About Enzymes in A Level Biology
Enzymes is one of the 11 main topics in our A Level Biology (Cambridge (CAIE) 9700) question bank, and about 5% of its real questions come from it. Tayyari Karo has 32 real questions on this topic from 2021 to 2026, from Paper 1, Paper 2; 27 are multiple choice and marked instantly, and the rest are structured questions with a model answer. By difficulty: 22 medium, 10 hard.
Work through the questions below, then open the explanation for each one. Redo the ones you missed a few days later.
Try 6 real questions
These are real past-paper questions. Think of your answer first, then open the explanation.
May/June 2026 · Paper 1 · Enzymes
A student listed some possible steps to measure the progress of a starch–amylase catalysed reaction.
1 Calibrate the colorimeter with a dilute iodine solution.
2 Measure the light transmission with a colour filter.
3 Measure the absorbance of the reaction at different wavelengths of light.
Which steps are required in their method?
- 1, 2 and 3
- 1 and 2 only
- 2 and 3 only
- 3 only
Show answer and explanation
Answer: B. 1 and 2 only
Calibrate with iodine solution and measure transmission with a suitable colour filter; absorbance at different wavelengths is not needed.
May/June 2025 · Paper 1 · Enzymes
Which statements about the mode of action of an enzyme are correct?
1 Some enzymes have a region on their surface to which the substrate has a complementary shape.
2 Some enzymes and their substrates can change shape slightly as the substrate enters the active site.
3 Enzymes permanently change shape when they react with their substrates to form an enzyme–substrate complex.
- 1, 2 and 3
- 1 and 2 only
- 1 and 3 only
- 2 and 3 only
Show answer and explanation
Answer: B. 1 and 2 only
The lock-and-key (statement 1) and induced-fit (statement 2) models both apply; the enzyme's shape change is temporary, not permanent, so statement 3 is wrong.
May/June 2024 · Paper 1 · Enzymes
Which statements about the Michaelis–Menten constant (Km) are correct?
1 The higher the Km, the higher the enzyme affinity for the substrate.
2 Km is a measure of the degree of enzyme affinity for the substrate.
3 Km is defined as the substrate concentration at which the enzyme functions at half its maximum rate.
- 1, 2 and 3
- 1 and 2 only
- 1 and 3 only
- 2 and 3 only
Show answer and explanation
Answer: D. 2 and 3 only
Statement 1 is wrong — a higher Km actually indicates lower enzyme affinity for its substrate; statements 2 and 3 are correct definitions of Km.
October/November 2024 · Paper 1 · Enzymes
Which statement is correct for a non-competitive inhibitor?
A The inhibitor binds to the active site of the enzyme and decreases Vmax.
B The inhibitor binds away from the active site and increases the Michaelis–Menten constant.
C The inhibitor decreases Vmax, but the Michaelis–Menten constant does not change.
D The inhibitor does not change Vmax but increases the Michaelis–Menten constant.
- The inhibitor binds to the active site of the enzyme and decreases Vmax.
- The inhibitor binds away from the active site and increases the Michaelis–Menten constant.
- The inhibitor decreases Vmax, but the Michaelis–Menten constant does not change.
- The inhibitor does not change Vmax but increases the Michaelis–Menten constant.
Show answer and explanation
Answer: C. The inhibitor decreases Vmax, but the Michaelis–Menten constant does not change.
A non-competitive inhibitor binds away from the active site, reducing the proportion of functional enzyme so Vmax falls, but does not affect substrate affinity, so Km is unchanged.
October/November 2023 · Paper 1 · Enzymes
How will the removal of a reversible non-competitive enzyme inhibitor affect an enzyme-catalysed reaction?
- The Km will decrease and the Vmax will increase.
- The Km will increase and the Vmax will not change.
- The Km will not change and the Vmax will increase.
- The Km will not change and the Vmax will not change.
Show answer and explanation
Answer: C. The Km will not change and the Vmax will increase.
A non-competitive inhibitor binds away from the active site, reducing the number of functional enzyme molecules and so lowering Vmax without affecting Km (affinity for substrate at unaffected active sites is unchanged). Removing the inhibitor therefore restores/increases Vmax while Km stays the same.
May/June 2022 · Paper 1 · Enzymes
A student measured how increasing substrate concentration affects the rate of an enzyme-controlled reaction, then repeated the experiment with a fixed quantity of a reversible competitive inhibitor added. Which row describes the effect of a reversible competitive inhibitor?
- no attachment at active site; little effect on rate as substrate increases
- yes attachment at active site; rate increases as substrate increases
- no attachment at active site; rate increases as substrate increases
- yes attachment at active site; little effect on rate as substrate increases
Show answer and explanation
Answer: B. yes attachment at active site; rate increases as substrate increases
A competitive inhibitor binds reversibly at the active site (yes); as substrate concentration increases it outcompetes the inhibitor, so the rate increases towards the uninhibited Vmax.
