions on land different from conditions in

the environment of water?(b) Describe two ways organisms have become adapted for living on

land.(ii) 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.

(iii) Humans are said to be closely related to chimpanzees.(a) Explain why this is so by comparing visible differences and

similarities between the two.(b) Are there any similarities in their behaviour? Explain fully.

4 (i) Define:(a) species;(b) speciation.

(ii) Explain how geographic isolation and reproductive isolation can leadto the formation of a new species.

(iii) Using examples, explain each of the following terms:(a) breed;(b) race;(c) variety.

5 (i) List two features common to the organisms shown on page 13.(ii) Using each feature, classify the organisms. List the members of each

group.

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13


A D B

E

F

C

GI H

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Life depends on photosynthesis which is carried out by plants (chapter 6). Mostanimals get their nutrients, or food, which is their energy source either directly orindirectly from plants. Plants photosynthesise or make food from water and carbon dioxide, using light energy from the Sun to carry out the process. So theSun is the ultimate source of energy for almost all life on Earth.

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Feed ing re la t ionsh ips betweenorgan ismsú 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;ú explain the role of decomposers;ú understand the terms herbivore, carnivore and omnivore;ú identify a food chain;ú identify predator/prey relationships;ú construct a food web that includes different trophic levels;ú understand that special relationships exist and discuss the advantages

and disadvantages of such relationships.



2

Around deep-ocean hot water vents, there arebacteria which get their nutrients and energyfrom the water. These bacteria are the food foranimals, and these food chains are the onlyones we know on Earth which do not depend onthe Sun for their energy.

decomposers

symbiosis – relationships between organisms of

different species

parasitismcommensalism

mutualismpredator/prey

food chain

food web – interlinkingof food chains

firsttrophic level

producer

plants

second trophic level

primary consumer

herbivore

third trophic level

secondary consumer

carnivore

fourthtrophic level

tertiaryconsumer

carnivore

Concept map

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Decomposers and detritivoresAll living organisms eventually die. Their bodies are composed of complexcompounds like carbohydrates, lipids and proteins that they stored when they werealive. Two groups of organisms called the decomposers and detritivores obtain theirfood or energy from the remains of the dead organisms. As they feed on the deadorganisms they cause their decay or decomposition. They help in the recycling ofnutrients (chapter 4) since they return the nutrients trapped in the dead organismsback to the environment. The nutrients become available again to living organisms.

Decomposers include bacteria and fungi. They secrete enzymes which breakdown dead plants and animal material into a substance called humus. Humusenriches and improves the structure of soils in which plants grow and from which

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2 · Feeding relationships between organisms

producer ©consumer ©

producerplant

consumercaterpillar

consumersmall bird

decomposersreturn nutrients to the soil

in the form of humus

producer consumer etc ......

nutrients (humus) madeavailable by decomposers

Sun they all die and their bodies are eaten

Figure 2.1 The relationship between producers, consumers and decomposers.

ammonium compounds in the soil

Dead organismcomplex compounds

(proteins, lipids, carbohydrates, etc.)

fungi and bacteriasimple substances

(carbon dioxide (CO2), compounds of ammonia (NH3) from the proteins)

carbon dioxide (CO2) released into the air as the fungi and bacteria respire

SOIL

fungi and bacteria live in the dead organism

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

Figure 2.3 A dead organism decaysor decomposes as fungi and bacteria'feed' on it.

decomposer ©detritivore ©

humus ©

Figure 2.2 Mould (a fungus) feedingon dead fruit.

ITQ1

Define the terms producer, consumer anddecomposer and give two named examples ofeach.

Phytoplankton are microscopic organisms, likealgae and blue-green bacteria, that live in theoceans. They are important since they startfood chains in the world’s oceans or seas.

Producers and consumersPlants are called producers because they ‘produce’ or make their own food. Theyinclude mosses and green plants on land, and algae, aquatic plants and phytoplankton in water. Organisms that consume the plants or producers, mainly the animals, are called consumers.

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they derive nutrients. Imagine the build-up of dead plants and animals on theEarth’s surface if there were no decomposers! All the vital chemical elements ornutrients trapped in these dead organisms would also not be able to return to living organisms or be recycled.

Detritivores also help in the removal and recycling of dead organisms 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 organicmaterial, so decomposers and detritivores are all saprophytes.

Herbivores, carnivores and omnivoresHerbivores are organisms that feed on plants only. Examples are some insects (likegrasshoppers, locusts, butterflies, bees), some birds (such as seed- and fruit-eatingbirds) and some mammals (cows, horses, elephants, giraffes). In water, herbivoresmay be very large like the manatee or very small like shrimp.

Carnivores are organisms that feed on animals only. They may hunt and killother 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).

Omnivores feed on both plants and animals. Examples are pigs and humans.

Food chainsA 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 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. Thesmall bird and the hawk are also consumers because they are getting their food orenergy 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 smallbird as it feeds on the caterpillar and finally to the hawk. So all the consumers inthe 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 theherbivores. An omnivore may feed on the producer or herbivore (and even carnivore in some cases).

producer →herbivore → carnivore(grass) (chicken) (mongoose)

↓omnivore(Man)

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 → secondary consumer → tertiary consumer

Example: waterweed → tadpoles → small fish → bigger fishproducer primary secondary tertiary

(1°ry) (2°ry) (3°ry)consumer consumer consumer

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food chain ©

The arrow shows the movement of energyalong the food chain.

saprophyte ©

ITQ2

Draw a diagram to show the feedingrelationship between a producer, a consumerand a decomposer using examples from youranswer to ITQ1.

herbivore ©

carnivore ©

omnivore ©

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Each organism in the food chain represents a trophic level. The three foodchains below each consist of four trophic levels.

Food chain I leaf → caterpillar → toad → snake

Food chain IIgrass → grasshopper → insect-eating bird → hawk

Food chain IIIalgae → snail → leech → fish

Table 2.1 shows how the organisms of these three different food chains can beclassified.

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A food chain is composed of trophic levels.

These are examples of terrestrial food chains.

This is an example of an aquatic food chain.

trophic level ©

Table 2.1 Different ways to classify organisms in food chains.

Food chain I Food chain II Food chain III Type of feeder Consumer level Trophic level

leaf grass algae producer producer first trophic level↓ ↓ ↓

caterpillar grasshopper snail herbivore primary consumer second trophic level↓ ↓ ↓

toad insect-eating bird leech carnivore secondary consumer third trophic level↓ ↓ ↓

snake hawk fish carnivore tertiary consumer fourth trophic level

predator ©prey ©

Table 2.2 Predator/preyrelationships in the rosebush foodchain.

Prey Predator

aphid → ladybird

ladybird → spider

spider → bird

food web ©

All food chains have certain characteristics in common, as seen in table 2.1.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(chapter 3).

Predators and preyAnimals also show predator/prey relationships. Predators are carnivores that feedon other animals that are called their prey. Predators hunt, capture, kill and eatother animals and those that are hunted and eaten are the prey. Food chains willtherefore include predators. They are the higher order consumers.

rosebush → aphid → ladybird → spider → insectivorous bird

In this food chain, whilst the spider is a predator because it kills and eats theladybird, it is also prey to the insectivorous bird. The food chain shows three predator/prey relationships as seen in table 2.2.

Animals that are prey have evolved to hide and escape predators, using characteristics such as camouflage, mimicry and speed. Predators, on the otherhand, have evolved characteristics to improve their chances of catching prey, likespeed, lures and traps.

When all these organisms die, decomposers return their nutrients to the plantsthrough the soil, and the nutrients return to other feeding animals in the foodchains.

Food websA food chain shows one organism feeding on one other organism only, but feedingrelationships are more complex than this. One organism may feed on a number oforganisms and in turn may be eaten by a number of organisms. The interlinking ofa number of food chains is called a food web (see figures 2.4 and 2.5).

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Special relationshipsThe environment supports a host of organisms all living together. In the previoussections we have looked at the usual feeding relationships. However, some organ-isms live in special relationships with each other. These relationships may beadvantageous to all the organisms involved but, sometimes, one organism cancause harm to another.

Symbiosis describes any relationship that exists when different species oforganisms live together. There are three types of symbiosis:• mutualism;• commensalism;• parasitism.

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warbine

coscorob

water beetle

duck

mayfly nymph

water boatman water-flea

pond weed algae

T

Figure 2.5 A freshwater (aquatic) food web.

symbiosis ©

mutualism ©

ITQ3

From the food web shown in figure 2.4:(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.

hawk

snake mongoose

frog kiskedee(bird)

tarantula(spider)

rat

ladybird hummingbird

hibiscus plant mango tree grass

T

beetle caterpillar grasshopper snailaphid butterfly

Figure 2.4 A terrestrial food web.

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MutualismIn this kind of association two organisms of different species live closely togetherand both benefit. For example:• 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 crabgains protection from predators as it is camouflaged by the anemone and protected from predators by the stinging tentacles.

• leguminous plants and the bacterium Rhizobium (chapter 4): The bacteria liveinside swellings on the roots of leguminous plants, like peas and beans. Thesebacteria convert nitrogen gas into ammonia, which is then converted intoamino acids and used by the plants for growth. The plants benefit because theycan thrive in all types of soil, even soil where nitrate is in short supply. Thebacteria also benefit by having a place to live and an energy supply which theyget from the plant.

• egret and cow: The egret perches on the cow’s back as it feeds on insects, espe-cially ticks, that can harm the cow. The egret is obtaining food and the cow benefits by having blood-sucking insects removed from its body.

CommensalismThis is a relationship between two species in which one clearly benefits and theother is not harmed. For example:• some orchids or ferns on trees: The orchids or ferns are small plants that grow

high on the tree to obtain sunlight for photosynthesis. They use the tree for support but not 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 on insects that flyup as the cow shakes the grass while it walks. The egret benefits but the cowdoes not.

• shark and remora: The remora attaches itself to the shark and moves aroundwith it. As the shark feeds, the remora also feeds on scraps of food that are floating around. The remora obtains food while the shark is not harmed, nordoes it benefit.

ParasitismA parasite is an organism which lives and feeds on or inside another organism,which is called the host. The parasite gains while the host is harmed.• Parasites which live on the outer surface of their hosts are called ectoparasites.

For example, ticks, fleas and leeches feed on the blood of their hosts such asdogs, cattle and fish.

• Parasites that live within a host are called endoparasites. An example in Man isthe organism which causes the disease malaria. A protozoan of the speciesPlasmodium enters the bloodstream of a human through the bite of an infectedfemale Anopheles mosquito. Once in the body, the parasite multiplies, causingbouts of fever, pain, shivering and sweating. Millions of people die each yearfrom malaria, although anti-malarial drugs like quinine and choroquinine havebeen developed.

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parasitism ©ectoparasite ©

endoparasite ©

Figure 2.6 A hermit crab and seaanemone.

commensalism ©

Figure 2.7 An orchid growing on atree.

Figure 2.8 A leech sucks blood froma human.

ITQ4

Using named examples, distinguish betweenmutualism and commensalism.

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Þ The Sun is the ultimate source of energy for most life on Earth.Þ Plants make food and are called producers.Þ Animals eat plants and are called consumers.Þ Decomposers feed on dead plants and animals.Þ A diagram which shows the sequence in which organisms feed on each other

is called a food chain.Þ A food web shows the interlinking of a number of food chains.Þ Herbivores feed on plants alone.Þ Carnivores feed on animals alone.Þ Omnivores feed on both plants and animals.Þ Symbiosis describes relationships between two different species.Þ Mutualism describes a relationship where both species benefit.Þ Commensalism is seen when one species benefits and the other is not

harmed neither does it benefit.Þ In a parasitic relationship one species benefits at the expense of the other.

ITQ1 A producer is an organism that produces or makes organic food. A plantmakes organic food during photosynthesis, so any plant is a producer.Examples are mango tree, hibiscus plant, but you may have thought ofmany others.

A consumer is an organism that eats or consumes organic food.Animals cannot make their own food, so they are consumers. Examplesare caterpillar, Man.

A decomposer is an organism that feeds on dead organic food (deadanimals and plants). The food is said to be decaying or rotting as thedecomposer feeds on it. Examples are bacteria, fungi.

ITQ2 hibiscus plant → caterpillar

bacteria

ITQ3 (i) (a) You could have chosen: aphid, butterfly, beetle, hummingbird,caterpillar, grasshopper or snail.

(b) You could have chosen: ladybird, frog, kiskedee, tarantula, rat,snake, mongoose or hawk.

(ii) (a) Aphid, butterfly, beetle, hummingbird, caterpillar, grasshopper orsnail

(b) Ladybird, frog, kiskedee, tarantula, rat(c) Hibiscus, mango, grass(d) Frog, snake, mongoose, hawk(e) Frog, hawk

(iii) (a) Ladybird, frog, kiskedee, tarantula, rat, snake, mongoose, hawk(b) snake, frog, ladybird, kiskedee, tarantula, rat, hummingbird,

aphid, beetle, caterpillar, grasshopper, snail(iv) (a) Hibiscus, mango, grass

(b) Ladybird, hawk, kiskedee, frog, tarantula, ratITQ4 Mutualism and commensalism are both relationships between two

species or partners that are beneficial or good. In mutualism, bothpartners benefit. In commensalism, one partner benefits while the other,though not benefiting from the relationship, is not harmed in any way.

An example of mutualism is between the pigeon pea plant(leguminous plant) and Rhizobium bacteria that live in swellings of itsroots. The pigeon pea plant gets amino acids for growth, and the bacteriaobtain shelter and energy.

An example of commensalism is seen with sharks and remora fish. Theremora fish obtain food and protection from the shark which benefitsnothing from the relationship and is also not harmed.

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Summary

Answers to ITQs

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1 (i) Construct a food web from the information given in the table.

(ii) Examine the food web constructed and describe three consequencesof 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 anddescribe:(a) how the predator is adapted to catch its prey;(b) any feature used by the prey to escape the predator.

2 (i) Using named examples, describe a:(a) parasite relationship;(b) mutualistic relationship.

(ii) (a) Draw a food chain with four trophic levels. (Use namedorganisms.)

(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?

(iii) Which organism in the food chain is a:(a) herbivore?(b) carnivore?(c) predator?(d) prey?

(iv) Describe the role of the decomposers in the food chain.(v) Copy the table below and use examples from these food chains to

complete it.root → earthworm → frog → foxpondweed → mayfly nymph → water beetle

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 (Ixora)

Stages of food chain Two examples of organisms, one from each food chain

producer

primary consumer

predator

prey

herbivore

second trophic level

third trophic level

first trophic level

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All living organisms need energy to carry out life processes; for example your bodyuses energy to grow, move, inhale and eat. The energy that your body is usingcame from your food. If you made a food web for everything you eat, you wouldfind that all the energy you use was trapped by plants from the Sun. Ultimatelyall energy for life comes from the Sun.

Trapping the Sun’s energyPlants use the Sun’s energy to make food during photosynthesis (chapter 6).During photosynthesis carbon dioxide and water are combined to make glucoseand oxygen.

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Energy f low wi th in a food cha in or food webú understand that the Sun is the ultimate source of energy for life on

Earth;ú explain why food is the source of energy needed by living organisms;ú understand that respiration is the process by which energy is

released from food;ú describe pyramids of energy;ú describe pyramids of numbers;ú describe pyramids of biomass.



3

photosynthesis ©

energy lost due to

respiration,in urine and

faeces

importance of photosynthesisto food chains

food – source of energy for all organisms

food chain

some energypassed on

some energypassed on

animal eats and obtainsfood (chemical energy)

plant makes food usinglight energy from Sun

feeding pyramids:• energy• numbers• biomass

animal

Concept map

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The glucose is then used to make carbohydrates, lipids and proteins and every-thing else the plant needs. These become the components of food (chapter 7) forconsumers. The term ‘food’ can thus be used for the term ‘energy’, since energyis obtained from food.

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) thenpasses on to the consumers as they feed on the plants.

Food (usually glucose) is ‘burnt’ during respiration by plants and animals torelease energy so that they can carry out all the processes necessary for life.

glucose + oxygen → energy + carbon dioxide + water

Respiration releases the energy trapped in the food so that it can be used bythe organism. Respiration also makes carbon dioxide and water.

How a plant gains and loses energy• A plant gains energy when 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 life processes. Some of the energy that is released is lost as heatenergy from the plant.

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3 · Energy flow within a food chain or food web

respiration ©

energy from the Sun

carbon dioxide + water oxygen + glucose

Plants(photosynthesis)

makefood/chemical energy

energy fromplants passed to

Animals(when they feed on plants)During RESPIRATION this

energy is made available to be used for everyday activities.

energy from Sun passed to

Sun

T

ITQ1

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

energytaken in

energystored

Rest of energystored in plant tissues.

Passed on to herbivoreswhen they feed on plant

light energy

Sun

energylost

Some energy changedto heat during respiration,

for life processes.Heat lost to the environment

Figure 3.2 Only some of the energy taken up by a plant can be passed on to aherbivore.

heat loss in respiration

urine

urine andfaeces

Energy passed on to carnivore

when it eats the cow

Energytaken in

food eatenEnergy stored in

the body of the animal

Energy loss

Energy loss

Energy loss

Figure 3.3 Only some of the energy that an animal gains through eating can bepassed on to a predator.

ITQ2

What happens to the energy that a plant gainsduring photosynthesis?

Figure 3.1 Energy from the Sun isused by plants and by animals.

033380368X.Text.qxd 1/8/06 23:08 Page 23

biomass ©

productivity ©

How an animal gains and loses energyFor each animal at each trophic level:• energy is gained as the organism feeds;• 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 released during respiration for the organism to stay

alive and some of that energy is lost as heat to the environment.

Movement of energy through a food chainEnergy flow through a food chain or web is related to the movement of foodthrough the chain. Figure 3.4 shows the movement of energy through a foodchain.

The diagram shows that energy is lost at every step in the food chain. Thismeans there is less energy at each level for the animals in that level than in thelevel below. The length of a food chain will be limited by the energy loss at eachlevel. There will come a point when there is not enough energy in one level tosupport another level. There are usually not more than five steps in any foodchain.

When the plants and animals die, the energy stored in the dead bodies will bepassed on to the detritivores and decomposers as they feed. They also feed on theurine and faeces made by animals.

Energy is not recycled, it moves through and out of the food chains. Energyenters a food chain as light energy from the Sun, and is lost from every trophiclevel as heat energy to the environment. Its flow is non-cyclical, which means thatthe energy cannot be returned to a living organism.

The length of a food chain depends on the energy in the biomass available ateach level. Ultimately this depends on how much energy is being trapped by theproducers in the chain (their productivity). If the whole ecosystem is highlyproductive, then the food chains will be longer because there will be more energyentering at the producer level of the chain. If there is only a small amount of

24


Unlike energy, the elements of which organismsare made, such as carbon and nitrogen, arerecycled (chapter 4).

ITQ3

What happens to the energy that an animalobtains?

Energy lost asheat due torespiration




PLANT CARNIVORE orTERTIARY

CONSUMER

CARNIVORE orSECONDARY CONSUMER

HERBIVORE orPRIMARY

CONSUMER

Energy storedin tissue

Energy lossin urine

and faeces

Energy lossin urine

and faeces

Energy lossin urine

and faeces

Sun




T

Figure 3.4 Movement of energy through a food chain.

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energy being trapped by the producers, then they can only support fewer trophiclevels (figure 3.5). Ecosystems in equatorial regions are generally more productivethan those in higher latitudes because they get more light.

Crop plants are mass harvested for human consumption. If these plants are eatendirectly by humans, a lot more energy can be obtained than if they were fed toother animals first and then the animals eaten by humans.

Efficient use of food chain for energy by Man.

Inefficient use of food chain: a lot of energy is lost that could be available to Man.

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Productivity ofecosystem

is high

Productivity ofecosystem

is low

energy lossenergy loss

energy loss

energy lossenergy loss

plant

plant

T

Figure 3.5 The productivity of the producers in an ecosystem limits the lengthof food chains that can be supported.

Figure 3.6 (a) Ecosystem of highproductivity. (b) Ecosystem of lowproductivity.

ITQ4

How is energy transferred through a foodchain?

ITQ5

What is the importance of the process ofrespiration in a food chain?

energy loss

energy

energy loss

energyenergy

T

(b)

(a)

energy loss

energyenergy

MANenergy

T

energy loss

energy

energy loss

energy

energy OTHERANIMAL

MAN

T

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Pyramids of energyA pyramid of energy is a good way of showing the energy relationships betweenorganisms in different trophic levels. Figure 3.5 shows the pyramid of energy fora simple food chain. Each block in the pyramid shows the amount of energy avail-able to the next trophic level.

Using figure 3.7 as an example, 90 000 units of energy are available to thegrasshoppers. The grasshoppers consume that energy as food and lose some of itto the environment as heat during respiration and activity, and some of it asfaeces. That leaves only 15 000 units for the insect-eating birds. The birds consumethat energy and lose some of it to the environment in faeces and as heat. So only2000 units are available to the next level, the cats. The cats lose energy to theenviroment 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 trophiclevels in this chain.

Pyramid of numbersA pyramid of numbers is like a pyramid of energy but shows the numbers of allthe organisms at each tropic level of a food chain within a given area. Look at thepyramid in figure 3.8. The pyramid shows that, within the area being studied,there were 80 leaves. On these leaves 8 caterpillars were feeding. Two birds wereseen feeding on the caterpillars and one cat ate both birds.

Ecosystems usually contain a large number of small organisms and a smallernumber of large animals. Predators are usually larger than their prey and must eata number of them to stay alive.

26


pyramid of energy ©

pyramid of numbers ©

Figure 3.7 A pyramid of energy shows that less and less energy is available to higher trophic levels in a food chain.

Figure 3.8 A pyramid of numbers is obtained by counting all the individuals at each trophic level.

A pyramid – each block getssmaller as you go up Pyramid of energy

90 000units of energy

PRODUCER

PRIMARYCONSUMER

TERTIARYCONSUMER

100units of energy

2000units of energy

15 000units of energy

grass grasshopper bird cat

SECONDARYCONSUMER

cat

10 leaves

10 leaves

10 leaves

10 leaves

10 leaves

10 leaves

10 leaves

10 leaves

grasshopper

grasshopper

grasshopper

grasshopper

grasshopper

grasshopper

grasshopper

grasshopper

Each grasshopper eats10 leaves each day

Each bird eats4 grasshoppers a day

bird

birdcat

grasshopper

leaves

Eats 2 birdsa day

T

bird

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With this type of ecological pyramid, no allowance is made for the size of theorganism. Each cat and each caterpillar is each counted as one. So sometimes wecan see different shapes in pyramids of numbers (figure 3.9). One tree may beeaten by many caterpillars, though we could have counted each leaf separately toget a ‘normal’ pyramid shape. One dog is host to many ticks, and each tick mayhave several parasites, but in this case each ‘predator’ is actually smaller than its‘prey’.

Pyramids of biomassInstead of estimating the numbers of organisms at each trophic level we canestimate their biomass or dry weight (chapter 17). From this we can construct apyramid 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 eachtrophic level.

At the start of the grazing food chain in figure 3.10 is a large biomass of greenleaves. The pyramid shows that a large amount of plant material supports asmaller mass of herbivores and an even smaller mass of carnivores.

BioaccumulationPesticides can spread through the environment in a food chain. Pesticides (such asfungicides, herbicides and insecticides) are chemicals that are toxic to some organ-isms. They work in one of two ways, on contact or once the chemical has enteredthe organism. For example, a grasshopper feeding on plants sprayed with insecti-cide will only need to take in a small amount to kill it.

This can harm other animals in the food chain. For example, a bird feeding onthe grasshoppers will accumulate in its body all the insecticide that the grass-hoppers have ingested. So the bird may end up with levels of insecticide highenough to poison it or harm it in some way. A hawk or other predator feeding onthe small birds could end up with even higher levels of pesticide in its body, againenough to poison or harm it. This is called bioaccumulation or biological magnification.

A well-known example is DDT (dichlorodiphenyl-trichloroethane) which is avery effective insecticide that was used in many countries in the 1950s and 1960sto control mosquitoes which carry malaria, and to control other insect pests.However, DDT is stored in fatty tissue so predators absorb the chemical when theyeat prey that contains it. Levels of DDT that accumulate in the bodies of top pred-ators may be enough to kill them or to harm them in other ways. In a study ofospreys (North American birds) adult birds were found to contain 8 million timesmore DDT than organisms at the bottom of the food chain. These high concen-trations did not kill the birds, but caused the females to lay eggs with very thinshells. Many eggs broke and so numbers of these birds dropped rapidly. Since1972 the use of DDT has been banned in many countries.

27


pyramid of biomass ©

Figure 3.9 Some ‘pyramids’ of numbers are of different shapes.

Figure 3.10 A pyramid of biomass.

tree

smallbird

hawk

dog

ticks

parasiteson ticks

T

caterpillar

mass oftertiary

consumers

mass ofsecondaryconsumers

mass ofprimary

consumers

mass of producers

T

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Þ Energy from the Sun is used by plants to make food during photosynthesis.Þ The equation for photosynthesis is:

carbon dioxide + water + light energy → glucose (food) + oxygen.Þ The energy that is stored in a plant is passed on to other organisms when

they feed on the plant.Þ Respiration releases energy in plants and animals to do all they need to stay

alive and grow.Þ The equation for respiration is:

food (glucose) + oxygen → energy + carbon dioxide + water.Þ Most of the energy released in respiration is lost as heat to the environment

and cannot be passed on to the next trophic level.Þ A pyramid of energy shows that less and less energy is passed on to the

higher trophic levels of a food chain.Þ A pyramid of numbers shows the number of organisms found in each

trophic level of a food chain. Þ If the dry mass of the organisms at each trophic level of a food chain is

measured, a pyramid of biomass can be produced.

ITQ1 The energy from the Sun is used by plants or producers to make organicfood that is used directly and indirectly by all animals, including Man.Without the Sun, plants would die so there would be no food for theanimals. They would also die and life, as we know it, would cease to exist.

ITQ2 A plant stores some of the energy in its tissues as it grows and uses someenergy to stay alive. This energy is lost as heat energy. Some energy isthus lost to the environment and some is kept in its body.

ITQ3 An animal uses some of the energy in respiration to stay alive. Much ofthis energy is lost to the environment as heat. The animal may use up

28


Summary

Answers to ITQs

Figure 3.11 Pesticides like DDTaccumulate in the tissues of eachtrophic level of a food chain.

DDT accumulatesin the top consumers

herbivore eatsphytoplankton andaccumulates DDT

DDT entersphytoplankton

Top predator

Consumer

Herbivore

124 ppm DDT

5 ppm DDT

1 ppm DDT

Producer 0.0025 ppm DDT

run-off from agricultural landcarries a dilute solution of

pesticides e.g. DDT

T

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1

(i) Copy the diagram above and, using arrows, annotate it to show themovement of energy into and out of each organism.

(ii) What is the importance of the following in a food chain:(a) respiration?(b) photosynthesis?(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 Acow → tick → egret

Food chain Bgrass → grasshopper → lizard

(i) Construct possible pyramids of numbers for food chains A and B. Ineach 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.

(iii) Look at the pyramid of energy above. Why do leaves contain thegreatest amount of energy?

(iv) What happens to the energy that is not passed on to thegrasshoppers?

(v) What will happen to the cats if all the grasshoppers were killed bythe use of insecticide?


more energy than a plant since it is more active. It also stores up some ofthe energy as chemical energy in its tissues as it grows.

ITQ4 Energy is transferred from one trophic or feeding level to another whenan organism feeds. Energy is obtained in the form of food. The food isneeded for respiration which makes energy available to the organism. Soenergy moves through a food chain when the organisms eat.

ITQ5 Some of the energy that is released during respiration is heat which is lostto the environment from the organism. It is important in a food chainbecause at each level in the food chain energy is lost. Only a proportionof the energy entering one trophic level is stored in the organism’s bodyand is thus available to the next trophic level.

grass grasshopper bird

T

cat

bird

grasshopper

leaves

T

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30

The cyc l ing o f nut r ientsú explain the carbon cycle;ú understand what is meant by the greenhouse effect and

global warming;ú explain the importance of nitrogen to plants and animals;ú explain the nitrogen cycle;ú describe the causes and effects of acid rain.



4

Biogeochemical cyclesLiving organisms are made up of different kinds of atoms. The most commonatoms are carbon, hydrogen and oxygen, with nitrogen following closely behind.Smaller amounts of other atoms, such as iron, calcium and sodium, are also foundin living organisms.

These atoms bond together to form larger structures such as proteins, carbo-hydrates and lipids. These larger structures are then arranged in particular ways

global warming

leaching

greenhouseeffect

acid rain

biogeochemical cycles

atoms inplants

carbonhydrogenoxygennitrogenothers

atoms in theenvironment


atoms inanimals


Concept map

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31

4 · The cycling of nutrients

to make up all the tissues needed to build a living organism. All living organismsare in essence complex structures of organic molecules. If we look at a person, wesee skin, hair and nails – it is difficult to imagine that basically we are just atomsof carbon, hydrogen, oxygen and nitrogen.

As an animal grows from birth to adulthood, the growing tissues come fromthe food it eats. The animal increases its store of these atoms as it eats and muscle,bone and all the tissues that make up the organism increase in mass. Then, whenthe organism dies, the body is broken down or decomposed, and the atoms arereleased back into the environment.

The atoms become part of the soil as the organism’s decomposed body becomesmixed into the soil. They may then be taken up by plants and built into the plant’stissues as the plant absorbs them from the soil with water. These plants are theneaten by animals and the atoms thus become part of an animal once again.

The cycling processes by which these essential atoms are released and reusedin nature are called biogeochemical cycles. The carbon and nitrogen cycles areexamples of such cycles.

The carbon cycleThe carbon cycle shows how carbon atoms are passed from one organism toanother and to their environment as they live, breathe, eat, die and decay.1 The atmosphere contains about 0.03% 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 apart of the plant’s body.

2 Animals then obtain their supply of carbon by eating plants or other animalsthat have eaten plants.

3 As plants and animals respire, molecules of carbon dioxide are released backinto the atmosphere.

4 Waste materials from living organisms (like urine and faeces) and their deadbodies (all organisms die), are used as food sources by decomposers.Decomposers, like bacteria and fungi, feed on dead organic matter. Carbonatoms then become incorporated into the bodies of the decomposers.

5 Respiration of the decomposers releases carbon dioxide into the atmosphere.6 In waterlogged soils where oxygen was in short supply, decomposers were not

able to break down tissues completely in dead bodies and the remains

Equation for respiration:food (glucose) + oxygen →

energy + carbon dioxide + water

biogeochemical cycles ©

A carbon atom present in Einstein’s body couldvery well be present in your body right now!

ITQ1

(i) What atoms are living organisms made upof?

(ii) How do they obtain these components?(iii) What happens to these components after

the organism dies?(iv) What is a biogeochemical cycle?

Figure 4.1 The carbon cycle shown in diagrammatic form. (The numbers refer to the numbers in the text.)

Remember that carbon is found in carbondioxide (CO2), carbohydrates, lipids andproteins, since carbon is an integral part ofthose compounds.

carbon dioxide (CO2)in the air [0.035%]

organiccompoundsin animals

organiccompounds

in greenplants

organic compoundsin bacteriaand fungi

organiccompounds

in fossilfuels

1 photosynthesis respiration 7 combustion

5 respiration

4 death and decay

6 fossilisation

6 fossilisation

T

2 eaten by animals

4 death and decay

3 3

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accumulated. For example, in the Carboniferous period (about 290 millionyears ago) huge areas of waterlogged swamps covered many parts of the world.When the swamp plants died, partially decomposed plant material accumulatedand 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 animalsthat died in oceans. Fossil fuels contain a large proportion of carbon.

7 The burning of fossil fuels (combustion) releases carbon dioxide into theatmosphere.

And so the cycle continues, carbon in the atmosphere is taken up by plants,which are eaten by animals, and returned to the atmosphere through respiration,decomposition or combustion of fossil fuels.

Note the importance of plants in this cycle. Without plants, the carbon stays inthe atmosphere and cannot be reused and incorporated into the bodies of animals.If there were no plants, there would be no animals.

The human effect on the carbon cycleFigure 4.3 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 manufac-turing and other types of industries. Since the Industrial Revolution humans havebeen burning fossil fuels to release energy for machines. This has added carbondioxide to the air at an alarmingly fast rate. The carbon was locked away in thesolid or liquid forms of fossil fuel for millions of years. Increased combustion ofthese fossil fuels increases the carbon dioxide concentration in the air. Increasedconcentration of carbon dioxide in the atmosphere is associated with an environ-mental problem known as global warming.

The Industrial Revolution is a term used todescribe when people started to make and usemachines to do a lot of their work. It beganabout two hundred years ago. Machines needenergy to make them work, and most of thisenergy comes from burning fossil fuels.

fossil fuels ©

Figure 4.2 The carbon cycle in more detail.

ITQ2

(i) What is the importance of photosynthesisin the carbon cycle?

(ii) What is the importance of respiration inthe carbon cycle?

(iii) What is combustion?(iv) What role do decomposers play in the

carbon cycle?

carbon dioxide in the air

decomposersin the soil

4 urine and faeces

2 eaten

6 fossilisation

6 fossilisation

7 combustion

coalgas

3 respiration

1 photosynthesis

5 respirationof decomposers

GAS

4 death and decay

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Sun

incoming solarradiation (ultraviolet,visible and infrared)

reflected back to space

atmosphere

infrared radiation(heat) radiated

back towards spaceabsorbed by

'greenhouse gases'

reradiatedinto space

reflectionfrom clouds

atmosphere heated– raising Earth's

temperature

Earth

T

surface warmed surface warmed

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The greenhouse effect and global warmingWhen heat from the Sun reaches the Earth much of it bounces straight back intospace. Within the Earth’s atmosphere there are gases like carbon dioxide andmethane that absorb some of the escaping heat and send it back to the Earth’ssurface, keeping it trapped around the Earth. They act like a ‘greenhouse’ aroundthe Earth and thus are called ‘greenhouse 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 ofthe organisms living on it.

A problem arises when the proportions of these gases in the atmosphereincrease. They bounce more of the heat back to the Earth’s surface. This is calledthe ‘enhanced’ greenhouse effect. As a result the temperature of the Earthincreases, which is known as global warming.

greenhouse gases ©

greenhouse effect ©

global warming ©

Figure 4.3 The levels of carbon dioxide in the atmosphere over the last 160 years.

Figure 4.4 Some solar radiation that reaches the Earth is absorbed by the atmosphere rather than going back out to space.

1840 1860 1880 1900 1920 1960 1980 20001940

Year

0

1

2

3

4

5

6

7 Fossil carbon emisions(billion tonnes)

fossil carbon emissions(combustion since

Industrial Revolution)

1840 1860 1880 1900 1920 1960 1980 20001940

Year

300

310

320

330

340

350

360

370 Atmospheric carbon dioxide(parts per million)

290

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Carbon dioxide concentration in the Earth’s atmosphere has increased byabout 20% over the last 100 years. This effect has also been worsened bydeforestation. Trees (forests) remove carbon dioxide from the atmosphere duringphotosynthesis. Large areas of forests are being removed by cutting or burning,and so there are fewer trees to remove carbon dioxide by photosynthesis from theatmosphere.

It is not proven that higher carbon dioxide levels cause temperature rises, butscientific research suggests that the two may be associated.

Some people think that global warming might cause the Earth’s temperatureto rise between 1.5 °C and 4.5 °C by the end of the 21st 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 thesemay be flooded, driving people from their homes.

• Fertile, crop-producing land could 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, tornadoes and typhoons may also be moresevere.

• Cold countries may become more temperate and fertile.

We must be very careful not to say that every example of extreme weather is dueto global warming. There have always been variations in climate over the years,and over centuries. Also, we must be careful not to make unjustified assumptionsabout future 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 morethan a mile in seven years. Some sea levels are said to be lower now than in theeighteenth century – for example mean sea level in the Cook Islands has appar-ently dropped by about 20 cm in 200 years. Globally, mean sea level is rising atabout 3 mm per year. So although global warming is a reality, and many expertsattribute this to the enhanced greenhouse effect, we should not be too quick topredict catastrophe!

The nitrogen cycleAbout 79% of the air around us is nitrogen gas. This gas is very unreactive – itpasses in and out of animal’s bodies unchanged when they breathe. However,nitrogen is an essential component of biological molecules such as proteins andDNA. Muscle is composed of long strands of protein and DNA is the molecule ineach nucleus of a cell which contains the information about how to build that celland make it work.

Plants manufacture protein by absorbing nitrogen from the soil mostly asnitrate ions. These are combined with carbon, hydrogen and oxygen taken fromglucose that was made during photosynthesis. The elements are then arranged inanother way as they combine with the nitrogen, to make the building blocks forproteins and DNA.

Animals obtain their nitrogen from the protein in their diet, through eatingplants or other animals. The protein they eat is digested, absorbed and used asneeded in the feeding animal. That is, the nitrogen obtained from the protein of apiece of plant material or meat can be used to build growing muscles, make DNA,enzymes and other proteins, and everything else requiring nitrogen.

34


Remember that glucose is made duringphotosynthesis and is composed of carbon,hydrogen and oxygen.

ITQ3

(i) Why are the carbon dioxide levels in theatmosphere rising?

(ii) What might be some consequences of thisrise?

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The nitrogen cycle consists of four main processes:• nitrogen fixation;• decay;• nitrification;• denitrification.

Nitrogen fixation (1)

This occurs in nitrogen-fixing bacteria which convert nitrogen gas in the air, tonitrate. Some of these bacteria, like Azotobacter and Clostridium, live in the soil andconvert the nitrogen gas found in the air in the soil to nitrate. Plants cannot absorbnitrogen gas, only substances that contain it, like nitrates. So nitrogen-fixingbacteria thus make nitrogen available to plants in a form they can absorb. Plantsuse the nitrogen from nitrates in the soil to make proteins and DNA.

Other kinds of nitrogen-fixing bacteria, called Rhizobium, live in the roots oflegumes (plants of the pea family). There, nitrogen gas is converted to nitrates andused directly inside the plant to make protein.

The relationship between the plants and the nitrogen-fixing bacteria is anexample of mutualism as discussed on page 19.

Decay (2)

When plants and animals die, their bodies are decomposed by decomposers tomake ammonium compounds in the soil. Animal wastes, like faeces and urine,are also decomposed by bacteria living freely in the soil.

Nitrification (3)

The ammonium compounds formed during decay are converted to nitrites andthen nitrates. The processes that lead to the formation of nitrates in the soil arecalled nitrification and are carried out by nitrifying bacteria like Nitrosomonas andNitrobacter. Plants take up nitrate ions from the soil and make proteins.

nitrogen fixation ©

nitrification ©

Figure 4.5 The nitrogen cycle shown in diagrammatic form. (The numbers refer to the numbers in the text.)

nitrogen in the air

nitrates in the soil

plant protein

animal protein

nitrogenoxide

ammoniumcompounds

nitritesin soil

nitrifying bacteriaNitrosomonas

lightning

rain(acid rain)

in root nodules

Rhizobium

in soil

Clostridium

plants eaten

nitrates absorbed

nitrogen-fixingbacteria

4 denitrifyingbacteria

2 death and decay

fertilisersleaching

of the soil

T

1

5

3 nitrifyingbacteria

Nitrobacter

7

3

6

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soil water to rivertakes nutrients with it

rain

river

T

36


Denitrification (4)

The nitrogen cycle is completed by denitrifying bacteria. They convert nitrates inthe 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.

In nature, a little nitrogen fixation occurs during thunderstorms (5). Lightningprovides the energy to convert nitrogen to nitrogen oxides. These gases dissolvein rain droplets to form nitrates that plants can use.

To make crops grow better, we add artificial and natural fertilisers to the soil toincrease the levels of nitrates (6).

As rain water passes through the soil on its way to the rivers, lakes or seas, itcarries with it dissolved nitrates and other soil nutrients. So the nitrates can bewashed out of the soil. This is called leaching.

The nitrogen cycle is thus essential to life as nitrogen is a vital component of everyliving organism. This biogeochemical cycle allows nitrogen to be reused over andover by living organisms. Nitrogen atoms cannot be created and there is only a

denitrification ©

leaching ©

Figure 4.6 Diagram showing hownitrates can be ‘leached’ from soil.

Figure 4.7 The nitrogen cycle in more detail.

ammonium compounds

in soil

Rhizobium

Clostridium

nitrates in soil death and decay

urine and faeces

nitrogen-fixing

bacteria

nitrites in soil decaybacteria

absorbed

root nodule

eaten

nitrogen in the air

protein inanimals

denitrifyingbacteria

lightning

nitrogen fixation

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acid pollutantsfrom vehicles, powerstations and industry

oxides of sulphur and nitrogenfrom pollution dissolve in waterin the cloud to make acid rain

pH of rain5–7

pH of rain4–5

37


certain amount on Earth. The importance of bacteria should be noted, since theyare an integral part of this cycle. Nitrifying bacteria can be considered ‘good’bacteria, without which living organisms would slowly become extinct.

Acid rainCombustion of fossil fuels in industry and from motor vehicles releases acidicgases such as sulphur dioxide and nitrogen dioxide. These gases dissolve in atmos-pheric water vapour in clouds and later fall as acid rain. Sulphur dioxide dissolvesin atmospheric water to give, eventually, dilute sulphuric acid. Oxides of nitrogendissolve to form dilute nitric acid.

acid rain ©

pH is a measure of how acidic or how alkaline asolution is. A pH of 7 is neutral. A solution witha pH less than this is acidic. If it has a pH above7, it is alkaline.

ITQ4

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

Nitrogen fixation

Decay

Nitrification

Denitrification

Figure 4.8 The formation of acid rain.

Figure 4.9 These trees have beenkilled by acid rain.

The acid clouds may be carried hundreds of miles away from the source of thepollution by air currents. It has been recorded that rain with a pH as low as 4 hasfallen over Scandinavia, Germany and Canada.• Acid rain may kill plants and trees. Some forests, like the Black Forest in

Germany, have been severely damaged (see figure 4.9). Recently, though, it hasbeen found that acid rain enhances the growth of pine forests in Scandinavia.

• Acid rain also dissolves some poisonous metals thus introducing them into lakesand rivers. This poisons organisms living in the water. About 400 lakes inNorway have been rendered fishless because of acid rain.

• In cities, stone (statues and carvings) and metal structures have been damagedbecause of erosion due to acid rain.

Governments are trying to reduce acid rain by introducing regulations thatdemand that industries do not release atmospheric pollutants. The design ofengines for motor vehicles is also important to reduce the amount of pollutantgases that they make.

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Þ Living organisms are built up from simple atoms, mostly carbon, hydrogen,oxygen, with some nitrogen, iron, sulphur and other elements.

Þ Biogeochemical cycles show how materials are reused in nature.Þ The carbon cycle shows how carbon passes between the air, soil, plants and

animals and back again.Þ The greenhouse effect is an important natural process, caused by greenhouse

gases in the atmosphere that absorb heat energy from the Sun and keep thesurface of the Earth warm enough for life as we know it.

Þ Increasing levels of carbon dioxide in the air could lead to global warmingwhich could affect sea levels and weather, with devastating consequences.

Þ The nitrogen cycle shows how nitrogen passes between air, soil, plants andanimals and back again. Bacteria are very important in this cycle.

Þ Acid rain forms when acidic gases such as sulphur dioxide and nitrogendioxide dissolve in atmospheric water vapour. It can be very damaging to life.

ITQ1 (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 organicand contains carbon, hydrogen, oxygen, nitrogen, sodium, calciumand iron, etc. Food is ingested, digested, absorbed into blood andtransported to all parts of the body to build tissues. Plants take insimple inorganic molecules, carbon dioxide and water from theatmosphere, and nitrates from the soil to build their tissues.

(iii) The large organic molecules in dead bodies are broken down bydecomposers and detritivores back to the smaller components. Thenthe components can return to the environment and be used again byother organisms. They are recycled through living tissue in differentorganisms in food chains.

(iv) A biogeochemical cycle is a cycling process by which an atom isreleased and reused in nature.

ITQ2 (i) Photosynthesis is an important part of the carbon cycle since it is theonly means by which carbon from the air is taken into an organism.Plants take in carbon dioxide and turn the carbon into glucose andother 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(as carbon dioxide) from living organisms.

(iii) Combustion is the burning of fuels, a process which uses oxygen.When fuels, such as wood, gas and coal, are burnt, carbon dioxide isproduced, thus returning carbon atoms to the air.

(iv) Dead plants and animals have carbon molecules, such ascarbohydrates, proteins and fats, trapped in their bodies.Decomposers feed on their dead bodies and release the carbon ascarbon dioxide back to the environment when they respire.

ITQ3 (i) Carbon dioxide levels in the atmosphere are rising because of thevast amount of combustion of fossil fuels to release energy, especiallyin industries. An increase in human population leads to a greaterdemand for energy. Widespread deforestation adds to the problem.

(ii) Global warming (or the ‘enhanced’ greenhouse effect) which couldlead to higher temperatures, melting of polar ice caps, flooding andchanges in weather patterns.

38


Answers to ITQs

Summary

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39


Process in nitrogen cycle Importance Examples of bacteria involved

Nitrogen fixation Nitrogen gas is converted to nitrates in the soil and Azotobacterabsorbed by plants; or to amino acids in the root Rhizobiumnodules and used by the plant to make protein

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

Nitrification Ammonium compounds are converted to a more Nitrosomonasusable form, nitrates. Nitrates are absorbed by Nitrobacterplants and used to make proteins.

Denitrification Nitrates are converted back to nitrogen gas in the air Denitrifying bacteria

ITQ4

1 (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 duringvarious processes. Copy and complete the table below to show themovement of carbon in the processes listed.

(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.

(ii) Describe what happens at A, B and C.(iii) How are some plants like the garden pea able to survive in soil

deficient in nitrates?(iv) Describe how nitrates are leached from the soil.(v) Describe an example of symbiosis as seen in the nitrogen cycle.(vi) Nitrogen is a vital component of every living organism. Describe its

importance to these organisms.


Process Movement of carbon

From To

Respiration in an animal

Combustion of coal

Photosynthesis

Decomposition

nitrogen in the air

nitrate in soil

decay

decay

C

A

B ammonium compounds

T

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The cell is the basic unit of life. A cell cannot be viewed by the naked eye since itis too small. It can only be seen with a microscope. Cells are thus described asbeing microscopic.

A microscope is used to produce a magnified image of an object. There aredifferent kinds of microscopes, for example light and electron. When lookingthrough 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 can seewith a microscope depends on how powerful its magnification is. A light micro-scope typically magnifies between 10 and 400 times real size; an electronmicroscope is more powerful and can magnify tens of thousand of times actual size.

The actual size of an object in a photograph can easily be calculated from theimage and the magnification given. If the length of the object in the photo ismeasured as X, and the magnification is 100× (that means the object is 100 timeslarger than in real life), then the actual size of the object is X/100.

40

Ce l l sú draw diagrams to show the structure of typical plant and

animal cells;ú understand the functions of the cell wall, cell membrane,

cytoplasm, mitochondrion, chloroplast, nucleus and vacuole;ú compare plant and animal cells;ú understand why specialisation is important in multicellular

organisms;ú understand how some substances move into and out of cells.



5

plant

microscope

calculating size of cells

animal

cell – basic unit of life

tissue

osmosis

diffusion

organ

system

organism

movementinto

and out of cell

Concept map

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6 image seen is 400 times biggerthan the actual specimen

5 light passes through eyepiece lens, magnification x10

4 light passes through lens, magnification x40

3 light passes through the slide(specimen)

2 light passes through the filter and condenser

1 light reflects on mirrorand travels up to the eyepiece

Plant and animal cellsPlant and animal cells have the same basic structure but each has its own charac-teristics that make it typically plant or typically animal.

The structures found within a cell are called the cell organelles. They havedifferent functions and, as they work together, they keep the cell (and thereforethe organism) alive.

Figure 5.2 shows diagrams of typical plant and animal cells and table 5.1describes the functions of some cell organelles.

41

5 · Cells

organelles ©

ITQ1

What is the purpose of a microscope?

Figure 5.1 Diagram of a light microscope showing how it works.

Figure 5.2 Typical plant and animal cells.

Figure 5.3 Photomicrograph of atypical plant cell (magnification×5000). Compare this with thedrawing of a plant cell in figure 5.2.

ITQ2

Measure the width of the largest chloroplast ofthe cell in figure 5.3, and calculate its actualsize using the magnification given.

Plantcell

cell wall

cell membrane

cytoplasm

nucleus containschromosomes which

carry genetic information

vacuole

chloroplast

mitochondrion

starch grain glycogengranule

Animalcell

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Specialisation in multicellular organismsWe saw in chapter 1 that organisms can be described as unicellular ormulticellular. Unicellular organisms like Amoeba (animal) and Chlorella (plant) arejust one cell in size. Multicellular organisms, like all the larger animals and plants,are made up of many (sometimes millions) of cells.

The cells of unicellular organisms, e.g. Amoeba and bacteria, are independentbut are still able to carry out all characteristics of life. Multicellular organisms,however, are made up of millions of cells. These cells work together and are oftendependent on each other to carry out all the characteristics of life.

In multicellular organisms each cell has the same basic structure, but there arevariations in the design. Within a single organism, such as a human, there aregreat differences between the cells. Each type of cell is specialised to carry out aparticular function well. For example, a muscle cell is concerned with contractionof the muscle, while a nerve cell is specialised to transmit nerve impulses.

In a multicellular organism, cells are arranged in groups to form tissues. Atissue is a structure made up of many similar or identical cells which are adaptedto perform one specific function. Muscle cells make up muscle tissue and all thesecells are concerned with the muscle function of contraction.

42

5 · Cells

tissue ©

Table 5.2 describes differences between plant and animal cells in more detail.

Table 5.1 The functions of some cell organelles. (Organelles shown in green areonly found in plant cells.)

Organelle Function

cell wall prevents bursting of the plant cell and gives it a fixed shape

cell membrane a selectively permeable barrier which controls exchange between the cell and its environment

cytoplasm site of many of the chemical reactions of life

nucleus controls the activities of the cell

chromosome carries genetic information in the form of DNA

mitochondrion site of energy production

vacuole important during exchange of water and minerals, and stores various substances including waste products

chloroplast where photosynthesis takes place

Table 5.2 The main differences between plant and animal cells.

Plant cell Animal cell

Cytoplasm is surrounded by a cell Cytoplasm is surrounded by a cell membrane as well as a cell wall. membrane only.

Chloroplasts are present. Chloroplasts are absent.

Carbohydrates are stored as starch. Carbohydrates are stored as glycogen.

A large, permanent vacuole is present Many small, temporary vacuoles are in most plant cells. It has a definite, present at a time. These have no fixed shape. fixed shape.

Cytoplasm is pushed to the edges of Cytoplasm is present throughout the cell.the cell by the vacuole, so it isnormally confined to a thin layer.

ITQ3

Make lists to show (i) all the organelles whichare found in both plant and animal cells,(ii) organelles which are only found in plantcells.

ITQ4

Name two organelles found in a plant cell butnot in an animal cell. What is the importance ofthese two organelles to a plant cell?

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nucleus

these cellsinsulate thenerve cell

Nerve cell

nucleus

striations in cellscan shorten or lengthen

Muscle cellsmake up a muscle fibre

transmitsnerve pulses

T Several different kinds of tissue may be grouped to form an organ. Forexample, the human brain contains nerve tissue, blood vessels with muscle andconnective tissue, and blood.

In animals, organs form parts of even larger functional units called systems.The digestive system is made up of several organs, including the stomach, intes-tines and liver.

Cells in plants are also grouped into tissues, and tissues grouped as organs.Table 5.3 shows examples of tissues, organs and systems that are found in plantsand animals.

A healthy organism is made up of all these parts working efficiently together,enabling it to do many things at the same time, such as use its energy source andmake the energy available for movement, reproduction, growth, response andexcretion. A total breakdown in the normal functioning of any one of thesesystems can lead to the death of the organism, such as a heart attack when thecirculatory system breaks down.

Most animals are either predator or prey in food chains. A healthy organismhas all its systems functioning efficiently and so is able to survive in the environ-ment or wild. Unhealthy organisms may be unable to capture food or fall prey topredators more easily. Survival is for the fittest, meaning that an organism with all

43

5 · Cells

organ ©

system ©

Figure 5.4 Specialised cells that are found in nerves and muscle.

Table 5.3 Examples of tissues, organs and systems in plants and animals.

Structure Examples in plants Examples in animals

tissue mesophyll (chapter 6), nerve tissue (chapter 14),phloem tissue (chapter 11), muscle tissue (chapter 13)xylem tissue (chapter 11)

organ leaf, root stomach, lung, brain, eye

system (not organised into systems) digestive system (chapter 7),respiratory system (chapter 8),nervous system (chapter 14)

Figure 5.5 The organ of theintestine is made up of differenttissues working together.

Epithelial cells

These mass together to form

an epithelial tissue.

CELLS

TISSUE

These cells masstogether to formmuscle tissue.

The epithelial and smooth

muscle tissues combine together

in the wall of an organ

such as the intestine.

CELLS

TISSUE

ORGAN DIGESTIVESYSTEM

ORGANISM

Muscle cells

T

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its systems functioning efficiently and continuously has an advantage for survival– an advantage for life.

Movement of substances into and out of cellsAll kinds of reactions take place within a cell. The organelles within a cell requiremany different substances to carry out these reactions. Waste products are formedduring these reactions and must be removed. The substances, needed andproduced, must pass into and out of the cell.

There is thus a constant movement of substances into and out of cells, forexample:• substances needed by the cell, like glucose and oxygen, must pass into the cell;• substances produced by the cell must be passed out of the cell. These may be

waste products like carbon dioxide and urea, or substances needed by anothercell, like enzymes. This is called secretion.

Substances can be taken in within small vesicles made from the cell membrane.Amoeba takes its food in this way. Substances can also be released from cells whenvesicles containing the substance join with the cell membrane (see figure 5.7).Hormones are released from cells like this.

Substances may also enter and leave cells as individual molecules. They do this byvarious mechanisms including diffusion. Water enters and leaves cells by osmosis.

Movement by diffusionDiffusion is the movement of molecules from a region of high concentration ofthose molecules to a region of lower concentration of those molecules. Diffusioncan happen in gases and in liquids.

A diffusion gradient or concentration gradient occurs when there is a differ-ence in the number of molecules, or the concentration of molecules between thetwo regions. For example, when a drop of dye is added to water, the dye mole-cules move around and between the water molecules and eventually are spreadevenly, even when not stirred. In other words, the dye molecules move fromwhere they are plentiful to where they are not so plentiful. We say they diffuse.

Substances can also diffuse across membranes if the concentrations are different on both sides and the membrane is permeable to those molecules (letsthem through).

44

5 · Cells

secretion ©

diffusion ©

concentration gradient ©

permeable ©

Figure 5.6 Grouping of cells to form tissues in the organ of a leaf.

ITQ5

Distinguish between unicellular andmulticellular organisms, giving two examplesof each.

ITQ6

Give an example of each of the following: cell,tissue, organ, system.

Epidermal cells

Palisademesophyll cells

Epidermal cells

Spongy mesophyll cells

Spongy tissue

Epidermal tissue

Palisade tissue

Epidermal tissue

The tissuescombine together

in an organsuch as the leaf.

leaf

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45

5 · Cells

Figure 5.8 Over time the dye molecules diffuse so they are evenly spreadthroughout the solution.

Figure 5.9 Diffusion can occur across thin cell membranes.

Some examples of diffusion in the human body• After a meal the end-products of digestion are at a high concentration in the gut.

They diffuse down their concentration gradient into the blood where they areat a lower concentration.

Substance moving out ofthe cell. (These were made

by the cell and are important, like hormones and enzymes.)

substances movinginto cell

waste products movingout of cell

T

more molecules movefrom left to right than from

right to left

no net movementof molecules

molecules ata higher

concentration

molecules ata lower

concentration

molecules atthe same

concentrationon both sides

Figure 5.7 Diagram showing substances moving into and out of a cell in smallvesicles.

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• Diffusion occurs in the lungs. Carbon dioxide diffuses from the blood where itis at high concentration into the lung where its concentration is lower. Oxygendiffuses in the other direction because it has a higher concentration in the lungsand a lower concentration in the blood.

• When the blood gets near the cells, the oxygen concentration in the blood ishigher than in the cells. The blood came from the lungs where it picked up oxygen. The oxygen concentration in the cell is low, since the oxygen that wasin the cell was used for respiration. The oxygen in the blood diffuses into thecell, where it can be used for energy production during respiration.

46

5 · Cells

blood richin the end

products ofdigestion

end products of digestion at a high concentration

ileum of the gut

blood capillary

Figure 5.10 Diffusion of small food molecules from gut to blood.

blood richin oxygen (O2)

blood capillary

O2

O2

O2

O2

oxygen at a higher concentration

in the alveolus

O2

O2

O2

O2

O2

O2

Figure 5.11 Diffusion of gasesbetween the lungs and the blood.

blood capillary with blood rich in oxygen

body cells – oxygen used up during respiration

and its concentration is low

O2

O2

O2

O2 O2

O2

O2

O2

O2

O2

O2

O2

O2

1 concentration gradient

higher concentrationof glucose in the gut

lower concentrationin the blood

2 surface area canbe increased in

many ways – moresurface area, more diffusion

3 one cell thick

T

3 blood capillaryone cell thick

Figure 5.13 Adaptations that help to speed up the rate of diffusion.

Figure 5.12 Diffusion of oxygen from blood into cells.

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• In the cells, carbon dioxide builds up as a waste product of respiration. It is at ahigher concentration than in the blood. Thus it diffuses out of the cell and intothe blood.

• Other wastes made by cells, such as ammonia, are at a higher concentration inthe cell than in the blood. They also diffuse out of the cell to the blood and aretaken away and expelled from the body.

Diffusion is a very slow process unless there is a large concentration gradient overa short distance. Tissues like the lungs and small intestine are especially adaptedto maximise the rate of diffusion by:1 keeping the difference between the concentrations on each side as high as

possible (maintaining a steep concentration gradient);2 having a large surface area to volume ratio so that gases have as large an area

of cells as possible to diffuse through;3 being very thin and thus minimising the distance over which diffusion must

take place.

Movement by osmosisOsmosis is a special kind of diffusion. It is the diffusion of water molecules acrossa selectively permeable membrane. Cell membranes are all selectively permeablemembranes. ‘Selectively permeable’ means that water and some substances canpass through the membrane but other substances do not.

Osmosis in plant cellsWhen a plant cell is put into a solution which has the same concentration as thecell contents (isotonic), some water molecules will move into the cell through thecell membrane and some will move out. There is no concentration gradient so themovements each way are the same and balance each other out. We say there isno net movement, or net flow, of water.

47

5 · Cells

osmosis ©

isotonic ©

net flow ©

• concentration of solution outside isotonic with (same as) inside cell• no net flow of water

• outside hypotonic to (less concentrated) inside cell• net flow of water into cell• cell is turgid

• outside hypertonic to (more concentrated) inside cell• net flow of water out of the cell • cell membrane pulls away from cell wall• cell is flaccid

turgid cell flaccid cell

Figure 5.14 The effect of differentconcentrations of solution on a plantcell.

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When a plant cell is put into a solution that is less concentrated (hypotonic)than the cell contents, there is a greater concentration of water molecules outsidethan inside. Some water molecules move out of the cell but more move into thecell, so there is a net flow of water into the cell. The cell becomes full of water andis 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 fewwater molecules will move into of the cell but many more move out of it, so thereis a net flow of water out of the cell. The cell loses water and is described as beingflaccid. Flaccid cells are easy to distinguish under the microscope because the cellmembrane and contents pull away from the cell wall.

Osmosis in animal cellsAn animal cell has no cell wall like a plant cell, so hypotonic and hypertonic solu-tions have different effects. In a hypotonic (dilute) solution there is a net flow ofwater into the cell. With no strong cell wall to prevent the membrane fromstretching too far, it eventually bursts. In a hypertonic (concentrated) solutionthere is a net flow of water out of the cell and the whole cell shrinks.

It is important for cells to be protected from large changes in concentration ofthe solutions around them. Animal bodies have complex mechanisms to do thiscalled osmoregulation and homeostasis (chapter 12).

48

5 · Cells

hypotonic ©

turgid ©hypertonic ©

flaccid ©

• cell in isotonic solution• no net movement of water

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

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

Figure 5.15 The effect of different concentrations of solution on an animal cell.

ITQ7

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

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49

5 · Cells

Þ The cell is the basic unit of life.Þ A cell contains smaller parts called organelles.Þ The nucleus, cell membrane, cytoplasm and mitochondrion are some

organelles found in typical plant and animal cells.Þ Plant cells also contain cell walls, chloroplasts and large central vacuoles.Þ Cells in multicellular organisms are often specialised for a particular function.Þ A group of specialised cells that have the same function is called a tissue.Þ An organ is a group of different tissues that work together.Þ Organs working together make up a system.Þ Systems co-ordinate with each other and work together in a living organismÞ Many substances can move into and out of a cell through the cell membrane

which is selectively permeable.Þ Diffusion is the movement of a substance from a high concentration to a low

concentration.Þ Osmosis is the movement of water across a selectively permeable membrane

from a solution where there is a high concentration of water molecules to asolution where the concentration of water molecules is lower.

Þ Diffusion and osmosis occur at many places in a living organism.Þ Different concentrations of solution have different effects on plant and

animal cells.

ITQ1 A microscope is an instrument used to produce a magnified image of anobject. Organisms and objects that cannot be seen by the human ornaked eye may be visible under a microscope.

ITQ2 The measured width of the chloroplast in the photograph is 14 mm (or 14 × 10–3m). The magnification is × 5000. This means that the measuredsize is 5000 times larger than in reality. So the actual size is (14 ÷ 5000) ×10–3m = 0.0028 × 10–3m (or 2.8 × 10–6m or 2.8 µm).

ITQ3 (i) Plant and animal cells have: cell membrane, nucleus, cytoplasm,mitochondria, small vacuoles.

(ii) Plant cells only have a cell wall*, chloroplasts, large central vacuole,starch grains. (*Fungal cells and some bacteria also have cell walls,but these have a completely different structure to those in plants.)

ITQ4 The cell wall has protective and structural functions. It protects the plantby protecting each plant cell from bursting when the plant takes upwater. It also helps to support stems and leaves of the plant when thecells are full of water, because plants have no skeleton like many animals.

The chloroplast contains the pigment chlorophyll which collects thelight energy of the Sun. Chloroplasts are the sites of photosynthesis, soanimals do not need them.

The large plant vacuole is important during exchange of water andminerals, and stores various substances including waste products.

ITQ5 A unicellular organism is an organism that is one cell in size. This smallcell shows all the characteristics of life and lives an independent life. Forexample, Amoeba and Chlorella.

A multicellular organism however, is made up of many cells. Thesecells work together, and the organism is thus able to show all thecharacteristics of life. For example, a human and a worm (there are manyother examples you could have chosen).

ITQ6 For example, muscle cell.Tissue: any group of one kind of cell working together, e.g. muscle cells inmuscle tissueOrgan: any group of tissues working together, e.g. stomach, made up ofsecretory tissue, muscle tissue and other tissues: or leaf made of palisadetissue, xylem tissue.

Summary

Answers to ITQs

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System: any group of one kind of organs working together, e.g. digestivesystem made up of stomach, liver, intestines and other organs

ITQ7 A plant cell has a cellulose cell wall around the cell membrane. The wallis strong and cannot stretch. When placed in water the cell will take upwater, but the cell membrane will not burst because the cellulose cell wallstops it stretching to bursting point. An animal cell does not have acellulose cell wall and so can stretch to the point where it bursts.

50

5 · Cells

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

(ii) The figure below shows how a section of a root or stem is mountedfor microscopic investigation.

Explain why it is necessary to cut a very thin section of the materialwhich is to be observed under the microscope.

(iii) (a) Name two types of microscope.(b) Why are cells described as being microscopic?

2 (i) Make labelled drawings of typical plant and animal cells.(ii) Use a table to compare typical plant and animal cells.(iii) Give one advantage of being multicellular.(iv) Name one difference between a tissue and an organ.(v) Give one named example of:

(a) a tissue;(b) an organ to be found in:

• an animal;• a plant.

3 The figure below shows onion rings A and B before immersion in water Cand a salt solution D.


A

very thin section

drop of water

onion ring A onion ring C

onion ring Bonion ring D

onion ringbefore immersion

in solution I

onion ringbefore immersion

in solution II

onion ring after immersion in solution I water

onion ring after immersion in solution II salt solution

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51

5 · Cells

(i) Copy and complete the table below to show the measurements ofthe rings.

(ii) (a) Using measurements from the table, describe what happened tothe 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.

Onion ring Outer diameter Inner diameter Mean diameter

A

B

C

D

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52

Photosynthes is ú understand the difference between heterotrophic and

autotrophic nutrition;ú describe photosynthesis;ú relate the structure of the leaf of a flowering plant to its

function in photosynthesis;ú understand how some simple controlled investigations

demonstrate that light and chlorophyll are necessaryconditions for photosynthesis.

6

Plants are the food supply for animalsIn the study of food chains it was seen that plants are producers and were at thestart of almost all food chains. Animals are consumers and feed on the plants oron other animals. Plants do not eat yet they are full of food! They are rich incarbohydrates, fats and proteins. Plants are full of food because they are able tomanufacture or make their own food. We call them autotrophs (self-feeders),because they are able to make organic substances (glucose) from simple inorganicsubstances (carbon dioxide and water). This process is called photosynthesis andrequires light from the Sun for the energy needed to carry it out. From glucose,the plant makes all the carbohydrates, fats and proteins it needs.

Consumers feed on the organic substances made by the plants. They are calledheterotrophs (other or different feeders). Heterotrophic nutrition is the intake ofcomplex organic substances when animals feed. Autotrophic nutrition is the‘intake’ of simple inorganic substances by plants during photosynthesis.

autotroph ©

heterotroph ©

ITQ1

Define the terms autotroph and heterotroph.CO2

H2Oinorganic

substances

Autotrophic nutrition

plants take in inorganicsubstances and makeorganic substances

Heterotrophic nutrition

animals take in organicsubstances when

they feed

Figure 6.1 Autotrophic nutrition must occur before heterotrophic nutrition canoccur. Food chains start with plants and animals feed on the plants.

ITQ2

Why must autotrophic nutrition occur beforeheterotrophic nutrition?

conditions

limiting factorsleaf structure

photosynthesis

autotrophic nutrition

inorganic substancesconverted to

organic substances

heterotrophic nutrition – animals

adaptations forphotosynthesis

Concept map

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53

6 · Photosynthesis

PhotosynthesisPhotosynthesis can be summarised by the simple equation:

Chlorophyll is a complex green pigment. At the centre of a chlorophyll moleculeis a single atom of magnesium and four atoms of nitrogen. Without supplies ofnitrogen a plant cannot make chlorophyll and so cannot photosynthesisesuccessfully.

Experiments show that there are two main stages in photosynthesis (figure6.2), namely:• the light-dependent stage;• the light-independent stage.

Light-dependent stageChloroplasts are organelles seen in green plant cells. They contain the greenpigment chlorophyll which ‘traps’ the light energy from the Sun. The energy isused to ‘split’ water (H2O) into hydrogen (H) and oxygen (O). The oxygen is awaste product and diffuses out of the leaf.

Light-independent stageThe hydrogen then combines with carbon dioxide (CO2) to make glucose(C6H12O6). This stage of photosynthesis does not need light and can happen whenit is dark.

The organ specialised for photosynthesis is the leaf. The transverse section of a leafreveals many cells, arranged in a manner that is ideally suited for photosynthesis.

Adaptations of the leaf for photosynthesisLeaves are adapted to carry out photosynthesis in a number of ways.• They are generally broad and flat with a large surface area to absorb a lot of light

and carbon dioxide.

photosynthesis equation ©

light-dependent stage ©light-independent stage ©

Sun

chlorophyllwater

oxygen

diffuses out of the leaf

carbon dioxide

light-dependent stage

light-independent stage

glucose

diffuses into the leafhydrogen

Figure 6.2 Light-dependent and light-independent stages of photosynthesis.

lightcarbon dioxide + water glucose + oxygen

chlorophyll

light6CO2 + 6H2O C6H12O6 + 6O2

chlorophyll

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54

6 · Photosynthesis

• They lie at 90° to the sunlight and are spaced around the stem to catch as muchlight as possible.

• The leaves are thin to allow light and carbon dioxide to reach all cells rapidly.• Stomata (small holes) are present in the lower epidermis to allow gases to get in

and out easily.• Air spaces around the cells in the lower half of the leaf allow oxygen to get to

the chloroplasts as quickly as possible.• Chloroplasts are most numerous in cells in the palisade layer which is in the top

part of the leaf, closest to the sunlight.• Xylem vessels transport water to the leaf cells.• Phloem sieve tubes carry away the food made to the rest of the plant.• A waxy cuticle prevents water loss from the top of the leaf, and is transparent

to let light through.

stomata ©

One hole is a stoma. Stomata is the plural.

palisade cell ©

waxy cuticle

chloroplast

air space

xylem vessels

phloem tubes

waxy cuticle

palisade layer

upper epidermis

spongy layer

lower epidermis

cell of upperepidermis – no

chloroplasts

palisade mesophyllcell – chloroplasts

present

spongymesophyll cell

vein

cell of lowerepidermis – no

chloroplasts

guard cell –thickenedinner wall stoma

A

B

apex

veins – runthroughout leaf

margin

midrib

petiole (leaf stalk)

A

B

spongy layerpalisade layer

midribupper

epidermis

lowerepidermis

veinphloem

xylem

magnification ofthis small section

(a transverse section)

the leaf is cut atA–B and magnified

stoma

Figure 6.3 A section of a leaf.

light

light

light

T

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6 · Photosynthesis

Guard cellsA stoma is surrounded by a pair of specialised epidermal cells called guard cells.The guard cells vary the size of the opening of the stoma by changing their shape,thus the size of the stomatal pore is regulated by the guard cells. The stoma is theroute by which water is lost from the plant during transpiration (chapter 11), andalso by which the gaseous exchange necessary for photosynthesis occurs. By controlling stomatal opening and closing, a plant controls the balance between theneed to conserve water and the need to exchange gases.

Stomatal opening varies as a result of changes in the turgidity of the guardcells:• when they are turgid, the stoma opens;• when they are flaccid, the stoma closes.

The following observations have been made:• most stomata open during the day and close at night;• stomata generally close when a plant suffers water stress, or when transpiration

rate exceeds the rate of water absorption by the roots;• the stomata of some desert plants close during the day and open at night.

How everything gets to the chloroplast1 Carbon dioxide diffuses from the surrounding air into the stomata or pores on

the underside of the leaf. It moves into the air space surrounding the meso-phyll cells, and then into the cells themselves. As the carbon dioxide is used upduring photosynthesis, its concentration drops. There is thus a greater concen-tration of carbon dioxide outside the cells than inside and carbon dioxide diffuses into them.

2 Water moves by osmosis from the soil into the roots of the plant. It then travelsup the xylem vessels in the stem and into the leaves. From the xylem in the leaf,

epidermal cell

guard cell (turgid)

stoma open

stoma closed

guard cell (flaccid)

Figure 6.4 The guard cells controlthe opening and closing of thestomatal pore.

3 light from the Sun

carbon dioxide in theair surrounding the leaf1

water moves by osmosisfrom cell to cell

2 water travels to theleaf via the xylem, from

the soil surroundingthe roots

typical photosynthesising cell(chlorophyll is present

in the chloroplasts)

Figure 6.5 All the requirements for photosynthesis must get to all thephotosynthesising cells. (The numbers relate to the numbers in the text.)

ITQ3

Why do you think that the stomata of somedesert plants close during the day?

CO2

• CO2 used up during photosynthesis• Its concentration is thus low in the cell

• CO2 in the air space. Its concentration is higher than in the cell• CO2 diffuses into the cell from the air space

Figure 6.6 Carbon dioxide diffusesdown its concentration gradient intothe leaf.

ITQ4

Describe how carbon dioxide gas in theatmosphere gets to a photosynthesising cellinside a leaf.

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56

6 · Photosynthesis

water moves by osmosis to the palisade cells where it is used duringphotosynthesis.

3 Light rays pass into the leaf from all around, especially from above.Chloroplasts are found mainly in the palisade cells where the chlorophyll caneasily intercept and trap the light energy.

4 Within the chloroplasts the light energy splits the water which then reacts withthe carbon dioxide.

Products of photosynthesisThe 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 carbohydrates, lipids and proteins and used for growth, or it canbe converted to starch and stored, as in potatoes.

Oxygen is a waste product of photosynthesis. The cells in the leaf will use somefor respiration, but the rest of the oxygen is not needed by the plant. Inside theleaf, photosynthesis is taking place and oxygen is being produced. It is thus at ahigher concentration inside the leaf than outside. So oxygen diffuses out of theleaf through the stomata.

O2

• Oxygen is produced during photosynthesis• Its concentration is therefore high in the cell

• Oxygen diffuses into the air space where its concentration is lower• Oxygen diffuses out of the cell through stomata

Figure 6.7 Oxygen moves out of aleaf.

ITQ5

Look at the tomato plant and describe fourways in which the plant is adapted forphotosynthesis.

xylem vessel in the stem

transports water

the leaves are thin and flat and lie

at right angles to the Sun's rays

leaves are green, contain chlorophyll

leaves are spread around

the stem

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57

6 · Photosynthesis

Limiting factors in photosynthesisPhotosynthesis is a chemical reaction, and the rate at which a reaction can happendepends on how fast the chemicals that are reacting can get together.

In photosynthesis a plant requires water, carbon dioxide and light. If any oneof these is in short supply, the rate of the reaction will slow down. For example, aplant may have sufficient carbon dioxide and water, but not enough light forphotosynthesis to take place at its maximum rate. Light is then said to be thelimiting 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 at its leastfavourable value (light in this example). Water, light and carbon dioxide may belimiting factors at different times for the plant.

The limiting factors which affect photosynthesis are:• temperature;• light intensity;• carbon dioxide concentration;• availability of water.

TemperatureThe rate of a reaction increases as temperature increases; with heat the moleculesmove about and come together faster.

Photosynthesis also involves a series of enzyme-catalysed reactions. Enzymeshave an optimum temperature or temperature at which they work best (chapter7), so this will also affect the rate of the reaction.

Temperature is often the limiting factor on the rate of photosynthesis in coolseasons in temperate regions.

Carbon dioxide concentrationThe concentration of carbon dioxide is relatively low in the atmosphere. So carbondioxide is usually the limiting factor when temperature and light levels are high.Commercial growers who grow their crops in large greenhouses often pump inextra carbon dioxide to the air in the greenhouse to increase the rate ofphotosynthesis in the crops.

Light intensityThe amount of light in the environment varies greatly between night and day.Light is usually the limiting factor from dusk until dawn.

Availability of waterThe availability of water varies in the environment. If the soil is dry, water may bethe limiting factor on photosynthesis.

rate slows down, some factor is limiting the rate0.13% CO2 at 30 oC

0.03% CO2 at 30 oC

greater CO2 concentration, rate increases

rate of photosynthesis slows down because of CO2concentration – CO2 is the limiting factor, not light

rate of photosynthesis increases as light intensity increases – light is the limiting factor

Light intensity

Rate of photosynthesis

Figure 6.8 How light and carbon dioxide may limit the rate of photosynthesis.

ITQ6

Which factor will most likely be limitingphotosynthesis 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.

limiting factor ©

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EtiolationIf the plant cannot get sunlight, for example it is shaded by a rock or anotherplant, it cannot photosynthesise. Without photosynthesis it cannot make food.But this does not mean that it cannot continue to grow. For a short while, it canuse some of the food stored within the plant to grow and lengthen. This gives it achance to get some leaves into light and so start to photosynthesise again.

The form of growth a plant shows when it is out of light is different to normal.All the energy is used to make long thin cells, so the stem becomes elongated andthin, and leaves are kept very small. The stems and leaves are also pale yellow asno chlorophyll is made. This form of growth is called etiolation. If it does not reachlight quickly the plant will run out of food reserves and die.

58

6 · Photosynthesis

etiolation ©

Figure 6.9 The etiolated plants on the left have long thin, yellow stems andsmall yellow leaves.

Summary

Answers to ITQs

Þ Plants make food in a process called photosynthesis.Þ Photosynthesis is the process whereby food is made from simple inorganic

substances.Þ Photosynthesis is an example of autotrophic nutrition.Þ Heterotrophic nutrition is the intake of organic food. Animals feed

heterotrophically.Þ Photosynthesis is made up of two stages. In the light-dependent stage, light

‘splits’ water into hydrogen and oxygen. In the light-independent stage, thehydrogen combines with carbon dioxide to make glucose.

Þ Photosynthesis occurs in the leaves of plants.Þ Plants show many adaptations for photosynthesis.Þ The food made in a plant is used in many ways.

ITQ1 An autotroph is an organism that is able to make its own food (organicsubstances) from simple substances (inorganic substances). A plant is anautotroph – when it photosynthesises it makes glucose from carbondioxide 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 foritself.

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ITQ2 Autotrophs make organic food which is eaten by heterotrophs.Autotrophic nutrition must therefore take place first so that heterotrophscan have something to eat.

ITQ3 Some desert plants close their stomata during the day to prevent loss oftoo much water from the leaf when it is hot. They open their stomata atnight to exchange gases for photosynthesis. (They have a specialmechanism which allows them to trap the energy from sunlight duringthe day and store it, until the stomata open at night and the energy canbe used to make glucose.)

ITQ4 Carbon dioxide is in the atmosphere around the leaf and gets to thephotosynthesising cell by diffusion. A photosynthesising cell uses carbondioxide, and so the carbon dioxide concentration decreases within thecell. Carbon dioxide diffuses into the cell from the surrounding air spacewhere its concentration is greater. The carbon dioxide concentration isthus lowered in the air space. Carbon dioxide from the atmosphere nowdiffuses into the air space through the stomata.

ITQ5 • The leaves are spread around the stem and lie at right angles to theSun’s rays so that they can intercept as much light as possible.

• Leaves are green because the cells contain chlorophyll. This captureslight 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.

ITQ6 (i) Carbon dioxide(ii) Temperature(iii) Water

59

6 · Photosynthesis

1 The diagram shows a transverse section of a leaf as seen under amicroscope.

(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, identify 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.


E

F

A

B

C

D

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60

6 · Photosynthesis

2 (i) Define:(a) autotrophic nutrition;(b) heterotrophic nutrition.

(ii) Photosynthesis is summarised in one equation, but described as twostages (a) light-dependent, and (b) light-independent. Describe thetwo stages of photosynthesis.

(iii) List five ways a plant is adapted for photosynthesis.

3 (i) The starch test can be summarised in a series of stages:1 A leaf is dipped in boiling water for 10 seconds.2 The leaf is immersed in ethanol which is placed in a water bath at

80 °C.3 The leaf is then dipped in tap water.4 The leaf is tested with iodine.

(a) Why was the leaf dipped in boiling water?(b) What is the role of ethanol?(c) Describe the iodine test for starch.(d) A starch test was performed on a leaf and positive results were

seen. What interpretations can be suggested about activities inthe leaf?

(ii) Explain the meaning of the term ‘destarched’ as it refers to a leaf.Give details of the process by which a leaf is destarched.

(iii) Describe an investigation to show that plants need CO2 forphotosynthesis.

4 The diagram below shows a leaf in its actual size.(i) Making a drawing of the leaf.(ii) Write a heading for the

drawing.(iii) Calculate the magnification of

your drawing.(iv) Label the parts of the leaf.

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All organisms, plants and animals, must be supplied with a source of energy formetabolism. This energy is used for maintenance, growth and repair of theirbodies to sustain their lives. Plants (autotrophs) are able to make their own foodusing energy from the Sun. They take in only very simple inorganic substanceslike water, carbon dioxide and nitrate ions. Animals (heterotrophs) can onlyobtain this energy when they feed on other living organisms made up of complexmaterials such as carbohydrates, proteins and lipids.

DietTo maintain their bodies in good heath, animals need various materials. Theseinclude carbohydrates, proteins, lipids, vitamins and minerals. Animals eat foodthat contain these materials or nutrients. The term diet is used to describe thequantity and quality of food eaten, that is which nutrients and how much of eachis present in the food being eaten every day.

61

Feed ing and d igest ionú understand the importance of a balanced diet to Man;ú describe food tests for carbohydrates, proteins and fats;ú relate a balanced diet to age, sex and activity of an individual;ú explain the meaning of the term malnutrition;ú describe health problems associated with food additives;ú understand why minerals are needed in plants;ú describe the tests to identify the different classes of food substances;ú describe the role and structure of teeth;ú understand the action of enzymes;ú understand how the alimentary canal of Man works;ú describe what happens to the products of digestion.



7

diet ©

diet

malnutrition

pregnancy

age

activity

sex

ingestion

digestion

absorption villus

assimilation liver

egestion constipation

balanceddiet

foodadditives

alimentarysystem

physical digestion – teeth

chemical digestion – enzymes

Concept map

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monosaccharide ©

disaccharide ©

A balanced diet is a diet which has the quality and proportions of nutrientsneeded to maintain good health. This includes water and fibre. Water is essentialbecause 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 thetough fibres that come from plant material. We cannot digest and absorb them,but they are essential to the healthy working of the gut. Without enough fibre wesoon suffer from constipation. Eventually this can lead to bowel disease.

Some nutrients that are needed are organic and some are inorganic.

Organic nutrientsThese are required in the diet in relatively large amounts (see table 7.2).

Carbohydrates are compounds of carbon, hydrogen and oxygen in the ratio1 C : 2 H : 1 O. An example is glucose. Figure 7.1 shows a ball-and-stick model ofa molecule of glucose. It can also exist as a ring formed from five carbon atomsand one oxygen atom. A –CH2OH group is attached to a ring carbon.

Compounds with one such ring structure are called monosaccharides. Theformula can be shortened to a symbol which can be

either or, for diagrams, just

Glucose and fructose are examples of monosaccharides. Monosaccharides areoften called simple sugars.

Two monosaccharides can combine to form a disaccharide. This happens in a condensation reaction because a water molecule is removed. Disaccharides can bebroken back down to monosaccharides by hydrolysis which is a chemical reactioninvolving recombination with water. Monosaccharides and most disaccharidesreduce Benedict’s solution to an orange/red compound. Sucrose is the only com-mon disaccharide which does not react in this way. This provides a distinguishingtest for sucrose. Disaccharides are called complex sugars.

62

7 · Feeding and digestion

balanced diet ©

fibre ©

ITQ1

Define the terms diet and balanced diet.

Table 7.1 The organic and inorganic nutrients needed by living organisms.

Organic nutrients Inorganic nutrients

Carbohydrates:contain carbon (C), hydrogen (H) and oxygen (O)

Minerals:such as calcium, iron,potassium, sodium, iodine,phosphorusProteins:

contain C, H, O and also nitrogen (N) and smallamounts of sulphur (S)

Lipids:contain C, H and O

Vitamins:contain C, H, and O and other essential elements

Figure 7.1 Three-dimensional ball-and-stick model of a glucosemolecule.

Figure 7.2 (a) Disaccharidemolecules are made when twomonosaccharide molecules jointogether. (b) Polysaccharidemolecules are made of manymonosaccharide molecules.

monosaccharides

disaccharide

polysaccharideOOOO

OHO

HO

H

OH

HH

OH HO OH

HH

HO

H H

condensation (water removed)

condensation

hydrolysis (water added)

H2O

hydrolysis

OH

H

O

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63


Table 7.2 The major organic nutrient groups.

Organic nutrient Major groups Structure Characteristics Importance

Carbohydrate Monosaccharide

e.g. glucose,fructose

five carbon atoms and anoxygen 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 havea sweet taste

starch is used as theenergy store in plant cellsand as a food source foranimals

cellulose is found inplant cell walls

glycogen is used as theenergy store in animalcells

Protein made up of long chains ofamino acids

there exists about 20different kinds of aminoacids

they can be arranged inthe protein chain in anyorder

a difference in the orderof amino acids in thechain results in a differentprotein

there are millions ofproteins, some soluble(e.g. haemoglobin, redpigment in blood) andsome insoluble (e.g.keratin, from which hairand nails are made)

used for making newcells, growth anddamaged parts of the body

antibodies, hormones andenzymes are also proteins

Lipids

(fats and oils)

four molecules (threefatty acids and oneglycerol molecule) joinedtogether

insoluble in water secondary energy supplyafter carbohydrates havebeen used up

important for storage (oilsin seeds)

also function as insulation(fat under the skin)especially for animalsliving in cold regions

foods like butter, oils andnuts are rich in lipids

vitamins A, B, C, D, E andK

each vitamin hasmany functions

small amounts needed forgood health

A – aids vision in dimlight

B – assists in respiration

C – keeps tissues healthy

D – aids absorption ofcalcium

K – aids in blood clotting

glycerol fatty acids

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NB Both Benedict’s and Fehling’s solutionscontain copper sulphate. Reducing sugarsreduce the copper(II) ions (Cu2+) present in thecopper sulphate to insoluble red-browncopper(I) oxide which contains Cu+ ions and is aprecipitate.

Many monosaccharides can be joined to form or synthesise a very large moleculecalled a polysaccharide. Since condensation (dehydration) reactions are involvedin the synthesis of these polymers, these reactions can be called dehydrationsynthesis. Starch, cellulose and glycogen are examples of polysaccharides. Theycan form very large molecules.

Food testsTable 7.4 shows the standard tests which can be made on a sample of food toindicate each of the main food groups.

64


Table 7.3 Some vitamins needed by Man for healthy growth.

Vitamins Sources Functions Symptoms of deficiency

A carrots, spinach, egg yolk, codliver oil, butter

keeps skin and mucousmembranes healthyaids vision in dim light

dry skin, mucous membranesdegeneratepoor ‘night’ vision

B1 liver, rice, cereals,wholewheat flour, yeast

helps in respiration beriberi – muscles becomeweak and painful, nervoussystem affected

B6 leafy vegetables, eggs, liver,fish, kidney

helps in metabolism depression and irritability

C citrus fruits, green vegetables keeps tissues healthy scurvy – gums bleed, woundstake long to heal, heart failure

D egg yolk, dairy products, codliver oil

also made by the action ofsunlight on the skin

controls calcium andphosphorus absorption,important in bone and toothformation

rickets – growing bones donot calcify, results in ‘bow'legs in young children, and‘knock-knee' in older children

Substance to be tested Test Observations

Reducing sugars – allmonosaccharides (e.g. glucose,fructose) and somedisaccharides (e.g. maltose)

(i) Benedict’s test: 2 cm3 of the solution to betested is put into a test-tube. 2 cm3 of Benedict’ssolution is then added. The mixture is shakenand brought gently to the boil.(ii) Fehling’s test: 2 cm3 of the solution to betested is put into a test-tube. 1 cm3 of Fehling’s Ais added. 1 cm3 of Fehling’s B is then added. Themixture is shaken and brought gently to the boil.

The initial blue colour of the mixtureturns green, then yellow and may forma brick-red precipitate.

Same as Benedict’s test.

Non-reducing sugars e.g.sucrose

1 cm3 of the solution is put into a test-tube and1 cm3 of dilute hydrochloric acid (HCl) is alsoadded. The mixture is boiled for 1 minute. 1 cm3

of aqueous NaOH (NaOH solution) is added,followed by 2 cm3 of Benedict’s solution. Themixture is then shaken and boiled gently.

A red-brown precipitate results as thesucrose is hydrolysed to fructose andglucose by the acid. Fructose and glucoseare reducing sugars, so Benedict’s testthen 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(I2/KI) solution is added. (Iodine may also beadded in its solid form).

A blue-black precipitate results.

polysaccharide ©

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65


Table 7.4 Tests for the main food groups.

Inorganic nutrientsMinerals are inorganic nutrients that are required in small amounts for goodhealth and development. Some are required in only trace (very small) amountsfor good health and thus are called trace elements. Table 7.5 shows some mineralelements required by plants and table 7.6 shows some minerals required byhumans.

Protein Biuret test: 2 cm3 of protein solution is put intoa test-tube. 2 cm3 of 5% potassium hydroxide(KOH) is then added. The mixture is stirred and 2drops of 1% copper sulphate (CuSO4) is added.

A mauve or purple colour slowlydevelops.

Fats Ethanol test: 2 cm3 of fat solution or oil is putinto a test-tube. 2 cm3 of absolute ethanol is thenadded. The mixture is shaken vigorously and3 cm3 of water is added.Grease spot test: A drop of the sample isdropped onto a piece of paper.

A cloudy white suspension can be seenwhen the water is added.

A permanent translucent spot is seen onthe paper.

trace element ©

Element One function Deficiency effects

nitrogen (N) (absorbed as nitrates)

make proteins small yellow leaves andpoor growth

magnesium (Mg) contained in chlorophyll leaves yellow betweenthe veins

iron (Fe) make chlorophyll new leaves yellowbetween veins

calcium (Ca) make cell walls poor stunted growth,leaves yellow, terminalbuds die

potassium (K) maintain the saltbalance in cells

yellow/brown edges onleaves, edges wither,plant dies early

sulphur (S) make proteins young leaves small,thin, yellow betweengreen veins

phosphorus (P) make some proteins poor growth, smallreddish-brown leaves

Table 7.5 Some elements needed by plants for healthy growth.

ITQ2

Give three named examples of foods whichcan be eaten to obtain (i) organic nutrients,and (ii) inorganic nutrients.

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Food additivesMany additives are used in preparing food, for many different reasons. Food addi-tives may be natural or artificial. Common natural additives include sugar, cornsyrup and pepper. Common artificial additives are some flavours and sweeteners.The major groups of additives include the following.

Dyes and colouringsThese are purely cosmetic and rarely add nutritional value. Tartrazine is used togive a yellow colour to foods and drinks, for example, orange juice, fish fingers. Itdoes, however, have some adverse effects as it is associated with:• hyperactivity in children;• allergic reactions;• adverse effects on asthmatics.

PreservativesThese make food less susceptible to bacterial infection, so food can be kept forlonger periods of time in tins, packets, spreads and bottles without spoiling. Whenfood is produced and packaged it may travel thousands of miles, over severalmonths, before it is used. The health of the general population has improvedbecause preservatives reduce the risk of bacterial poisoning. They are perhaps themost easily justified additive, but only make up 1% of all additives used.

Synthetic flavouringsDuring preparation food can lose some of its flavour, so these are added toimprove or even change the flavour.

Flavour enhancers and sweeteners Saccharin is often used to sweeten prepared foods. Monosodium glutamate (MSG,Ali-jo-moto, Vet-sin) is a commonly used flavour enhancer found in processedfoods including soups, fast foods and Chinese foods. Young children and pregnant

66


Minerals Sources Functions Symptoms of deficiency

calcium milk, cheese formation of bonesand teeth

brittle bones and teeth

iron red meat, greenleafy vegetable

formation ofhaemoglobin

anaemia – tiredness, lack ofenergy because of areduction in the number ofred blood cells

iodine sea foods,iodised table salt

formation of thehormone thyroxin

goitre (adults) – reducedmetabolic rate, swelling ofthe thyroid glandcretinism (children) –physical and mentalretardation

phosphorus meat, fish combine withcalcium in theformation of bonesand teeth

brittle bones and teeth

Table 7.6 Some elements needed by Man for healthy growth.

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and lactating women are advised not to eat foods containing MSG as it may berelated to asthma, attention deficit disorder, acute headaches, extreme moodswings, depression and paranoia.

PropellantsCarbon dioxide may be used to form an aerosol, forcing food out of containers.

AcidsThese are added to give a sour taste to prepared food.

Firming agentsAluminium salts are used to retain crispness and gums increase the thickness ofsauces and soups.

A balanced dietWe can group all the foods available to Man into six food groups:• staple foods – cereal grain (such as rice), cornmeal, wheat flour, oats, starchy

fruits, roots, tubers;• peas and beans (legumes) – red beans, pigeon peas, black eyed peas, broad

beans;• dark green, leafy and yellow vegetables – cabbage, patchoi, lettuce,

spinach, pumpkins, carrots;• foods from animals – fish, poultry, meat, milk, eggs, cheese;• fruits – citrus, bananas, apples;• fats – oils, butter, margarine and food with high proportion of fat such as cakes,

biscuits.Figure 7.4 shows the components of a balanced diet. Each block represents a foodgroup and the size of the block indicates the proportion of the diet which that foodgroup should comprise.

Balanced diet related to age, sex and activity of anindividualNutritional requirements vary with age, sex and activity.

Energy requirementEnergy requirements are generally greater for men. They usually have moremuscle, relatively less fat and weigh more than women. In women, the energy

67


Ingredients: sugar, enriched bleach flour (wheat flour, niacin, reduced iron, thiamin 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

Figure 7.3 There are many additivesin the ingredients of manufacturedfood as seen in this list of ingredientsfor a blueberry muffin mix.

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

21% legumesand nuts

11% dark green leafy and/or

yellowvegetables

11%food from

animals11% fruits

5%fa

tsan

dsu

bstit

utes

Figure 7.4 Pie chart showing therelative proportions of foods in a gooddiet.

12 000

8000

10 000

6000

4000

2000

0

Energy (kcal per day)

Age (years)

1–34–6

7–1011–14

15–1819–22

23–2627–30

31–3435–38

39–4243–46

47–5051–59

60–6465–74

75+

male female

Figure 7.5 Energy requirements for men and women vary as they get older.

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requirements are higher during the last three months of pregnancy. Extra energyis needed for growth of the fetus and deposition of fat in preparation for breast-feeding. This requires extra energy because the energy needed by the baby forrapid growth in its early life comes from the mother’s milk.

Energy requirements for a growing individual increase up to about age 18years, when the energy requirements are the greatest. It then decreases as theperson gets older.

Physical activity of an individual varies with both occupation and leisure. Somepeople are mostly sedentary (sitting for much of the time, such as in an office) andothers are very active. Energy requirements for different levels of activity can varygreatly.

Protein requirementMen require more protein than women from the around the age of eleven yearsonwards. This is when the muscle to fat ratio starts to differ because of the devel-opment of secondary sexual characteristics. Women start to store fat in their hipsand breasts and men develop more muscle, especially on their shoulders and legs.Extra protein is required by women during pregnancy and breast-feeding.

Requirements of minerals and vitaminsMineral intake is especially important during pregnancy and lactation. The mother’sdiet must contain sufficient iron, calcium, vitamin C, folic acid, and everythingneeded to make the baby’s tissues, such as blood, bone and muscle. Extra folic acidmay be given to the mother to reduce the risk of spina bifida in the baby.

MalnutritionMalnutrition means bad nutrition, and can be applied to under-eating, over-eatingand bad eating habits. Malnutrition is the cause of many diseases like deficiencydiseases, obesity, heart diseases and anorexia. Education on balanced diet and goodhealth is very important in preventing the occurrence of many diseases.

Under-eatingStarvation is one kind of under-eating and is most often associated with developingcountries. It means not eating enough food to supply the energy requirements fordaily activities. Also not enough protein and vitamins are eaten which arenecessary for growth, development, resistance to infection and a healthy life.Marasmus and kwashiorkor are common conditions caused by under-eating.

Some signs and symptoms of marasmus and kwashiorkor:• very underweight (less than 60% for age);• thin muscles, thin arms and legs;• reduced growth may led to reduced mental development;• reduced resistance to infection;• sometimes swelling of the body tissues with fluid (oedema);• shrunken features giving the face the appearance of an old person;• hair becomes thin, sparse and easily removed;• rough skin;• little interest in surroundings.Anorexia is another kind of under-eating but is associated with developed countries. It is the voluntary refusal to eat and is most commonly found in teenagegirls, though occasionally teenage boys. It is as much a psychological illness as aphysical one, because the refusal to eat is based on a poor self image, the sufferercontinuing to think that they are fat even when they are underweight. Recoveryrequires treatment for the psychological condition as well as improving the diet.

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anorexia ©

malnutrition ©

ITQ3

Describe a meal which includes all thenutrients necessary for good health.

Figure 7.6 A child suffering fromkwashiorkor or marasmus.

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ObesityObesity results from over-eating, especially of fatty foods, and a lack of exercise.Excess fat accumulates in the body and body mass increases to well above normal.Obese people are predisposed to many diseases like diabetes, hypertension (highblood pressure), coronary heart disease, arthritis, cancer and stroke. Over-eatingcan be prevented by eating sensibly, and engaging in regular aerobic exercise.

Heart diseases and cardiovascular diseases Some diseases of the heart and cardiovascular system develop slowly after yearsof living on a diet of fatty foods and not much exercise. Atherosclerosis is a diseaseof blood vessels. It is a thickening of the inner layers of artery walls, eventually theartery may become blocked. If it is a coronary artery, then the heart muscle willnot be supplied with food and oxygen and that part of the heart dies. This couldresult in a heart attack (coronary heart disease). A similar blockage in a bloodvessel in the brain results in a stroke. The rough surface of the thickened wallcould also encourage formation of a blood clot which may block blood vessels.

Utilisation of food in ManMammals, which include humans, feed by taking in or ingesting organic food.This particular type of heterotrophic nutrition is termed holozoic nutrition, andincludes ingestion, digestion, absorption, assimilation and egestion.• Ingestion – This is the act of taking in food (into the mouth in humans).• Digestion – This is the process of breaking down large, complex, insoluble

material into small, simple, soluble molecules. The teeth physically break thefood into pieces, and enzymes then chemically break down the large moleculesinto smaller ones.

• Absorption – This is the diffusion of soluble food molecules (glucose, aminoacids, fatty acids, glycerol, vitamins, minerals and water) into the bloodstream.

• Assimilation – This is when these food molecules are taken from the blood andused by the body cells for respiration, growth and development.

• Egestion – This is the process by which the undigested part of the food isremoved from the body. It is also known as defaecation.

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holozoic nutrition ©

coronary heart disease ©

obesity ©

Figure 7.7 A person is described as‘obese’ if they weigh at least 20%more than the average for someonetheir height.

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Mammals differ from other animals in that they have more than one type oftooth. In Man there are four kinds of teeth. Table 7.6 summarises the structuresand functions of these different types of teeth.

Milk teeth are the first set of teeth in humans. They cut singly or in pairs fromthe time a child is approximately six months old. By age three, most children haveabout 20 teeth. These begin to fall out when a child is about seven years old.

Permanent teeth are the teeth which replace the ones that have fallen out. Anadditional 12 new teeth also erupt, which make up the complete set of permanentteeth by about age 17. Most adults have eight incisors, four canines, eight premolars and twelve molars. The four molars at the end of the jaw are the lastset to grow through the gum and are called wisdom teeth.

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milk teeth ©

permanent teeth ©

wisdom teeth ©

fibres

enamel (hard material)

dentine

pulp cavity (containsblood vessels andnerve endings)

cement (holds thetooth in the bone)

jaw bone

gum

root

crown

nerve

Figure 7.8 A section through a tooth showing the general structure.

incisorscanine

premolars

molars

duct fromsalivary gland

salivary glands

tongue

physical digestion ©

Digestion

TeethTeeth help with the physical breakdown or mechanical breakdown of food. Thisis called physical digestion. The structure of a typical tooth is shown in figure 7.8.

Figure 7.9 There are four types ofteeth in a human’s mouth.

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The use of fluoridesThe use of fluorides in toothpaste or in water supplies helps to prevent tooth decayin humans. Fluorides are compounds of the element fluorine which improveresistance to tooth decay by hardening the enamel. When permanent teeth aredeveloping in children, the use of fluorides is effective in helping these ‘new’ teethresist decay.

Compounds of fluorine can act as serious pollutants in the environment. In theproduction and extraction of aluminium from bauxite, sodium aluminiumfluoride (Na3AlF3) is used to lower the melting temperature of alumina from 2050 °C to 950 °C, thus using less energy. The exhaust gases from the manufactureof aluminium then contains fluorides. Fluorides seem to affect trees, and on grassthey can enter food chains. The teeth and bones of grazing animals are affectedbadly.

Although fluoride provides resistance to tooth decay in humans, an excess inthe environment can be harmful. Also fluorides can be dangerous to youngchildren and they should never swallow fluoridated toothpaste.

The action of enzymes in digestionTeeth physically break down food; however food must also be chemically brokendown. Chemical digestion involves enzymes. Enzymes are organic catalysts,which means they speed up chemical reactions occurring in living cells. Duringdigestion, enzymes speed up the rate at which the large, insoluble food moleculesare broken down into small, soluble food molecules.

chemical digestion ©enzyme ©catalyst ©

Type Shape Function

incisor chisel-shaped forcutting, one root

cutting foodbiting off bits of food

canine pointed or ‘dagger'-shaped, one root

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

premolar flat with cusps orbumps on the surface,fairly broad surface2 pointed cusps, 2 roots

crush and grind food

molar flat, with cusps on thesurface, broad surface

4 or 5 cusps, 2 or 3roots

large back teeth

crush and grind food

Table 7.6 The shape and function of human teeth

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saliva ©

All enzymes have similar properties which include the following.• They are all proteins.• There are thousands of enzymes, and each one is specific for the type of

chemical reaction it will speed up.• They are required in small amounts.• They are inhibited or prevented from working by poisons like cyanide and

arsenic.• They work best at a particular temperature called the optimum temperature (see

figure 7.10).• They are denatured or destroyed by high temperatures.• They work best at a particular pH, called the optimum pH (see figure 7.11).The substance that the enzyme breaks down is called the substrate and thesubstances that are made are known as the products.

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substrate ©products ©

rate increases as the temperature increases

optimum temperature– enzyme works bestat this temperature

enzyme breaks downat higher temperature

Rate of reaction

20 30 40 50 60Temperature (oC)

Figure 7.10 The effects oftemperature on an enzyme-catalysedreaction. The activity of the enzyme issmall below 20 °C, rises steadily to amaximum near 50 °C, then fallssharply.

• pepsin works best at pH 2.5• trypsin works best at pH 8.0• most enzymes in cells work best at pH 7.2

Rate of reaction

lower pH higher pHoptimum pH for enzyme

2 4 6 8 10pH

optimum pH ofmost enzymesin human cellsoptimum pH

of pepsin optimum pHof trypsin

pH at whichenzyme works best

Rate of reaction

Figure 7.11 The effects of pH on an enzyme-catalysed reaction. The activity ofthe enzyme rises sharply near the optimum pH and falls just as sharply as that pHis exceeded.

ITQ4

Define the terms physical digestion andchemical digestion.

enzyme (amylase)polysaccharides disaccharides + monosaccharides

enzyme (protease)proteins amino acids

enzyme (lipase)lipids fatty acids + glycerol

Digestion and absorption in the alimentarycanalThe alimentary canal, or gut, is a long muscular tube, which extends from thethroat to the anus. It consists of the major parts of the digestive system wheredigestion and absorption of food take place.

The mouthDigestion begins in the mouth, after food is ingested using the hands, lips andtongue. The teeth break the food down into smaller pieces. This is done with thehelp of saliva, which moistens the food. Saliva is secreted from the salivary glands

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and is a mixture of water, mucus and salivary amylase. The water and mucus soften the food, while the enzyme salivary amylase begins to digest the starch inthe food. The mucus also helps food to move easily down the alimentary canal.

Salivary amylase breaks down the starch bonds by hydrolysis, and so breaksdown starch into smaller and smaller chains, eventually to glucose molecules.

So both physical digestion and chemical digestion, by enzymes, occur in themouth. The tongue churns food and rolls it into a bolus, or a ball-like structure,which is then swallowed.

The oesophagusWhen food is swallowed it enters the oesophagus. The trachea, or windpipe,which opens to the lungs lies to the front of the oesophagus. If you press yourhands gently on your throat, you can feel the rings of cartilage of the trachea. Theoesophagus is directly behind this.

When food is swallowed, it is prevented from going into the trachea by a small,flat-like structure called the epiglottis, which covers the trachea as you swallow. Itis thus impossible to swallow and inhale at the same time. Try it!

However, when a person is eating while talking and laughing, the food couldbecome stuck in the trachea. The Heimlich manoeuvre, which forces air rapidlyout of the lungs, can be applied to remove the stuck food.

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salivary amylase ©

bolus ©

oesophagus ©

epiglottis ©

stomach

pyloric sphincter

pancreaspancreatic duct

colon

rectum

anus

largeintestine

oesophagus from mouth

duodenum

ileumsmall

intestine

caecumappendix

gall bladder

liver

bile duct

diaphragmcardiac sphincter

Figure 7.12 The human alimentary canal.

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peristalsis © The oesophagus is a muscular tube and food moves down by peristalsis. This isa wave of muscle contractions that moves downward and squeezes the food intothe stomach.

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starch – long chain of glucose

addition of water(hydrolysis)

eventually, all the bonds will be broken

glucose

amylase

bondbroken

smaller chains

bondbroken

bondbroken

glucose

Figure 7.13 Starch is broken down to glucose by the enzyme amylase.

epiglottis

air can gointo the trachea

bolus goes intothe oesophagus

tracheaclosed

food

tongue

trachea oesophagus

Figure 7.14 The epiglottis stops food entering the trachea when you are eating.

stomach

bolus is pushed downand into the oesophagus

muscles contract, narrowing the oesophagus and pushing

the bolus down

oesophagus

Figure 7.16 A bolus moves down the oesophagus to the stomach by peristalsis.

(a) give five sharp slapsbetween the

shoulder blades

(b) clasp both handsaround the waist

(c) pull sharply upwardsand below the ribs

Figure 7.15 The Heimlichmanoeuvre.

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The stomachThe muscular walls of the stomach relax and contract to churn the food as itarrives. Food is mixed with enzymes, mucus and hydrochloric acid. This mixtureis called chyme. The stomach walls are dotted with pits leading to gastric juiceglands that secrete gastric juice into the stomach.

Gastric juice consists of:• mucus;• hydrochloric acid;• pepsin.Digestion of proteins begins in the stomach as the long protein chains are brokendown by the enzyme pepsin into amino acids. Hydrochloric acid provides theacidic medium in which pepsin works most efficiently. It also kills any pathogensthat may have entered the body with the food.

The stomachs of young mammals produce the enzyme rennin, which curdlesor clots the milk that they get from their mother. The milk proteins are thenbroken down by pepsin.

After one or two hours in the stomach, the sphincter muscles at the bottom ofthe stomach relax and open, so that small amounts of chyme pass into the nextregion of the alimentary canal, the duodenum.

Peptic ulcersA peptic ulcer is a hole or ‘sore’ in the stomach lining. It was thought to be causedby excessive secretions of hydrochloric acid and pepsin as the stomach wall iseaten away, but recent research has shown it to be caused by the presence ofHelicobacter pylori, a species of bacterium that lives in the gut. Some patients havebeen successfully treated with antibiotics; others have been successfully treatedwith a drug which suppresses the production of stomach acid.

The duodenumThe duodenum is the first region of the small intestine. It receives chyme from thestomach and secretions from the gall bladder and pancreas. Bile, which isproduced by liver cells and stored in the gall bladder, breaks down large lumps offat into tiny droplets. This process, called emulsification, increases the surface areaof the fats making it much easier for the enzyme lipase to digest the fat.

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chyme ©gastric juice ©

rennin ©

pepsin ©

duodenum ©bile ©

emulsification ©

part of the stomach wall

magnified

pits in the stomachwall secrete gastric juices

Figure 7.17 Gastric juice pours out of pits in the stomach wall.

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villus wall, one cellthick – diffusion of foodmolecules can occur readily

capillary network – foodmolecules are absorbedinto the blood

lacteal – larger foodmolecules (fatty acids)are absorbed here

ileum

finger-likeprojections called

villi

digestedfood

one villusmagnified

Pancreatic juice is secreted from the pancreas and contains many enzymes:• amylase continues the digestion of starch, digesting the maltose into glucose;• lipase digests fats or lipids into fatty acids and glycerol;• trypsin is a protease (an enzyme which digests protein) that breaks down long

protein chains into shorter ones, or polypeptides, so that they can be brokendown into amino acids by other proteases.

These enzymes work best in a neutral environment, but the chyme, which camefrom the stomach is acidic because it contains hydrochloric acid. Pancreatic juicealso contains sodium hydrogencarbonate which neutralises the hydrochloric acidto create a pH 7–8, which is optimum for pancreatic enzymes. Bile also containsbile pigments which are waste products from the liver that need to be excreted.

The food is now fully broken down physically and chemically into the end-products of digestion.

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pancreatic juice ©

ITQ5

Describe, giving examples, the role of enzymesin digestion.

ITQ6

Copy and complete the table. Part of alimentary canal Importance

mouth

stomach

duodenum

Figure 7.18 The wall of the ileum is made up of villi that increasethe surface area.

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The ileumThe ileum is the second part of the small intestine and is the site of absorption inthe alimentary canal. By the time food reaches the ileum it has been broken downinto glucose, fatty acids, glycerol, amino acids, vitamins, minerals and water.These nutrients are small enough to be absorbed and used by the body.

The structure of the ileum has many adaptations which make it good forabsorption.• It is about 6 metres long and has a large surface area.• There are folds in the intestinal walls that increase the surface area even more

for efficient absorption of nutrients (see figure 7.18). These finger-like projec-tions in the wall are called villi. They are covered with epithelial cells whichthemselves have microscopic folds on their surface, called microvilli. These further increase the surface area for absorption.

• The epithelial cells have large numbers of mitochondria, which provide theenergy for transport of the nutrients from the ileum to the blood.

• Each villus has a good blood supply in the form of a dense network of capillariesthat transport the nutrients rapidly away from the ileum to the liver for processing.

• Each villus contains a lacteal, or lymph capillary, which absorbs fatty acids fromthe digestion of fat.

The colonAfter most of the nutrients have been absorbed into the blood in the ileum, theremaining intestinal contents, now called faeces, continue to move slowly alongthe colon, or large intestine. Faeces consist of undigested cellulose and plant fibre,dead bacteria and intestinal cells scraped off the gut walls. The main function ofthe colon is to reabsorb water from the faeces into the bloodstream so that waterloss from the body is minimised.

The rectumThe faeces are stored temporarily in the rectum. As faeces accumulate, pressureincreases in the rectum which results in a desire to defaecate, or expel faeces,through the anus.

About 24 hours after eating, most of the nutrients have been absorbed fromthe food and the undigested part is ready to be gotten rid of.

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ileum ©

villi ©microvilli ©

lacteal ©

faeces ©colon ©

rectum ©

anus ©

(a) undigested food with fibre is bulkyand pushed easily along the colon

stored temporarily inthe rectum then egested

water reabsorptionoccurs in colon

rectum anusperistalsis

faeces stays in colon and formsa hard solid mass that is

difficult to move – this is constipation

peristalsis does not take placereadily – undigested food has no fibre

water reabsorption occurs

(b)

Figure 7.20 Dietary fibres help to prevent constipation. In (a) the faecescontaining fibre stay bulky and soft and are easy to egest. (b) Without fibre, thefaeces become hard and solid and are difficult to get rid of.

rectumanus

largeintestine

smallintestine

Figure 7.19 The arrangement of theintestines in Man.

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assimilation ©

constipation ©

ConstipationConstipation results from poor eating habits. A diet lacking fibre can lead to ablockage of the alimentary canal. Egestion of undigested waste material thencannot occur normally.

Constipation sometimes results in haemorrhoids, which are protrusion oftissues through the anus because of forced ‘pushing’. Constipation also increasesthe chance of developing colon cancer. Dietary fibre, the undigestible part of foodfrom plants (mainly cellulose), aids peristalsis and prevents constipation.

AssimilationAssimilation is the process of incorporating and making use of the digested foodinto the body. These absorbed food molecules may be stored by the body forfuture use, broken down to produce energy or used for growth, repair and tomaintain good health.

Monosaccharides (such as glucose)These are taken to the liver, then to the rest of the body where:• they are used in respiration;• excess amounts are converted into glycogen in the liver, and stored in liver and

muscle cells;• excess amounts are converted to fat and stored under the skin or around organs.

Amino acidsThese are taken to the liver and then to the rest of the body where:• they are used by the body cells for growth and repair;• they are used to make hormones and enzymes;• excess amounts are converted to glycogen or fat;• excess amounts are broken down, or deaminated, in the liver and converted to

urea to be excreted by the kidneys.

Fatty acidsFat molecules are carried by the lymph to the blood and are:• stored under the skin and around the organs;• used to form new membranes in cells and organelles;• used for respiration in some circumstances.

Functions of the liverThe liver is one of the most important organs in the body as it has many functionsthat are essential to keeping the body healthy.• Carbohydrate metabolism – Excess glucose is stored up as glycogen and

reconverted to glucose when blood sugar levels fall. Excess carbohydrate mayalso be converted to fat.

• Lipid metabolism – Excess cholesterol is excreted into the bile and removedfrom the body

• Protein metabolism – Excess amino acids are broken down to form ammoniaand then converted to the less toxic substance urea. Urea is transported to thekidneys by the blood and excreted in urine.

• Production of bile – Bile salts are produced and temporarily stored in the gallbladder. They then travel to the duodenum to aid in digestion.

• Storage of vitamins – A number of vitamins are stored in the liver andreleased if deficient in the diet.

• Storage of minerals – The liver also acts as a store for some essential minerals,such as iron and potassium. (This is why liver is a nutritious food.) They can bereleased into the body if the diet lacks these minerals.

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ITQ7

Describe the route taken by a bolus from themouth to the anus.

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• Synthesis of plasma proteins – These important proteins are found in bloodplasma. For example prothrombin and fibrinogen are needed for blood clotting.

• Detoxification – Toxic materials absorbed from the intestines are stored,broken down or gotten rid of by the liver.

• Breakdown of red blood cells – Red blood cells are broken down (they liveonly for three months) and the iron components may be stored, reused, orexcreted as bile pigments.

• Production of heat – A lot of metabolic activity occurs in the liver, whichrequires a considerable amount of energy. Much of the energy from respirationis lost as heat, so the liver generates a lot of heat. In mammals (including Man)and birds, the liver also plays an important role in keeping the body at the righttemperature inside.

Þ A balanced diet is important for good health.Þ A balanced diet has appropriate proportions of carbohydrate, proteins, lipids,

vitamins and minerals. Water and fibre are also important.Þ Plants need minerals for healthy growth.Þ Ingestion in Man is the intake of food using the mouth, hands and lips.Þ Physical digestion involves breakdown of food by teeth, and chemical

digestion is the breakdown of food by enzymes.Þ In Man, there are four different kinds of teeth: incisors, canines, premolars

and molars.Þ Incisors are chisel-shaped and used for biting food.Þ Canines are dagger-shaped and are used to tear or rip food.Þ The premolars and molars are used to chew food into smaller pieces.Þ Enzymes are biological catalysts: they speed up the chemical breakdown of

food.Þ Food is broken down from insoluble to soluble substances by enzymes.Þ Digestion takes place in the mouth, stomach and duodenum.Þ Absorption is the movement of the end-products of digestion into blood.

It occurs in the ileumÞ The ileum has several adaptations for absorption, such as the villi.Þ The food is assimilated when the body cells make use of it.Þ The liver has many important functions relating to the assimilation of food.

ITQ1 (i) Diet is the quantity and quality of food eaten every day by anindividual.

(ii) A balanced diet has the quantity and quality of food needed tomaintain good health.

ITQ2 (i) Organic foods include: chicken, bread, liver.(ii) Foods which contain inorganic nutrients include: lettuce, liver,

banana.ITQ3 The answer needs to include examples of each of the six food groups,

such as:

ITQ4 Physical digestion is the mechanical breakdown of food into smallerpieces by the teeth.

Chemical digestion is the breakdown of food by enzymes into solublecompounds.

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Summary

Answers to ITQs

Staple Peas andbeans

Dark green leafyvegetables

Food fromanimals

Fruit

rice red beans steamed pumpkin,carrot

chicken orange juice

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ITQ5 Enzymes speed up the breakdown of food molecules into their respectiveend-products, without being used up themselves. Some examples are:• amylase, which breaks down starch eventually to glucose;• pepsin, which breaks down proteins into polypeptides;• lipase, which converts lipids into fatty acids and glycerol.

ITQ6

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Part of alimentarycanal

Importance

Mouth Food is moistened and lubricated. Physical digestiontakes place here. The conversion of starch to maltose(chemical digestion) begins.

Stomach Acid contents kill bacteria in food. Food is churnedinto chyme. In babies, curdling of the milk occurs. Inadults, protein digestion begins in the stomach, asproteins are converted to polypeptides.

Duodenum Here the enzymes amylase, trypsin, and lipase aresecreted, which break down starch to maltose, whichis further broken down into glucose and fructose;polypeptides to amino acids, and lipids into fatty acidsand glycerol respectively. Chemical digestion takesplace in the duodenum.

1 (i) Five main processes occur in holozoic nutrition. Define each in theorder in which they occur.

(ii) Give two functions of the tongue during eating.(iii) Describe how food moves down the oesophagus.(iv) Name the enzyme found in saliva and describe its action.

2 (i) List the functions of these substances in the stomach:(a) mucus; (b) hydrochloric acid; (c) the enzyme pepsin.

(ii) A peptic ulcer is a damage to the stomach wall. It can be very painfuland is easily infected.(a) How can the stomach wall be damaged? Give details of how

ulcers are formed.(b) Why is an ulcer painful?(c) Why is an ulcer easily infected?

(iii) What are the products of digestion of:(a) starch? (b) lipids? (c) protein?

3 (i) Explain how the structure of the wall of the small intestine is adaptedfor its function of absorption.

(ii) The table below refers to some enzymes involved in digestion of foodin the digestive system. Copy and complete the table.


Name of enzyme Site of production Products of reaction

Fatty acids and glycerol

Salivary gland

Stomach wall

Maltase

ITQ7 Mouth → oesophagus → stomach → duodenum → ileum → colon →rectum → anus

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(iii) The diagrams show different types of teeth found in a human mouth.Copy and complete the table below to show the type and function ofeach tooth.

4 (i) List the components of a balanced diet.(ii) Define (a) obesity; (b) malnutrition;

(c) deficiency disease; (d) food additive.(iii) Vitamins and minerals are essential to a healthy life. Explain why:

(a) pregnant women must include calcium and iron in their diet;(b) it is recommended that we eat an orange a day.

(iv) Nutritional requirements vary with age, sex and activity. Describe thenutritional requirements of:(a) a 19-year-old male who plays competitive football;(b) a 19-year-old female who loves to read.

5 The figure below shows the graphs obtained from an investigation into anenzyme-controlled reaction. Each represents an experiment performed tostudy the time taken for the enzyme to break down the substance. Graph 1shows the time taken under different temperature conditions with thereaction at a constant pH of 6.7. Graph 2 shows the time taken underdifferent pH conditions at a constant temperature of 40 °C.

Tooth

A B C

Type of tooth

Function of tooth

A B C

10 20 30 40 50 60 70 8000

2

4

6

8

10

12

14

16at a constant pH 6.7

a

Time (minutes)

Temperature (oC)

T

Time (minutes)

1 2 3 4 5 6 7 800

2

4

6

8

10

12

14

16

at a constant temperature of 40 o C

Temperature (oC)

81

Graph 1 Graph 2

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82


Study the graphs and answer the following.(i) (a) At what temperature did the reaction occur in the shortest time?

(b) At what pH did the reaction occur in the shortest time?(ii) In graph 1:

(a) Why did the reaction slow down at higher temperatures?(b) What effect on the reaction rates is shown by a steady increase

from low to medium temperatures?

6 An experiment was carried out to investigate the effect of temperature onthe rate of an enzyme-controlled reaction. The concentration of enzymes andsubstrate were kept constant at all the temperatures investigated. The resultswere as follows.

(i) Plot the results on graph paper.(ii) Interpret and explain them as fully as you can.(iii) If the enzyme used was amylase, name the substrate used in the experi-

ment.(iv) If the enzyme used was amylase, what effect would pH have on its

activity?

7 The table below shows the activity on an enzyme in relation to pH.

(i) At which pH is most activity seen?(ii) At which pH is least activity seen?(iii) What is the optimum pH for this enzyme?(iv) Give an example of an enzyme that might give these results.(v) Give examples of enzymes that would not be expected to give these

results.

Temperature (°C)

Rate of reaction (mg of products perunit time)

51015202530354045505560

0.30.50.91.42.02.73.33.63.62.30.90

pH 4.5 5.5 6.5 7.5

Units of enzymeactivity

3.1 9.6 14.5 10.1

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Aerobic respirationRespiration is the process by which the energy in food is made available for a cellto do the work necessary to keep it alive. When oxygen is used in the reaction,we call it aerobic respiration. The process is catalysed by enzymes and is also calledcellular, internal or tissue respiration.

83

Resp i ra t ion ú understand that respiration takes place at the level of the cell;ú understand the function of ATP;ú describe the process of aerobic respiration;ú distinguish between aerobic and anaerobic respiration;ú describe the uses of anaerobic respiration to Man;ú understand simple investigations that show the products of

respiration.



8

aerobic respiration ©

food

oxygen

cellrespiration energy

energy for building up materials

energy for breakdown of materials

energy for secretion

energy for contraction

can move

can grow

can reproduce

can respond

Some of the uses ofthe energy made by

a cell

Some of activitiesof an organism.

They all require energy

T

organism isalive when allthe cells are

alive and workingFigure 8.1 An organism is alivewhen all its cells are respiring.

ITQ1

What is the purpose of respiration?

ITQ2

When do animal cells and plant cells respire?

yeast

Man

bacteria

respiration

aerobic anaerobic

breadproduction

alcoholproduction

yoghurtproduction

oxygendebt

mitochondrionADP→ ATP

Concept map

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body cellwhere respiration

occurs

glucose

lungs

blood rich in O2 is taken to all body cells

blood rich in glucose is taken to all body cells

intestine

blood rich inCO2 is taken to

lungs to get rid of CO2

O2

How do the food and oxygen get to respiring cells?Food – In animals food eaten is digested and absorbed into the bloodstream. Theend-products of digestion eventually reach all the body cells. In plants the foodmade in photosynthesis in the leaves travels around in phloem tubes and even-tually reaches all body cells.

Oxygen – In vertebrates, oxygen comes from the air that is inhaled into the lungs.It diffuses into the bloodstream and is transported to all the body cells. In plantssome of the oxygen comes from photosynthesis and some through diffusion inthrough the leaves and other parts of the plant.

In both plants and animals the type of food used for making energy is usuallyglucose. Energy is released when it combines with oxygen (the oxidation ofglucose). Carbon dioxide is a waste product of this reaction. In vertebrates itdiffuses back into the bloodstream, to be taken to the lungs and exhaled out of thebody. In plants it is used for photosynthesis during daylight.

Respiration or cellular respiration occurs in a series of steps, each of which iscatalysed by enzymes. The overall process can be summarised in words or by theequation below:

glucose + oxygen → energy + carbon dioxide + waterEquation: C6H12O6 + 6O2 → energy + 6CO2+ 6H2O

During aerobic respiration glucose is broken down completely into carbondioxide and water.

At each step in the breakdown of glucose, energy is released. This is used toconvert a chemical called ADP into adenosine triphosphate (ATP). Each moleculeof ATP acts as a little ‘packet’ of energy. The energy can be stored and used laterwhen needed.

There are many advantages of storing and using energy in small packets likethis.• The energy can be released from ATP wherever and whenever it is required by

a cell.• The energy can be released rapidly.• Energy is not wasted. A large amount of energy is released by oxidising one

glucose molecule and many ATP molecules are formed. A cell may not requirevery much energy at once. By storing the energy in small packets of ATPmolecules, the cell can use small amounts of energy as required.

• 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 transported and used else-

where without causing reactions in between.

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8 · Respiration

respiration equation ©

ATP ©

Figure 8.2 A respiring cell in a mammal is supplied with food and oxygen.

ITQ3

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

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Energy production and utilisation are very efficiently and carefully controlled bythe cell.

Respiration occurs in an organelle called the mitochondrion. Mitochondria arepresent in all cells, animal and plant, and are sometimes referred to as the ‘powerhouses‘ of the cell.

The energy stored in ATP (adenosine triphosphate) is released when it isconverted to ADP (adenosine diphosphate).

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8 · Respiration

mitochondrion ©

C6H12O2 + O2

oxidationof

glucose

energy

energy

energy

energy

ADP

ATP

ADP

ATP

ADP

ATP

CO2 + H2O

ADP

ATP

the energy from thebreakdown of glucose is

stored in this high energy bond

ADP

ATP

adenosine diphosphate

adenosine triphosphate

adenosine phosphate

high energy bond

ATP is a packet of energy!

Figure 8.3 The oxidation of glucose results in the formation of many moleculesof ATP.

ATP ADP

energy + ++ phosphate

energy usedby the cell

P P P P P P( )

O2 transportedto cell

red blood cell

capillary bringingblood from lungs

ATPEnergy

O2 CO2

energy is madeduring respiration inthe mitochondrion

CO2 transportedto lungs

CO2O2

O2

Figure 8.4 Energy can be released from ATP during respiration in themitochondrion.ITQ4

Where does respiration occur?

ITQ5

Give three reasons why it is advantageous tostore energy in small packets.

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Habitats such as stagnant ponds and deep underground have no oxygen.Organisms living there have adapted to survive without oxygen; they must respireanaerobically all the time. These organisms include some worms, some bacteriaand some fungi. Parasites that live inside other organisms, such as the gut parasitetapeworm and bacteria, also live in conditions that lack oxygen. They must alsorespire anaerobically.

Living cells that normally respire aerobically can also respire anaerobically ifoxygen is lacking. Animal and plant cells do this in different ways (see table 8.1).

Table 8.1 summarises the differences between aerobic and anaerobic respiration.

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8 · Respiration

ITQ6

Give two examples of organisms that respireaerobically, and two that respire anaerobicallyonly.

Aerobic respiration Anaerobic respiration

uses oxygen does not use oxygen

• in plants and animals:C6H12O6 + 6O2 → energy + 6H2O +

6CO2

water and carbon dioxide are wasteproducts

• in animal cells:C6H12O6 → energy + 2C3H6O3

lactic acid is the waste product• in plant cells:

C6H12O6 → energy + 2C2H5OH + 2CO2

ethanol and carbon dioxide are wasteproducts

large amounts of energy produced(2880 kJ) for the breakdown of eachmolecule of glucose

small amounts of energy are produced(150 kJ per glucose molecule in animalsand 210 kJ in plants)

glucose is broken down completely toinorganic molecules

glucose is not broken down completely –ethanol and lactic acid are organicmolecules that still contain useful energy

occurs in the mitochondria of the cell occurs in the cytoplasm of the cell

Table 8.1 Differences between aerobic and anaerobic respiration.

(a) Plant and animal cellscan respire aerobically.

(b) Animal and plant cells can respire anaerobically but do so in different ways.

C6H12O6 + 6O2 energy + 6H2O + 6CO2

oxygen

glucose

oxygen

glucose

water

energy

energy

C6H12O6 energy + 2C3H6O3

glucose

glucose

energy

energy

lactic acid

carbon dioxide

carbon dioxide

ethanol

C6H12O6 energy + 2C2H5OH + 2CO2

water

carbon dioxide

anaerobic respiration ©

Figure 8.5 Cells can respire anaerobically and aerobically.

Anaerobic respirationRespiration can also occur without oxygen and this type of respiration is calledanaerobic respiration. Both anaerobic and aerobic respiration involve thebreakdown of glucose, however in anaerobic respiration it is not completelybroken down.

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cell duringanaerobic respiration

e.g. muscle cells, during prolonged strenuous exercise

energy (smaller amount)

lactic acid

series of reactions leading to breakdown to

CO2 + H2O

oxygen

used for contraction etc.

T

Anaerobic respiration in humansHuman cells respire normally aerobically. However, during strenuous exercise,muscle cells need much more energy for the extra work that they are doing.Breathing rate and heart rate increase in an attempt to get more oxygen to thesecells. Sweating occurs to help lose some of the extra energy as heat. Withincreased respiration, a lot of heat is produced which is lost from the skin (seechapter 16). After a while of sustained exercising, the oxygen supply becomesinadequate, even with the panting for air and the increased heart rate. The musclecells then respire anaerobically.

Energy is still produced when cells respire anaerobically, although it is a muchsmaller amount for each molecule of glucose. This means that they can continueto do work (contract and relax).

Lactic acid is a waste product of this reaction. It builds up in the muscles andcauses them to ache. This is often called fatigue. After exercise, the body has to getrid of the lactic acid as quickly as possible. This is done by using oxygen to changeit back to a chemical like glucose so that it can be broken down completely inaerobic respiration.

A person continues to ‘breathe hard‘ or pant for some time after exercise asoxygen is needed to get rid of the lactic acid. The oxygen required to get rid of thelactic acid is called the oxygen debt.

Anaerobic respiration in yeastDuring anaerobic respiration in yeast, ethanol and carbon dioxide are produced aswaste products. Ethanol is an alcohol and the process is known as alcoholicfermentation. Yeast is very important in the making of alcohol and bread. Theethanol can be produced in many ways to make a wide range of alcoholic drinks,including beer and wine, which are enjoyed by Man.

The production of carbon dioxide is used in breadmaking to make dough rise.The carbon dioxide produced by the yeast as it respires accumulates inside thedough in small pockets. The dough is seen to get bigger or rise. The ethanol whichis also produced evaporates when the bread is baking in the oven.

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8 · Respiration

oxygen debt ©

alcoholic fermentation ©

lactic acid ©

fatigue ©

Figure 8.6 The oxygen debt is the oxygen needed to break down the lactic acidformed during exercise.

Figure 8.7 The build-up of lacticacid in muscle cells after strenuousexercise can be painful.

ITQ7

Humans respire aerobically normally. When dohumans respire anaerobically?

anaerobic respirationglucose lactic acid + energy

in muscle cells

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milk contains lactose

store at 2 oC

pasteurisationheat treatment (90 oC) to kill disease-causing organisms

inoculationcooled to 40 oC and a 'starter' culture of bacteria added

e.g. Lactobacillus bulgaris

fermentationincubated in large vats (40 oC for about 5 hours) –

lactose converted to lactic acid producing natural yoghurt

cool, add fruits, etc.

package and distributeat 4.5 oC the bacteria remain alive but no more

fermentation occurs at this temperature

Anaerobic respiration in bacteriaSome bacteria also respire anaerobically. Like animal cells, they make lactic acidas a waste product. We make use of this in the manufacture of yoghurt andcheese.

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8 · Respiration

Figure 8.9 The manufacture of yoghurt depends on theanaerobic respiration of Lactobacillus bacteria.

ITQ9

Some bacteria can be found in canned foods ortins, despite the fact that the cans and tins aresealed so that no air can enter. How is thispossible?

Þ All cells respire to make energy to carry out the processes of life.Þ Respiration takes place in the mitochondria of cells.Þ Food is oxidised during respiration, and carbon dioxide and water are

produced as waste products:C6H12O6 + 6O2 → energy + 6H2O + 6CO2

Þ Energy is stored in phosphate bonds in ATP (adenosine triphosphate).Þ There are many advantages to storing energy as small packets of ATP.Þ There are two types of respiration: aerobic and anaerobic.Þ Aerobic respiration uses oxygen and produces a lot of energy.Þ Anaerobic respiration produces a small amount of energy without the use of

oxygen.Þ Humans usually respire aerobically but their muscle cells can respire

anaerobically during prolonged exercise.Þ Lactic acid is produced during anaerobic respiration in animals and creates an

oxygen debt which has to be repaid.Þ Anaerobic respiration in yeast produces ethanol which is used in the alcohol

industry and carbon dioxide which is used in making bread.Þ Anaerobic respiration in bacteria is used in the making of yogurt and cheese.

Summary

sugar ethanol + carbon dioxidefermentation

if the sugarused comes from

• barley seeds

• cane sugar or molasses

beer

rum

• 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

fermentation

fermentation

after some time

• dough rises as bubbles of CO2get caught in the dough

• baking kills the yeast and evaporates the ethanol

Figure 8.8 Uses of fermentation.

ITQ8

Describe the process of alcoholic fermentationand list two of its uses.

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8 · Respiration

ITQ1 During respiration, the energy from the food eaten by an organism ismade available. This energy can be used to carry out all the characteristicsof life, movement, growth, reproduction, and so on.

ITQ2 Animal cells respire all the time because the animal is in constant need ofenergy. Plant cells also respire all the time. During the day, while sunlightis available, plants also photosynthesise, but they never stop respiring.

ITQ3 The important product of respiration is energy, which an organism needsto carry out the characteristics of life. The waste products of respirationare carbon dioxide and water.

ITQ4 Respiration occurs in the mitochondria of cells.ITQ5 Energy is released only when necessary; only as much energy as is

needed is used; energy is released rapidly when it is needed.ITQ6 Two examples of organisms that respire aerobically are humans and birds

(there are many others). Two examples of organisms that respire onlyanaerobically are yeast and the tapeworm.

ITQ7 Human muscle cells use anaerobic respiration during prolonged exercise,when oxygen cannot be supplied fast enough for aerobic respiration totake place. As a result, energy is produced to do the work necessarywhen exercising, although less energy is produced from each glucosemolecule used than in aerobic respiration.

ITQ8 Fermentation is anaerobic respiration in plant and fungal cells. Alcoholicfermentation occurs when yeast respires anaerobically to produceethanol. This process is important in the bread, beer and wine industries.

ITQ9 The bacteria that are found in cans and tins respire anaerobically, that isthey do not need oxygen to release energy for all their living processes.So the fact that there is no air in the can does not affect them and theycan live in that environment.

Answers to ITQs

1 (i) Respiration is described as a characteristic of life. What is theimportance of respiration to plants and animals?

(ii) Although respiration occurs in a series of steps, it can be summarisedin 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 forbreakfast or lunch and that she felt ‘weak’. Explain to her why she isfeeling weak and why it is important not to skip meals.

2 (i) Using a table, outline the differences and similarities betweenanaerobic 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 breadmaking industry;(b) the alcohol industry.

(v) Describe how yoghurt is made.


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8 · Respiration

3 Diagrams A and B show investigations to demonstrate the products ofrespiration and photosynthesis.

(i) Copy the diagrams and, using annotated labels only, completediagram:(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 usedup 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) Explain fully how the rate of respiration would change if more

organisms were put in the flask.

A

B

at the start after 30 minutes

capillarytube

oil drop

wire gauze

soda lime

small animalse.g. woodliceor millipedes

T

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Importance of gaseous exchange in ManRespiring cells need a continuous supply of oxygen. They must also be able to getrid of the carbon dioxide that is being produced constantly. The blood system isthe 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,exchange these two gases. In Man, gaseous exchange takes place in the lungs.

91

Gaseous exchangeú understand the function and mechanism of gaseous exchange in Man;ú understand the function and mechanism of gaseous exchange in

plants;ú identify characteristics common to gaseous exchange surfaces;ú discuss the effects of cigarette smoking in Man.



9

gaseous exchange ©

cell membrane

gill

inhalation exhalation

leaf

lungs Man

Amoeba

plant

fish

gaseous exchange

respiratorysystem

cigarettesmoking

respiratorysurface

characteristics

thin

large surface area

rich blood supply

constantly movingtransport medium

Concept map

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lungs

GASEOUS EXCHANGE

oxygen net diffusioninto the bloodcarbon dioxide net diffusion out of the blood

air inhaled – hasmore oxygen

than air exhaled

mouth, nose

air exhaled –has more carbon dioxide than the

air inhaled

respiring body cell uses oxygen,

produces carbon dioxide

blood rich in carbon dioxide flows towards the lungs

blood vessels in the lungs

blood rich in oxygen flows to the body cells

CO2

O2

O2

CO2

Mechanism of gaseous exchange in ManThe human respiratory system is involved in the exchange of gases in Man. Thelungs are very important and are made up of many tiny air spaces or air sacs calledalveoli (see figure 9.2).

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9 · Gaseous exchange

alveoli ©

Figure 9.1 The role of the lungs in exchanging gases with the environment.

movement of air

nose

mouth

trachea opens to mouth and nose

trachea

larynx (voice box)

ring of cartilage

left bronchus

internal intercostal muscle

external intercostal muscle

left lung

heart

diaphragm

bronchiole

pleural membrane

pleural fluid

rib

air sacs/alveoli

Figure 9.2 The human respiratory system.

ITQ1

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

human body.

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network of capillaries surrounding alveoli – gases are exchanged here

capillary, transporting blood that has little oxygen (deoxygenated

blood) to the alveoli – blood has high concentration of carbon dioxide

bronchiole

capillary transporting oxygenated blood from lungs –

blood has low concentration of carbon dioxide

air sac or alveolus

section of one alveolus

Air enters the nose and/or mouth and moves down the trachea (windpipe).The trachea is supported by rings of cartilage so that it is kept ‘open’ at all times.

The trachea then divides into two bronchi, the right and left. These are alsosupported by rings of cartilage. Each bronchus branches into smaller and smallertubes called bronchioles. At the end of each bronchiole are the many tiny sacscalled alveoli. Gaseous exchange occurs in the alveoli.

The walls of the alveoli are the gaseous exchange surfaces or the respiratorysurfaces. The smallest blood vessels, capillaries, are closely wrapped around eachalveolus. Blood is thus brought to and taken away from each alveolus. Oxygendiffuses across the walls of the alveolus into the capillary and the blood in thecapillary becomes oxygenated. Carbon dioxide diffuses from the capillary into thealveolus and is exhaled out of the body. The walls of the alveolus and capillary arevery thin (actually one cell thin) so that diffusion can occur readily.

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trachea ©bronchus ©

bronchiole ©

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

trachea branches into two

bronchus branches into smaller and smaller branches

bronchiole

alveolus or air sac found at the end

site of gaseous exchange

Figure 9.3 The route taken by air into and out of the lungs.

Figure 9.4 The blood supply to one alveolus.

alveolus (the wall is one cell thick)

flow of blood in capillary

oxygenated blooddeoxygenated blood

carbon dioxide oxygen

out in

air

capillary(the wall is

one cell thick)

Figure 9.5 Gaseous exchange between an alveolus andthe blood in a capillary.

ITQ2

State the importance of the rings of cartilagethat surround the trachea.

ITQ3

Describe the passage taken by an oxygenmolecule from the air to a capillary in thelungs.

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The continuous exchange of gases in the lungs is extremely important. Bodycells can obtain a constant supply of oxygen for respiration and the carbon dioxidethat is constantly being produced is exhaled out of the body.

Gaseous exchange also occurs at the level of the cells. Here oxygen leaves theblood and diffuses into the cells. Carbon dioxide moves in the opposite direction(see figure 9.1).

The trachea is lined with mucus, a slimy substance which traps and holds dustand micro-organisms. The trachea is also lined with microscopic hair-likeextensions called cilia. These beat in a wave-like manner, moving the mucuscontaining dust and micro-organisms upwards and out of the lungs.

Pathogens can enter the lungs with air as it is breathed in. The mucus and ciliaafford some protection by trapping and moving them out of the lungs.

If an irritating substance like dust is breathed in, this can stimulate a sneezeduring which the irritant is ejected out of the lungs.

The other parts of the respiratory system, namely the ribs, intercostal musclesand diaphragm, are also involved in gaseous exchange. They help to move air inand out of the lungs. Breathing in is called inspiration and breathing out is calledexpiration.

Importance and mechanism of gaseousexchange in plantsDuring the day plants photosynthesise and need carbon dioxide. Oxygen is awaste product and must be gotten rid of. Plants respire all the time but, during theday, photosynthesis is also being carried out. More oxygen is made inphotosynthesis than is used up in respiration and more carbon dioxide is usedthan is made. So there is a net flow of oxygen out of the leaf and a net flow ofcarbon dioxide into it.

At night photosynthesis stops, since there is no light, and respiration contin-ues. Now oxygen only moves into the leaf and carbon dioxide moves out.

The leaf is the respiratory surface or gaseous exchange surface. There are tinypores called stomata on the underside of the leaf through which these gases pass.From the air space inside the leaf, the gases diffuse into and out of the plant cells.The gases move down their concentration gradients as discussed in chapter 6.

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inspiration ©expiration ©

Inspiredair

Expired air

Reason

Oxygen 21% 16% some of the oxygen is used by thecells of the body during respiration

Carbondioxide

0.03% 4% carbon dioxide is made by the cellsand is transported by blood to thelungs

Nitrogen 78% 78% not used

Watercontent

variable always higherthan whenrespired

the alveolar surface has a thin filmof moisture to aid gas exchange, andsome of this evaporates

Temperature variable always higherthan wheninspired

air is warmed by the body heatwhilst within the body

Table 9.1 A comparison of inspired and expired air.

ITQ4

List the differences between the blood in thecapillary coming to, and the blood leaving, thealveolus in figure 9.5.

ITQ5

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

Man?

ITQ6

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

cilia ©

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Inspiration Expiration

• External intercostal muscles contract (internal intercostalsrelax) and the ribcage is raised.

• The muscles of the diaphragm contract and the diaphragmmoves downwards.

• These two movements increase the volume of the thorax.• The pressure inside the thorax is lowered to below

atmospheric pressure. This pulls on the lungs so theyexpand.

• Air rushes into the lungs through the mouth/nose andtrachea.

• Internal intercostal muscles contract (external intercostalsrelax), and the ribcage is lowered.

• The diaphragm muscles relax and the diaphragm movesupwards.

• These two movements decrease the volume of the thorax.• The pressure inside the thorax increases which squeezes the

lungs.• Air is pushed out of the lungs. It passes out through the

trachea and the mouth or nose, out of the body.

ribs move upand out

air in a

sternum movesupwards and

forwards

s

diaphragm contracts diaph

ribs

vertebralcolumn

air in a

diaphragm contracts(moves down)

volume increased volume decr

air out

sternum movesdownwards and

backwards

diaphragm recoils

air out

diaphragm recoilsupwards

volume decreased

ribs move downwardand in

T

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thin wall of capillary

thin wall of alveolus

layer of moisture – oxygen dissolves in this moisture and there is alwaysa high concentration of oxygen

next to the capillary

blood brought to thealveolus – it is low in O2

and high in CO2

blood taken away –it is rich in O2

and low in CO2

blood flows constantly

O2 O2O2

lungs, highly foldedto increase surface area

CO2

CO2CO2

Characteristics common to gaseous exchangesurfacesGaseous exchange or respiratory surfaces are those surfaces where the exchangeof oxygen and carbon dioxide occur. These surfaces must have certain character-istics that encourage:• a lot of gaseous exchange to take place,• gaseous exchange to take place quickly,• gaseous exchange to take place continuously,so that organisms respiring aerobically can get a constant supply of oxygen andremove carbon dioxide. Without oxygen, cells die and carbon dioxide, if allowedto accumulate in cells, could poison and kill them.

Adaptations for efficient gaseous exchange

Large surface areaFor gaseous exchange to take place quickly and in large amounts, respiratorysurfaces must have a large surface area or a large area over which the exchange

96


• respiration occurs• photosynthesis is the main activity

• respiration occurs• no light, therefore no photosynthesis

6CO2 + 6H2O C6H12O6 + 6O2photosynthesis equation

O2

O2

O2

CO2

C6H12O6 + 6O2 energy + 6H2O + 6CO2respiration equation

CO2

CO2

Day Night

T

Figure 9.6 The net flow of gasesdiffusing in and out of a leaf duringthe day and at night is different.

Figure 9.7 Adaptations of the lungs in Man for efficient gaseous exchange.

gaseous exchange surface ©

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CO2

air space

CO2

CO2

CO2

CO2

palisademesophyll

spongymesophyll

leaf is about4–5 cells

thick

wind

wind

leaf is thin and flat forlarge surface area

oxygen leaving leaf is blown away, leaving a

low concentrationaround the leaves

O2

O2

O2

O2

O2

O2

CO2

CO2CO2

O2

O2O2

O2

of gases can occur. In Man, the lungs are made up of thousands of sacs calledalveoli, that, if laid out side by side, could cover a lawn-tennis court!

In fish, gill lamellae, which are part of the gills, form the respiratory surface.There are thousands of lamellae in each gill creating a large surface area forexchange.

In plants, the respiratory surface is the leaf. They are thin and flat to create thelargest area possible for gaseous exchange. On a tree the thousands of broad flatleaves show what a large surface area is available for gaseous exchange.

Protozoans, like Amoeba, are microscopic unicellular organisms. Their surfacearea to volume ratio is thus large. Gaseous exchange occurs across the cellmembrane by diffusion and, because it is so small, the entire ‘body’ of the Amoebacan be supplied with oxygen.

Thin surface for gaseous exchangeFor gaseous exchange to take place quickly, the respiratory surfaces must be thinso that diffusion of the gases can take place rapidly. In Man, the walls of the alveoliand capillaries are just one cell thin. The walls of the alveoli are also moist, so thatthe gases dissolve in the moisture before they diffuse.

In fish, the lamellae and capillaries are also one cell thin and diffusion can thusreadily occur across the gills.

The cell membranes of protozoans are very thin and diffusion readily occurs.Air spaces inside leaves ensure that the gases can get close to most of the cells

into and out of which they must diffuse.

97


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

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

surface area greatly increased

water flowing around gills

gill lamella

blood capillaries – take awayblood rich in O2 and lowin CO2, bring blood to

pick up O2 and lose CO2

Figure 9.8 Adaptations of the lamellae on the gill of a fishfor efficient gaseous exchange.

Figure 9.9 Adaptations of leaves on a plant for efficientgaseous exchange.

small volume relative to large surface area

thin membrane flow of water takes CO2 away

flow of water brings more O2

O2CO2

movement of water

Figure 9.10 The unicellular Amoebaneeds no special organ for gaseousexchange. It is so small that the gasescan be exchanged efficiently across itscell membrane.

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Constantly moving transport mediumFor gaseous exchange to take place continuously, the medium which brings thegases to the respiratory surface must be constantly moving. This ensures that aconcentration gradient is always maintained and diffusion will take placeconstantly. For example, in Man and in fish there is a rich blood supply constantlyflowing past the respiratory surface. In Man, breathing continuously refreshes theair in the lungs, and in fish water is continuously forced across the gills.

In plants, the wind blows or moves the gases away from the leaves ensuringthat a concentration gradient is always maintained and that diffusion occursreadily. In unicellular protozoans, the water around the organisms constantlytakes away and supplies the gases which dissolve easily in water.

The effects of cigarette smokingTobacco may be the cause of over 3 million deaths worldwide a year. Death fromcigarette smoking comes mainly from lung cancer, but heart disease is alsoassociated with smoking. The products of cigarette smoke, directly from smokinga cigarette or from inhaling another person’s cigarette smoke, include nicotine, tarand carbon monoxide.

Nicotine• Makes cigarettes highly addictive.• Reduces air flow in and out of the lungs.• Paralyses the cilia lining the trachea, which remove dirt and bacteria.• Raises blood pressure.• Raises heart rate.• Increases the risk of osteoporosis.

Osteoporosis is the loss of calcium carbonate from the bones which can happen inolder people. It makes the bones brittle, so they break more easily and are moredifficult to heal.

Tar• Sticks to cells in the lungs.• Causes the development of cancer.• Damages lung tissue.• Breaks down the alveoli, thus decreasing the surface area for gaseous exchange.• Causes bronchitis or inflammation of the lining of the air passages.• Causes ‘smokers cough’.

Carbon monoxideLike car exhaust smoke, cigarette smoke contains carbon monoxide.• Combines irreversibly with haemoglobin in the blood.• Causes less oxygen to be transported by blood.• Reduces the smoker’s ability to take strenuous exercise.• Causes breathlessness.• If a pregnant woman smokes, carbon monoxide gets into the blood of the fetus

and combines with the haemoglobin. Less oxygen gets to the growing tissues,resulting in a smaller birthweight which is associated with greater risk of healthproblems during and after birth

Although studies show that there is a connection between cigarette smoking andlung cancer, millions of people worldwide continue to smoke. A large percentageof smokers are young people who become addicted very quickly and continue tosmoke throughout their lives. Statistics show that 25% of smokers die of lungcancer. Figure 9.11 shows the effects of smoking on human lungs.

98


ITQ7

What is the respiratory surface for each of thefollowing organisms: Man, a fish, plant,Amoeba?

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Þ Living cells need a constant supply of oxygen, and need to get rid of carbondioxide as respiration takes place.

Þ Gaseous exchange is the exchange of these gases and takes place at therespiratory surface.

Þ In Man, the respiratory surface is the lungs.Þ Plants also photosynthesise and thus, during the day, require carbon dioxide

and must get rid of oxygen.Þ In plants the respiratory surface is the leaf.Þ Characteristics common to respiratory surfaces are:

(a) a large surface area;(b) thin walls;(c) rich blood supply;(d)presence of moisture.

Þ In Man, the lungs are adapted for gaseous exchange.Þ The gills of fishes are adapted for gaseous exchange.Þ In plants, the leaves are adapted for gaseous exchange.Þ Gaseous exchange occurs across the cell membrane of Amoeba.Þ The components of cigarette smoke include nicotine, tar and carbon

monoxide.Þ Cigarette smoking causes lung cancer and is dangerous to good health.

ITQ1 (i) Gaseous exchange is the exchange of gases, in particular oxygen andcarbon dioxide.

(ii) In the lungs, where gases are exchanged between the alveoli andblood. In the tissues, where gases are exchanged between the bloodand cells.

ITQ2 These rings are solid rings and keep the trachea open. They also supportthe trachea.

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Figure 9.11 A normal lung (left) and a cancerous lung (right). The cancerous lung camefrom a heavy smoker.

Summary

Answers to ITQs

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ITQ3 Nose → trachea → bronchus → bronchiole → alveolus → capillary

ITQ4 Blood in the capillary approaching the alveolus contains a higher concen-tration of carbon dioxide and lower concentration of oxygen than bloodleaving the alveolus.

ITQ5 (i) Breathing is the process whereby air is pulled into and pushed out ofthe lungs. It is important because it brings a supply of oxygen, whichis needed for respiration, and it also takes away carbon dioxide, awaste gas.

(ii) The muscles involved in breathing are: the diaphragm muscles andthe external and internal intercostal muscles.

ITQ6 (i) At noon oxygen is leaving the plant and carbon dioxide is enteringthe leaf. Although respiration is also occurring, photosynthesis is hap-pening at a much faster rate, so the carbon dioxide produced andoxygen used in respiration are not significant.

(ii) At midnight there is no light available and photosynthesis cannottake place. Respiration is the only process that is occurring so carbondioxide is leaving and oxygen is being taken in by the leaf.

ITQ7 Man – alveolus; fish – gill; plant – leaf; Amoeba – cell membrane.

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ribs

diaphragm

T

1 (i) Inhalation and exhalation are movements that ventilate the lungs.The diagram on the left shows the ribs and diaphragm.(a) Copy the diagram on the left and use arrows to show the

movements of the diaphragm, the ribs and air during exhalation.(b) Explain fully how the volume of the thorax is increased by giving

details of contraction and relaxation of muscles involved inraising of the ribs and lowering of the diaphragm.

(ii) List three ways inhaled air differs from exhaled air.(iii) A boa constrictor kills its prey by squeezing it to death. This is termed

asphyxiation. Explain how asphyxiation results in death.

2 (i) Define the following terms:(a) gaseous exchange;(b) respiratory surface.

(ii) List three characteristics of respiratory surfaces. Describe how thelungs of Man are adapted in these three ways to increase the rate ofgaseous exchange.

(iii) The nicotine found in tobacco smoke can prevent the beating of ciliain the trachea. Suggest how this contributes to the development oflung diseases.

(iv) List two effects of each of the following products of cigarette smoke:(a) nicotine; (b) tars.

(v) How are plants adapted to exchange gases efficiently by diffusion?

3 The apparatus shown in the diagram on the left is used to investigate thechemicals in cigarette smoke.(i) After some time, the cotton wool turns black and appears oily.

(a) What are the black particles trapped in the cotton wool?(b) What chemical causes the oily appearance?

(ii) Describe and explain the colour change of the litmus paper.(iii) If a cigarette with a filter is used, what difference in the appearance

of the cotton wool would you expect?(iv) One of the gases in cigarette smoke is carbon monoxide. What is the

effect of carbon monoxide on the body?(v) How do chemicals in cigarette affect the cilia in the trachea?(vi) Name two respiratory diseases that may be caused by prolonged

smoking.


cotton wool

lightedcigarette

airoutlet

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