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 Theory of Knowledge connections TOK boxes included in this title have been reviewed and written by our TOK expert and bestselling author of our coursebook, John Sprague: Much like how the IB Diploma is a real “programme” in the sense that all of its moving parts mesh togeth...

 Theory of Knowledge connections TOK boxes included in this title have been reviewed and written by our TOK expert and bestselling author of our coursebook, John Sprague: Much like how the IB Diploma is a real “programme” in the sense that all of its moving parts mesh together, Hodder Education is working to incorporate a genuine collaborative and unified vision among its author team. We’ve brought together writers from the science specialists and DP Core to provide opportunities for students to experience the integrative approach to knowledge that the IBDP captures. We believe that every new publication provides us an opportunity to show our readers how the construction and transfer of knowledge is a collaborative adventure. Theory of Knowledge for the IB Diploma Theory of Knowledge for the IB Diploma: Fourth Edition 9781510474314 Skills for Success Second Edition 9781510474956 Confidently navigate the Build confidence in a range of Theory of Knowledge key Theory of Knowledge Guide with a set of rich skills with this practical and engaging resources, companion, full of advice and grounded in conceptual guidance from an considerations and experienced TOK expert. illustrated with real- Learn to apply analytical world examples. skills with Deeper Guide students by Thinking, showing you helping them examine the nature of how to go beyond simply knowledge and their own status as a identifying and explaining. knower. Develop awareness of the practical application Develop diverse and balanced arguments of knowledge with In Practice pointers, offering with a variety of activities, case studies guidance on how topics can be used in TOK and Deeper Thinking features. activities. Aid understanding with in-depth Improve your ability to respond to knowledge discussions of the twelve course concepts questions, a crucial part of assessment success. and detailed definitions of all key terms. Avoid making the mistakes that others make in Provide assessment support with the assessments with TOK Traps that highlight guidance relating to the TOK Exhibition common errors and misconceptions. and Essay. www.hoddereducation.com/tok Also available for Diploma Core Extended Essay for the IB Diploma: Skills for Success 9781510415126 Build confidence in a range of key essay writing techniques and skills with this practical companion, full of advice and guidance from experienced EE experts. Build essay writing techniques and skills through a range of strategies, serving as a useful companion throughout the writing process – from the development of a research question, critical-thinking, referencing and citation to reflecting on the process and final essay. Concise, clear explanations help you navigate the IB requirements, including advice on assessment objectives and academic honesty. Learn what is required to get the best EE grades and write an excellent essay with detailed examiner advice and expert tips and hints, including common mistakes to avoid. Explicit reference to the IB Learner profile and the importance of reflection. www.hoddereducation.com/ibdiploma  FOR THE IB DIPLOMA PROGRAMME Biology THIRD EDITION C. J. Clegg Andrew Davis Christopher Talbot 364240_00_IB_Biology_3rd_Edn_Prelims.indd 1 23/03/2023 15:14  This book marks 60 years since the marriage of my mother and father, Mary and Brian Davis, on 10 August 1963. It is dedicated to them both. Andrew Davis 2023 IB advisers: The Publishers would like to thank the following for their advice and support in the development of this project: Marcela Rodriguez. The Publishers would also like to thank the International Baccalaureate Organization for permission to re-use their past examination questions in the online materials. 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 6424 0 © C. J. Clegg and Andrew Davis 2023 First published in 2007 Second edition published in 2014 This edition published in 2023 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 2027 2026 2025 2024 2023 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 art © Thierry – stock.adobe.com Illustrations by Barking Dog, Aptara, Inc., and Oxford Designers & Illustrators Typeset in Berkeley Oldstyle 10/14pt by DC Graphic Design Limited, Hextable, Kent Printed in Slovenia by DZS Grafik A catalogue record for this title is available from the British Library. 364240_00_IB_Biology_3rd_Edn_Prelims.indd 2 23/03/2023 15:14  Contents Each of the four themes covered in this book is broken down into four levels of organization. These four levels are colour coded as follows: 1 Molecules 2 Cells 3 Organisms 4 Ecosystems Introduction.............................................. v How to use this book.................................... vi Tools and Inquiry........................................ viii A Unity and diversity A1.1 Water............................................... 2 A1.2 Nucleic acids......................................... 14 A2.1 Origins of cells (HL only)................................ 34 A2.2 Cell structure........................................ 49 A2.3 Viruses (HL only)...................................... 88 A3.1 Diversity of organisms................................. 102 A3.2 Classification and cladistics (HL only)....................... 125 A4.1 Evolution and speciation............................... 138 A4.2 Conservation of biodiversity............................ 156 B Form and function B1.1 Carbohydrates and lipids............................... 181 B1.2 Proteins........................................... 205 B2.1 Membranes and membrane transport..................... 223 B2.2 Organelles and compartmentalization..................... 244 B2.3 Cell specialization.................................... 256 B3.1 Gas exchange....................................... 273 B3.2 Transport.......................................... 295 B3.3 Muscle and motility (HL only)............................ 326 B4.1 Adaptation to environment............................ 341 B4.2 Ecological niches.................................... 363 364240_00_IB_Biology_3rd_Edn_Prelims.indd 3 23/03/2023 15:14  C Interaction and interdependence C1.1 Enzymes and metabolism.............................. 380 C1.2 Cell respiration...................................... 405 C1.3 Photosynthesis...................................... 425 C2.1 Chemical signalling (HL only)............................ 450 C2.2 Neural signalling..................................... 467 C3.1 Integration of body systems............................. 490 C3.2 Defence against disease................................ 517 C4.1 Populations and communities........................... 548 C4.2 Transfers of energy and matter.......................... 578 D Continuity and change D1.1 DNA replication..................................... 602 D1.2 Protein synthesis...................................... 615 D1.3 Mutation and gene editing............................. 637 D2.1 Cell and nuclear division............................... 648 D2.2 Gene expression (HL only).............................. 670 D2.3 Water potential...................................... 681 D3.1 Reproduction....................................... 693 D3.2 Inheritance......................................... 725 D3.3 Homeostasis........................................ 760 D4.1 Natural selection..................................... 779 D4.2 Stability and change.................................. 798 D4.3 Climate change...................................... 820 Acknowledgements................................ 838 Index................................................ 841 Free online content Go to our website www.hoddereducation.co.uk/ib-extras for free access to the following: n Practice exam-style questions for each chapter n Glossary n Answers to self-assessment questions and practice exam-style questions n Answers to linking questions n Tools and Inquiries reference guide n Internal Assessment – the scientific investigation 364240_00_IB_Biology_3rd_Edn_Prelims.indd 4 23/03/2023 15:14  Introduction Welcome to Biology for the IB Diploma Third Edition, updated and designed to meet the criteria of the new International Baccalaureate (IB) Diploma Programme Biology Guide. This coursebook provides complete coverage of the new IB Biology Diploma syllabus, with first teaching from 2023. Differentiated content for SL and HL students is clearly identified throughout. The aim of this syllabus is to integrate concepts, topic content and the nature of science through inquiry. This book comprises four main themes, each made up of two broad integrating concepts: l Theme A: Unity and diversity l Theme B: Form and function l Theme C: Interaction and interdependence l Theme D: Continuity and change Each theme is then further divided into four levels of biological organization. In this coursebook, each level is colour coded as follows: 1 Molecules 2 Cells 3 Organisms 4 Ecosystems About the authors Chris Clegg is an experienced teacher and examiner of biology and has written many internationally respected textbooks for pre-university courses. He was encouraged to write by his colleague and mentor at his school, textbook writer and teacher D.G. Mackean, in the 1970s and became his co‑author on numerous books. He eventually took over the biology coursebook mantle from Don in the 1980s. Andrew Davis has taught biology for over 20 years. He is the author of several IB textbooks, including Biology for the IB Diploma Study and Revision Guide, IB Diploma: Internal assessment for Biology: Skills for success, and Biology for the MYP 4 & 5: By Concept. He is also author of online teaching and learning resources: Biology for the IB Diploma Teaching and Learning and Biology for the IB MYP 4 & 5 Dynamic Learning. IB advisors Chris Talbot graduated in Biochemistry from the University of Sussex in the United Kingdom. He has Masters Degrees in Life Sciences (Chemistry) and in Science Education from the National Technological University in the Republic of Singapore. He has taught IB Chemistry, IB Biology and Theory of Knowledge (TOK) in a number of local and international schools in Singapore. He is the author of numerous science textbooks, including Chemistry for the MYP 4&5: By Concept. John Sprague has been teaching TOK for 20 years, in the UK, Switzerland and Singapore. Previously Director of IB at Sevenoaks School in the UK, he now teaches philosophy and TOK at Tanglin Trust School, Singapore. The ‘In cooperation with IB’ logo signifies that this coursebook has been rigorously reviewed by the IB to ensure it fully aligns with the current IB curriculum and offers high-quality guidance and support for IB teaching and learning. Introduction v 364240_00_IB_Biology_3rd_Edn_Prelims.indd 5 23/03/2023 15:14  How to use this book The following features of this book will help you to consolidate and develop your understanding of biology, through concept-based learning: Guiding questions There are two guiding questions at the start of every chapter, as signposts for inquiry. These questions will help you to view the content of the syllabus through the conceptual lenses of both the themes and the levels of biological organization. SYLLABUS CONTENT  This coursebook follows the order of the contents of the IB Biology Diploma syllabus.  At the beginning of each chapter is a list of the content to be covered, with all subsections clearly linked to the content statements and showing the breadth and depth of understanding required. Key terms Concepts ◆ Definitions appear throughout the margins The four themes that underpin the IB Biology Diploma course (A Unity and diversity, to provide context and B Form and function, C Interaction and interdependence, and D Continuity and change) to help you understand the language of biology. are integrated into the conceptual understandings of all the units to ensure that a There is also a glossary conceptual thread is woven throughout the course. of all key terms at www. Conceptual understanding therefore enhances your overall understanding of the hoddereducation.co.uk/ ib-extras. course, making the subject more meaningful. This understanding assists you in developing clear evidence of synthesis and evaluation in your responses to questions asked in the assessment, and helps you make connections across the course. Concepts are explored in context and can be found throughout the chapter. Common mistake These detail Tools some common The Tools features explore the skills and techniques that you require and are integrated misunderstandings and into the biology content to be practised in context. The skills in the study of biology can typical errors made by be assessed through internal and external assessment. students, so that you can avoid making the same mistakes yourself. Inquiry The application and development of the Inquiry process is supported throughout this Top tips! coursebook, in close association with the Tools. This feature includes advice relating to the content being WORKED EXAMPLES discussed and tips to These provide a step-by-step guide showing you how to answer the kind of help you retain the knowledge you need. quantitative questions that you might encounter in your studies and in the assessment. vi Biology for the IB Diploma Programme 364240_00_IB_Biology_3rd_Edn_Prelims.indd 6 23/03/2023 15:14  TOK Nature of science Links to Theory of Nature of science (NOS) is an overarching theme in the biology course that seeks to explore conceptual Knowledge (TOK) understandings related to the purpose, features and impact of scientific knowledge. It can be allow you to develop examined in biology papers. NOS explores the scientific process itself, and how science is represented critical-thinking skills and understood by the general public. It covers 11 aspects: Observations, Patterns and trends, and deepen scientific Hypotheses, Experiments, Measurements, Models, Evidence, Theories, Falsification, Science as a shared understanding by endeavour and the Global impact of science. It also examines the way in which science is the basis for discussing the subject technological developments and how these new technologies, in turn, drive developments in science. beyond the scope of the curriculum. ATL ACTIVITY Links Approaches to learning (ATL), including learning through inquiry, are integral to IB pedagogy. Due to the conceptual These activities are designed to get you to think about real-world applications of biology. nature of biology, many topics are connected. The Links Going further feature states where relevant material is Written for students interested in further study, this optional feature contains material covered elsewhere in that goes beyond the IB Diploma Biology Guide. the coursebook. They may also help you to start creating your own linking questions. LINKING QUESTIONS These questions are listed at the end of each chapter and are for all students to attempt (apart from those in HL-only chapters). They are designed to strengthen your understanding by making connections across the themes. The linking questions encourage you to apply broad, integrated and discipline-specific concepts from one topic to another, ideally networking your knowledge. Practise answering the linking questions first, on your own or in groups. Sample answers and structures are provided online at www.hoddereducation.co.uk/ib-extras. The list in this coursebook is not exhaustive; you may encounter other connections between concepts, leading you to create your own linking questions. Self-assessment questions appear throughout the chapters, phrased to assist comprehension and recall, but also to help familiarize you with the assessment implications of the command terms. These command terms are defined in the online glossary. Practice exam-style questions for each chapter allow you to check your understanding and prepare for the assessments. The questions are in the style of those in the examination so that you get practise seeing the command terms and the weight of the answers with the mark scheme. Practice exam-style questions and their answers, together with self-assessment answers, are on the accompanying website, IB Extras: www.hoddereducation.co.uk/ib-extras Skills are highlighted with this icon. Students are expected to be able to show these skills in the examination, so we have explicitly pointed these out when they are mentioned in the Guide. International mindedness is indicated with this icon. It explores how the exchange of information and ideas across national boundaries has been essential to the progress of science and illustrates the international aspects of biology. HE I B L E AR N The IB learner profile icon indicates material that is particularly useful to help you towards ER T PROFILE developing the following attributes: to be inquirers, knowledgeable, thinkers, communicators, principled, open-minded, caring, risk-takers, balanced and reflective. When you see the icon, think about what learner profile attribute you might be demonstrating – it could be more than one. How to use this book vii 364240_00_IB_Biology_3rd_Edn_Prelims.indd 7 23/03/2023 15:14  Tools and Inquiry Skills in the study of biology The skills and techniques you must experience through this biology course are encompassed within the tools. These support the application and development of the inquiry process in the delivery of the course. Tools Inquiry process l Tool 1: Experimental techniques l Inquiry 1: Exploring and designing l Tool 2: Technology l Inquiry 2: Collecting and processing data l Tool 3: Mathematics l Inquiry 3: Concluding and evaluating Throughout the programme, you will be given opportunities to encounter and practise the skills; and instead of stand-alone topics, they will be integrated into the teaching of the syllabus when they are relevant to the topics being covered. You can see what the Tools and Inquiry boxes look like in the How to use this book section on page vi. The skills in the study of biology can be assessed through internal and external assessment. The approaches to learning provide the framework for the development of these skills. Thinking skills Experimental Technology Mathematics techniques Social skills Research skills Exploring and designing Collecting and processing data Communication Self-management skills Concluding and skills evaluating Skills for biology From IB Diploma Programme Biology Guide, page 29 Visit the following website to view the online Tools and Inquiries reference guide: www.hoddereducation.co.uk/ib-extras viii Biology for the IB Diploma Programme 364240_00_IB_Biology_3rd_Edn_Prelims.indd 8 23/03/2023 15:14  Tool 1: Experimental techniques Skill Description Addressing safety of Recognize and address relevant safety, ethical or environmental issues in self, others and the an investigation. environment Measuring variables Understand how to accurately measure the following to an appropriate level of precision: mass volume time temperature length. Make careful observations, including the following: counts drawing annotated diagrams from observation making appropriate qualitative observations classifying. Applying techniques Show awareness of the purpose and practice of: paper or thin layer chromatography colorimetry or spectrophotometry serial dilutions physical and digital molecular modelling a light microscope and eyepiece graticule preparation of temporary mounts identifying and classifying organisms using a variety of sampling techniques/using random and systematic sampling karyotyping and karyograms cladogram analysis. Tool 2: Technology Skill Description Applying technology to Use sensors. collect data Identify and extract data from databases. Generate data from models and simulations. Applying technology to Use spreadsheets to manipulate data. process data Represent data in a graphical form. Use computer modelling. Carry out image analysis. Tools and Inquiry ix 364240_00_IB_Biology_3rd_Edn_Prelims.indd 9 23/03/2023 15:14  Tool 3: Mathematics Skill Description Applying general Use basic arithmetic and algebraic calculations to solve problems. mathematics Carry out calculations involving: decimals, fractions, percentages, ratios, proportions, frequencies (including allele frequencies), densities, approximations and reciprocals. Calculate measures of central tendency: mean, median and mode. Apply measures of dispersion: range, standard deviation (SD), standard error (SE), interquartile range (IQR). Use and interpret scientific notation (for example, 3.5 × 10 6). Use approximation and estimation. Calculate scales of magnification. Calculate rates of change from graphical or tabulated data. Understand direct and inverse proportionality between variables, as well as positive and negative correlations between variables. Calculate and interpret percentage change and percentage difference. Distinguish between continuous and discrete variables. Calculate the actual size from a micrograph that has a scale bar. Apply Simpson's reciprocal index. Apply the Lincoln index. Apply the chi-squared test. Apply the t-test. Using units, symbols and Apply and use SI prefixes and units or non-SI metric units. numerical values Express quantities and uncertainties to an appropriate number of decimal places. Processing uncertainties Understand the significance of uncertainties in raw and processed data. Record uncertainties in measurements as a range (±) to an appropriate precision. Express ranges, degrees of precision, standard error or standard deviations as error bars. Express measurement and processed uncertainties to an appropriate number of decimal places or level of precision. Apply the coefficient of determination (R 2) to evaluate the fit of a trend line. Interpret values of the correlation coefficient and identify correlations as positive or negative. Apply and interpret appropriate tests of statistical significance (for example, chi-squared test). Graphing Sketch graphs, with labelled but unscaled axes, to qualitatively describe trends. Construct and interpret tables, charts and graphs for raw and processed data including bar charts, histograms, scatter graphs, line and curve graphs, logarithmic graphs, pie charts and box-and-whisker plots. Plot linear and non-linear graphs showing the relationship between two variables with appropriate scales and axes. Draw lines or curves of best fit. Interpret features of graphs including gradient, changes in gradient, intercepts, maxima and minima. Draw and interpret uncertainty/error bars. Extrapolate and interpolate graphs. Design dichotomous keys. Represent energy flow in the form of food chains, food webs and pyramids of energy. Represent familial genetic relationships using pedigree charts. x Biology for the IB Diploma Programme 364240_00_IB_Biology_3rd_Edn_Prelims.indd 10 23/03/2023 15:14  Inquiry process Inquiry 1: Exploring and designing Skill Description Exploring Demonstrate independent thinking, initiative and insight. Consult a variety of sources. Select sufficient and relevant sources of information. Formulate research questions and hypotheses. State and explain predictions using scientific understanding. Designing Demonstrate creativity in the designing, implementation and presentation of the investigation. Develop investigations that involve hands-on laboratory experiments, databases, simulations, modelling and surveys. Identify and justify the choice of dependent, independent and control variables. Justify the range and quantity of measurements. Design and explain a valid methodology. Pilot methodologies. Controlling variables Appreciate when and how to: calibrate measuring apparatus maintain constant environmental conditions of systems choose representative random samples and minimize sampling errors set up a control run where appropriate. Inquiry 2: Collecting and processing data Skill Description Collecting data Identify and record relevant qualitative observations. Collect and record sufficient relevant quantitative data. Identify and address issues that arise during data collection. Processing data Carry out relevant and accurate data processing. Interpreting results Interpret qualitative and quantitative data. Interpret diagrams, graphs and charts. Identify, describe and explain patterns, trends and relationships. Identify and justify the removal or inclusion of outliers in data (no mathematical processing is required). Assess accuracy, precision, reliability and validity. Inquiry 3: Concluding and evaluating Skill Description Concluding Interpret processed data and analysis to draw and justify conclusions. Compare the outcomes of an investigation to the accepted scientific context. Relate the outcomes of an investigation to the stated research question or hypothesis. Discuss the impact of uncertainties on the conclusions. Evaluating Evaluate hypotheses. Identify and discuss sources and impacts of random and systematic errors. Evaluate the implications of methodological weaknesses, limitations and assumptions on conclusions. Explain realistic and relevant improvements to an investigation. Tables from IB Diploma Programme Biology Guide, pages 29–33 Tools and Inquiry 1 364240_00_IB_Biology_3rd_Edn_Prelims.indd 1 23/03/2023 15:14  A1.1 Water Concept: Unity Guiding questions and diversity What physical and chemical properties of water make it essential for life? Common ancestry What are the challenges and opportunities of water as a habitat? has given living organisms many SYLLABUS CONTENT shared features while evolution has This chapter covers the following syllabus content: resulted in the rich  A1.1.1 Water as the medium for life biodiversity of life  A1.1.2 Hydrogen bonds as a consequence of the polar covalent bonds within water on Earth. molecules  A1.1.3 Cohesion of water molecules due to hydrogen bonding and consequences for organisms  A1.1.4 Adhesion of water to materials that are polar or charged and impacts for organisms  A1.1.5 Solvent properties of water linked to its role as a medium for metabolism and for transport in plants and animals  A1.1.6 Physical properties of water and the consequences for animals in aquatic habitats  A1.1.7 Extraplanetary origin of water on Earth and reasons for its retention (HL only)  A1.1.8 The relationship between the search for extraterrestrial life and the presence of Concept: Unity water (HL only) All living organisms require water to Water: the medium for life exist. Enzymes – biological The Earth is covered mainly by water and so appears a mostly blue planet when viewed from space. molecules that Approximately 71% of our planet’s surface is water, with 97% found in oceans and only 3% as fresh increase the water. Evidence from the geological record indicates that water has existed on Earth for 3.8 billion rate of chemical years. The Earth formed an estimated 4.5 billion years ago, so water has existed on its surface for reactions – need most of its history. The first cells originated in water, where the oceans blocked harmful ultraviolet to be dissolved radiation from the Sun, allowing the first life to evolve. Water remains the medium in which most in water to work. processes of life occur. Water provides Water forms a large proportion of living organisms – between 65% and 95% by mass of most a chemically stable medium multicellular plants and animals (about 80% of a human cell consists of water). Despite this, and for life processes the fact that water has some unusual properties, water is a substance that is often taken for granted. to operate. As we will see in this chapter, the properties of water allow life to exist at a range of scales – from the smallest bacteria to the tallest tree – and without water life would not exist on Earth. I B L E AR N HE ATL A1.1A ER T PROFILE Freshwater is a limited resource globally. Work in a group to produce an informative poster on the threats to freshwater sources and the solutions available for providing sufficient, clean drinking water for all. 2 Theme A: Unity and diversity – Molecules 364240_01_IB_Biology_3rd_Edn_A_11.indd 2 13/03/2023 11:22  ◆ Covalent bond: a bond between atoms in Hydrogen bonds which pairs of electrons The water molecule consists of one atom of oxygen and two atoms of hydrogen combined by sharing are shared. pairs of electrons (covalent bonding). However, the molecule is V-shaped rather than linear. ◆ Polar molecule: a molecule where there is The nucleus of the oxygen atom draws electrons (negatively charged) away from the hydrogen nuclei an unequal distribution (positively charged) with an interesting consequence. Although overall the water molecule is of electrical charge: one electrically neutral, there is a net negative charge on the oxygen atom and a net positive charge on the end is slightly positive hydrogen atoms. The water molecule therefore carries an unequal distribution of electrical charge and the other end is slightly negative. within it. This arrangement is known as a polar molecule (Figure A1.1.1). ◆ Hydrogen bond: With water molecules, the positively charged hydrogen atoms of one molecule are attracted a weak attractive to negatively charged oxygen atoms of nearby water molecules, causing attractive forces called intermolecular force; a hydrogen atom in a hydrogen bonds (Figure A1.1.1). These intermolecular forces are weak compared to covalent bonds, molecule is attracted to yet they are strong enough to hold water molecules together and to attract water molecules to charged an electronegative atom, particles or to a charged surface. Hydrogen bonds largely account for the unique properties of water. such as oxygen, in a We will examine these properties next. different molecule. 1 Distinguish between ionic and covalent bonding. one oxygen atom combines unshared electrons – lone pairs with two hydrogen atoms by (negative charge) sharing pairs of electrons (covalent bond) nucleus of oxygen atom shared electrons the oxygen nucleus draws electrons (negatively charged) away from the nucleus of hydrogen hydrogen nucleus (positively atom (proton – charged) positive charge) the water molecule carries angle about an unequal distribution of electrical charge, even though overall it is electrically neutral Note the water molecule is V-shaped, not linear small negative δ– polar water molecule charge there is electrostatic attraction between the small hydrogen bond hydrogen bond positively charged region of positive δ+ one water molecule and the charge negatively charged region of a neighbouring one, giving rise to weak bonds or intermolecular forces called hydrogen bonds Figure A1.1.1 The water molecule and the hydrogen bonds it forms A1.1 Water 3 364240_01_IB_Biology_3rd_Edn_A_11.indd 3 13/03/2023 11:22  δ– Figure A1.1.2 (left) shows how to indicate polarity in a water molecule. O H δ+ H δ+ Top tip! H δ+ δ– δ– You need to be able to represent two or more δ+ Figure A1.1.2 The O H O water molecules and hydrogen bonds between polarity of water them. Delta (δ) symbols indicate a small charge. δ+ δ+ H H Figure A1.1.3 Hydrogen bonds between water molecules; the dashed Common line between the oxygen and hydrogen atoms represents a hydrogen bond mistake A common mistake 2 List the important properties of water that are due to its polar nature. is suggesting that hydrogen bonding occurs within water Tool 2: Technology molecules. Do not confuse intra- (within) Using computer modelling and inter- (between) Computer modelling allows scientists to explore how the structure of water is essential molecular bonding. for maintaining its properties and, therefore, in maintaining life. Covalent bonding Ruth Lynden-Bell and co-workers at Queen’s University Belfast used computer acts within a water simulations to model changes in water’s properties. The bond angle in water molecules molecule; hydrogen is 104.5°: they found that if this was changed to 90°, or if the hydrogen bonds were bonds are formed about 15% weaker, the three-dimensional network of hydrogen bonds – crucial to the between water liquid’s unique properties – would be severely disrupted or fall apart. molecules. TOK Confirmation bias The central principle of homeopathy is that water can retain a Confirmation bias refers to the tendency to search for, interpret ‘memory’ of substances previously dissolved in it, even after any and favour information or data in a way that confirms your pre- number of serial dilutions. Such claims about the ‘memory of existing beliefs or hypotheses. You may be guilty of this when water’ are categorized as ‘pseudoscientific’, meaning that while you use an internet search engine to settle an argument and the theories or ideas might look as if they follow the scientific only look for results that confirm what you already think. method as normally applied by expert scientists, they do not. The concept of water having ‘memory’ of what it has previously HE I B L E AR N What are the criteria that can be used to distinguish scientific encountered contradicts current scientific understanding of ER T PROFILE claims from pseudoscientific claims? physical chemistry. Another characteristic of pseudoscientific theories is that they are at odds with well-established scientific The scientific method uses hypothesis, observations and findings; they are wildly surprising. The responsible scientific falsification to develop new scientific ideas. This means that approach is therefore to replicate the tests to see whether the scientists set out to challenge hypotheses and look for evidence same results are found. With the cooperation of Benveniste’s that might prove them false. If researchers only seek more and own team, a group from Nature tried to repeat Benveniste’s more confirmation of their ideas, rather than trying to find findings but failed, ultimately showing that there was no how their ideas might be false, it is possible that their results evidence that water had any sort of chemical ‘memory’. could be biased. The results may appear well established, but Subsequent investigations did not support Benveniste’s findings. really the research is either irrelevant or ignores false results. Given the scientific evidence, then (as opposed to anecdotal One characteristic of ‘pseudoscience’ is that it only looks for evidence), there is no reason to believe that water has a evidence that supports its claims. chemical memory. 4 Theme A: Unity and diversity – Molecules 364240_01_IB_Biology_3rd_Edn_A_11.indd 4 13/03/2023 11:22  ◆ Cohesion: force by which individual Cohesion of water molecules and molecules of the same the consequences for organisms type attract and associate (‘stick together’). Cohesion is the force by which individual molecules of the same type attract and associate (stick ◆ Surface tension: together). Water molecules stick together because of hydrogen bonding. These bonds continually property of the surface break and reform with surrounding water molecules, although at any one moment a large number are of a liquid that allows it to resist an external held together by their hydrogen bonds. Cohesive forces allow water molecules to be drawn up xylem force, due to the vessels in plants by the evaporative loss of water from the leaves (Figure A1.1.4). Compared with cohesion between water other liquids, water has extremely strong cohesive properties that prevent it ‘breaking’ under tension. molecules. Water can be drawn up heat in sunlight evaporation of water to a great height without in the leaves the column breaking or (transpiration) pulling apart. the column of water xylem vessels run from coheres (does not break), roots to leaves, as adheres to the walls of the continuous narrow tubes xylem vessels and flows water drawn up the tree smoothly through them trunk by force generated (because its viscosity is low) in transpiration water taken from soil by root cells Figure A1.1.4 Water is drawn up a tree trunk through xylem vessels: cohesive forces stop the water column from breaking and help draw water up the tree Link The transport of water Common mistake from roots to leaves A common mistake is suggesting that hydrogen bonds are strong – this is not the case. A single during transpiration hydrogen bond is a weak interaction. It is only because there are many hydrogen bonds in water is covered in that they collectively exert large cohesive forces. Chapter B3.2, page 303. Related to the property of cohesion is the property of surface tension. The outermost molecules of water form hydrogen bonds with the water molecules below them. This gives water a very high surface tension (Figure A1.1.5), higher than any other liquid except mercury. The water molecules on the surface have no neighbouring water molecules above and therefore exhibit stronger attractive forces upon their nearest neighbours on and below the surface. Water’s strong surface tension allows it to form almost completely spherical droplets. A1.1 Water 5 364240_01_IB_Biology_3rd_Edn_A_11.indd 5 13/03/2023 11:23  a water molecule O H a hydrogen bond between hydrogen H and oxygen O H a hydrogen bond between hydrogen H and oxygen 04_06 Cam/Chem AS&A2 Barking Dog Art Figure A1.1.5 Hydrogen bonding on the surface of water forms a hexagonal lattice that provides a high surface tension Within a body of liquid, there is no net force on a molecule because the cohesive forces exerted by the neighbouring molecules all cancel O H out (see Figure A1.1.5). However, for a molecule on the surface a water molecule a hydrogen bond of the liquid, there is a net inward cohesive force since there is no attractive between force acting from above. hydrogen H and oxygen This inward net force causes the molecules on the surface to contract and to resist being stretched or broken. Thus, the surface is under tension, hence the name ‘surface tension’. The surface tension of water is exploited by insects that ‘surface skate’ (Figure A1.1.6). The insect’s waxy cuticle prevents the wetting of its body, and the mass of the insect is not great enough to break 04_06 Cam/Chem AS&A2 the surface tension. Barking Dog Art a Surface H2O H2O H2O H2O increased depth H2O H2O H2O H2O H2O H2O H2O H2O Surface tension – molecules at the surface form stronger bonds b Figure A1.1.6 a) A pond skater moving over the water surface; b) the surface tension supports the pond skater – the surface is depressed but the hydrogen bonds hold it together 6 04_07b Cam/Chem AS&A2 Theme A: Unity and diversity – Molecules Barking Dog Art 364240_01_IB_Biology_3rd_Edn_A_11.indd 6 13/03/2023 11:23  ATL A1.1B What other examples of surface tension are there? How does the knowledge of surface tension help you understand everyday phenomena and experiences? For example: l Why are droplets of water pulled into a spherical shape? l Why is it better to wash in hot water rather than cold water? l Why are soaps and detergents used to clean clothes? You could use the website below or other sources to research other examples of surface tension and why the property is useful to know about. www.usgs.gov/special-topics/water-science-school/science/surface-tension-and-water#overview ◆ Viscosity: a measure Below the surface, water molecules slide past each other very easily. This property is described as low of a fluid’s resistance viscosity. Consequently, water flows readily through narrow capillaries, tiny gaps and pores. to flow. Top tip! The diffusion of molecules through a solvent, such as water, is inversely proportional to the viscosity of the solvent. Temperature affects the viscosity of liquids, for example, the viscosity of water at 25 °C is approximately half that than when the temperature is 4 °C. As we will see in Theme B2.1, the diffusion rate of molecules is extremely important for the processes that are needed to sustain life. HE I B L E AR N Inquiry 1: Exploring and designing ER T PROFILE Designing Use the following equipment: Surface tension is one of water’s most l drinking glass important properties. It causes water to l water collect in drops. l liquid dishwashing detergent Design an investigation to show the l paper clips properties of water's surface tension l piece of paper towel. using a paper clip. ◆ Adhesion: the force by which individual molecules stick to surrounding materials and surfaces. Adhesion of water and the impacts for organisms ◆ Hydrophilic: attracted Adhesion is the force by which individual molecules cling to surrounding materials and to water; e.g. hydrogen bonds are readily formed surfaces. Materials and substances with an affinity for water are described as hydrophilic between a molecule (page 8). Water adheres strongly to most surfaces and can be drawn up long columns, for and water. example through narrow tubes such as the xylem vessels of plant stems, without danger of the water column breaking (Figure A1.1.4). It should be noted that cohesion is a far more significant δ+ wall of xylem vessel force in xylem transport and explains how tensions can be resisted. Adhesion is only significant H δ+ water when air-filled xylem vessels refill with aqueous sap under positive pressures, which is something H O molecule δ– δ+ that happens only rarely (no more than once a year). Figure A1.1.7 shows both adhesive and H δ– cohesive forces at work in a xylem vessel. O adhesion δ+ δ– H δ+ H O cohesion Common mistake H δ+ The terms ‘cohesion’ and ‘adhesion’ are sometimes treated as if their meanings are Figure A1.1.7 Adhesive interchangeable, but this is not the case. If they were, we would have one word for these forces and cohesive forces rather than two! Cohesion (‘co’ means ‘together’) is attraction between water molecules, while supporting a column of adhesion (‘ad’ means ‘toward’) is attraction to a surface. water in a xylem vessel A1.1 Water 7 364240_01_IB_Biology_3rd_Edn_A_11.indd 7 13/03/2023 11:23  capillary force water Capillary molecules action in soils and plant cell walls Soilcohesion contains between water molecules many vertical, thin channels known as capillary tubes, in which plant cohesion between water molecules roots onare the located. surface When water enters capillary tubes, adhesion between the water adhesionand molecules between waterofmolecules the wall the capillary draws water up the small tube: this is called and capillary wall capillary action. In this way, plants bring water up from the water table to the roots when the ground becomes dry. capillary wall capillary wall The cell walls of plants are made from a fibrous material called cellulose (see page 193). Cellulose is polar/hydrophilic to a certain degree. Fibrous materials can act like wicks, drawing water up into the material by capillary action (see Figure A1.1.8). Cell walls can draw water by capillary action from nearby xylem vessels, keeping water flowing through plant tissue. Cells that are directly exposed to the air, such as those found in weight of the water column the spongy mesophyll tissue of leaves (page 283), remain constantly wetted by capillary action into these cells. Water evaporates from the moist, blotting-paper-like cell walls of orce water molecules the mesophyll and then diffuses out of leaves through pores on the surface of the leaf cohesion between water molecules (stomata), enabling water to be transported up the plant. cohesion between water molecules on the surface adhesion between water molecules and capillary wall Solvent properties of water Figure A1.1.8 Channels in soils and Hydrogen bonds pull water molecules very close to each other because the potential capillary wall spaces between cellulose fibres in the cell wall act as capillary tubes, energy of the hydrogen bonds is greater than the kinetic energies of the water drawing water through the plant molecules up to 100 °C (at atmospheric pressure). This is why water is a liquid at the temperatures and pressure that exist over much of the Earth’s surface. As a result, we have a liquid medium with distinctive thermal and solvent properties. ◆ Capillary tubes: Water is a powerful solvent for polar substances such as ionic substances like sodium chloride ater column channels with a very small (Na+ and Cl−). All cations (positively charged ions) and anions (negatively charged ions) become internal diameter. surrounded by a layer of orientated water molecules (Figure A1.1.9). ◆ Capillary action: the tendency of a liquid to There is a diverse range of hydrophilic molecules that dissolve in water, such as carbon-containing move up against gravity (organic) molecules with ionized groups (for example, amino acids have a negatively charged when confined within a carboxyl group, –COO−, and a positively charged amino group, –NH3+); soluble organic molecules narrow tube (capillary). like sugars dissolve in water due to the formation of hydrogen bonds with their slightly charged Also known as capillarity. hydroxyl groups (–OH). Once they have dissolved, molecules or ions (the solute) are free to move ◆ Solute: dissolved molecule or ion in around in water (the solvent) by diffusion and, as a result, are more chemically reactive than when a solution. in the undissolved solid. ◆ Solvent: a liquid in On the other hand, non-polar substances are repelled by water, as in the case of oil on the surface which another substance can be dissolved. of water. Non-polar substances are hydrophobic. The functions of some molecules in cells ◆ Hydrophobic: depends on them being hydrophobic and insoluble. For example, the cell membrane is made from repelled by water.