IGCSE Biology: Enzymes — Practice Questions (Cambridge 0610)

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — Cambridge International holds copyright in its own papers. Use these alongside the official past papers available from your board.

Each question practises a skill tested in the June 2024 Paper 42. After each answer there is an examiner insight, a mark-scheme insight or a tip, and (where a close match exists) the real question to try next. Questions marked (Extended) test Supplement content.

Tier note: questions marked (Extended) go beyond 0610 Core: explaining enzyme action and specificity with the enzyme–substrate complex, and explaining the effects of temperature and pH in terms of kinetic energy, collisions, shape and fit (5.1, Supplement outcomes 6–9). All other questions are answerable by a 0610 Core candidate.

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Questions

1. Enzymes are described as biological catalysts.

(a) Define the term catalyst.

(b) Explain why enzymes are important to all living organisms. [3]

2. A student investigates a protease enzyme. She adds the enzyme to a cloudy suspension of cooked egg white and times how long the mixture takes to go clear. Her results are: 20 °C, 14 minutes; 30 °C, 7 minutes; 40 °C, 4 minutes; 50 °C, 9 minutes; 70 °C, the mixture never goes clear.

(a) State which of the temperatures she tested was closest to the optimum temperature for this enzyme.

(b) Explain why the mixture never goes clear at 70 °C. [4]

3. (Extended) A protease enzyme digests egg white much faster at 40 °C than at 20 °C. Explain why, referring to kinetic energy and collisions in your answer. [3]

4. (Extended) Amylase breaks down starch, but it has no effect on protein molecules. Explain why. [3]

5. (Extended) The enzyme catalase breaks down hydrogen peroxide into water and oxygen. Use the terms active site, substrate, enzyme–substrate complex and product to explain how catalase does this. [3]

6. (Extended) Pepsin digests protein in the stomach, where the contents are strongly acidic. When the stomach contents pass into the duodenum, bile makes them alkaline and pepsin stops working. Explain why pepsin stops working in alkaline conditions. [3]

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Answers

1. (a) A substance that increases the rate of a chemical reaction [1] and is not changed by the reaction [1].

(b) Enzymes speed up metabolic reactions so that they happen fast enough to sustain life [1]. Without enzymes, reactions such as respiration and digestion would be far too slow at body temperature.

Tip: A catalyst is not “used up” and is not a reactant. Keep both parts of the definition: it speeds up the reaction, and it is unchanged at the end.

2. (a) 40 °C [1]: it gave the shortest time to go clear, so the fastest rate.

(b) At 70 °C the enzyme is denatured [1]. The shape of the active site has changed [1], so the egg-white protein (the substrate) no longer fits / is no longer complementary to the active site, and no product is formed [1].

Mark-scheme insight (Cambridge 0610 June 2024 mark scheme, Paper 42, Question 1(c)(iv)): In the yeast question at 95 °C, each of the three marks was for a separate idea, from: enzymes denatured, active site changed shape, the active site no longer fitting (complementary to) the substrate, and the yeast being unable to respire. Write each idea as its own point to collect all three marks.

Source for the mark-scheme insights on this page: Cambridge IGCSE Biology 0610 June 2024 mark scheme for Paper 42 (0610/42), paraphrased. Cambridge’s 0610 past papers page publishes the Paper 41 mark scheme from this series, not the Paper 42 one.

Try the real question next: Cambridge IGCSE Biology 0610, June 2024, Paper 42, Question 1.

3. (Extended) At 40 °C the enzyme and substrate molecules have more kinetic energy and move faster [1]. So there are more frequent (effective) collisions between enzyme and substrate molecules [1], and more enzyme–substrate complexes are formed each minute, so the egg white is digested faster [1].

Tip: Below the optimum, explain the rise in rate with kinetic energy and collisions; above the optimum, switch to denaturation and active-site shape. Using “denatured” for a low temperature is wrong: cold enzymes are just slow.

4. (Extended) The active site of amylase has a shape that is complementary to starch [1]. Protein molecules have a different shape, so they do not fit the active site [1]. No enzyme–substrate complex can form, so protein is not broken down [1]. This is called enzyme specificity.

Tip: Say “complementary shape”, not “the same shape”. The substrate and active site fit together like a key in a lock, not like two identical objects.

5. (Extended) Hydrogen peroxide is the substrate; it fits into the active site of catalase because their shapes are complementary [1]. An enzyme–substrate complex forms [1]. The hydrogen peroxide is broken down and the products (water and oxygen) are released from the active site, leaving the enzyme unchanged and able to act again [1].

Tip: Put the steps in order: substrate enters active site, complex forms, products leave. Each step is a separate mark point.

6. (Extended) The change in pH changes the shape of the active site of pepsin / denatures the enzyme [1]. Protein (the substrate) no longer fits the active site [1], so fewer or no enzyme–substrate complexes form and digestion stops [1].

Tip: Each enzyme has its own optimum pH. Pepsin’s optimum is acidic, so an alkaline pH moves it away from its optimum; “the enzyme dies” never earns credit because enzymes are not alive.

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Where marks are usually lost

• Saying enzymes are “killed” or “die”; enzymes are proteins and are denatured.
• Writing that the active site has “the same shape” as the substrate instead of a complementary shape.
• Using denaturation to explain a low rate at a low temperature.
• Leaving out the enzyme–substrate complex when explaining enzyme action.
• Treating a whole organism, such as yeast, as if it were the enzyme.