Cambridge IGCSE Biology Teacher's Guide PDF

The Cambridge IGCSE™ Biology series consists of a Student’s Book, Boost eBook, Workbooks and Teacher’s Guide with Boost Subscription. To explore the entire series, visit www.hoddereducation.com/cambridge-igcse-science Cambridge IGCSE™ Biology Teacher’s Guide with Boost Subscription Created with teachers and students in schools across the globe, Boost is the next generation in digital learning for schools and colleges, bringing quality content and new technology together in one interactive website. The Teacher’s Guide includes a print handbook and a subscription to Boost, where you will find a range of online resources to support your teaching. Confidently deliver the revised syllabus: guidance on how to approach the syllabus from experienced authors, practical support to help you work scientifically and safely, as well as lesson plans based on the provided scheme of work. Develop key concepts and skills: let students see how their skills are developing with a range of worksheets, formative knowledge tests and detailed answers to all the questions in the accompanying Student’s Book, Workbook and Practical Skills Workbook. Enhance learning: videos and animations on key concepts, mathematical skills and practicals plus audio of technical terms to support vocabulary flashcards. To purchase Cambridge IGCSE™ Biology Teacher’s Guide with Boost Subscription, visit www.hoddereducation.com/cambridge-igcse-science Cambridge International copyright material in this publication is reproduced under licence and remains the intellectual property of Cambridge Assessment International Education. 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. Third-party websites and resources referred to in this publication have not been endorsed by Cambridge Assessment International Education. We have carried out a health and safety check of this text and have attempted to identify all recognised hazards and suggest appropriate cautions. However, the Publishers and the authors accept no legal responsibility on any issue arising from this check; whilst every effort has been made to carefully check the instructions for practical work described in this book, it is still the duty and legal obligation of schools to carry out their own risk assessments for each practical in accordance with local health and safety requirements. For further health and safety information (e.g. Hazcards) please refer to CLEAPSS at www.cleapss.org.uk. Every effort has been made to trace all copyright holders, but if any have been inadvertently overlooked, the Publishers will be pleased to make the necessary arrangements at the first opportunity. Although every effort has been made to ensure that website addresses are correct at time of going to press, Hodder Education cannot be held responsible for the content of any website mentioned in this book. It is sometimes possible to find a relocated web page by typing in the address of the home page for a website in the URL window of your browser. Hachette UK’s policy is to use papers that are natural, renewable and recyclable products and made from wood grown in well-managed forests and other controlled sources. The logging and manufacturing processes are expected to conform to the environmental regulations of the country of origin. Orders: please contact Hachette UK Distribution, Hely Hutchinson Centre, Milton Road, Didcot, Oxfordshire, OX11 7HH. Telephone: +44 (0)1235 827827. Email [email protected] Lines are open from 9 a.m. to 5 p.m., Monday to Friday. You can also order through our website: www.hoddereducation.com ISBN: 978 1 3983 1045 2 eISBN: 978 1 3983 1070 4 © D G Mackean and Dave Hayward 2021 First published in 2002 Second edition published in 2009 Third edition published in 2014 This edition published in 2021 by Hodder Education, An Hachette UK Company Carmelite House 50 Victoria Embankment London EC4Y 0DZ www.hoddereducation.com Impression number 10 9 8 7 6 5 4 3 2 1 Year 2024 2023 2022 2021 All rights reserved. Apart from any use permitted under UK copyright law, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying and recording, or held within any information storage and retrieval system, without permission in writing from the publisher or under licence from the Copyright Licensing Agency Limited. Further details of such licences (for reprographic reproduction) may be obtained from the Copyright Licensing Agency Limited, www.cla.co.uk Cover photo © Werner Dreblow - stock.adobe.com Typeset by Integra Software Services Pvt. Ltd., Puducherry, India Printed in Slovenia A catalogue record for this title is available from the British Library. Contents Acknowledgements How to use this book Scientific enquiry 1 Characteristics and classification of living organisms Characteristics of living organisms Classification systems Features of organisms 2 Organisation of the organism Cell structure and organisation Size of specimens 3 Movement into and out of cells Diffusion Osmosis Active transport 4 Biological molecules Biological molecules Structure of DNA 5 Enzymes Enzyme action 6 Plant nutrition Photosynthesis Mineral requirements Leaf structure 7 Human nutrition Diet Digestive system Physical digestion Chemical digestion Absorption 8 Transport in plants Xylem and phloem Leaf, stem and root structure Water uptake Transpiration Translocation 9 Transport in animals Circulatory systems Heart Blood vessels Blood 10 Diseases and immunity Pathogens and transmission Defences against disease 11 Gas exchange in humans Gas exchange in humans 12 Respiration Respiration Aerobic respiration Anaerobic respiration 13 Excretion in humans Excretion 14 Coordination and response Coordination and response Nervous control in humans Hormones Homeostasis Tropic responses 15 Drugs Drugs 16 Reproduction Asexual reproduction Sexual reproduction Sexual reproduction in plants Sexual reproduction in humans Sexual hormones in humans Sexually transmitted infections 17 Inheritance Chromosomes, genes and proteins Mitosis Meiosis Monohybrid inheritance 18 Variation and selection Variation Adaptive features Selection 19 Organisms and their environment Energy flow Food chains and food webs Nutrient cycles Populations 20 Human influences on ecosystems Food supply Habitat destruction Pollution Conservation 21 Biotechnology and genetic modification Biotechnology and genetic modification Biotechnology Genetic modification Theory past paper and exam-style questions Alternative to Practical past paper questions Glossary Acknowledgements Author acknowledgements I have really appreciated the persistence and hard work of the team who supported me in the production of this new edition. They include Anthony Muller, Catherine Perks, Rosie Stewart, Carol Usher and Christine Graham. With special thanks to Margaret Mackean for her continued support in the publication of this book. Artwork and text acknowledgements Original illustrations by D.G. Mackean, prepared and adapted by Wearset Ltd. Additional illustrations by Ethan Danielson, Richard Draper and Mike Humphries. Natural history artwork by Chris Etheridge. Full colour illustrations on pages 9, 10, 17 and 18 by Pamela Haddon. p.62 Figure 3.27 from J.K. Brierley, Plant Physiology (The Association for Science Education, 1954); p.64 Figure 4.4 from J. Bonner and A.W. Galston (1952), Principles of Plant Physiology (W.H. Freeman and Co., 1952); p.95 Figure 6.15 from Verma, S. B., & Rosenberg, N. J. (1979). Agriculture and the atmospheric carbon dioxide build-up, (Span; Progress in Agriculture); p.109 Table 7.2 from National Nutrient Database, Agricultural Research Service (United States Department of Agriculture); p.153 Figure 9.13 from Smoking or Health: a Report from the Royal College of Physicians of London, 1977 (Pitman Medical Publishing Co. Ltd); p.171 Figure 10.3 from Brian Jones, Introduction to Human and Social Biology, 2/e (John Murray, 1985); p.197 (table) Emslie-Smith, D., Paterson, C. R., Scratcherd, T., & Read, N. W. (Eds.). (1988). Textbook of Physiology: BDS. (Edinburgh: Churchill Livingstone); p.274 Figure 16.53 from Corner, G. W. (2015). Hormones in human reproduction, © Princeton University Press; p.320 Figure 19.6 from Whittaker, R. H., Lovely, R. A. (1975). Communities and Ecosystems 2nd edition. (Macmillan College Textbooks 1975); p.330 Figure 19.19 from Trevor Lewis and L.R. Taylor. Introduction to Experimental Ecology (Academic Press, 1967); p.332 Figure 19.21 from F. M. Burnett Natural History of Infectious Disease, 3rd edition (Cambridge University Press, 1962); p.334 Figure 19.23 from Trevor Lewis and L.R. Taylor. Introduction to Experimental Ecology (Academic Press, 1967); p.335 Figure 19.24 from Trevor Lewis and L.R. Taylor. Introduction to Experimental Ecology (Academic Press, 1967); p.341 Figure 20.6 from Clive A. Edwards, Soil Pollutants and Soil Animals (Scientific American, 1969); p.354 Figure 20.31 from © 1988 New Scientist Ltd. All rights reserved. Distributed by Tribune Content Agency; p.388 Figure 6.6 © NOAA. Every effort has been made to trace or contact all rights holders. The publishers will be pleased to rectify any omissions or errors brought to their notice at the earliest opportunity. 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Krasemann/Science Photo Library; p.412 © Cotswolds Photo Library/Alamy Stock Photo; p.413 © Frans Lanting, Mint Images/Science Photo Library; p.416 © UCLES; p.417 l © eyewave – Fotolia, r © Svetlana Kuznetsova – Fotolia; p.420 © Power And Syred/Science Photo Library; p.422 © Biophoto Associates/Science Photo Library; p.424 © Dr. Gladden Willis, Visuals Unlimited/Science Photo Library. Every effort has been made to trace or contact all rights holders. The publishers will be pleased to rectify any omissions or errors brought to their notice at the earliest opportunity. How to use this book To make your study of Biology for Cambridge IGCSE™ as rewarding and successful as possible, this textbook, endorsed by Cambridge Assessment International Education, offers the following important features: Focus Each chapter starts with a short outline of the topic so you know what to expect within each chapter. Focus points Each topic starts with a bullet point summary of what you will encounter over the next few pages. Test yourself These questions appear regularly throughout the topic so you can check your understanding as you progress. Revision checklist At the end of each chapter, a revision checklist will allow you to recap what you have learnt in each chapter and double check that you understand the key concepts before moving on. Exam-style questions Each chapter is followed by exam-style questions to help familiarise learners with the style of questions they may see in their examinations. These will also prove useful in consolidating your learning. Past paper questions are also provided in the back of the book. As you read through the book, you will notice that some text is shaded yellow. This indicates that the highlighted material is Supplement content only. Text that is not shaded covers the Core syllabus. If you are studying the Extended syllabus, you should look at both the Core and Supplement sections. Key definitions These provide explanations of the meanings of key words as required by the syllabus. Practical work These boxes identify the key practical skills you need to be able to understand and apply as part of completing the course. Worked example These boxes give step-by-step guidance on how to approach different sorts of calculations, with follow-up questions so you can practise these skills. Going further These boxes take your learning further than is required by the Cambridge syllabus so that you have the opportunity to stretch yourself. Answers are provided online with the accompanying Cambridge IGCSE™ Biology Teacher Guide. A Practical Skills Workbook is also available to further support you in developing your practical skills as part of carrying out experiments. Scientific enquiry During your course you will have to carry out several experiments and investigations which will help you to develop some of the skills and abilities that scientists use to solve real-life problems. Simple experiments may be designed to measure, for example, your pulse rate while you are resting. Longer investigations may be designed to establish or confirm a relationship between two or more physical quantities, for example, the effect of increasing temperature on the rate of transpiration in a plant shoot. Investigations will likely be generated from the topic you are currently studying in class. Any investigation will involve the following five aspects: 1 Selecting and safely using techniques, apparatus and materials – you need to be aware of any hazards presented by an investigation and how you will minimise any possible risks. Your teacher should help you with any risk assessments before you start. You also need to be able to identify the best materials and equipment in order to make sure you stay safe and that your observations or data are accurate. 2 Planning experiments – when planning you need to consider what procedure will help you find answers to the questions you are investigating. When choosing the apparatus or technique you will use, think about the reasons for your choice. Once you’ve selected these it will be useful to make predictions and hypotheses (informed guesses) of the results you’d expect. It will help to write down your plan as it develops. Variables are also very important in planning – you will need to identify both the independent variables and the dependent variable so you can make sure the results are valid. You will need to consider how the independent variables will be controlled and what range of values you intend to collect. Decide how you will process the results in order to form a conclusion or to evaluate your prediction. 3 Making and recording observations, measurements and estimates – you must make sure you measure and collect the necessary experimental data with suitable precision. This involves selecting and using the most appropriate measuring instruments available to you. You need to choose an appropriate number of readings or observations, remembering to include repeats to check your data is reliable. The results will need to be recorded systematically (e.g. in a suitable table). If you’re recording observations be sure to be detailed. 4 Interpreting and evaluating the observations and data – when evaluating results you need to do so in a way that enables any relationships between quantities to be formed. As part of this, you will need to process the information you have collected. This may involve calculations, such as working out the percentage change in mass of samples of potato when placed in a range of sucrose solutions. Alternatively, you may need to plot a graph, for example, to show the relationship between temperature and the rate of enzyme action. Always make sure any graph axes are labelled with the descriptor and units on both axes (these details can be taken from your table headers). When forming conclusions, you need to state the relationship your data has established (what happens to the dependent variable as the independent variable is changed) and give a scientific explanation for why it has happened. You should be aware of the possibility of any anomalous results and decide how to treat these. If you were to evaluate the data, how could you have improved the accuracy, reliability or quality? What would you do differently next time? 5 Evaluating experimental methods and suggesting possible improvements – this is different than evaluating your data and requires you to look at the investigation as a whole. You should assess the techniques you used and decide whether your use of a control was adequate. You should also identify any possible sources of error, deciding how you could have overcome these. It may be that you had difficulty measuring the change in length of a piece of potato with a ruler in an osmosis investigation. Instead of measuring length, weighing the potato pieces on a digital balance could provide more accurate data. A written report of the investigation is normally made, and this should include: An aim – what you are trying to find out. A plan of what you intend to do. This should include the apparatus needed for your experiments, including justifications for your choice, safety precautions, identification of the variables and how you will control them, and predictions of expected results. You should draft out a table with suitable headings (and don’t forget to state units) for recording any data you intend to collect. When listing items of apparatus you will use, make a record of the smallest division of the scale of any measuring device. For example, the smallest division on a metre rule is 1 mm. The scale of the rule can be read to the nearest mm. So, when used to measure a length of 100 mm (0.1 m), the length is measured to the nearest 1 mm, and the degree of accuracy of the measurement is 1 part in 100. When used to measure 10 mm (0.01 m), the degree of accuracy of the measurement is 1 part in 10. As another example, a thermometer is calibrated in degrees Celsius and may be read to the nearest 1°C. A temperature may be measured to the nearest 1°C. So, when used to measure a temperature of 20°C, the degree of accuracy is 1 part in 20 (this is 5 parts in 100). If a digital thermometer is available, this may be a better choice for accuracy. A method – this should include the details of any procedures, observations and measurements you carry out. A clearly labelled diagram of the apparatus is a good way of supporting your method. When labelling the apparatus, avoid label lines crossing each other. Presentation of results and calculations. Any data you collected should be clearly presented (most likely in a table). The column headings, or start of rows, should include a descriptor (naming the measurement) and its unit; for example, ‘temperature / oC’. If you have repeated any measurements, calculate an average value. Numerical values should be given to the number of significant figures appropriate to the measuring device you used in collecting the data. If it is appropriate to plot a graph of your results, you will need at least five data points taken over as large a range as possible. Remember to label each axis of a graph with the descriptor and unit of the quantity being plotted. Also put a title on the graph. This will refer to both axes of the graph, for example, ‘Graph to show the effect of increasing temperature on the rate of action of amylase on starch.’ A concise conclusion should be drawn from the evidence. This can be based on the prediction, stating the relationship between the two quantities you investigated. Note that sometimes experiments do not achieve the intended objective. If this is the case, a conclusion is still important. A conclusion must be a description of the pattern as well as a scientific explanation for this trend. In order to produce an evaluation and discussion of the result of the investigation you need to look critically at your procedure. Points to include are: – commenting critically on the original plan – evaluating the procedures used – deciding how reliable the results are. The reliability can be indicated by how close any repeated readings are. You could compare your results with secondhand evidence (such as from a graph in a textbook) to consider whether or not the evidence can be trusted – using a systematic approach to dealing with unexpected results. For example, if you took a reading that was unexpected, did you take a further reading? Can you identify why that reading might have been incorrect? – considering how appropriate the apparatus used in the investigation was and suggesting improvements where appropriate. Suggestions for investigations Some of the suggested investigations in this book include: 1 The factors that influence diffusion (Chapter 3). 2 Osmosis, using dialysis tubing, and the effects of different concentrations of solutions on plant tissues (Chapter 3). 3 Food tests (Chapter 4). 4 The effects of changes in temperature and pH on enzyme activity (Chapter 5). 5 The requirements for photosynthesis, rate of photosynthesis, and the effect of light and dark conditions on gas exchange in an aquatic plant (Chapter 6). 6 The pathway of water through the above-ground parts of a plant (Chapter 8). 7 The effects of variation of temperature and wind speed on transpiration rate (Chapter 8). 8 The effect of physical activity on the heart rate (Chapter 9). 9 The differences in composition between inspired and expired air (Chapter 11). 1 0 The effects of physical activity on the rate and depth of breathing (Chapter 11). 1 1 The effect of temperature on respiration in yeast (Chapter 12). 1 2 Gravitropism and phototropism in shoots and roots (Chapter 14). 1 3 The environmental conditions that affect germination of seeds (Chapter 16). 1 4 Continuous and discontinuous variation (Chapter 18). 1 5 The use of biological washing powders that contain enzymes (Chapter 21). Ideas and evidence in science When you share the results of an investigation you have performed with your friends and compare their findings with your own, you may find that you do not interpret your data in the same way as your friends do. This could generate a discussion about the best way to explain your results. You may even try to persuade them that your interpretation is the right one. Scientific ideas often change through people interpreting evidence differently, or through new discoveries being made. Jean-Baptiste van Helmont was a Dutch scientist working in the 17th century. At that time scientists did not know much about the process of photosynthesis. He carried out an experiment using a willow shoot. He planted the shoot in a container with 90.8 kg of dry soil. He placed a metal grill over the soil to stop any accidental gain or loss of mass. He then left the shoot for 5 years in an open place, giving it only rainwater and distilled water for growth. After 5 years he reweighed the tree and the soil (see Figure 1). He concluded that the increase in mass of the tree (74.7 kg) was because of the water it had received. However, he did not know that plants also take in mineral ions and carbon dioxide, or that they use light as a source of energy. Indeed, carbon dioxide was not discovered until one hundred years later, by a Scottish scientist called Joseph Black. We now know that plants take in water and carbon dioxide, using energy from light to make carbohydrates. We call the process photosynthesis. Plants also need minerals to form other molecules, such as magnesium to make chlorophyll and nitrates to form proteins. ▲ Figure 1 Van Helmont’s experiment Scientists are constantly carrying out research which provides new evidence, and they evaluate that evidence. However, that can generate controversy and sometimes there is an unwillingness to accept the scientific findings. This can be the case especially if there are vested social or economic interests involved. Current examples of this are the issues of global warming (Chapter 20) and the control of the virus Covid-19. 1 Characteristics and classification of living organisms Focus In this chapter you will be introduced to the characteristics that are common to all living organisms and an effective way of remembering them. You will also be shown why it is necessary to classify organisms (there are 8.7 million species). You will learn about why biologists use the internationally agreed system to organise organisms into groups and the main features used to place animals and plants into the appropriate kingdoms and subgroups. Can you name many of the plants and animals you see around you? Do you know any of their scientific names or the groups they belong to? FOCUS POINTS What are the characteristics of living organisms? Characteristics of living organisms All living organisms, whether they are single-celled or multicellular, plants or animals, show the characteristics included in the definitions above: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. You can remember this list of the characteristics of living things by using the mnemonic MRS GREN. The letters stand for the first letters of the characteristics. Mnemonics work because they help to make the material you are learning more meaningful. They give a structure that is easier to recall later. This structure may be a word, a name (such as MRS GREN) or a phrase. Key definitions Movement is an action by an organism or part of an organism causing a change of position or place (see Chapter 14). Respiration describes the chemical reactions in cells that break down nutrient molecules and release energy for metabolism (see Chapter 12). Sensitivity is the ability to detect and respond to changes in the internal or external environment (see Chapter 14). Growth is a permanent increase in size and dry mass (see Chapter 16). Reproduction is the processes that make more of the same kind of organism (see Chapter 16). Excretion is the removal of waste products of metabolism and substances in excess of requirements (see Chapter 13). Nutrition is the taking in of materials for energy, growth and development (see Chapters 6 and 7). Classification systems FOCUS POINT How are organisms classified? What is a species? What is the binomial system? How do you make a dichotomous key? How do classification systems reflect evolutionary relationships? How is DNA used for classifying organisms? Why do closely related organisms have more similar base sequences than those that share more distant ancestors? Key definitions A species is a group of organisms that can reproduce to produce fertile offspring. The binomial system of naming organisms is an internationally agreed system in which the scientific name of an organism is made up of two parts showing the genus and species. There are millions of different organisms living on the Earth. Biologists sort them into a meaningful order, they classify them. There are many possible ways of classifying organisms. You could group all aquatic organisms together or put all black and white creatures into the same group. However, these do not make very meaningful groups; a seaweed and a porpoise are both aquatic organisms, a magpie and a zebra are both black and white. Neither of these pairs has much in common apart from being living organisms and the magpie and zebra being animals. These would be artificial systems of classification. Biologists look for a natural system of classification using important features that are shared by as large a group as possible. In some cases it is easy. Birds all have wings, beaks and feathers; there is rarely any doubt about whether an animal is a bird or not. In other cases it is not so easy. As a result, biologists change their ideas from time to time about how living things should be grouped. New groupings are suggested and old ones abandoned. Species The smallest natural group of organisms is the species. A species is a group of organisms that can reproduce to produce fertile offspring. Members of a species also often look very similar to each other in appearance, Common mynas, eagles and parrots are three different species of bird. Apart from small variations, members of a species are almost identical in their anatomy, physiology and behaviour. Animals may look quite different if humans have been involved in their breeding programmes. For example, all cats belong to the same species, but there are wide variations in the appearance of different breeds (see ‘Variation’ in Chapter 18). An American Longhair and a Siamese may look very different but they breed together successfully. Closely related species are grouped into a genus (plural: genera). For example, there are 45 species of bronzeback snake, all in the same genus Dendrelaphis. Binomial nomenclature Species must be named in such a way that the name is recognised all over the world. ‘Money Plant’ and ‘Devil’s Ivy’ are two common names for the same wild plant. If you are not aware that these are alternative names this could lead to confusion. If the botanical name, Epipremnum aureum, is used there is no chance of error. The Latin form of the name allows it to be used in all the countries of the world regardless of language barriers. People living in the Indian subcontinent are familiar with the appearance of a robin. The male is mainly black, with some red–brown bottom feathers (although some more northern populations are more brown than black). Males also have a white flash across their shoulder. The female has completely brown upper feathers and grey–brown underparts. Its scientific name is Copsychus fulicatus and the adult is about 17 cm long (see Figure 1.1). However, someone living in Britain would describe a robin very differently. It has the species name Erithacus rubecula, and is very distinctive. It has a round body with a bright orange–red breast, a white belly and olive–brown upper feathers. It is only 14 cm long (see Figure 1.2). A British scientist could get very confused talking to an Indian scientist about a robin! Again, the use of the scientific name avoids any confusion. The binomial system of naming species is an internationally agreed system in which the scientific name of an organism is made up of two parts, showing the genus and the species. Binomial means ‘two names’; the first name gives the genus and the second gives the species. For example, the Egyptian mongoose and Indian grey mongoose are both in the genus Herpestes but they are different species; the Egyptian mongoose is Herpestes ichneumon and the Indian grey mongoose is Herpestes edwardsii. The name of the genus (the generic name) is always given a capital letter and the name of the species (the specific name) always starts with a lowercase letter. Often, the specific name is descriptive, for example, edulis means ‘edible’, aquatilis means ‘living in water’, bulbosus means ‘having a bulb’, serratus means ‘having a jagged (serrated) edge’. ▲ Figure 1.1 Indian robin, Copsychus fulicatus ♂ ▲ Figure 1.2 British robin, Erithacus rubecula ♂ Test yourself 1 In the mnemonic MRS GREN, what do the letters N, S and M stand for? 2 Explain the meaning of the term binomial system. Give an example in your answer. Dichotomous keys We use dichotomous keys to identify unfamiliar organisms. Keys simplify the process of identification. Each key is made up of pairs of contrasting features (dichotomous means two branches), starting with quite general characteristics and moving on to more specific ones. When we follow the key and make suitable choices it is possible to identify the organism correctly. Figure 1.3 shows an example of a dichotomous key that could be used to place an unknown vertebrate in the correct class. Item 1 gives you a choice between two alternatives. If the animal is cold-blooded, you move to item 2 and make a further choice. If it is warm-blooded, you move to item 4 for your next choice. The same technique may be used for assigning an organism to its class, genus or species. However, the important features may not always be easy to see, so you must make use of less basic characteristics. ▲ Figure 1.3 A dichotomous key for vertebrate classes Figure 1.4 is a key for identifying some of the possible invertebrates to be found in a compost heap. Of course, you do not need a key to identify these familiar animals, but it does show you how a key can be constructed. ▲ Figure 1.4 A dichotomous key for some invertebrates in a compost heap You need to be able to develop the skills to make simple dichotomous keys, based on easily identifiable features. If you know the main characteristics of a group, it is possible to draw up a logical plan for identifying an unfamiliar organism. One such plan is shown in Figure 1.5. ▲ Figure 1.5 Identification plan Figure 1.6 shows five different items of laboratory glassware. If you were unfamiliar with the resources in a science laboratory you may not be able to name them. We are going to design a dichotomous key to help with identification. All the items have one thing in common – they are made of glass. However, each has features that make it distinctive and we can write questions based on these features. First you need to study the items, to work out what some of them have in common and what makes them different from others. For example, some have a pouring spout, others have graduations marked on the glass for measuring, some have a neck (where the glass narrows to form a thinner structure), some can stand without support because they have a flat base, and so on. ▲ Figure 1.6 Items of laboratory glassware The first question should be based on a feature that will split the group into two. The question is going to give a ‘yes’ or ‘no’ answer. For each of the two subgroups formed, a further question based on the features of some of that sub-group should then be written. Figure 1.7 shows one possible solution. ▲ Figure 1.7 Dichotomous key for identifying laboratory glassware Test yourself 3 The animals X and Y shown in Figure 1.8 are found in a compost heap. Use the key in Figure 1.4 to identify them. ▲ Figure 1.8 Two invertebrates found in a compost heap 4 The example on the left is not the only way that a dichotomous key could be set up for the laboratory glassware shown. Make your own key and test it for each object. Classification and evolutionary relationships Scientists make it possible to understand evolutionary relationships when they classify organisms. Vertebrates all have the presence of a vertebral column, along with a skull protecting a brain and a pair of jaws (usually with teeth). Studying the anatomy of different groups of vertebrates helps us to learn about their evolution. The skeletons of the front limb of five types of vertebrate are shown in Figure 1.9. Although the limbs have different functions, such as holding on to objects, flying, running and swimming, the arrangement and number of the bones is almost the same in all five. There is a single top bone (the humerus), with a ball and socket joint at one end and a hinge joint at the other. It makes a joint with two other bones (the radius and ulna) that join to a group of small wrist bones. The limb skeleton ends with five groups of bones (the hand and fingers), although some of these groups are missing in the bird. The argument for evolution says that, if these animals are not related, it seems very odd that such a similar limb skeleton should be used to do such different things as flying, running and swimming. However, if all the animals came from the same ancestor, the ancestral skeleton could have changed in small stages in different ways in each group. So, we would expect to find that the basic pattern of bones was the same in all these animals. There are many other examples of this kind of evidence among the vertebrate animals. Pangolins and armadillos (Figure 1.10) may look very closely related, but appearances can be misleading. At one stage scientists placed them in the same group together with anteaters, but we now know that some species have evolved similar characteristics completely independently and have no close links at all. ▲ Figure 1.9 Skeletons of a limb of five different vertebrates ▲ Figure 1.10 Pangolin (above) and armadillo (below) If organisms share a common ancestor this will be reflected in how they are classified. However, if they are found not to share a common ancestor, as is the case with the pangolin and armadillo, their classification will be different. Although at first glance the pangolin and armadillo may appear to share a common ancestor, a closer study of the two species reveals major differences. The pangolin has a body covered in scales made of keratin (the same material as our nails), as shown in Figure 1.11. It has no teeth but uses its long tongue to feed on ants and termites. It can roll into a tight ball for protection. The armadillo has an armoured body covering, made up of hard bony plates (see Figure 1.11). It has long claws which it uses for digging and making a burrow. Also, it has small teeth, which are not covered in enamel, and feeds on grubs and insects. Some armadillo species can roll up into a ball when threatened by predators. The two animals are both mammals but the differences between them mean that they are not classified in the same group. ▲ Figure 1.11 Pangolin body scales (above) and armadillo bony plates (below) Use of DNA sequencing in classification The use of DNA has revolutionised the process of classification. Most organisms contain chromosomes made up of strings of genes. The chemical that forms these genes is called DNA (which is short for deoxyribonucleic acid). DNA is made up of a sequence of bases, coding for amino acids and, therefore, proteins (see Chapters 4 and 17). Each species has a distinct number of chromosomes and a unique sequence of bases in its DNA, making it identifiable and distinguishable from other species. This helps particularly when different species are very similar morphologically (in appearance) and anatomically (in internal structure). Human and primate evolution is a good example of how DNA has been used to make a process of evolution clear. Traditional classification of primates (into the groups of monkeys, apes and humans) was based on their anatomy, particularly their bones and teeth. This placed humans in a separate group, while placing the other apes together into one family called Pongidae. However, genetic evidence using DNA provides a different understanding – humans are more closely related to chimpanzees (1.2% difference in the genome – the complete set of genetic material of the organism) and gorillas (1.6% different) than to orang- utans (3.1% different). Also, chimpanzees are closer to humans than to gorillas (see Figure 1.12). Bonobos and chimps are found in Zaire and were only identified as different species in 1929. The two species share the same percentage difference in the genome from humans. ▲ Figure 1.12 Classification of primates based on DNA evidence Test yourself 5 a Describe what a chromosome is made from. b State the name of the chemical that forms genes. c What do bases code for? d a Explain how DNA can be used to identify different species. b When is the use of DNA to identify a species especially useful? e Explain how scientists know that humans are more closely related to chimpanzees than they are to gorillas. Features of organisms FOCUS POINT What are the main features used to place animals and plants into the appropriate kingdoms? What are the main features used to place organisms into groups in the animal kingdom? How do you classify organisms using their features? What are the main features used to place all organisms into one of the five kingdoms? What are the main features used to place organisms into groups in the plant kingdom? What are the main features of viruses? All living organisms have certain features in common, including the presence of cytoplasm and cell membranes, and DNA as genetic material. A kingdom is a category of living organisms. The animal kingdom Animals are multicellular organisms whose cells have no cell walls or chloroplasts. Most animals ingest solid food and digest it internally. An outline classification of plants and animals follows and is illustrated in Figures 1.13 and 1.14. ▲ Figure 1.13 The animal kingdom; examples of arthropods – one of the invertebrate groups (phyla) ▲ Figure 1.14 The animal kingdom; the vertebrate classes (Only two groups out of 23 are listed here.) Each group is called a phylum (plural = phyla). *All the organisms that do not have a vertebral column are often called invertebrates. Invertebrates are not a natural group, but the term is convenient to use. Arthropods The arthropods include the crustacea, insects, centipedes and spiders (see Figure 1.13 on page 9). The name arthropod means ‘jointed limbs’, and this is a feature common to them all. They also have a hard, firm, external skeleton, called a cuticle, which encloses their bodies. Their bodies are segmented (made up of several sections), and, between the segments (sections), there are flexible joints which allow movement. In most arthropods, the segments are grouped together to form distinct regions, the head, thorax (the middle section of the body) and abdomen (the part of the body behind the thorax). Table 1.1 outlines the key features of the four classes of arthropod. Crustacea Marine crustacea are crabs, prawns, lobsters, shrimps and barnacles. Freshwater crustacea are water fleas, Cyclops, the freshwater shrimp (Gammarus) and the water louse (Asellus). Woodlice are land-dwelling crustacea. Some of these crustacea are shown in Figure 1.13 on page 9. Like all arthropods, crustacea have an exoskeleton (a rigid external skeleton) and jointed limbs. They also have two pairs of antennae (long thin feelers attached to the head) which are sensitive to touch and to chemicals, and they have compound eyes. Compound eyes are made up of tens or hundreds of separate lenses with light-sensitive cells underneath. They can form a simple image and are very sensitive to movement. Most crustacea have a pair of jointed limbs on each segment of the body, but those on the head segments are modified to form antennae or specialised mouth parts for feeding (see Figure 1.15). ▲ Figure 1.15 External features of a crustacean (lobster ×0.2) Insects The insects form a very large class of arthropods. Some are shown in Figure 1.13. Wasps, butterflies, mosquitoes, houseflies, earwigs (which you identified from Figure 1.8), greenflies (shown in Figure 1.16) and beetles (e.g. ladybird) are just a few of the subgroups in this class. Insects have segmented bodies with a firm exoskeleton, three pairs of jointed legs, compound eyes and, usually, two pairs of wings. The segments are grouped into distinct head, thorax and abdomen regions (see Figure 1.16). ▲ Figure 1.16 External features of an insect (greenbottle ×5). Flies, midges and mosquitoes have only one pair of wings Insects are different from crustacea because they have wings, only one pair of antennae and only three pairs of legs. There are no limbs on the abdominal segments. The insects have very successfully colonised the land. One reason for their success is that their cuticle stops water loss from inside the body and stops water entering the body. So the body of an insect is prevented from drying out even in very hot, dry climates. It can survive in these extreme conditions. Arachnids These are spiders, scorpions, mites and ticks. Their bodies are divided into two regions, a combined head and thorax region, called the cephalothorax, and the abdomen (see Figure 1.17). They have four pairs of limbs on their cephalothorax. In addition, there are two pairs of pedipalps. One pair is used in reproduction; the other is used to pierce their prey and paralyse it with a poison secreted by a gland at the base. There are usually several pairs of simple eyes. ▲ Figure 1.17 External features of an arachnid (×2.5) Myriapods These are millipedes and centipedes. They have a head and a segmented body that is not clearly divided into thorax and abdomen. There is a pair of legs on each body segment but in the millipede the abdominal segments are merged together in pairs and it looks as if it has two pairs of legs per segment (see Figure 1.18). As the myriapod grows, extra segments are formed. The myriapods have one pair of antennae and simple eyes. Centipedes are carnivores, feeding on other animals, but millipedes are herbivores, feeding on plant material. ▲ Figure 1.18 External features of a myriapod (×2.5) ▼ Table 1.1 Key features of the four classes of arthropods Test yourself 1 What features do all arthropods have in common? 2 State two features for each of the arthropod classes (insects, crustaceans, arachnids and myriapods) that distinguishes it from the others. Vertebrates Vertebrates are animals which have a vertebral column. The vertebral column is sometimes called the spinal column, or just the spine, and consists of a chain of cylindrical bones (vertebrae) joined end to end. Each vertebra carries an arch of bone on its dorsal (upper) surface. This arch protects the spinal cord (see Chapter 14), which runs most of the length of the vertebral column. The front end of the spinal cord is expanded to form a brain, which is enclosed and protected by the skull. The skull carries a pair of jaws which, in most vertebrates, contain rows of teeth. The five classes of vertebrates are fish, amphibia, reptiles, birds and mammals. Table 1.2 summarises the key features of these classes. Body temperature Fish, amphibia and reptiles are often referred to as ‘cold-blooded’. This is a misleading term. A fish in a tropical pool or a lizard basking in the sun will have warm blood. The point is that these animals have a variable body temperature that, to some extent, depends on the temperature of their surroundings. Reptiles, for example, move into sunlight or hide in shade to control their temperature, but there is no internal mechanism for temperature control. Warm-blooded animals usually have a body temperature that is higher than their surroundings. The main difference, however, is that these temperatures are kept mainly constant despite any variation in external temperature. There are internal regulatory mechanisms (see Chapter 14) that keep the body temperature within narrow limits. The advantage of being warm-blooded is that an animal’s activity is not dependent on the surrounding temperature. A lizard’s body movements may become slow if the surrounding temperature falls. This could be a disadvantage if the lizard is being chased by a warm- blooded predator whose speed and reactions are not affected by low temperatures. Fish Fish are cold-blooded vertebrates. Many of them have a smooth, streamlined shape that allows them to move through the water easily (see Figure 1.19). Their bodies are covered with overlapping scales and they have fins, which are important in movement. Fish have filamentous gills to breathe. The gills are protected by a bony plate called the operculum. Fish reproduce sexually but fertilisation usually takes place externally; the female lays eggs and the male sheds sperms on them after they have been laid. ▲ Figure 1.19 Rawas (Eleutheronema ×0.1) Amphibia Amphibia are cold-blooded vertebrates with four limbs and no scales. The class includes frogs, toads and newts. The name, amphibian, means ‘double life’ and refers to the fact that the organism spends part of its life in water and part on the land. Most frogs, toads and newts spend their life on the land where it is moist and return to water only to lay eggs. The external features of the common frog are shown in Figure 1.20. Figure 1.14 on page 10 shows the toad and the newt. ▲ Figure 1.20 Rana (×0.75) The toad’s skin is drier than a frog’s skin and it has glands that can release an unpleasant-tasting chemical to put off predators. Newts differ from frogs and toads in having a tail. All three groups are carnivorous. Amphibia have four limbs. In frogs and toads, the hind feet have a web of skin between the toes. This provides a large surface area to push against the water when the animal is swimming. Newts swim by a wriggling, fish-like movement of their bodies and make less use of their limbs for swimming. Amphibia have moist skins with a good supply of capillaries, which can exchange oxygen and carbon dioxide with the air or water. They also have lungs that can be inflated by a kind of swallowing action. They do not have a diaphragm or ribs. Frogs and toads migrate to ponds where the males and females pair up. The male climbs on the female’s back and grips firmly with his front legs (see Figure 1.21). When the female lays eggs, the male immediately releases sperms over them. Fertilisation, therefore, is external even though the frogs are in close contact for the event. ▲ Figure 1.21 Frogs pairing. The male clings to the female’s back and releases his sperm as she lays the eggs Reptiles Reptiles are land-living vertebrates. Their skins are dry and the outer layer of epidermis forms a pattern of scales. This dry, scaly skin helps reduce water loss. Also, the eggs of most species have a tough, paper- like shell. So, reptiles are not limited to damp habitats, and they do not need water in which to breed. Reptiles are cold-blooded, but they can try to regulate their temperature. They do this by lying in the sun until their bodies warm up. When reptiles warm up, they can move about rapidly to chase insects and other prey. Reptiles include lizards, snakes, turtles, tortoises and crocodiles (see Figure 1.22, and Figure 1.14 on page 10). ▲ Figure 1.22 Lacerta (×1.5) Apart from snakes, reptiles have four limbs, each with five toes. Some species of snake still have the traces of limbs and girdles. Male and female reptiles mate, and sperms are passed into the female’s body. So, the eggs are fertilised internally before being laid. In some species, the female keeps the eggs in the body until they are ready to hatch. Birds Birds are warm-blooded vertebrates. The vertebral column in the neck is flexible but the rest of the vertebrae are merged to form a rigid structure. This is probably an adaptation to flight, as the powerful wing muscles need a rigid frame to work against. The epidermis over most of the body produces a covering of feathers but, on the legs and toes, the epidermis forms scales. The feathers are of several kinds. The fluffy down feathers form an insulating layer close to the skin; the contour feathers cover the body and give the bird its shape and coloration; the large quill feathers on the wing are vital for flight. Birds have four limbs, but the forelimbs are modified to form wings. The feet have four toes with claws, which help the bird to perch, scratch for seeds or capture prey, according to the species. The upper and lower jaws are extended to form a beak, which is used for feeding in various ways. Figure 1.23 shows the main features of a bird. In birds, fertilisation is internal and the female lays hard-shelled eggs in a nest where she incubates them (keeps them warm and safe). ▲ Figure 1.23 The main features of a pigeon (×0.14) Mammals Mammals are warm-blooded vertebrates with four limbs. They differ from birds because they have hair rather than feathers. Unlike the other vertebrates, they have a diaphragm, which plays a part in breathing (see Chapter 11). They also have mammary glands and suckle their young on milk. A sample of mammals is shown in Figure 1.14 on page 10 and Figure 1.24 shows some of the mammalian features. Humans are mammals. All mammals give birth to fully formed young instead of laying eggs. The eggs are fertilised internally and go through a period of development in the uterus (see ‘Sexual reproduction in humans’ in Chapter 16). ▲ Figure 1.24 Mammalian features. The furry coat, the external ear flaps (pinnae) and the facial whiskers are visible mammalian features in this gerbil The young may be blind and helpless at first (e.g. cats), or they may be able to stand up and move about soon after birth (e.g. sheep and goats). In either case, the youngster’s first food is the milk that it sucks from the mother’s teats. The milk is made in the mammary glands and contains all the nutrients that the offspring need for the first few weeks or months, depending on the species. As the youngsters get older, they start to feed on the same food as the parents. In the case of carnivores, the parents bring the food to the young until they can get food for themselves. ▼ Table 1.2 Key features of the five classes of vertebrates Test yourself 8 Make up a mnemonic involving the five classes of vertebrates. Test yourself using your mnemonic to see if it helps you remember the classes. 9 Which of the vertebrate classes have a cold blood b scaly skin c external fertilisation d gills for breathing? 1 0 Why do you think cold-blooded animals are slowed down by low temperatures? (See Chapter 5.) The five-kingdom scheme The kingdom is the largest group of organisms recognised by biologists. But how many kingdoms should there be? Most biologists used to opt for the use of two kingdoms: Plant and Animal. This, however, caused problems in trying to classify fungi, bacteria and single-celled organisms, which do not fit obviously into either kingdom. Many biologists now favour the five-kingdom scheme. This is a scheme that consists of Animal, Plant, Fungus, Prokaryote and Protoctist. It is still not easy to fit all organisms into the five-kingdom scheme. For example, many Protoctista with chlorophyll (the protophyta) show important similarities to some members of the algae, but the algae are classified into the plant kingdom. Viruses are not included in any kingdom – they are not considered to be living organisms because they do not have cell membranes (made of protein and fat), cytoplasm and ribosomes, and do not demonstrate the characteristics of living things: they do not feed, respire, excrete or grow. Although viruses do reproduce, this only happens inside the cells of living organisms, using materials provided by the host cell. This kind of problem will always occur when we try to come up with rigid classification schemes with clear boundaries between groups. The process of evolution cannot be expected to result in a tidy scheme of classification for biologists to use. Going further The three-domain scheme As scientists learn more about organisms, classification systems change. Genetic sequencing has provided scientists with a different way of studying relationships between organisms. The three-domain scheme was introduced by Carl Woese in 1978 and involves grouping organisms using differences in ribosomal RNA structure. Under this system, organisms are classified into three domains and six kingdoms, rather than five. Splitting the Prokaryote kingdom into two has created a sixth kingdom. The domains are: 1 Archaea: containing ancient prokaryotic organisms which do not have a nucleus surrounded by a membrane. They have an independent evolutionary history to other bacteria and their biochemistry is very different to other forms of life. 2 Eubacteria: prokaryotic organisms that do not have a nucleus surrounded by a membrane. 3 Eukarya: organisms that have a membrane-bound nucleus. This domain is subdivided into the kingdoms Protoctist, Fungus, Plant and Animal. A summary of the classification schemes proposed by scientists is shown in Figure 1.25. ▲ Figure 1.25 A summary of the classification schemes proposed by scientists The plant kingdom Plants are made up of many cells – they are multicellular. Plant cells have an outside wall made of cellulose. Many of the cells in plant leaves and stems contain chloroplasts with photosynthetic pigments (e.g. chlorophyll). Plants make their food by photosynthesis. The syllabus only requires knowledge of two groups – ferns and flowering plants. ▲ Figure 1.26 The plant kingdom; flowering plants Ferns Ferns are land plants with well-developed structures. Their stems, leaves and roots are very similar to those of the flowering plants. The stem is usually completely below ground. In bracken, the stem grows horizontally below ground, sending up leaves at intervals. Roots grow directly from the stem. The stem and leaves have sieve tubes and water-conducting cells like those in the xylem and phloem of a flowering plant (see Chapter 8). The leaves of ferns vary from one species to another (see Figures 1.27 and 1.28), but they are all several cells thick. Most of them have an upper and lower epidermis, a layer of palisade cells and a spongy mesophyll, like the leaves of a flowering plant. ▲ Figure 1.27 Young fern leaves. Ferns do not form buds like those of the flowering plants. The midrib and leaflets of the young leaf are tightly coiled and unwind as it grows Ferns produce gametes but no seeds. The zygote gives rise to the fern plant, which then produces single-celled spores from many sporangia (spore capsules) on its leaves. The sporangia are formed on the lower side of the leaf, but their position depends on the species of fern. The sporangia are usually arranged in compact groups (see Figure 1.29). ▲ Figure 1.28 The plant kingdom; ferns – one group of plants that does not bear seeds ▲ Figure 1.29 Polypody fern. Each brown patch on the underside of the leaf is made up of many sporangia Flowering plants Flowering plants reproduce by seeds that are formed in flowers. The seeds are enclosed in an ovary. The general structure of flowering plants is described in Chapter 8. Examples are shown in Figure 8.1 on page 130. Flowering plants are divided into two subclasses: monocotyledons and dicotyledons. Monocotyledons (monocots for short) are flowering plants that have only one cotyledon in their seeds. A cotyledon is an embryonic leaf which often contains food stores. Most, but not all, monocots also have long, narrow leaves (e.g. grasses, daffodils, bluebells) with parallel leaf veins (see Figure 1.30(a)). The dicotyledons (dicots for short) have two cotyledons in their seeds. Their leaves are usually broad, and the leaf veins form a branching network (see Figure 1.30(b)). The key features of monocots and dicots are summarised in Table 1.3. ▲ Figure 1.30 Leaf types in flowering plants ▼ Table 1.3 Summary of the key features of monocots and dicots Feature Monocotyledon Dicotyledon leaf shape long and narrow broad leaf veins parallel branching cotyledons one two grouping of flower parts (petals, sepals and carpels) threes fives Test yourself 1 1 The white deadnettle is Lamium album; the red deadnettle is Lamium purpureum. Would you expect these two plants to cross-pollinate successfully? Explain your answer. 1 2 If a fire destroys all the above-ground vegetation, the bracken (a type of fern) will still grow well in the next season. Suggest why this is so. As well as knowing the features used to place animals and plants into the appropriate kingdoms, you also need to know the main features of the following kingdoms: Fungus, Prokaryote and Protoctist. The Fungi kingdom Most fungi are made up of thread-like hyphae (see Figure 1.31), rather than cells, and there are many nuclei scattered throughout the cytoplasm in their hyphae (see Figure 1.32). ▲ Figure 1.31 The branching hyphae form a mycelium ▲ Figure 1.32 The structure of fungal hyphae The fungi include organisms such as mushrooms, toadstools, puffballs and the bracket fungi that grow on tree trunks (Figure 1.33). There are also the less obvious, but very important, mould fungi, which grow on stale bread, cheese, fruit or other food. Many of the mould fungi live in the soil or in dead wood. The yeasts are single- celled fungi that have some features similar to moulds. Some fungal species are parasites, as is the bracket fungus shown in Figure 1.33. A parasite is an organism living on another organism (the host), gaining food and shelter from it. It is a very one-sided relationship. Fungal parasites live in other organisms, particularly plants, where they cause diseases that can affect crop plants, such as the mildew shown in Figure 1.34. (See also Chapter 10.) ▲ Figure 1.33 A parasitic fungus. The ‘brackets’ are the reproductive structures. The mycelium in the trunk will eventually kill the tree ▲ Figure 1.34 Mildew on wheat. Most of the hyphae are inside the leaves, digesting the cells, but some grow out and produce the powdery spores seen here The Prokaryote kingdom These are the bacteria and the blue-green algae. They consist of single cells but are different from other single-celled organisms because their chromosomes are not organised into a nucleus. The structure of bacterial cells is described in Chapter 2, page 29. The Protoctist kingdom These are single-celled (unicellular) organisms which have their chromosomes enclosed in a nuclear membrane to form a nucleus. Some examples are shown in Figure 1.35. Some of the Protoctista (e.g. Euglena) have chloroplasts and make their food by photosynthesis. These Protoctista are often referred to as unicellular. Organisms like Amoeba and Paramecium take in and digest solid food and so are animal-like in their feeding. They may be called unicellular ‘animals’. Amoeba is a protozoan that moves by a flowing movement of its cytoplasm. It feeds by picking up bacteria and other microscopic organisms as it moves. Vorticella has a stalk that can contract and feeds by making a current of water with its cilia (tiny hair-like organelles which project from the cell surface). The current brings particles of food to the cell. Euglena and Chlamydomonas have chloroplasts in their cells and feed, like plants, by photosynthesis. ▲ Figure 1.35 Protoctista. Chlamydomonas and Euglena have chloroplasts and can photosynthesise. The others are protozoa and ingest (take in) solid food Viruses There are many different types of virus and they vary in their shape and structure. All viruses, however, have a central core of RNA or DNA (see Chapter 4) surrounded by a protein coat. Viruses have no nucleus, cytoplasm, cell organelles or cell membrane, though some forms have a membrane outside their protein coats. So, virus particles are not cells. They do not feed, respire, excrete or grow, and it is arguable whether they can be classed as living organisms. Viruses do reproduce, but only inside the cells of living organisms, using materials provided by the host cell. A generalised virus particle is shown in Figure 1.36. The nucleic acid core is a coiled single strand of RNA with a protein coat. The protein coat is called a capsid. ▲ Figure 1.36 Generalised structure of a virus One example of a virus is the influenza virus (Figure 1.37). ▲ Figure 1.37 Structure of the influenza virus Test yourself 1 3 Figure 1.35 shows some Protoctista. Using only the features shown in the drawings, construct a dichotomous key that could be used to identify these organisms. 1 4 Classify the following organisms: beetle, sparrow, weasel, gorilla, bracken, buttercup. For example, butterfly: Kingdom, animal; Group, arthropod; Class, insect. 1 5 Which kingdoms contain organisms with a many cells b nuclei in their cells c cell walls d hyphae e chloroplasts? Revision checklist After studying Chapter 1 you should know and understand the following: ✔ Living things show seven characteristics. ✔ A species is a group of organisms that can reproduce to produce fertile offspring. ✔ The binomial system of naming species is an internationally agreed system in which the scientific name of an organism is made up of two parts, showing the genus and species. ✔ Classification systems are ways of sorting organisms into groups by the features that they share, and such systems try to reflect evolutionary relationships. ✔ Keys are used to identify unfamiliar organisms, with dichotomous keys having two branches at each stage. ✔ The sequences of bases in DNA are used as a means of classification. ✔ Groups of organisms that share a more recent ancestor (are more closely related) have more similar base sequences in their DNA than those that share only a distant ancestor. ✔ Animals get their food by eating plants or other animals. ✔ Plants make their food by photosynthesis. ✔ Arthropods have a hard exoskeleton and jointed legs. The classes of arthropods are arachnids, insects, crustaceans and myriapods. ✔ Vertebrates have a spinal column and skull. The classes of vertebrates are amphibia, birds, fish, mammals and reptiles. ✔ Prokaryotes are microscopic organisms; they have no proper nucleus. ✔ Protoctists are single-celled organisms containing a nucleus. ✔ Fungi are made up of thread-like hyphae. They reproduce by spores. ✔ Ferns have well developed stems, leaves and roots. They reproduce by spores. ✔ Seed-bearing plants reproduce by seeds. ✔ Flowering plants have flowers; their seeds are in an ovary which forms a fruit. They are subdivided into monocots and dicots. ✔ Viruses do not possess the features of a living organism. Exam-style questions 1 Amoeba is a single-celled organism which shows several characteristics of living things, such as excretion, irritability and nutrition. State three other characteristics of living things you would expect this organism to show. 2 a Define the term species. b With reference to the tiger, Panthera tigris, what does the term binomial system mean? 3 a State the main characteristics of arthropods. b By means of a table, state three differences between insects and myriapods. 4 List the main characteristics of a a fungus b a bacterium 5 Design a dichotomous key to divide the vertebrates into classes. Give one example of a species of each of the classes named in your key. Start with the question, ‘Is the animal warm-blooded?’ 6 The table shows the proportions of all known species in each of the main groups of organisms. Group of organisms Proportion of all known species / % arachnids 4.5 bacteria, viruses 0.5 crustaceans 2.4 fungi 4.2 insects 56.3 other arthropods 1.2 other invertebrates (not arthropods) 9.1 plants 14.3 protoctists 4.8 vertebrates 2.7 a i) Apart from insects, which group of organisms has the most known species? ii) Assuming that there are four groups of arthropods, what percentage of all known species are myriapods? iii ) Fungi are listed separately from plants. State two reasons why fungi are not classified as plants. b Using data from the table, calculate what percentage of arthropods are arachnids. c i) Birds and fish are two classes of vertebrates. State the names of the other three classes. ii) State one feature that distinguishes fish from all the other vertebrate classes. d It is estimated that 1.9 million species of organisms have been named. Use data from the table to calculate the total number of insects known. Show your working. 7 a Distinguish between the following groups of organisms: i) monocotyledons and dicotyledons ii) amphibians and reptiles b Define the term binomial system. State one example to support your answer. 2 Organisation of the organism Focus In the previous chapter you recognised the characteristics present in all living organisms and used a mnemonic to help you remember these. You were introduced to reasons for classifying organisms into groups and the use of the binomial system of naming species. You had the opportunity to develop your own dichotomous keys based on identifiable features. Then you learned about some of the main animal and plant groups. In this chapter you will discover the main differences between animal, plant and bacterial cells, as well as the functions of their parts. Within an organism there are levels of organisation. By the end of the chapter you will be able to name these and describe examples from animals and plants. Why are cells different shapes? What jobs do they do? How can we work out their magnification when looking at them? By studying the chapter carefully and following the practical suggestions you should be able to answer these questions. Cell structure and organisation FOCUS POINT What are the structures and functions of plant, animal and bacterial cells? How do you identify cell structures in diagrams and images of animal, plant and bacterial cells? What are the differences between a plant and an animal cell? How are new cells produced? What are the specific functions of these specialised cells: ciliated cells root hair cells palisade mesophyll cells neurones red blood cells sperm and egg cells (gametes)? What are the meanings of the terms cell, tissue, organ, organ system and organism? Cell structure If a very thin slice of a plant stem is cut and studied under a microscope, the stem appears to consist of thousands of tiny, box-like structures. These structures are called cells. Figure 2.1 is a thin slice taken from the tip of a plant shoot and photographed through a microscope. It is 60 times larger than life, so a cell which appears to be 2 mm long in the picture is only 0.03 mm long in reality. ▲ Figure 2.1 Longitudinal section through the tip of a plant shoot (×60). The slice is only one cell thick, so light can pass through it and allow the cells to be seen clearly Thin slices like this are called sections. If you cut along the length of the structure, you are taking a longitudinal section (Figure 2.2(b)). Figure 2.1 shows a longitudinal section, which passes through two small developing leaves near the tip of the shoot, and two larger leaves below them. The leaves, buds and stem are all made up of cells. If you cut across the structure, you make a transverse section (Figure 2.2(a)). ▲ Figure 2.2 Cutting sections of a plant stem You can cut sections through plant structures quite easily just by using a razor blade. Cutting sections of animal structures is more difficult because they are mostly soft and flexible. Pieces of skin, muscle or liver, for example, first must be soaked in melted wax. When the wax goes solid it is then possible to cut thin sections. The wax is dissolved away after making the section. When sections of animal structures are examined under the microscope, they too are seen to be made up of cells, but they are much smaller than plant cells and need to be magnified more. The photomicrograph of kidney tissue in Figure 2.3 has been magnified 700 times to show the cells clearly. The sections are often treated with dyes, called stains, in order to make the structures inside the cells show up more clearly. ▲ Figure 2.3 Transverse section through a kidney tubule (×700). A section through a tube will look like a ring (see Figure 2.7(b)). In this case, each ‘ring’ consists of about 12 cells Making sections is not the only way to study cells. Thin strips of plant tissue, only one cell thick, can be pulled off stems or leaves (experiment 1, pages 30–31). Plant or animal tissue can be squashed or smeared on a microscope slide (experiment 2, page 31), or treated with chemicals to separate the cells before studying them. There is no such thing as a typical plant or animal cell because cells vary a lot in their size and shape depending on their function. However, it is possible to make a drawing, like that in Figure 2.4, to show the features that are present in most cells. All cells have a cell membrane, which is a thin boundary enclosing the cytoplasm. Most cells have a nucleus. ▲ Figure 2.4 A group of liver cells. These cells have all the characteristics of animal cells Cytoplasm Under the ordinary microscope (light microscope), cytoplasm looks like a thick liquid with particles in it. In plant cells it may be seen to be flowing about. The particles may be food reserves like oil droplets or granules (small particles) of starch. Other particles are structures known as organelles, which have special functions in the cytoplasm. In the cytoplasm, large numbers of chemical reactions are taking place, which keep the cell alive by providing energy and making substances that the cell needs. The liquid part of cytoplasm is about 90% water, with molecules of salts and sugars dissolved in it. Suspended in this solution there are larger molecules of fats (lipids) and proteins (see Chapter 4). Fats and proteins may be used to build up the cell structures, like the membranes. Some of the proteins are enzymes (see Chapter 5). Enzymes control the rate and type of chemical reactions that take place in the cells. Some enzymes are attached to the membrane systems of the cell, while others float freely in the liquid part of the cytoplasm. Cell membrane This is a thin layer of cytoplasm around the outside of the cell. It stops the cell contents from escaping and controls which substances can enter and leave the cell. In general, oxygen, food and water are allowed to enter; waste products are allowed to leave; and harmful substances are kept out. In this way the cell membrane maintains the structure and chemical reactions of the cytoplasm. Nucleus (plural: nuclei) Most cells contain one nucleus, which is usually seen as a rounded structure covered by a membrane and fixed in the cytoplasm. In drawings of cells, the nucleus may be shown darker than the cytoplasm because, in prepared sections, it takes up certain stains more strongly than the cytoplasm. The function of the nucleus is to control the type and quantity of enzymes produced by the cytoplasm. In this way it regulates the chemical changes that take place in the cell. As a result, the nucleus controls what the cell will be, for example, a blood cell, a liver cell, a muscle cell or a nerve cell. When existing cells divide, new cells are produced. The nucleus controls cell division, as shown in Figure 2.5. A cell without a nucleus cannot reproduce. Inside the nucleus are thread-like structures called chromosomes, which can be seen most easily at the time when the cell is dividing (see Chapter 17 for a fuller account of chromosomes and cell division). ▲ Figure 2.5 Cell division in an animal cell Plant cells A few generalised animal cells are shown in Figure 2.4, while Figure 2.6 is a drawing of two palisade cells from a plant leaf. (See ‘Leaf structure’ in Chapter 6.) ▲ Figure 2.6 Palisade cells from a leaf Plant cells differ from animal cells in several ways because they have extra structures: a cell wall, chloroplasts and sap vacuoles. Cell wall The cell wall, which is outside the membrane, contains cellulose and other compounds. It is non-living and allows water and dissolved substances to pass through it. The cell wall is not selective like the cell membrane. (Note that plant cells do have a cell membrane, but it is not easy to see or draw because it is pressed against the inside of the cell wall (see Figure 2.7).) Under the microscope, plant cells are quite distinct and easy to see because of their cell walls. In Figure 2.1 it is only the cell walls (and in some cases the nuclei) that can be seen. Each plant cell has its own cell wall but the boundary between two cells side by side does not usually show up clearly. So, cells next to each other appear to be sharing the same cell wall. Vacuole Most mature plant cells have a large, fluid-filled space called a vacuole. The vacuole contains cell sap, a watery solution of sugars, salts and sometimes pigments. This large, central vacuole pushes the cytoplasm outwards so that it forms just a thin lining inside the cell wall. It is the outward pressure of the vacuole on the cytoplasm and cell wall that makes plant cells and their tissues firm (see ‘Osmosis’ in Chapter 3). Animal cells may sometimes have small vacuoles in their cytoplasm, but they are usually produced to do a special job and are not permanent. Chloroplasts Chloroplasts are organelles that contain the green substance chlorophyll (see Chapter 6). ▲ Figure 2.7 Structure of a palisade mesophyll cell. It is important to remember that, although cells look flat in sections or in thin strips of tissue, they are three- dimensional and may seem to have different shapes depending on the direction in which the section is cut. If the cell is cut across it will look like (b); if cut longitudinally it will look like (a) The shape of a cell when seen in a transverse section may be quite different from when the same cell is seen in a longitudinal section, and Figure 2.7 shows why this is so. Figures 8.4(b) and 8.4(c) on page 132 show the appearance of cells in a stem vein as seen in transverse and longitudinal sections. ▼ Table 2.1 Summary: the parts of a cell When studied at much higher magnifications with the electron microscope, the cytoplasm of animal and plant cells no longer looks like a structureless jelly. It appears to be organised into a complicated system of membranes and vacuoles. Ribosomes are one of the organelles present. They may be held on a membrane but can also be found free in the cytoplasm. They build up the cell’s proteins (see Chapter 4). Mitochondria are tiny organelles, which may appear slipper- shaped, circular or oval when viewed in section. In three dimensions, they may be spherical, rod-like or extended. They have an outer membrane and an inner membrane with many inward-pointing folds. Mitochondria are most frequent in regions of rapid chemical activity. They are responsible for releasing energy from food substances through the process of aerobic respiration (see Chapter 12). Note that prokaryotes do not possess mitochondria in their cytoplasm. Figure 2.8(a) is a diagram of an animal cell magnified 10 000 times. Figure 2.8(b) is an electron micrograph of a liver cell. Organelles in the cytoplasm can be seen clearly. They have recognisable shapes and features. Figure 2.8(c) is an electron micrograph of a plant cell. As well as the organelles already named and described, other organelles are also present, like chloroplasts and a cell wall. ▲ Figure 2.8 Cells at high magnification Test yourself 1 a What structures are usually present in both animal and plant cells? b What structures are present in plant cells but not in animal cells? 2 What cell structure is mainly responsible for controlling the entry and exit of substances into or out of the cell? 3 How does a cell membrane differ from a cell wall? Bacterial cell structure Bacteria (singular: bacterium) are very smallorganisms that are single cells not often more than0.01 mm in length. They can be seen only at high magnification under a microscope. They have a cell wall made of a complicatedmixture of proteins, sugars and fats. (You willremember that plant cell walls are made of cellulose.) Inside the cell wall is the cytoplasm, which may contain granules (small particles) of glycogen, fat and other food reserves (see Figure 2.9). Large numbers of ribosomes float freely in the cytoplasm. They are smaller than the ribosomes found in plant and animal cells but have the same function of protein synthesis. ▲ Figure 2.9 Generalised diagram of a bacterium ▲ Figure 2.10 Longitudinal section through a bacterium(×27 000). The light areas are coiled DNA strands. There are three of them because the bacterium is about to divide twice (see Figure 2.11) ▲ Figure 2.11 Bacterium reproducing. This is asexual reproduction by cell division (see ‘Asexual reproduction’ in Chapter 16 and ‘Mitosis’ in Chapter 17) Each bacterial cell contains a single chromosome made of a circular strand of DNA (see Chapter 4 and ‘Chromosomes, genes and proteins’ in Chapter 17). The chromosome is not surrounded by a nuclear membrane but is coiled up to fill a small part of the cell, asshown in Figure 2.10. There are also smaller circular structures called plasmids, which are also made of DNA. Plasmids are used by scientists in the process of genetic modification because it is relatively easy to insert genetic material into them (see Chapter 21). Bacteria can be different shapes: they may be spherical, rod- shaped or spiral. Some have filaments, called flagella, projecting from them. The flagella can flick and so move the bacterial cell about. The functions of the structures in a bacterium are shown in Table 2.2. ▼ Table 2.2 Summary: the parts of a bacterial cell Test yourself 4 How is a bacterial cell different from a plant cell? 5 Bacteria and plant cells both have a cell wall. In what way are the cell walls different? Practical work For safe experiments/demonstrations which are related to this chapter, please refer to the Biology Practical Skills Workbook that is also part of this series. Safety Eye protection must be worn. Take care when using a scalpel, follow your teacher’s guidance. Take care using the iodine and methylene blue stains – they will stain skin and clothing. Looking at cells 1 Plant cells – preparing a slide of onion epidermis cells The onion contains a very useful source of epidermal plant tissue which is one cell thick. This makes it quite easy to set up as a temporary slide. The onion is made up of fleshy leaves. On the incurve of each leaf there is an epidermal layer which can be peeled off (Figure 2.12(a)). Using forceps, peel a piece of epidermal tissue from the incurve of an onion bulb leaf. Place the epidermal tissue on a glass microscope slide. Using a scalpel, cut out a 1 cm square of tissue (throw away the rest) and arrange it in the centre of the slide. Add two to three drops of iodine solution. (This stains any starch in the cells and makes different parts of the cells distinct.) Using forceps, a mounted needle or a wooden splint, support a cover-slip with one edge resting near to the onion tissue, at an angle of about 45° (Figure 2.12(b)). Gently lower the cover-slip over the onion tissue. Try to avoid trapping any air bubbles.(Air bubbles reflect light when viewing under the light microscope, hiding the features you are trying to see.) Leave the slide for about 5 minutes. This allows the iodine stain to react with the specimen. The iodine stains the cell nuclei pale yellow and the starch grains blue. Place the slide on to the microscope stage, choose the lowest power objective lens and focus on the specimen. Increase the magnification using the other objective lenses. Under high power, the cells should look like those shown in Figure 2.13. An alternative tissue is rhubarb epidermis (Figure 2.12(c)). You can strip this off from the surface of a stalk and treat it in the same way as the onion tissue. If you use red epidermis from rhubarb stalk, you will see the red cell sap in the vacuoles. ▲ Figure 2.12 Looking at plant cells ▲ Figure 2.13 Onion epidermis cells 2 Plant cells – preparing cells with chloroplasts Using forceps, remove a leaf from a moss plant. Place the leaf in the centre of a microscope slide and add one or two drops of water. Place a cover-slip over the leaf. Examine the leaf cells with the high power objective of a microscope. The cells should look like those shown in Figure 2.14. ▲ Figure 2.14 Cells in a moss leaf (×500). The vacuole occupies most of the space in each cell. The chloroplasts are limited to the layer of cytoplasm lining the cell wall 3 Animal cells – preparing human cheek cells Human cheek cells are constantly being wiped off the inside of the mouth when the tongue and food rub against them, so they can be collected easily for use in a temporary slide. Note: Check local guidance to whether observing cheek cells is permitted. Use appropriate precautions to treat contaminated items with disinfectant or by autoclaving. Rinse your mouth with water. This will remove any fragments of food. Take a cotton bud from a freshly opened pack. Rub the cotton bud lightly on the inside of your cheek and gums to collect some cheek cells in saliva. Rub the cotton bud on to the centre of a clean microscope slide, leaving a sample of saliva. Repeat if the sample is too small. Then drop the cotton bud into a container of absolute alcohol or disinfectant. Add two to three drops of methylene blue dye. (This stains parts of the cheek cells to make nuclei more visible.) Using forceps, a mounted needle or wooden splint, support a cover- slip with one edge resting near to the cheek cell sample, at an angle of about 45°. Gently lower the cover-slip over the tissue. Try to avoid trapping any air bubbles. (Air bubbles reflect light when viewing under the light microscope, hiding the features you are trying to see.) Leave the slide for a few minutes. This allows the methylene blue stain to react with the specimen. ▲ Figure 2.15 Cells from the lining epithelium of the cheek (×1 500) Place the slide on to the microscope stage, choose the lowest power objective lens and focus on the specimen. Increase the magnification using the other objective lenses. Under high power, the cells should look like those shown in Figure 2.15, but less magnified. When you have completed the ‘Test yourself’ section, place your used slide in laboratory disinfectant before washing. 4 Animal cell – preparing human skin cells You can try another method of obtaining cells if the previous method is not suitable. Wash your wrist well, then press some transparent sticky tape on to the cleaned area of skin. Remove the tape and stick it to a microscope slide. Place the slide on the microscope stage. Look for cells. You should be able to see nuclei in them. If you add a few drops of methylene blue solution before putting the tape on the slide, the cells take up the stain and it makes the nuclei more distinct. Practical work questions 1 In experiment 1, what cell structures could you identify in the onion cells you observed? 2 a For experiment 2, explain why the chloroplasts appear to be pressed against the cell wall of the cell. b Why are the chloroplasts green? 3 For experiment 2, explain why it is necessary to use a stain when preparing specimens of cells. 4 In experiments 3 and 4, the skin cells are animal epidermal cells. Plants also have epidermal cells. Compare a human skin epidermal cell with an upper epidermal cell of a leaf (see Figure 6.26 on page 104). What cell structures do leaf epidermal cells have which are not present in human epidermal cells? Test yourself 6 Make a large drawing of one cell and labelthe following parts: cell wall, cell membrane,cytoplasm, nucleus. 7 Make a note of the magnification of the eyepiece and objective lenses of your microscope. 8 Copy and complete the table by a writing the magnification of the eyepiece lens b writing the magnification of the objective lens of your microscope which you used to make your drawing c calculating total magnification provided by the microscope. magnification of the eyepiece lens magnification of the objective lens total magnification provided by the microscope 9 Estimate how much bigger your drawing is than the image you see through the microscope. Use these figures to calculate the total magnification of your drawing. Specialisation of cells When cells have finished dividing and growing, most become specialised and have specific functions. When cells are specialised: they do one special job they develop a distinct shape special kinds of chemical changes take place in their cytoplasm. The changes in shape and the chemical reactions enable the cell to carry out its special function. Red blood cells and root hair cells are just two examples of specialised cells. Figure 2.16 shows a variety of specialised cells. The specialisation of cells to carry out special functions in an organism is sometimes called ‘division of labour’ within the organism. Similarly,the special functions of mitochondria, ribosomes and other cell organelles may be called division oflabour within the cell. ▲ Figure 2.16 Specialised cells (not to scale) Test yourself 1 0 In what way does the red blood cell shown in Figure 2.16(e) differ from most other animal cells? 1 1 Why does the cell shown in Figure 2.7(b) appear to have no nucleus? Tissues, organs, organ systems and the organism FOCUS POINT Definitions of tissues, organs, organ systems and organism Some microscopic organisms are made of one cell only (see ‘Features of organisms’ in Chapter 1). These can carry out all the processes needed to keep them alive. The cells of the larger plants and animals cannot survive on their own. A muscle cell could not obtain its own food and oxygen. Other specialised cells provide the food and oxygen needed for the muscle cell to live. Unless these cells are grouped together in large numbers and made to work together, they cannot stay alive. Tissues A tissue, like bone, nerve or muscle in animals, and epidermis, xylem or pith in plants, is made up of large numbers of cells. These are often just a single type. The cells of each type have a similar structure and function so that the tissue itself has a special function. For example, muscles contract to cause movement, xylem carries water in plants. Figure 2.17 shows how some cells are arranged to form simple tissues. Some forms of tissues

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