IGCSE生物复习language.doc

Understanding exam languageWhy do some questions ask me to state whilst others ask me to explain and yet others describe? Does the examiner just use different words to make the exam paper look better?In the syllabus book there is a section entitled Glossary of Terms used in Science Papers, which describe the command words in science papers. It is important that you understand what each of the command words requires and that you tailor your answers to questions accordingly.The list is quite a long one and can be off-putting. In this section, the most commonly used command words are discussed in more detail to help you identify what is required.DefineThis requires a formal definition. The syllabus shows words for which you should know a definition in italics. The syllabus generally includes the definition that you should learn.Your textbook also includes these definitions. You need to learn these by heart but do make sure that you fully understand the meaning of the definition as well.StateThis tests whether you have a basic knowledge of a fact, or if you can find a particular piece of information in data that has been provided to you. You need only to give a very short answer. You do not need to support your statement in any way. Often, the answer can be just one word.Example: State the name of the blood vessel that transports digested food from the small intestine to the liver.Answer: Hepatic portal vein.DescribeThis is a very common word used in examination questions. It could be used when the examiner wants you to: describe the pattern or trend shown in a graph in your own words describe how you would do something, for example carry out a food test or an experiment describe the structure of something (you can sometimes use a labelled diagram for this) describe how something works.Example: With reference to the graph below, describe how temperature affects the activity of enzymes.Answer: As temperature increases from 0 C to 40 C, the rate of enzyme activity increases exponentially. It reaches a maximum at 40 C, which is the optimum temperature for this enzyme. As temperature increases above 40 C, enzyme activity drops steeply, becoming completely inactive at 60 C.Example: Describe how you would test a food for starch.Answer: Place a sample of the food on a white tile. Add iodine solution. If it goes blue-black then starch is present. If it stays brown, then there is no starch present.Example: Describe how the structure of a plant cell differs from an animal cell.Answer: A plant cell always has a cellulose cell wall, which animal cells never have. Most plant cells also have a large vacuole containing cell sap, which animal cells do not have. Some plant cells also have chloroplasts containing chlorophyll, which are never found in animal cells.Example: Describe how the biceps and triceps cause the arm to bend at the elbow joint.Answer: The biceps muscle contracts. This pulls on the tendons attaching the muscle to the ulna and scapula. The ulna is therefore pulled closer to the scapula and the arms bends at the elbow. The triceps relaxes while this happening.ExplainThis does require some explanation of how or why something happens, and is often more difficult to do than just describing something. Candidates often lose marks in exams because they describe something when they are asked to explain it.Example: With reference to the graph, explain how temperature affects the activity of enzymes.Answer: At low temperatures, the molecules of the enzyme and its substrate have little kinetic energy. They move about slowly, and do not collide with each other very often. As temperature increases, they move faster and faster, so they collide more frequently. This increases the rate of the reaction.Above a temperature of about 40 C, the high temperature causes the enzyme molecule to lose its shape. It is said to be denatured. The substrate can no longer fit into the active site of the enzyme, so the enzyme no longer catalyses the reaction. Once a temperature of 60 C is reached, all the enzyme molecules are completely denatured, so the reaction stops.SuggestThis is often used where the examiner wants you to use your understanding of biology to make a prediction from information given, or where they want you to show your ability to use your own judgement stepping beyond the narrow confines of the syllabus.It can also be used where there is no single answer and you must use your judgement to make a decision. When you see this word, expect to see something unfamiliar following it and dont panic! Just remember that you are not expected to have learnt an answer to this question. You just have to use your common sense and biological knowledge to think of a good answer.Example: Suggest why jackals survive better when they hunt in packs. (This question followed a short passage about jackals hunting in Africa.)Answer: A group of jackals will be able to kill larger prey animals than one jackal on its own, so they will each have a better chance of getting plenty of food.CalculateThis used where a numerical answer is required. A few words of warning: Always show your working clearly, even if the question does not specifically ask for it. There are usually two or more marks for a calculation. You might score both for just the giving correct answer, but if you make a simple mistake there will be no credit at all unless the examiner can follow your working and see where you made the error. If you do make an error in the arithmetic, you may still get some marks for doing the working in the correct way. No answer is complete without a unit every year unnecessary marks are lost because units are forgotten or wrongly given.DiscussThis requires you to look at a topic from more than one angle. You need to be able to put forward two different points of view.Example: Discuss the advantages and disadvantages of breast-feeding and bottle-feeding of human b