endorsed for edexcel = Edexcel AS/A level BIOLOGY B ] Ann Fullick PEARSONPublished by Pearson Education Limited, 0 Strand, London WC2R ORL. ‘www pearsonschoolsandfecclleges co uk Copies of oficial specifications for all Edexcel qualifications may be found on the website vveww.edexcelcom Text © Ann Fullick ‘Exam-style questions © Pearson Education Limited Baited by Natalie Bayne and Jo Egré Designed by Elizabeth Amous for Pearson Education Limited ‘Typeset by Tech-Set Ltd, Gateshead (Oniginalilustrations © Pearson Education Limited 2015 Mlustrated by Tech-Set Lid, Gateshead and Peter Bull Art Suidio Cover design by Elizabeth Amoux for Pearson Education Limited Picture research by Caitlin Swain (Cover photo illustration © Science Photo Library/King's College London ‘The rights of Ann Fullick and Graham Hartland to be identified as authors ofthis work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988, First published 2008 ‘Second ecition published 2015 191817 16 10987654 British Library Cataloguing in Publication Data ‘A catalogue record for this book is available from the British Library ISBN 9781447001144 Copyright notice All rights reserved. No part ofthis publication may be reproduced in any form or by any means {including photocopying or storing tin any medium by electronic means and whether or not transiently Or incidentally to some other use ofthis publication) without the written permission ofthe copyright ‘owner, except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms ofa licence issued by te Copyright Licensing Agency, Sallron House, 6-10 Kirby Street London ECIN 8TS (www.claco.uk). Applications Tor he copyright owner's written permission should be addressed to the publisher. Printed in Italy by Lego SpA Acknowledgements [Every effort has Been made to contact copyright holders ef material reproduced in this book. Any (omissions will be rectified in subsequent printings if notice is given to the publishers. Anote from the publisher In order to ensure that this resource offers high-quality support forthe associated Edexcel qualification, it hhas been through a review process by the awarding body to confirm that it fully covers the teaching and Jeaming content ofthe specification or part ofa specification at which itis aimed, and demonstrates an ‘appropriate balance between the development of subject skis, knowledge and understanding, in addition to preparation for assessment, ‘While the publishers have made every attempt to ensure that advice on the qualification and its ‘assossment is accurate, the oficial specification and associated agsesement guidance materials are the only authoritative source of infomation and should always be veferred to for definitive gudance. ‘Edexcel examiners have not contributed to any sections in this resource relevant to examination papers {or which they have responsibilty [No material from an endorsed book: willbe used verbatim in any assessment set by Edexcel Endorsement ef a Book does not mean thatthe book is required to achieve this Edexcel qualification, nor does it mean that its the only suitable material available to support the qualification, and any resource lists produced by the awarding body shall include this and other appropriate resources,Picture credits “The publisher would like to thank the following for thei kind emission to reproduce their photograph: (Key: bottom; e-centve: [eft right; -19p) ‘Alamy Images: Ann and Steve Toon 168cr, Arco Images GmbH 208, BSIP SA T3br, Custom Life Science Images 33, Edwin Remsberg ?0br, INTERFOTO 70bl, Jeremy Sutton-tibbert 102, ‘Mark Conlin 239, Nigel Catlin 243bl, Patrick J. Endres 150-151, Picture Partners 120cl, Roberto Nistri 186, Scott Camazine 248, Steve Bloom Images 198, Stone Nature Photography 184c, The Natural History Museum 160br, 16Gcl; Anthony Short: 8-9, 10cr, 13, 265, 110-111. 118, 126-127, 152, 153, 155, 155cl. 1568, 159U, 1591 163, 172-173, 175, 17601, 179, 184cl, 188, 192-193, 1941, 1960, 204, 206el, 213, 234-225, 232, 233, 238br, 290, Ardea: Bill Coster 4 78br/b, John Mason 178br/t; Biodiversity Institute of Ontario: 159r; Copperhead Institute: Chuck Smith 119; Corbis: Carolina Biological/ Visuals Unlimited 116, CDC/PHIL 160cr, Vierame 436-47, David Grémillet: 177d, 177i; DK Images: Liicy Claxton 15601, FLPA Images of Nature: Fritz Poking 205cr, Fotolia, ‘com: Simone Werner-Ney 202, tomatito26 2601: Getty Images: (Chuis Jackson 205, De Agostini Picture Library 288, Dr. Brad Mogen. 292, Glyn Kirk/AFP 20dbr, Matt Cardy 203, moodboard 133, Peter ‘sei 54-55, Photolibraty 76cl, Photolibrary/Ed Reschke 83, Ralph Slepecky/Visuals Unlimited, Inc. 98tr, Roland Birke 226, Veta 23: Pearson Education Ltd: 2251, Photoshot Holdings Limited: ‘Oceans-Image 251; Phototake, Inc: ISM abl: Professor Legesse ‘Negash: SEC 207; Science Photo Library Ltd: A. Dowsett. Health Protection Agency 106, 70bc, 74, 130/2, 244-245, 258, 284cr, Adrian Bicker 156br. AM Images 138, Andzew Lambert Photography 20, 31, ‘Asa Thoresen 880i, Athenais, SM 264d, Biology Pics 83. Biophoto Associates 84, 8b, 0b, 130 (al), 176r, 255br, Chuck Brown 254, CCNRI 9041, 98, D Phllips 140, 256tr, David M. Phillips 217c, David ‘MeCarthy 122, David Schaef 117, Dirk Wiersma 157, Don W. Fawoett 7atl, Dr. P. Marazzi 261, Dr. Yorgos Nikas 143, Dr. Gopal Mut 76bi, 82, 90r, Dr. Jeremy Burgess 86, 87.237, Dr. John Brackenbury 176cl, Dr. Kari Lounatmaa 75tr, Dr. Keith Wheeler 23801, Dr. Richard Kessel & Dr. Gene Shib, Visuals Unlimited 243br, Dr. Rosalind King. br, Dr. Stanley Flegler/Visuals Unlimited, Ine, 217, 217bl, Dr. Stanley Flegler, Visuals Unlimited 210-211, Bye of Science 16-17, 101, 182er, Herve Conge, ISM. 124b), 278, Innerspace Imaging @8be, JC. Revy, ISM 217t, Jackie Lewin, Royal Free Hospital 5, James King-Hoimes 146, Juan Gaeriner 40, Lee D, Simon 100, Lack at Sciences 112, Louise Hughes 56, Louise Murray 15Str, MI. Walker 121, Martin Oeggerli 92-98, Martin Shields 22, Martyn F. Chillmaid 18, Medimage 23b, Mehau Kulyk 1200, Michael Abbey 217cr, NIBSC 275bl, Omikron 98, Pascal Goetgheluck 162, PHOTOTAKE Inc. 78, Power and Syred 23t, 91, 279, Pr. G. Gimenez-Martin 1244, Professor . Motta & D. Palermo 115, Professors P. Motta & T. Naguro 77, Richard J. Green 95cr, Science Source 221, Scott Camazine 285, Sovereign, ISM 265, Steve Gschmeisaner 72, 727,231, 253, Ted ‘Kinsman 243be, Thomas Ames Jr, Visuals Unlimited 243tr, Thomas Deerinck, NCMIR 68-69, Tom Kinsbergen 182t! Shutterstock. ‘com: Alexey Repka 154, Debra James 206tr, Egon Zitter 194c, ‘éreamphoto 12, Image Point Fr 267, Jeff Dalton 15Scr, Jim Lopes 284, Joe! Bit 163t, Nicky Rhodes 197. Picsfive 272, Svtlana S. 144, Viad61 194r; The University of California: Alex MePherson, levine/National Institute of General Medical Sciences 97: US. Department of Agriculture: Agricultural Research Service 158: Veer/Corbis: Backyard Productions 174, enjoylfe2® 183, gbrouwer 200, goce risteski 128, lchschen 276-27, marilyna 64, Nyker 184cr, prochassen fiederic 10b); Wellcome Trust Sanger Institute: 4 Cover images: Front Science Photo Library Ltd: King’s College Londen All other images © Pearson Education Limited Picture Research by: Caitlin Swain ‘We are grateful to the following for permission to reproduce copyright materia: i Figure on page 22 from “Trehalase’ an intriguing disaccharide ‘with potential for medical application in ophthalmology’. Clinical ‘ophthalmology, 5, 577 (2011), Clinieal Ophthalmology by Society for Clinical Ophthalmology (Creat Britain) Reproduced with permission ‘of ove Medical Press Limited in the format Republish in a book via Copyright Clearance Center, Figure on page 203 from the front cover of the DEFRA publication "What nature can do for you’, hitps:/ ‘wwe gov.uk/ government/uploads/system/uploads/attachment_ {ata /fle/221097 pb 3897-nature-do-for-you pa, Published by the Depariment for Environment, Food and Rural Affairs. © Crown Copyright 2010; Figure on page 240 from http://uwwwabpischools, ‘orguk/ page/modules/breathingandasthma/asthma? cfzn, ABPL Resources for Schools, Association ofthe British Pharmaceutical Industry (ABPI) with permission, Text Article on page 32 fiom "Trehalose: an intriguing diseccharide ‘with potential for medical application in ophthalmology’. Clinical ophthalmology, 5, 577 (2011), Clinical Ophthalmology by Society for Clinical Ophthalmology (Great Britain) Reprocuced with permission of Dove Medical Press Limited in the format Republish in a book via Copyright Clearance Center, Article on page 106 adapted trom ‘Deadly Ebola virus “could spread globally” alter plane brings itto Nigeria’, Daily Mail 28/07/2014 (Nick Fagge). Daily Mail: Article on page 106 adapted from ‘Epidemiology and surveillance http://www, afro who int/en/ clusters-a-programmes/dpe/epidemic-a-pandemic- alert-and- response outbreak-nevis/4236-ebola-virus-disease-west- afrea-29-july-2014 him. © Copyright World Health Organization (WHO) ~ Regional Office for Arica. 2013. Al rights reserved. Article on page 106 from hip: /ww wales.nhsuk/sitesplus/888/ page/74508, Public Health Wales; Extact on page 146 adapted from ‘In vitro ferlisation’, Heinemann Library (Flick, A); Poetry con page 168 trom "Oxford Ragwor’ (Short, G), with permission from Anthony Short; Article on page 188 adapted from ‘Quagga rebreeding 2 success story, Farmer's Weekly (Harvey. K), © 2014 Fonner’s Weekly Magazine: Anicle on page 188 from ‘A vapid loas cf stipes: the evolutionary history ofthe extinct ouages’. September 2008 Volume: 1 Issue: 3 (Jennifer A. Leonard etal), Copyright © 2014, The Royal Society, Anicle on page 240 from hitp.// www. abpischools org uk/page/modales/ breathingandasthnna/asthma7, clin, ABP] Resources for Schools, Association of the British Pharmaceutical Industry (ABPI) with permission; Article on page 200 from Encyclopedia of Life Sciences, John Wiley & Sons, Lid (Turgor Prossure by Jeremy Prichard, University of Birmingham 2001) © 2001, John Wiley & Sons, Lid Reproduced with permission of Blackwell Publishing, ‘The Publisher would lke to thank Chris Curtis and Wade Nottingham {for their contributions tothe Maths skills section of this book. ‘The author would like to acknowledge and thank the teams at Science and Plants for Schools (SAPS), the Wellcome Trust Sanger Institute ‘and the ABPI for thei valuable input. The author would also like to thank the following for their support and individual contributions Dr Jeremy Pritchard: Alice Kelly: Amy Ekins-Coward: Tony Short ‘Wiliam Pulls Thomas Pulick: James Fulick, Eéward Pulick Chris Short Every effort has been madle to contact copyright holders of material reproduced in this book. Any omissions will be rectified in subsequent printings i notice is given to the publishersContents How to use this book TOPIC 1 Biological molecules 1.1 Chemistry for life 1 Chemistry for lie Exam-style questions 1.2 Biological molecules 1 1 Carbohydrates 1 ~ monosaccharides and. disaccharides 2 Catbchydrates 2 ~ polysaccharides 3. Lipids 4 Proveins Thinking Bigger Exam-style questions 1.3 Biological molecules 2 Nucleotides and ATP Nucleic acids How DNA works ‘The genetic code DNA and protein synthesis Gene mutation ‘zam-style questions 1.4 Enzymes 1 Emymes 2. How enzymes work 3 Enzyme inhibition Thinking Bigger Exam-style questions 6 10 14 16 18 at 25 28 32 34 36 38 42 47 50 52 54 56 58 62 64 66 TOPIC 2 Cells and viruses 2.1 Eukaryotic cells 1 Observing cells 2 Cell membranes 3 Eukaryotic cells 1 ~ common cellular structures 4. Bukaryotic cells 2 ~ protein transport 5 Eukaryotic cells 3 ~ plant cell structures 6 Bukaryotic cells 4 — plant organelles 7 The organisation of cells Exam-style questions 2.2. Prokaryotic cells 1 Prokaryotic cells 2 Viruses 3. Controlling viral infeetiona ‘Thinking Bigger Exam-style questions 2.3. Eukaryotic cell division - mitosis 1 The cell eycle 2 Mitosis 3 Asexual reproduction 4 Growth and repair ‘Thinking Bigger Exam-style questions 24. Meiosis and sexual reproduction 1 Sexual reproduction and meiosis, 2 Mutations 3 Gametogenesis, 4 Fentlsation in mammals and plants 5 Embryo development in mammals, ‘Thinking Bigger Exam-style questions 68 70 14 76 20 83 86 88 90 92, 94 98 102 106 108 ao 12 ana uur 120 122 124 126 128 132 135 140 143 148 148a TOPIC 3 Classification 3.1 Classification 32 33 1 Principles of classification What isa species? Identifying individual species New evidence for evolution Domains, kingdoms or both? ‘Thinking Bigger Exam-style questions Natural selection 1 2 3 4 Evolution and adaptation ‘Natural selection in action ‘The evolutionary race between pathogens and medicines Speciation ‘Thinking Bigger Examvstyle questions Biodiversity 1 2 3 4 ‘The importance of biodiversity Biodiversity within a species Beosystem services Ex-situ and in-situ conservation Examstyle questions 150 152 154 158 160 162 168 170 172 174 178 181 183 188 190 192 194 199 202 204 208 TOPIC 4 Exchange and transport aq 42 a3 aa Cell transport mechanisms 1 Trangpor in cells 2. Diffusion and facilitated diffusion 3. Osmosis ~a special case of diffusion 4 Active transport Exam-style questions Gas exchange 1 The need for gas exchange surfaces 2. The mammalian gas exchange system 3 Gas exchange in insects 4 Gas exchange in fish 5 Gas exchange in plants ‘Thinking Bigger Exam-style questions Circulation 1 Principles of circulation 2 The roles of the blood 3 Transporting oxygen and carbon dioxide 4 Blood circulation 5 The human heart 6 Controlling the heart 7 Atherosclerosis 8 Risk factors for atherosclerosis 9 Tissue fluid and lymph ‘Thinking Bigger Exam-style questions Transport in plants 1 Transport tissues in plants 2 The uptake of water by plants 3 Translocation of sucrose Thinking Bigger Exam-style questions Maths sills Preparing for your exams Glossary Index 210 212 214 216 220 222 224 226 228 232 234 236 240 242 244 246 248 250 254 257 260 263 266 270 272 274 276 278 281 287 290 292 294 300 306 316How to use this book ‘Welcome to your Edexcel AS/A level Biology B course. In this book you will ind a number of features designed to support your learning, Chapter openers Each chapter starts by setting the context for that haprer’s lernings + Link to other areas of Biology are shown. incuding previous knowledge tha isbult on inthe capex, fn tue earring that you wl cover ater in yur course + The All the maths you need checklist helps you 9 Yoon hat maths sls le equ Main content ‘The main part of each chapter covers all the points from the spectfication that you need to learn, The text is supported by diagrams and photos that will help you understand the concepts, Within each section, you will ind the following features: + Learning objectives at the beginning of each section, highlighting what you need to know and understand, + Key definitions shovm in bold and collated at the ‘end of each section for easy reference. + Worked examples showing you how to work through questions, and how your calculations should be set out, + Learning tips to help you focus your learning and avoid common errors. + Did you know? boxes featuring interesting facts to help you remember the key concepts. + Questions to help you check whether you have understood what you have just read, and whether there is anything that you need to look at again,Thinking Bigger “The book festures a number of Thinking Bigger spreads that give you an opportunity to ead and work with real Iie esearch and ‘writing abour science. The timeline atthe bottom of the spreads highlights which of the chapters the material relates to, These spreads will help you to + read realife material that’s relevant to your course + analyse how scientists write + think eitcally and consider the issues + develop your own writing + understand how diferent aspeets of your leaming piece together Exam-style questions ‘At the end of each chapter there are also exam-style questions [0 help you to: + test how filly you have understood the leaming + practise for your exams. Getting the most from your online ActiveBook ‘This book comes with 3 years’ access to ActiveBook* ~ an online, digital version of your textbook. Follow the instructions printed on the inside front cover to start sing your ActiveBook. Your ActiveBook is the perfect way to personalise your learning, as you progress through your Edexcel AS/A level Biology course. You can: + access your content onlin, anytime, anywhere + use the inbuilt highlighting and annotation tools to personalise the content and make it really relevant to you + search the content quickly using the index. Highlight tool Use this to pick out key terms or topics so you are ready and prepared for revision Annotations tool Use this to add your own notes, for example links to your wider reading, such as websites or other files. Ormake a note to remind yourself about work that you need to do. “For new purchases cay IF this access code has already been revealed may no longerbe val. If you have bought his testhock secondhand, the code may steady have been usec by te fist ner of the Bookwe % Chemistry for life ‘A alt spider Dolomedes fimbriatus sits on the surface of the water, hidden by the stems of water plants, waiting for the vibrations in the surface tension that alert her tothe presence of her prey. She is lage - up ta 23 mm actoss - yet water-repelient hairs enable her to run across the surfac her victims. These ae usually agus vital for this semiaquatic spider = and for ebrates that als live on or near the lite on Earth, er surface. Water is nit of life is the call, and un Biology isthe study of living things. The chemistry! The way atoms ace bonded together affects the way chemicals work int atfects everything, from the way plants make food by photosynthesis to the way your eyes respond to light In this chapter you will be looking at some ofthe key ways in which atoms and molecules interact to ‘make up the chemistry of life. You wil be using these basic principles the course, because they underpin the structures and functions of al the of ms you wil stud Around Jo-thitds ofthe surface of the Earth is covered in water and around two-thirds of your body is water. The oceans, rivers and lakes of the world are teeming with lifeand all the reactions in your cells take place in solution in water. In this chapter you will be applying your knowledge of the basic ‘chemical principles to help you understand just why war ie £0 vital for life Recognise and make appropriate use of units in calculation iNimetres) (eg, Use ratios (eg. representing the relationLife processes depend on molecules whose Punatieeenee Teed end eee eee Peat Eon eee eee eee rc Water is needed for photosynthesis Cee eae ene Pata ccd How carbohydrates, lipids and proteins are ere eriaed Cee eee) eee a ‘The importance of hydrogen bonding in the tertiary and quaternary structure of proteins and eee eter eres How waters taken into and moved around plants See an oes tissues and vessels in animal, plants and fungi Seta ey ‘The role of waterin the reactions of cellular co Perea er) NW irani cr Mau Ree Tied payer ence enn Creer ees bonding 3 See eer : Reeve irpreurenneter fineraerenries bare Saeed cae : eek ererte teehee mea Roe reer ne Teron Stet paperprereernesemreiy seri) . . Pieces Soe eee ae Ree ay tsChemistry for life By the end of this section, you should be able to.. © explain the role of inorganic ions in plants © explain the importance of the dipole nature of water in the formation of hydrogen bonds and ‘the significance of some of the properties of water to the organisms lonic and covalent bonding Biology isthe study of living things but ving tings are made up of chemical If you understand Some of the basi principles of chemistry you vil also develop a'mch beter understanding of biological systems, The chemical bonds within nd between molecls affect the properis ofthe compounds they form, Thsin tur affects the fonctions within te cell andthe orgasm, fig Ai i cependson some very fundamental chemistry The single basic unit ofall elements is the atom, When the atoms of two or more elements react they form a compound. An atom is made up of a nucleus containing postive protons and neutral neutrons surrounded by negative electrons, We model these electrons as orbiting around the nucleus in shells. When an atom has a full outer shell of electrons itis stable and does not react. However, ‘most atoms do not have @ full cuter shell of electrons, In chemical reactions, they are involved in changes that give them a stable outer shell. There are to ways they can achieve this: + Tonic bonding: the atoms involved in the reaction donate or receive eleetrons.‘The atom, or part of the molecule, gains one or more electrons and becomes a negative ion (anion). The other atom, of part of the molecule, loses one or more electrons and becomes a positive ion (cation). ‘Strong forces of attraction called ionic bonds hold the oppositely charged ions together game + +s» [i] Fa] an oa, ea ea {ig The formation of sadium chloride (salt an inorganic substance that is very important in ving organisms, an sample ofan bondi 10sr + Covalent bonding: the atoms involved in the reaction share electrons, Covalent bonds are very strong and the molecules formed are usually neural. However. in some covalent compounds, the ‘molecules are slightly pola'sed, The electrons inthe covalent bonds are not quite evenly shared. “This means the molecule has a part tha ssightly negative and a part that is slightly positive. This, separation of charge is called a dipole, and te tiny charges are represented as 6° and 6 (see fig D). “The molecule is described as.a polar molecule. This polarity is particularly common if ane or more hydrogen atoms are involved in the bond + He ——> HH (1) a) Q) hydrogen hydrogen hydrogen atom atom molecule © © © a Hot oH + Oe ae at ) } és) (2a) hycrogen hydrogen oxygen atom vrater molecule atom) ‘atom fig The formation ef hydrogen molecules and water molecules are examples of covalent bonding Be clear about the difference between ionic substances, charged particles and polar molecules. The importance of inorganic ions When ionic substances are dissolved in water the ions separate, Cells are 60-70% water and s0 in living organisms most onie substances exis: as postive and negative ions, Many ofthese ions play specialized role in individual ells ana inthe functioning of entire organisms. Here are some ofthe snorganic ions you vill meet as you sty biclogy, with an inciction oF one or mone oftheir roles Important anions + nitrate ions (NO;") ~ needed in plants for the formation of amino acids and therefore proteins from the products of photosynthesis, and also for the formation of DNA «+ phosphate ions (PO,?~) ~ needed in all living organisms including plants and animals in the formation of ATP and ADP as well as DNA and RNA + chloride ions (Cl+)— needed in nerve impulses and many secretory systems + hydrogen carbonate ions (HCO,”) — needed for buffering the blood to prevent it from becoming 100 acidic Important cations + sodium ions (Na*) — needed in nerve impulses and many secretory systems + calcium ions (Ca?) — needed for the formation of calcium pectate for the middle lamella between two cell walls in plants, and forbone formation and muscle contraction in animals + hydrogen ions (H)~ needed in cellular respiration and photosynthesis, and in numerous pumps and systems in organisms as well as pH balance “+ magnesium jons (IMg’)- needed for production of chlorophyll in plants uThe chemistry of water Water isthe medium in which all the reactions take place in living cells, Without it, substances could not move around the body ‘Waters one of the reactants in the process af photosynthesis, con which almost all life depends. And water is a major habitat = itsupports more life than any other part of the planet. Understanding the properties of water will help you understand rmany key systems in living organisms fig Water etal for fe on Earth in many aierent way ‘The importance of water 19 biological systems is due to the basie chemistry ofits molecules. The simple chemical formula of water isH,O. This tells us that two atoms of hydrogen are joined to cone atom of axygen to make up each water molecule (see fig F) However because the electrons are held closer to the oxygen ator than to the hycragen ators, water is a polar molecule. i N\A i 1045" {ig F Armedel ofa water molecule One of the most important results ofthis charge separation is that water molecules form hydrogen bonds. The slightly negative axygen atom of one water molecule wil attract the slightly positive hydrogen atoms of other water molecules in a ‘weak electrostatic araction called a hydragen bond. This means thatthe molecules of water ‘sick together’ more than you might otherwise expect, because although each individual hydrogen bond is weak. there area great many of them (as shown in fig G) Water has relatively high melting and boiling points eompared ‘with other substances that have molecules ofa similar siz — it takes more energy to overcome the attractive forges of all the hydrogen bonds. Hydrogen bonds are an important concept in biochemistry —for example they play an important part in protein structure (see Section 1.2.4) and in the structure and functioning of DNA (see Section 1.3.2) R figG Hydrogen bonding in water molecules. The importance of water ‘The properties of water make it very important in biological systems for several reasons + Water isa polar solvent. Because water i apolar molecule ‘many ionic substances lke sodium chioride will dissolve in it Mary covalently bonded substances are also polar and they +00 will dissolve in water although they often do not dissolve in other covalently bonded solvents such as ethanol. As a result most of the chemical reactions within cells occur in water (in ‘aqueous solution). + Water isan excellent transport medium because so many different substances will dissolve init, Water also carries other substances such as starch that form colloids rather than solutions. + As water cools to 4°C, it reaches its maximum density, As it cools further the molecules become more widely spaced, ‘Asa resul, ice is less dense than water and floats, forming an insulating layer and helping to prevent the water underneath it from freezing. Italso melts quickly because itis at the top, exposed to the sun. Itis very unusual for the solid form of chemical to be less dense than the liquid, but as a result ofthis ‘unusual property, organisms can live in water even in countries where it gets cold enough to freeze in winter + Water is slow to absorb and release heat ~ithas a high specific heat capacity. The hydrogen bonds between the molecules mean it takes a lot of energy to separate them. This means the temperature of large bodies of water such as lakes and seas does not change much throughout the year, making them good habitats for living organisms, + Water isa liquid and soit cannot be compressed. ‘This is an Jmportant factor in many lyeraulic mechanisms in ving organisms. + Water molecules are cohesive — the forces hetween the ‘molecules mean they tick together. This is very important for the movement of water from the rots to the leaves of plants.sr + Water molecules are adhesive ~ they are attracted to other different molecules. This is also Jmportant in plant transport systems and in surface tension. + Water has a vory high surface tension because the attraction berween the water molecules, including hydrogen bonds, is greater than the atraction between he water molecules and the air As a result the water molecules hold together forming a thin skin of surface tension. Surf sion is of great importance in plant transport systems, and also affects lf atthe surface of ponds, lakes and other water masses. fig wehout surace tension a rat spider could not move 1 How do ionic bonds and covalent bonds differ? 2. what are the ciferences between ini substances and polar substances? 3 How are ydrogen bonds formed between water molecules nd what eet do they have onthe proper ovate? 4. Te propenesef water fect ts lin ving organisms Discuss ‘Keydefinitions 0 ‘An anion isa negative ion, formed when an atom gains electron(s). ‘cation is a positive ion, formed when an atom loses electrons) lonic bonds are attractive forces between oppositely charged ions. Covalent bonds are formed when atoms share electrons. ‘A dipole isthe separation of charge in a molecule when the electrons in covalent bonds are not evenly shared. Apolar molecule is a molecule containing a dipole Hydrogen bonds are weak electrostatic intermolecular bonds formed between polar molecules Containing atleast one hydragen atom, BA Biology has 2 lot of application of scientific knowledge, so i's a ‘200d idea to remind yourself of the basics learnt at GCSE. 1 Remind yourself of ionic bonds by answering these questions, (2) Draw a diagram of a sodium ator, including the protons, neutrons and electrons, 2} (b) Draw 2 diagram of a chlorine atom, including the protons. neutrons and electrons, ia (c) Now show how sodium and chlorine atoms can be turned into sodium and chloride ions to form the ionic bond. [2] (Total: 6] 2 a) Draw one water molecule. ii (b) Using the atomic structure of oxygen and hydrogen, explain ‘hy the electrons are held closer to the oxygen atom. [2] (6) Explain how a molecule of sodium chloride can dissolve in water (3) [Total: 6] 3. Read through the following account about water. then write ‘on the dotted lines the most appropriate word or words to ‘complete the account Water molecules are described as. Se because they have a slight positive charge at one end of the ‘molecule and a slight negative charge atthe other end. This ‘makes water a good, for salts and substances such as sugars. Bonds that form between water molecules are called bonds. ‘Waters a good coolant because it has @ high sey which means that it takes a lot of heat to changeit rom a liquid to a gas. Water also has a high... which means that a lot of ‘energy is needed to cause a small rise inits temperature. [5] (Total: 5] Exam-style questions 4 Fill in this table to show which ion is used for which purpose. POF ca Needed to produce chlorophyll (4) (Total: 4} 5. There are many substances important to living organisms ‘These can be classified as A. cations B anions polar molecules D_ non-polar molecules Identity the following molecules using one of the terms above. (2) water (0) chloride ion (CI>) (c) sodium ion (Na*) {d) hydrogen carbonate ion {e) methane (phosphate fon (6) [Total: 6] 6 Acids release hydnogen ions (H") into solution. Explain how !hyimagen carbonate ions (HCO. act to prevent the Hood ‘becoming too acidkc. 2) (Total: 2} 7 {a} A.student wrote a tte to her table of results in a water based ink. and then underlined in ballpoint pen. Her lab partner then accidentally spilled water over the page. The tle smudge, but the underining dirt Using your nowledge of the properties of water explain these observations. (2] {b) Having done some research, the student decided that it would be more sensible ro do her tables of results using @ pencil, Use your knowledge of solvents to explain wy this 's 8 good idea @ (Total: 4)8 w (@) Draw the electron shells ofthe following atoms: (carbon (i) oxygen (i) sodium () argon 4) (&) Use the information from the electron shells to state how many protons each of the above elements has. (4) (6) Use the information of the number of protons to explain ‘why CHqis a non-polar molecule but HO isa polar molecule. a (@) Use the periodic table to find the relative atomic mass of ‘each element. Why is this number always bigger than the proton number? i) (€) Looking again atthe electron shells, explain why eaxbon ‘can form four bonds, oxygen can form two, sodium only forms one bond, but argon can form no bonds 4) [Total: 15] Marion wanted to build a pond to breed fish in the north of England. Temperatures inthis region can fall below 0°C in the winter She was advised to make sure the pond was at teast 3m deep and held 3500 00 res of water. Use your lrnowledge of the properties of water to explain why such a large pond was necessary 4) (Total: 4] Pond skaters are insects that can travel on the surface of water Using your knowledge of the properties of water, explain how these insects can travel ike this, By [Total: 3]A sivall child with a swollen belly sts istlesly inthe Caribbean sun. Like many millions of newy weaned infants she is suffering from kuashiorkor. She is ‘sugar baby - socalled because she isnot lacking in calories but in protein, A breastfed baby gets ll the carbohydrates, lipids and proteins it needs from its ‘mother’s milk. Butin many countries, the main foods used to wean babies are cereals. Cereals contain around 12-14% water, 65-75% carbohydrate, 2-6% lipids and 7-12% protein. In contrast, the human | body ie made up of arounc 64% water, 20% pratein, 10% fat, 1% carbohydrate and 5% minerals. Cells depend on proteins to work so ifthe diets severely lacking in protein, over time the health of the child (or adult al fal In this chapter you wil be studying some ofthe key biological molecules that make up the cells of you conn bedy, and those of other organisms. You will laokat carbohydrates, from the simplest sugars tothe ‘most complex polysaccharides, These molecules have a wide variety of uses in organisms, from the fuel for cellular respiration to the main structural material in plants. As you discover how the molecules are joined together you wil recognise the relationships between the structure ofthe molecules and their functions in the body. “The same links between structure and function are lear when you look at the way lipid molecules build up. B For example lipids are used as energy stores in bath animals and plants Lipids are non-polar molecules but ‘you will discover how they can become polar in combination with other inorganic groups such as phosphates. This polarity has great importance forthe characteristics ofthe cel membrane. Proteins are key molecules in cells both as part of the structure of the membranes and as the enzymes that contal the metabolism ofthe cell and the whole organism, Proteins are long chains of amino acids that are hheld together to make complex structures by chemical bonds, including the covalent bonds, ionic bonds and hydrogen bonds you discovered in Chapter 1.1 IR ecognise and make appropriate use of units in caeulations (eg nanometre) Use ratios (¢9, representing the eatonships Between atoms in an ion or molecule)Peet ard Sypgygeere Maine ere i fates Se ogra cae | rng ae seca rie have | studied before? eee ee ree od epee snd ee en ca Pent ur ene tee Ts een een puna Be cee cel eee ee eee ees Dn ee te aac How earbohyaratesand proteins actas signaling ee eee eet eae Sete eee ead eee el eee Pr ‘ ee ee ce EE eer et ete cian een ee eas eee ees > ns ee, ee ee) age ee amen) Want vaunted paren erevnestiaser ett] ts neers Se ny Hosalioanenetsporaueeritaer neh or et re ree ee ere eer renner Pee eee Se ee importanceas storage molecilesin plats and cts ester bonds +The structure ofamino acids peptides and Pec beo Tent mnn nity Ta Seen oceans os eres bee ever ant near eter eee ~SCarbohydrates 1 - monosaccharides and disaccharides By the end of this section, you should be able to.. © describe the difference between monosaccharides and disaccharides © describe the structure of the hexose glucose (alpha and beta) and the pentose ribose © explain how monosaccharides join to form disaccharides thraugh condensation reactions forming glycosidic bonds, and how they can be split through hydrolysis reactions © explain how the structure of glucose relates to its function fig Corbohy-ses a rmeleculesin planta ake ~ and they a What are organic compounds? Biological molecules ae the key tothe structure and function of living things. Biological molecules are olten organic compouns. Organic compounds all contain carbon atom, They also contain atoms of hyéragen, oxygen and, leas frequent trogen, sulfur and phosphorus, Mest of the material in your body that is not water is made up of these organic molecules. An understanding of why onganic molecues ae special il ep you to understand the chemistry of biological molecules inducing carbohydrates, lipids and proteins Each carbon atom can make four bonds and so it can join up with four other atoms. Carbon atom bond particulary strongly to other carbon atoms to make long chains, The four bands of a carbon ‘atom usually form a tetrehedral shape and this leads to the formation of branched chains, or rings, cor any number of three-dimensional (3D) shapes. In some carbon compounds small molecules (monomers) bond with many other similar units to make a very large molecule called a polymer. The ability of carbon to combine and make macromolecules (large molecules) isthe basis of all biological molecules and provides the great variety and complexity found in living things. , cee Sa plane of the paper two bonds This bond sticks out of the plane of the paper HHH H can be shown with corners chan meee ryeogns ignore MwA YY ormoreofenas: HH HH H fig A The bonds in a carbon atom havea complicate diagame we use one of everal dfenert ways Carbohydrates Carbohydrates are important in cells as a usable energy source. They are also used for storing ‘energy, and in plants, fungi and bacteria they form an important part of the cell wall. The best kn carbohydrates are sugars and starch, Suerase isthe white crystalline sugar familia to us all, while ‘glucose is the energy supplier in sports and health drinks, Starch is found in flour and potatoes. But the group of chemicals knowm as carbohydrates contains many more compounds, as you will discover‘The basic structure of all carbohyctrates is the same. They are ‘made up of carbon, hydrogen and oxygen. There are three main groups of carbohydrates with varying complexity of molecules: monosaccharides, disaccharides and polysaccharides, Monosaccharides - the simple sugars Monosaccharides are simple sugars in which there is one oxygen ‘atom and two hydrogen atoms for each carbon atom present in the molecule, A general forrnula for this can be written (CH.O), Here n can be any number but it is usually low: + Triose sugars (n=3) have three carbon atoms and the molecular formula CsH,O:, They are important in the ‘mitochondria, where glucose is broken down inta triose sugars during respiration. + Pentose sugars (n=5) have five earbon atoms and the ‘malecular formula C:H,0;, Ribose and deoxyribose are Jmportant in the nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), vhich make up the genetic ‘material (see Sections 1.3.1 and 1.3.2), + Hexose sugars (n=6) have six carbon atoms and the molecular formula C:H.,O,. They are the best known monosaccharides. often taste sweet and include glucose, galactose and fructose. Molecular formulae show you how many atoms there are in the ‘molecule, and what type they are, but they do net tell you what the molecule looks like and way it behaves asit does. To show this you can use displayed formulae, Although these do not fallow ‘every wiggle and kink in the earbon chain, chey can give you a good idea of how the molecules are arranged in three dimensions. ‘This can reveal all sorts of secrets about why biological systems behave as they do (see fig D). ribose CHOH OH As | a \ ~ I dy figD Howse sugarshavea hese crainscan make a significant ference othe way imwhich the molecule can be used by the body Werumber the carbon ators 30 = can denafy the diferent arangements og truce. The arrangement te aloms on a-glucose and f-glucose Glucose comes in different forms (isomers), including a-giucose and -glucose. These two isomers result from different arrangements of the atoms on the side chains of the molecule (see fig E) ‘The afferent isomers form cifferent bands between neighbouring ‘glucose molecules, and this affects the polymers that are made. ex-glucose Begiucese (chon HOH HY Q mh 4. EH } > Ce Ho 7 HHO H OH or even more simply: av-glucose H, H. HO Xs HO In these diagrams, the positions of carbon atoms are represented by their numbers only Note carefully he different arrangernent of atorns around the carbon 1 atom in aeglicose and B-glucose, fig The difrence in suture becusen a-ucose and fg siall, bu ithas a big impact onthe function ofeach molecule Did vou know? Hydrogenating some sugars reduces the energy they provide. When slucase is hydrogenated it forms sorbitol (C,H, 0). Sorbital tastes Lp to 60% sweeter than glucose but it provides less energy when is used in the body (11 Kg" compared to 17klg"). The combination of the very sweet taste and the lower energy count makes it useful as a sweetener for people who want to lose weight. small change in the chemical structure has abi effect on function. Disaccharides - the double sugars Disaccharides are made up of two monosaccharides joined together ~ for example sucrose (ordinary table sugar) is formed bby a molecule of a-glucose joining with a molecule of fructose. ‘Two monosaccharides join in a condensation reaction to form a disaccharide, and a molecule of water (H,O) is removed. “The link between the two monosaccharides results in a covalent bond known as a glyeosidie bond (see fig F). We use numbers to show which carbon molecules are involved in the bond, If carbon 1 on one monosaccharide joins to carbon 4 on another ‘monosaccharide, we call ita 1.4-giycosidic bond. If the bond is bbetiveen carbon | and carbon 6, its a 1.6-glycosidic bond. 19orglucose a-glucose HY pou feo on Hi Y bund of “OH ‘condensation yoo a: 1h H wo SSN 1 delycosidic bond fig The formation glycoside b condensation reaction betwen two monosaccharides results na dsacchatide anda molecule cl water When different monosaccharides join together different disaccharides result. Many disaccharides taste sweet. Petree eet sucrose stored in plants such as arglucose + sugar cane uctose lactose milk isthe man | erglucose + carbohy yund in ile maltose malt sugar ~ found in germinating seed such as barley table A [hres common dieacsnandes Did you know? Testing for sugars + Benedict solution isa chemical test for reducing sugars. Itisa bright blue solution thet contains capper) ans Some sugars react resdiy with this solution when heated gently and reduce the copper() ions to copper} ions, forming a precipitate and giving a colour change from biue to orange. They are known as reducing sugars. All ofthe monosaccharides and some disaccharides are reducing sugars, + Some sugars do not react with Benedict solution, They are known as non-reducing sugars. You can heat a non-reducing sugar such as sucrose with afew drops of hydrochloric acto hydrolyse the ycosidic bonds. Allow it to cool and then neutralise the solution With sodium hydrogen carbonate. This produces the monosaccharide Units ofthe sugar, which wil now give a positive Benedict test. fig Benecic’ tet for reducing sugars 20 ‘| What are the properties of organic compounds that make them so Important iving organisms? 2 pescrvehowa ghcsiicbondistormed beween two Imonosecharie orm a saccharide Keydefinitions ‘Amonomer isa small molecule thats a single unit ofa larger molecule called a polymer polymers along chain molecule made up of many smaller, repeating monomer units joined together by chemical bonds. -Amacromolecule isa very large molecule often formed by polymerisation. Starch sa long chain polymer formed of a-glucose monomers. Sucrose isa sweet tasting disaccharide formed by the joining of ce-glucose and fructose by a glycosidic bond. Glucose isa hexose sugar ‘Amonosacchatide isa single sugar monomer. ‘disaccharides a sugar made up of two monosaccharide units joined by a glycosidic bond, formed in a condensation reaction ‘A polysaccharide is a polymer made up of long chains of monosaccharide units joined by glycosidic bonds [Atriose sugars a sugar with three carbon atoms. [pentose sugar isa suger with five carbon atoms Ribose isa pentose sugar that makes up part ofthe structure of RNA. Deoxyribose sa pentose sugar that makes up part ofthe structure of DNA. Deoxyribonucleic acid (ONA) isa nucleicacidthatactsas the genetic material in many organisms. Ribonucleic acd (RNA) sa nuceic acid which can actas the genetic material in some organisms and is involved in protein synthesis Alhexose sugars a sugar with siccarbon atoms. Isomers are molecules that have the same chemical formula, but different molecular sructures ‘Acondensation reaction isa reaction in which a molecule of water is removed from the reacting molecules asa bond isformed between them glycosidic bond isa covalent bond formed between two monosaccharides in condensation reaction Reducing sugars are sugars that react with blue Benedict’ solution and reduce the copper(I) ions to copper ons giving an orangey: red precipitate. Non-reducing sugars are sugars that do not react with Benedict’ solution.Carbohydrates 2 - polysaccharides By the end of this section, you should be able to... ‘© explain how monosaccharides join to form polysaccharides through condensation reactions forming glycosidic bonds; and how these can be split through hydrolysis reactions © explain how the structure of polysaccharides relates to their functions The most complex carbohydrates are the polysaccharides. They are made of many monosaccharide units joined by condensation reactions that form glycosidic bonds (see Section 1.2.1, fig F), Molecules with 3-10 sugar units are known as oligosaccharides, wiile molecules containing 11 or Remember that glycosidic more monosaccharides are known as true polysaccharides Polysaccharides do not have the sweet bonds are formed withthe taste of many mono- and disaccharides, but these complex polymers form some very important removal of a molecule of water iiolegieal molecules in condensation reactions and broken withthe addition of a nakes them ideal as storage molecules: molecule af water in hydrolysis reactions The structure of polysaccha + They can form very compact molecules, so large numbers can be stored ina cel + The glycosidic bonds are easily broken, allowing rapid release of monosaccharide units for celular respiration. + They are not very soluble in water so have litle effect on water potential within a cell and cause ‘no esmotic water movement The glycosidic bond between two monosaccharides is split by a process known as hydrolysis, {see fig A). The hydrolysis reaction is the opposite of the condensation reaction that formed th ‘molecule, so water is added to the bond, Polysaccharides are gradually broken dow into shor shorter chains and eventually single sugars are lef. Disaccharides break down to form two and monosaccharides. Hydrolysis takes place during digestion in the gut, and also in the muscle and lve cells when the carbohydrate stores are broken down to release sugars for use in cellular respiration a-glucose a-glucose } 0, 1 ut \ HH stein | mon, HO ROY OHH H Ho ° ‘OH maltose ~I.t-ycosiic bond fig Gycosicic bonds a ade by condensation reactions and broken down by hydolss aDidyouknow? __ Testing for starch Ifyouadd afew drops of reddish-brown iodine solution toa solid sample ora sample Insolution, when starch is present the solution will un 4 blue-biack, 22 sa Carbohydrates as energy stores Starch Starch is particulary important as en energy store in plants The sugars produced by photosynthesis are Tape convertedinte starch, wich sinsohble and compact but canbe broken dlown rapidly to release glucose wien is needed. Storage organs such a potatoes ae pacar tichin starch ‘Starch is made up of long chains of e-glucose. But if you look at it more elosely you will see that it is ‘actually a misture of two compounds: Amylose: an unbranched polymer made up of between 200 and 5000 glucose molecules As the chain lengthens the molecule spirals, which makes it more compact for storage. Amylopectin: a branched polymer of glucose molecules. The branching chains have many terminal glucose molecules that can be broken off rapidly when energy is needed. Amylose and amylopectin are both long chains of a-glucose molecules — so wy are the molecules, 0 different? It all depends on the carbon atoms involved in the glycosidic bonds Amylose is made up purely of exglucose molecules joined by 1.4-glycosidic bonds, which is way the ‘molecules are long unbranched chains. In amylopectin many of the glucose molecules are jeined by 1.4-glycosidic bonds, but there are also afew L.6-glycosidic bonds. This results inthe branching chains that change the properties of the molecule, So starch has a combination of straight chain amylose and branched chain amylopectin ‘molecules. This combination explains why carbohydrate foods lice pasta are so good for you. ‘when you are doing sport. The amylopectin releases glucose for cellular respiration rapidly ‘when needed. Amylose releases glucose more slowly ever @ longer period, Keeping you going longer side group amylose 3 xy [os cchain forms a spiral cooondonnn amylopectin oS tO seas tai We 1 drglycosidiec took —S ° fig Amylose and amylopectin - a smal ference in the postion ofthe glycosidic bonds inthe molecule makes a big liference tothe propories ofthe compound.Glycogen Glycogen is sometimes referred to as ‘animal starch’ because itis the only carbohydrate energy store found in animals (see fig C). Its also an important storage carbohydrate in fungi ‘Chemically, glycogen is very similar to the amylopectin molecules in starch, and is also made up of many a-glucose units Like starch, its very compact, but the glycogen molecule has more 1,6-lycosidic bonds, giving it many side branches. As a result, glycogen can be broken down very rapidly: This makes it an ideal source of glucose for active tissues with a constantly high rate of celular respiration, such as muscle and liver tissue Carbohydrates in plants Polysaccharides ar very important in plants, Starch isthe main eneigy storage material in plants Atypical tarch grain in a plant tel contains 70-20% amylopectin, withthe rest beng amylose {a) starch grains ina plant cell (b) glycogen granules in liver cells fig€ Storage carnonycrates in plant and animal cals Cellulose is an important structural material in plants, The cell ‘wall (see Section 2.1.8) is an important feature that gives plants ‘their strength and support It is made up langely of insoluble cellulose. Cellulose has much in common with starch and _slycogen. It consists of long chains of glucose joined by glycosidic bonds. However, as you will remember there are two structural isomers of glucose, a-glucose and f-glicose, In starch, the monomer units are aeglucose. In cellulose, they are B glucose and are held together by 14-glycosidic bonds where ‘one of the monomer units has to be turned round (inverted) s0 the bonding can take place. Ths linking of -glucose molecules, means thar the hydraxyl (-OH) groups stick out on both sides of the molecule (see fig D). This means hydrogen bonds can pony form berween the partially positively charged hydrogen atoms of ‘the hydroxy] groups and the partially negatively charged exygen atoms in other areas of the glucose molecules. This is knowin as cross-linking and it holds neighbouring chains firmly together Many of these hydrogen bonds form, making celiulose a material ‘with considerable strength, Cellulose molecules do not coil or spiral - they remain as very long, straight chains. In contrast, starch molecules, with L4- and 1,6-glycosidic bonds between, ‘eeglucose monomers, form compact globular molecules that are ‘useful for storage. Beshucose Prglucose # OOH B. OOH HO! HHO H H OH Hi H HO! HOW ‘OH condensation lass x Hot HO O H “SX : HO. xX HO, 0, anes 2 XX Pro i fay ve OH KX, hydrogen bonds fig Calulose molecules consist of @ glucose moromes joined together by “egyczsidc bonds, ‘This difference in structure between starch and cellulose ‘gives them very different properties and functions, Starch is an important source of energy in the diet for many animals However, most animals do not possess the enzymes needed to break the 1,4-glycosidie bonds between the molecules of Beglucose and so they cannot digest cellulose. Ruminants such as cows and sheep, have bacteria, fungi and protozoa living in their _gut which produce cellulose-digesting enzymes. Itis the cellulose in plant food that acts as roughage or fibre in the human diet ~ an important part of a healthy diet even though you cannot digest it 2B24 Be clear about the diferences between a glucose and glucose and between 1L4-glycosidic bonds and ‘e-glycosidic links ~ is easy toiget them wrong and lose marks asa resut. at Explain how the structure of carbohydrates is rlated to their function as storage molecules providing the fue for cellular respiration in animals and plants, 2. Explain how the chemical structure of cellulose differs from that of starch and how this affects the way they can be used to supply enezgy in animals. IDR, ty ° HO, 2 38 xx cellulose ‘Oligosaccharides are molecules with 3-10 monosaccharide unis Hydrolysis ia reaction in which bonds are broken by the addition of a molecule of water. “Amylose isa complex carbohydrate containing onty glucose monomers joined together by 14- glycosidic bonds o the molecules form long unbranched chains. Amylopectin isa complex carbohydrate made up of glucose monomers joined by both 1,4-glycosidic bonds and I,6-glycosidic bonds so the molecules branch repeatedly Glycogen is made up of many «glucose unis joined by 1,4glycosidic bonds but also has 1/S-lycosidic bonds, giving it many side branches. Cellulose is 2 complex carbohydrate with fglucose monomers held together by 1-glycosidic bands. is very important in plant cel walBy the end of this section, you should be able to... ‘© explain the synthesis ofa triglyceride, including the formation of ester bonds during condensation reactions between glycerol and three fatty acids © describe the differences between unsaturated and saturated fatty acids ‘© explain how the structure of lipids relates to their role in energy storage, waterproofing and insulation © explain the structure and properties of phospholipids in relation to their function in the cell membranes. The lipids are another group of organic chemicals that play a vital role in organisms. They form an integral part of all cell membranes and are also used as an energy store. Many plants and animals cconwert spare food into oils or fats to use when they are needed. For example, the seeds of plants contain lipids to provide energy for the seedling when it starts to grow, which is why seeds are such {an important food source for many animals. 4 4 Fats and oils | | Fats and ol are important groups of lipids Chemical they ar extremely simi but ats suchas. HCCC (eee ar omen ok aateunlcopatiget he Glas pe | oneal meal Hp ie a fetal ler cea ej re ue co ety en gropnrion nf. cnr tan entobyrmaes: tsae 9 nares oc OH OM OH {poe bes ri, Aly Asa ad gifcee ris 1h Tey mu carmn Tey dew orpupsltemlectare presente Gioved kaa ic dlcnied teense, ake neeoaess Ally acids have along hydrocarbon chain a plated backbone ofcatbon atoms wth hydrogen atoms attached, a carboxy group (-COOH) at one end. Living tissues contain more than 70 different kinds of fatty acids. Fatty acids vary in two ways: + The length of the carbon chain can differ (although often 19-17 carbon atoms long in organisms) + The fatty acid may be a saturated fatty acid or unsaturated fatty acid [na saturated fatty acid, each carbon atom is joined to the one next co it by a single covalent bond, A common example is stearic acid (see fig B). In an unsarurated fatty acid, the carbon chains have ‘one or more double covalent bonds in them. A monounsaturated fatty acid has one double bond anda polyunsaturated fatty acid has more than one double bond (see fig C). Linoleic acid is an ‘example of a polyunsaturated fatty acid. Itis an essential fatty acid in our diet because we cannot make it from other chemical cH, (cH), coor fg 8 Displayed formula of stearic acid, a satura 2526 fig Displayed formula of nolec a Forming ester bonds AA fator oil results when glycerol combines with one, twa or three fatty acids to form a monoglyeeride, a diglyceride or a triglyceride. A bond is formed in @ condensation reaction between the carboxyl group (COOH) of a fatty acid and one of the hydroxy! groups (-OH) of the glycerol. A molecule of water is removed and the resulting bond is known as an ester bond. This type of condensation reaction is, called esterification (see fig D). The nature of the lipid formed depends on which fatty acids are present, So, for example, lipids containing saturated fatty acids are more likely to be solid at room {temperature than those containing unsaturated fatty acids For simplicity faty acids are represented by this general ° formula where represents the hydrocarbon chain The faty acids below are drawn in reversed form, R—C—OH glycerol 3 fatty acids triglyceride o lester bond, 9° 30) H o: I OH HOMC—R < hydrolysis H—C—oH HO-C—R <— © condensation H—C—OH HOLC—R K #H Note: there are only 6 ators of. ‘oxygen in a triglyceride molecule fig Formation of este The nature of lipids Lipids contain many caborhydogen bonds and ite cxygen. When iis are oxidised in respiration, the bonds are broken and cabo dioxide and water are the ultimate products. This reaction canbe used to dive the production of lt of ATP (ee Section 1.3.1) Lcies, especially triglycerides, store about thee times as mach energy asthe same mas of carbohydrates ‘The hydrophobic nature of lipids is @ key feature of their role in waterproofing organisms. Oils ae ‘important in waterproofing the fur and feethers of mammals and birds, wile insects and plants use waxes for waterproofing their outer surfaces (see fig B). Lipids are good insulators fatty sheath insulates your nerves 50 the electrical impulses travel faster They also insulate animals against heat loss — the thick layer Of blubber in whales isa good example. Lipids have a very low density, so the body fat of water marnmals ‘helps them to float easily All lipids dissolve in organic solvents, but are insoluble in water, so lipids do not interfere with the many water-based reactions that go on in the cytoplasm of a cell (@) (b) fig Ol onthe fatness, nthe sua af hess png leaves makes them very watrprcotPhospholipids Inorganic phosphate ions (-PO,) are preset in the cytoplasm of every cell Sometimes one of the yeas groups oF aycee) undergoes an esterification reaction witha phosphate group instead of with fatty acid and a simple phospholipids formed, Phospholpi are important because te lp ane the phosphate pars ofthe molecule get very diferent properties “The fatty acid chains of a phospholipid are neutral and insoluble in water In contrast the phosphate head carries a small negative ‘charge and is soluble in water When these phospholipids come into contact with water the two parts of the molecule behave differently The polar phosphate partis hydrophilic and cissolves readily in water (sce fig F). The lipid tails are hydrophobic, so they do not dissolve in water Ifthe molecules are tightly packed in ‘water they either form a monolayer, with the hyrophilic heads in the water and the hydrophobic lipid tails in the air or clusters called micelles. In a micelle. all the hydrophilic heads point ‘outwards and all the hydrophobic tals are inside (see fig G) hydrophilic head hydrophobic tall fig phospholipid hydrophobic air end ‘aqueous hydrophilic ~~ solution sy fig Phospholipids form 2 mo ‘A phospholipid monolayer may form ata surface between air and ‘water but this isa feisty rare situation in living cells where there {are water-based solutions on either side of the membranes. With ‘water on each side, the phospholipid molecules form a bilayer with the hydrophilic heads pointing into the water protecting the hydrophobic tas in the middle (see fig H).‘Thisstructure, the unit ‘membrane, isthe bass of all membranes aqueous solution Phosphate heads move hesphate head oyands aqueous solution “hydrophilic phospholipid aqueous solution of all mem fig Alpi bilayer ithe boc ‘| Describe the main difference between a saturated and an ‘unsaturated lipid, and the effect of ths difference on the properties, ofthe lipids. 2 explain how ghee are fou Key definitions Lipids are a large family of organic molecules that are important in cell membranes and as an energy store in many organisms, They include triglycerides, phospholipids and steroids. Alatty acid is an organic acid with along hydrocarbon chain. Giycerol is propane-1.23-triol, an important component of triglycerides [An ester bond is a bond formed in a condensation reaction between the carboxy group (-COOH) of a Fatty acid and one of the hydroxy groups (-OH] of glycerol ‘saturated fatty acid isa fatty acid in which each carbon atom is joined tw the one next to it nthe hydrocarbon chain by asngle covalent bond. [An unsaturated fatty acid is fatty acid in which the carbon atoms in the hycracarbon chain have one or more double covalent bondsin them. ‘A monounsaturated fatty acid is fatty acid with only one double covalent bond between carbon atoms in the hydracarbon chain, ‘polyunsaturated fatty acid is. fatty acid with two oF more double covalent bonds between carbon atoms in the hydrocarbon chain, Esterfication is the formation of ester Bonds phospholipid isa chemical in which glycerol bonds with two fatty acids and an inorganic phosphate group, Hydrophilic molecules dissolve readily in water. Hydrophobie molecules will not cissolve in water Amonolayer isa single closely packed ayer of atoms or molecules. [Armicelle isa spherical aggregate of molecules in water with hydrophobic areas in the mide and hydrophilic areas outside Abilayer isa double layer of closely packed atoms or molecules, ‘Aunit membrane i a bilayer structure formed by phospholipids in an aqueous environment, with the hydrophobic tailsin the middle and the hydrophilic heads on the outside. 27Proteins By the end of this section, you should be able to.. © outline the structure of an amino acid © explain the formation of polypeptides and proteins and the nature of the bonds in proteins © explain the significance of the primary, secondary, tertiary and quaternary structure of protein in determining the properties. of fibrous and globular proteins © explain how the structure of collagen and haemoglobin is. related to their function About 18% of your body is made up of protein. Proteins form hrar, skin and nails, the enzymes needed for metabolism and digestion, and many of the hormones that control various body systems, They enable muscle fibres to contract, form antibodies that protect you from disease, help clot your blood and transport ‘oxygen in the form of haemoglobin, Understanding the structure of proteins helps you develop an insight into the detailed biology of cells and organisms, Like carbohydrates and lipids, proteins contain carbon, hydrogen and oxygen. In addition they all contain nitrogen and many proteins also contain sulfur Proteins are another group of macromolecules made up of many small monomer units called amino acids joined together by condensation reactions, Amino acids combine in long chains to [produce proteins, There are about 20 different naturally occurring amino acids that can combine in different ways to form a vast range of different proteins. Amino acids ‘All amino acids have the same basic structure, which is represented as a general formula. There is always an amino group (-NH,) and a carboxyl group (COOH) attached to a carton atom (see fig A), The group known as the R group varies between amino acids. This is where sulfur and selenium are found in the structure of a few amino acids. The structure of the R group affects the way the amino acid bonds with others in the protein, depending largely on whether the R group is polar or not general formula R ‘This pat is como the examples blow. slyeine cysteine H cH, sit fig A. Some ciferenc amino acids. In te simpos amino aie gcine Risa single hydrogen atom, ina larger amine aca such a eystelne, Ris muc 28 Forming proteins from amino acids Amino acids join together by 2 reaction between the amino group Cf one amino acid, and the carbaxy/ group of another They join in a condensation reaction and a molecule of water is ost A peptide bond is formed when two amino acids jon and a dipeptide isthe result (see fig B)."The R group isnot involved inthis reaction, Mor land maze amino acids join to form polypeptide chains which contain from a hundred to many thousands of arn acids. When the polypeptide folds or coils or associates with other polypeptide chains it forms @ protein. amino acid 1 amino acid 2 (inverted) H HO N Ry condensation |) hydrolysis 0 So Ry H HO H bobo H—N—C—CwwN—C—C—OH a I Peptide HOH OO PRS Ry dipeptide fig Amino acid pide bones King blocks of proteins jon Bonds in proteins The peptide bond between amino acs isa strong bond, Other bonds also form between the aminoacids ina chain o form the 30 Sartre ofthe protein, They depend onthe atoms in he group and include hydrogen bonds disulfide bonds end ionic bonds Hydrogen bonds in amino acids, tiny negative charges are present on the oxygen cf the carboxyl groups and tiny positive charges are present on, the hydrogen atoms of the amino groups. When these charge ‘groups are close to each other, the of forming a hydrogen bond, Hydrogen bonds are weak, but they ‘can potentially form between any two amino acids positioned correctly so there are lots of them holding the protein together very firmly. Hydrogen bonds break easily and reform if pH or ‘temperature conditions change. They are very important in the folding and coiling of the polypeptide chains (see fig C). pposite charges attractDisulfide bonds Disulfide bonds form when two cysteine molecules are close together in the structure of a polypeptide (see fig C). ‘An oxidation reaction takes place between the two sulfur containing groups, resulting in a strong covalent bond known asa disulfide bond. These disulfide bonds are muuch stronger than hydrogen bonds, but they occur much less often. They are ‘important for holding the folded polypeptide chains in place. B pleated sheet av-helix fig Hydrogen bond and dude bonds maintain the shape of protein moles and this determines ther uncron lonic bonds [onic bonds can form between some of the strongly postive and negative amino acid side chains found buried deep inthe protein molecules, Those links are known as salt bridges, They are strong, bonds, but they are not as common as the other structural bonds, figD Straightening your so the hair curs ina diferent ramon sgement ofthe hygen bonds Your hair is made of the protein keratin. Some methods of styling the hair actually change the bonds within the protein ‘molecules. Blow drying or straightening your hair breaks the Iydrogen bonds and reforms them with the hair curling in a different way temporarily until the hydrogen bonds reform in their original places. Perming breaks the disulfide bonds between the polypeptide chains «and reforms them in a different place. This effect is permanent — your hair will stay styled in that particular way unt itis cut Protein structure Proteins can be described by their primary, secondary, tertiary and ‘quaternary structure (see fig E) + The primary structure of a protein is the sequence of amino acids that make up the polypeptide chain held together by peptide bonds + The secondary structure of a protein is the arrangement of the polypeptide chain into a regular, repeating structure, held together by hydrogen bonds. One example isthe right-handed helix (a-helix), a spiral col with the peptide bonds forming the backbone and the R groups sticking out in all directions Another is the f-pleated sheet, in which the polypeptide chain, folds into regular pleats hele together by hydrogen bonds between the amino and carboxyl ends of different amino acids. Most fibrous proteins have this sort of structure, Sometimes there is no regular secondary structure and the polypeptide forms a random call + The tertiary structure is a evel of 3D organisation imposed on top of the secondary structure in many proteins The amino acid chain, incuding any a-helices and A-pleated sheets, is folded further into complicated shapes. Hydrogen bonds, disulfide bonds and ionic bonds between amino acids hold these 3D shapes in place (see page 30). Globular proteins are an example of tertiary structures. + The quaternary structure of a protein is only seen in proteins consisting of several polypeptide chains, The quaternary structure describes the way these separate polypeptide chains fit together in three dimensions. Examples include some very important enzymes and the blood pigment haemoglobin. ‘The bonds that hold the 3D shapes of proteins together are affected bby changes in conditions such as temperature or pH. Even small changes can cause the bonds to break. resulting inthe loss of the 3D shape of the protein, We say thatthe protein is denatured. [Because the 3D seructure of these proteins is important ta the way they work, changing conditions inside the body can cause proteins such as enzymes to stop working propery. Primary structure — the linear sequence of amino acids in a peptide. BBE PO PD Pe Secondary structure ~ the repeating pattern in the structure of the peptide chains, such as an ce-helix ar B-pleated sheets, Tertiary structure secondary structure, the three-dimensional folding of the Be (Quaternary structure ~ the thrze-dimensional arrangement of more than one tertiary polypeptide. fig The 30 stucture of proteins 29Fibrous and globular proteins Fibrous proteins “The complex structures of lage protein molecules relate closely to their functions inthe body. Fibrous proteins have lite or no tertiary structure. They are long, parallel polypeptide chains with cccasional croselnkages that form ito flares. They are insoluble in water and ate very Lough, which makes them ideally suited to their structural unetions win organisms. Fibrous proteins appear inthe structure of connective tise in tendons and the ‘matrix of bones, in the structure of muscles asthe silk of spiders’ ‘webs and sileworm cocoons, and as the Kerstin that makes Up hai nail, hos and feathers. Collagen isa fibrous protein that gives strength to tendons, ligaments, bones and skin. It isthe most common structural protein found in animals — up to 35% of the protein in your body is collagen. Collagen is extremely strong ~ the fibres have a tensile strength comparable to that of steel. This is due to the unusual structure of the collagen molecule. Its made up of three polypeptide chains, which are each up to 1000 amino acids long, The primary structure of these chains is repeating ‘sequences of giycine with two other amino acids - often proline and hydroxyproline, The three a-chains are arranged in a unique triple helix, held together by a very large number of hydrogen bonds. These collagen molecules, which can be up to several millimetres long, are often found together in fibrils that in turn are hheld together to form collagen fibres. Collagen fibres are found combined with the bone tissue. giving ittensile strength rather like the steel rods in reinforced concrete, Inthe genetic disease osteogenesis imperfecta, the collagen triple helix may not form properly. The bone lacks tensile strength as & result, and itis brittle and breaks very easily. _Geusia Ren i seu er escrton a seme AMM i Pcusar Procolages Coban r fon (@bleles mole (ule eb Caagen Cag tre ea) fig Collagen sa ibrous protein of wth an unusual ple heicsucure and Globular proteins Globular proteins have complex tertiary and sometimes quaternary structures. They fold into spherical (globular) shapes. The large size of these globular protein molecules affects their behaviour in water 30 Because thei carboxyl and amino ends give them ionic properties yyou might expect them to dissolve in water and form a solution. In fact the molecules are so big that instead they form a colloid, Globular proteins play an important role in holding molecules in position in the cytoplasm. Giobular protein ae aso important in your immune system — for example, antibodies ae globular proteins Globular proteins form enzymes and some hormones and are involved in maintaining the structure of the eytoplasm (see Section 1.4.1 for details of proteins as enzymes) Haemoglobin is one of the best known globular proteins Itisa very large molecule made up of 574 amino acids arranged in four polypeptide chains which are held together by disulfide bonds. ach chain is arranged around an iron-containing haem group Hoemogicbin is « conjugated protein as vell as a globular protein, [tis the iron that enables the haemoglobin to bind and, Telease oxygen molecules, and itis the arrangement of the polypeptide chains that determines how easily the oxygen binds or in released (see Section 4.3.3) Conjugated proteins Some protein molecules are joined vith or conjugated ro another rmolecile called a prosthetic group. Ths strucaral change tsually affects the performance anc functions of te molecules You have already looked at haemoglobin, a large protein with an iron-containing prosthetic group Chloropyl the molecule involved in the eapture of light energy in photosynthesis, is another conjugated protein, with a prosthetic group that contains magnesium, Glycoproteins are proteins with a carbohydrate prosthetic group, ‘The carbohydrate part of the molecule helps them to hold on toa lot of water and also makes it harder for protein-digesting ‘enzymes (proteases) to break them down. Lots of lubricants used by the human bedy ~ such as mucus and the synovial uid in the joints ~ are glycoproteins waose water-nclding properties make them slippery and viscous, which reduces friction. This also helps to explain wity the mucus produced in the stomach protects the protein walls from digestion Lipoproteins are proteins conjugated with lipids and are very important in the transport of cholesterol in the blood. The lipid part Of the molecule enables it © combine with the lipid cholesterol "There are two main forms of lipoproteins in your blood ~ low- ensity lpoproteins (LDLs) (around 22 nm in diameter) and high- ensity lipoproteins (HDLs) (around &-L1 nmin diameter). The DLs contain more protein than LDLs, which is partly why they are denser ~ proteins are more compact molecules than lipids. Remember that amina acids are joined together by peptide bonds to form dipeptides and then polypeptides, but the 40 strutures of proteins are the result of hydrogen bonds, disuifide bonds and ionic bonds between amino acids within the polypeptide chains