Cambridge IGCSE Biology
Cambridge IGCSE Biology 5 Enzymes Exam Questions
Free Cambridge IGCSE Biology revision for 5 Enzymes: what you need to know, common mistakes and exam-style questions with answers. Covers enzymes as biological catalysts, the lock and key model, active sites, and the effects of temperature and pH on enzyme activity.
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About Cambridge IGCSE Biology 5 Enzymes
Enzymes are proteins that act as biological catalysts, and this topic underpins digestion, respiration, photosynthesis and biotechnology in Cambridge IGCSE Biology. Core candidates describe enzymes, explain why they are needed to keep reactions fast enough to sustain life, and investigate the effects of temperature and pH. Extended candidates explain enzyme action using the shape of the active site, complementary substrates and enzyme-substrate complexes, and explain the effects of temperature and pH in terms of kinetic energy, collisions and denaturation. Examiners frequently set graph-based questions and practical questions on amylase and starch or catalase and hydrogen peroxide, and penalise vague phrases such as 'the enzyme is killed'.
What you need to know
- Definitions A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction. Enzymes are proteins that are involved in all metabolic reactions, where they function as biological catalysts.
- Lock and key model The substrate fits into the enzyme's active site and products are released. Each enzyme is specific because its active site has a shape complementary to only one substrate.
- Enzyme-substrate complex (Extended) The substrate binds to the active site, forming an enzyme-substrate complex. The reaction takes place and the products leave, so the enzyme can be used again.
- Temperature As temperature increases towards the optimum, molecules have more kinetic energy, so there are more frequent effective collisions and more enzyme-substrate complexes. Above the optimum, the active site changes shape (the enzyme is denatured) and the substrate no longer fits. Many human enzymes have an optimum near 37 degrees Celsius.
- pH Each enzyme has an optimum pH, for example pepsin in the stomach works best in acidic conditions. pH values far from the optimum change the shape of the active site and denature the enzyme.
- Investigations Typical practicals use amylase and starch, testing samples with iodine at intervals until the blue-black colour no longer appears, or catalase with hydrogen peroxide, measuring the volume of oxygen produced. Control variables include enzyme concentration, substrate concentration and volume.
Common mistakes in 5 Enzymes
- Saying the enzyme is killed at high temperature. Enzymes are proteins, not living things; they are denatured.
- Saying the active site is the same shape as the substrate. It is complementary in shape.
- Explaining that low temperatures denature enzymes. At low temperatures molecules have less kinetic energy and collide less often; the enzyme is not denatured.
- Saying enzymes are used up in reactions. They are unchanged and can be reused.
- Describing a graph without data. Quote the optimum value and readings from the graph where asked.
Try three 5 Enzymes exam-style questions
1. State what is meant by the term catalyst. (2 marks)
Show the mark scheme answer
A substance that increases the rate of a chemical reaction; and is not changed by the reaction.
2. Explain why the rate of an enzyme-controlled reaction falls above the optimum temperature. (3 marks)
Show the mark scheme answer
The enzyme is denatured; the shape of the active site changes; the substrate no longer fits (is no longer complementary) so fewer enzyme-substrate complexes form.
3. In an investigation, amylase took 120 seconds to break down all the starch at 40 degrees Celsius. Calculate the rate of reaction using rate = 1000 / time. (1 mark)
Show the mark scheme answer
1000 / 120 = 8.3 (arbitrary units).
5 Enzymes FAQs
What does Cambridge IGCSE Biology 5 Enzymes cover?
It covers enzymes as biological catalysts, the lock and key model and active sites, and how temperature and pH affect enzyme activity, with practical investigations.
Which paper is 5 Enzymes examined in?
It is examined in the theory and multiple choice papers: Papers 1 and 3 for Core candidates, or Papers 2 and 4 for Extended candidates. Practical skills linked to it can also appear in Paper 5 (Practical Test) or Paper 6 (Alternative to Practical).
How should I explain temperature graphs for enzymes?
Split your answer into three parts: rising rate due to more kinetic energy and more effective collisions, the optimum where activity is highest, then the fall due to denaturation and the change in active site shape. Use figures from the graph.
