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Cambridge International AS and A Leve

BiologyCoursebook

Fourth Edition

Original material Cambridge University Press 2014



Mary Jones, Richard Fosbery,

Jennifer Gregory and Dennis Taylor

Cambridge International AS and A Level

BiologyCoursebook

Fourth Edition




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5 The mitotic cell cycle 93

Chromosomes 94

Mitosis 97

The significance of telomeres 102

Stem cells 103

Cancer 103

End-of-chapter questions 107

6 Nucleic acids and protein synthesis 110

The structure of DNA and RNA 111

DNA replication 113

Genes and mutations 118

DNA, RNA and protein synthesis 109


7 Transport in plants 126

The transport needs of plants 127

Two systems: xylem and phloem 128

Structure of stems, roots and leaves 128

The transport of water 134

Transport of mineral ions 146

Translocation 146

Differences between sieve tubes and

xylem vessels 151


8 Transport in mammals 157

Transport systems in animals 158

The mammalian cardiovascular system 158

Blood vessels 160

Blood plasma and tissue fluid 164

Lymph 164

Blood 166

Haemoglobin 168

Problems with oxygen transport 171

The heart 173

The cardiac cycle 175Control of the heart beat 177


How to use this book vi

Introduction viii

1 Cell structure 1

Why cells? 3

Cell biology and microscopy 3

Animal and plant cells have features in common 5

Differences between animal and plant cells 5

Units of measurement in cell studies 6

Electron microscopy 6

Ultrastructure of an animal cell 13

Ultrastructure of a plant cell 18

Two fundamentally different types of cell 21End-of-chapter questions 23

2 Biological molecules 27

The building blocks of life 28

Monomers, polymers and macromolecules 29

Carbohydrates 29

Lipids 36

Proteins 39

Water 46


3 Enzymes 53Mode of action of enzymes 54

Factors that affect enzme action 57

Enzyme inhibitors 61

Comparing the affinity of different enzymes for

their substrates 62

Immobilising enzymes 64


4 Cell membranes and transport 72

Phospholipids 73

Structure of membranes 74

Cell signalling 77

Movement of substances into and out of cells 79


Contents




v

9 Gas exchange and smoking 185

Gas exchange 186

Lungs 186

Trachea, bronchi and bronchioles 187

Alveoli 189

Smoking 190Tobacco smoke 190

Lung diseases 190

Short-term effects on the cardiovascular system 193


10 Infectious diseases 198

Worldwide importance of infectious diseases 200

Cholera 200

Malaria 202

Acquired immune deficiency syndrome (AIDS) 205

Tuberculosis (TB) 209

Measles 212Antibiotics 213

End of chaper questions 219

11 Immunity 222

Defence against disease 223

Cells of the immune system 224

Active and passive immunity 232

Autoimmune diseases a case of

mistaken identity 237

End of chaper questions 242

P1 Practical skills for AS 246Experiments 247

Variables and making measurements 247

Estimating uncertainty in measurement 255

Recording quantitative results 255

Constructing a line graph 255

Constructing bar charts and histograms 258

Making conclusions 259

Describing data 259

Making calculations from data 259

Explaining your results 261

Identifying sources of error and suggesting

improvements 261Drawings 262

End of chapter questions 264

12 Energy and respiration 267

The need for energy in living organisms 268

Work 269

ATP 270

Respiration 272

Mitochondrial structure and function 276Respiration without oxygen 277

Respiratory substrates 278

Adaptations of rice for wet environments 281


13 Photosynthesis 286

An energy transfer process 287

The light dependent reactions of photosynthesis 288

The light independent reactions of

photosynthesis 290

Chloroplast structure and function 290

Factors necessar y for photosynthesis 291C4 plants 293

Trapping light energy 295


14 Homeostasis 299

Internal environment 300

Control of homeostatic mechanisms 301

The control of body temperature 302

Excretion 304

The structure of the kidney 305

Control of water content 312

The control of blood glucose 315Urine analysis 319

Homeostasis in plants 320


15 Coordination 329

Nervous communication 330

Muscle contraction 344

Hormonal communication 349

Birth control 351

Control and coordination in plants 353


Contents




Contents

16 Inherited change 364

Homologous chromosomes 365

Two types of nuclear division 367

Genetics 374

Genotype affects phenotype 374

Inheriting genes 375Multiple alleles 378

Sex inheritance 378

Sex linkage 379

Dihybrid crosses 380

Interactions between loci 382

Autosomal linkage 383

Crossing over 384

The 2(chi-squared) test 386

Mutations 387

Gene control in prokaryotes 389

Gene control in eukaryotes 391


17 Selection and evolution 397

Variation 398

Natural selection 402

Evolution 404

Artificial selection 409

The DarwinWallace theory of evolution by

natural selection 412

Species and speciation 413

Molecular comparisons between species 416

Extinctions 417


18 Biodiversity, classification and conservation 423

Ecosystems 425

Biodiversity 426

Simpsons Index of Diversity 430

Systematic sampling 431

Correlation 433

Classification 435

Viruses 440

Threats to biodiversity 441

Why does biodiversity matter? 444Protecting endangered species 445

Controlling alien species 451

International conservation organisations 452

Restoring degraded habitats 453


19 Genetic technology 462

Genetic engineering 463

Tools for the gene technologist 464

Genetic technology and medicine 475

Gene therapy 477

Genetic technology and agriculture 480End-of-chapter questions 487

P2 Planning, analysis and evaluation 490

Planning an investigation 491

Constructing a hypothesis 491

Using the right apparatus 491

Identifying variables 492

Describing the sequence of steps 495

Risk assessment 495

Recording and displaying results 495

Analysis, conclusions and evaluation 495

Pearsons linear correlation 501Spearmans rank correlation 503

Evaluating evidence 504

Conclusions and discussion 506


Appendix 1: Amino acid R groups 512

Appendix 2: DNA and RNA triplet codes 513

Glossary 514

Index 529




i

How to use this bookEach chapter begins with a short

list o the acts and concepts thatare explained in it.

There is a short

context at thebeginning o each

chapter, containingan example o howthe material covered

in the chapter relatesto the 'real world'.

This book does not contain detailedinstructions or doing particularexperiments, but you will ind

background inormation aboutthe practical work you need to do

in these Boxes. There are also twochapters, P1 and P2, which providedetailed inormation about the

practical skills you need to developduring your course.

The text and illustrations describe and

explain all o the acts and conceptsthat you need to know. The chapters,

and ofen the content within themas well, are arranged in the samesequence as in your syllabus.

Important equations and

other acts are shown inhighlight boxes.

Questions throughout the text give you a chance to check that you have understoodthe topic you have just read about. You can ind the answers to these questions on

the CD-ROM.




How to use this book

Wherever you need to know how to use a ormula to carry out a calculation,there are worked example boxes to show you how to do this.

Key words are highlighted in the textwhen they are irst introduced.

You will also ind deinitions o

these words in the Glossary.

Deinitions that are required by the

syllabus are shown in highlight boxes.

There is a summary o keypoints at the end o eachchapter. You might ind

this helpul when you arerevising.

Questions at the end o each chapter begin with a ew multiple choice questions, then move onto questions that will help you to organise and practice what you have learnt in that chapter.

Finally, there are several more demanding exam-style questions, some o which may require use oknowledge rom previous chapters. Answers to these questions can be ound on the CDROM.




ii

Introduction

Tis ourth edition o Cambridge International AS and

A Level Biologyprovides everything that you need to

do well in your Cambridge International Examinations

AS and A level Biology (9700) courses. It provides ull

coverage o the syllabus or examinations rom 2016

onwards.

Te chapters are arranged in the same sequence as the

material in your syllabus. Chapters 1 to P1 cover the AS

material, and Chapters 12 to P2 cover the extra material

you need or the ull A level examinations. Te various

eatures that you will find in these chapters are explained

on the next two pages.In your examinations, you will be asked many

questions that test deep understanding o the acts and

concepts that you will learn during your course. Its

thereore not enough just to learn words and diagrams that

you can repeat in the examination; you need to ensure that

you really understand each concept ully. rying to answer

the questions that you will find within each chapter, and

at the end, should help you to do this. Tere are answers

to all o these questions on the CD-ROM that comes with

this book.

Although you will study your biology as a series odifferent topics, its very important to appreciate that all o

these topics link up with each other. Some o the questions

in your examination will test your ability to make links

between different areas o the syllabus. For example, in

the AS examination you might be asked a question that

involves bringing together knowledge about protein

synthesis, inectious disease and transport in mammals.

In particular, you will find that certain key concepts come

up again and again. Tese include:

cells as units o lie

biochemical processes

DNA, the molecule o heredity

natural selection

organisms in their environment

observation and experiment

As you work through your course, make sure that you

keep on thinking about the work that you did earlier, and

how it relates to the current topic that you are studying.

On the CD-ROM, you will also find some suggestions

or other sources o particularly interesting or useul

inormation about the material covered in each chapter.

Do try to track down and read some o these.

Practical skills are an important part o your biology

course. You will develop these skills as you do experiments

and other practical work related to the topic you are

studying. Chapters P1 (or AS) and P2 (or A level) explain

what these skills are, and what you need to be able to do to

succeed in the examination papers that test these skills.



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Chapter 1: Cell structure

Chapter 1:

Cell structureLearning outcomes

You should be able to:

describe and compare the structure of animal,

plant and bacterial cells, and discuss the non-

cellular nature of viruses

describe the use of light microscopes and

electron microscopes to study cells

draw and measure cell structures

discuss the variety of cell structures and their

functions

describe the organisation of cells into tissues

and organs

outline the role of ATP in cells




AS Level Biology

Progress in science ofen depends on people thinking

outside the box original thinkers who are ofen

ignored or even ridiculed when they irst put orward

their radical new ideas. One such individual, who

battled constantly throughout her career to get herideas accepted, was the American biologist Lynn

Margulis (born 1938, died 2011: Figure 1.1). Her

greatest achievement was to use evidence rom

microbiology to help irmly establish an idea that had

been around since the mid-19th century that new

organisms can be created rom combinations

o existing organisms which are not necessarily

closely related. The organisms orm a symbiotic

partnership, typically by one enguling the other,

a process known as endosymbiosis. Dramatic

evolutionary changes result.

The classic examples, now conirmed by laterwork, were the suggestions that mitochondria and

chloroplasts were originally ree-living bacteria

(prokaryotes) which invaded the ancestors o modern

eukaryotic cells (cells with nuclei). Margulis saw

such symbiotic unions as a major driving cause o

evolutionary change. She continued to challenge the

Darwinian view that evolution occurs mainly as a

result o competition between species.

In the early days o microscopy an English scientist,

Robert Hooke, decided to examine thin slices o plant

material. He chose cork as one o his examples. Lookingdown the microscope, he was struck by the regular

appearance o the structure, and in 1665 he wrote a book

containing the diagram shown in Figure 1.1.

I you examine the diagram you will see the pore-

like regular structures that Hooke called cells. Each cell

appeared to be an empty box surrounded by a wall. Hooke

had discovered and described, without realising it, the

undamental unit o al l living things.

Although we now know that the cells o cork are dead,

urther observations o cells in living materials were

made by Hooke and other scientists. However, it was

not until almost 200 years later that a general cell theoryemerged rom the work o two German scientists. In 1838

Schleiden, a botanist, suggested that all plants are made

o cells, and a year later Schwann, a zoologist, suggested

the same or animals. Te cell theorystates that the basic

unit o structure and unction o all living organisms is the

cell. Now, over 170 years later, this idea is one o the most

amiliar and important theories in biology. o it has been

added Virchows theory o 1855 that all cells arise rom

pre-existing cells by cell division.

Figure 1.2 Drawing o cork cells published by Robert Hooke

in 1665.

Figure 1.1 Lynn Margulis: My work more than didnt it in.

It crossed the boundaries that people had spent their lives

building up. It hits some 30 sub-ields o biology,

even geology.

Thinking outside the box





Why cells?A cell can be thought o as a bag in which the chemistry

o lie is allowed to occur, partially separated rom the

environment outside the cell. Te thin membrane which

surrounds all cells is essential in controlling exchange

between the cell and its environment. It is a very effective

barrier, but also allows a controlled traffi c o materials

across it in both directions. Te membrane is thereore

described as partially permeable. I it were freely

permeable, lie could not exist, because the chemicals o

the cell would simply mix with the surrounding chemicals

by diffusion, (page 73).

Cell biology and microscopyTe study o cells has given rise to an important branch o

biology known as cell biology. Cells can now be studied

by many different methods, but scientists began simplyby looking at them, using various types o microscope.

Tere are two undamentally different types o

microscope now in use: the light microscope and the

electron microscope. Both use a orm o radiation in order

to create an image o the specimen being examined. Te

light microscope uses light as a source o radiation, while

the electron microscope uses electrons, or reasons which

are discussed later.

Light microscopyTe golden age o light microscopy could be said to be

the 19th century. Microscopes had been available since

the beginning o the 17th century but, when dramatic

improvements were made in the quality o glass lenses in

the early 19th century, interest among scientists became

widespread. Te ascination o the microscopic world

that opened up in biology inspired rapid progress both in

microscope design and, equally importantly, in preparing

material or examination with microscopes. Tis branch

o biology is known as cytology. Figure 1.3 shows how the

light microscope works.

By 1900, all the structures shown in Figures 1.4, 1.5 and

1.6, had been discovered. Figure 1.4 shows the structure oa generalised animal cell and Figure 1.6 the structure o a

generalised plant cell as seen with a light microscope.

(A generalised cell shows allthe structures that are

typically ound in a cell.) Figure 1.5 shows some actual

human cells and Figure 1.6 shows an actual plant cell

taken rom a lea. Figure 1.4 Structure o a generalised animal cell (diameterabout 20m) as seen with a very high quality light microscope.

Golgi body

cytoplasm

mitochondria

small structures that

are difficult to identify

nuclear envelope

chromatin

deeply staining

and thread-likenucleus

nucleolus

deeply staining

cell surface

membrane

centriole always

found near nucleus,

has a role in nuclear

division

Figure 1.3 How the light microscope works.

eyepiece

light beam

objective

glass slide

condenser

iris diaphragm

light sourceCondenser iris

diaphragmis closed

slightly to produce a

narrow beam of light.

Condenser lensfocuses

the light onto the

specimen held between

the cover slip and slide.

Objective lenscollects

light passing through the

specimen and produces a

magnified image.

Eyepiece lensmagnifies

and focuses the image

from the objective onto

the eye.

pathway of light

cover slip




AS Level Biology

QUESTION

1.1 Using Figures 1.4 and 1.6, name the structures

that animal and plant cells have in common, those

ound in only plant cells, and those ound only in

animal cells.

Figure 1.5 Cells rom the lining o the human cheek (500),

each showing a centrally placed nucleus which is a typical

animal cell characteristic. The cells are part o a tissue known

as squamous (lattened) epithelium.

Figure 1.6 Structure o a generalised plant cell (diameter about 40m) as seen with a very high quality light microscope.

Golgi apparatus

cytoplasm

chromatin deeply staining

and thread-like

nucleussmall structures that

are difficult to identify

nucleolus

deeply staining

nuclear envelope

mitochondriachloroplast

grana just visible

tonoplast membrane

surrounding vacuole

vacuole large

with central position

plasmodesma

connects cytoplasm

of neighbouring cells

cell wall

cell wall of

neighbouring

cell

cell surface membrane

(pressed against cell wall)

middle lamella thin layer

holding cells together,

contains calcium pectate

Figure 1.7 Photomicrograph o a cell in a moss lea (1200).





Animal and plant cellshave features in commonIn animals and plants each cell is surrounded by a very

thin cell surface membrane. Tis is also sometimes

reerred to as the plasma membrane.Many o the cell contents are colourless and

transparent so they need to be stained to be seen. Each

cell has a nucleus, which is a relatively large structure

that stains intensely and is thereore very conspicuous.

Te deeply staining material in the nucleus is called

chromatinand is a mass o loosely coiled threads.

Tis material collects together to orm visible separate

chromosomes during nuclear division (page 86). It

contains DNA(deoxyribonucleic acid), a molecule which

contains the instructions that control the activities o the

cell (see Chapter 6). Within the nucleus an even more

deeply staining area is visible, the nucleolus, which ismade o loops o DNA rom several chromosomes. Te

number o nucleoli is variable, one to five being common

in mammals.

Te material between the nucleus and the cell surace

membrane is known as cytoplasm. Cytoplasm is an

aqueous (watery) material, varying rom a fluid to a

jelly-like consistency. Many small structures can be seen

within it. Tese have been likened to small organs and

hence are known as organelles. An organelle can be

defined as a unctionally and structurally distinct part

o a cell. Organelles themselves are ofen surrounded

by membranes so that their activities can be separated

rom the surrounding cytoplasm. Tis is described as

compartmentalisation. Having separate compartments

is essential or a structure as complex as an animal or

plant cell to work efficiently. Since each type o organelle

has its own unction, the cell is said to show division of

labour, a sharing o the work between different

specialised organelles.

Te most numerous organelles seen with the light

microscope are usually mitochondria(singular:

mitochondrion). Mitochondria are only just visible,

but films o living cells, taken with the aid o a lightmicroscope, have shown that they can move about,

change shape and divide. Tey are specialised to carry

out aerobic respiration.

Te use o special stains containing silver enabled the

Golgi apparatusto be detected or the first time in 1898 by

Camillo Golgi. Te Golgi apparatus is part o a complex

internal sorting and distribution system within the cell

(page 16). It is also sometimes called the Golgi bodyor

Golgi complex.

Differences between animaland plant cellsTe only structure commonly ound in animal cells which

is absent rom plant cells is the centriole. Plant cells also

differ rom animal cells in possessing cell walls, largepermanent vacuoles and chloroplasts.

CentriolesUnder the light microscope the centrioleappears as a small

structure close to the nucleus (Figure 1.5, page 4). Centrioles

are discussed on page 18.

Cell walls and plasmodesmataWith a light microscope, individual plant cells are more

easily seen than animal cells, because they are usually

larger and, unlike animal cells, surrounded by acell wall

outside the cell surace membrane. Tis is relatively rigidbecause it contains fibres o cellulose, a polysaccharide

which strengthens the wall. Te cell wall gives the cell a

definite shape. It prevents the cell rom bursting when

water enters by osmosis, allowing large pressures to

develop inside the cell (page 77). Cell walls may also be

reinorced with extra cellulose or with a hard material

called lignin or extra strength (page 00 in Chapter 7).

Cell walls are reely permeable, allowing ree movement o

molecules and ions through to the cell surace membrane.

Plant cells are linked to neighbouring cells by means o

fine strands o cytoplasm called plasmodesmata(singular:

plasmodesma), which pass through pore-like structures in

their walls. Movement through the pores is thought to be

controlled by the structure o the pores.

VacuolesAlthough animal cells may possess small vacuoles such

as phagocytic vacuoles (page 80), which are temporary

structures, mature plant cells ofen possess a large,

permanent, centralvacuole. Te plant vacuole is

surrounded by a membrane, the tonoplast, which controls

exchange between the vacuole and the cytoplasm. Te

fluid in the vacuole is a solution o pigments, enzymes,sugars and other organic compounds (including some

waste products), mineral salts, oxygen and carbon dioxide.

Vacuoles help to regulate the osmotic properties o cells

(the flow o water inwards and outwards) as well as having

a wide range o other unctions. For example, the pigments

which colour the petals o certain flowers and parts o

some vegetables, such as the red pigment o beetroots, may

be located in vacuoles.




AS Level Biology

ChloroplastsChloroplasts are ound in the green parts o the plant,

mainly in the leaves. Tey are relatively large organelles

and so are easily seen with a light microscope. It is even

possible to see tiny grains or grana(singular: granum)

inside the chloroplasts using a light microscope. Teseare the parts o the chloroplast that contain chlorophyll,

the green pigment which absorbs light during the process

o photosynthesis, the main unction o chloroplasts.

Chloroplasts are discussed urther on page 19.

Points to note You can think o a plant cell as being very similar to an

animal cell, but with extra structures.

Plant cells are ofen larger than animal cells, although

cell size varies enormously.

Do not conuse the cell wallwith the cell surace

membrane. Cell walls are relatively thick and

physically strong, whereas cell surace membranes are

very thin. Cell walls are reely permeable, whereas cell

surace membranes are partially permeable. Allcells

have a cell surace membrane.

Vacuoles are not confined to plant cells; animal cells

may have small vacuoles, such as phagocytic

vacuoles (page 80), although these are not usually

permanent structures.

We return to the differences between animal and plant

cells as seen using the electron microscopeon page 18.

Units of measurement incell studiesIn order to measure objects in the microscopic world, we

need to use very small units o measurement, which are

unamiliar to most people. According to international

agreement, the International System o Units (SI units)

should be used. In this system, the basic unit o length is

the metre(symbol, m). Additional units can be created

in multiples o a thousand times larger or smaller, using

standard prefixes. For example, the prefix kilo

means1000times. Tus 1 kilometre =1000 metres. Te units

o length relevant to cell studies are shown in able 1.1.

It is difficult to imagine how small these units are,

but, when looking down a microscope and seeing cells

clearly, we should not orget how amazingly small the

cells actually are. Te smallest structure visible with the

human eye is about 50100

m in diameter. Your bodycontains about 60 million million cells, varying in size

rom about 5m to 40m. ry to imagine structures like

mitochondria, which have an average diameter o 1 m.

Te smallest cell organelles we deal with in this book,

ribosomes, are only about 25 nm in diameter! You could

line up about 20 000 ribosomes across the ull stop at the

end o this sentence.

Electron microscopyAs we said on page 3, by 1900 almost all the structures

shown in Figures 1.5 and 1.6 (pages 2and 3) had been

discovered. Tere ollowed a time o rustration or

microscopists, because they realised that no matter how

much the design o light microscopes improved, there was

a limit to how much could ever be seen using light.

In order to understand why this is, it is necessary to

know something about the nature o light itsel and to

understand the difference between magnification

and resolution.

MagnificationMagnificationis the number o times larger an image is,

than the real size o the object.magnification =

observed size o the imageactual size

orM=

IA

HereI=observed size o the image (that is, what you

can measure with a ruler) andA=actual size (that is, the

real size or example, the size o a cell beore it

is magnified).

I you know two o these values, you can work out the

third one. For example, i the observed size o the image

and the magnification are known, you can work out the

Fraction of a metre Unit Symbol

one thousandth = 0.001 = 1/1000 = 103 millimetre mm

one millionth = 0.000 001 = 1/1000 000 = 106 micrometre m

one thousand millionth = 0.000 000 001 = 1/1000 000 000 = 109 nanometre nm

Table 1.1 Units o measurement relevant to cell studies: is the Greek letter mu; 1 micrometre is a thousandth o a millimetre;

1 nanometre is a thousandth o a micrometre.





actual size:A= IM

. I you write the ormula in a triangle

as shown on the right and cover up the value you want to

find, it should be obvious how to do the right calculation.

Some worked examples are now provided.

I

M A

Measuring cellsCells and organelles can be measured with a microscope

by means o an eyepiece graticule. This is a transparent

scale. It usually has 100 divisions (see Figure 1.8a). The

eyepiece graticule is placed in the microscope eyepiece

so that it can be seen at the same time as the object to

be measured, as shown in Figure 1.8b. Figure 1.8b shows

the scale over a human cheek epithelial cell. The cell

lies between 40 and 60 on the scale. We thereore say it

measures 20 eyepiece units in diameter (the difference

between 60 and 40). We will not know the actual size othe eyepiece units until the eyepiece graticule scale is

calibrated.

To calibrate the eyepiece graticule scale, a miniature

transparent ruler called a stage micrometerscale is

placed on the microscope stage and is brought into ocus.

This scale may be etched onto a glass slide or printed on

a transparent ilm. It commonly has subdivisions o 0.1

and 0.01 mm. The images o the two scales can then be

superimposed as shown in Figure 1.8c.

In the eyepiece graticule shown in the igure, 100 units

measure 0.25 mm. Hence, the value o each eyepiece

unit is:

0.25

= 0.0025 mm100

Or, converting mm to m:

0.251000

= 2.5m100

The diameter o the cell shown superimposed on the scale

in Figure 1.8b measures 20 eyepiece units and so its actual

diameter is:

202.5m = 50m

This diameter is greater than that o many human cells

because the cell is a lattened epithelial cell.

WORKED EXAMPLE 1

Figure 1.8 Microscopical measurement. Three ields o view

seen using a high-power (40) objective lens. aAn eyepiece

graticule scale. bSuperimposed images o human cheek

epithelial cells and the eyepiece graticule scale.

cSuperimposed images o the eyepiece graticule scale

and the stage micrometer scale.

0 10 20 30 40 50 60 70 80 90 100

0 10 20 30 40 50 60 70 80

0 0.1 0.2

90 100

cheek cells on a slideon the stage of the

microscope

0 10 20 30 40 50 60 70 80 90 100

eyepiecegraticule

scale (arbitrary

units)

eyepiece

graticule inthe eyepiece

of the

microscope

stage micrometer

scale (marked in

0.01mm and 0.1mm divisions)

a

b

c




AS Level Biology

WORKED EXAMPLE 2

Figure 1.9 Photographs o the same plant cells seenawith a light microscope,bwith an electron microscope,

both shown at a magniication o about 750.

a

b

Calculating the magnification of a photographor objectTo calculateM, the magniication o a photograph or an

object, we can use the ollowing method.Figure 1.9 shows two photographs o a section

through the same plant cells. The magniications o the two

photographs are the same. Suppose we want to know the

magniication o the plant cell labelled P in Figure1.9b. I we

know its actual (real) length we can calculate itsmagniication using the ormula

IM =

A.

The real length o the cell is 80m.

Step 1 Measure the length in mmo the cell in thephotograph using a ruler. You should ind that it is about

60 mm.

Step 2 Convert mm to m. (It is easier i we irst convertall measurements to the same units in this case

micrometres,m.)

1 mm =1000m

so 60 mm =60 1000m

=60 000m

Step 3 Use the equation to calculate the magniication.

=750m

image size, Imagniication, M =

actual size,A

60 000m= 80m

The multiplication sign in ront o the number 750 meanstimes. We say that the magniication is times 750.

P

QUESTION

1.2 a Calculate the magniication o the drawing o the

animal cell in Figure 1.4 on page 3.

b Calculate the actual (real) length o the

chloroplast labelled Xin Figure 1.29 on page 20.





WORKED EXAMPLE 3

Figure 1.10 A lymphocyte.

6 m

Calculating magnification from a scale barFigure 1.10shows a lymphocyte.

We can calculate the magniication o the lymphocyte by

simply using the scale bar. All you need to do is measure

the length o the scale bar and then substitute this and the

length it represents into the equation.

Step 1 Measure the scale bar. Here, it is 36 mm.

Step 2 Convert mm to m: 36 mm=361000m = 36 000m

Step 3 Use the equation to calculate the magniication:

= 6000

image size, Imagniication, M =

actual size,A

36 000m= 6m

WORKED EXAMPLE 4

Calculating the real size of an object from itsmagnificationTo calculateA, the real or actual size o an object, we can use

the ollowing method.

Figure 1.27 on page 19 shows parts o three plant cells

magniied 5600. One o the chloroplasts is labelled

chloroplast in the igure. Suppose we want to know

the actual length o this chloroplast.

Step 1 Measure the observed length o the image o thechloroplast (I), in mm, using a ruler. The maximum length is

40 mm.

Step 2 Convert mm to m: 40 mm=40 1000m =40 000m

Step 3 Use the equation to calculate the actual length:

Background informationBiological material may be examined live or in a preserved

state. Prepared slides contain material that has been killed

and preserved in a lie-like condition. This material is ofencut into thin sections to enable light to pass through the

structures or viewing with a light microscope. The sections

are typically stained and mounted on a glass slide, orming

a permanent preparation.

Temporary preparations o resh material have the

advantage that they can be made rapidly and are useul or

quick preliminary investigations. Sectioning and staining

may still be carried out i required. Sometimes macerated

(chopped up) material can be used, as when examining the

structure o wood (xylem). A number o temporary stains are

commonly used. For example, iodine in potassium iodide

solution is useul or plant specimens. It stains starch blue-

black and will also colour nuclei and cell walls a pale yellow.A dilute solution o methylene blue can be used to stain

animal cells such as cheek cells.

Viewing specimens yoursel with a microscope will help

you to understand and remember structures more ully.

This can be reinorced by making a pencil drawing on good

quality plain paper, using the guidance given later in

Chapter 7 (Box 7.1, page 131). Remember always to draw

what you see, and not what you think you should see.

ProcedureThe material is placed on a clean glass slide and one or two

drops o stain added. A cover slip is careully lowered over

the specimen to protect the microscope lens and to help

prevent the specimen rom drying out. A drop o glycerinemixed with the stain can also help prevent drying out.

Suitable animal material: human cheek cells

Suitable plant material: onion epidermal cells, lettuce

epidermal cells, Chlorellacells. moss leaves

BOX 1.1: Making temporary slides

=7.1m (to one decimal place)

image size, Iactual size,A=

magniication, M

40 000m=

5600




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AS Level Biology

ResolutionLook again at Figure 1.9 (page 8). Figure 1.9a is a light

micrograph(a photograph taken with a light microscope,

also known as a photomicrograph). Figure 1.9b is an

electron micrographo the same cells taken at the same

magnification (an electron micrograph is a picture takenwith an electron microscope). You can see that Figure 1.9b,

the electron micrograph, is much clearer. Tis is because

it has greater resolution. Resolutioncan be defined as the

ability to distinguish between two separate points. I the

two points cannot be resolved, they will be seen as one

point. In practice, resolution is the amount o detail that

can be seen the greater the resolution, the greater

the detail.

Te maximum resolution o a light microscope is

200 nm. Tis means that i two points or objects are

closer together than 200 nm they cannot be distinguished

as separate.It is possible to take a photograph such as Figure 1.9a

and to magniy (enlarge) it, but we see no more detail; in

other words, we do not improve resolution, even though

we ofen enlarge photographs because they are easier to

see when larger. With a microscope, magnification up to

the limit o resolution can reveal urther detail, but any

urther magnification increases blurring as well as the size

o the image.

Figure 1.11 Diagram o the electromagnetic spectrum (the waves are not drawn to scale). The numbers indicate the wavelengths

o the different types o electromagnetic radiation. Visible light is a orm o electromagnetic radiation. The arrow labelled uv is

ultraviolet light.

400 nm 500 nm 600 nm 700 nm

blueviolet yellow orange red

0.1 nm 10 nm 1000 nm 105nm 107nm 109nm 1011nm 1013nm

visible light

radio and TV waves

infrared

microwaves

gamma rays

green

X-rays

uv

The electromagnetic spectrumHow is resolution linked with the nature o light? One

o the properties o light is that it travels in waves. Te

length o the waves o visible light varies, ranging rom

about 400 nm (violet light) to about 700 nm (red light).

Te human eye can distinguish between these differentwavelengths, and in the brain the differences are converted

to colour differences. (Colour is an invention o the brain!)

Te whole range o different wavelengths is called the

electromagnetic spectrum. Visible light is only one part o

this spectrum. Figure 1.11 shows some o the parts o the

electromagnetic spectrum. Te longer the waves, the lower

their requency (all the waves travel at the same speed, so

imagine them passing a post: shorter waves pass at higher

requency). In theory, there is no limit to how short or how

long the waves can be. Wavelength changes with energy:

the greater the energy, the shorter the wavelength.

Now look at Figure 1.12, which shows a mitochondrion,some very small cell organelles called ribosomes (page 13)

and light o 400 nm wavelength, the shortest visible

wavelength. Te mitochondrion is large enough to

interere with the light waves. However, the ribosomes

are ar too small to have any effect on the light waves. Te

general rule is that the limit o resolution is about one hal

the wavelength o the radiation used to view the specimen.

In other words, i an object is any smaller than hal the

wavelength o the radiation used to view it, it cannot be

seen separately rom nearby objects. Tis means that the

best resolution that can be obtained using a microscopethat uses visible light (a light microscope) is 200 nm,

since the shortest wavelength o visible light is 400 nm

(violet light). In practice, this corresponds to a maximum

useul magnification o about 1500 times. Ribosomes are

approximately 25 nm in diameter and can thereore never

be seen using light.

Resolutionis the ability to distinguish between two

objects very close together; the higher the resolution o an

image, the greater the detail that can be seen.

Magniicationis the number o times greater that an image

is than the actual object;magniication = image size actual (real) size o the object.





Figure 1.12 A mitochondrion and some ribosomes in the patho light waves o 400 nm length.

stained ribosomes of diameter 25nm

do not interfere with light waves

stained mitochondrion

of diameter 1000 nm

interferes with light waves

wavelength

400nm

I an object is transparent, it will allow light waves to

pass through it and thereore will still not be visible. Tis

is why many biological structures have to be stained beore

they can be seen.

wavelength is extremely short (at least as short as that o

X-rays). Second, because they are negatively charged, they

can be ocused easily using electromagnets (a magnet can

be made to alter the path o the beam, the equivalent o a

glass lens bending light).

Using an electron microscope, a resolution o 0.5 nmcan be obtained, 400 times better than a light microscope.

Transmission and scanning electronmicroscopes

wo types o electron microscope are now in common use.

Te transmission electron microscope, or EM, was the

type originally developed. Here the beam o electrons is

passed throughthe specimen beore being viewed. Only

those electrons that are transmitted

(pass through the

specimen) are seen. Tis allows us to see thin sections o

specimens, and thus to see inside cells. In the scanning

electron microscope(SEM), on the other hand, theelectron beam is used to scan the surfaceso structures,

and only the reflected

beam is observed.

An example o a scanning electron micrograph is

shown in Figure 1.13. Te advantage o this microscope is

that surace structures can be seen. Also, great depth o

field is obtained so that much o the specimen is in ocus

at the same time and a three-dimensional appearance

is achieved. Such a picture would be impossible to

obtain with a light microscope, even using the same

magnification and resolution, because you would have to

keep ocusing up and down with the objective lens to see

different parts o the specimen. Te disadvantage o theSEM is that it cannot achieve the same resolution as

a EM. Using an SEM, resolution is between 3 nm

and 20 nm.

QUESTION

1.3 Explain why ribosomes are not visible using a light

microscope.

The electron microscopeBiologists, aced with the problem that they would never see

anything smaller than 200 nm using a light microscope,

realised that the only solution would be to use radiation o

a shorter wavelength than light. I you study Figure 1.11,

you will see that ultraviolet light, or better still X-rays,

look like possible candidates. Both ultraviolet and X-ray

microscopes have been built, the latter with little success

partly because o the difficulty o ocusing X-rays. A much

better solution is to use electrons. Electrons

are negativelycharged particles which orbit the nucleus o an atom.

When a metal becomes very hot, some o its electrons

gain so much energy that they escape rom their orbits,

like a rocket escaping rom Earths gravity. Free electrons

behave like electromagnetic radiation. Tey have a very

short wavelength: the greater the energy, the shorter the

wavelength. Electrons are a very suitable orm o radiation

or microscopy or two major reasons. Firstly, their Figure 1.13 False-colour scanning electron micrograph of the headof a cat flea (100).




2

AS Level Biology

Viewing specimens with the electronmicroscopeFigure 1.14 shows how an electron microscope works

and Figure 1.15 shows one in use.

It is not possible to see an electron beam, so to make

the image visible the electron beam has to be projectedonto a fluorescent screen. Te areas hit by electrons shine

brightly, giving overall a black and white picture. Te

stains used to improve the contrast o biological specimens

or electron microscopy contain heavy metal atoms, which

stop the passage o electrons. Te resulting picture is like

an X-ray photograph, with the more densely stained parts

o the specimen appearing blacker. False-colour images

can be created by colouring the standard black and white

image using a computer.

Figure 1.14 How an electron microscope (EM) works.

electron gun and anode, which

produce a beam of electrons

condenser electromagnetic

lenswhich directs the electron

beam onto the specimen

specimen which is

placed on a grid

objective electromagnetic

lens which produces an

image

projector electromagnetic

lenses which focus themagnified image onto

the screen

screen or photographic

plate which shows the

image of the specimen

electron beam

vacuum

pathway of electrons

Figure 1.15 A transmission electron microscope (TEM) in use.

o add to the diffi culties o electron microscopy,

the electron beam, and thereore the specimen and the

fluorescent screen, must be in a vacuum. I electrons

collided with air molecules, they would scatter, making it

impossible to achieve a sharp picture. Also, water boils at

room temperature in a vacuum, so all specimens must bedehydrated beore being placed in the microscope. Tis

means that only dead material can be examined. Great

efforts are thereore made to try to preserve material in a

lie-like state when preparing it or electron microscopy.





Golgi bodyGolgi body

lysosomelysosome

cell surface

membrane

cell surface

membrane

mitochondriamitochondria

nucleolusnucleolus

nucleusnucleus

chromatinchromatin

ribosomes

endoplasmic

reticulum

endoplasmic

reticulum

glycogen granulesglycogen granules

microvillusmicrovillus

ribosomes

nuclear envelopenuclear envelope

Figure 1.16 Representative animal cells as seen with a TEM. The cells are liver cells rom a rat (9600). The nucleus is clearly

visible in one o the cells.

Ultrastructure of an animal cellTe fine (detailed) structure o a cell as revealed by the

electron microscope is called its ultrastructure.

Figure 1.16 shows the appearance o typical animal cells

as seen with an electron microscope, and Figure 1.17 is a

diagram based on many other such micrographs.




4

AS Level Biology

QUESTION

1.4 Compare Figure 1.17 with Figure 1.5 onpage 3. Name

the structures in an animal cell which can be seen

with the electron microscope but not with the light

microscope.

Figure 1.18 Transmission electron micrograph o the nucleus

o a cell rom the pancreas o a bat (7500). The circular

nucleus is surrounded by a double-layered nuclear envelope

containing nuclear pores. The nucleolus is more darkly

stained. Rough ER (page 15) is visible in the surrounding

cytoplasm.

microvilli

rough endoplasmic reticulum

nucleus

Golgi vesicle

Golgi body

mitochondrion ribosomes

lysosome

cytoplasm

smooth endoplasmic reticulum

nucleolus

chromatin

nuclear envelope

(two membranes)

nuclear pore

microtubules radiating from centrosome

centrosome with two centrioles close to the

nucleus and at right angles to each other

cell surface membrane

Figure 1.17 Ultrastructure o a typical animal cell as seen with an electron microscope. In reality, the ER is more extensive than

shown, and ree ribosomes may be more extensive. Glycogen granules are sometimes present in the cytoplasm.

Structures and functions of organellesCompartmentalisation and division o labour within the

cell are even more obvious with an electron microscope

than with a light microscope. We will now consider the

structures and unctions o some o the cell components inmore detail.

NucleusTe nucleus(Figure 1.18) is the largest cell organelle. It

is surrounded by two membranes known as the nuclear

envelope. Te outer membrane o the nuclear envelope is

continuous with the endoplasmic reticulum (Figure 1.17).





Figure 1.19 Transmission electron micrograph o rough ER

covered with ribosomes (black dots) (17 000). Some ree

ribosomes can also be seen in the cytoplasm.

Te nuclear envelope has many small pores called nuclear

pores. Tese al low and control exchange between the

nucleus and the cytoplasm. Examples o substances leaving

the nucleus through the pores are mRNA and ribosomes

or protein synthesis. Examples o substances entering

through the nuclear pores are proteins to help makeribosomes, nucleotides, AP (adenosine triphosphate) and

some hormones such as thyroid hormone 3.

Within the nucleus, the chromosomes are in a loosely

coiled state known as chromatin (except during nuclear

division, Chapter 5). Chromosomes contain DNA, which is

organised into unctional units called genes. Genes control

the activities o the cell and inheritance; thus the nucleus

controls the cells activities. When a cell is about to divide,

the nucleus divides first so that each new cell will have its

own nucleus (Chapters 5 and 19). Also within the nucleus,

the nucleolus

makes ribosomes, using the inormation in

its own DNA.

Endoplasmic reticulum and ribosomesWhen cells were first seen with the electron microscope,

biologists were amazed to see so much detailed structure.

Te existence o much o this had not been suspected. Tis

was particularly true o an extensive system o membranes

running through the cytoplasm, which became known

as the endoplasmic reticulum (ER)(Figures 1.18, 1.19

and 1.22). Te membranes orm an extended system

o flattened compartments, called sacs, spreading

throughout the cell. Processes can take place inside these

sacs, separated rom the cytoplasm. Te sacs can be

interconnected to orm a complete system (reticulum)

the connections have been compared to the way in which

the different levels o a car park are connected by ramps.Te ER is continuous with the outer membrane o the

nuclear envelope (Figure 1.17).

Tere are two types o ER: rough ER and smooth

ER. Rough ERis so called because it is covered with

many tiny organelles called ribosomes. Tese are just

visible as black dots in Figures 1.18 and 1.19. At very high

magnifications they can be seen to consist o two subunits:

a large and a small subunit. Ribosomes are the sites o

protein synthesis (see pages 111112). Tey can be ound

ree in the cytoplasm as well as on the rough ER. Tey are

very small, only about 25 nm in diameter. Tey are made

o RNA (ribonucleic acid) and protein. Proteins made bythe ribosomes on the rough ER enter the sacs and move

through them. Te proteins are ofen modified in some

way on their journey. Small sacs called vesicles can break

off rom the ER and these can join together to orm the

Golgi body. Tey orm part o the secretory pathway

because the proteins can be exported rom the cell via the

Golgi vesicles (page 80).

Smooth ER, so called because it lacks ribosomes, has a

completely different unction. It makes lipids and steroids,

such as cholesterol and the reproductive hormones

oestrogen and testosterone.

Golgi body (Golgi apparatus or Golgicomplex)Te Golgi bodyis a stack o flattened sacs (Figure 1.20).

More than one Golgi body may be present in a cell. Te

stack is constantly being ormed at one end rom vesicles

which bud off rom the ER, and broken down again at the

other end to orm Golgi vesicles. Te stack o sacs together

with the associated vesicles is reerred to as the Golgi

apparatus or Golgi complex.

Te Golgi body collects, processes and sorts molecules

(particularly proteins rom the rough ER), ready ortransport in Golgi vesicles either to other parts o the cell

or out o the cell (secretion). wo examples o protein

processing in the Golgi body are the addition o sugars

to proteins to make molecules known as glycoproteins,

and the removal o the first amino acid, methionine, rom

newly ormed proteins to make a unctioning protein.

In plants, enzymes in the Golgi body convert sugars into

cell wall components. Golgi vesicles are also used to

make lysosomes.




6

AS Level Biology

LysosomesLysosomes(Figure 1.21) are spherical sacs, surrounded

by a single membrane and having no internal structure.

Tey are commonly 0.1 0.5m in diameter. Tey contain

digestive (hydrolytic) enzymes which must be kept

separate rom the rest o the cell to prevent damage rom

being done. Lysosomes are responsible or the breakdown

(digestion) o unwanted structures such as old organelles

or even whole cells, as in mammary glands afer lactation

(breast eeding). In white blood cells, lysosomes are used

to digest bacteria (see endocytosis, page 80). Enzymes aresometimes released outside the cell or example, in the

replacement o cartilage with bone during development.

Te heads o sperm contain a special lysosome, the

acrosome, or digesting a path to the ovum (egg).

Mitochondria

StructureTe structure o the mitochondrion as seen with the

electron microscope is visible in Figures 1.16, 1.22, 12.13

and 12.14. Mitochondria (singular: mitochondrion)are usually about 1 m in diameter and can be various

shapes, ofen sausage-shaped as in Figure 1.22. Tey are

surrounded by two membranes (an envelope). Te inner

o these is olded to orm finger-like cristaewhich project

into the interior solution, or matrix. Te space between the

two membranes is called the intermembranespace. Te

outer membrane contains a transport protein called porin,

which orms wide aqueous channels allowing easy access

o small, water-soluble molecules rom the surrounding

cytoplasm into the intermembrane space. Te inner

membrane is a ar more selective barrier and controls

precisely what ions and molecules can enter the matrix.Te number o mitochondria in a cell is very variable.

As they are responsible or aerobic respiration, it is not

surprising that cells with a high demand or energy,

such as liver and muscle cells, contain large numbers o

mitochondria. A liver cell may contain as many as 2000

mitochondria. I you exercise regularly, your muscles will

make more mitochondria.

Function of mitochondria and the role of ATP

As we have seen, the main unction o mitochondria is

to carry out aerobic respiration, although they do have

other unctions, such as the synthesis o lipids. During

Figure 1.20 Transmission electron micrograph o a Golgi body.

A central stack o saucer-shaped sacs can be seen budding

off small Golgi vesicles (green). These may orm secretory

vesicles whose contents can be released at the cell surace by

exocytosis (page 80).

Figure 1.21 Lysosomes (orange) in a mouse kidney cell

(55 000). They contain cell structures in the process o

digestion and vesicles (green). Cytoplasm is coloured blue here.

Figure 1.22 Mitochondrion (orange) with its double

membrane (envelope); the inner membrane is olded to orm

cristae (20 000). Mitochondria are the sites o aerobic cell

respiration. Note also the rough ER.Original material Cambridge University Press 2014




respiration, a series o reactions takes place in which

energy is released rom energy-rich molecules such as

sugars and ats. Most o this energy is transerred to

molecules o AP. AP (adenosine triphosphate) is the

energy-carrying molecule ound in all living cells. It is

known as the universal energy carrier.Te reactions o respiration take place in solution in the

matrix and in the inner membrane (cristae). Te matrix

contains enzymes in solution, including those o the Krebs

cycle (Chapter 12) and these supply the hydrogen and

electrons to the reactions that take place in the cristae.

Te flow o electrons along the precisely placed electron

carriers in the membranes o the cristae is what provides

the power to generate AP molecules, as explained

in Chapter 12. Te olding o the cristae increases the

efficiency o respiration because it increases the surace

area available or these reactions to take place.

Once made, AP leaves the mitochondrion and, as it isa small, soluble molecule, it can spread rapidly to all parts

o the cell where energy is needed. Its energy is released

by breaking the molecule down to ADP (adenosine

diphosphate). Tis is a hydrolysis reaction. Te ADP can

then be recycled into a mitochondrion or conversion back

to AP during aerobic respiration.

The endosymbiont theory

In the 1960s, it was discovered that mitochondria and

chloroplasts contain ribosomes which are slightly smaller

than those in the cytoplasm and are the same size as those

ound in bacteria. Te size o ribosomes is measured inS units, which are a measure o how ast they sediment

in a centriuge. Cytoplasmic ribosomes are 80S, while

those o bacteria, mitochondria and ribosomes are 70S.

It was also discovered in the 1960s that mitochondria

and chloroplasts contain small, circular DNA molecules,

also like those ound in bacteria. It was later proved that

mitochondria and chloroplasts are, in effect, ancient

bacteria which now live inside the larger cells typical

o animals and plants (see prokaryotic and eukaryotic

cells, page 18). Tis is known as the endosymbiont

theory. Endo means inside and a symbiont is an

organism which lives in a mutually beneficial relationship

with another organism. Te DNA and ribosomes o

mitochondria and chloroplasts are still active and

responsible or the coding and synthesis o certain vital

proteins, but mitochondria and chloroplasts can no longer

live independently.

Mitochondrial ribosomes are just visible as tiny dark

dots in the mitochondrial matrix in Figure 1.22.

Cell surface membraneTe cell surace membrane is extremely thin (about 7 nm).

However, at very high magnifications, at least 100 000, it

can be seen to have three layers, described as a trilaminar

appearance. Tis consists o two dark lines (heavily

stained) either side o a narrow, pale interior (Figure1.23). Te membrane is partially permeable and controls

exchange between the cell and its environment. Membrane

structure is discussed urther in Chapter 4.

Figure 1.23 Cell surace membrane (250 000). At this

magniication the membrane appears as two dark lines at the

edge o the cell.

MicrovilliMicrovilli (singular: microvillus) are finger-like extensions

o the cell surace membrane, typical o certain epithelial

cells (cells covering suraces o structures). Tey greatly

increase the surace area o the cell surace membrane

(Figure 1.17 on page 14). Tis is useul, or example, or

absorption in the gut and or reabsorption in the proximal

convoluted tubules o the kidney (page 307).

Microtubules and microtubule organisingcentres (MTOCs)Microtubulesare long, rigid, hollow tubes ound in the

cytoplasm. Tey are very small, about 25 nm in diameter.

ogether with actin filaments and intermediate filaments

(not discussed in this book), they make up the cytoskeleton,

an essential structural component o cells which helps to

determine cell shape.

Microtubules are made o a protein called tubulin.

ubulin has two orms, -tubulin (alpha-tubulin) and

-tubulin (beta-tubulin). - and -tubulin molecules

combine to orm dimers (double molecules). Tese dimers

are then joined end to end to orm long protofilaments.Tis is an example o polymerisation. Tirteen

protofilaments then line up alongside each other in a ring

to orm a cylinder with a hollow centre. Tis cylinder is

the microtubule. Figure 1.24 (overlea) shows the

helical pattern ormed by neighbouring - and

-tubulin molecules.




8

AS Level Biology

Apart rom their mechanical unction o support,

microtubules have a number o other unctions. Secretory

vesicles and other organelles and cell components can be

moved along the outside suraces o the microtubules,

orming an intracellular transport system. Membrane-

bound organelles are held in place by the cytoskeleton.During nuclear division (Chapter 5), the spindle used or

the separation o chromatids or chromosomes is made o

microtubules, and microtubules orm part o the structure

o centrioles.

Te assembly o microtubules rom tubulin

molecules is controlled by special locations in cells called

25nm

5nm

appearance in

cross section

dimers can reversibly

attach to a microtubule

dimer

triplet of microtubules (one

complete microtubule and

two partial microtubules)

500nm

200nm

The dimers have a

helical arrangement. The dimers form 13 protofilaments

around a hollow core.

Figure 1.24 a The structure o a microtubule and bthe

arrangement o microtubules in two cells. The microtubules

are coloured yellow.

Figure 1.25 The structure o a centriole. It consists o nine

groups o microtubules arranged in triplets.

Figure 1.26 Centrioles in transverse and longitudinal section

(TS and LS) (86 000). The one on the lef is seen in TS and

clearly shows the nine triplets o microtubules which make up

the structure.

b

a

microtubule organising centres (MOCs). Tese are

discussed urther in the ollowing section on centrioles.

Because o their simple construction, microtubules can

be ormed and broken down very easily at the MOCs,

according to need.

Centrioles and centrosomesTe extra resolution o the electron microscope reveals

that just outside the nucleus o animal cells there are

really two centrioles and not one as it appears under the

light microscope (compare Figures 1.5 and 1.17). Tey lie

close together and at right angles to each other in a region

known as the centrosome. Centrioles and the centrosome

are absent rom most plant cells.

A centriole is a hollow cylinder about 500 nm long,

ormed rom a ring o short microtubules. Each centriole

contains nine triplets o microtubules (Figures 1.25

and 1.26).





cell surface

membrane

cell wall

endoplasmic

reticulum

mitochondrion

chloroplast

Golgi body

starch grain

ribosome

vacuole

tonoplastnuclear envelope

chromatin

nucleolus

nuclear pore

Figure 1.27 A representative plant cell as seen with a TEM. The cell is a palisade cell rom a soya bean lea (5600).

Te unction o the centrioles remains a mystery. Until

recently, it was believed that they acted as MOCs or the

assembly o the microtubules that make up the spindle

during nuclear division (Chapter 5). It is now known that

this is done by the centrosome, but does not involve

the centrioles.Centrioles ound at the bases o cilia (page 000) and

flagella, where they are known as basal bodies, do act as

MOCs. Te microtubules that extend rom the basal

bodies into the cilia and flagella are essential or the

beating movements o these organelles.

Ultrastructure of a plant cellAll the structures so ar described in animal cells are also

ound in plant cells, with the exception o centrioles and

microvilli. Te plant cell structures that are not ound in

animal cells are the cell wall, the large central vacuole, and

chloroplasts. Tese are all shown clearly in Figures 1.27

and 1.28. Te structures and unctions o cell walls and

vacuoles have been described on page 5.

ChloroplastsTe structure o the chloroplast as seen with the electron

microscope is visible in Figures 1.271.29 and at a

higher resolution in Figure 13.6. Chloroplasts tend to

have an elongated shape and a diameter o about 3 to

10 m (compare 1m diameter or mitochondria). Likemitochondria, they are surrounded by two membranes,

orming the chloroplast envelope. Also like mitochondria,

chloroplasts replicate themselves independently o cell

division by dividing into two.

Te main unction o chloroplasts is to carry

out photosynthesis. Chloroplasts are an excellent

example o how structure is related to unction, so a

brie understanding o their unction will help you to

understand their structure.

During the first stage o photosynthesis (the

light dependent stage) light energy is absorbed by

photosynthetic pigments, particularly the green pigmentchlorophyll. Some o this energy is used to manuacture

AP rom ADP. An essential stage in the process is the




0

AS Level Biology

splitting o water into hydrogen and oxygen. Te hydrogen

is used as the uel which is oxidised to provide the energy

to make the AP. Tis process, as in mitochondria,

requires electron transport in membranes. Tis explains

why chloroplasts contain a complex system o membranes.

Te membrane system is highly organised. It consistso fluid-filled sacs called thylakoids which spread out like

sheets in three dimensions. In places, the thylakoids orm

flat, disc-like structures that stack up like piles o coins

many layers deep, orming structures called grana (rom

their appearance in the light microscope; grana means

grains). Tese membranes contain the photosynthetic

pigments and electron carriers needed or the light

dependent stage o photosynthesis. Both the membranes

and whole chloroplasts can change their orientation

within the cell in order to receive the maximum amount

o light.

Te second stage o photosynthesis (the lightindependent stage) uses the energy and reducing power

generated during the first stage to convert carbon dioxide

into sugars. Tis requires a cycle o enzyme-controlled

reactions called the Calvin cycle and takes place in

solution in the stroma (the equivalent o the matrix in

QUESTION

1.5 Compare Figure 1.28 with Figure 1.6 on page 4. Name

the structures in a plant cell which can be seen

with the electron microscope but not with the light

microscope.

Figure 1.28 Ultrastructure o a typical plant cell as seen with the electron microscope. In reality, the ER is more extensive than

shown. Free ribosomes may also be more extensive.

cytoplasm

nucleolus

smooth ER

cell surface

membrane

(pressed against

cell wall)

tonoplast

cell sapvacuole

cell walls of

neighbouring cells

Golgi body

Golgi

vesicle

chloroplast

ribosomes

rough ER microtubule

nucleus

envelope grana

chloroplast

mitochondrion

nuclear pore

plasmodesma

middle lamella

nuclear envelope

chromatin

mitochondria). Te sugars made may be stored in the orm

o starch grains in the stroma (Figures 1.27 and 13.6). Te

lipid droplets also seen in the stroma as black spheres in

electron micrographs (Figure 1.29) are reserves o lipid or

making membranes or rom the breakdown o membranes

in the chloroplast.Like mitochondria, chloroplasts have their own protein

synthesising machinery, including 70S ribosomes and

a circular strand o DNA. In electron micrographs, the

ribosomes can just be seen as small black dots in the

stroma (Figure 13.6, page 000). Fibres o DNA can also

sometimes be seen in small, clear areas in the stroma.

As with mitochondria, it has been shown that

chloroplasts originated as endosymbiotic bacteria, in this

case photosynthetic blue-green bacteria. Te endosymbiont

theory is discussed in more detail on page 17.





Figure 1.29 Chloroplasts (16 000). Thylakoids (yellow) run

through the stroma (dark green) and are stacked in places

to orm grana. Black circles among the thylakoids are lipiddroplets. See also Figure 13.2, page 288. Chloroplast X is

reerred to in Question 1.2.

Figure 1.30 Diagram o a generalised bacterium showing the

typical eatures o a prokaryotic cell.

flagellum

for locomotion;

very simple structure

capsule

additional protection

infolding of cell

surface membrane

may form a

photosynthetic

membrane or carry out

nitrogen fixation

plasmid

small circle of DNA;

several may be present

pili

for attachment to

other cells or surfaces;

involved in sexual

reproduction

cell wall

containing

murein, a

peptidoglycan

cell surface

membrane

cytoplasm

circular DNA

sometimes referred

to as a chromosome

ribosomes

additional structures

sometimes present

structures always

present

Two fundamentally differenttypes of cellAt one time it was common practice to try to classiy

allliving organisms as either animals or plants. With

advances in our knowledge o living things, it has

become obvious that the living world is not that simple.

Fungi and bacteria, or example, are very different rom

animals and plants, and rom each other. Eventually itwas discovered that there are two undamentally different

types o cell. Te most obvious difference between

these types is that one possesses a nucleus and the other

does not.

Organisms that lack nuclei are called prokaryotes

(pro means beore; karyon means nucleus). Tey are,

on average, about 1000 to 10 000 times smaller involume

than cells with nuclei, and are much simpler in structure

or example, their DNA lies ree in the cytoplasm.

Organisms whose cells possess nuclei are called

eukaryotes(eu means true). Teir DNA lies inside a

nucleus. Eukaryotes include animals, plants, fungiand

a group containing most o the unicellular eukaryotes

known as protoctists. Most biologists believe that

eukaryotes evolved rom prokaryotes, one-and-a-hal

thousand million years afer prokaryotes first appeared on

Earth. We mainly study animals and plants in this book,

but alleukaryotic cells have certain eatures in common.

A generalised prokaryoticcellis shown in Figure 1.30.

A comparison o prokaryotic and eukaryotic cells is given

in able 1.2.

X

Viruses

In 1852, a Russian scientist discovered that certain diseasescould be transmitted by agents that, unlike bacteria, could

pass through the finest filters. Tis was the first evidence

or the existence o viruses, tiny organisms which are

much smaller than bacteria and are on the boundary

between what we think o as living and non-living. Unlike

prokaryotes and eukaryotes, viruses do not have a cell

structure. In other words, they are not surrounded by

a partially permeable membrane containing cytoplasm

with ribosomes. Tey are much simpler in structure. Most

consist only o:

a sel-replicating molecule o DNA or RNA which acts

as its genetic code

a protective coat o protein molecules.

Figure 1.31 shows the structure o a simple virus. It has

a very symmetrical shape. Its protein coat (or capsid) is

made up o separate protein molecules, each o which is

called a capsomere.

Viruses range in size rom about 20300 nm (about 50

times smaller on average than bacteria).




2

AS Level Biology

Prokaryotes Eukaryotes

average diameter o cell is 0.55mcells commonly up to 40m diameter and commonly 100010 000 times the volumeo prokaryotic cells

DNA is circular and lies ree in thecytoplasm

DNA is not circular and is contained in a nucleus the nucleus is surrounded by anenvelope o two membranes

DNA is naked DNA is associated with protein, orming structures called chromosomes

slightly smaller (70S) ribosomes (about20 nm diameter) than those o eukaryotes

slightly larger (80S) ribosomes (about 25 nm diameter) than those o prokaryotes

no ER present ER present, to which ribosomes may be attached

very ew cell organelles no separatemembrane-bound compartments unlessormed by inolding o the cell suracemembrane

many types o cell organelle present (extensive compartmentalisation and divisiono labour): some organelles are bounded by a single membrane, e.g. lysosomes, Golgi body,

vacuoles some are bounded by two membranes (an envelope), e.g. nucleus,

mitochondrion, chloroplast some have no membrane, e.g. ribosomes, centrioles, microtubules

cell wall present wall contains murein,a peptidoglycan(a polysaccharidecombined with amino acids)

cell wall sometimes present, e.g. in plants and ungi contains cellulose or lignin inplants, and chitin (a nitrogen-containing polysaccharide similar to cellulose) in ungi

Table 1.2 A comparison o prokaryotic and eukaryotic cells.

protein

molecules

capsid

DNA or RNA

genetic code

Figure 1.31 The structure o a simple virus.

All viruses are parasitic because they can only

reproduce by inecting and taking over living cells. Te

virus DNA or RNA takes over the protein synthesising

machinery o the host cell, which then helps to make new

virus particles.

Summary

The basic unit o lie, the cell, can be seen clearly onlywith the aid o microscopes. The light microscope uses

light as a source o radiation, whereas the electron

microscope uses electrons. The electron microscope has

greater resolution (allows more detail to be seen) than

the light microscope, because electrons have a shorter

wavelength than light.

With a light microscope, cells may be measured using

an eyepiece graticule and a stage micrometer. Using

the ormula IA = Mthe actual size o an object (A) or its

magniication (M) can be ound i its observed (image)

size (I) is measured andAor M, as appropriate, is known.

All cells are surrounded by a partially permeable cell

surace membrane that controls exchange between

the cell and its environment. All cells contain genetic

material in the orm

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