Describe the role of enzymes in digestion and explain why a protease enzyme cannot break down starch.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Enzymes are biological catalysts that speed up the breakdown of large food molecules into smaller, soluble molecules that can be absorbed.
In digestion each enzyme has a specific active site that is complementary in shape to its substrate.
A protease has an active site that recognises and binds proteins.
Because the active site of a protease is not complementary to starch, starch cannot bind and form an enzyme‑substrate complex with the protease.
Therefore a protease cannot break down starch.
In digestion each enzyme has a specific active site that is complementary in shape to its substrate.
A protease has an active site that recognises and binds proteins.
Because the active site of a protease is not complementary to starch, starch cannot bind and form an enzyme‑substrate complex with the protease.
Therefore a protease cannot break down starch.
Examiner tips
- Use the terms "biological catalysts" and "active site". Explain the shape‑complementarity and why it matters. Show the logical chain: specific active site → only proteins bind → starch cannot bind → no breakdown.
Common mistakes
- Using vague terms like "helps digestion" without mentioning catalysis. Confusing protease with amylase. Forgetting to state that the active site is not complementary to starch.
Mark scheme (5 marks)
- Enzymes are biological catalysts
- Enzymes break down large food molecules into smaller, soluble molecules (that can be absorbed)
- Each enzyme has a specific active site with a complementary shape to its substrate
- The active site of a protease is complementary to proteins (not starch)
- Therefore starch cannot bind to / form an enzyme-substrate complex with protease, so it is not broken down
Key terms in this question
Related
- Revision notes: Proteins and Enzymes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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