Cambridge International AS & A Level Biology Coursebook PDF

Biology for Cambridge International AS & A Level COURSEBOOK Mary Jones, Richard Fosbery, Dennis Taylor & Jennifer Gregory Fifth edition Digital Access  Biology for Cambridge International AS & A Level...

Biology for Cambridge International AS & A Level COURSEBOOK Mary Jones, Richard Fosbery, Dennis Taylor & Jennifer Gregory Fifth edition Digital Access  Biology for Cambridge International AS & A Level COURSEBOOK Mary Jones, Richard Fosbery, Dennis Taylor & Jennifer Gregory University Printing House, Cambridge CB2 8BS, United Kingdom One Liberty Plaza, 20th Floor, New York, NY 10006, USA 477 Williamstown Road, Port Melbourne, VIC 3207, Australia 314–321, 3rd Floor, Plot 3, Splendor Forum, Jasola District Centre, New Delhi – 110025, India 79 Anson Road, #06–04/06, Singapore 079906 Cambridge University Press is part of the University of Cambridge. It furthers the University’s mission by disseminating knowledge in the pursuit of education, learning and research at the highest international levels of excellence. www.cambridge.org Information on this title: www.cambridge.org/9781108859028 © Cambridge University Press 2020 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First edition 2003 Second edition 2007 Third edition 2012 Fourth edition 2014 Fifth edition 2020 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 Printed in Dubai by Oriental Press A catalogue record for this publication is available from the British Library ISBN 978-1-108-85902-8 Coursebook Paperback with Digital Access (2 Years) ISBN 978-1-108-79651-4 Digital Coursebook (2 Years) ISBN 978-1-108-79653-8 Coursebook eBook Additional resources for this publication at www.cambridge.org/9781108859028 Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Information regarding prices, travel timetables, and other factual information given in this work is correct at the time of first printing but Cambridge University Press does not guarantee the accuracy of such information thereafter. NOTICE TO TEACHERS IN THE UK It is illegal to reproduce any part of this work in material form (including photocopying and electronic storage) except under the following circumstances: (i) where you are abiding by a licence granted to your school or institution by the Copyright Licensing Agency; (ii) where no such licence exists, or where you wish to exceed the terms of a licence, and you have gained the written permission of Cambridge University Press; (iii) where you are allowed to reproduce without permission under the provisions of Chapter 3 of the Copyright, Designs and Patents Act 1988, which covers, for example, the reproduction of short passages within certain types of educational anthology and reproduction for the purposes of setting examination questions. NOTICE TO TEACHERS Cambridge International copyright material in this publication is reproduced under licence and remains the intellectual property of Cambridge Assessment International Education. W Cambridge Assessment International Education bears no responsibility for the example answers to questions taken from its past question papers which are contained in this publication. Exam-style questions and sample answers have been written by the authors. In examinations, the way marks are awarded may be different. References to assessment and/or assessment preparation are the publisher’s interpretation of the syllabus requirements and may not fully reflect the approach of Cambridge Assessment International Education. Cambridge International recommends that teachers consider using a range of teaching and learning resources in preparing learners for assessment, based on their own professional judgement of their students’ needs.  Teachers play an important part in shaping futures. Our Dedicated Teacher Awards recognise the hard work that teachers put in every day. Thank you to everyone who nominated this year; we have been inspired and moved by all of your stories. Well done to all our nominees for your dedication to learning and for inspiring the next generation of thinkers, leaders and innovators. WINNER WINNER Ahmed Saya Sharon Kong Foong Abhinandan Bhattacharya Anthony Chelliah Candice Green Jimrey Buntas Dapin Cordoba School for A-Level, Sunway College, JBCN International School Oshiwara, Gateway College, St Augustine’s College, University of San Jose-Recoletos, Pakistan Malaysia India Sri Lanka Australia Philippines For more information about our dedicated teachers and their stories, go to dedicatedteacher.cambridge.org  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK Contents Introduction vii 4 Cell membranes and transport 4.1 The importance of membranes 98 How to use this series ix 4.2 Structure of membranes 98 How to use this book xi 4.3 Roles of the molecules found in membranes 101 1 Cell structure 4.4 Cell signalling 102 1.1 Cells are the basic units of life 3 4.5 Movement of substances across 1.2 Cell biology and microscopy 4 membranes 104 1.3 Plant and animal cells as seen with a light microscope 4 5 The mitotic cell cycle 1.4 Measuring size and calculating 5.1 Growth and reproduction 124 magnification 11 5.2 Chromosomes 125 1.5 Electron microscopy 14 5.3 The cell cycle 126 1.6 Plant and animal cells as seen with an 5.4 Mitosis 127 electron microscope 17 5.5 The role of telomeres 132 1.7 Bacteria 32 5.6 The role of stem cells 133 1.8 Comparing prokaryotic cells with 5.7 Cancers 134 eukaryotic cells 34 1.9 Viruses 34 6 Nucleic acids and protein synthesis 6.1 The molecule of life 144 2 Biological molecules 6.2 The structure of DNA and RNA 144 2.1 Biochemistry 45 6.3 DNA replication 149 2.2 The building blocks of life 45 6.4 The genetic code 150 2.3 Monomers, polymers and macromolecules 45 6.5 Protein synthesis 151 2.4 Carbohydrates 46 6.6 Gene mutations 153 2.5 Lipids 53 7 Transport in plants 2.6 Proteins 57 7.1 The transport needs of plants 163 2.7 Water 66 7.2 Vascular system: xylem and phloem 163 3 Enzymes 7.3 Structure of stems, roots and leaves and the distribution of xylem and phloem 164 3.1 What is an enzyme? 75 7.4 The transport of water 170 3.2 Mode of action of enzymes 76 7.5 Transport of assimilates 180 3.3 Investigating the progress of an enzyme-catalysed reaction 79 8 Transport in mammals 3.4 Factors that affect enzyme action 81 8.1 Transport systems in animals 194 3.5 Comparing enzyme affinities 84 8.2 The mammalian circulatory system 194 3.6 Enzyme inhibitors 85 8.3 Blood vessels 195 3.7 Immobilising enzymes 87 iv  Contents 8.4 Tissue fluid 200 13 Photosynthesis 8.5 Blood 202 13.1 An energy transfer process 332 8.6 The heart 209 13.2 Structure and function of chloroplasts 333 13.3 The light-dependent stage of 9 Gas exchange photosynthesis 337 9.1 Gas exchange 224 13.4 The light-independent stage of 9.2 Lungs 225 photosynthesis 339 9.3 Trachea, bronchi and bronchioles 226 13.5 Limiting factors in photosynthesis 340 9.4 Warming and cleaning the air 226 9.5 Alveoli 228 14 Homeostasis 14.1 Homeostasis 349 10 Infectious disease 14.2 The structure of the kidney 352 10.1 Infectious diseases 238 14.3 Control of water content 360 10.2 Antibiotics 253 14.4 The control of blood glucose 364 14.5 Homeostasis in plants 371 11 Immunity 11.1 Defence against disease 267 15 Control and coordination 11.2 Cells of the immune system 268 15.1 Hormonal communication 388 11.3 Active and passive immunity 277 15.2 Nervous communication 389 15.3 Muscle contraction 406 P1 Practical skills for AS Level 15.4 Control and coordination in plants 413 P1.1 Practical skills 292 P1.2 Experiments 292 16 Inheritance P1.3 Variables and making measurements 292 16.1 Gametes and reproduction 428 P1.4 Recording quantitative results 298 16.2 The production of genetic variation 433 P1.5 Displaying data 299 16.3 Genetics 435 P1.6 Making conclusions 301 16.4 Monohybrid inheritance and genetic P1.7 Describing data 301 diagrams 437 P1.8 Making calculations from data 301 16.5 Dihybrid inheritance 441 P1.9 Identifying sources of error and 16.6 The chi-squared (χ2) test 449 suggesting improvements 303 16.7 Genes, proteins and phenotype 451 P1.10 Drawings 304 16.8 Control of gene expression 453 12 Energy and respiration 17 Selection and evolution 12.1 The need for energy in living organisms 312 17.1 Variation 465 12.2 Aerobic respiration 313 17.2 Natural selection 469 12.3 Mitochondrial structure and function 319 17.3 Genetic drift and the founder effect 474 12.4 Respiration without oxygen 320 17.4 The Hardy–Weinberg principle 476 12.5 Respiratory substrates 322 17.5 Artificial selection 478 17.6 Evolution 482 17.7 Identifying evolutionary relationships 486 v  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK 18 Classification, biodiversity and P2 Practical skills for A Level conservation P2.1 Practical skills 582 18.1 Classification 497 P2.2 Planning an investigation 582 18.2 Biodiversity 507 P2.3 Constructing a hypothesis 582 18.3 Maintaining biodiversity 521 P2.4 Identifying variables 583 18.4 Protecting endangered species 524 P2.5 Describing the sequence of steps 586 18.5 Controlling alien species 530 P2.6 Risk assessment 586 18.6 International conservation P2.7 Recording and displaying results 587 organisations 531 P2.8 Analysis, conclusions and evaluation 587 P2.9 Evaluating evidence 600 19 Genetic technology P2.10 Conclusions and discussion 600 19.1 Genetic engineering 544 19.2 Tools for the gene technologist 545 Appendix 1: Amino acid R groups 608 19.3 Gene editing 552 19.4 Separating and amplifying DNA 555 Appendix 2: DNA and RNA triplet codes 609 19.5 Analysing and storing genetic information 560 Glossary 611 19.6 Genetic technology and medicine 564 Index 631 19.7 Genetic technology and agriculture 570 Acknowledgements 643 vi  Introduction Introduction This is the fifth edition of the Cambridge International AS & A Level Biology Coursebook, and it provides everything that you need to support your course for Cambridge AS & A Level Biology (9700). It provides full coverage of the syllabus for examinations from 2022 onwards. The chapters are arranged in the same sequence as the topics in the syllabus. Chapters 1 to P1 cover the AS material, and Chapters 12 to P2 cover the material needed for A Level. The various features that you will find in these chapters are explained on the next two pages. Many questions will test a deeper understanding of the facts and concepts that you will learn during your course. It is therefore not enough just to learn words and diagrams that you can repeat in your examinations; you need to ensure that you really understand each concept fully. Trying to answer the questions that you will find within each chapter, and at the end of each chapter, should help you to do this. Although you will study your biology as a series of different topics, it is very important to appreciate that all of these topics link up with each other. You need to make links between different areas of the syllabus to answer some questions. For example, you might be asked a question that involves bringing together knowledge about protein synthesis, infectious disease and transport in mammals. In particular, you will find that certain key concepts come up again and again. These include: Cells as units of life Biochemical processes DNA, the molecule of heredity Natural selection Organisms in their environment Observation and experiment. As you work through your course, make sure that you keep reflecting on the work that you did earlier and how it relates to the current topic that you are studying. Some of the reflection questions at the ends of the chapters suggest particular links that you could think about. They also ask you to think about how you learn, which may help you to make the very best use of your time and abilities as your course progresses. You can also use the self-evaluation checklists at the end of each chapter to decide how well you have understood each topic in the syllabus, and whether or not you need to do more work on each one. Practical skills are an important part of your biology course. You will develop these skills as you do experiments and other practical work related to the topics you are studying. Chapters P1 (for AS Level) and P2 (for A Level) explain what these skills are and what you need to be able to do. vii  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK You may like to look at two other books in this series – the Workbook and the Practical Workbook. The Workbook provides clear guidance on many of the skills that you need to develop as you work through the course – such as constructing and analysing graphs, and planning experiments – with exercises for you to try. The Practical Workbook is full of detailed explanations of how to carry out all the practicals required in the syllabus, and many others too, that will help you to become more confident in practical work. This is an exciting time to be studying biology, with new discoveries and technologies constantly finding their way into the news. We very much hope that you will enjoy your biology course, and that this book will help you not only to prepare for your examinations but also to develop a life-long interest in this subject. viii  How to use this series How to use this series This suite of resources supports students and teachers following the Cambridge International AS & A Level Biology syllabus (9700). All of the books in the series work together to help students develop the necessary knowledge and scientific skills required for this subject. With clear language and style, they are designed for international learners. The coursebook provides comprehensive support Biology for the full Cambridge International AS & A for Cambridge International AS & A Level Level Biology syllabus (9700). It clearly explains COURSEBOOK facts, concepts and practical techniques, and Mary Jones, Richard Fosbery, Dennis Taylor & Jennifer Gregory uses real-world examples of scientific principles. Two chapters provide full guidance to help students develop investigative skills. Questions within each chapter help them to develop their understanding, while exam-style questions provide essential practice. Fifth edition Digital Access The workbook contains over 100 exercises and exam-style questions, Biology carefully constructed to help learners for Cambridge International AS & A Level develop the skills that they need as they WORKBOOK progress through their Biology course. Mary Jones & Matthew Parkin The exercises also help students develop understanding of the meaning of various command words used in questions, and provide practice in responding appropriately to these. Second edition Digital Access ix  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK Biology for Cambridge International AS & A Level This write-in book provides students with a wealth PRACTICAL WORKBOOK of hands-on practical work, giving them full guidance Mary Jones & Matthew Parkin and support that will help them to develop all of the essential investigative skills. These skills include planning investigations, selecting and handling apparatus, creating hypotheses, recording and displaying results, and analysing and evaluating data. Second edition The teacher’s resource supports and enhances the questions and practical activities in the coursebook. This resource includes detailed lesson ideas, as well as answers and exemplar data for all questions and activities in the coursebook and workbook. The practical teacher’s guide, included with this resource, provides support for the practical activities and experiments in the practical workbook. Teaching notes for each topic area include a suggested teaching plan, ideas for active learning and formative assessment, links to resources, ideas for lesson starters and plenaries, differentiation, lists of common misconceptions and suggestions for homework activities. Answers are included for every question and exercise in the coursebook, workbook and practical workbook. Detailed support is provided for preparing and carrying out for all the investigations in the practical workbook, including tips for getting things to work well, and a set of sample results that can be used if students cannot do the experiment, or fail to Biology collect results. for Cambridge International AS & A Level Biology for Cambridge International AS & A Level DIGITAL TEACHER’S RESOURCE ACCESS CARD Digital Teacher’s Resource DO NOT Code inside is required to activate your DISCARD purchase of the Teacher’s Resource x  How to use this book How to use this book Throughout this book, you will notice lots of different features that will help your learning. These are explained below. LEARNING INTENTIONS KEY WORDS These set the scene for each chapter, help with navigation through the Key vocabulary coursebook and indicate the important concepts in each topic. is highlighted in the text when it is first introduced. BEFORE YOU START Definitions are then given in the margin, This contains questions and activities on subject knowledge you will need which explain the before starting this chapter. meanings of these words and phrases. You will also find SCIENCE IN CONTEXT definitions of these This feature presents real-world examples and applications of the content in words in the Glossary a chapter, encouraging you to look further into topics. There are discussion at the back of this questions at the end which look at some of the benefits and problems of these book. applications. COMMAND WORDS PRACTICAL ACTIVITY Command words This book does not contain detailed instructions for doing particular that appear in the experiments, but you will find background information about the practical work syllabus and might you need to do in these boxes. There are also two chapters, P1 and P2, which be used in exams are provide detailed information about the practical skills you need to develop highlighted in the during the course. exam-style questions when they are first introduced. In the margin, you will Questions find the Cambridge Appearing throughout the text, questions give you a chance to check that you have International understood the topic you have just read about. You can find the answers to these definition. You will questions in the digital version of the Coursebook. also find these definitions in the Glossary at the back of the book with some further explanation on the meaning of these words.* *The information in this section is taken from the Cambridge International syllabus (9700) for examination from 2022. You should always refer to the appropriate syllabus document for the year of your examination to confirm the details and for more information. The syllabus document is available on the Cambridge International website at www.cambridgeinternational.org. xi  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK WORKED EXAMPLE Wherever you need to know how to use a formula to carry out a calculation, there are worked examples boxes to show you how to do this. REFLECTION IMPORTANT These activities ask you to look back on the topics covered in the chapter and Important equations, test how well you understand these topics and encourage you to reflect on facts and tips are your learning. given in these boxes. EXAM-STYLE QUESTIONS Questions at the end of each chapter provide more demanding exam-style questions, some of which may require use of knowledge from previous chapters. Some questions are taken from past papers. Where this is the case, they include references to the relevant past paper. All other questions are written by the authors. Answers to these questions can be found in the digital version of the Coursebook. SUMMARY There is a summary of key points at the end of each chapter. SELF-EVALUATION The summary checklists are followed by ‘I can’ statements which match the Learning intentions at the beginning of the chapter. You might find it helpful to rate how confident you are for each of these statements when you are revising. You should revisit any topics that you rated ‘Needs more work’ or ‘Almost there’. See Needs Almost Ready to I can section more work there move on These boxes tell you where information in the book is extension content, and is not part of the syllabus. xii  Chapter 1 Cell structure LEARNING INTENTIONS In this chapter you will learn how to: explain that cells are the basic units of life use the units of measurement relevant to microscopy recognise the common structures found in cells as seen with a light microscope and outline their structures and functions compare the key structural features of animal and plant cells use a light microscope and make temporary preparations to observe cells recognise, draw and measure cell structures from temporary preparations and micrographs calculate magnifications of images and actual sizes of specimens using drawings or micrographs explain the use of the electron microscope to study cells with reference to the increased resolution of electron microscopes recognise the common structures found in cells as seen with an electron microscope and outline their structures and functions outline briefly the role of ATP in cells describe the structure of bacteria and compare the structure of prokaryotic cells with eukaryotic cells describe the structure of viruses.  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK BEFORE YOU START Make a list of structures that could be found in a cell. Try to write down the functions of the structures you have listed. Which structures are found in plant cells and which are found in animal cells? Are there any cells that are not animal or plant cells? THINKING OUTSIDE THE BOX Progress in science often depends on people thinking ‘outside the box’ – original thinkers who are often ignored or even ridiculed when they first put forward their radical new ideas. One such individual, who battled constantly throughout her career to get her ideas accepted, was the American biologist Lynn Margulis (1938–2011; Figure 1.1). Her greatest achievement was to use evidence from microbiology to help firmly establish an idea that had been around since the mid-19th century – that new organisms can be created from combinations of existing organisms. Importantly, the existing organisms are not necessarily closely related. The organisms form a symbiotic partnership (they live together in a partnership in which both partners benefit). Margulis imagined that one organism engulfed (‘ate’) another. Normally the engulfed organism would be digested and killed, Figure 1.1: Lynn Margulis: ‘My work more than didn’t fit but sometimes the organism engulfed may survive in. It crossed the boundaries that people had spent their and even be of benefit to the organism in which lives building up. It hits some 30 sub-fields of biology, it finds itself. This type of symbiosis is known as even geology.’ endosymbiosis (‘endo’ means inside). A completely new type of organism is created, representing a traditional view, first put forward by Charles Darwin, dramatic evolutionary change. that evolution occurs mainly as a result of competition between species. The best-known example of Margulis’ ideas is her suggestion that mitochondria and chloroplasts Questions for discussion were originally free-living bacteria (prokaryotes). Can you think of any ideas people have had She suggested that these bacteria invaded the which were controversial at the time but ancestors of modern eukaryotic cells, which are are now accepted? Try to think of scientific much larger and more complex cells than bacteria, examples. You may also like to consider why and entered into a symbiotic relationship with the ideas were controversial. the cells. This idea has been confirmed as true by later work. Margulis saw such symbiotic unions Can you think of any scientific ideas people as a major driving cause of evolutionary change. have now which are controversial and not Throughout her life, she continued to challenge the accepted by everybody? 2  1 Cell structure animals. It was soon also realised that all cells come from 1.1 Cells are the basic pre-existing cells by the process of cell division. This raises the obvious question of where the original cell units of life came from. There are many hypotheses, but we still have no definite answers to this question. Towards the middle of the 19th century, scientists made a fundamental breakthrough in our understanding of how life ‘works’. They realised that the basic unit of life is the cell. Why cells? The origins of this idea go back to the early days of A cell can be thought of as a bag in which the chemistry microscopy when an English scientist, Robert Hooke, of life occurs. The activity going on inside the cell is decided to examine thin slices of plant material. He therefore separated from the environment outside the cell. chose cork as one of his examples. Looking down the The bag, or cell, is surrounded by a thin membrane. The microscope, he made a drawing to show the regular membrane is an essential feature of all cells because it appearance of the structure, as you can see in Figure 1.2. controls exchange between the cell and its environment. In 1665 he published a book containing It can act as a barrier, but it can also control movement this drawing. of materials across the membrane in both directions. The membrane is therefore described as partially permeable. If it were freely permeable, life could not exist, because the chemicals of the cell would simply mix with the surrounding chemicals by diffusion and the inside of the cell would be the same as the outside. Two types of cell During the 20th century, scientists studying the cells of bacteria and of more complex organisms such as plants and animals began to realise that there were two fundamentally different kinds of cells. Some cells were very simple, but some were much larger and more complex. The complex cells contained a nucleus (plural: nuclei) surrounded by two membranes. The genetic material, DNA, was in the nucleus. In the simple cells the DNA was not surrounded by membranes, but apparently free in the cytoplasm. Figure 1.2: Drawing of cork cells published by Robert KEY WORDS Hooke in 1665. cell: the basic unit of all living organisms; it is surrounded by a cell surface membrane and If you examine the drawing you will see the regular contains genetic material (DNA) and cytoplasm structures that Hooke called ‘cells’. Each cell appeared containing organelles to be an empty box surrounded by a wall. Hooke had discovered and described, without realising it, the organelle: a functionally and structurally distinct fundamental unit of all living things. part of a cell, e.g. a ribosome or mitochondrion Although we now know that the cells of cork are dead, nucleus (plural: nuclei): a relatively large Hooke and other scientists made further observations of organelle found in eukaryotic cells, but absent cells in living materials. However, it was not until almost from prokaryotic cells; the nucleus contains the 200 years later that a general cell theory emerged from cell’s DNA and therefore controls the activities the work of two German scientists. In 1838 Schleiden, of the cell; it is surrounded by two membranes a botanist, suggested that all plants are made of cells. A which together form the nuclear envelope year later Schwann, a zoologist, suggested the same for 3  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK Organisms made of cells with membrane-bound are unfamiliar to most people. Before studying light and nuclei are now known as eukaryotes, while the simpler electron microscopy further, you need to become familiar cells lacking membrane-bound nuclei are known as with these units. prokaryotes (‘eu’ means true, ‘karyon’ means nucleus, According to international agreement, the International ‘pro’ means before). Eukaryotes are thought to have System of Units (SI units) should be used. In this system, evolved from prokaryotes more than two billion years the basic unit of length is the metre (symbol, m). More ago. Prokaryotes include bacteria. Eukaryotes include units are created by going a thousand times larger or animals, plants, fungi and some other organisms. smaller. Standard prefixes are used for the units. For example, the prefix ‘kilo’ means 1000 times. Thus, KEY WORDS 1 kilometre = 1000 metres. The units of length relevant to cell studies are shown in Table 1.1. eukaryote: an organism whose cells contain a nucleus and other membrane-bound organelles The smallest structure visible with the human eye is about 50–100 μm in diameter (roughly the diameter of prokaryote: an organism whose cells do not the sharp end of a pin). The cells in your body vary in contain a nucleus or any other membrane-bound size from about 5 μm to 40 μm. It is difficult to imagine organelles how small these cells are, especially when they are clearly visible using a microscope. An average bacterial cell is about 1 µm across. One of the smallest structures you will study in this book is the ribosome, which is only 1.2 Cell biology and about 25 nm in diameter! You could line up about 20 000 ribosomes across the full stop at the end of microscopy this sentence. The study of cells has given rise to an important branch of biology known as cell biology. Cell biologists study cells using many different methods, including the use of various types of microscope. 1.3 Plant and animal There are two fundamentally different types of cells as seen with a light microscope: the light microscope and the electron microscope. Both use a form of radiation in order to see the specimen being examined. The light microscope microscope uses light as a source of radiation, while the electron Microscopes that use light as a source of radiation are microscope uses electrons, for reasons which are called light microscopes. Figure 1.3 shows how the light discussed later. microscope works. Note: the structure of a light microscope is Units of measurement extension content, and is not part of the syllabus. In order to measure objects in the microscopic world, we need to use very small units of measurement, which Fraction of a metre Unit Symbol –3 one thousandth = 0.001 = 1/1000 = 10 millimetre mm –6 one millionth = 0.000 001 = 1/1000 000 = 10 micrometre μm one thousand millionth = 0.000 000 001 = nanometre nm 1/1000 000 000 = 10–9 Table 1.1: Units of measurement relevant to cell studies: 1 micrometre is a thousandth of a millimetre; 1 nanometre is a thousandth of a micrometre. 4  1 Cell structure Eyepiece lens magnifies and showing the structure of a generalised plant cell, both eyepiece focuses the image from the as seen with a light microscope. (A generalised cell objective onto the eye. shows all the structures that may commonly be found in a cell.) Figures 1.6 and 1.7 are photomicrographs. light beam A photomicrograph is a photograph of a specimen as seen with a light microscope. Figure 1.6 shows some human cells. Figure 1.7 shows a plant cell taken from a leaf. Both figures show cells magnified 400 times, which objective Objective lens collects light is equivalent to using the high-power objective lens on coverslip passing through the specimen a light microscope. See also Figures 1.8a and 1.8b for and produces a magnified image. labelled drawings of these figures. glass slide Many of the cell contents are colourless and transparent Condenser lens focuses the so they need to be stained with coloured dyes to be seen. condenser light onto the specimen held The human cells in Figure 1.6 have been stained. The between the coverslip and slide. chromatin in the nuclei is particularly heavily stained. iris diaphragm The plant cells in Figure 1.5 have not been stained light source Condenser iris diaphragm is because the chloroplasts contain the green pigment closed slightly to produce a pathway of light narrow beam of light. chlorophyll and are easily visible without staining. Figure 1.3: How the light microscope works. The coverslip is a thin sheet of glass used to cover the specimen. It protects specimens from drying out and also prevents the objective Question lens from touching the specimen. 1 Using Figures 1.4 and 1.5, name the structures that: a animal and plant cells have in common small structures that b are found only in plant cells Golgi apparatus are difficult to identify c are found only in animal cells. cytoplasm mitochondria Features that animal and plant cells have in common Cell surface membrane cell surface All cells, including those of both eukaryotes and membrane prokaryotes, are surrounded by a very thin cell surface membrane. This is also sometimes referred to as the plasma membrane. As mentioned before, it is partially permeable and controls the exchange of materials between the cell and its environment. nuclear envelope chromatin – Nucleus centriole – always deeply staining nucleus All eukaryotic cells contain a nucleus. The nucleus found near nucleus and thread-like is a relatively large structure. It stains intensely and nucleolus – deeply staining KEY WORD Figure 1.4: Structure of a generalised animal cell (diameter about 20 μm) as seen with a very high quality light cell surface membrane: a very thin membrane microscope. (about 7 nm diameter) surrounding all cells; it is partially permeable and controls the exchange of Figure 1.4 is a drawing showing the structure of a materials between the cell and its environment generalised animal cell and Figure 1.5 is a drawing 5  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK middle lamella – thin layer tonoplast – membrane holding cells together surrounding vacuole cell surface membrane plasmodesma – (pressed against cell wall) connects cytoplasm of neighbouring cells vacuole – large cell wall of with central position neighbouring cell cytoplasm cell wall mitochondria chloroplast nucleolus – deeply staining grana just visible nuclear envelope nucleus chromatin – small structures that deeply staining are difficult to identify and thread-like Golgi apparatus Figure 1.5: Structure of a generalised plant cell (diameter about 40 μm) as seen with a very high quality light microscope. Figure 1.6: Cells from the lining of the human cheek (×400). Figure 1.7: Cells in a moss leaf (×400). Many green chloroplasts Each cell shows a centrally placed nucleus, which is typical are visible inside each cell. The grana are just visible as black of animal cells. The cells are part of a tissue known as grains inside the chloroplasts (‘grana’ means grains). Cell walls squamous (flattened) epithelium. are also clearly visible (animal cells lack cell walls). 6  1 Cell structure is therefore very easy to see when looking down the microscope. The deeply staining material in the nucleus KEY WORDS is called chromatin (‘chroma’ means colour). Chromatin chromatin: the material of which chromosomes is a mass of coiled threads. The threads are seen to are made, consisting of DNA, proteins and small collect together to form chromosomes during nuclear amounts of RNA; visible as patches or fibres division (Chapter 5, Section 5.2, Chromosomes). within the nucleus when stained Chromatin contains DNA (deoxyribonucleic acid), the molecule which contains the instructions (genes) that chromosome: in the nucleus of the cells of control the activities of the cell (Chapter 6). eukaryotes, a structure made of tightly coiled chromatin (DNA, proteins and RNA) visible during Inside the nucleus an even more deeply staining area cell division; the term ‘circular DNA’ is now also is visible, the nucleolus. This is made of loops of DNA commonly used for the circular strand of DNA from several chromosomes. The number of nucleoli is present in a prokaryotic cell variable, one to five being common in mammals. One of nucleolus: a small structure, one or more of the main functions of nucleoli is to make ribosomes. which is found inside the nucleus; the nucleolus is usually visible as a densely stained body; its Cytoplasm function is to manufacture ribosomes using the All the living material inside the cell is called protoplasm. information in its own DNA It is also useful to have a term for all the living material protoplasm: all the living material inside a cell outside the nucleus; it is called cytoplasm. Therefore, (cytoplasm plus nucleus) cytoplasm + nucleus = protoplasm. cytoplasm: the contents of a cell, excluding Cytoplasm is an aqueous (watery) material, varying the nucleus from a fluid to a jelly-like consistency. Using a light microscope, many small structures can be seen within mitochondrion (plural: mitochondria): the it. These have been likened to small organs and are organelle in eukaryotes in which aerobic therefore known as organelles (meaning ‘little organs’). respiration takes place An organelle can be defined as a functionally and cell wall: a wall surrounding prokaryote, plant structurally distinct part of a cell. Organelles are often, and fungal cells; the wall contains a strengthening but not always, surrounded by one or two membranes material which protects the cell from mechanical so that their activities can be separated from the damage, supports it and prevents it from bursting surrounding cytoplasm. Organising cell activities in by osmosis if the cell is surrounded by a solution separate compartments is essential for a structure as with a higher water potential complex as an animal or plant cell to work efficiently. Mitochondria (singular: mitochondrion) Differences between The most numerous organelles seen with the light microscope are usually mitochondria (singular: animal and plant cells mitochondrion). Mitochondria are only just visible using One of the structures commonly found in animal cells a light microscope. Videos of living cells, taken with the which is absent from plant cells is the centriole. Plant aid of a light microscope, have shown that mitochondria cells also differ from animal cells in possessing cell walls, can move about, change shape and divide. They are large permanent vacuoles and chloroplasts. specialised to carry out aerobic respiration. Centrioles Golgi apparatus Under the light microscope the centriole appears as The use of special stains containing silver resulted in the a small structure close to the nucleus (Figure 1.4). Golgi apparatus being discovered in 1898 by Camillo Centrioles are discussed later in this chapter. Golgi. The Golgi apparatus collects and processes molecules within the cell, particularly proteins. Cell walls and plasmodesmata With a light microscope, individual plant cells are more easily seen than animal cells. This is because they are Note: you do not need to learn this structure. It is usually larger and, unlike animal cells, are surrounded sometimes called the Golgi body or Golgi complex. by a cell wall. Note that the cell wall is an extra 7  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK structure which is outside the cell surface membrane. of the plant, mainly in the leaves. They are relatively The wall is relatively rigid because it contains fibres large organelles and so are easily seen with a light of cellulose, a polysaccharide which strengthens the microscope. It is even possible to see tiny ‘grains’ or wall. The cell wall gives the cell a definite shape. It grana (singular: granum) inside the chloroplasts using prevents the cell from bursting when water enters by a light microscope (Figure 1.7). These are the parts osmosis, allowing large pressures to develop inside the of the chloroplast that contain chlorophyll, the green cell (Chapter 4, Section 4.5, Movement of substances pigment which absorbs light during the process of across membranes). Cell walls may be reinforced with photosynthesis. Chloroplasts are discussed further in extra cellulose or with a hard material called lignin Chapter 13 (Section 13.2, Structure and function of for extra strength (Chapter 7). Cell walls are freely chloroplasts). permeable, allowing free movement of molecules and ions through to the cell surface membrane. KEY WORDS Plant cells are linked to neighbouring cells by means of pores containing fine strands of cytoplasm. plasmodesma (plural: plasmodesmata): a These structures are called plasmodesmata (singular: pore-like structure found in plant cell walls; plasmodesma). They are lined with the cell surface plasmodesmata of neighbouring plant cells line membrane. Movement through the pores is thought to up to form tube-like pores through the cell walls, be controlled by the structure of the pores. allowing the controlled passage of materials from one cell to the other; the pores contain ER and are lined with the cell surface membrane Vacuoles Vacuoles are sac-like structures which are surrounded vacuole: an organelle found in eukaryotic cells; by a single membrane. Although animal cells may a large, permanent central vacuole is a typical possess small vacuoles such as phagocytic vacuoles feature of plant cells, where it has a variety of (Chapter 4, Section 4.5, Movement of substances functions, including storage of biochemicals such across membranes), which are temporary structures, as salts, sugars and waste products; temporary mature plant cells often possess a large, permanent, vacuoles, such as phagocytic vacuoles (also central vacuole. The plant vacuole is surrounded by known as phagocytic vesicles), may form in a membrane, the tonoplast, which controls exchange animal cells between the vacuole and the cytoplasm. The fluid tonoplast: the partially permeable membrane in the vacuole is a solution of pigments, enzymes, that surrounds plant vacuoles sugars and other organic compounds (including some waste products), mineral salts, oxygen and carbon chloroplast: an organelle, bounded by an dioxide. envelope (i.e. two membranes), in which In plants, vacuoles help to regulate the osmotic photosynthesis takes place in eukaryotes properties of cells (the flow of water inwards and photosynthesis: the production of organic outwards) as well as having a wide range of other substances from inorganic ones, using energy functions. For example, the pigments which colour from light the petals of certain flowers and the parts of some vegetables, such as the red pigment of beetroots, may grana (singular: granum): stacks of membranes be found in vacuoles. inside a chloroplast Chloroplasts Chloroplasts are organelles specialised for the process of photosynthesis. They are found in the green parts 8  1 Cell structure IMPORTANT You can think of a plant cell as being very similar to an animal cell but with extra structures. Plant cells are often larger than animal cells, although cell size varies enormously. Do not confuse the cell wall with the cell surface membrane. Cell walls are relatively thick and physically strong, whereas cell surface membranes are very thin. Cell walls are freely permeable, whereas cell surface membranes are partially permeable. All cells have a cell surface membrane, but animal cells do not have a cell wall. Vacuoles are not confined to plant cells; animal cells may have small vacuoles, such as phagocytic vacuoles, although these are not usually permanent structures. PRACTICAL ACTIVITY 1.1 Making temporary slides black and will also colour nuclei and cell walls a pale yellow. A dilute solution of methylene blue can be A common method of examining material with a light used to stain animal cells such as cheek cells. microscope is to cut thin slices of the material called ‘sections’. The advantage of cutting sections is that Viewing specimens yourself with a microscope will they are thin enough to allow light to pass through help you to understand and remember structures. the section. The section is laid (‘mounted’) on a glass Your understanding can be reinforced by making slide and covered with a coverslip to protect it. Light a pencil drawing on good quality plain paper. passing through the section produces an image Remember always to draw what you see, and not which can then be magnified using the objective and what you think you should see. eyepiece lenses of the microscope. Procedure Biological material may be examined live or in a preserved state. Prepared slides contain material that Place the biological specimen on a clean glass slide has been killed and preserved in a life-like condition. and add one or two drops of stain. Carefully lower a cover over the specimen to protect the microscope Temporary slides are quicker and easier to prepare lens and to help prevent the specimen from drying and are often used to examine fresh material out. Adding a drop of glycerine and mixing it with containing living cells. In both cases the sections the stain can also help prevent drying out. are typically stained before being mounted on the glass slide. Suitable animal material: human cheek cells obtained by gently scraping the lining of the Temporary preparations of fresh material are useful cheek with a finger nail for quick preliminary investigations. Sometimes Suitable plant material: onion epidermal cells, macerated (chopped up) material can be used, lettuce epidermal cells, Chlorella cells, moss as when examining the structure of wood (xylem). slip leaves A number of temporary stains are commonly used. For example, iodine in potassium iodide solution (See Practical Investigation 1.1 in the Practical is useful for plant specimens. It stains starch blue- Workbook for additional information.) 9  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK PRACTICAL ACTIVITY 1.2 Biological drawing sections of Practical Activity 7.1 before answering the question below, which is relevant to this chapter. To reinforce your learning, you will find it useful to Figures 1.8a and b show examples of good drawing make labelled drawings of some of your temporary and labelling technique based on Figures 1.6 and permanent slides, as well as labelled drawings and 1.7. Note that it is acceptable to draw only of photomicrographs. a representative portion of the cell contents of Practical Activity 7.1 in Chapter 7 provides general Figure 1.7, but add a label explaining this. guidance on biological drawing. Read the relevant a cytoplasm nucleus chromatin small structures (organelles?) visible (not all drawn) Question 2 A student was asked to make a high-power drawing of three neighbouring cells from Figure 1.6. Figure 1.9 shows the drawing made by the student. Using Practical Activity 7.1 to help you, suggest how b the drawing in Figure 1.9 could be improved. eus cytoplasm nucl representative portion of chloroplast cytoplasm cytoplasm drawn grana visible? cell wall Figure 1.8: Examples of good drawing technique: a high-power drawing of three neighbouring animal cells from Figure 1.6; b high-power drawing of two neighbouring plant cells from Figure 1.9: A student’s high-power drawing of Figure 1.7. three neighbouring cells from Figure 1.6. (See Practical Investigation 1.1 in the Practical Workbook for additional information.) 10  1 Cell structure 1.4 Measuring size and Measuring cell size Cells and organelles can be measured with a microscope calculating magnification by means of an eyepiece graticule. This is a transparent scale. It usually has 100 divisions (see Figure 1.10a). Magnification is the number of times larger an image of The eyepiece graticule is placed in the microscope an object is than the real size of the object. eyepiece so that it can be seen at the same time as observed size of the image the object to be measured, as shown in Figure 1.10b. magnification = actual size Figure 1.10b shows the scale over one of a group of or six human cheek epithelial cells (like those shown in I Figure 1.6). The cell selected lies between 40 and 60 on M = the scale. We therefore say it measures 20 eyepiece units A M = magnification in diameter (the difference between 60 and 40). We will not know the actual size of the eyepiece units until the I = observed size of the image (what you can measure eyepiece graticule is calibrated. with a ruler) A = actual size (the real size – for example, the size of a KEY WORDS cell before it is magnified). If you know two of the values M, I and A, you can work magnification: the number of times larger an out the third one. For example, if the observed size of image of an object is than the real size of the the image and the magnification are known, you can object; magnification = image size ÷ actual (real) I size of the object work out the actual size A =. If you write the formula M eyepiece graticule: small scale that is placed in a in a triangle as shown below and cover up the value you want to find, it should be obvious how to do the right microscope eyepiece calculation. I M × A a cheek cells on a slide b eyepiece graticule c on the stage of the scale (arbitrary microscope units) 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 0.1 0.2 eyepiece graticule in the eyepiece stage micrometer of the scale (marked in microscope 0.01 mm and 0.1 mm divisions) Figure 1.10: Microscopical measurement. Three fields of view seen using a high-power (×40) objective lens: a an eyepiece graticule scale; b superimposed images of human cheek epithelial cells and the eyepiece graticule scale; c superimposed images of the eyepiece graticule scale and the stage micrometer scale. 11  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK To calibrate the eyepiece graticule, a miniature a transparent ruler called a stage micrometer is placed on the microscope stage and is brought into focus. This scale may be etched onto a glass slide or printed on a transparent film. It commonly has subdivisions of 0.1 and 0.01 mm. The images of the stage micrometer and the eyepiece graticule can then be superimposed (placed on top of one another) as shown in Figure 1.10c. Calculating magnification Figure 1.11 shows micrographs of two sections through the same plant cell. The difference in appearance of the two micrographs is explained in the next section. If we know the actual (real) length of a cell in such a micrograph, we can calculate its magnification, M, using b the formula: I M = A epidermal cell cell wall KEY WORDS chloroplast stage micrometer: very small, accurately drawn P starch grain scale of known dimensions, engraved on a microscope slide vacuole nucleus micrograph: a picture taken with the aid of a microscope; a photomicrograph (or light mitochondrion micrograph) is taken using a light microscope; cytoplasm an electron micrograph is taken using an electron microscope Figure 1.11: Micrographs of two sections of the same plant cells, as seen a with a light microscope, and b with an electron microscope. Both are shown at the same magnification (about ×750). WORKED EXAMPLE 1 In the eyepiece graticule shown in Figure 1.10, The diameter of the cell shown superimposed 100 units measure 0.25 mm. Hence, the value of on the scale in Figure 1.8b measures 20 eyepiece each eyepiece unit is: units and so its actual diameter is: 0.25 = 0.0025 mm 20 × 2.5 μm = 50 μm 100 This diameter is greater than that of many human Or, converting mm to μm: cells because the cell is a flattened epithelial cell. 0. 25 1000 2. 5 µm 100 12  1 Cell structure WORKED EXAMPLE 2 Suppose we want to know the magnification of the Step 3 Use the equation to calculate the plant cell labelled P in Figure 1.11b. The real length magnification. of the cell is 80 μm. image size, l Step 1 Measure the length in mm of the cell in the magnification, M = actual size, A micrograph using a ruler. You should find 50 000 µm that it is about 50 mm. = 80 µm Step 2 Convert mm to μm. (It is easier if we first = × 625 convert all measurements to the same units – in this case micrometres, μm.) The multiplication sign (×) in front of the number 625 means ‘times’. We say that the magnification So: 1 mm = 1000 µm is ‘times 625’. 50 mm = 50 × 1000 µm = 50 000 µm Question 3 a Calculate the magnification of the drawing of b Calculate the actual (real) length of the the animal cell in Figure 1.4. chloroplast labelled X in Figure 1.34. WORKED EXAMPLE 3 Figure 1.12 shows a lymphocyte with a scale Step 1 Measure the scale bar. Here, it is 36 mm. bar. We can use this scale bar to calculate the Step 2 Convert mm to μm: magnification. 36 mm = 36 × 1000 μm = 36 000 μm Step 3 The scale bar represents 6 µm. This is the actual size, A. Use the equation to calculate the magnification: image size, l magnification, M = actualsize, A 36 000µm = 6µm = × 6000 6 µm Figure 1.12: A lymphocyte. 13  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK Calculating the real size of an separate points. If the two points cannot be resolved, they will be seen as one point. In practice, resolution is object from its magnification the amount of detail that can be seen – the greater the resolution, the greater the detail. To calculate the real or actual size of an object, we can use the same magnification equation. The maximum resolution of a light microscope is 200 nm. The reason for this is explained in the next section, ‘The electromagnetic spectrum’. A resolution of 200 nm means WORKED EXAMPLE that, if two points or objects are closer together than 200 nm, they cannot be distinguished as separate. 4 Figure 1.20 shows parts of three plant cells You might imagine that you could see more detail in magnified ×5600. Suppose we want to know the Figure 1.11a by magnifying it (simply making it larger). actual length of the labelled chloroplast in this In practice you would be able to see what is already electron micrograph. there more easily, but you would not see any more detail. The image would just get more and more blurred Step 1 Measure the observed length of the as magnification increased. The resolution would not image of the chloroplast (I), in mm, be greater. using a ruler. The maximum length is 25 mm. Step 2 Convert mm to μm: The electromagnetic spectrum 25 mm = 25 × 1000 μm = 25 000 μm How is resolution linked with the nature of light? One of the properties of light is that it travels in waves. The Step 3 Use the equation to calculate the lengths of the waves of visible light vary, ranging from actual length: about 400 nm to about 700 nm. The human eye can distinguish between these different wavelengths, and image size, l actual size, A = in the brain the differences are converted to colour magnification, M differences. Waves that are 400 nm in length are seen as 25 000 µm violet. Waves that are 700 nm in length are seen = 5600 as red. = 4.5 µm (to one Visible light is a form of electromagnetic radiation. decimal place) The range of different wavelengths of electromagnetic radiation is called the electromagnetic spectrum. Visible light is only one part of this spectrum. Figure 1.13 shows some of the parts of the electromagnetic 1.5 Electron microscopy spectrum. The longer the waves, the lower their frequency. (All the waves travel at the same speed, so Before studying what cells look like with an electron imagine them passing a post: shorter waves pass at microscope, you need to understand the difference higher frequency.) In theory, there is no limit to how between magnification and resolution. short or how long the waves can be. Wavelength changes with energy: the greater the energy, the shorter the wavelength. Magnification and resolution Look again at Figure 1.11. Figure 1.11a is a light KEY WORD micrograph. Figure 1.11b is an electron micrograph. Both micrographs are of the same cells and both have resolution: the ability to distinguish between the same magnification. However, you can see that two objects very close together; the higher the Figure 1.11b, the electron micrograph, is much clearer. resolution of an image, the greater the detail that This is because it has greater resolution. Resolution can be seen can be defined as the ability to distinguish between two 14  1 Cell structure X-rays infrared microwaves gamma rays UV radio and TV waves 5 7 9 11 13 0.1 nm 10 nm 1000 nm 10 nm 10 nm 10 nm 10 nm 10 nm visible light 400 nm 500 nm 600 nm 700 nm violet green orange red Figure 1.13: Diagram of the electromagnetic spectrum. The numbers indicate the wavelengths of the different types of electromagnetic radiation. Note the waves vary from very short to very long. Visible light is part of the spectrum. The double- headed arrow labelled UV is ultraviolet light. wavelength stained mitochondrion The general rule when viewing specimens is that the 400 nm of diameter 1000 nm limit of resolution is about one half the wavelength interferes with light waves of the radiation used to view the specimen. In other words, if an object is any smaller than half the wavelength of the radiation used to view it, it cannot be seen separately from nearby objects. This means that the best resolution that can be obtained using a microscope that uses visible light (a light microscope) is 200 nm, since the shortest wavelength of visible light is 400 nm (violet light). Ribosomes are approximately 25 nm in diameter and can therefore never be seen using a light microscope. If an object is transparent, it will allow light waves to pass through it and therefore will still not be visible. This is why many biological structures have to be stained before they can be seen. Question stained ribosomes of diameter 25 nm 4 Explain why ribosomes are not visible using a light do not interfere with light waves microscope. Figure 1.14: A mitochondrion and some ribosomes in the path of light waves of 400 nm length. The electron microscope So how can we look at things smaller than 200 nm? Now look at Figure 1.14. It shows a mitochondrion The only solution to this problem is to use radiation and some very small cell organelles called ribosomes. of a shorter wavelength than visible light. If you study It also shows some wavy blue lines that represent light Figure 1.13, you will see that ultraviolet light or X-rays of 400 nm wavelength. This is the shortest visible look like possible candidates. A much better solution, wavelength. The mitochondrion is large enough to though, is to use electrons. Electrons are negatively interfere with the light waves. However, the ribosomes charged particles which orbit the nucleus of an atom. are far too small to have any effect on the light waves. When a metal becomes very hot, some of its electrons 15  CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK gain so much energy that they escape from their orbits, similar to a rocket escaping from Earth’s gravity. Free Viewing specimens with the electrons behave like electromagnetic radiation. They have a very short wavelength: the greater the energy, electron microscope the shorter the wavelength. Electrons are a very suitable Figure 1.16 shows how a TEM works and Figure 1.17 form of radiation for microscopy for two major reasons. shows one in use. First, their wavelength is extremely short (at least as short as that of X-rays). Second, unlike X-rays, they are electron gun and anode – negatively charged, so they can be focused easily using produce a beam of electrons electromagnets (a magnet can be made to alter the path of the beam, the equivalent of a glass lens bending light). electron beam Using an electron microscope, a resolution of 0.5 nm can vacuum be obtained, 400 times better than a light microscope. pathway of electrons Transmission and scanning condenser electromagnetic electron microscopes lens – directs the electron beam Two types of electron microscope are now in common onto the specimen use. The transmission electron microscope (TEM) was the type originally developed. The beam of electrons is passed through the specimen before being viewed. Only specimen is placed on a those electrons that are transmitted (pass through the grid specimen) are seen. This allows us to see thin sections of specimens, and thus to see inside cells. In the scanning electron microscope (SEM), the electron beam is used objective electromagnetic to scan the surfaces of structures and only the reflected lens – produces an image beam is observed. An example of a scanning electron micrograph is shown in Figure 1.15. The advantage of this microscope is that surface structures can be seen. Because much of the projector electromagnetic specimen is in focus at the same time, a three-dimensional lenses – focus the magnified appearance is achieved. A disadvantage of the SEM is image onto the screen that it cannot achieve the same resolution as a TEM. Using an SEM, resolution is between 3 nm and 20 nm. screen or photographic film or sensor – shows the image of the specimen Figure 1.16: How a TEM works. Note: the structure of an electron microscope is extension content, and is not part of the syllabus. Figure 1.15: Scanning electron micrograph (SEM) of a tardigrade. Tardigrades or water bears, are about 0.5 mm It is not possible to see an electron beam, so to make long, with four pairs of legs. They are common in soil and the image visible the electron beam has to be projected can survive extreme environmental conditions (×86). onto a fluorescent screen. The areas hit by electrons 16  1 Cell structure The electron beam, and therefore the specimen and the fluorescent screen, must be in a vacuum. If the electrons collided with air molecules, they would scatter, making it impossible to achieve a sharp picture. Also, water boils at room temperature in a vacuum, so all specimens must be dehydrated before being placed in the microscope. This means that only dead material or