Explain why two different enzymes, such as amylase and lipase, cannot catalyse each other's reactions.

Edexcel GCSE Biology (1BI0) — 1.4 Enzymes · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Amylase is an enzyme that breaks down starch, while lipase is an enzyme that breaks down lipids. Each enzyme can only catalyse one type of reaction.

Model answer (4 marks)

Enzymes have an active site with a specific shape.
The substrate must be complementary to, and fit, that active site.
Starch, the substrate of amylase, cannot fit the active site of lipase (and lipids cannot fit the active site of amylase).
Therefore no enzyme–substrate complex is formed and the reaction cannot be catalysed.

Examiner tips

  • Use the ‘specific shape’ and ‘complementary fit’ wording from the mark scheme. Show the logical chain: shape → fit → no complex → no reaction. Keep the answer concise – 4 marks can be earned with 4 short sentences.

Common mistakes

  • Confusing the enzyme with the substrate; saying the enzyme can act on any substrate. Using vague terms like ‘different’ or ‘different type’ without linking to shape. Failing to mention that no enzyme–substrate complex is formed.

Mark scheme (4 marks)

  1. Enzymes have an active site with a specific shape
  2. The substrate must be complementary to / fit the active site
  3. Starch/the substrate of amylase cannot fit the active site of lipase (or vice versa)
  4. Therefore no enzyme–substrate complex is formed so the reaction cannot be catalysed

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