Cambridge IGCSE™ Biology 4th Edition
Genres: Biology
Book Preface
To make your study of Biology for Cambridge IGCSE™ as rewarding and successful as possible, this textbook, endorsed by Cambridge Assessment International Education, offers the following important features:
Focus
Each chapter starts with a short outline of the topic so you know what to expect within each chapter.
Focus points
- Each topic starts with a bullet point summary of what you will encounter over the next few pages.
Test yourself
These questions appear regularly throughout the topic so you can check your understanding as you progress.
Revision checklist
At the end of each chapter, a revision checklist will allow you to recap what you have learnt in each chapter and double check that you understand the key concepts before moving on.
Exam-style questions
Each chapter is followed by exam-style questions to help familiarise learners with the style of questions they may see in their examinations. These will also prove useful in consolidating your learning. Past paper questions are also provided in the back of the book.
As you read through the book, you will notice that some text is shaded yellow. This indicates that the highlighted material is Supplement content only. Text that is not shaded covers the Core syllabus. If you are studying the Extended syllabus, you should look at both the Core and Supplement sections.
Key definitions
These provide explanations of the meanings of key words as required by the syllabus.
Practical work
These boxes identify the key practical skills you need to be able to understand and apply as part of completing the course.
Worked example
These boxes give step-by-step guidance on how to approach different sorts of calculations, with follow-up questions so you can practise these skills.
Going further
These boxes take your learning further than is required by the Cambridge syllabus so that you have the opportunity to stretch yourself.
Answers are provided online with the accompanying Cambridge IGCSE™ Biology Teacher Guide . A Practical Skills Workbook is also available to further support you in developing your practical skills as part of carrying out experiments.
Scientific enquiry
During your course you will have to carry out several experiments and investigations which will help you to develop some of the skills and abilities that scientists use to solve real-life problems.
Simple experiments may be designed to measure, for example, your pulse rate while you are resting. Longer investigations may be designed to establish or confirm a relationship between two or more physical quantities, for example, the effect of increasing temperature on the rate of transpiration in a plant shoot. Investigations will likely be generated from the topic you are currently studying in class.
Any investigation will involve the following five aspects:
- Selecting and safely using techniques, apparatus and materials – you need to be aware of any hazards presented by an investigation and how you will minimise any possible risks. Your teacher should help you with any risk assessments before you start. You also need to be able to identify the best materials and equipment in order to make sure you stay safe and that your observations or data are accurate.
- Planning experiments – when planning you need to consider what procedure will help you find answers to the questions you are investigating. When choosing the apparatus or technique you will use, think about the reasons for your choice. Once you’ve selected these it will be useful to make predictions and hypotheses (informed guesses) of the results you’d expect. It will help to write down your plan as it develops. Variables are also very important in planning – you will need to identify both the independent variables and the dependent variable so you can make sure the results are valid. You will need to consider how the independent variables will be controlled and what range of values you intend to collect. Decide how you will process the results in order to form a conclusion or to evaluate your prediction.
- Making and recording observations, measurements and estimates – you must make sure you measure and collect the necessary experimental data with suitable precision. This involves selecting and using the most appropriate measuring instruments available to you. You need to choose an appropriate number of readings or observations, remembering to include repeats to check your data is reliable. The results will need to be recorded systematically (e.g. in a suitable table). If you’re recording observations be sure to be detailed.
- Interpreting and evaluating the observations and data – when evaluating results you need to do so in a way that enables any relationships between quantities to be formed. As part of this, you will need to process the information you have collected. This may involve calculations, such as working out the percentage change in mass of samples of potato when placed in a range of sucrose solutions. Alternatively, you may need to plot a graph, for example, to show the relationship between temperature and the rate of enzyme action. Always make sure any graph axes are labelled with the descriptor and units on both axes (these details can be taken from your table headers).
When forming conclusions, you need to state the relationship your data has established (what happens to the dependent variable as the independent variable is changed) and give a scientific explanation for why it has happened. You should be aware of the possibility of any anomalous results and decide how to treat these. If you were to evaluate the data, how could you have improved the accuracy, reliability or quality? What would you do differently next time?
- Evaluating experimental methods and suggesting possible improvements – this is different than evaluating your data and requires you to look at the investigation as a whole. You should assess the techniques you used and decide whether your use of a control was adequate. You should also identify any possible sources of error, deciding how you could have overcome these. It may be that you had difficulty measuring the change in length of a piece of potato with a ruler in an osmosis investigation. Instead of measuring length, weighing the potato pieces on a digital balance could provide more accurate data.
A written report of the investigation is normally made, and this should include:
- An aim – what you are trying to find out.
- A plan of what you intend to do. This should include the apparatus needed for your experiments, including justifications for your choice, safety precautions, identification of the variables and how you will control them, and predictions of expected results. You should draft out a table with suitable headings (and don’t forget to state units) for recording any data you intend to collect.
- When listing items of apparatus you will use, make a record of the smallest division of the scale of any measuring device. For example, the smallest division on a metre rule is 1 mm. The scale of the rule can be read to the nearest mm. So, when used to measure a length of 100 mm (0.1 m), the length is measured to the nearest 1 mm, and the degree of accuracy of the measurement is 1 part in 100. When used to measure 10 mm (0.01 m), the degree of accuracy of the measurement is 1 part in 10. As another example, a thermometer is calibrated in degrees Celsius and may be read to the nearest 1°C. A temperature may be measured to the nearest 1°C. So, when used to measure a temperature of 20°C, the degree of accuracy is 1 part in 20 (this is 5 parts in 100). If a digital thermometer is available, this may be a better choice for accuracy.
- A method – this should include the details of any procedures, observations and measurements you carry out. A clearly labelled diagram of the apparatus is a good way of supporting your method. When labelling the apparatus, avoid label lines crossing each other.
- Presentation of results and calculations. Any data you collected should be clearly presented (most likely in a table). The column headings, or start of rows, should include a descriptor (naming the measurement) and its unit; for example, ‘temperature / oC’. If you have repeated any measurements, calculate an average value. Numerical values should be given to the number of significant figures appropriate to the measuring device you used in collecting the data. If it is appropriate to plot a graph of your results, you will need at least five data points taken over as large a range as possible. Remember to label each axis of a graph with the descriptor and unit of the quantity being plotted. Also put a title on the graph. This will refer to both axes of the graph, for example, ‘Graph to show the effect of increasing temperature on the rate of action of amylase on starch.’
- A concise conclusion should be drawn from the evidence. This can be based on the prediction, stating the relationship between the two quantities you investigated. Note that sometimes experiments do not achieve the intended objective. If this is the case, a conclusion is still important. A conclusion must be a description of the pattern as well as a scientific explanation for this trend.
- In order to produce an evaluation and discussion of the result of the investigation you need to look critically at your procedure. Points to include are:
- commenting critically on the original plan
- evaluating the procedures used
- deciding how reliable the results are. The reliability can be indicated by how close any repeated readings are. You could compare your results with secondhand evidence (such as from a graph in a textbook) to consider whether or not the evidence can be trusted
- using a systematic approach to dealing with unexpected results. For example, if you took a reading that was unexpected, did you take a further reading? Can you identify why that reading might have been incorrect?
- considering how appropriate the apparatus used in the investigation was and suggesting improvements where appropriate.
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