contents module 4 module 1 disease - pearson · pdf filemodule 2 foundations in biology ......

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Contents

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Contents

How to use this book 4

Module 1 Development of practical skills in Biology 1.1 Practical skills assessed in a written

examination 61.1.1 Planning 81.1.2 Implementing an investigation 111.1.3 Analysis 1: qualitative and quantitative data 151.1.4 Analysis 2: graphs 171.1.5 Evaluation 20 Thinking Bigger: How science can go wrong 22 Exam-style questions 24

Module 2 Foundations in biology 2.1 Cell structure 262.1.1 Microscopes 282.1.2 Slides and photomicrographs 322.1.3 Measuring objects seen with a light microscope 352.1.4 The ultrastructure of eukaryotic cells:

membrane-bound organelles 372.1.5 Other features of eukaryotic cells 422.1.6 How organelles work together in cells 452.1.7 Prokaryotic cells 47 Thinking Bigger: Cell theory 50 Exam-style questions 52

2.2 Biological molecules 542.2.1 Molecular bonding 562.2.2 Properties of water 582.2.3 Carbohydrates 1: sugar 602.2.4 Carbohydrates 2: polysaccharides as energy stores 622.2.5 Carbohydrates 3: polysaccharides as structural units 652.2.6 Lipids 1: triglycerides 682.2.7 Lipids 2: phospholipids and cholesterol 712.2.8 Proteins 1: amino acids 732.2.9 Proteins 2: protein structure and bonding 752.2.10 Proteins 3: fi brous and globular proteins 792.2.11 Inorganic ions 822.2.12 Practical biochemistry 1: Qualitative tests for biological

molecules 84

2.6.6 Animal tissues 181

2.6.7 Plant tissues and organs 184

2.6.8 Organs and organ systems in animals 186

2.6.9 Stem cells and their potential uses 188

Thinking Bigger: The Eastslack skeleton 192

Exam-style questions 194

Module 3 Exchange and transport 3.1 Exchange 1963.1.1 Exchange surfaces 1983.1.2 Mammalian gaseous exchange system 2003.1.3 Tissues in the gaseous exchange system 2023.1.4 Measuring lung volumes 2043.1.5 Gas exchange in other organisms 206 Thinking Bigger: Asthma 208 Exam-style questions 210

3.2 Transport in animals 2123.2.1 Transport in animals 2143.2.2 Blood vessels 2163.2.3 Exchange at the capillaries 2183.2.4 The structure of the heart 2203.2.5 The cardiac cycle 2223.2.6 Coordination of the cardiac cycle 2243.2.7 Transport of oxygen 2263.2.8 Transporting carbon dioxide 228 Thinking Bigger: Living at altitude 230 Exam-style questions 232

3.3 Transport in plants 2343.3.1 Transport in plants 2363.3.2 Transport tissues 2393.3.3 Movement of water through plants 2423.3.4 Transpiration 2443.3.5 The transpiration stream 2463.3.6 The adaptations of plants to the availability of water 2483.3.7 Translocation 250 Thinking Bigger: Guttation 252 Exam-style questions 254

2.2.13 Practical biochemistry 2: quantitative tests for biological molecules 86

2.2.14 Practical biochemistry 3: chromatography 89

Thinking Bigger: Biological molecules 92


2.3 Nucleic acids 96

2.3.1 DNA: deoxyribonucleic acid 98

2.3.2 How DNA replicates 102

2.3.3 How DNA codes for polypeptides 105

Thinking Bigger: The RNA revolution 108


2.4 Enzymes 112

2.4.1 Enzymes – biological catalysts 114

2.4.2 Cofactors 118

2.4.3 The mechanism of enzyme action 121

2.4.4 The effect of temperature on enzyme activity 124

2.4.5 The effect of pH on enzyme activity 127

2.4.6 The effect of substrate concentration on the rate of enzyme-catalysed reactions 130

2.4.7 The effects of enzyme concentration on the rate of reaction 132

2.4.8 Enzyme inhibitors 134

2.4.9 Enzyme inhibition: poisons and medicinal drugs 138

Thinking Bigger: The bite that heals 142


2.5 Biological membranes 146

2.5.1 The structure of plasma membranes 148

2.5.2 Diffusion across membranes 150

2.5.3 Osmosis 153

2.5.4 How substances cross membranes using active processes 156

2.5.5 Factors affecting membrane structure and permeability 159

Thinking Bigger: Red blood cell membrane disorders 162


2.6 Cell division 166

2.6.1 The cell cycle and its regulation 168

2.6.2 Mitosis 170

2.6.3 Meiosis 173

2.6.4 Diversity in animal cells 176

2.6.5 Cell diversity in plants 179

Module 4 Biodiversity, evolution and disease 4.1 Communicable diseases 2564.1.1 Organisms that cause disease 2584.1.2 Transmission of pathogens 2604.1.3 Plant defences against pathogens 2624.1.4 Primary defences against disease 2644.1.5 Secondary non-specifi c defences 2674.1.6 The specifi c immune response 2694.1.7 Antibodies 2714.1.8 Vaccination 2744.1.9 Development and use of drugs 276 Thinking Bigger: HIV 278 Exam-style questions 280

4.2 Biodiversity 2824.2.1 Biodiversity 2844.2.2 Sampling plants 2864.2.3 Sampling animals 2884.2.4 Calculating biodiversity 2904.2.5 What affects biodiversity? 2924.2.6 Reasons to maintain biodiversity 2944.2.7 Conservation in situ 2964.2.8 Conservation ex situ 2984.2.9 Protection of species and habitats 300 Thinking Bigger: Conservation 302 Exam-style questions 304

4.3 Classifi cation and evolution 3064.3.1 Biological classifi cation 3084.3.2 Features used in classifi cation 3104.3.3 Evidence used in in classifi cation 3124.3.4 Classifi cation and phylogeny 3144.3.5 The evidence for natural selection 3164.3.6 Variation 3184.3.7 Applying statistical techniques 3204.3.8 Adaptation 3224.3.9 Natural selection and evolution 324 Thinking Bigger: Antibiotics and superbugs 326 Exam-style questions 328

Maths skills 330 Preparing for your exams 336 Glossary 342 Index 346

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How to use this book

How to use this book Thinking BiggerAt the end of each chapter there is an opportunity to read and work with real-life research and writing about science. These sections will help you to expand your knowledge and develop your own research and writing techniques. The questions and tasks will help you to apply your knowledge to new contexts and to bring together different aspects of your learning from across the whole course.

These spreads will give you opportunities to:

• read real-life material that’s relevant to your course

• analyse how scientists write

• think critically and consider relevant issues

• develop your own writing

• understand how different aspects of your learning piece together.

Exam-style questionsAt the end of each chapter, there are exam-style questions to test how fully you have understood the learning and to help you practise for your exams.

Getting the most from your ActiveBookYour ActiveBook is the perfect way to personalise your learning as you progress through your OCR AS/A level Biology course. You can:

• access your content online, anytime, anywhere

• use the inbuilt highlighting and annotation tools to personalise the content and make it really relevant to you

• search the content quickly.

Highlight toolUse this to pick out key terms or topics so you are ready and prepared for revision.

Annotations toolUse this to add your own notes, for example, links to your wider reading, such as websites or other fi les. Or make a note to remind yourself about work that you need to do.

Welcome to your OCR AS/A level Biology student book. In this book you will fi nd a number of features designed to support your learning.

Chapter openersEach chapter starts by setting the context for that chapter’s learning:

• Links to other areas of Biology are shown, including previous knowledge that is built on in the chapter and future learning that you will cover later in your course.

• The All the maths I need checklist helps you to know what maths skills will be required.

Main contentThe main part of the chapter covers all of the points from the specifi cation you need to learn. The text is supported by diagrams and photos that will help you understand the concepts.

Within each topic, you will fi nd the following features:

• Learning objectives at the beginning of each topic highlight what you need to know and understand.

• Key terms are shown in bold and defi ned within the relevant topic for easy reference.

• Worked examples show you how to work through questions, and how your calculations should be set out.

• Investigations provide a summary of practical experiments that explore key concepts.

• Learning tips help you focus your learning and avoid common errors.

• Did you know? boxes feature interesting facts to help you remember the key concepts.

At the end of each topic, you will fi nd questions that cover what you have just learned. You can use these questions to help you check whether you have understood what you have just read, and to identify anything that you need to look at again.

Atoms and reactions 2.1

Foundations of biology

Introduction In 1995 the actor Christopher Reeve, best known for playing the part of Superman, fell from his horse and sustained a spinal injury that led to him being paralysed and wheelchair-bound. He spent a lot of money backing research efforts into the use of stem cells for medical therapies, such as repairing spinal cord injuries. However, at that time the main source of stem cells was from embryos. This raised ethical concerns and held up research into the use of stem cells. In 2006 a team led by Shinya Yamanaka at Kyoto University, Japan, found that they could reprogram human skin cells to become stem cells. The use of such induced pluripotent (capable of becoming any kind of cell) stem cells is less controversial than using embryonic stem cells and research into stem cell therapy is gathering pace again. Most people today appreciate that we are made of billions of cells. However, this was not always the case. Cells are too small to be seen with the naked eye so it was not until microscopes were available that people could see that animals and plants were made of cells. Scientists also observed single-celled organisms for the first time. As microscopes improved, biologists were able to see the even smaller structures inside cells and sophisticated biochemical techniques enabled them to work out what each part of the cell actually did.

All the maths you need To unlock the puzzles of this chapter you need the following maths:

• Units of measurement

• How to calculate magnification

• How to calculate surface area

• How to calculate volume

• How to calculate ratios

What have I studied before? You should already know from GCSE:

• Cells are the building blocks of all living organisms

• Living organisms’ metabolic processes, such as respiration, are carried out in their cells

• Cells are very small and can only be studied using a microscope

• All cells have common features such as:

○ a surface membrane that separates the cell’s interior from the external environment and regulates what goes into and out of a cell

○ a jelly like cytoplasm and a cytoskeleton

○ DNA that makes up the cell’s genetic content (genome)

○ ribosomes where proteins are assembled

• There are differences between plant, animal and bacterial cells

• Within a developing organism undifferentiated stem cells become differentiated and specialised to carry out certain specific functions – we all began life as one cell and we all develop many types of cells in our bodies

What will I study later? • In chapter 2.2 you will learn about the structure

and properties of key biological molecules, including the phospholipids, proteins and carbohydrates that make up the cell membranes

• In chapter 2.3 you will learn about enzymes, many of which catalyse chemical reactions inside cells

• In chapter 2.4 you will learn more about the structure and functions of cell membranes – the membranes around the outside of cells and the membranes around some of their internal organelles

• In chapter 2.5 you will learn about the properties of nucleic acids, DNA and RNA, which are found in cells

• In chapter 2.6 you will learn how cells reproduce and pass on their genetic material to their daughter cells

• In chapter 3.1 you will learn more about the tissues involved in exchanging substances between organisms and their environments

• In chapters 3.2 and 3.3 you will learn how substances needed for life are transported to all the cells of large multicellular plants and animals

• In chapter 4.1 you will learn about the roles of special cells involved in defence against infectious disease

What will I study in this chapter? • Microscopes, optical and electron, plus their

advantages and disadvantages (2.1.1)

• How slides and photomicrographs help us study cells (2.1.2)

• How electron micrographs have helped scientists to study cells (2.1.3)

• The ultrastructure of cells – the structure and functions of the smaller parts within cells (2.1.4)

• How organelles within cells work together, for example to make proteins (2.1.5)

• How cells become differentiated and specialised for particular functions, how they are organised into tissues and that tissues are organised into organs (2.1.6)

• More about the structure of prokaryotic cells and how they differ from eukaryotic cells (2.1.7)

You will also learn how to:

• make slides of cells to examine using an optical microscope

• correctly draw low-power plans and high-power drawings of prepared slides of tissues

• calculate the size of cells and organelles as seen in photomicrographs or electron micrographs

2 MO

DU

LE

CHAPTER

2.1 CELL STRUCTURE

28 2928 29

31

Cell structure 2.1

3030

Microscopes

By the end of this topic, you should be able to demonstrate and apply your knowledge and understanding of:

∗ the use of microscopy to observe and investigate different types of cell and cell structure in a range of eukaryotic organisms

∗ the difference between magnification and resolution

KEY DEFINITION S

Magnification: the number of times larger an image appears, compared with the size of the object. Resolution: the clarity of an image; the higher the resolution, the clearer the image. Organelles: small structures within cells, each of which has a specific function. Cytology : the study of cell structure and function. Photomicrograph: photograph of an image seen using an optical microscope. Electron micrograph: photograph of an image seen using an electron microscope.

Magnifi cation Magnifi cation describes how much bigger an image appears compared with the original object. Microscopes produce linear magnifi cation, which means that if a specimen is seen magnifi ed ×100, it appears to be 100 times wider and 100 times longer than it really is.

Resolution Resolution is the ability of an optical instrument to see or produce an image that shows fi ne detail clearly. You may have a high-resolution television (called ‘ultra-high defi nition’ or UHD) and have noticed how clear and sharp the images on its screen are.

Optical microscopes The development of optical (light) microscopes played a key role in our understanding of cell structure. They were the fi rst sort to be used, and are still used in schools, colleges, hospitals and research laboratories because they are:

• relatively cheap

• easy to use

• portable and able to be used in the fi eld as well as in laboratories

• able to be used to study whole living specimens.

Present-day light microscopes look different from the ones used in the seventeenth century, but both types rely on lenses to focus a beam of light.

Notice that the image is laterally (sideways) inverted. This means that as you look down the microscope at the image and move your slide to your left, the image appears to move to the right.

D

eyepiecelens

image 1

objectivelens

objectimage 2

A The object is placed on a slide on the microscope stage.

B Light rays passing through the object are bent and focused by the objective lens, to form a real laterally inverted image, image 1.

C Light rays from the real image (image 1) are bent by the eyepiece lens.

D Diverging light rays entering the eye are perceived as coming from a magnified, laterally inverted, virtual image, image 2.

C

BA

Figure 2 Diagram showing how the compound optical microscope produces a magnifi ed image.

LEARNING TIP

You do not need to learn how an image is formed by an optical microscope. The information is given here so that you can understand why, when you move a slide one way, the image moves in the opposite direction, and also to help you appreciate why total magnification is the magnifying power of the objective lens × the magnifying power of the eyepiece lens.

Objective lens magnifi cation

Eyepiece lens magnifi cation

Total magnifi cation

×4 ×10 ×40

×10 ×10 ×100

×40 ×10 ×400

×4 ×15 ×60

×10 ×15 ×150

×40 ×15 ×600

×100 (oil immersion)

×10 ×1000


×15 ×1500

Table 1 The magnifi cations produced by most optical microscopes.

A photograph of the image seen using an optical microscope is called a photomicrograph . You will see an example of one in this chapter. Modern digital microscopes display the image on a computer screen.

2.1 1

Optical microscopes allow magnifi cation up to ×1500, or in some types ×2000, which enables us to see clearly some of the larger structures inside cells. However, because their resolution is limited, they cannot magnify any higher while still giving a clear image. See Table 1 for more details of the magnifi cations produced by most optical microscopes.

• Optical microscopes use visible light, a part of the electromagnetic spectrum that has a wavelength of between 400 and 700 nm.

• The light waves cannot pass between small particles that are less than 200 nm apart, so such objects within a cell will appear blurred.

• Ribosomes are very small, non-membrane-bound, cell organelles of about 20 nm diameter, and so they cannot be examined using a light microscope.

Arm

2. By rotating the nosepiece,the lowest power (smallest)objective lens is placedover the specimen.

3. Adjust thecoarse focus knob,while lookinginto theeyepiece, untilthe image yousee is clearand in focus.

Fine focus knob(see step 5)

6. Repeat step 5. using the ×40 objective lens.

1. The specimen on a slide isplaced here on the stage andclipped into place.

CondenserLight source

4. Whilst viewingthe image adjustthe iris diaphragmfor optimum light.

5. Make sure that theobject you wish toview is directly overthe hole in the stage.Now rotate thenosepiece and bringthe ×10 objective intoplace over the specimen. Look down the oculartube and use thefine focus knob tofocus the image.

Figure 1 Annotated diagram showing how to use a light microscope. Note that when you carry a microscope you should hold it by its arm in one of your hands, whilst having your other hand under the base of the microscope.

You may have used microscopes with just one eyepiece, but they work in the same way as the one shown in Figure 1. For people such as cytology screeners, who spend a lot of time staring down a microscope, the type shown here produces less eye-strain.

You can see from Figure 2 that the objective lens produces the fi rst (real) magnifi ed image, and the eyepiece lens(es) magnify this image to give an even larger (virtual) image. Therefore:

total magnifi cation = the magnifying power of the objective lens × magnifying power of the eyepiece lens

Laser scanning microscopes Laser scanning microscopes are also called confocal microscopes (see Figure 3).

• They use laser light to scan an object point by point and assemble, by computer, the pixel information into one image, displayed on a computer screen.

• The laser beams are focused by lenses and a concave mirror.

• The images are high resolution and show high contrast.

• These microscopes have depth selectivity and can focus on structures at different depths within a specimen. Such microscopy can therefore be used to clearly observe whole living specimens, as well as cells.

• They are used in the medical profession, for example to observe fungal fi laments within the cornea of the eye of a patient with a fungal corneal infection, in order to give a swift diagnosis and earlier, and therefore more effective, treatment.

• They are also used in many branches of biological research.

(a) (b)

Figure 3 (a) A laser scanning microscope; (b) cells in the retina of the eye as seen with a laser scanning microscope.

Electron microscopes Electron microscopes use a beam of fast-travelling electrons that has a wavelength of about 0.004 nm. This means that they have much greater resolution than optical microscopes and can be used to give clear and highly magnifi ed images.

• The electrons are fi red from a cathode and focused, by magnets rather than glass lenses, on to a screen or photographic plate.

• The faster electrons travel, the shorter is their wavelength.

• Fast-travelling electrons have a wavelength about 125 000 times smaller than that of the central part of the visible light spectrum. Therefore, they can pass between cell structures that are very close together.

Transmission electron microscopes These were the fi rst type, developed during the 1930s.

• The specimen has to be chemically fi xed by being dehydrated and stained. It is then mounted on a small copper grid and placed in a vacuum chamber because air molecules can defl ect the beam of electrons.

• The beam of electrons passes through the specimen, which is stained with metal salts. Some electrons pass through and are focused on the screen or photographic plate.

36 37

22.1Cell structure 2.1Cell structureCell structureCell structureCell structureCell structure 2.12.12.12.1Cell structureCell structure

Measuring objects seen with a light microscope


∗ the preparation and examination of microscope slides for use in light microscopy

∗ the use and manipulation of the magnification formula

∗ photomicrographs of cellular components in a range of eukaryotic cells

KEY DEFINITIONS

A micrometre ( mm) is equal to one-millionth (10 –6 ) of a metre. It is the standard unit for measuring cell dimensions. A nanometre (nm) is one-thousandth (10 –3 ) of a micrometre. It is therefore one-thousand-millionth (10 –9 ) of a metre. It is a useful unit for measuring the sizes of organelles within cells and the size of large molecules. A stage graticule is a precise measuring device. It is a small scale that is placed on a microscope stage and used to calibrate the value of eyepiece divisions at different magnifications. An eyepiece graticule is a measuring device. It is placed in the eyepiece of a microscope and acts as a ruler when you view an object under the microscope. However, the divisions appear larger under higher magnification, and so they have a different real length value at each magnification. A stage graticule is used to calibrate the eyepiece graticule and find the real value of each division at those magnifications.

Using graticules • A microscope eyepiece can be fi tted with a graticule.

• This graticule is transparent with a small ruler etched on it.

• As the specimen is viewed, the eyepiece-graticule scale is superimposed on it and the dimensions of the specimen can be measured (just as you can measure a large object by placing a ruler against it) in eyepiece units (epu) (Figure 1).

The scale of the eyepiece graticule is arbitrary – it represents different lengths at different magnifi cations. The image of the specimen looks bigger at higher magnifi cations, but the actual specimen has not increased in size. The eyepiece scale has to be calibrated (its value worked out) for each different objective lens. See Table 1 for values of eyepiece divisions at different magnifi cations.

A stage graticule is used only to calibrate the eyepiece graticule.

0

0 10

(a) (b)

1 2 3 4eyepiecegraticule

stagegraticule

5 6 7 8 910 0 1 2 3 4 5 6 7 8 910

Figure 1 Eyepiece graticule and stage graticule at (a) ×40 magnifi cation, and (b) ×100 magnifi cation.

2.1 3

INVESTIGATION

Using a stage graticule to calibrate the eyepiece graticule A microscopic ruler on a special slide, called a stage graticule, is placed on the microscope stage. This ruler is 1 mm long and divided into 100 divisions. Each division is 0.01 mm or 10 mm ( micrometres ). 1. Insert an eyepiece graticule into the ×10 eyepiece of your

microscope. This ruler has a total of 100 divisions. 2. Place a stage graticule on the microscope stage and bring it into focus,

using the low-power (×4) objective. Total magnification is now ×40. 3. Align the eyepiece graticule and the stage graticule as shown

in Figure 1(a). Check the value of one eyepiece division at this magnification on your microscope.

4. In the example shown here, the stage graticule (which is 1 mm or 1000 mm) corresponds with 40 eyepiece divisions.

5. Therefore each eyepiece division = 1000

_____ 40

mm = 25 mm.

6. Now use the ×10 objective lens on your microscope (total magnification is ×100) and focus on the stage graticule.

7. Align them both as shown in Figure 1(b). 8. In the example shown here, 100 eyepiece divisions now correspond

with 1 mm or 1000 mm.

9. Therefore one eyepiece division = 1000

_____ 100

mm = 10 mm.

10. Now use the ×40 objective lens, giving a total magnification of ×400, and repeat steps 6 and 7. You should find that each eyepiece division corresponds with 2.5 mm.

11. If you have a ×100 objective lens, repeat steps 6 and 7. One eyepiece division should equal 1 mm.

12. You can write down the eyepiece division values at each magnification for your microscope on to a label and stick this to the base of the microscope for future use.

Magnifi cation of eyepiece lens

Magnifi cation of objective lens


Value of one eyepiece division (epu)/mm

×10 ×4 ×40 25

×10 ×10 ×100 10

×10 ×40 ×400 2.5

×10 ×100 (oil-immersion lens)

×1000 1.0

Table 1 Values of eyepiece divisions at different magnifi cations for most modern microscopes used in schools.

Calculations involving magnification On the photomicrograph in Figure 3 (showing a leaf in transverse section), because you know the magnifi cation, you can then fi nd the actual size of the structures.

• Measure the thickness of the leaf, in mm, in its widest part.

• Convert that measurement to mm by multiplying by 1000.

• Now divide this fi gure by the magnifi cation. This tells you the actual thickness of the leaf at this point.

If you are told the actual size of a structure on a photomicrograph ( A ), and you measure its image size on the photomicrograph ( I ), in mm [mm × 1000], you can calculate the magnifi cation factor ( M ) using the formula:

M = I __

A

There are no units for magnifi cation but if, for example, the magnifi cation factor is 1000, then you must write it as ×1000.

LEARNING TIPS

When you observe structures in a microscope section, bear in mind the following: • cells have a 3D structure and you are looking at a 2D section

• depending on whereabouts in the cell the section was cut, some structures may be absent from your slide section

• depending how certain structures were oriented in the cell, they will appear as different shapes. For example, mitochondria sliced lengthways (in longitudinal section ) would appear sausage-shaped, but if sliced transversely (crossways) will appear round and if sliced obliquely (slanting) will appear elliptical.

When drawing your specimens, always draw what you see, not what you think the specimen should look like as remembered from a textbook diagram.

or any liquid, just look at it under the microscope and see the hyphae. If you try this, then make sure that you place used slides into a pot of bleach or disinfectant and wash your hands after the practical, as you should always do.

Figure 3 Bread mould, Mucor mucedo , seen under a light microscope (×40). Note the thread-like hyphae and the round fruiting bodies.

Questions

1 Write an equation, using the symbols A for actual size, I for image size and M for magnifi cation, in order to show how you can calculate the actual size of a structure on a photomicrograph.

2 If a nucleus diameter measures 10 mm on a photomicrograph with a magnifi cation of ×1000, what is the actual diameter of this nucleus?

3 On a photomicrograph of a human blood smear, an erythrocyte has a diameter of 3.2 mm. Erythrocytes have an actual diameter of 8 mm. What is the magnifi cation of this photomicrograph?

4 Explain why biological specimens to be examined under a microscope may be stained.

5 Explain why the mitochondria shown in a photomicrograph of a liver cell will not all be the same shape.

6 Draw a diagram of a plant tap-root (for example, a carrot) and show where you would cut it to make (a) a longitudinal section, and (b) a transverse section.

INVESTIGATION

An easy way to make a slide of a fungusThere is a very easy way to make temporary slides of some specimens, such as fungi. If you grow some mould fungi, such as Penicillium or Mucor (see Figure 3), on an agar plate, you can simply place a piece of SellotapeTM lightly on to the surface of the mould growth, then place this same piece of Sellotape, face-down, on to a slide. No need for a coverslip

6.1


THINKING

Where else will I encounter these themes?

Cell structure 2.1

52 53

2.11.1 2.62.2 3.12.3 3.22.4 3.3 4.22.5 4.1 4.3YOU ARE HERE

5352

CELL THEORY In 1839 two scientists developed cell theory that said all living organisms are made of cells. Since then we have learned a lot about prokaryotic and eukaryotic cells. Viruses have also been discovered, and they do not have cells.

Use your own knowledge, information in the text above and in the rest of Chapter 2.1, plus research using the internet, to answer these questions.

1 What are the main statements of cell theory?

2 Which statements in cell theory can be applied to viruses?

3 When Linnaeus developed his classifi cation system, all living things were classifi ed into one of two groups – Animalia and Plantae. How many kingdoms are there in the present day biological classifi cation system?

4 Explain the phrase ‘viruses are nature’s genetic engineers’.

5 Suggest why it is unlikely that viruses were the fi rst forms of biological entity on Earth.

6 By what process do you think rabies virus particles are moved along neurone axons to the brain or spinal cord?

7 Which organelles in the host cell are involved in replicating the virus particles?

8 Discuss, using your biological knowledge, whether you think viruses should be classifi ed as living or non-living. Do you think that ‘cell theory’ may need to be amended in the future? Give reasons for your answer.

Activity

The branch of biology dealing with DNA technology such as genetic profi ling and genetic modifi cation of living organisms relies on knowledge and understanding of bacteria and viruses, as well as knowledge of the structure of DNA.

(a) Investigate, using the internet, exactly how viruses can provide scientists working in this fi eld with certain chemicals that they need.

(b) Write an article or make a poster showing how viruses can be helpful to humans.

DID YOU KNOW?

Viruses outnumber all other biological entities put together. In just one teaspoon of seawater, for example, there are one million virus particles.Recently a virus large enough to be seen with the light microscope was unearthed from the Siberian permafrost, where it had lain dormant for 30 000 years. It was reactivated and its genome, of several thousand genes, was sequenced. It infects amoebae.


You will recall from the introduction to this topic of cells, in the opener pages, that cell theory was developed in 1839. At that time, all living things were classifi ed as either animals or plants. Bacteria and viruses had not yet been discovered.

However, in the late 1700s, Dr Edward Jenner, the man who developed the fi rst vaccine against smallpox, used the term ‘virus’ to describe the contents of pustules in people suffering from smallpox. However, he used it in its Latin meaning – poison.

By 1885 Louis Pasteur had observed small single-celled entities, that he called germs, which could cause certain diseases. In fact these were bacteria. Until that time people thought that miasmas, (bad air) spread such infections. Pasteur also used a very fi ne mesh to fi lter the spinal fl uid of rabbits infected with rabies. This mesh had very small holes that prevented bacteria from passing through. However, this fi ltered spinal fl uid, when injected into healthy rabbits, gave them rabies. He concluded that the infecting agent which caused rabies was smaller than bacteria.

We now know that rabies is caused by a virus belonging to a group of viruses called Lyssavirus . This type of virus is shaped rather like a bullet and contains a single strand of RNA. Its length is about 180 nm and its diameter is about 75 nm. It has a lipoprotein envelope and on the outside of this envelope are knob-like spikes of glycoprotein. Underneath the envelope is the protein coat. Rabies virus particles enter the neurones of mammals, including humans, by endocytosis. The glycoprotein spikes on their surfaces dock with complementary-shaped receptors on the host cell membranes and trick the cells into taking them in. Once inside a host cell, the virus uses the cell nucleus and organelles to make copies of itself. Virus particles move back along neurone axons to the brain and spinal cord. They also move to the salivary glands and can therefore be transmitted to another host in bites.

As is often the case, the advances in technology help scientifi c knowledge to advance. A relatively large virus, the tobacco mosaic virus was studied with a powerful light microscope in the 1890s. However, it was not until the advent of electron microscopes, in the 1930s, that smaller viruses could be seen and their structure discovered. In the 1950s the scientist, Rosalind Franklin, worked out the structure of the tobacco mosaic virus using X-ray crystallography. These virus particles are about 300 nm long and 18 nm wide. The protein units making up the virus coat are arranged in coils. Its genome is a single strand of RNA.

In general, viruses have the following properties: ● they are about one-hundredth the size of bacteria, and therefore most are too

small to be seen with a light microscope ● they do not have a cell structure – they have no cytoplasm, membranes or

organelles ● they have genetic material – either DNA or RNA, but not both ● their genome is enclosed in a protein coat made of capsomeres ● some have a lipid membrane, around the protein coat, derived from the mem-

brane of cells that they infect ● many also contain a few enzymes ● they can only reproduce when inside a living host cell ● they can remain dormant, in some cases for a very long time ● they are the most abundant types of biological entity, with many different types ● some types infect plant cells; some infect animal cells; some, known as bacte-

riophages, infect bacteria; some infect fungi; some infect protoctists; and some can infect other viruses

● they can be spread from host to host by direct contact (shaking hands), droplets (sneezing), body fluids, the faecal–oral route, and by insects

● although we associate them with diseases, some are useful as they help regulate other life forms, for example they can kill algal blooms. Some persistent (those that pass from generation to generation in their host) plant viruses can make their host drought-resistant or heat resistant. Species of grass that live in a geyser field in Yellowstone Park, USA, have symbiotic fungi that contain symbiotic viruses. If the viruses are removed, then the grasses cannot grow in this hot place

WILL CELL THEORY NEED TO BE MODIFIED ONE DAY?

Exam-style questions 2.1

5554

2.1

1. Which of the following organelles are found in both eukaryotic and prokaryotic cells? [1]

A. Chloroplasts

B. Golgi bodies

C. Mitochondria

D. Ribosomes [Total: 1]

2. Figure 1 shows the general structure of an animal cell. 1 3

4

2

Figure 1

Which row correctly identifi es the structures labelled in the diagram? [1]

Structure 1 Structure 2 Structure 3 Structure 4

A mitochondria Golgi rough

endoplasmic

reticulum

smooth

endoplasmic

reticulum

B centrioles mitochondria vesicles cytoskeleton

C mitochondria centrioles ribosomes endoplasmic

reticulum

D mitochondria centrosome ribosome Golgi

[Total: 1]

3. Read the following statements:

(i) The wavelength of visible light is about 12 × 10 5 times longer than the wavelength of an electron beam used in an electron microscope.

(ii) Scanning electron microscopes can be used to observe whole living specimens.

(iii) Laser scanning microscopes can focus on structures at different depths within a specimen.

Which statement(s) is/are true? [1]

A. (i) only

B. (i) and (ii) only

C. (i) and (iii) only

D. (i), (ii) and (iii) [Total: 1]

4. Which of the following is not a function of the rough endoplasmic reticulum? [1]

A. It is the site of protein synthesis.

B. It is an intracellular transport system.

C. It gives a large surface area for ribosomes.

D. It can channel newly synthesised proteins to the Golgi body.

[Total: 1]

5. Read the following statements.

(i) Cilia are formed from centrioles and each is surrounded by the plasma membrane.

(ii) Cilia contain microtubules that enable them to move.

(iii) Cilia are only found on epithelial cells lining the airways.


A. (i) and (ii) only

B. (ii) and (iii) only



6. Figure 2 shows a goblet cell from the epithelium (lining) of the stomach. Other cells in the stomach lining produce hydrochloric acid, and the pH inside a human stomach is between 1 and 2.

(a) The protein mucin is synthesised within the cell and secreted, in mucus, at the position marked Z.

(i) Place the appropriate letters in the correct order to show the passage of newly synthesised molecules of mucin as they are moved from the place where they were made to position Z. [2]

R

Z

S

T

U

V

W

X

Y

Figure 2

(ii) There are amino acids in the cell cytoplasm that may be used to make mucin. Describe precisely where in this cell the mucin molecules will be assembled from these amino acid monomers. Give a reason for your answer. [2]

(iii) By what process do mucin molecules pass out of this cell? [1]

(b) The structures labelled Z are extensions of the plasma membrane and are called microvilli. Suggest why this type of cell has microvilli. [2]

(c) Suggest why this cell has many of the structures labelled W inside it. [4]

(d) Why do you think mucus needs to be produced by the stomach? [1]

[Total: 12]

7. Some scientists wanted to study the structure and functions of chloroplasts (see Figure 3). They macerated some spinach leaves in a food blender, adding 2% sucrose solution and kept the mixture cold. They fi ltered the mixture to remove debris and then spun the mixture in a centrifuge, which increases the force of gravity and, after a short spin, the cell nuclei are pulled to the bottom of the tube, forming a sediment.

macerated spinach leaves

sediment containing nuclei supernatant

supernatant

10 minutes at600 g force

10 minutes at10 000 g force

sediment containing chloroplasts

chloroplasts re-suspended in ice-cold 2% sucrose solution

Figure 3

The supernatant liquid is then taken out of the tube, placed in another centrifuge tube and spun again at a higher speed. The chloroplasts were seen at the bottom of the tube as a green sediment.

After decanting off the supernatant liquid, the chloroplasts were resuspended in ice-cold 2% sucrose solution before being used for investigations.

(a) Suggest why the fi rst organelles to sediment out during centrifugation were the nuclei? [1]

(b) Suggest why chloroplasts were the second type of organelles to sediment out by centrifugation? [2]

(c) Explain why leaves were used as a source of chloroplasts. [1]

(d) Suggest why, prior to their use, the isolated chloroplasts were:

(i) suspended in 2% sucrose solution [1]

(ii) kept ice cold. [2]

(e) Name the substance that gives the chloroplasts their green colour. [1]

(f) Briefl y outline the function of chloroplasts. [2] [Total: 10]

8. The electron micrograph in Figure 4 shows some plant cells.

Figure 4

(a) Identify the structures labelled A–G. [7]

(b) The true diameter, across line WX, of that organelle is 10 mm.

(i) What is the magnifi cation of this electron micrograph? Show your working. [2]

(ii) Calculate the length of structure H, along the line PQ. Show your working. [2]

(iii) The organelle with diameter 10 µm is spherical. Calculate its volume. Express your answer to the nearest whole number. Show your working. [3]

(c) Explain why this electron-micrograph image is grey-scale (has no colour). [1]

(d) State two functions of structure D. [2][Total: 17]

D

F E

G

B

A

C

HX

W

P

Q

X

W

P

Q

Exam-style questions


Foundations of biology

Introduction In 1995 the actor Christopher Reeve, best known for playing the part of Superman, fell from his horse and sustained a spinal injury that led to him being paralysed and wheelchair-bound. He spent a lot of money backing research efforts into the use of stem cells for medical therapies, such as repairing spinal cord injuries. However, at that time the main source of stem cells was from embryos. This raised ethical concerns and held up research into the use of stem cells. In 2006 a team led by Shinya Yamanaka at Kyoto University, Japan, found that they could reprogram human skin cells to become stem cells. The use of such induced pluripotent (capable of becoming any kind of cell) stem cells is less controversial than using embryonic stem cells and research into stem cell therapy is gathering pace again. Most people today appreciate that we are made of billions of cells. However, this was not always the case. Cells are too small to be seen with the naked eye so it was not until microscopes were available that people could see that animals and plants were made of cells. Scientists also observed single-celled organisms for the first time. As microscopes improved, biologists were able to see the even smaller structures inside cells and sophisticated biochemical techniques enabled them to work out what each part of the cell actually did.

All the maths you need To unlock the puzzles of this chapter you need the following maths:

• Units of measurement

• How to calculate magnification

• How to calculate surface area

• How to calculate volume

• How to calculate ratios

What have I studied before? You should already know from GCSE:

• Cells are the building blocks of all living organisms

• Living organisms’ metabolic processes, such as respiration, are carried out in their cells

• Cells are very small and can only be studied using a microscope

• All cells have common features such as:

○ a surface membrane that separates the cell’s interior from the external environment and regulates what goes into and out of a cell

○ a jelly like cytoplasm and a cytoskeleton

○ DNA that makes up the cell’s genetic content (genome)

○ ribosomes where proteins are assembled

• There are differences between plant, animal and bacterial cells

• Within a developing organism undifferentiated stem cells become differentiated and specialised to carry out certain specific functions – we all began life as one cell and we all develop many types of cells in our bodies

What will I study later? • In chapter 2.2 you will learn about the structure

and properties of key biological molecules, including the phospholipids, proteins and carbohydrates that make up the cell membranes

• In chapter 2.3 you will learn about enzymes, many of which catalyse chemical reactions inside cells

• In chapter 2.4 you will learn more about the structure and functions of cell membranes – the membranes around the outside of cells and the membranes around some of their internal organelles

• In chapter 2.5 you will learn about the properties of nucleic acids, DNA and RNA, which are found in cells

• In chapter 2.6 you will learn how cells reproduce and pass on their genetic material to their daughter cells

• In chapter 3.1 you will learn more about the tissues involved in exchanging substances between organisms and their environments

• In chapters 3.2 and 3.3 you will learn how substances needed for life are transported to all the cells of large multicellular plants and animals

• In chapter 4.1 you will learn about the roles of special cells involved in defence against infectious disease

What will I study in this chapter? • Microscopes, optical and electron, plus their

advantages and disadvantages (2.1.1)

• How slides and photomicrographs help us study cells (2.1.2)

• How electron micrographs have helped scientists to study cells (2.1.3)

• The ultrastructure of cells – the structure and functions of the smaller parts within cells (2.1.4)

• How organelles within cells work together, for example to make proteins (2.1.5)

• How cells become differentiated and specialised for particular functions, how they are organised into tissues and that tissues are organised into organs (2.1.6)

• More about the structure of prokaryotic cells and how they differ from eukaryotic cells (2.1.7)

You will also learn how to:

• make slides of cells to examine using an optical microscope

• correctly draw low-power plans and high-power drawings of prepared slides of tissues

• calculate the size of cells and organelles as seen in photomicrographs or electron micrographs

2 MO

DU

LE

CHAPTER

2.1 CELL STRUCTURE

28 2928 29

31

Cell structure 2.1

3030

Microscopes


∗ the use of microscopy to observe and investigate different types of cell and cell structure in a range of eukaryotic organisms

∗ the difference between magnification and resolution

KEY DEFINITION S

Magnification: the number of times larger an image appears, compared with the size of the object. Resolution: the clarity of an image; the higher the resolution, the clearer the image. Organelles: small structures within cells, each of which has a specific function. Cytology : the study of cell structure and function. Photomicrograph: photograph of an image seen using an optical microscope. Electron micrograph: photograph of an image seen using an electron microscope.

Magnifi cation Magnifi cation describes how much bigger an image appears compared with the original object. Microscopes produce linear magnifi cation, which means that if a specimen is seen magnifi ed ×100, it appears to be 100 times wider and 100 times longer than it really is.

Resolution Resolution is the ability of an optical instrument to see or produce an image that shows fi ne detail clearly. You may have a high-resolution television (called ‘ultra-high defi nition’ or UHD) and have noticed how clear and sharp the images on its screen are.

Optical microscopes The development of optical (light) microscopes played a key role in our understanding of cell structure. They were the fi rst sort to be used, and are still used in schools, colleges, hospitals and research laboratories because they are:

• relatively cheap

• easy to use

• portable and able to be used in the fi eld as well as in laboratories

• able to be used to study whole living specimens.

Present-day light microscopes look different from the ones used in the seventeenth century, but both types rely on lenses to focus a beam of light.

Notice that the image is laterally (sideways) inverted. This means that as you look down the microscope at the image and move your slide to your left, the image appears to move to the right.

D

eyepiecelens

image 1

objectivelens

objectimage 2

A The object is placed on a slide on the microscope stage.

B Light rays passing through the object are bent and focused by the objective lens, to form a real laterally inverted image, image 1.

C Light rays from the real image (image 1) are bent by the eyepiece lens.

D Diverging light rays entering the eye are perceived as coming from a magnified, laterally inverted, virtual image, image 2.

C

BA

Figure 2 Diagram showing how the compound optical microscope produces a magnifi ed image.

LEARNING TIP

You do not need to learn how an image is formed by an optical microscope. The information is given here so that you can understand why, when you move a slide one way, the image moves in the opposite direction, and also to help you appreciate why total magnification is the magnifying power of the objective lens × the magnifying power of the eyepiece lens.

Objective lens magnifi cation

Eyepiece lens magnifi cation


×4 ×10 ×40

×10 ×10 ×100

×40 ×10 ×400

×4 ×15 ×60

×10 ×15 ×150

×40 ×15 ×600


×10 ×1000


×15 ×1500

Table 1 The magnifi cations produced by most optical microscopes.

A photograph of the image seen using an optical microscope is called a photomicrograph . You will see an example of one in this chapter. Modern digital microscopes display the image on a computer screen.

2.1 1

Optical microscopes allow magnifi cation up to ×1500, or in some types ×2000, which enables us to see clearly some of the larger structures inside cells. However, because their resolution is limited, they cannot magnify any higher while still giving a clear image. See Table 1 for more details of the magnifi cations produced by most optical microscopes.

• Optical microscopes use visible light, a part of the electromagnetic spectrum that has a wavelength of between 400 and 700 nm.

• The light waves cannot pass between small particles that are less than 200 nm apart, so such objects within a cell will appear blurred.

• Ribosomes are very small, non-membrane-bound, cell organelles of about 20 nm diameter, and so they cannot be examined using a light microscope.

Arm

2. By rotating the nosepiece,the lowest power (smallest)objective lens is placedover the specimen.

3. Adjust thecoarse focus knob,while lookinginto theeyepiece, untilthe image yousee is clearand in focus.

Fine focus knob(see step 5)

6. Repeat step 5. using the ×40 objective lens.

1. The specimen on a slide isplaced here on the stage andclipped into place.

CondenserLight source

4. Whilst viewingthe image adjustthe iris diaphragmfor optimum light.

5. Make sure that theobject you wish toview is directly overthe hole in the stage.Now rotate thenosepiece and bringthe ×10 objective intoplace over the specimen. Look down the oculartube and use thefine focus knob tofocus the image.

Figure 1 Annotated diagram showing how to use a light microscope. Note that when you carry a microscope you should hold it by its arm in one of your hands, whilst having your other hand under the base of the microscope.

You may have used microscopes with just one eyepiece, but they work in the same way as the one shown in Figure 1. For people such as cytology screeners, who spend a lot of time staring down a microscope, the type shown here produces less eye-strain.

You can see from Figure 2 that the objective lens produces the fi rst (real) magnifi ed image, and the eyepiece lens(es) magnify this image to give an even larger (virtual) image. Therefore:

total magnifi cation = magnifying power of the objective lens × magnifying power of the eyepiece lens

Laser scanning microscopes Laser scanning microscopes are also called confocal microscopes (see Figure 3).

• They use laser light to scan an object point by point and assemble, by computer, the pixel information into one image, displayed on a computer screen.

• The laser beams are focused by lenses and a concave mirror.

• The images are high resolution and show high contrast.

• These microscopes have depth selectivity and can focus on structures at different depths within a specimen. Such microscopy can therefore be used to clearly observe whole living specimens, as well as cells.

• They are used in the medical profession, for example to observe fungal fi laments within the cornea of the eye of a patient with a fungal corneal infection, in order to give a swift diagnosis and earlier, and therefore more effective, treatment.

• They are also used in many branches of biological research.

(a) (b)

Figure 3 (a) A laser scanning microscope; (b) cells in the retina of the eye as seen with a laser scanning microscope.

Electron microscopes Electron microscopes use a beam of fast-travelling electrons that has a wavelength of about 0.004 nm. This means that they have much greater resolution than optical microscopes and can be used to give clear and highly magnifi ed images.

• The electrons are fi red from a cathode and focused, by magnets rather than glass lenses, on to a screen or photographic plate.

• The faster electrons travel, the shorter is their wavelength.

• Fast-travelling electrons have a wavelength about 125 000 times smaller than that of the central part of the visible light spectrum. Therefore, they can pass between cell structures that are very close together.

Transmission electron microscopes These were the fi rst type, developed during the 1930s.

• The specimen has to be chemically fi xed by being dehydrated and stained. It is then mounted on a small copper grid and placed in a vacuum chamber because air molecules can defl ect the beam of electrons.

• The beam of electrons passes through the specimen, which is stained with metal salts. Some electrons pass through and are focused on the screen or photographic plate.

32 33

2.1Cell structure 2.11

• The electrons form a 2D black-and-white (grey-scale) image. When photographed this is called an electron micrograph . Transmission electron microscopes can produce a magnifi cation of up to 2 million times, and a new generation is being developed that can magnify up to 50 million times.

Figure 4 A scanning electron microscope.

Scanning electron microscopes (Figure 4)

These were developed during the 1960s. Electrons do not pass through the specimen, which is whole, but cause secondary electrons to ‘bounce off ’ the specimen’s surface and be focused on to a screen. This gives a 3D image with a magnifi cation from ×15 up to ×200 000. The image is black and white, but computer software programmes can add false colour. However, the specimen still has to be placed in a vacuum and is often coated with a fi ne fi lm of metal (see Figure 5).

Figure 5 Scanning electron micrograph of an ant grasping a microprocessor (×12).

Figure 6 False-colour electron micrograph of blood cells. Erythrocytes are coloured red, lymphocytes blue and platelets yellow (×4000).

Both types of electron microscope:

• are large and very expensive

• need a great deal of skill and training to use.

Specimens, even whole ones for use in SEMs, have to be dead, as they are viewed while in a vacuum. The metallic salt stains used for staining specimens may be potentially hazardous to the user.

Range of objects seen with and without microscopes The eye, and optical and electron microscopes, are all optical instruments. Figure 7 shows the sizes of some objects that biologists may study, using these instruments. Note that the scale is logarithmic – it goes up in steps, where each is a 10-fold increase. See Table 2 for the units of measurement that biologists use.

Ele

ctro

n m

icro

scop

e

Ligh

t mic

rosc

ope

Human cheek cells

0.1 nm

1 nm

10 nm

100 nm

1 µm

10 µm

100 µ m

1 mm

10 mm

100 mm

0.1 m

Lipids

Ribosome Protein

Influenza virus

Mitochondrion Chloroplast Bacterium

Human ovum

Hen’s egg

Onion epidermis cell

Hum

an e

ye

Atom

Amoeba

Figure 7 Relative sizes of some biological structures on a logarithmic scale, showing the scope of the electron microscope, light microscope and human eye for studying them.

Unit Symbol Equivalent in metres Fractions of a metre

metre m 1 one

centimetre cm 10 –2 one-hundredth

millimetre mm 10 –3 one-thousandth

micrometre mm 10 –6 one-millionth

nanometre nm 10 –9 one-billionth

Table 2 Units of measurement that biologists use.

Bear in mind that there are 1000 mm in a metre. You can visualise this just by looking at a metre ruler and the mm scale on it. Now imagine one millimetre divided into 1000 parts; each of those is a micrometre. Now try to imagine a micrometre divided into 1000 parts; each of those is 1 nanometre.

DID YOU KNOW?

Your eye can distinguish objects that are about 0.3–0.5 mm apart. This is the limit of its resolution, but it gives you quite good visual acuity for ‘everyday’ objects. In the retina, at the back of the eye, are photosensitive cells called cones that work in bright light and produce this visual acuity (sharpness). You have about 200 000 cones per mm 2 . Eagles and hawks have many more cones in their retinas, around 1 million per mm 2 , and therefore have greater resolution and visual acuity. When you see a hawk hovering 20 m high over a roadside grass verge, it can clearly see an insect scurrying amongst that vegetation. An eagle can spot a rabbit 2 miles away and, although it is much smaller than you, its eyes are about the same size as yours.

LEARNING TIP

You need to really get to grips with the units mm, mm and nm and be able to convert one to the other. In the next topic you will carry out some maths exercises that require you to use and convert these units.

Questions

1 If you were to examine a slide of a protoctist, using a ×40 objective lens and a ×15 eyepiece lens, what would be the total magnifi cation of the protoctist?

2 List or make a table to show the advantages and disadvantages of optical microscopes.

3 Suggest the most useful type of microscope to observe each of the following:

(a) living water-fl eas in pond water during a biology fi eld trip

(b) cells taken from a cervical smear to be examined for abnormalities that may indicate cancer

(c) virus particles

(d) the inner structure of a mitochondrion

(e) the ribosomes in a liver cell.

4 List or make a table to show the advantages and disadvantages of electron microscopes.

5 The wavelength of red light is 700 nm. How many times larger is this than the wavelength of electrons?

6 What is a logarithmic scale? Why do you think it is used for comparing sizes of biological structures?

34 35

Cell structure 2.1

KEY DEFINITIONS

Transverse section : section cut crossways. Longitudinal section : section cut lengthways. Low-power plan : drawing showing distribution of cells but no individual cells shown. High-power drawing : drawing showing detail of some individual cells.

Making slides You can use an optical microscope to view a wide range of specimens including:

• living organisms such as Paramecium and Amoeba

• smear preparations of human blood- and cheek-cells

• thin-sections of animal, plant and fungal tissue, such as bone, muscle, leaf, root or fungal hyphae.

Observing unstained specimens Many biological structures, including single-celled organisms such as Paramecium , are colourless and transparent. Some microscopes use light interference, rather than light absorption, in order to produce a clear image without staining. Some use a dark background against which the illuminated specimen shows up. These microscopes are particularly useful for studying living specimens. You can observe living specimens with a school light microscope by adjusting the iris diaphragm to reduce the illumination of the specimen.

Staining specimens Stains are coloured chemicals that bind to chemicals in or on the specimen, making the specimen easy to see. Methylene blue is an all-purpose stain. Some stains bind to specifi c cell structures:

• acetic orcein binds to DNA and stains chromosomes dark red

• eosin stains cytoplasm; Sudan red stains lipids

• iodine in potassium iodide solution stains the cellulose in plant cell-walls yellow, and starch granules blue/black (these will look violet under the microscope).

Slides and photomicrographs



∗ the use of staining in light microscopy


∗ the representation of cell structure as seen under the light microscope, using drawings and annotated diagrams of whole cells or cells in sections of tissue

2 2.1

INVESTIGATION

Making slides of onion epidermis You can make temporary slides, such as of onion epidermis, and stain them with iodine solution (see Figure 1).

4. Use a piece of paper tissue to mop up any excess stain that is outside of the coverslip. You can now observe your specimen under the microscope.

1. Place a drop of iodine solution on to a clean microscope slide.

2. Place a thin piece of onion epidermis onto the stain. Make sure it stays flat. You may need to add another drop of iodine solution.

3. Use a mounted needle to carefully lower a coverslip onto the specimen. Lower the coverslip SLOWLY, otherwise you will get air bubbles in the solution on your slide.

Figure 1 The main steps in preparing a microscope slide.

Making a smear of cheek cells

• To obtain the cheek cells, simply rub a clean cotton bud gently against the inside of your cheek (in your mouth) and then smear the cotton bud on to the central part of a microscope slide.

• Dispose of the cotton bud into a pot of bleach solution.

• Allow the smear to dry on the slide, add a drop of methylene blue and, after 1 minute, rinse off.

• Dry and then add a drop of distilled water and a coverslip.

Observing prepared specimens You will have access to many prepared and permanently fi xed slides. They have been made by experts in a laboratory by:

• dehydrating the specimens

• embedding them in wax to prevent distortion during slicing

• using a special instrument to make very thin slices called sections – these are stained and mounted in a special chemical to preserve them.

INVESTIGATION

Making drawings of slides You need to be able to make clear, labelled drawings of specimens you examine under the light microscope. For success, follow some simple rules: 1. Use a prepared slide such as a transverse section through a dicot

leaf. Set it up on the microscope, following the advice in topic 2.1.1. Focus the specimen under low power.

2. Use a sharp HB pencil. 3. Use a title that explains exactly what the drawing is and the

magnification used. 4. Indicate the scale – i.e. how much bigger your drawing is than the

size of the image. 5. Make a low-power plan of the specimen to show where the

different tissue areas are, and do not draw any individual cells. Use clear unbroken lines and do not shade any areas.

6. Label the areas shown on the low-power plan. 7. Indicate on the plan a portion of the tissues that you will include in

a high-power drawing. 8. Make sure that this area of the specimen on the slide is directly

over the hole in the microscope stage. 9. Turn the nosepiece and bring the bigger objective lens into place

over it. Make sure that it fully clicks into place. 10. Use the fine-focus knob to bring the specimen into sharp focus. 11. Make a separate drawing of two or three cells from each region that

you highlighted in step 5. Draw clear unbroken lines and do not shade.

12. Label as many structures as you can see and identify. Use a ruler to draw the label lines and make sure that each label points exactly to the structure identified.

Figure 2 (a) A picture (photomicrograph) of a section through a dicot leaf (×100). (b) A low-power plan of the TS leaf slide and (c) a high-power drawing of some cells, from the photomicrograph of a leaf in transverse section shown in part (a).

(a)

xylem

L.P. Plan 3 100 scale 3 1upper epidermis

(b)

palisade mesophyll

spongy mesophyll

lower epidermis

vascularbundle

cambium

phloem

H.P. Drawing 3 100 scale 3 2

palisade mesophyll cell

spongy mesophyll cell

air space

(c)

cellulose cell wall

upper epidermal cell

lower epidermal cell

DID YOU KNOW?

How to observe single-celled organisms moving If you want to observe living single-celled organisms in pond water, you can use a cavity slide. This allows a larger drop of water to be placed on the slide. If you want to slow the organisms down so you can see how they move, add some methyl cellulose to the pond water. This increases the viscosity of the water. For the organisms, it is rather like them having to swim in treacle.

36 37

22.1Cell structure 2.1Cell structureCell structureCell structureCell structureCell structure 2.12.12.12.1Cell structureCell structure

Measuring objects seen with a light microscope




∗ photomicrographs of cellular components in a range of eukaryotic cells

KEY DEFINITIONS

A micrometre (mm) is equal to one-millionth (10 –6 ) of a metre. It is the standard unit for measuring cell dimensions. A nanometre (nm) is one-thousandth (10 –3 ) of a micrometre. It is therefore one-thousand-millionth (10 –9 ) of a metre. It is a useful unit for measuring the sizes of organelles within cells and the size of large molecules. A stage graticule is a precise measuring device. It is a small scale that is placed on a microscope stage and used to calibrate the value of eyepiece divisions at different magnifications. An eyepiece graticule is a measuring device. It is placed in the eyepiece of a microscope and acts as a ruler when you view an object under the microscope. However, the divisions appear larger under higher magnification, and so they have a different real length value at each magnification. A stage graticule is used to calibrate the eyepiece graticule and find the real value of each division at those magnifications.

Using graticules • A microscope eyepiece can be fi tted with a graticule.

• This graticule is transparent with a small ruler etched on it.

• As the specimen is viewed, the eyepiece-graticule scale is superimposed on it and the dimensions of the specimen can be measured (just as you can measure a large object by placing a ruler against it) in eyepiece units (epu) (Figure 1).

The scale of the eyepiece graticule is arbitrary – it represents different lengths at different magnifi cations. The image of the specimen looks bigger at higher magnifi cations, but the actual specimen has not increased in size. The eyepiece scale has to be calibrated (its value worked out) for each different objective lens. See Table 1 for values of eyepiece divisions at different magnifi cations.

A stage graticule is used only to calibrate the eyepiece graticule.

0

0 10

(a) (b)

1 2 3 4eyepiecegraticule

stagegraticule

5 6 7 8 910 0 1 2 3 4 5 6 7 8 910

Figure 1 Eyepiece graticule and stage graticule at (a) ×40 magnifi cation, and (b) ×100 magnifi cation.

2.1 3

INVESTIGATION

Using a stage graticule to calibrate the eyepiece graticule A microscopic ruler on a special slide, called a stage graticule, is placed on the microscope stage. This ruler is 1 mm long and divided into 100 divisions. Each division is 0.01 mm or 10 mm ( micrometres ). 1. Insert an eyepiece graticule into the ×10 eyepiece of your

microscope. This ruler has a total of 100 divisions. 2. Place a stage graticule on the microscope stage and bring it into focus,

using the low-power (×4) objective. Total magnification is now ×40. 3. Align the eyepiece graticule and the stage graticule as shown

in Figure 1(a). Check the value of one eyepiece division at this magnification on your microscope.

4. In the example shown here, the stage graticule (which is 1 mm or 1000 mm) corresponds with 40 eyepiece divisions.

5. Therefore each eyepiece division = 1000

_____ 40

mm = 25 mm.

6. Now use the ×10 objective lens on your microscope (total magnification is ×100) and focus on the stage graticule.

7. Align them both as shown in Figure 1(b). 8. In the example shown here, 100 eyepiece divisions now correspond

with 1 mm or 1000 mm.

9. Therefore one eyepiece division = 1000

_____ 100

mm = 10 mm.

10. Now use the ×40 objective lens, giving a total magnification of ×400, and repeat steps 6 and 7. You should find that each eyepiece division corresponds with 2.5 mm.

11. If you have a ×100 objective lens, repeat steps 6 and 7. One eyepiece division should equal 1 mm.

12. You can write down the eyepiece division values at each magnification for your microscope on to a label and stick this to the base of the microscope for future use.

Magnifi cation of eyepiece lens

Magnifi cation of objective lens


Value of one eyepiece division (epu)/mm

×10 ×4 ×40 25

×10 ×10 ×100 10

×10 ×40 ×400 2.5

×10 ×100 (oil-immersion lens)

×1000 1.0

Table 1 Values of eyepiece divisions at different magnifi cations for most modern microscopes used in schools.

Calculations involving magnification On the photomicrograph in Figure 3 (showing a leaf in transverse section), because you know the magnifi cation, you can then fi nd the actual size of the structures.

• Measure the thickness of the leaf, in mm, in its widest part.

• Convert that measurement to mm by multiplying by 1000.

• Now divide this fi gure by the magnifi cation. This tells you the actual thickness of the leaf at this point.

If you are told the actual size of a structure on a photomicrograph ( A ), and you measure its image size on the photomicrograph ( I ), in mm [mm × 1000], you can calculate the magnifi cation factor ( M ) using the formula:

M = I __

A

There are no units for magnifi cation but if, for example, the magnifi cation factor is 1000, then you must write it as ×1000.

LEARNING TIPS

When you observe structures in a microscope section, bear in mind the following: • cells have a 3D structure and you are looking at a 2D section

• depending on whereabouts in the cell the section was cut, some structures may be absent from your slide section

• depending how certain structures were oriented in the cell, they will appear as different shapes. For example, mitochondria sliced lengthways (in longitudinal section ) would appear sausage-shaped, but if sliced transversely (crossways) will appear round and if sliced obliquely (slanting) will appear elliptical.

When drawing your specimens, always draw what you see, not what you think the specimen should look like as remembered from a textbook diagram.

INVESTIGATION

An easy way to make a slide of a fungusThere is a very easy way to make temporary slides of some specimens, such as fungi. If you grow some mould fungi, such as Penicillium or Mucor (see Figure 3), on an agar plate, you can simply place a piece of SellotapeTM lightly on to the surface of the mould growth, then place this same piece of Sellotape, face-down, on to a slide. No need for a coverslip or any liquid, just look at it under the microscope and see the hyphae. If you try this, then make sure that you place used slides into a pot of bleach or disinfectant and wash your hands after the practical, as you should always do.

Figure 3 Bread mould, Mucor mucedo , seen under a light microscope (×40). Note the thread-like hyphae and the round fruiting bodies.

Questions

1 Write an equation, using the symbols A for actual size, I for image size and M for magnifi cation, in order to show how you can calculate the actual size of a structure on a photomicrograph.

2 If a nucleus diameter measures 10 mm on a photomicrograph with a magnifi cation of ×1000, what is the actual diameter of this nucleus?

3 On a photomicrograph of a human blood smear, an erythrocyte has a diameter of 3.2 mm. Erythrocytes have an actual diameter of 8 mm. What is the magnifi cation of this photomicrograph?

4 Explain why biological specimens to be examined under a microscope may be stained.

5 Explain why the mitochondria shown in a photomicrograph of a liver cell will not all be the same shape.

6 Draw a diagram of a plant tap-root (for example, a carrot) and show where you would cut it to make (a) a longitudinal section, and (b) a transverse section.

38 39


INVESTIGATION

Measuring the size of onion epidermis cells Onion epidermis cells are quite large and so are easy to see. 1. Place a slide of onion epidermis cells (see topic 2.1.2 for how to make

such a slide) under the microscope and use the ×4 objective lens to bring these cells into focus.

2. Find two cells that you wish to measure and make sure that they are in the centre of your fi eld of view.

3. Now use the ×10 objective lens and bring these cells into focus.

4. Draw the two cells. Label the structures that you can see, such as cellulose cell wall, vacuole and nucleus.

5. Now use a higher magnifi cation and bring some cells into focus.

6. Using the eyepiece graticule in your microscope, measure the dimensions of those two cells. Measure the length and width of the cells; if possible, measure all of the following: the width of the cellulose cell walls; the length of the vacuoles; and the diameter of the nuclei. If you can see nucleoli inside the nucleus, then measure those as well.

Observing and measuring starch grains (amyloplasts) in potato tuber cells 1. Using a sharp knife, gently scrape a little material from the surface of a

peeled raw potato and place it on to a microscope slide. You need a very thin layer on the slide. In that material will be potato tuber cells.

2. Place two drops of iodine/potassium iodide (KI) solution onto them and carefully add a coverslip.

3. Examine this slide under the microscope. Use low power fi rst and then use high-power magnifi cation. The amyloplasts stained with iodine solution will appear violet in colour.

4. Measure the length and width of three amyloplasts.

WORKED EXAMPLE

If you are asked to measure a structure as it would appear with a light microscope with a graticule, then you will also be told the value of the divisions on the graticule. It is just like a ruler and allows you to measure the specimen.

Figure 2 shows a plant root in transverse section, seen under a light microscope. The smallest divisions on the graticule are 0.01 mm (10 mm) apart.

1. Use the graticule to measure the actual width of this root along the line AB.

Answer The length of the line AB is 59 small divisions of the graticule × 10 mm = 590 mm = 0.59 mm.

2. Calculate the magnifi cation of this image.

Answer Measure the length of the image on the page with a ruler. It is

approx. 59 mm = 59 000 mm. So the magnifi cation = 59 000

______ 590

= 100.

3. The eyepiece used in this microscope had a magnifi cation factor of ×10. What was the magnifi cation of the objective lens?

Answer If the total magnification is ×100 and the eyepiece magnification

is ×10, then the objective lens magnifi cation is 100

____ 10

= 10.

Figure 2 Transverse section of a plant root as seen under a light microscope and with a graticule in place.

LEARNING TIP

You have already seen the equation that A ctual size = I mage size/ M agnification. Therefore M agnification = I mage size/ A ctual size The IMA triangle shown in Figure 3 may help you to use and substitute in that equation.

I

M

Actual size 5Image size

Magnification

A Magnification 5Image sizeActual size

Figure 3

Questions

1 If a nucleus measures 100 mm on a diagram, with a magnifi cation of ×10 000, what is the actual size of the nucleus?

2 Draw up a table to show each of the following measurements in metres (m), millimetres (mm) and micrometres (mm): 5 mm, 0.3 m, 23 mm, 75 mm.

3 Express the following measurements in micrometres: (a) 5 cm, (b) 25 mm, and (c) 100 nm.

4 Express the following measurements in nanometres : (a) 0.5 mm, (b) 0.4 mm, and (c) 0.1 cm.

5 You want to examine and measure the sizes of some living, single-celled eukaryotic organisms in pond water.

(a) Describe how you would make the slide, and how you would illuminate the specimen under the microscope.

(b) Explain why it may be diffi cult to focus this specimen under the highest magnifi cation of your microscope (see topic 2.1.2 as well to help you answer this question).

A B

The ultrastructure of eukaryotic cells: membrane-bound organelles


∗ the ultrastructure of eukaryotic cells and the functions of the different cellular components

∗ the importance of the cytoskeleton

All animal, plant, fungal and protoctist cells are eukaryotic. This means that they have (see Figure 1):

• a nucleus surrounded by a nuclear envelope and containing DNA organised and wound into linear chromosomes

• an area inside the nucleus called the nucleolus, containing RNA, where chromosomes unwind; the nucleolus is also involved in making ribosomes

• jelly-like cytoplasm in which the organelles are suspended

• a cytoskeleton – a network of protein fi laments (actin or microtubules) within the cytoplasm that move organelles from place to place within the cell; allow some cells (amoebae and lymphocytes) to move; and allow contraction of muscle cells

• a plasma membrane (also called cell surface membrane or cytoplasmic membrane)

2.1 4

• membrane-bound organelles, other than the nucleus, such as mitochondria, the Golgi apparatus and endoplasmic reticulum

• small vesicles

• ribosomes, which are organelles without membranes, where proteins are assembled.

Organelles

Cells are the fundamental units or building blocks of all living organisms. You will see in topics 2.6.4 and 2.6.5 that cells become specialised to do particular jobs. Within every cell there are various organelles, each having specifi c functions. This provides a division of labour, which means that every cell can carry out its many functions effi ciently.

cellmembrane

nucleus

mitochondrion

rough endoplasmicreticulum

(a) (b)

smoothendoplasmicreticulum

Golgiapparatus

lysosome

nucleolus

nuclearenvelope

ribosome mitochondrionchloroplast

vacuole

nucleolus

roughendoplasmicreticulum


Golgiapparatus

cell wallcellmembrane

amyloplastcontainingstarch

ribosomes

nuclearenvelope

nucleus

Figure 1 Structure of (a) generalised animal cell and (b) generalised plant cell, showing structures visible with a transmission electron microscope.

40 41


Membrane-bound organelles Most of the organelles within eukaryotic cells are membrane bound, which means they are covered by a membrane (similar in structure to the plasma membrane or cell surface membrane); see topic 2.5.1. This keeps each organelle separate from the rest of the cell, so that it is a discrete compartment. Membrane-bound organelles are a feature of eukaryotic cells; prokaryotic cells do not have them.

Nucleus, nuclear envelope and nucleolus

Structure Function

• The nucleus is surrounded by a double membrane, called the nuclear envelope. There are pores in the nuclear envelope.

• The nucleolus does not have a membrane around it. It contains RNA.

• Chromatin is the genetic material, consisting of DNA wound around histone proteins. When the cell is not dividing, chromatin is spread out or extended. When the cell is about to divide, chromatin condenses and coils tightly into chromosomes (see topic 2.6.2) . These make up nearly all the organism’s genome (see Figure 2).

nucleus

Figure 2

• The nuclear envelope separates the contents of the nucleus from the rest of the cell.

• In some regions the outer and inner nuclear membranes fuse together. At these points some dissolved substances and ribosomes can pass through.

• The pores enable larger substances, such as messenger RNA (mRNA) to leave the nucleus (see topic 2.3.3). Substances, such as some steroid hormones, may enter the nucleus, from the cytoplasm, via these pores.

• The nucleolus is where ribosomes are made. • Chromosomes contain the organism’s genes.

In summary, the nucleus: • is the control centre of the cell • stores the organism’s genome • transmits genetic information • provides the instructions for protein synthesis.

Rough endoplasmic reticulum (RER)

Structure Function

Rough endoplasmic reticulum (RER) • This is a system of membranes, containing fl uid-fi lled cavities

(cisternae) that are continuous with the nuclear membrane. • It is coated with ribosomes.

• RER is the intracellular transport system: the cisternae form channels for transporting substances from one area of a cell to another.

• It provides a large surface area for ribosomes, which assemble amino acids into proteins (see topic 2.3.3). These proteins then actively pass through the membrane into the cisternae and are transported to the Golgi apparatus for modifi cation and packaging.

Electron microscopy has enabled scientists to ascertain the structure of these organelles by making and examining several sections through an organelle in order to build up a 3D picture of it. Biochemistry research has enabled scientists to fi nd the functions of each organelle.

Cell structure 2.1

Smooth endoplasmic reticulum (SER)

Structure Function

• This is a system of membranes, containing fl uid-fi lled cavities (cisternae) that are continuous with the nuclear membrane.

• There are no ribosomes on its surface (see Figure 3).

nuclearenvelope

cisternae


roughendoplasmicreticulum

ribosome

cisterna

Figure 3

• SER contains enzymes that catalyse reactions involved with lipid metabolism, such as: s synthesis of cholesterol s synthesis of lipids/phospholipids needed by the cell s synthesis of steroid hormones.

• It is involved with absorption, synthesis and transport of lipids (from the gut).

• It carries out detoxifi cation of drugs such as alcohol and medicines.

• In muscle cells, modifi ed SER, called sarcoplasmic reticulum, stores calcium ions and releases them for muscle contraction.

Golgi apparatus

Structure Function

This consists of a stack of membrane-bound fl attened sacs. Secretory vesicles bring materials to and from the Golgi apparatus (see Figure 4).

vesicles bringing materials to and from the Golgi apparatus

Figure 4

• Proteins are modifi ed for example by: s adding sugar molecules to make glycoproteins s adding lipid molecules to make lipoproteins s being folded into their 3D shape.

• The proteins are packaged into vesicles that are pinched off and then: s stored in the cell or s moved to the cell surface membrane, either to be

incorporated into the cell surface membrane, or exported outside the cell.

Lysosomes

Structure Function

• These are small bags, formed from the Golgi apparatus. Each is surrounded by a single membrane.

• They contain powerful hydrolytic (digestive) enzymes. • They are abundant in phagocytic cells such as neutrophils and

macrophages (types of white blood cell) that can ingest and digest invading pathogens such as bacteria.

• Lysosomes keep the powerful hydrolytic enzymes separate from the rest of the cell.

• These enzymes can digest bacteria. • Lysosomes can engulf old cell organelles, digest them and

return the digested components to the cell for reuse. • They are important in apoptosis (programmed cell death) – for

example, the breaking down of webs between digits in the feet and hands of developing human embryo.

• Before fertilisation, sperm release hydrolytic enzymes from a large lysosome to digest the barrier around the ovum and enable a sperm to penetrate.

42 43


LEARNING TIP

Sometimes the undulipodium of eukaryotic cells is called a flagellum (from the Latin word meaning ‘whip’). However, some bacteria (prokaryotes) also have a flagellum or flagella (more than one flagellum), and as the internal structure of the prokaryotic flagellum is different from that of the eukaryotic ‘flagellum’, the eukaryotic structure should really be called an undulipodium.

DID YOU KNOW?

Scientists have recently discovered that nearly all of our cells have at least one cilium. Cells lining the kidney tubules each have one, and these cilia monitor the flow of urine. Brain cells also have one, and these are important for enabling learning. Individuals with genetic diseases that prevent the formation of these cilia have learning difficulties.

LEARNING TIP

Remember that there is a level of organisation within an organism. Organelles are small structures inside cells. Do not confuse them with organs, which are large structures made of many cells and tissues.

Cell structure 2.1

Questions

1 Name one substance that passes out of pores in the nuclear envelope, to the cell cytoplasm.

2 Name one substance that passes into the nucleus via the pores in the nuclear envelope.

3 Describe the role of the nucleolus.

4 State three functions of the nucleus.

5 Name one type of human cell that does not contain a nucleus.

6 Plasma cells (a type of white blood-cell) produce antibodies during an immune response. Liver cells metabolise drugs, such as antibiotics. If you have a bacterial infection, which types of endoplasmic reticulum would you expect to be abundant in (a) your plasma cells, and (b) your liver cells? Explain your answer.

7 Suggest why hydrolytic enzymes within cells need to be inside a vesicle.

8 If you carried out a physical training programme, how and why would you expect the number of mitochondria in your muscle cells to change?

9 Use the Internet and fi nd out what peroxisomes are.

Vacuole

Structure Function

The vacuole is surrounded by a membrane called the tonoplast, and contains fl uid (see Figure 7).

cell sap insidevacuole

tonoplast membranesurrounds vacuole andseparates it from restof cell

Figure 7

• Only plant cells have a large permanent vacuole. • It is fi lled with water and solutes and maintains cell stability,

because when full it pushes against the cell wall, making the cell turgid.

• If all the plant cells are turgid then this helps to support the plant, especially in non-woody plants.

There is a practical involving the pigments in beetroot cell vacuoles in topic 2.5.5.

Cilia and undulipodia

Structure Function

• These are protrusions from the cell and are surrounded by the cell surface membrane.

• Each contains microtubules (see ‘Cytoskeleton’, in topic 2.1.5). • They are formed from centrioles (see ‘Centrosome and

centrioles’, in topic 2.1.5).

• The epithelial cells lining your airways each have many hundreds of cilia that beat and move the band of mucus.

• Nearly all cell types in the body have one cilium that acts as an antenna. It contains receptors and allows the cell to detect signals about its immediate environment.

• The only type of human cell to have an undulipodium (a longer cilium) is a spermatozoon. The undulipodium enables the spermatozoon to move.

Mitochondria (singular: mitochondrion)

Structure Function

• These may be spherical, rod-shaped or branched, and are 2–5 mm long.

• They are surrounded by two membranes with a fl uid-fi lled space between them. The inner membrane is highly folded into cristae.

• The inner part of the mitochondrion is a fl uid-fi lled matrix. Inside there are small loops of DNA and some RNA (see Figure 5).

cristae matrix

intermembranespace

outermembrane

innermembrane

Figure 5

• Mitochondria are the site of ATP (energy currency) production during aerobic respiration.

• They are self-replicating, so more can be made if the cell’s energy needs increase.

• They are abundant in cells where much metabolic activity takes place, for example in liver cells and at synapses between neurones where neurotransmitter is synthesised and released.

Chloroplasts

Structure Function

• These are large organelles, 4–10 mm long. • They are found only in plant cells and in some protoctists; they

are absent from animal cells and fungi. • They are surrounded by a double membrane or envelope.

The inner membrane is continuous with stacks of fl attened membrane sacs called thylakoids (resembling piles of plates), which contain chlorophyll. Each stack or pile of thylakoids is called a granum (plural: grana). The fl uid-fi lled matrix is called the stroma.

• Chloroplasts contain loops of DNA and starch grains (see Figure 6).

innermembrane

outer membrane stroma

granum

thylakoidsintergranallamallae

intermembranecompartment

Figure 6

• Chloroplasts are the site of photosynthesis. • The fi rst stage of photosynthesis, when light energy is trapped

by chlorophyll and used to make ATP, occurs in the grana. Water is also split to supply hydrogen ions.

• The second stage, when hydrogen reduces carbon dioxide, using energy from ATP, to make carbohydrates, occurs in the stroma. Chloroplasts are abundant in leaf cells, particularly the palisade mesophyll layer.

44 45

Cell structure 2.1

KEY DEFINITIONS

Synthesis : making of large structures or molecules from smaller ones. Hydrolysis : breaking down large molecules by addition of water and breaking bonds.

Ribosomes

Structure Function

• Small spherical organelles, about 20 nm in diameter. • Made of ribosomal RNA. • Made in the nucleolus, as two separate subunits, which pass

through the nuclear envelope into the cell cytoplasm and then can combine.

• Some remain free in the cytoplasm and some attach to the endoplasmic reticulum.

• The two subunits come together when proteins are to be made. Amino acids are joined together, in a specifi c sequence, at ribosomes, using instructions carried from genes in the nucleus, via mRNA.

• Ribosomes bound to the exterior of RER are mainly for synthesising proteins that will be exported outside the cell.

• Ribosomes that are free in the cytoplasm, either singly or in clusters, are primarily the site of assembly of proteins that will be used inside the cell.

Topic 2.3.3 outlines how proteins are made.

Centrosome and centrioles • The centrosome is a region of the cell, close to the nucleus.

It is the centriole-organising centre. • The centrioles consist of two bundles of microtubules at right

angles to each other. The microtubules are made of tubulin protein subunits, and are arranged to form a cylinder (see Figure 1).

nucleustwo centriolescentrosome

Figure 1 Centrioles.

Paramecium ., single-celled protoctist is covered with cilia, which it uses to move, and to waft food particles into its ‘mouth’.

• Before a cell divides, the spindle, made of threads of tubulin, forms from the centrioles.

• Chromosomes attach to the middle part of the spindle (see topic 2.6.2) and motor proteins walk along the tubulin threads, pulling the chromosomes to opposite ends of the cell.

Centrioles are involved in the formation of cilia (see topic 2.1.4): • Before the cilia form, the centrioles multiply and line up

beneath the cell surface membrane. • Microtubules then sprout outwards from each centriole, forming

a cilium. • If the sprouting extension is longer, then it is called an

undulipodium (previously called a fl agellum).

Organelles without membranes Ribosomes and the cytoskeleton, including centrioles, are not covered by membranes.

The tables below explain how the structures of organelles without a membrane help them to carry out their functions.

Other features of eukaryotic cells


∗ the ultrastructure of eukaryotic cells and the function of the different cellular components

∗ the importance of the cytoskeleton

5 2.1 Cytoskeleton

Structure Function

A network of protein structures within the cytoplasm. It consists of: • rod-like microfi laments made of subunits of the protein actin;

they are polymers of actin and each microfi lament is about 7 nm in diameter

• intermediate fi laments about 10 nm in diameter • straight, cylindrical microtubules, made of protein subunits

called tubulin; they are polymers of tubulin; about 18–30 nm in diameter (see Figure 2).

ph_bio/2.1.1.5/Fig1

Figure 2 SEM of part of the cytoskeleton in a cell (×51 400).

The protein microfi laments within the cytoplasm give support and mechanical strength, keep the cell’s shape stable and allow cell movement. Microtubules also provide shape and support to cells, and help substances and organelles to move through the cytoplasm within a cell. • They form the track along which motor proteins walk and drag

organelles from one part of the cell to another. • They form the spindle before a cell divides. These spindle

threads enable chromosomes to be moved within the cell. • Microtubules also make up the cilia, undulipodia and centrioles. Intermediate fi laments are made of a variety of proteins. They: • anchor the nucleus within the cytoplasm • extend between cells in some tissues, between special

junctions, enabling cell–cell signalling and allowing cells to adhere to a basement membrane, therefore stabilising tissues.

Myosin proteins attach and detach from actin fi laments, causing the actin fi lament to move and making muscle cells contract for movement. Kinesins and dyneins ‘walk’, in opposite directions to each other, along cell microtubules. They are key for: • active transport of proteins, RNA molecules, vesicles and other

organelles within the cytoplasm • forming the spindle before cell division • separating chromosomes during cell division • enabling cilia to move • enabling spermatozoa to move via their undulipodia.

The cytoskeletal motor proteins , myosins, kinesins and dyneins, are molecular motors. They are also enzymes and have a site that binds to and allows hydrolysis of ATP as their energy source (see Figure 3).

organelle

tail domain ofmotor protein

motor protein

microtubuletrack

feet

Figure 3 A motor protein walking along a microtubule track in a cell, dragging an organelle. The tail domain is the part attached to the organelle, which is its cargo. The feet of the motor protein attach to and detach from the microtubule, moving along the microtubule track. At each step, ATP is hydrolysed to provide energy.

ph_bio/2.1.1.5/Fig1

47

Cell structure 2.1

46

2.1 5

Cellulose cell wall

Structure Function

The cell wall of plants is on the outside of the cell surface membrane. It is made from bundles of cellulose fi bres. Cellulose is a polymer of beta glucose residues (see Figure 4).

Figure 4

There is more information on the structure of cellulose in Chapter 2.2.

Absent from animal cells, the cell wall is strong and can prevent plant cells from bursting when turgid (swollen).

The cell walls of plant cells: • provide strength and support • maintain the cell’s shape • contribute to the strength and support of the whole plant • are permeable and allow solutions (solute and solvent) to pass

through.

DID YOU KNOW?

There are many different types of kinesin. In the human genome there are 45 genes for kinesin proteins. Each type has a differently shaped tail domain so that it can attach to a different type of ‘cargo’ to move it within the cell.

LEARNING TIP

You will notice a recurring theme running through your whole A level biology course – that of structure and function and adaptation. All biological structures are well adapted to carry out their function(s). They are ‘fit for purpose’. If a structure was not well adapted, then it would not carry out its function well; the organism would be at a disadvantage and would not be able to compete with those that were better adapted. The species might eventually become extinct. Organisms that are well adapted are more likely to survive and reproduce, passing on the well-adapted characteristics, be they organelles, cells or organs, to their offspring. This is the essence of natural selection, one of the main theories as to how evolution occurs. (You will learn more about evolution by natural selection and the role of genetic drift in evolution later in your course – see Chapter 4.3.)

2.1 6 How organelles work together in cells


∗ the interrelationship between the organelles involved in the production and secretion of proteins

• Many copies of this mRNA are made and they pass out of the pores in the nuclear envelope to the ribosomes.

• At the ribosomes, the instructions are translated and insulin molecules are assembled.

• The insulin molecules pass into the cisternae of the rough endoplasmic reticulum (RER) and along these hollow sacs.

• Vesicles with insulin inside are pinched off from the RER and pass, via microtubules and motor proteins, to the Golgi apparatus.

• The vesicles fuse with the Golgi apparatus, where the insulin protein molecules may be modifi ed for release.

• Inside vesicles pinched off from the Golgi apparatus, these molecules pass to the plasma membrane.

• The vesicles and plasma membrane fuse, and the insulin is released to the outside of the cell.

This is a type of bulk transport called exocytosis, an active process for which energy is needed. (See topic 2.5.4 for more on exocytosis.)

DID YOU KNOW?

When can cells make proteins? Cells cannot make proteins when they are dividing. (See topic 2.6.1 on the cell cycle for more about this.) When the coded instructions in a gene (length of DNA) are transcribed and translated so that a protein is made, we say that the gene is being expressed. (See topic 2.3.3 for more on transcription and translation.) Transcription can only occur in the nucleus when the cell is not dividing or about to divide, because the DNA has to be in the form of chromatin, and therefore spread out or extended. If it is condensed and tightly wound into chromosomes, then the gene cannot unwind and be transcribed. So, when cells are dividing, genes are not being expressed, and products like insulin are not being made. However, when cells divide, the whole process only takes up to an hour or two, and then the cells can get back to making proteins. The spindle threads, needed for the process of cell division, are made of protein, but they are produced by the cell before the chromosomes condense and tightly coil.

LEARNING TIP

Proteins are made at ribosomes. They may pass into the RER to be taken to the Golgi apparatus for modification, but they are not actually made in the RER.

KEY DEFINITIONS

Hormone: chemical produced in glands and secreted directly into blood to travel to its target cells. Later broken down in the liver. Involved with communication and control. Gene: length of DNA that codes for one or more polypeptides or for a length of RNA . Transcription: formation of a length of messenger RNA, using one of the DNA strands of a gene as a template . Translation: the process by which a protein is made from the information contained in messenger RNA.

The hormone insulin is a small protein. Each molecule consists of two polypeptide strands. When needed, insulin is secreted by special cells, called beta cells, within patches of tissue in the pancreas called islets of Langerhans. Insulin does many jobs, but the one you are familiar with is regulating blood-glucose levels (see Figures 1 and 2).

How insulin regulates the blood-glucose level After you eat a meal, your blood-glucose level increases.

• This increase is detected by the beta cells in your pancreas and they begin to secrete insulin.

• The insulin passes straight into your bloodstream and travels around the body.

• Its molecular shape is complementary to receptors on certain types of cell, particularly liver and muscle cells. As the insulin molecules bind on to these receptors of their target cells, they cause the surface membranes of these cells to become more permeable to glucose.

• These cells take up extra glucose and convert it to glycogen for storage and later use.

• Other cells in the body may also take up more glucose and use it for respiration to release energy.

For this coordinated response to happen, many organelles within the beta cells of the pancreas work together.

Making and secreting insulin • The gene that has the coded instructions for insulin, housed on

chromatin in the nucleus, is transcribed into a length of RNA, called messenger RNA (mRNA).

Questions

1 Describe how the functions of ribosomes that are free in the cytoplasm differ from the functions of ribosomes that are attached to RER.

2 Name the monomeric units of the rod-like fi laments that occur in the cytoskeleton.

3 Name the monomeric units of microtubules that occur in the cytoskeleton.

4 What is the difference between centrosomes and centrioles?

5 Describe the functions of the cellulose cell wall found in plant cells.

6 On a dull day, the chloroplasts inside palisade leaf cells are moved up to near the surface, to absorb more light. By what mechanism do you think the chloroplasts are moved?

7 Describe the functions of intermediate fi laments that form part of the cytoskeleton.

48


49

1 mRNA copy of theinstructions (gene)for insulin is madein the nucleus.

2 mRNA leaves the nucleusthrough a nuclear pore.

3 mRNA attaches to a ribosome, in thiscase attached to endoplasmic reticulum.Ribosome reads the instructions toassemble the protein (insulin).

4 Insulin molecules are‘pinched off’ in vesiclesand travel towardsGolgi apparatus.

5 Vesicle fuseswith Golgiapparatus.

6 Golgi apparatus processes and packages insulin molecules ready for release.

7 Packaged insulin molecules are ‘pinched off’ invesicles from Golgi apparatus and move towardscell surface membrane.

8 Vesicle fuses withcell surface membrane.

9 Cell surface membraneopens to release insulinmolecules outside.

instructionsfor insulin

DNA molecule

nuclear envelope

Golgi apparatus

Figure 1 How insulin is made in a beta cell in an islet of Langerhans in the pancreas.

DID YOU KNOW?

You have probably heard of type 1 and type 2 diabetes. People with type 1 are dependent on insulin injections because their own immune system has damaged or destroyed their beta cells that make insulin. People with type 2 may be insulin resistant, sometimes due to poor diet or being overweight. Their bodies do not respond to insulin or may not produce enough insulin. However, a small number of people have diabetes from birth. This is called neonatal diabetes and is due to a gene mutation. These people do not make a properly functioning membrane channel – an ATP-dependent potassium-ion channel. This prevents potassium ions entering their beta cells and prevents insulin, that their cells can make, from being released. They can be treated with medication, sulfonylurea, to help release their insulin, rather than being given insulin injections.

Questions

Hint: You may need to refer back to information in topics 2.1.4 and 2.1.5 in order to answer these questions.

1 Why do you think beta cells of the islets of Langerhans in the pancreas contain many mitochondria?

2 Explain how the vesicles containing insulin, that pinch off from the Golgi apparatus, are moved to the plasma membrane of the beta cells.

3 At which stage on the diagram in Figure 1 does exocytosis occur?

4 The diagram in Figure 3 shows some of the events occurring in a liver cell that responds to insulin by taking up more glucose. Suggest how the vesicles containing glucose channels move towards the plasma membrane.

glucose channel

glucose channel

vesicle made of plasma membraneand containing more glucose channels

as vesicles fuse with plasma membrane, more glucose channelsare on the plasma membrane

vesicles move to plasmamembrane and fuse with it

Figure 3

Figure 2 Light micrograph of pancreas tissue (×50). The central region is an islet of Langerhans. The beta cells that produce insulin are in this patch of cells.

2.1 7 Prokaryotic cells


∗ the similarities and differences in the structure and ultrastructure of prokaryotic and eukaryotic cells

KEY DEFINITIONS

Bacteria: prokaryotic microorganisms. Binary fission: type of division found in prokaryotic cells and organelles such as chloroplasts and mitochondria. Symbiosis: relationship where two organisms coexist for their mutual benefit.

Comparing prokaryotic and eukaryotic cells Bacteria are microorganisms. They have prokaryotic cells (see Figure 1).

Their cells are similar to eukaryotic cells in that they have:

• a plasma membrane

• cytoplasm

• ribosomes for assembling amino acids into proteins

• DNA and RNA.

They are different from eukaryotic cells, as they:

• are much smaller

• have a much less well-developed cytoskeleton with no centrioles

• do not have a nucleus

• do not have membrane-bound organelles such as mitochondria, endoplasmic reticulum, chloroplasts or Golgi apparatus

• have a wall that is made of peptidoglycan and not cellulose

• have smaller ribosomes

• have naked DNA that is not wound around histone proteins but fl oats free in the cytoplasm, as a loop (not linear chromosomes).

Some prokaryotic cells also have:

• a protective waxy capsule surrounding their cell wall

• small loops of DNA, called plasmids, as well as the main large loop of DNA

• fl agella – long whip-like projections that enable them to move. The structure of these fl agella differs from that of eukaryotic undulipodia

• pili – smaller hair-like projections that enable the bacteria to adhere to host cells or to each other, and allow the passage of plasmid DNA from one cell to another.

cytoplasm pili – small, hair-likestructures thatallow bacteriato adhere to eachother or to host cells

plasmid – small loopof DNA

nucleoid – area within thecytoplasm where theDNA is positioned

ribosomeflagella – allow bacteria to move

cell surface membrane cell wall, madeof peptidoglycan

waxy, protectivecapsule

Figure 1 Many of the features of prokaryotic cells. Prokaryotic cells are usually between 1 and 5 mm long.

50 51


Some bacteria are rod-shaped (bacillus) (see Figure 2); some are round (coccus) (see Figure 3); some are comma shaped ( Vibrio ); and some are like corkscrews (spirochetes, Spirillum ). Some rarer types are square or star-shaped. Cyanobacteria can form fi laments.

Figure 2 SEM of rod-shaped (bacillus) bacteria (×5000).

Figure 3 False-colour SEM of Staphylococcus aureus bacteria being engulfed by a neutrophil (×30 000).

DID YOU KNOW?

Prokaryotic cells divide by binary fission and not by mitosis. They do not have linear chromosomes, so could not carry out mitosis. However, before they divide, their DNA is copied so that each new cell receives the large loop of DNA and any smaller plasmids.

LEARNING TIP

Prokaryotes (bacteria) do not have any membrane-bound organelles, but they do have organelles that are not covered by a membrane, such as ribosomes.

DID YOU KNOW?

Bacteria are microorganisms because they are very small. They are also prokaryotes because of their cell structure. However, not all microorganisms are prokaryotes. Yeast (which is a single-celled fungus) and amoebae have eukaryotic cells.

Viruses are microscopic but they do not have cells.

The origins of eukaryotic cells Prokaryotic cells have been present on Earth for about 3.5 billion years. Some bacteria have many infolded membranes with chlorophyll, and they can photosynthesise. These bacteria are about the same size as, and resemble, chloroplasts. Other bacteria have many infolded membranes for making ATP. They are about the same size as mitochondria and they resemble these organelles.

Fossil records indicate that eukaryotic cells evolved from prokaryotes 1.5–2 billion years ago when some prokaryote cells with infolded membranes (for making ATP, or containing chlorophyll) invaded, or were engulfed by, some other prokaryotes, but not digested. As the invaded prokaryote’s plasma membrane folded inwards around the invading cell, this produced the double membrane of what are now chloroplasts and mitochondria.

Both chloroplasts and mitochondria share characteristics with prokaryotic cells. They: • have small ribosomes • have loops of DNA • also contain RNA • can divide by binary fi ssion.

This theory as to how eukaryotic cells arose from prokaryotic cells is called the endosymbiont theory.

DID YOU KNOW?

We usually refer to prokaryotic cells as being single-celled microorganisms. However, bacteria can sense the presence of other bacteria and send out signals according to the density of their neighbours. Some bacteria form biofilms. In response to such chemical signals, gene expression in the bacteria can alter so that their metabolism alters. This phenomenon is called quorum sensing, and may have been an early step in the evolution of multicellular organisms.

LEARNING TIP

Viruses are not prokaryotes. Viruses do not have a cell structure; they consist of nucleic acid (DNA or RNA but not both) enclosed in a protein coat. Some also have a lipid envelope around them. They have no cytoplasm, membranes or organelles and are described as akaryotic.

DID YOU KNOW?

Some of the genes in mitochondria code for the enzymes and other proteins that mitochondria need. Because we rely on these organelles, some of the errors (mutations) in the mitochondrial genes may cause diseases in us, although others do not appear to harm us. All the mitochondria in your body came originally from your mother. This is because the mitochondria in the sperm (which are located in the body of the sperm, just above the tail) do not enter the ovum. The mitochondria in the cells of the developing embryo divide every time the embryonic cells divide, so that each new cell receives mitochondria. The sequencing of genes and any accumulated changes to them, in mitochondrial DNA, can be used for ancestry tracing, but only along the maternal line.

Questions

1 Make a list to show the similarities between prokaryotic and eukaryotic cells.

2 Make a table to show the differences between prokaryotic and eukaryotic cells.

3 Explain what is meant by the endosymbiont theory.

4 What features of chloroplast and mitochondrial structures support the endosymbiont theory of the origin of eukaryotic cells?

5 Explain why viruses are not classed as either prokaryotes or eukaryotes.

6 By what process do prokaryotic cells divide?

7 Name three diseases of humans caused by bacteria.

8 State four ways in which prokaryotes are useful to other life forms on Earth.

6.1


THINKING


Cell structure 2.1

52 53

2.11.1 2.62.2 3.12.3 3.22.4 3.3 4.22.5 4.1 4.3YOU ARE HERE

5352

CELL THEORY In 1839 two scientists developed cell theory that said all living organisms are made of cells. Since then we have learned a lot about prokaryotic and eukaryotic cells. Viruses have also been discovered, and they do not have cells.

Use your own knowledge, information in the text above and in the rest of Chapter 2.1, plus research using the internet, to answer these questions.

1 What are the main statements of cell theory?

2 Which statements in cell theory can be applied to viruses?

3 When Linnaeus developed his classifi cation system, all living things were classifi ed into one of two groups – Animalia and Plantae. How many kingdoms are there in the present day biological classifi cation system?

4 Explain the phrase ‘viruses are nature’s genetic engineers’.

5 Suggest why it is unlikely that viruses were the fi rst forms of biological entity on Earth.

6 By what process do you think rabies virus particles are moved along neurone axons to the brain or spinal cord?

7 Which organelles in the host cell are involved in replicating the virus particles?

8 Discuss, using your biological knowledge, whether you think viruses should be classifi ed as living or non-living. Do you think that ‘cell theory’ may need to be amended in the future? Give reasons for your answer.

Activity

The branch of biology dealing with DNA technology such as genetic profi ling and genetic modifi cation of living organisms relies on knowledge and understanding of bacteria and viruses, as well as knowledge of the structure of DNA.

(a) Investigate, using the internet, exactly how viruses can provide scientists working in this fi eld with certain chemicals that they need.

(b) Write an article or make a poster showing how viruses can be helpful to humans.

DID YOU KNOW?

Viruses outnumber all other biological entities put together. In just one teaspoon of seawater, for example, there are one million virus particles.Recently a virus large enough to be seen with the light microscope was unearthed from the Siberian permafrost, where it had lain dormant for 30 000 years. It was reactivated and its genome, of several thousand genes, was sequenced. It infects amoebae.


You will recall from the introduction to this topic of cells, in the opener pages, that cell theory was developed in 1839. At that time, all living things were classifi ed as either animals or plants. Bacteria and viruses had not yet been discovered.

However, in the late 1700s, Dr Edward Jenner, the man who developed the fi rst vaccine against smallpox, used the term ‘virus’ to describe the contents of pustules in people suffering from smallpox. However, he used it in its Latin meaning – poison.

By 1885 Louis Pasteur had observed small single-celled entities, that he called germs, which could cause certain diseases. In fact these were bacteria. Until that time people thought that miasmas, (bad air) spread such infections. Pasteur also used a very fi ne mesh to fi lter the spinal fl uid of rabbits infected with rabies. This mesh had very small holes that prevented bacteria from passing through. However, this fi ltered spinal fl uid, when injected into healthy rabbits, gave them rabies. He concluded that the infecting agent which caused rabies was smaller than bacteria.

We now know that rabies is caused by a virus belonging to a group of viruses called Lyssavirus . This type of virus is shaped rather like a bullet and contains a single strand of RNA. Its length is about 180 nm and its diameter is about 75 nm. It has a lipoprotein envelope and on the outside of this envelope are knob-like spikes of glycoprotein. Underneath the envelope is the protein coat. Rabies virus particles enter the neurones of mammals, including humans, by endocytosis. The glycoprotein spikes on their surfaces dock with complementary-shaped receptors on the host cell membranes and trick the cells into taking them in. Once inside a host cell, the virus uses the cell nucleus and organelles to make copies of itself. Virus particles move back along neurone axons to the brain and spinal cord. They also move to the salivary glands and can therefore be transmitted to another host in bites.

As is often the case, the advances in technology help scientifi c knowledge to advance. A relatively large virus, the tobacco mosaic virus was studied with a powerful light microscope in the 1890s. However, it was not until the advent of electron microscopes, in the 1930s, that smaller viruses could be seen and their structure discovered. In the 1950s the scientist, Rosalind Franklin, worked out the structure of the tobacco mosaic virus using X-ray crystallography. These virus particles are about 300 nm long and 18 nm wide. The protein units making up the virus coat are arranged in coils. Its genome is a single strand of RNA.

In general, viruses have the following properties: ● they are about one-hundredth the size of bacteria, and therefore most are too

small to be seen with a light microscope ● they do not have a cell structure – they have no cytoplasm, membranes or

organelles ● they have genetic material – either DNA or RNA, but not both ● their genome is enclosed in a protein coat made of capsomeres ● some have a lipid membrane, around the protein coat, derived from the mem-

brane of cells that they infect ● many also contain a few enzymes ● they can only reproduce when inside a living host cell ● they can remain dormant, in some cases for a very long time ● they are the most abundant types of biological entity, with many different types ● some types infect plant cells; some infect animal cells; some, known as bacte-

riophages, infect bacteria; some infect fungi; some infect protoctists; and some can infect other viruses

● they can be spread from host to host by direct contact (shaking hands), droplets (sneezing), body fluids, the faecal–oral route, and by insects

● although we associate them with diseases, some are useful as they help regulate other life forms, for example they can kill algal blooms. Some persistent (those that pass from generation to generation in their host) plant viruses can make their host drought-resistant or heat resistant. Species of grass that live in a geyser field in Yellowstone Park, USA, have symbiotic fungi that contain symbiotic viruses. If the viruses are removed, then the grasses cannot grow in this hot place

WILL CELL THEORY NEED TO BE MODIFIED ONE DAY?

Exam-style questions 2.1

5554

2.1

1. Which of the following organelles are found in both eukaryotic and prokaryotic cells? [1]

A. Chloroplasts

B. Golgi bodies

C. Mitochondria

D. Ribosomes [Total: 1]

2. Figure 1 shows the general structure of an animal cell. 1 3

4

2

Figure 1

Which row correctly identifi es the structures labelled in the diagram? [1]

Structure 1 Structure 2 Structure 3 Structure 4

A mitochondria Golgi rough

endoplasmic

reticulum

smooth

endoplasmic

reticulum

B centrioles mitochondria vesicles cytoskeleton

C mitochondria centrioles ribosomes endoplasmic

reticulum

D mitochondria centrosome ribosome Golgi

[Total: 1]

3. Read the following statements:

(i) The wavelength of visible light is about 12 × 10 5 times longer than the wavelength of an electron beam used in an electron microscope.

(ii) Scanning electron microscopes can be used to observe whole living specimens.

(iii) Laser scanning microscopes can focus on structures at different depths within a specimen.


A. (i) only

B. (i) and (ii) only



4. Which of the following is not a function of the rough endoplasmic reticulum? [1]

A. It is the site of protein synthesis.

B. It is an intracellular transport system.

C. It gives a large surface area for ribosomes.

D. It can channel newly synthesised proteins to the Golgi body.

[Total: 1]

5. Read the following statements.

(i) Cilia are formed from centrioles and each is surrounded by the plasma membrane.

(ii) Cilia contain microtubules that enable them to move.

(iii) Cilia are only found on epithelial cells lining the airways.


A. (i) and (ii) only

B. (ii) and (iii) only



6. Figure 2 shows a goblet cell from the epithelium (lining) of the stomach. Other cells in the stomach lining produce hydrochloric acid, and the pH inside a human stomach is between 1 and 2.

(a) The protein mucin is synthesised within the cell and secreted, in mucus, at the position marked Z.

(i) Place the appropriate letters in the correct order to show the passage of newly synthesised molecules of mucin as they are moved from the place where they were made to position Z. [2]

R

Z

S

T

U

V

W

X

Y

Figure 2

(ii) There are amino acids in the cell cytoplasm that may be used to make mucin. Describe precisely where in this cell the mucin molecules will be assembled from these amino acid monomers. Give a reason for your answer. [2]

(iii) By what process do mucin molecules pass out of this cell? [1]

(b) The structures labelled Z are extensions of the plasma membrane and are called microvilli. Suggest why this type of cell has microvilli. [2]

(c) Suggest why this cell has many of the structures labelled W inside it. [4]

(d) Why do you think mucus needs to be produced by the stomach? [1]

[Total: 12]

7. Some scientists wanted to study the structure and functions of chloroplasts (see Figure 3). They macerated some spinach leaves in a food blender, adding 2% sucrose solution and kept the mixture cold. They fi ltered the mixture to remove debris and then spun the mixture in a centrifuge, which increases the force of gravity and, after a short spin, the cell nuclei are pulled to the bottom of the tube, forming a sediment.

macerated spinach leaves

sediment containing nuclei supernatant

supernatant

10 minutes at600 g force

10 minutes at10 000 g force

sediment containing chloroplasts

chloroplasts re-suspended in ice-cold 2% sucrose solution

Figure 3

The supernatant liquid is then taken out of the tube, placed in another centrifuge tube and spun again at a higher speed. The chloroplasts were seen at the bottom of the tube as a green sediment.

After decanting off the supernatant liquid, the chloroplasts were resuspended in ice-cold 2% sucrose solution before being used for investigations.

(a) Suggest why the fi rst organelles to sediment out during centrifugation were the nuclei? [1]

(b) Suggest why chloroplasts were the second type of organelles to sediment out by centrifugation? [2]

(c) Explain why leaves were used as a source of chloroplasts. [1]

(d) Suggest why, prior to their use, the isolated chloroplasts were:

(i) suspended in 2% sucrose solution [1]

(ii) kept ice cold. [2]

(e) Name the substance that gives the chloroplasts their green colour. [1]

(f) Briefl y outline the function of chloroplasts. [2] [Total: 10]

8. The electron micrograph in Figure 4 shows some plant cells.

Figure 4

(a) Identify the structures labelled A–G. [7]

(b) The true diameter, across line WX, of that organelle is 10 mm.

(i) What is the magnifi cation of this electron micrograph? Show your working. [2]

(ii) Calculate the length of structure H, along the line PQ. Show your working. [2]

(iii) The organelle with diameter 10 µm is spherical. Calculate its volume. Express your answer to the nearest whole number. Show your working. [3]

(c) Explain why this electron-micrograph image is grey-scale (has no colour). [1]

(d) State two functions of structure D. [2][Total: 17]

D

F E

G

B

A

C

HX

W

P

Q

X

W

P

Q

Exam-style questions

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