Cambridge IGCSE®

Biology
Revision Guide
Ian J. Burton
University Printing House, Cambridge CB2 8BS, United Kingdom
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notice to teachers

Teachers and students are reminded of the importance of safe laboratory practice and, while every
care has been taken to ensure that all experiments and demonstrations appearing in the text are
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Table of Contents

PrefaceIV
How to use this book VIII
AcknowledgementsX
Chapter 1: Classification 1
Chapter 2: Cells 15
Chapter 3: Movement in and out of cells 25
Chapter 4: The chemicals of life 34
Chapter 5: Enzymes 39
Chapter 6: Plant nutrition 43
Chapter 7: Animal nutrition 56
Chapter 8: Transport in plants 67
Chapter 9: Transport in animals 75
Chapter 10: Diseases and immunity 86
Chapter 11: Respiration and gas exchange 89
Chapter 12: Excretion 96
Chapter 13: Coordination and response 101
Chapter 14: Homeostasis 109
Chapter 15: Drugs 117
Chapter 16: Reproduction in plants 122
Chapter 17: Reproduction in humans 132
Chapter 18: Inheritance 143
Chapter 19: Variation and natural selection 153
Chapter 20: Organisms and their environment 160
Chapter 21: Biotechnology and genetic engineering 171
Chapter 22: Human influences on ecosystems 179
Progress check answers 189
Answers to Exam-style questions 197
Glossary214
Index223

III
 Preface
Revision tips for IGCSE biology 6. Continue until you have a list of questions and
answers to the section you are trying to learn.
students
7. Take a second sheet of paper (folded if writing
Understanding IGCSE Biology is not usually a problem, would otherwise show through it) and use this to
but committing facts to memory can often be a major cover the answers. Test yourself again, writing your
obstacle to success. Many students are at a loss to know answers on the folded sheet, and continue this until
exactly how to set about what seems to them to be you are able to score over 80%. (You can, of course,
a task of immense proportions. I offer the following set your own target. Some will not be content until
method, one that I devised myself when, as a student, they can score 100%.)
I was faced with the same problem. It has the advantage,
8. File away your Question/Answer sheet for further
if followed carefully, of improving one’s factual
revision at a later date.
knowledge as a result of time spent.
9. Continue this process systematically until you have,
This revision guide is full of important terms and
effectively, a full set of revision notes for later use.
phrases. The method that I offer for learning it is as
follows: 10. In the last few weeks before the examination, it is
better to revise by reading the text of this book
1. Take a sheet of file paper and divide it with a
carefully, section at a time. Concentrate on every
vertical line such that three quarters of the sheet is
sentence, making sure you understand what you have
on the left of the line.
read. It is so easy to get to the bottom of a page in
2. Read a page of the revision text and, each time you a book and realise that your mind was elsewhere as
come to an important word or phrase, think up you were reading it and, as a result, nothing registered
your own simple question to which that word or at all. If that happens, be honest with yourself. Go
phrase is the answer. back to the top of the page and start again.
3. Write these simple questions on the left hand side 11. In the last few days before the examination, your
of your sheet of file paper, leaving a space between Question/Answer sheets should now prove
each, and number them. Continue on further sheets invaluable for last-minute consolidation of your facts.
of paper if necessary.
It cannot be stressed too strongly that examination
4. If there is a diagram in the text, then draw a quick results depend on knowledge. It is important that you
sketch of the diagram on the left-hand side of your have a very good grasp of simple knowledge to do well,
sheet with numbered label lines vertically above and interpretation questions rely heavily on a sound
each other extended towards the right-hand side of knowledge of the subject matter.
your sheet. The advantage of this revision method is based so
5. When you have reached the bottom of the page firmly on the student phrasing the questions to which
of text, close the book and see how many of the he or she will already know the answer that it would
answers you can write down on the right hand defeat the object if more than a short example of the
side of your sheet. When you have attempted technique were given. The success of the method relies
all answers, check them against the text. You will only on the student following the technique carefully.
probably be surprised at how well you do, but since It does work, but you must be prepared to spend the
you wrote the questions, carefully phrased around necessary time. You may even enjoy the experience!
the required answer, perhaps it is not so surprising
after all.

IV
Example of a Revision Sheet, based on information in this revision guide:

Preface
1. What word is used for organisms containing only one cell? unicellular

2. Give an example of a one-celled organism. a bacterium

3. What word is used for organisms made of many cells? multicellular

4. What structure controls the passage of substances into and out of a cell? cell membrane

5. In what state must all chemicals be before they can enter or leave a cell? in solution

6. What is the jelly-like substance where chemical reactions occur in a cell? cytoplasm

7. What is the correct term for the chemical reactions in a cell? metabolic reactions

8. Whereabouts in a cell are chromosomes found? the nucleus

9. What do chromosomes contain? genes

10. Of what chemical are chromosomes made? DNA

11. What makes up protoplasm? cytoplasm +nucleus

12. What is the space in the centre of a plant cell? vacuole

13. What does the space in the centre of a plant cell contain? cell sap

14. What is the name of the box in which a plant cell is contained? cell wall

15. What chemical is this box made of? cellulose

16. Name the green structures in photosynthesising cells. chloroplasts

17. What pigment do they contain? chlorophyll

V
 Here is how you can test yourself on the labels to a diagram.
Preface

cell wall

surface membrane

cytoplasm

chloroplast

vacuole

nucleus

As the examination approaches and a greater amount People vary as to how long they can work at a stretch.
of time is spent on revision, it is usually more productive It is important to have a break from time to time (again,
to set aside a certain time each day for revision. Do preferably, the same time each day). When you stop, set
not allow yourself to be persuaded to do anything else yourself a time to resume your revision and stick to it.
during that time.
It would indeed be a pity if, armed with a sound factual
Work on your own with no distractions around knowledge, you then failed to use that knowledge
you. Some people say they can work better listening effectively in the examination. You may find the following
to music. If that really is so in your case, then keep advice useful:
the music quiet and, at least, it may shut out other
distractions!
You may find it helpful to make a calendar by dividing
Teacher’s tips for answering
a piece of paper into a space for each day during your examination questions
revision period before the examination. Then you can
divide the syllabus into the same number of parts as 1. It can never be said often enough: read the
there are days for revision and enter one such part per question. You must answer the question asked, not
day on your calendar. In this way you will know exactly any other. It is probable that, on first reading, you
what you are going to revise on each day. Your day’s feel worried that you can’t answer it. You may think
revision will not be complete until you have revised you have never learnt that particular topic. But, wait
everything on your calendar for that day. a moment, then read it again and you will often
be surprised how much more sense it makes the
second time!

VI
2. Do not omit facts simply because you consider 5. If a question involves the interpretation of a graph,

Preface
them too obvious to mention. They will often be try to include some numerical information read
credited. from the graph and, if they are available, remember
to include the appropriate units.
3. The space provided for your answer on the
question paper is a guide to how long your answer Finally, good luck with your revision. This method can
should be. Don’t waste time or space writing work. I know, because it did so for me!
irrelevant material. Ian J. Burton
4. Make sure that you do not contradict yourself. You
are unlikely to get credit for a correct statement
when you have also stated the contradiction.

VII
 How to use this book
Classification
Chapter 1

Learning outcomes
Learning outcomes –
By the end of this chapter you should understand:
set the scene of each
The characteristics of living organisms The characteristics of some invertebrates chapter, help with
How to use the binomial system for naming Viruses, prokaryotes (bacteria), protoctists and navigation through
organisms fungi the book and give a
How living organisms are classified The construction and use of a dichotomous reminder of what's
key important about
The characteristics of some vertebrates
each topic.

1.01 Characteristics of living TIP

Breathing and respiration are not the
organisms same thing. When we take air into and
expel air from our lungs, we are breathing.
All living organisms
Supplement possess –theindicated
material ‘characteristics of life’. This process is to supply oxygen to the
The one group of organisms that does not show all blood that takes it to the cells where
by a bold vertical line. This is
the characteristics of life is the viruses. Thus they are respiration occurs.
for students
considered to bewho areborder
on the taking the living and
between energy input

Chapter 20 Organisms and their environment

from the Sun
Extended
non-living. syllabus covering the during energy lost
photosynthesis in respiration
energy lost energy lost energy lost
in respiration in respiration in respiration
Core
All trulyand
livingSupplement content.
organisms display the following • Sensitivity. This is the ability to detect and respond
characteristics: to changes in theproducersenvironment (known as stimuli).
herbivores carnivores top carnivores
The stimuli may be from the internal environment
• Movement. This may be the movement of a part of
– for example, the effectdefecation of hormones
excretion and
on a cell
excretion and excretion and
an organism in relation to the rest of its body (such defecation defecation energy lost
or tissue, or from death the external environment
death – for
death death in respiration
as the movement of an arm or of a shoot tip), or it
example, light. The internal environment is a term
may involve the movement of the whole organism decomposition
that refers to the conditions inside an organism.
from one place to another – when it is called Figure 20.1 Energy flow in an ecosystem
Sensitivity is also the ability to detect or sense stimuli
locomotion. It commonly involves the contraction of
in the internal or external environment and to make
muscles (as in the arm) or cells growing at different • Each organism unlocks some of this energy to long ones. In order to supply enough energy in food to
appropriate responses.
rates (as in the shoot tip). use for various processes within its body, for maintain an ever increasing world population, it must be
example, making new cells and the large organic realised that far less energy is lost when man eats green
• Growth. It isthem
customary for muscular
organisms toplants start
thanlife
Terms – words
It is thus defined asinanbold indicate
action by an organism (or part molecules within
small in size
contraction
(during growth),
and gradually
(and movement), become
generating electrical largerarewith
when crop plants are fed to animals, which
time.by a human.
then eaten
of an organism)
important terms causing
and adefichange of position or place.
nitions impulses in the nervous system and raising body
Some organisms
temperature. The chemical grow tothat
reaction a certain
unlocks thesize then stop,
In any one habitat, such as a pond or mangrove swamp,
that are explained
• Respiration. This is aclearly
chemicalinreaction
each that takes while
chemicalothers
energy forgrows continuously
conversion into other formsthroughout
is
there will be many organisms living together. In some
way theytheir
will all be interconnected by way of different
place in living cells. It involves the breakdown of large, respiration.
topic. lives. Growth is defined as a permanent food increase
chains.
• Energy is used up in most of these processes. Only
nutrient organic molecules (usually carbohydrates, in size.
in the form of heat from an organism’s body is it
A network of interconnected food chains is known as
a food web (Figure 20.2).
such as glucose) to release (not to ‘make’, released to the environment outside the food chain.
Growth
This includesinvolves
energy fromantheincrease inbacteria
respiration of dry massWhile
by anall food chains (and thus all food webs) begin
‘manufacture’ or ‘produce’) the energy contained
Tips – quick
within suggestions
the molecule. to molecule contains
The glucose
increase
and fungi thatineventually
cell number
decay deadororganisms.
cell size or both.
with aDry mass
producer, food webs may begin with several
different species of producer.
is the mass of all the components within an object
remind
energy you
in theabout
form ofkey factsenergy,
chemical and which is exceptAvoidanysaying
water present.
A producer is an organism that makes its own organic
TIP

that energy is ‘needed’ for nutrients, usually using energy from sunlight, through
converted
highlight into other forms
important for use in doing work –
points. respiration (it is released by respiration). photosynthesis.
such as electrical energy in nerve impulses. • Reproduction. In order to maintain (or Inincrease) a food chain or food web, producers are eaten by
their numbers, all organisms have the ability to make
consumers.
Respiration is defined as the chemical reactions Much of the energy is still present in the faeces and
more
in theof the same wastekind. A consumer in a food chain is an organism that gets its
in cells that break down nutrient molecules and some nitrogenous of animals. This energy is
energy by feeding on other organisms.
available to decomposers. Not all herbivores are eaten,
release energy (for metabolism). thus the amount of energy left within herbivores to be An animal that gets its energy by eating plants is an
passed on to carnivores is small – 20% (only 2% of the herbivore (or primary consumer).
original amount in the producer).
An animal that gets its energy by eating other animals is
For this reason, food chains are limited in length, as 1
a carnivore (or secondary consumer − the consumer
there is insufficient energy remaining to sustain a that feeds on the secondary consumer is a tertiary
succession of carnivores. Five trophic levels are usually consumer − and so on). Thus all consumers above the
Glossary terms – terms in green can the limit for a food chain (Figure 20.1). level of herbivore, that is, all meat eaters, are carnivores.
The longer the food chain, the less the energy available When all organisms in a food chain or web die
also be found in the Glossary to the top carnivore at the end of the chain. Short food they are decomposed largely by bacteria and fungi.
chains are therefore much more energy efficient than

161

VIII
 The importance of photosynthesis to The importance of magnesium ions
the living universe As with nitrates and all other ions, magnesium ions
are also absorbed from the soil through the root
Photosynthesis produces the carbohydrate with its hair. Magnesium is the central atom in a chlorophyll
stored chemical energy. Almost all other forms of life molecule. Plants lacking in magnesium ions are unable
rely on carbohydrate, although plants may convert this to make chlorophyll and thus their leaves are yellow in
carbohydrate into protein or fat before it is passed on. colour (a condition known as chlorosis).

PRACTICAL
Practical skills – reinforce your

How to use this book

practical knowledge and skills
A demonstration of the effects of a lack of nitrates
and of magnesium ions in a growing plant with clear explanations and
Apparatus: 2 small cuttings or seedlings seedling
diagrams.
(e.g. sorghum)
2 containers cotton
wool
2 There are noCottonrods wool
or cones at the point where the When a person is viewing a distant object:

Chapter 13 Coordination and response

Black paper or black polythene
retina is joined to the optic nerve. Images formed dark cover
Culture solutionsare not converted into 1 The (circular) ciliary muscles relax, increasing their
(to prevent
on this part of the retina the growth of
Method:
culture circumference.
single-celled
impulses and relayed to the brain. This region solution
green algae)
Two seedlings or small cuttings, with the same 2 The suspensory ligaments are pulled tight.
is therefore called the blind spot. We are not
number of leaves, are selected from a quick-
(usually)
growingaware ofheld
plant and theinblind
the topspot since the blind
of two Figure 6.10 Experiment
3 Theto lens show iseffects of a lackto
stretched Make sure you turn the page
of become longer and
spots in our(Atwo
containers and eyes docotton
B) using not wool
recordas the same partand of magnesium
nitrates ions in a growing plant
shown in Figure 6.10.
of our field of view and one eye thus covers for
thinner – increasing its when you see this arrow in the
focal length and decreasing
➔ is refracted. Thiscorner!
the amount that the light is helped
the other. by muscles at the back of the eyeball that contract
to increase the liquid pressure in53the eye.
Progress check 13.3 4 Rays from the distant object are brought to focus
on the retina.
1 It is easier to see the outline of an object
in dim light by looking to the side of it. Can Progress check questions – check
you explain why this is? The pupil (or iris)
your reflex
own knowledge and see how
2 Why do you think that a ‘round-arm’, well you're
This reflex is usually described as thegetting onbut,
pupil reflex by answering
punch aimed at the side of a boxer’s face regular
since the pupil is a space, questions.
it is the muscles of the iris that
is often more successful than one aimed respond. Therefore, it is more accurate to call it the iris
straight at him? reflex.
Bright light could seriously damage the delicate light
sensitive cells of the retina. The intensity of light Worked
falling on examples – a step by
Accommodation
Worked example

Chapter 3 Movement in and out of cells

the energy is usedthe to move
retinatheiscarrier proteins.controlled by the iris. In general
therefore
First the carrier protein opens to the outside and step approach to answering
Describe how the structure of a cell membrane is terms,
to be the size to
ofattach
the pupil
(bind) automatically adjusts to the
Accommodation is theofability
adapted to the process active uptake.
allows the molecule
to change the focalto length intensity
the protein. Only
absorbed
one typeofof light reaching
molecule
questions, guiding you through
will bindthe eye. In bright light, the iris
of the
Celllens so thatcontain
membranes we can seemolecules
protein both near calledand distant
as the site is not suitable
increasesfor any
in other
size molecule.
to make the pupil small, reducing
The protein then changes shape again (again using
from thestart to finish.
‘channel
objects. proteins’. These molecules
Accommodation depends fit loosely
on: light thatopening,
reaches the retina. In dim light, the iris opens up
together leaving minute channels between them energy) – closing the outer and opening into
• the elasticity
extending from of
thethe lensof the cell to the cell
outside the cytoplasm of thethecell.
pupil to allowis as
The molecule much light into the eye as possible.
released
cytoplasm. Suggest how these channel proteins may into the cell cytoplasm, then changes shape again,
• the
play existence
a part is the of ciliaryofmuscles,
processes which
diffusion and closing
are used
osmosis. to to the inside and opening to the outside
alter
Answerthe shape of the lens How the iris muscles
ready to bind with another molecule to be absorbed.
(Note that the complete cycle of carrier protein
work to
The suspensory
• the question calls filigaments,
rst for a realisation
which thattransfer
there the effect of control pupil size
movements is described as a continuous process.)
is a special structural feature of the cell membrane Diffusion requires pores in the membrane before
the
makingciliary
it ablemuscles
to undergo toactive
the lens.
uptake. That feature molecules can enter.
TheTheiris
channel
has proteins would
an antagonistic arrangement of circular and
is the presence of carrier proteins. Since they have provide the pores. However, the size of the pores
to work against a concentration gradient (there radial
might prevent larger muscles.
molecules from entering. Water is
Near and distant objects
may be a lower concentration of the chemical
to be absorbed outside than inside the cell) then
a small molecule and thus could enter by osmosis, and
the pores may be too small to allow larger molecules
When
energy must be used. All energy within a cell is
a person
initially released by is respiration
viewing aand, near object:
in this case,
In bright light
to enter – making the membrane semi-permeable.

1 The (circular) ciliary muscles contract reducing their 1 Light sensitive cells in the retina detect the light
circumference. intensity.
Chapter summary
2 TheyYoureduce
have learntpull
howoncellsthe (elastic)
are involved suspensory
in the
2
You have learnt how to demonstrate these Chapter
Impulses are sent along the optic (a sensory) summary – at the end
nerve
ligaments. to the brain.
processes of diffusion, osmosis and active transport. processes experimentally. of each chapter so you can check
3 With
Youless
have force on these
learnt how the lens, its elasticity
processes are to have also3learntThe
allows itYou brain
about returns
the factors thatimpulses
affect along a motoroff
nerve to as you revise them.
topics
become wider (bulge) – decreasing its focal length
important to living structures. them. the circular muscles of the iris.
and refracting light rays to a greater degree. 4 The circular iris muscles contract while the radial
4 Rays from the questions
near object produce a focused image iris muscles relax.
Exam-style
on the retina.
1 Describe how different substances in a leaf move Describe5whatThe diameter
will happen to theof the and
tubing pupil, the hole in the centre, Exam-style questions – prepare
by diffusion during a 24-hour period. [6] decreases,
its contents over the next 20allowing less light to enter, decreasing the
minutes. [3]
2 a Figure 3.13 shows a piece of partially b After 20 minutes,
riskapart from whatto
of damage happens
the retina. for examinations by completing
permeable tubing, tightly tied at each end,
and containing a concentrated sugar solution
to the tubing, the water in the beaker has
turned blue. With reference to diffusion
the Exam-style questions and
that is coloured with blue dye. It has been and osmosis, explain the results of this checking your105answers, which are
placed in a beaker of pure water. experiment. [7]
3 a Explain how a plant root absorbs from the
provided at the back of the book.
concentrated sugar
solution coloured soil:
with blue dye i) water [6]
tightly pure water ii) essential mineral ions that are in very
tied partially short supply. [4]
permeable
tubing b Suggest why a plant may have great difficulty
in absorbing essential mineral ions that are in
Figure 3.13 very short supply in a water-logged soil. [4]

33

IX
 Acknowledgements
Every effort has been made to trace the owners of Photo Library; P.19 Dr. Robert Calentine, Visuals
copyright material included in this book. The publishers Unlimited/Science Photo Library; P.20r Dr David
would be grateful for any omissions brought to their Furness, Keele University/Science Photo Library;
notice for acknowledgement in future editions of the P.50r Dr Keith Wheeler/Science Photo Library;
book. The authors and publishers acknowledge the P.83b Dr. Fred Hossler, Visuals Unlimited/Science
following sources of copyright material and are grateful Photo Library; P.84 Eye of Science/Science Photo
for the permissions granted. Library; P.126tl Dr Jeremy Burgess/Science
Photo Library; P.126tr Susumu Nihhinaga/Science
Cover image Frans Lanting, Mint Images/SPL;
Photo Library; P.156t AlexSmith/Shutterstock;
P.7c Alhovik/Shutterstock; P.11 AS Food studio/
P.156b small1/Shutterstock.
Shutterstock; P.15b Jose Luis Calvo/Shutterstock;
P.17t Keith R. Porter/Science Photo Library; SPL = Science Photo Library
P.17b clusterx/Alamy; P.18l Power and Syred/Science

X
 Classification
Chapter 1

Learning outcomes
By the end of this chapter you should understand:
The characteristics of living organisms The characteristics of some invertebrates
How to use the binomial system for naming Viruses, prokaryotes (bacteria), protoctists and
organisms fungi
How living organisms are classified The construction and use of a dichotomous
key
The characteristics of some vertebrates

1.01 Characteristics of living Breathing and respiration are not the

TIP
organisms same thing. When we take air into and
expel air from our lungs, we are breathing.
All living organisms possess the ‘characteristics of life’. This process is to supply oxygen to the
The one group of organisms that does not show all blood that takes it to the cells where
the characteristics of life is the viruses. Thus they are respiration occurs.
considered to be on the border between living and
non-living.
All truly living organisms display the following • Sensitivity. This is the ability to detect and respond
characteristics: to changes in the environment (known as stimuli).
The stimuli may be from the internal environment
• Movement. This may be the movement of a part of
– for example, the effect of hormones on a cell
an organism in relation to the rest of its body (such
or tissue, or from the external environment – for
as the movement of an arm or of a shoot tip), or it
example, light. The internal environment is a term
may involve the movement of the whole organism
that refers to the conditions inside an organism.
from one place to another – when it is called
Sensitivity is also the ability to detect or sense stimuli
locomotion. It commonly involves the contraction of
in the internal or external environment and to make
muscles (as in the arm) or cells growing at different
appropriate responses.
rates (as in the shoot tip).
• Growth. It is customary for organisms to start life
It is thus defined as an action by an organism (or part
small in size and gradually become larger with time.
of an organism) causing a change of position or place.
Some organisms grow to a certain size then stop,
• Respiration. This is a chemical reaction that takes while others grows continuously throughout their
place in living cells. It involves the breakdown of large, lives. Growth is defined as a permanent increase
nutrient organic molecules (usually carbohydrates, in size.
such as glucose) to release (not to ‘make’,
Growth involves an increase in dry mass by an
‘manufacture’ or ‘produce’) the energy contained
increase in cell number or cell size or both. Dry mass
within the molecule. The glucose molecule contains
is the mass of all the components within an object
energy in the form of chemical energy, which is
except any water present.
converted into other forms for use in doing work –
such as electrical energy in nerve impulses. • Reproduction. In order to maintain (or increase)
their numbers, all organisms have the ability to make
Respiration is defined as the chemical reactions
more of the same kind.
in cells that break down nutrient molecules and
release energy (for metabolism).

1
 • Excretion. This is the removal from organisms of toxic • Plant
Chapter 1 Classification

materials and substances in excess of requirements.

• Animal.
The material removed includes the waste products
Each kingdom is divided into sub-groups and each sub-
of metabolism – chemical reactions in cells including
group is divided into smaller groups. The last two groups
respiration.
in this succession are the genus and finally, the species.
(The plural of genus is genera.)
Remember that excretion does NOT
TIP

A species is defined as a group of organisms that can

include the removal of undigested waste interbreed (reproduce) and produce fertile offspring.
from the intestines since it has never taken A species is therefore said to be ‘reproductively isolated’.
part is a chemical reaction within the
body’s cells. Organisms within a species are not identical and the
differences between them are called variations.

• Nutrition. In order to provide the raw materials The binomial system of classification
and the energy for all the other characteristics of
life listed previously, organisms must take in energy- ‘bi’ = two and ‘nomial’ from the Latin nomen = name
containing materials that are required for growth
and development. All living organisms are usually known by the binomial
system, an internationally agreed system using two
Nutrition is thus defined as the taking in of names.
materials for energy, growth and development.
These two names indicate the genus and the species to
Plants require light, carbon dioxide, water and ions; which the organism belongs.
animals need organic compounds and ions and
usually need water. The genus is always written with an upper-case first
letter and the species is written with all lower-case
letters. Both names are always underlined when hand-
The first letters of each of the written and appear in italics in print. Both names often
TIP

characteristics together spell the name have a Latin or Greek origin. Thus, the lion is
of ‘MRS GREN’ – a lady well known
to students trying to remember the Panthera leo (hand-written) and
characteristics of living organisms! Panthera leo (in print).
The binomial system is useful because:

1.02 The concept and use of a • Sometimes, different species in different parts of
the world share the same name. When different
system of classification countries work together on schemes to conserve
endangered species, it is vital that they are all
The living universe comprises well over 10 million considering the same organisms (e.g. there are three
different types of organism, which are sorted into different species of arthropod all called ‘Daddy Long-
groups based on common features. This is called legs’ in different parts of the world.)
classification (or taxonomy). Those organisms that
share many similar features are placed in the same • The same species may have different names in
group. Those that share few features are placed in different languages.
separate groups. The number of shared features • The common name may be misleading (a jellyfi sh is
between different groups gives an indication of how not a fish).
closely related the groups may be.
• All organisms placed in the same genus will share a
The largest groups are called kingdoms, of which there set of features common only to that group. Knowing
are five: the genus, even without actually seeing the organism,
• Prokaryote (Bacteria) therefore tells the biologist a great deal about
organisms and about their evolutionary history and
• Protoctist relationships (i.e. how recently they separated from
• Fungus one another as they have evolved).

2
For many years, the classification of organisms was 1.03 Features of organisms

Chapter 1 Classification
based on studies of their morphology, that is, their
outward appearance, for example, the number and All living organisms share the possession of a cellular
type of limbs, or the shape of the flowers produced by structure, that is, they are all made up of one or more
a plant. It may include internal morphological features, living units called cells.
such as the skeleton (useful when classifying fossils,
for example). These studies were also supported by Cells include the following features:
consideration of shared anatomical features, that is, • Cytoplasm – a jelly-like substance that contains
internal features visible as a result of dissection of smaller structures (organelles) and in which all the
organisms. metabolic chemical reactions occur.
• DNA – the chemical that forms the genes of the cell
RNA and DNA sequencing that are responsible for the nature of the proteins
made within the cell and also for handing on this
The sequence of chemical bases in the DNA and information to future generations.
RNA molecules found in different organisms gives a • Cell membrane – the living, selectively permeable
very accurate indication of how closely related those structure that encloses the cell contents and is
organisms are. Mutations are constantly changing this responsible for the entry of substances into and exit
sequence and those changes are handed on to the next of substances from the cell.
generation. (See Chapter 18.)
Two of the most familiar kingdoms in the living universe
The sequence of bases in the DNA molecule are the animals and the plants. The distinguishing
determines the sequence of amino acids in the features of these two kingdoms are as follows:
proteins made by the organism. Thus, a mutation in
an organism’s DNA leads to a change in its protein
structure. The longer ago the two different organisms Animals
separated from a common ancestor, the larger the
number of mutations will have occurred, and the • Animals take in (ingest) and use organic materials
greater the differences in the sequence of bases there from other living organisms as their source of energy
will be in these organisms’ DNA and RNA. This, in turn, for growth and development.
leads to a greater difference in the amino acid sequence • Animals are able to move from one place to another
in their proteins. (movement known as locomotion). (Sponges are
Data from the analysis of DNA/RNA base sequences exceptions to this as they are animals that remain
is now so accurate that we are able to identify human fixed to the surface on which they live.)
beings in the same family. • Sexual reproduction – animals reproduce using
specialized reproductive cells (gametes). The male
gamete is the sperm and the female gamete the egg
Progress check 1.1 cell (or ovum). Few animals reproduce by asexual
reproduction.
1 Find out what you can about a DNA
molecule. How many bases are there? • Most animals have diploid nuclei. That is to say that
each nucleus has two full sets of genetic material
2 Make sure you know what each of the contained in matching chromosomes. Only the X
letters in ‘Mrs Gren’ stand for. and Y chromosomes (the sex chromosomes) do not
3 What describes respiration? exactly match.
A breathing in oxygen • There is no rigid cell wall surrounding the cell
membrane.
B breathing out carbon dioxide
C releasing energy from nutrient molecules
D using energy to construct nutrient
molecules

3
 Plants Fish
Chapter 1 Classification

• Plants manufacture their own food from carbon All fish share the following characteristics (Figure 1.1):
dioxide and water, using energy from sunlight that is
• A skeleton made of bone or of the more pliable
trapped by the green pigment called chlorophyll. The
material, cartilage
process is called photosynthesis.
• A skin covered with scales
• Plant cells are surrounded by a rigid cell wall made
of cellulose. Pressure within the cell caused by the • Fins that present a large surface area to push against
entry of water keeps the cell firm and supplies the water when swimming
rigidity to the plant. • Gills for extracting oxygen from water and supplying
• Plants have a complex reproductive cycle, involving it to the blood.
various agents to bring about the processes of
body streamlined and
pollination and, later, fruit or seed dispersal. fins covered with scales
• Most plants have only a few, but easily identifi able
organs – leaves, flowers, stems and roots.
• Asexual reproduction, where a parent plant gives rise
to may offspring without the involvement of gametes,
is relatively common in plants.
• Although most plants in their familiar form have
diploid nuclei, very few have the non-matching XY
sex chromosomes.
NB Both plants and animals are made up of many cells
powerful tail gills under
(they are thus both described as being multicellular). for swimming gill cover

Figure 1.1 A fish

Classification within the animal
kingdom Amphibians
Animals either possess or do not possess a vertebral Frogs, toads, newts and salamanders all have the
column. following characteristics (Figure 1.2):
• A soft skin with no scales
Avoid the word ‘backbone’. It is not a very
TIP

• Live or can survive on land but always return to

accurate term as there are many bones in water to lay eggs
the vertebral column.Those possessing a
vertebral column are called vertebrates. • Adults have lungs to breathe air
(Those without are called invertebrates.) • Eggs hatch into larvae called tadpoles that live
in water
There are five groups of vertebrates: • Tadpoles breathe using gills
• Fish • Birds • Tadpoles change into adults by metamorphosis
(metamorphosis = a change in form and feeding
• Amphibians • Mammals.
habits from larva to adult).
• Reptiles

4
 nostril ear drum smooth moist skin • A beak for feeding

Chapter 1 Classification
• Have scales on their legs and toes
• Lungs for breathing
• Lay hard-shelled eggs on land
• Maintain body at a constant temperature – usually
above atmospheric temperature.

Use the terms ‘warm-blooded’ and ‘cold-

TIP
blooded’ with care. They are not very
helpful terms, since some reptiles when
basking in the sun may have a blood
temperature higher than that of birds.
one pair each of front and rear limbs

Figure 1.2 An amphibian (frog)

wings for flight
Reptiles
Lizards, snakes, tortoises and turtles all have the
following characteristics (Figure 1.3):
bill or beak
• A tough, dry, scaly skin
• Lay eggs with leathery shells on land
• Have lungs for breathing air.
body covered
with feathers

scales on legs

Figure 1.4 A bird

dry scaly skin
Mammals
Mammals, including kangaroos, cows, whales and human
beings, have the following characteristics (Figure 1.5):
• Have hair on at least some part of the skin
• Internal fertilisation and internal development of the
embryo
Figure 1.3 A reptile (crocodile)
• Young ones fed on milk from mammary glands
Birds • Lungs for breathing
• Maintain body at a constant temperature (‘warm-
Birds including the flightless birds (such as the ostrich) blooded’ – but sometimes not as warm as the
have the following characteristics (Figure 1.4): surrounding atmosphere).
• Skin covered with feathers
• Forelimbs modifi ed to form wings

5
 hair covering body The invertebrates
Chapter 1 Classification

These are the animals that do not have vertebral columns.

Like the vertebrates, they are divided into phyla (the plural
of ‘phylum’), but such is the diversity of the invertebrates
that they include over 30 different phyla.The largest group
(phylum) of invertebrates, by far, is the arthropods.

mammary The arthropods

glands
These include several Classes of which the largest and
better-known ones are:
Figure 1.5  A cow • The insects

Test yourself by writing at least two characteristic • The crustaceans (crabs and lobsters)
features of each of the five vertebrate classes. • The arachnids (spiders)
• The myriapods (centipedes and millipedes).
Worked example All arthropods have the following features:
a A student is told that an animal he is about • They have segmented bodies.
to be shown is either an amphibian, a • They have limbs with clearly visible joints.
reptile or a mammal. Describe the features
common these three groups. • They have an exoskeleton (i.e. a skeleton on the
outside of the body). (Muscles are attached internally
b Describe the external features that would to the exoskeleton – the opposite of ourselves, where
indicate to which group it belongs. muscles are attached externally to our endoskeleton.)
Answer • The exoskeleton is composed of the chemical chitin.
a Since these are all vertebrates, then the (See fungi.)
question is asking for vertebrate features
shared by the three groups. All vertebrates The insects
have a vertebral column (avoid calling it a
‘backbone’) and a bony skeleton. They will In addition to the characteristics of arthropods
possess eyes, a mouth, two front legs (‘arms’) listed, insects have the following features
and two rear legs. They will all have the (Figure 1.6):
following internal organs: lungs, heart, blood
vessels, liver, kidneys and an alimentary canal. • The body is divided into three parts – head, thorax
and abdomen. The head, thorax and abdomen are
b If it is an amphibian, it will have a soft, smooth not segments.
and most probably moist skin. If it is a reptile,
its skin will be tough, dry and will be covered • They have three pairs of (jointed) legs – attached to
with scales. If it is a mammal, it will have a the thorax.
skin covered, or partly-covered, with hair. It is • They usually have wings – one or two pairs attached
likely to be warm to the touch. If it is a female, to the thorax.
then it will have at least one pair of mammary
• They have one pair of antennae – attached to the head.
glands on the front of its thorax (chest).
• They have compound eyes – each one with
(Note that part (b) asks about external features.
hundreds of small units called ocelli.
Mention of internal or external fertilization, laying of
unprotected eggs in water that hatch into tadpoles • Breathing is through small holes (spiracles),
(amphibian) or of shelled eggs on land (reptiles) are occurring in pairs, one each side of the abdominal
facts that are not relevant to the question. segments and two on the thoracic segments. The
spiracles lead into branched tubes called tracheae.

6
 compound eye antennae (1 pair) The arachnids

Chapter 1 Classification
thorax wings
This group includes the spiders and the scorpions and,
as well as those features possessed by arthropods, the
arachnids also have the following (Figure 1.8):
• A body divided into two parts (the head and thorax,
called the cephalothorax, and the abdomen).
• Four pairs of jointed legs joined to the cephalothorax.
head
segments of • No antennae.
first leg of spiracles
3 pairs abdomen
for breathing

Figure 1.6 An insect

The crustaceans
These live mostly in water (e.g. a lobster) and those that
live on land (e.g. some crabs) live in damp places.
As well as the characteristic of arthropods, crustaceans
have other features: (Figure 1.7):
• There are two pairs of antennae that are attached
to the head.
• There are three pairs of mouthparts that, with the
antennae, make up the five pairs of appendages
attached to the head.
• The exoskeleton is often strengthened with calcium Figure 1.8 A spider
salts. (This protects the animal from predators,
but can make the animal very heavy. The additional The myriapods
mass is supported by the water in which most
crustaceans live.) Myriapod means ‘countless legs’ and includes the
centipedes and millipedes. As well as possessing the
• The head and thorax are often joined to form the features common to arthropods, they also possess
cephalothorax. (Figure 1.9):
• The abdomen often has a pair of limbs on each • One pair of antennae.
segment, which are modified for many purposes, but
often for swimming. • One or two pairs of legs attached to every segment.
One pair of legs
second pair
One pair of antennae per segment
of antennae
hard
exoskeleton
first pair of
antennae
(each one divides)

other limbs
underneath
abdomen
head and thorax
(cephalothorax) Figure 1.9 A centipede

Figure 1.7 A lobster

7
 Occupying a position below the plants and animals in
Progress check 1.2
Chapter 1 Classification

the evolutionary tree, the prokaryotes, protoctists and

the fungi each form their own kingdoms (see earlier).
1 Name the chemical found in the
exoskeleton of arthropods.
2 Which animals possess visible body Viruses
segments?
Viruses are not truly living organisms. They have the
A birds following main characteristics (Figure 1.10):
B insects 1 They are less than 300 nm in size – around 50 times
C mammals smaller than a bacterium. (1 nm, or nanometre, is
1 thousand millionth of a metre). They can be seen
D reptiles only with an electron microscope.
2 They contain nucleic acid (DNA or RNA).
3 The nucleic acid is surrounded by a protein coat
Other features present in cells (known as the capsid).

As well as cytoplasm, DNA and a cell membrane, 4 They can reproduce only inside living (host) cells.
the cells of all truly living organisms also contain 5 Since they are parasites, they cause disease
within their cytoplasm structures (‘organelles’) called (i.e. they are pathogenic). Examples of diseases
ribosomes. caused by viruses are influenza, measles and AIDS.
Ribosomes are: 6 Viruses are not affected by antibiotics.
• about 20 nm in diameter (1 nm = 1 millionth of a DNA or RNA
millimetre)
• the place where amino acids are joined together to
make proteins.
• made of protein and the nucleic acid RNA.
Some of the proteins made will be enzymes and
amongst the enzymes will be those used in the
process of anaerobic respiration. This occurs in the protein
cytoplasm of all cells with the release of a relatively coat
small amount of energy. If oxygen is available
within the cell, then the end-products of anaerobic Figure 1.10 A virus
respiration will be further broken down (oxidised
during aerobic respiration) to release greater Their extremely small size allows them to be easily
amounts of energy. transmitted from host to host in very considerable
numbers, both by air currents as well as by contact.The
protein coat gives the nucleic acid considerable protection.
The main features used in classifying Once inside a living host cell, they take over the host cell’s
metabolism and use it for their own reproduction. Some
viruses, prokaryotes (bacteria), viruses (e.g. influenza virus) have a high mutation rate;
protoctists and fungi thus a person may recover from flu, but still fall victim to
the next epidemic caused by a mutated strain of the virus
Since a microscope is required to study viruses and to which they have no immunity.
prokaryotes, protoctists and also for some fungi – such
as yeast – they are referred to as microorganisms.

8
Prokaryotes (or bacteria) Protoctista

Chapter 1 Classification
Prokaryotes are truly living organisms, with the following These are a group of largely microscopic, truly living
characteristics (Figure 1.11): organisms.
1 They have a size in the range of 0.5–5 µm 1 They are mostly unicellular but some are multicellular.
(1 µm = 1/1000 mm).
2 They are either free-living or parasitic.
2 They are unicellular (made of one cell only).
3 They have aerobic respiration involving mitochondria.
3 They have no true nucleus (their DNA lies ‘loose’
4 Unlike the prokaryotes, structures that lie in their
in the cytoplasm).
cytoplasm are surrounded by membranes.
NB Some prokaryotes contain a loop of DNA
5 They have true nuclei (i.e. they are eukaryotes).
called a plasmid – a feature sometimes employed in
biotechnology. 6 They reproduce both sexually and asexually.
4 They have a cell wall. 7 They are grouped into three categories: animal-
like (protozoa) that have animal-like nutrition;
5 They may be (pathogenic) parasites or they
plant-like (algae) that feed by photosynthesis and
may be saprotrophs. Some may be involved in
fungus-like.
nitrogen fixation and denitrification. (See the
Nitrogen Cycle.)
Pathogenic = disease-causing; Saprotrophic = feeding
Fungi
on dead organic matter causing it to decay. Fungi are usually much larger organisms, mostly visible
6 They are killed by antibiotics. to the naked eye. For example: yeasts, moulds and
mushrooms. They have the following characteristics
slime layer (Figure 1.12):
(sometimes present)
• They have no chlorophyll. (They release enzymes
to digest large molecules externally, then absorb
cell wall the soluble products.) They are thus parasites or
saprotrophs.
• They have a ‘cell’ wall made of chitin.
• They are usually made of a large number of tubular
threads (hyphae) intertwined to form a mycelium.
DNA
(no nucleus) • Hyphae are not divided into individual cells. The lining
of cytoplasm has many nuclei and the central space
Figure 1.11 A bacterium in the hyphae is a vacuole full of (vacuolar) sap.
• If they store carbohydrate, they store glycogen.
They can form resistant spores that are easily carried
by air currents and by contact. Once within a suitable • They reproduce by producing spores.
substrate, they reproduce quickly – dividing every half
an hour. spores

Progress check 1.3

1 In suitable conditions, one bacterium could hypha
become how many in 5 hours?
mycelium
2 What is the purpose of ribosomes?

Figure 1.12 A fungus

9
 The branching mycelium of a fungus ensures that the the monocotyledons and the dicotyledons (usually
Chapter 1 Classification

maximum amount of food substance (substrate) is shortened to ‘monocots’ and ‘dicots’).

digested as quickly as possible before it dries up, or is
(NB The dicots have more recently been divided into
digested by bacteria. Fungi are important in the decay
several separate smaller groups, the most important of
of dead, organic matter. Fungal spores are light and
which is called the ‘eudicotyledons’.)
easily carried by air currents from one substrate or host
to another.
1.05 Classes of flowering plant
1.04 Classification of the plant Monocotyledons
kingdom
Mono = one, cotyledon = a leaf that forms part of the
In plant classification, the term ‘division’ is often used structure of the seed.
instead of phylum, but like phyla, divisions are divided
into classes. Two major divisions of plants are the ferns This class includes the grasses, cereals, lilies and
and the flowering plants. orchids, all of which share the following characteristics
(Figure 1.14):
• One cotyledon inside each seed
Characteristics of ferns
• Leaves that are narrow and strap-like
Ferns have the following characteristics (Figure 1.13):
• Leaves that have parallel veins
• They are green photosynthesising plants.
• A mass of equally sized (fibrous) roots
• They have conducting tissue (xylem and phloem)
• Flower parts that are usually arranged in threes
forming veins.
(i.e. three petals etc.).
• They have, often compressed, stems called rhizomes.
• They do not produce flowers.
• Instead, they produce spores that are light and easily
carried away by the wind.
• Spores are released from spore cases (sporangia)
that are found on the lower surfaces of fronds.
• Frond is the term for the leaves of ferns.

spore cases on
frond
under surface
of leaflet

stem (or rhizome)

roots

Figure 1.13 A fern plant Figure 1.14 A monocotyledon (Iris) with thin strap-like
leaves that have parallel veins
By far the largest division of the plant kingdom is the
flowering plants that are sub-divided into two classes:

10
The (eu)dicotyledons

Chapter 1 Classification
2 An organism possesses xylem but never
This class includes cabbage, hibiscus, geranium and sweet produces flowers. Which of the following
potato, all of which have features that differ from those will it be?
of the monocotyledons mentioned earlier (Figure 1.15). A a dicotyledon
• Two cotyledons are present inside each seed. Not B a fern
only do these become the first photosynthesizing
leaves when the seedling emerges above ground, but C a fungus
generally store food used during the process of seed D a monocotyledon
germination.
3 An organism has three petals. Which of the
• The leaves are broad. following will it be?
• The leaves have branched veins usually radiating A a dicotyledon
from a central thicker vein called the midrib with the
B a fern
branches linked by a network of veins.
C a fungus
• Fewer, thicker roots which are often joined to one
long central root called the tap root. D a monocotyledon
• Flowers have parts usually arranged in fours or fives.

1.06 Dichotomous keys

Dichotomous means cutting (or dividing) into two.
Organisms are often identified using a book of
illustrations. This is possible only if such a book is available,
and this is the case only with certain organisms such as
common plants, birds and butterflies. Even when such a
book is available, identification will rely on the accuracy
of the illustration, and it can be a time-consuming
process if the organism is at the back of the book! For
these reasons, biologists use dichotomous keys.
Figure 1.15 Flower of a dicotyledonous plant (flax) A dichotomous key consists of a series of questions.
Each question has two alternative answers. Depending
on which answer is chosen, the user is directed to the
next question. Thus, by starting at the first question, and
then by a process of elimination, a specimen may be
identified.
Progress check 1.4
This process is reliable because it directs the user to
1 Which chemical is found in the cell walls of observe particular characteristic features. Also it is
fungi? quicker since, at each question, possible alternatives are
A cellulose eliminated.
B chitin Dichotomous keys are usually presented in the following
format. The example chosen is a key in its simplest
C glycogen
form – namely to identify the kingdom into which an
D protein organism should be placed. More detailed keys are used
➔ when determining precisely to which species from many
within the same genus an organism belongs.

11
 1 Is it unicellular (i.e. made of When identifying one organism from amongst a
Chapter 1 Classification

only one cell)? Yes go to 2 large number of possibilities, the most effective
No go to 3 dichotomous key asks questions that each time divides
the remaining possibilities into roughly equal halves. In
2 Does it have a nucleus? Yes protoctist
this way, half the possible organisms are discarded at
No bacterium
each step.
3 Does it have hyphae? Yes fungus
No go to 4
4 Does it have cell walls? Yes plant
No animal

Progress check 1.5

1 Six different geometrical shapes, identified by Construct a dichotomous key to identify the
the letters A to F are shown in Figure 1.16. six shapes.
A B 2 Now construct a key to identify the shapes
using different features from those you have
used already.
You should now find that several different
keys may be constructed, all of which may
C D be perfectly suitable for the purposes of
identification.

E F

Figure 1.16

Chapter summary
You now know the characteristics of living You are now able to write down the names and
organisms. characteristics of the four phyla of invertebrates
You have learnt how to use the binomial system and the four classes of arthropods that are
of naming organisms. described.

You are able to list the five classes of vertebrate You have learnt the differences between the
and know how to distinguish between them. two classes of flowering plant.

You have learnt the differences between the You have learnt how to use and also to
microorganisms viruses and bacteria and how construct a dichotomous key for identifying
they differ from fungi. organisms.

12
Exam-style questions

Chapter 1 Classification
1 Figure 1.17 shows six arthropods. Use the key to identify each of the arthropods. Copy
Table 1.1 and write the name of each arthropod in
A B C
the correct box. As you work through the key, put
a tick (✓) in the boxes to show how you identified
each arthropod.
As an example, the appropriate boxes for arthropod
A have been ticked for you.

Figure 1.17
Key
Arthropod
1 a segments on abdomen clearly visible ………………………………………… go to 3
b segments on abdomen not clearly visible …………………………………… go to 2
2 a 3 pairs of legs ……………………………………………………………… Pediculus
b 4 pairs of legs ……………………………………………………………… Ornithodorus
3 a wings present ………………………………………………………………. go to 4
b wings absent ……………………………………………………………… Pulex
4 a wings clearly longer than abdomen ………………………………………. Musca
b wings not clearly longer than abdomen ……………………………………… go to 5
5 a antennae curved …………………………………………………………… Periplaneta
b antennae straight …………………………………………………………… Anopheles

1a 1b 2a 2b 3a 3b 4a 4b 5a 5b name of arthropod

A ✓ ✓ Pediculus

F
[10]
Table 1.1

13
Chapter 1 Classification

2 Figure 1.18 shows a centipede. compared with most other arthropods, its outer
body covering is thin and permeable.
One pair of legs
One pair of antennae per segment a Name two other groups of arthropod.
For each group, state one feature found only
in arthropods of that group. [4]
b Suggest and explain two reasons why
centipedes are often found under stones,
decaying wood and leaves. [4]
3 a Explain the fact that viruses are almost
Figure 1.18 always harmful to other organisms. [6]

The centipede is a myriapod, one of the four groups b Describe how viruses differ structurally from
of arthropod. It is a carnivore that lives on land and, bacteria. [5]

14
 Cells
Chapter 2

Learning outcomes
By the end of this unit you should understand:

The structure of animal and plant cells and the How cells work together to form tissues,
differences between them organs and organ systems within living
organisms
How certain plant and animal cells are adapted
to the functions they perform

2.01 Cell structure and nucleus

organisation
The basic unit of life is the cell. The simplest living
organisms have one cell only. Such organisms are
described as unicellular. Bacteria are examples of
unicellular organisms. Most other living organisms have
many cells. They are described as multicellular.
All cells have the following structural features in
common (Figure 2.1 and Figure 2.2):
cell surface cytoplasm
1 Cell surface membrane: This surrounds the cell membrane
and controls the passage of substances into and
out of the cell. One of the most important of those Figure 2.2  Animal cell (liver)
substances is water. All other substances that pass
through, do so in solution. Plant cells have the following additional structures
(Figure 2.3):
2 Cytoplasm: A jelly-like substance in which the
chemical reactions of the cell (metabolic reactions) 1 A large, central vacuole: a space full of cell sap
take place and which contains the nucleus. (thus sometimes called the sap vacuole), which is
a solution mostly of sugars. It is separated from the
3 Nucleus: This contains a number of chromosomes cytoplasm by the vacuolar membrane. (Plant cells
made of the chemical DNA. undergoing cell division do not have a vacuole.)
NB Cytoplasm and nucleus together may be referred to 2 Cell wall, which is a ‘box’ made of cellulose that
as protoplasm. contains the cell.
3 Chloroplasts. These are present only if the cell is
involved in the process of photosynthesis. These
are small bodies lying in the cytoplasm, which are
green in colour because of the pigment chlorophyll
that they contain.

Figure 2.1  Stained liver cells

15
 cellulose cell wall 2.02 How the structural
Chapter 2 Cells

features in a cell are related to

cell surface membrane
their functions
cytoplasm
(containing mitochondria) Cell surface membrane
chloroplast
(containing chlorophyll) This controls the passage of materials in and out of the
cell since it is partially (or selectively) permeable. That
vacuole
(containing cell sap)
means that it allows some chemicals to pass through
nucleus it but not others. Some small molecules pass through
(containing chromosomes) minute pores in the membrane, and the process does
not require energy. Other, usually larger, molecules pass
Figure 2.3  Palisade mesophyll cells from a leaf through the membrane via specific pathways, and may
pass through when there is a greater concentration
In plants, the cell membrane is not usually easily visible
inside compared with outside the cell. This process does
as it fits tightly against the cell wall.
require energy.

Progress check 2.1 Cytoplasm

1 Figure 2.4 shows a cell from the leaf of a
plant. This is where the chemical reactions of the cell occur
such as respiration and protein manufacture. It
contains a range of small structures called organelles
for example, the chloroplasts in plant cell cytoplasm
A
where photosynthesis and carbohydrate (often starch
in plants) storage takes place.
B
Further examples of structural features inside cells are:
C
• Mitochondria, the largest of which may just be visible
D
using a light microscope and which are involved
in the storage of energy (in the form of ATP)
released by respiration (Figure 2.5, 2.6). Although
mitochondria are present in almost all cells, they
are not present in the cells of prokaryotes. A
mitochrondrion has many folds (called cristae) on its
inner walls, and it is on these folds that the process
of aerobic respiration takes place. It is for this reason
that mitochondria are referred to as the ‘power
houses’ of the cells.
Figure 2.4  Palisade mesophyll cells from a leaf

Which labelled feature is also found in an

animal cell?
2 Why do cells from the root of a plant rarely
contain chloroplasts?

16
 rough endoplasmic vesicles
containing enzymes cell membrane

Chapter 2 Cells
reticulum
for use in the cell

storage proteins
within exported
cytoplasm vesicles from cell
containing
vesicles proteins cell cytoplasm
containing protein
ribosomes budded-off from the
rough endoplasmic
reticulum
Figure 2.7 Endoplasmic reticulum and vesicles

Mitochondria are present in the largest numbers in

cells that release large amounts of energy. For example,
those involved in active uptake (absorption) of
chemicals and in movement, for example, sperm cells.
Ribosomes are made of RNA and protein.
Figure 2.5 Photomicrograph of a mitochondrion
with endoplasmic reticulum beneath with ribosomes
on its walls Nucleus
ribosome
This can be regarded as the organelle that controls the
cell’s activities. It contains thread-like structures called
chromosomes that are made largely of DNA.
DNA DNA forms genes that are responsible for programming
peripheral
space the cytoplasm to manufacture particular proteins.
crista
When any cell other than those that produce
reproductive cells (gametes) divides, its nucleus does
so by a process called mitosis during which each
chromosome forms an exact replica of itself. The two
cells formed are therefore genetically identical both
inner with themselves and with the original cell.
membrane
outer
matrix
granules
Matrix membrane
Cell wall
Mitochondrion
Plant cells are enclosed in a ‘box’ of flexible but tough
Figure 2.6 Diagram of a mitochondrion carbohydrate called cellulose, which is a completely
permeable substance. Thus it does not in any way affect
• Ribosomes are where amino acids are linked materials passing in and out, but it helps the cell to:
together to form proteins. Ribosomes lie attached • keep its shape
to the walls of a system of many-folded small tubes
(microtubules) known as the endoplasmic reticulum • prevent the cell from bursting as it absorbs water
– often shortened to ER (Figure 2.7). The ER runs • maintain a pressure within the cell (turgor pressure)
throughout the cytoplasm of cells. Endoplasmic to keep the plant firm and upright.
reticulum with ribosomes attached is known as
rough endoplasmic reticulum.
Sap vacuole
• Vesicles are spherical bodies that break away from
the endoplasmic reticulum and which contain proteins The importance of this structure is to contain a
that have been made by the cell. They are used to solution more concentrated than the solution in the
transport and, sometimes, export these proteins.

17
 soil water around the plant. A more concentrated   Animal cell Plant cell
Chapter 2 Cells

solution has a lower water potential and this causes

Similarities cell membrane
water molecules to enter the plant from the soil by
osmosis. (See Chapter 3, Osmosis.)   cytoplasm
  nucleus
Similarities and differences between plant and animal
Differences no sap vacuole sap vacuole
cells are shown in Table 2.1.
  no cell wall cell wall
  no chloroplasts may have chloroplasts
  never stores starch may store starch
TIP

l   around 10–20 µm in around 40–100 µm in

Remember that l µm = mm
1000 diameter diameter

Table 2.1 Comparison of plant and animal cells

PRACTICAL
Make sure you are familiar with the procedure for 2 To observe plant cells:
preparing animal and plant cells for viewing under a
Peel off the dry outer leaves of an onion bulb.
microscope.
Remove one of the fleshy leaves beneath.
1 To observe animal cells: Preferably using forceps, but fingers would
Cut a cube of fresh liver, in section, do, peel away the outer skin-like covering
approximately 1.5 cm square. (epidermis) of the fleshy leaf.
Scrape one of the cut surfaces of the cube with Place three drops of dilute iodine solution on a
the end of a spatula (the end of a teaspoon clean, dry microscope slide (iodine solution is a
would do). suitable, temporary stain for plant cells).
Transfer a small piece of the epidermis
Transfer the cells removed to a clean
(a 50–75 mm square is large enough) to the
microscope slide. Add one drop of methylene
iodine solution. (Make sure it lies flat and is
blue (a suitable stain for animal cells) and one
completely covered by the iodine solution.)
drop of glycerine.
Carefully place a glass cover slip on top of the
Mix the cells, stain and glycerine together gently
preparation, remove any excess liquid with a
and leave for 30 seconds. (This time can be
piece of filter paper and transfer the slide to the
adjusted according to the depth of staining
stage of a microscope.
required.)
The structural features shown in Figure 2.8
Carefully place a clean, dry cover slip over should be visible (owing to the large size of the
the preparation, and then wrap a filter paper onion cells, it may not be necessary to use the
around the slide and cover slip. high power of your microscope).
Place the slide on a bench and press hard with
your thumb on the filter paper over the cover cellulose cell wall
slip. The filter paper should absorb any surplus cell surface
stain and glycerine, and the slide is then ready membrane
nucleus
for viewing with a microscope (medium to high cytoplasm
power).
Figures 2.1 and 2.2 show structures that should
be visible. Figure 2.8 Photomicrograph and labelled drawing
of onion cells

18
 Root hair cells
Progress check 2.2

Chapter 2 Cells
The tip of a root with its many root hair cells is shown
1 Why are mitochondria referred to as the in Figure 2.8.
power houses of a cell?
2 Make a list of those features you would find
in a plant cell that are absent from an animal
cell.
2
3 How long is = mm expressed in µm?
100

2.03 Levels of organisation

Specialised cells, tissues and organs
In unicellular organisms, one cell must be able to carry
out all the functions of a living organism. In multicellular
organisms, cells are usually modified to carry out one
main function. The appearance of the cell will vary
depending on what that main function is.
Thus, there is a relationship between the structure and
the particular function of a cell.
Examples of this relationship are discussed here. Figure 2.9 A root tip showing root hairs

Function
Ciliated cells The absorption of water and mineral ions (salts) from
Function the soil.
To sweep mucus, in which dust and bacteria are
How they are adapted to this function
trapped, up the bronchi and trachea towards the throat
where it is swallowed. The outer part of the cell wall of each root hair cell
(i.e. the part in direct contact with the soil) is in the
How they are adapted to this function form of a long, tubular extension (the root hair).
Ciliated cells are found lining the walls of the trachea This root hair:
(wind-pipe) in the respiratory tract. Each cell bears a
• is able to form a very close contact with the water
fringe of minute projections (cilia). The singular of cilia is
film surrounding many soil particles and
cilium.
• it greatly increases the surface area of the cell
The cilia perform an upward-beating motion that
available for uptake of water and ions.
carries the mucus, made and released by neighbouring
cells, upwards like a ‘moving carpet’.
Xylem vessels
Functions
TIP

Remember that cilia do not form a

network to ‘trap’ dust and bacteria. 1 To conduct water and ions (dissolved salts) from
the roots to the stem, leaves, flowers and fruits.
2 To provide support for the aerial parts of the
plant.

19
 How they are adapted to these functions 2 Xylem vessels are part of the vascular bundles
Chapter 2 Cells

(Figure 2.9), which run the entire length of the

Conduction stems of plants thus resisting bending strains caused
Xylem vessels are elongated dead cells forming long, by the wind.
narrow tubes, stretching from the roots, via the stem, to
the leaves. They are stacked end to end like drain pipes.
Vascular bundles help to strengthen

TIP
Support a stem since they work like iron
1 Their walls have been strengthened by the addition reinforcements in concrete pillars.
of the chemical lignin. (As the lignin in the walls
builds up, it eventually kills the xylem vessels. There
is then no layer of cytoplasm to restrict the flow of
water and dissolved salts.)

a vascular
bundle
cortex
phloem
xylem

a vascular
bundle
Figure 2.10 3D section of stem showing xylem as part of the vascular bundles and a photomicrograph of xylem vessels

Palisade mesophyll cells Nerve cells (or neurones)

The inner structure of a plant leaf includes two These may be long, thin cells that carry electrical
specialised types of cell known as mesophyll cells. impulses from the receptor organs to the central
nervous system, or from the central nervous system
The lower cells are called spongy cells and the upper
to the organs that are required to respond (the
layer are the palisade mesophyll cells (Figure 2.3). The
effectors) (see Chapter 13, Figure 13.3). Neurones in
palisade mesophyll cells form the main photosynthesising
the central nervous system, especially the brain, are
tissue of the leaf, helped by the fact that they:
more compact.
• contain the greatest number of chloroplasts
• they are the first to receive the Sun’s rays as they
enter the leaf Red blood cells
• their chloroplasts are able to move, within the Function
cytoplasm of the cell, towards the upper surface of To carry oxygen around the body.
the leaf to receive more sunlight.

20
How they are adapted to this function 3 They have a biconcave shape, increasing their

Chapter 2 Cells
1 The cytoplasm of red blood cells contains the surface area for absorption still further.
pigment hemoglobin, which combines (in the lungs) 4 They are flexible allowing them to be pushed more
with oxygen to become oxyhemoglobin. easily through capillaries.
2 They are small (7 µm × 2 µm) (Figure 2.11), and Points 2, 3 and 4 are also relevant to their other
there are many of them. This gives them a very function of carrying carbon dioxide.
large surface area for oxygen absorption.

surface side sectional

vi
view view view

7 m 2 m

Figure 2.11 Red blood cells

Sperm and egg cells • It does not possess the ability to move on its own,
but is moved by the action of cilia and muscles in the
Together known as gametes, these are the cells used female reproductive system.
in sexual reproduction.They are produced by a form • It is spherical and much larger than the sperm,
of cell division (meiosis) that halves the number of measuring between 0.10 and 0.15 mm in diameter.
chromosomes they possess, that is they become haploid.
In this way, when they unite at fertilisation, the full number • Egg cells are released one at a time.
of chromosomes (the diploid number) is restored.

The sperm Progress check 2.3

This is the male gamete, it is made of:
1 In what way are cells in the trachea adapted
• a head containing the haploid nucleus to carry mucus up to the throat?
• a middle piece that supplies energy for movement A They are biconcave in shape.
• a tail that beats to provide propulsion as it swims B They contain many fibrils.
through the female reproductive system to locate
C They have strengthened cell walls.
the egg cell.
D They possess many cilia.
Sperms are released in millions at a time to increase the
chances of fertilisation. A human sperm is about 100 µm 2 What features do root hair cells and red
in length. blood cells have in common and what
common purpose do these features serve?
The egg cell (or ovum)
3 Which cells in a plant leaf have the greatest
This is the female gamete with the following features: number of chloroplasts and why is this?
• The haploid nucleus is contained within a large
amount of nutritive cytoplasm.

21
 2.04 How cells combine to An organism is a collection of organ systems working
Chapter 2 Cells

together.
improve their efficiency The increasing order of cell organisation found within
One cell working on its own would achieve very little in any living organism is thus:
an individual plant or animal, thus it is usual to find many cell tissues organs organ systems organisms
similar cells lying side by side and working together,
performing the same function.
A tissue is therefore: many cells with similar structure Progress check 2.4
working together and performing the same (shared) 1 The following are structures found in living
function. organisms:
Examples of tissues: xylem tissue in the vascular bundles P the eye
of a plant, muscular tissue in the intestine wall of an
animal. Q the muscles in the intestine wall

Different types of tissue often work together in R a flower

order to achieve a combined function. Several tissues Which shows the level of organisation in
working together to perform specific functions form a these three structures?
structure called an organ.
Tissue Organ Organ system
Examples of organs: the leaf of a plant – an organ
A P Q R
for the manufacture of carbohydrates during
photosynthesis, the eye of an animal – the organ B Q R P
of sight. C R P Q
D Q P R
Several different organs may be necessary in order to
carry out a particular function. A group of organs with 2 Make a list of two different types, other than
related functions working together in order to perform those mentioned in the text, of cell, tissue,
a particular body function is called an organ system. organ and organ system. (Check with your
teacher that you have correctly classified
Examples of organ systems: the sepals + petals +
you chosen examples.)
stamens + carpels (i.e. the flowers) of a plant for
reproduction; the heart + arteries + veins + capillaries
in an animal, i.e. the circulatory system.

Worked example functions – palisade cells contain chloroplasts for

photosynthesis and xylem cells become modified for
Explain how the terms cell, tissue and organ may be support and conduction.
applied to the structure of a leaf.
The cells are found working beside many other similar
Answer cells, each one of which is modified in the same way
It is easiest to begin with the fact that leaves are and performs the same function. Such a group of cells
made up of a large number of cells. This would then forms a tissue – for example, the palisade tissue for
be followed by some different examples of types making starch during photosynthesis (and the xylem
of cells found in a leaf – for example, palisade cells tissue for bringing water and ions into the leaf and for
and, if you have looked up leaf structure, you could supporting the leaf). A leaf thus contains a number
include spongy cells, epidermal cells and cells found of groups of different cells, each group performing
in the vascular bundles (e.g. in the xylem – which its own job, but with each group performing one of
start out as living cells). The answer should then refer the necessary functions of a leaf. Thus, the leaf is an
to the fact that the cells are modified for different organ, since it have several tissues working together
performing specific functions.

22
The size of specimens Remember, if the specimen is large, then the

Chapter 2 Cells
magnification will almost certainly be less than 1.
Biologists deal with specimens with a wide variety of • Magnification should be written, e.g. × 4.4.
sizes. If a specimen is rare, then a description of it is
invaluable to other biologists in particular (and other • Don’t round off too much, × 4.4 is not × 4.
people in general). A drawing is often the best way to • No calculated magnification should include more
record observations, but such a drawing is of little value significant figures than the least accurate of the
if it does not give an indication of the size of the object measurements used to calculate it, e.g. 61 mm/
being observed. 14 mm = × 4.4 (not × 4.3571429).
All drawings of biological specimens should, therefore, • If the subject is a photograph it may also have
include a reference to the magnification of the drawing. a stated magnification that needs to be taken
into account – and shown – in the calculation.
When drawing, measure in mm – avoid Example: a photograph may bear the caption ‘× 4’
TIP

cm – so give a measurement as 5 mm thus the measurement taken from the photograph
rather than 0.5 cm, 50 mm rather than 5 cm. must be divided by four in your calculation,
e.g. 14 mm / (12 mm/4) = × 4.7).
Clearly state the linear dimension
measured from the drawing over the (Remember that diagrams and photographs of specimens
as seen using a microscope may have dimensions in
matching (linear) dimension measured
micrometres (µm) and 1 micrometer = 1000th of a
on the specimen when calculating the
millimetre.)
magnification, e.g. 42 mm/18 mm = × 2.3.

Chapter summary
You have learnt the component parts of a cell. You have learnt how cells are modified to
You have also learnt how plant and animal cells perform different functions.
are similar and how they differ. You have also learnt what is required when
You know how cells, tissues, organs and organ drawing a specimen.
systems are related.

23
 Exam-style questions
Chapter 2 Cells

2 Figure 2.13 shows the structures that produce

urine and excrete it from the body.
1 Figure 2.12 shows two cells.

structure A

cell A cell B

Figure 2.12

a i) State where, in a human, Cell A would

normally be found. [1] Figure 2.13
ii) Name the chemicals responsible in each
a In Table 2.2, put a tick (✓) in the box next to
case for the colour of the two cells: Cell
the term that states the level of organisation
A and Cell B. [2]
of structure A.
b Using only words from the list provided,
organism
complete the statements about Cell B.
organ system
air spaces cellulose organ
chloroplasts membrane tissue

mitochondrion nucleus cell [1]

starch vacuole Table 2.2
cell wall cytoplasm b With reference to the kidney, ureters and
bladder, explain the difference between the
Photosynthesis occurs in the terms organ and organ system. [6]
situated in the of 3 Explain how a plant’s xylem is particularly suited
the cell. It uses carbon dioxide absorbed to its functions. [10]
from the in the leaf.
The , which is made of
, under pressure from the
in the ,
helps the cell to maintain its shape. [7]
[Total 10]

24
 Movement in and out of cells
Chapter 3

Learning outcomes
By the end of this chapter you should understand diffusion
the processes responsible for the movement of
osmosis
substances into and out of cells:
active transport

3.01 Diffusion In plants

Chemicals must be able to move from one part of a 1 The movement of carbon dioxide, first as a gas
cell to another, into and out of a cell and from one cell from the atmosphere into a leaf, then in solution,
to another if an organism is to remain alive. from the water film surrounding the mesophyll cells
in a leaf, to the chloroplasts during photosynthesis.
It is an advantage if this movement requires no effort
(or, more correctly, no ‘expenditure of energy’) on 2 The movement of water vapour and oxygen (both
the part of the organism, and, so long as there is no in gaseous form) released from the water film
obstruction, chemical molecules carry out this process surrounding the mesophyll cells inside a leaf, and
by diffusion. both then passing through the intercellular spaces
of the leaf, and out through the stomata. Oxygen
Diffusion relies on the fact that all molecules are in a first diffuses in solution from the photosynthesising
constant state of random (kinetic) motion, but before cells into the water film surrounding the cells.
diffusion can occur, there must be a concentration
gradient of the molecules, that is, a region of their The loss of water vapour in this way is called transpiration.
(relatively) high concentration immediately beside a
region of their (relatively) low concentration.
In animals
Diffusion can then be defined as the net movement
of molecules and ions from a region of their 1 The movement of oxygen after it has dissolved in
higher concentration to a region of their lower the moisture lining the air sacs of the lungs through
concentration, down a concentration gradient as a the walls of the air sacs (alveoli) into the blood.
result of their random movement. 2 The movement of carbon dioxide, in solution, from
The energy that causes the movement of chemicals is the cells through tissue fluid, into the blood in
the kinetic energy that results from the movement of blood capillaries.
molecules and ions. In all the examples given here – apart from water
vapour, the substance that diffuses has first dissolved in
3.02 Examples of diffusion water (i.e. it is the solute molecules that are diffusing).

Living organisms make regular use of the diffusion of

gases or chemicals in solution (solutes). Only those
Progress check 3.1
molecules small enough to pass through the cell surface 1 How many examples of diffusion can you
membrane are able to pass in and out of a cell. think of – first in the world around you,
Some examples of diffusion are described here. then in plants and in animals? (Check your
list with your teacher to see how many of
your examples were correct.)
2 What causes molecules to move by diffusion?

25
 The rate at which a substance diffuses is controlled be a • when the temperature is higher, increasing the
Chapter 3 Movement in and out of cells

number of different factors: amount of kinetic energy in the molecules involved

Chemicals are absorbed faster by diffusion: • when the concentration gradient across which the
diffusion is occurring is greatest
• when the structure absorbing them has a large
surface area compared with its volume. This is • when less distance has to be travelled by the diffusing
achieved by the absorbing structure being long and molecules. For this reason, respiratory surfaces, for
thin (e.g. in root hair cells of plants or the villi in the example, the walls of the alveoli in the lungs that
small intestines of mammals), or simply by being absorb oxygen and release carbon dioxide are only
very small one cell thick.

PRACTICAL
Investigations of the factors that influence the rate hydrochloric acid (the beaker should be large
of diffusion enough to very easily accommodate the agar
cylinders mentioned later).
1 The effect of surface area
Using each cork borer, in turn, bore out three
Apparatus: A petri dish cylinders of agar from the petri dish.
Three cork borers of different sizes (If you have trouble in removing the agar from
Two medium-sized beakers the cork borers, you can, instead, using a sharp
Two dropping pipettes blade, cut the agar into three different-sized
Safety goggles and thin protective cubes, which you should measure carefully.)
gloves Transfer the three cylinders/cubes to the beaker
of hydrochloric acid and note the time.
Materials: Agar tablets or powdered agar
Keep a constant watch on the cylinders/cubes
Phenolphthalein pH indicator and record the time at which each cylinder
1 mol/dm3 hydrochloric acid loses its pink colouration.
Bench dilute sodium hydroxide Calculate the volume and the surface area of
A supply of water (preferably each agar cylinder/cube.
ionised or distilled) and a means of
The volume of each cylinder can be calculated by
heating it.
measuring the depth of the agar in the petri dish
Method: and the diameter of the cork borer and using the
Following the instructions on the bottle, formula πr2h (where h is the depth of the agar
dissolve some agar in hot water. When it has and r is half the diameter of the cork borer).
dissolved, using a pipette, add a few drops of
phenolphthalein indicator to the agar solution. The surface area of each cylinder is calculated
If the agar solution is alkaline, it will turn pink*; using the formula 2πr2 + 2πrh.
if not, using the second pipette, add drops of Results:
sodium hydroxide until it does so. Pour the pink The larger the ratio of surface area to volume
agar solution into the petri dish and allow it to for the cylinders/cubes, the shorter time it takes
cool and set in a refrigerator. for the pink colour to disappear.
*Colour-blind students may find this investigation Explanation: The greater the surface area to
easier if they use a pH indicator that does not volume ratio, the more H+ ions are able to
include pink as a significant colour. diffuse from the hydrochloric acid into the agar
Wearing the gloves and goggles, fill the second blocks changing their pH thus the quicker the
beaker to a depth of at least 3 cm with the indicator loses its pink alkaline colouration.
➔

26
 Chapter 3 Movement in and out of cells
2 The effect of temperature Method:
Prepare the agar as in Investigation 1 – The
The investigation of surface area previously
effect of surface area.
can be adapted to demonstrate the effect of
temperature on diffusion. Fill the three beakers to the same depth
Apparatus: 1 cork borer (around 3 cm), one with 2 mol / dm3, one
4 medium-sized beakers with 1 mol / dm3 and one with 0.2 mol / dm3
(size dependent of the size of the hydrochloric acid. All three beakers should be
cork borer) kept at the same (laboratory) temperature.
A container of ice and water and a Bore three cylinders each with the same borer,
water bath set at 40 °C or cut three identical cubes of agar.
Materials: As before
Transfer one of the cylinders/cubes to each of
Method: the beakers and note the time.
Prepare the agar as in Investigation 1 – The
effect of surface area. Keep a constant watch on the cylinders/cubes
and record the time at which each one loses its
Wearing the gloves and goggles, fill three beakers
pink colouration.
with the hydrochloric acid each to a depth of 3 cm.
Results and explanation:
Place one of the three beakers in the container
The agar in the most concentrated acid is first
of ice and water, one on the bench at
to lose its colour and last to lose its colour
laboratory temperature and one in the water
in the least concentrated. There is a steeper
bath, leave for 10 minutes.
concentration gradient of H+ ions, the more
Bore three cylinders of agar each with the same concentrated the acid, hence there is also a
cork borer (or, cut three identical cubes of agar, faster rate of diffusion of the H+ ions.
as described previously).
4 The effect of distance
Transfer one of the cylinders/cubes to each of
the three beakers and note the time. This can be easily investigated by using two glass
Keep a constant watch on the cylinders/cubes containers of significantly different volume. Each
and record the time at which each one loses its is filled with water, then placed side by side and
pink colouration. left for the water to become still. Two of three
drops of a concentrated solution of potassium
Results and explanation:
permanganate are added, at the same time,
The cylinder/cube in the water bath loses its
to both containers, and the time is noted. The
colour first and the one in the ice and water is
containers are then observed until the water in
last to lose its colour. Thus diffusion occurs faster
each of them is uniformly purple in colour and
the higher the temperature.
the time for each is recorded.
3 The effect of concentration gradient Results:
Investigation 2 can be adapted to demonstrate It takes longer for the water in the larger
this effect. container to become uniformly coloured.
Materials: As in 1 and 2 previously, but a Explanation: It is not just that the potassium
supply of 2 mol / dm3 hydrochloric acid is permanganate has further to travel, but also
required, which, by dilution, should be used that its concentration gradient is progressively
also to provide a supply of 1 mol / dm3 and declining as it spreads out in the container.
0.2 mol / dm3 hydrochloric acid. (Although these The effect of distance can also be demonstrated
concentrations are suggested, the investigation by spraying aerosol deodorant, or fly-killer at
should work well enough with three other one side of a classroom, and asking students to
differing concentrations.) note the time taken before they can smell it.
Apparatus: As in Investigation 2, but no The further away they are, then, perhaps not
container of ice and water or a surprisingly, the longer it takes for the chemical
water bath are required. to reach them (by diffusion through the air).

27
 3.03 Osmosis All cell membranes are partially permeable, therefore
Chapter 3 Movement in and out of cells

water will move into or out of cells by osmosis

If we were to take a container, completely divided into depending on the concentration gradient of water
two sections by a piece of cloth, and then, at the same molecules inside and outside the cell.
time pour a dilute sugar solution into one side and a Dilute solutions, which have a relatively large number
concentrated sugar solution into the other side, within of water molecules, are said to have a high water
several minutes, by diffusion, both the water molecules potential (i.e. a high concentration of water molecules).
and the sugar molecules would move down their Concentrated solutions, with fewer water molecules,
respective concentration gradients, until both sides were are said to have a low water potential.
at the same concentration (Figure 3.1). The pores in the
cloth would form no obstruction to the movement of During osmosis, since water molecules are in a
the molecules in either direction. constant state of motion, they will be moving through
the partially permeable membrane in both directions,
dilute concentrated but always more will be moving from where they
sugar sugar
solution solution
are in greater concentration towards the lower
cloth concentration.
Osmosis can then be defined as the net movement
of water molecules from a region of higher water
potential (a dilute solution) to a region of lower water
potential (a concentrated solution), through a partially
permeable membrane.
uniform distribution The cell sap of root hair cells has relatively low
net water molecules of water and sugar
movement molecules as a water potential. Soil water has a relatively high water
sugar molecules
of result of diffusion potential. Thus water will move into the vacuole
of root hair cells by osmosis (Figure 3.3). The cell
Figure 3.1  Sugar molecules moving one way, water
wall offers no obstruction to the passage of water,
molecules moving in the opposite direction until both
since cellulose, of which it is made, is completely
sides were at the same concentration permeable. But, where the walls of neighbouring
cells touch, water can pass into the root by simple
If we now carry out a similar experiment but this time,
diffusion – through the cellulose of the cell walls.
instead of the piece of cloth, we separate the two sides of
the container with a membrane with microscopic holes water following cell wall
in it (Figure 3.2) − so small that they allow the passage pathway (by simple diffusion)
of water molecules but not the sugar molecules, then to xylem
water molecules will diffuse down their concentration of root root hair
gradient while the sugar molecules stay where they are. cell
This specialised case of diffusion is called osmosis and the
separating membrane is described as partially permeable.
water following vacuolar
cortex of epidermis pathway (by osmosis)
root
sugar molecule

water molecule Figure 3.3  Uptake of water by osmosis (vacuolar

pathway) and by diffusion (cell wall pathway)

Once water molecules have entered the root hair cell,

partially permeable their presence increases the water potential of that cell,
membrane
compared with the next cell in towards the centre of
water but not sugar molecules the root. Osmosis will thus cause water to move into
pass through the membrane that cell from the root hair cell. This process continues
Figure 3.2  Partially permeable membrane allowing
water but not sugar molecules to pass through

28
until water molecules reach the xylem vessels in the water enters vacuole by osmosis

Chapter 3 Movement in and out of cells

centre of the root and are then transported away to
the stem. This can be demonstrated in the laboratory vacuole increases in
volume and pressure
using a piece of Visking tube that has been tied tightly at
its lower end, that contains sucrose solution, and is tied pushes against
tightly at its upper end to a piece of glass tubing. The cytoplasm
apparatus is then supported in a beaker of water as cytoplasm pushes
shown in Figure 3.4. against cell wall —
stretching the cell and
making it firm (turgid)

Figure 3.5  Plant cell in water

glass sucrose solution rises in
tube the glass tube as water The increase in pressure resulting from osmosis can be
passes from the beaker demonstrated as shown in Figure 3.5 using a tightly-tied
through the partially
permeable membrane
bag made of Visking tube, filled with sugar solution and
by osmosis placed in water for 20 minutes (Figure 3.6).
‘sausage’ becomes ‘sausage’ becomes
turgid and swells flaccid and shrinks
tightly tied
water sugar sugar
Visking tubing (sucrose) (sucrose)
sucrose (partially permeable solution solution
solution membrane) tightly tied
at the bottom and water water
around the glass tube partially
permeable
membrane
Figure 3.4  A demonstration of osmosis using an water enters (‘Visking’ tube) water leaves
artificial partially permeable membrane by osmosis by osmosis

Figure 3.6  A firm, turgid, Visking sausage and one that

has become flaccid
Progress check 3.2 The pressure caused by the water pushing outwards on
1 What part of a cell is partially permeable? the cell wall (turgor) in plant cells helps:
2 What is the difference between the 1 to keep stems upright
diffusion and osmosis?
2 to keep leaves flat so they can better absorb sunlight.
The water potential inside most animal cells is often the
same as the solution in which the cells are naturally bathed.
The effect of osmosis on plant cells (See Chapter 12, Kidney Function.) Thus there is little
movement of water by osmosis into or out of the cell.
The intake of water by osmosis
However, a red blood cell placed in a solution with a
Water will always tend to enter plant (root) cells by
relatively high water potential, starts to take in water
osmosis, since soil water will always be likely to have a
by osmosis, and since there is no cell wall to resist the
higher water potential than cell sap (Figure 3.5). As the
increased pressure that results, the cell bursts.
vacuole thus increases in volume, it increasingly presses
the cytoplasmic lining of the cell against the flexible, The loss of water by osmosis
box-like cell wall.
Plant cells placed in a solution of relatively low water
(This pressure is called turgor pressure and it gives potential, lose water from their vacuoles. As a result they
plant cells a firmness called turgidity.) lose their internal pressure since the cytoplasm is no
longer being forced against the inelastic cell wall. They
become soft (Figure 3.7).
When a cell loses its internal pressure, it is said to be
flaccid.

29
 vacuole decreases in water leaves vacuole Animal cells placed in solutions of lower water
by osmosis
Chapter 3 Movement in and out of cells

volume and pressure potential, lose their shape and what turgidity they have,
as water moves out of their cytoplasm. A red blood cell
shrinks in size and its cell membrane becomes unevenly
creased (‘crenated’) as shown in Figure 3.9.
Animal cells placed in a solution with a high water
potential (e.g. pure water) take in water by osmosis.
They have no inelastic cell wall to resist the intake and
to make them turgid, so they burst.
Pressure is no longer exerted on cell walls the cell
becomes flaccid and decreases in length and width. normal red blood cell
red blood cell in
Figure 3.7 A flaccid plant cell concentrated sugar solution
Further exposure to a bathing solution of lower water
potential (such as a sugar solution) will draw so much
water from the vacuole that the cytoplasm is pulled
away from the cell wall (Figure 3.8). (Such a condition is
called plasmolysis.)
‘crenated’ after water
partially permeable water cell shrinks in size
leaves the cell by osmosis
cell membrane pulled leaves and becomes soft
away from cell wall by osmosis (flaccid)
Figure 3.9 A normal and a crenated red blood cell

In animals, the cells of the body are usually bathed in

a fluid that is maintained at a constant water potential
(one of the jobs of the kidneys, in vertebrates). This
ensures that the concentration of chemicals in the cell
cytoplasm remains constant and, therefore, so too is the
rate of metabolism.

sugar solution diffuses

through permeable vacuole cytoplasm
cell wall to occupy the decreases pulled away
space between the wall in volume from cell wall
and the cell membrane

Figure 3.8 A (plasmolysed) plant cell after it has been left

in a sugar solution of low water potential for some time

PRACTICAL
Investigation of the effect on plant tissues of the Method:
entry and exit of water by osmosis Peel one or both potatoes to provide six similar
strips (‘chips’) at least 7 cm long, and no more than
Apparatus Two large potatoes 0.5 cm × 0.5 cm in cross-section.Then cut all the
and materials: Two containers (e.g. beakers) at strips to the same length and record that length.
least 10 cm deep
Fill one beaker to within 2 cm of the top with
A sharp knife
water, and the other with a concentrated
A supply of sucrose (table sugar)
sucrose or sodium chloride solution.
or table salt (sodium chloride)
A ruler measuring in mm Submerge three of the potato strips in each
A supply of water of the beakers. ➔
30
 Chapter 3 Movement in and out of cells
Leave the strips for at least half an hour (the than the water the strips were submerged in,
more dilute your sugar/salt solution, the longer thus water has entered the cells, stretching them
you should leave them), then remove them, and increasing the pressure inside them, making
measure them and record their lengths. Note the potato feel firm.
the texture of the strips.
For the strips in the sucrose or salt solution, the
Results: water potential of the cell sap is higher than the
The strips in the water will have increased in water potential of the solution, thus water has
length and will be firm to the touch; those in moved out of the cells. The cells have decreased
the sugar or salt solution will have decreased in size and lost their firmness. Thus the strips are
in length and be soft to the touch. shorter and are soft to the touch.
Explanation:
For the strips in water, the cell sap in the cells
of the potato will have a lower water potential

The effect of the gain or loss of Loss of water from cells makes stems soft and no
longer able to support the plant and leaves curl and are
water on a plant less able to photosynthesise.
When a plant has access to water and absorbs it by Place a fruit sweet between your teeth and your cheek
osmosis, its cells become firm as the water enters the and leave it there for about a quarter of an hour
cell vacuoles and presses outwards on the cell walls. without biting or sucking it. Then remove it and feel the
Like the potato strips, the tissues of the plant become inside of your cheek with your tongue. Write down a
firm and, when this happens in the stem and leaves, the description of how it feels and attempt an explanation.
plant is supported and held upright and the leaves are
firm and held open to the sunlight for photosynthesis.

Progress check 3.3 Which part of Figure 3.11 shows and explains
the results 20 minutes later?
1 Why, when a plant cell and an animal cell are
both placed in water, does only the animal cell A B C D

burst?
2 Figure 3.10 shows two liquids separated by a
partially permeable membrane at the start of
P Q P Q P Q P Q
an experiment. water moves water and water molecules sugar moves
from P to Q sugar move in move from Q to P from P to Q
partially permeable membrane by osmosis both directions by diffusion by osmosis

Figure 3.11

dilute 3 Why do the leaves and stems of a plant

concentrated sugar shoot, whose cells have started to undergo
sugar solution solution
plasmolysis, begin to droop?
P Q
Figure 3.10

31
 3.04 Active transport The situation also arises in the small intestine of
Chapter 3 Movement in and out of cells

an animal, when digested food (such as glucose) is

We have seen how chemicals move into and out absorbed by the epithelial cells of the villi by active
of cells by diffusion down a concentration gradient. transport.
Sometimes, a living cell may be in need of a chemical,
even though there is a lower concentration of it outside
than inside the cell. In such a case, the chemical would 3.05 How particles are moved
have to be absorbed against a concentration gradient. by active transport across a
This can be achieved only with the use of energy
released by respiration inside the cell in conjunction
cell membrane
with the cell membrane. This energy allows the cell to Particles, usually in molecular form, are taken up by
absorb the chemical through the cell membrane and cells against a concentration gradient. This is achieved
prevent any of the chemical already inside the cell from by proteins, called carrier proteins, that are bedded
leaving. in the cell membrane (Figure 3.12). They are the same
This process is called active transport and is defined as thickness as the membrane and thus they are in contact
the movement of particles through a cell membrane with the surrounding of the cell on the outside and the
from a region of lower concentration to a region of cytoplasm of the cell on the inside.
higher concentration using energy from respiration. Energy is used to open up a channel on the part of the
The energy released in respiration is stored in the molecule facing the outside of the cell. The molecule or
chemical ATP in the mitochondria. Cells that undergo ion to be absorbed enters the channel and binds to a
active transport characteristically have a large number special site in the centre of the carrier protein molecule.
of mitochondria. Each carrier protein will bind to only one particular
molecule. Thus, like enzymes, they are specific. Further
The energy used in this process not only affects the energy is then used to close off the opening to the
kinetic energy of the molecules that are being absorbed, outside of the cell, and to open up a similar channel, this
but it is used to move molecules in the cell membrane time into the cytoplasm of the cell. The molecule being
so they actively ‘pump’ the required molecules into absorbed is then released from its binding site into the
the cell. cell’s cytoplasm.
This situation arises in plant roots where the ions The carrier protein then reverses the process, again
needed for a plant’s metabolism may be in very short using energy so that it is available to take in another
supply in the soil water. Ions are thus absorbed by root molecule.
hair cells by active transport.

using energy, the

carrier protein molecule or ion carrier protein
ions or opens to the binds (attaches) changes shape
molecules to be outside of the to the functional and opens into
absorbed cell group the cell

cell
membrane

functional group ion or

of the carrier cytoplasm molecule
protein released
into the cell

Figure 3.12 Diagram of carrier protein at work

32
Worked example

Chapter 3 Movement in and out of cells

the energy is used to move the carrier proteins.
First the carrier protein opens to the outside and
Describe how the structure of a cell membrane is allows the molecule to be absorbed to attach (bind)
adapted to the process of active uptake. to the protein. Only one type of molecule will bind
Cell membranes contain protein molecules called as the site is not suitable for any other molecule.
‘channel proteins’. These molecules fit loosely The protein then changes shape again (again using
together leaving minute channels between them energy) – closing the outer opening, and opening into
extending from the outside of the cell to the cell the cytoplasm of the cell. The molecule is released
cytoplasm. Suggest how these channel proteins may into the cell cytoplasm, then changes shape again,
play a part is the processes of diffusion and osmosis. closing to the inside and opening to the outside
ready to bind with another molecule to be absorbed.
Answer (Note that the complete cycle of carrier protein
The question calls first for a realisation that there movements is described as a continuous process.)
is a special structural feature of the cell membrane Diffusion requires pores in the membrane before
making it able to undergo active uptake. That feature molecules can enter. The channel proteins would
is the presence of carrier proteins. Since they have provide the pores. However, the size of the pores
to work against a concentration gradient (there might prevent larger molecules from entering. Water is
may be a lower concentration of the chemical a small molecule and thus could enter by osmosis, and
to be absorbed outside than inside the cell) then the pores may be too small to allow larger molecules
energy must be used. All energy within a cell is to enter – making the membrane semi-permeable.
initially released by respiration and, in this case,

Chapter summary
You have learnt how cells are involved in the You have learnt how to demonstrate these
processes of diffusion, osmosis and active transport. processes experimentally.
You have learnt how these processes are You have also learnt about the factors that affect
important to living structures. them.

Exam-style questions
1 Describe how different substances in a leaf move Describe what will happen to the tubing and
by diffusion during a 24-hour period. [6] its contents over the next 20 minutes. [3]
2 a Figure 3.13 shows a piece of partially b After 20 minutes, apart from what happens
permeable tubing, tightly tied at each end, to the tubing, the water in the beaker has
and containing a concentrated sugar solution turned blue. With reference to diffusion
that is coloured with blue dye. It has been and osmosis, explain the results of this
placed in a beaker of pure water. experiment. [7]
3 a Explain how a plant root absorbs from the
concentrated sugar
solution coloured soil:
with blue dye i) water [6]
tightly pure water ii) essential mineral ions that are in very
tied partially short supply. [4]
permeable
tubing b Suggest why a plant may have great difficulty
in absorbing essential mineral ions that are in
Figure 3.13 very short supply in a water-logged soil. [4]

33
 The chemicals of life
Chapter 4

Learning outcomes
By the end of this chapter you should understand:
The structure of important chemicals The structure of the different chemicals that
found in organisms, including fats, proteins, organisms use in their nutrition
carbohydrates and DNA
How to carry out food tests to detect their
presence

4.01 Biological molecules Fats

Three of the most common organic molecules found in

TIP
living organisms are: Watch out for questions that refer to oils
and remember that oils are fats that are
• carbohydrates
liquid at 20°C.
• fats
• proteins. Fats are organic chemical substances that contain the
elements carbon, hydrogen and oxygen only. This time,
however, the ratio of H to O in the molecule is very
Carbohydrates much higher than 2:1. They are all insoluble in water and
are formed by the joining of a glycerol molecule with
Carbohydrates are organic chemicals containing the fatty acid molecules.
elements carbon, hydrogen and oxygen only. The
ratio of atoms of hydrogen to atoms of oxygen in a
carbohydrate molecule is always 2 : 1. Proteins
(It may help to remember this to know that ‘carbo’
refers to the carbon, ‘hydrate’ refers to water – where Proteins contain the elements carbon, hydrogen, oxygen
the ratio of H to O is also 2:1.) and nitrogen (and often other elements such as sulfur
and phosphorus).
Carbohydrates with large molecules, such as starch,
glycogen and cellulose, are insoluble. (Although it is They are large, usually insoluble, molecules that are built
common to refer to a ‘starch solution’, it is really a up from simple, soluble units known as amino acids, of
starch suspension.) which up to 20 are used in the production of protein in
living organisms.
They are synthesised in living organisms by linking
together molecules of simple sugar (glucose). Large )(

carbohydrate molecules can be broken down into Figure 4.1 Amino acids linking to form a protein
simple sugars. molecule (each different symbol represents one of the
Smaller carbohydrate molecules are soluble and 20 different amino acids)
occur as:
A relatively few amino acids link together to form a
• ‘complex’ sugars, such as maltose and sucrose (table polypeptide, while polypeptides link together to form
sugar), all with the formula C12H22O11 or protein.
• ‘simple’ sugars, such as glucose or fructose, with the Amino acids are of different sizes and shapes, thus,
formula C6H12O6. when linked together to form protein molecules,
the proteins formed will also be of different shapes.

34
This structural difference is very important when the Tests to show the presence of

Chapter 4 The chemicals of life

protein in question is an enzyme since part of the
molecule, known as the active site is of exactly the right carbohydrates, fat and protein
shape to fit the molecule (the substrate molecule) of
These are often referred to as ‘food tests’. The tests are
the chemical reaction that it will catalyse.
shown in Table 4.1 (carbohydrates), Table 4.2 (fat) and
Similarly, if the protein is an antibody in the blood, part Table 4.3 (protein).
of its molecule (the binding site) will be exactly the
right shape to stick to (‘bind with’) the pathogen or
toxin that the antibody has been made to counteract. Carbohydrates
Benedict’s solution does not distinguish between
Progress check 4.1 the complex sugar such as maltose, and the simple
sugar such as glucose. In their reaction with Benedict’s
1 Name the elements found in i) carbohydrates
solution, both sugars work as chemicals known as
ii) fats iii) proteins.
reducing agents, and thus belong to a group of sugars
2 Name the units from which i) fats, ii) starch referred to as reducing sugars all of which react in
and iii) proteins are made. this way.

Carbohydrate Chemical How test is carried out Result

reagent used
Starch iodine solution put a few drops on the substance to be blue-black colour if starch is present
tested
brown if starch is absent
Maltose and glucose Benedict’s solution add a few drops to a solution of the red/orange/yellow/green if either of the
substance to be tested and heat in a sugars is present
water bath at 90 °C
blue if neither of the sugars is present
Table 4.1  Tests for carbohydrates

Worked example
a Explain the chemical similarities and The differences are that one, glucose, is a
differences between glucose and sucrose. simple sugar and is the basic unit of many larger
b Explain how you could distinguish between carbohydrates (such as starch). Sucrose is a
them by performing a simple food test. complex sugar and one molecule of sucrose
(C12H22O11) is approximately twice as big as one
Answer molecule of glucose (C6H12O6).
a It is best to split this part into its two natural b This part hinges on the fact that the simple test
sections – similarities and differences. The first for sugars identifies only simple (or ‘reducing’)
similarity is that they are both sugars and they sugars. Glucose is a reducing sugar but sucrose is
are both soluble so that they can be more easily neither a reducing nor a simple sugar. Therefore,
moved from place to place within an organism. the Benedict’s test will identify the glucose but
The fact that they are sugars is indicated by not the sucrose. You are asked how you would
the ending ‘-ose’. They will therefore both also distinguish between them, so you will need to
be carbohydrates – a term you should explain. give a description of the test and state the result
The clue is, again, in the word: ‘carbo-’ because you would expect. The test is to heat a little
they contain carbon and ‘hydrate’ because they of each sugar in test-tube together with a few
contain the same elements as water – hydrogen drops of Benedict’s solution. The glucose will turn
and oxygen, and in the same ratio as well – 2 red, but the sucrose will remain the colour of the
Hs to each O. Some carbohydrates can be Benedict‘s solution – blue. (NB If you mention a
very large molecules, but these sugars are bunsen burner for heating, you would be advised
comparatively small ones. also to mention the use of goggles.)

35
 Fat – the ethanol emulsion test important constituent of foods is vitamin C and there
Chapter 4 The chemicals of life

is a test that is used to decide whether a food contains

vitamin C. The reagent used is known as DCPIP which
Reagent How test is Result
is in the form of a blue solution.
used carried out
Ethanol a dried sample of water turns cloudy if
the substance to be fat is present Reagent How test is Result
tested is mixed with carried out
water remains clear if
ethanol, which is then DCPIP add solution of DCPIP changes from
fat is not present
poured into a test- substance drop by blue to colourless if
tube of water drop to DCPIP vitamin C is present

Table 4.2 Tests for fat Table 4.4 Test for vitamin C

When performing ethanol emulsion test,

TIP

the test is more reliable if a dried sample Progress check 4.2

is used, but if it is not possible to dry the 1 Name the chemicals used to test for:
sample, then the ethanol drained from i) starch; ii) vitamin C.
it may already be cloudy, indicating the
presence of a fat. 2 A food test gives a purple colour. What
reagent has been used and what chemical is
present?
Proteins − the biuret test 3 When using Benedict’s solution, why is it not
possible to be absolutely sure that a food
contains glucose?
Reagent How test is Result
used carried out
biuret add equal volumes purple colour if
solutions of solution 1 and protein is present DNA
1 and 2 2 to a solution of blue colour if protein
the substance to be is absent Chromosomes, situated in the nuclei of all living cells
tested
(except bacteria, which have no true nucleus) have, as
Table 4.3 Test for protein their major component, the chemical substance DNA.The
DNA molecule, looking rather like a very long, twisted
Biuret 1 contains sodium hydroxide that is harmful to rope ladder, is made up of two strands (of alternating
the skin and thus appropriate safety precautions should sugar and phosphate units) held together by pairs of
be taken. chemical units called bases (these bases form the ‘rungs’
of the ladder).The molecule is described as a double
Since ‘biuret’ is a chemical – not a person as in helix (because the Greek word helix means a ‘spiral’.)
‘Benedict’s solution’ – it does not begin with a capital
letter, unless it is the first word in a sentence. There are only four bases, known by their initial letters
A, T, C and G, and the sequence in which they occur
in one of the two DNA strands is responsible for the
TIP

Never say that a food test is ‘positive’ or sequence of amino acids in the protein molecules that
‘negative’. State the observed colour and are made in the cell. Since the sequence of bases on a
the conclusion you draw from that result. DNA molecule is likely to be different for each (sexually
produced) individual, it follows that no two individuals
will make protein molecules with exactly the same
sequence of amino acids.
The test for vitamin C
As stated previously, the bases are always paired when
As indicated, these tests are usually referred to as ‘food forming the ‘rungs’ of the DNA double helix, but there
tests’ because they are mostly carried out to investigate is a ‘rule of base pairing’, since A always pairs with T,
the presence of chemicals in various foods. Another and C always with G.

36