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© C. J. Clegg, Ed Lees, Martin Rowland 2015
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Contents
Get the most from this book
Introduction
Biological molecules
1 Introducing the chemistry of life
2 Proteins and enzymes
3 Nucleic acids and protein synthesis
Cells and viruses
4 Cell structure and viruses
5 The eukaryotic cell cycle and cell division
6 Sexual reproduction in mammals and plants
Classification and biodiversity
7 Classification
8 Natural selection and biodiversity
Exchange and transport
9 Cell transport mechanisms
10 Gas exchange and transport
11 Mammalian circulation
12 Transport In plants
Appendix
13 Mathematics for biology
14 Preparing for the exams
Index
Free online material
Acknowledgements
iv
vi
Test yourself questions
These short questions, found throughout each chapter, are useful for checking your understanding as you progress through a topic.
Activities and Core practicals
These practical-based activities will help consolidate your learning and use your practical skills. Edexcel core practicals are clearly highlighted.
In this edition the authors describe many important experimental procedures to conform to recent changes in the A level curriculum.
Teachers should be aware that, although there is enough information to inform students of techniques and many observations for exam purposes, there is not enough prioritisation for teachers to replicate the experiments themselves, or with students, without recourse to CLEAPSS Hazards or laboratory worksheets which have undergone a risk assessment process.
Exam practice questions
You will find Exam practice questions at the end of every chapter. These follow the style of the different types of questions you might see in your examination and are coloured coded to highlight the level of difficulty. Test your understanding even further with Maths questions and Stretch and Challenge questions.
Tips
These highlight important facts, common misconceptions and signpost you towards other relevant topics.
[Image showing graphs and diagrams related to exam questions]
Dedicated chapters for developing your Maths and Preparing for your exam are also included in this book.
Get the most from this book
Introduction
Welcome to Edexcel A level Biology I. This book has been written to cover the first year of the Pearson Edexcel Level 3 Advanced GCE in Biology B (9BI0). Since the subject content is the same, it also covers the content of the Pearson Edexcel Level 3 Advanced Subsidiary GCE in Biology B (8BI0).
The first twelve chapters of this book cover the essential concepts, facts, principles and terminology of the four Biology topics in the AS qualification and the first year of A Level course. The final chapters contain advice about developing skills that are needed to fulfil your final examinations. Since you will need to put into practice the principles they contain, you are advised to read these final chapters at the start of your course. You will also find it useful to dip into them during the course, especially when preparing your examination strategy.
The biological content of each chapter has been written to ensure you cover everything that you can be expected to recall with understanding in your examinations. To help you and to encourage you to take an active part in your learning, the chapters contain the following features:
- Clear definitions of technical terms that you will be expected to understand and use correctly.
- Tips that offer reminders, hints or warnings.
- Regularly spaced Test yourself questions that encourage recall and understanding.
- Activities that encourage you to apply your knowledge and analyse information and make judgements.
At the end of each chapter you will find a series of Exam Practice Questions. By simulating the types of questions that you will encounter in your examinations, these questions will help you to develop, and maintain, the skills that will be assessed. To help you further, these questions have been graded to indicate their accessibility level: (= AS/A level grade E-C), (= AS/A level grade C and C= AS grade A/A level grade C-A). In addition, each chapter contains Stretch and Challenge questions. Some
of these are similar to questions in an A level paper that are targeted at the more able students. Others encourage you to use the learning resource centre in your college or school to carry out further research. Don’t feel you have to do this alone; group research will develop your skills and can be more satisfying than working alone.
Practical work is an essential part of science. The Edexcel Biology specification contains core practicals that you are expected to carry out and on which you can be tested in examinations. This book covers these core practicals in a way that will encourage you to think about what you are doing, analyse results and make judgements. Again, these are all skills that can be assessed in your examinations.
Above all else, Biology is a fascinating subject which we hope you enjoy. We hope that some of our own enjoyment of Biology is reflected in this book and in the supporting material.
Acknowledgements
This book is an extensively revised, restructured and updated version of Edexcel Biology for AS by C J Clegg. We have relied heavily on the original book and are most grateful that C J Clegg would also like to encourage us to build on his work. We would also like to acknowledge the value of the detailed comments and suggestions from Liz Jones. The team at Hodder Education, and initially by Harriet Rimming and then by Emma Bethwaite, has made an extremely valuable contribution to the development of the book and the website resources. In particular, we would like to thank Abigail Woodman, the project manager, for her expert advice and encouragement. We are also grateful for the skillful work on the print and electronic resources by Lydia Young.
Ed Lees and Martin Rowland
January 2015
Introducing the chemistry of life
1
Prior knowledge
In this chapter you will need to recall that:
- Carbohydrates contain the chemical elements carbon, hydrogen and oxygen in the ratio C, H, O.
- Carbohydrates include simple sugars, such as glucose, and complex carbohydrates, such as cellulose and starch.
- simple sugars are used in respiration; complex carbohydrates might be glucose stores (starch) or structural components of cells (cellulose).
- lipids also contain the chemical elements carbon, hydrogen and oxygen, but there is much less oxygen in lipids than in carbohydrates.
- Ions can be useful storage compounds and can also be structural components of cells.
- Organic ions are present in the cytoplasm of cells and in body fluids; each type of ion has a specific function.
- water is essential for life and most of the mass of an organism comprises water.
- water is a reactant in many cell reactions; these reactions also occur in solution in water.
Test yourself on prior knowledge
1 Give one way in which the composition of a carbohydrate molecule is similar to that of a lipid and one way in which it is different from that of a lipid.
2 Name one type of carbohydrate used for energy storage in an animal and one used for energy storage in a plant.
3 How do animal lipids differ from plant lipids at room temperature?
4 Give two functions of lipids.
5 Calcium ions are essential for healthy growth in animals and plants. Give one function of calcium ions in humans and one function of calcium ions in plants.
6 In cells, water is a reactant in hydrolysis reactions and in condensation reactions. Describe the difference between these two types of reaction.
Some basic concepts
Chemical elements are the units of pure substance that make up our world. The Earth is composed of about 92 stable elements; living things are built from some of them. Table 1.1 shows a comparison between the most common elements in the Earth’s crust and in us. You can see that the bulk of the Earth is composed of the elements oxygen, silicon, aluminium and iron. Of these, only oxygen is a major component of our cells.
Table 1.1 Most common elements
| Earth’s crust | % of atoms | Human body | % of atoms |
|—|—|—|—|
| Element | | Element | |
| Oxygen | 47.0 | Hydrogen | 63.0 |
| Silicon | 28.0 | Oxygen | 25.5 |
| Aluminium | 7.9 | Carbon | 9.5 |
| Iron | 4.5 | Nitrogen | 1.4 |
| Calcium | 3.5 | Calcium | 0.3 |
| Sodium | 2.5 | Phosphorus | 0.2 |
In fact, about 16 elements are required to build up all the molecules of the cell, and are therefore essential for life. Consequently, the full list of essential elements is a relatively short one. Furthermore, about 99 per cent of living matter consists of just four elements: carbon, hydrogen, oxygen and nitrogen.
The elements carbon, hydrogen and oxygen predominate because living things contain large quantities of water, and also because most other molecules present in cells and organisms are compounds of carbon combined with hydrogen and oxygen, including the carbohydrates and lipids. We will examine the structures and roles of carbohydrates and lipids shortly.
The element nitrogen is combined with carbon, hydrogen and oxygen in compounds called proteins, from which proteins are constructed (Chapter 2). First, we will introduce some inorganic ions essential for organisms, and then discuss water.
Atoms, molecules and ions
The fundamental unit of chemical structure is the atom. Atoms group together to form molecules and molecules are the smallest part of most molecules or compounds that can exist alone under normal conditions. For example, both oxygen and nitrogen naturally combine with another atom of the same type to form a molecule (O, and N, respectively).
If an atom gains or loses an electron, an ion is formed. Positively charged ions migrate to the poles of an electric field. Positively charged ions migrate to the negative pole (cathode) and are so called cations. In contrast, negatively charged ions migrate to the negative pole (anode) and so are called anions.
Acids and bases
An acid is a compound that releases hydrogen ions in solution. We are familiar with the sharp taste that acids such as lemon juice or vinegar give to the tongue. These are relatively weak acids, weak enough to use on foods. The stronger the acid the more dangerous and corrosive it is, and the more hydrogen ions it releases. An example of a strong acid is hydrochloric acid. HCl dissociates readily to release hydrogen ions in solution.
HCl → H⁺ + Cl⁻
Hydrochloric acid → Hydrogen ion (proton) + Chloride ion
Key term
- Atom: The smallest part of an element that can take part in a chemical change.
- Ions: Charged particles formed when atoms gain or lose electrons.
- Cations: Positively charged ions; whereas anions are negatively charged ions.
- Acid: A compound that releases hydrogen ions in solution. Acidic solutions have a pH value below 7.
Key term
- Buffer solution: A solution that resists changes in pH; usually a mixture of a weak acid and one of its soluble salts.
pH is very important in living organisms, largely because pH affects the shape of enzymes, almost all of which are proteins (page 34). In a mammal’s body there are mechanisms that stabilise pH at a value just slightly above pH 7.0. If the pH varies much from this value the lack of stabilisation is quickly fatal. For plants that obtain essential mineral ions from the soil solution, the pH of the soil affects the availability of the ions for absorption.
A buffer solution is one that will resist pH change when diluted, or if a little acid or alkali is added. Many buffers used in laboratory experiments contain a weak acid (such as ethanoic acid) and one of its soluble salts, for example sodium ethanoate. In this case, if acid is added, the excess hydrogen ions are immediately removed by being combined with ethanoic ions to form undissociated ethanoic acid. Alternatively, if alkali is added, the excess hydroxyl ions immediately combine with hydrogen ions, forming water. At the same time, more of the ethanoic acid dissociates, adding more hydrogen ions to the solution. The pH does not change in either case.
In the body of a mammal, the blood is very powerfully buffered by the presence of a mixture of phosphate ions, hydrogen carbonate ions and blood proteins (page 226). The blood’s pH is held between pH 7.35 and 7.45.
Test yourself
1 Distinguish between a sodium atom and a sodium ion.
2 A pH value is calculated as –log₁₀ hydrogen ion concentration. By how many times is the concentration of hydrogen ions in a solution with a pH value of 2 greater than one with a pH value of 8?
3 Explain the importance of buffer solutions during investigations into the rate of enzyme-controlled reactions.
4 Explain the meaning of the term dissociation.
5 Explain why a positively charged ion is called a cation.
Inorganic ions used by plants
Metabolism allows a range of inorganic ions, in addition to those mentioned above. Table 1.2 shows four inorganic ions whose roles in plants you are required to know.
Table 1.2 The role of selected ions in plants
| Inorganic ion | Role in plants |
|—|—|
| Nitrate (NO₃⁻) | Used to synthesise the nitrogenous bases in DNA and RNA nucleotides and to synthesise the amino groups of amino acids. |
| Calcium (Ca²⁺) | Used to synthesise calcium pectate, which exists as a layer, called the middle lamella, between the walls of adjacent plant cells. |
| Magnesium (Mg²⁺) | Used to synthesise the photosynthetic pigment, chlorophyll. |
| Phosphate (PO₄³⁻) | Used to synthesise adenosine triphosphate (ATP) and to synthesise DNA and RNA. |
Water
Living things are typically solid, substantial objects, yet water forms the bulk of their plants and animals (about 80 per cent of the mass of a human cell consists of water). Despite this, and the fact that water has some unusual properties, it is a substance that is often taken for granted.
Inorganic ions used by plants
Metabolism allows a range of inorganic ions, in addition to those mentioned above. Table 1.2 shows four inorganic ions whose roles in plants you are required to know.
Table 1.2 The role of selected ions in plants
| Inorganic ion | Role in plants |
|—|—|
| Nitrate (NO₃⁻) | Used to synthesise the nitrogenous bases in DNA and RNA nucleotides and to synthesise the amino groups of amino acids. |
| Calcium (Ca²⁺) | Used to synthesise calcium pectate, which exists as a layer, called the middle lamella, between the walls of adjacent plant cells. |
| Magnesium (Mg²⁺) | Used to synthesise the photosynthetic pigment, chlorophyll. |
| Phosphate (PO₄³⁻) | Used to synthesise adenosine triphosphate (ATP) and to synthesise DNA and RNA. |
Water
Living things are typically solid, substantial objects, yet water forms the bulk of their plants and animals (about 80 per cent of the mass of a human cell consists of water). Despite this, and the fact that water has some unusual properties, it is a substance that is often taken for granted.