Explain why two different enzymes, such as amylase and lipase, can each only catalyse one type of reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Enzymes are biological catalysts found in living organisms. Amylase catalyses the breakdown of starch, while lipase catalyses the breakdown of lipids. Neither enzyme can catalyse the other's reaction.
Model answer (5 marks)
Each enzyme has a specific active site
The active site is complementary to only one specific substrate
An enzyme‑substrate complex forms when the substrate fits the active site
Starch cannot fit the active site of lipase (or lipids cannot fit the active site of amylase) because the shapes are not complementary
Therefore no enzyme‑substrate complex forms and no reaction is catalysed
The active site is complementary to only one specific substrate
An enzyme‑substrate complex forms when the substrate fits the active site
Starch cannot fit the active site of lipase (or lipids cannot fit the active site of amylase) because the shapes are not complementary
Therefore no enzyme‑substrate complex forms and no reaction is catalysed
Examiner tips
- Use the ‘specific active site’ phrase first to show understanding of specificity
- Explain the complementary fit to justify why only one substrate works
- Mention the enzyme‑substrate complex to link structure to function
- Conclude that lack of complex means no catalysis
Common mistakes
- Confusing enzyme specificity with substrate concentration
- Saying the enzyme is ‘stronger’ or ‘weaker’ instead of describing active site shape
- Omitting the step that no complex forms, so no reaction occurs
Mark scheme (5 marks)
- Each enzyme has a specific active site
- The active site is complementary to only one specific substrate
- An enzyme-substrate complex forms when the substrate fits the active site
- Starch cannot fit the active site of lipase (or lipids cannot fit the active site of amylase) because the shapes are not complementary
- Therefore no enzyme-substrate complex forms and no reaction is catalysed
Related
- Revision notes: Proteins and Enzymes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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