Explain how a rise in temperature above an enzyme's optimum affects the rate of reaction. In your answer, refer to the active site and enzyme-substrate complexes.

WJEC A-Level Biology (Wales) — 1.3 Cell membranes and transport · Explain · 5 marks · View as Markdown

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

Enzymes are biological catalysts that control chemical reactions within cells. Each enzyme has an active site with a specific shape that is complementary to its substrate.

Model answer (5 marks)

Above the optimum temperature the enzyme becomes denatured.
The heat changes the shape of the active site permanently.
The substrate can no longer fit into the altered active site.
Fewer or no enzyme‑substrate complexes can form.
Therefore the rate of reaction decreases.

Examiner tips

Use the word ‘denatured’ to show understanding of irreversible change. Show the chain: temperature ↑ → shape change → no complexes → rate ↓.

Common mistakes

Saying the enzyme is ‘less efficient’ without mentioning denaturation. Forgetting to link the shape change to loss of complexes.

Mark scheme (5 marks)

Above the optimum temperature, the enzyme becomes denatured
The shape of the active site changes (permanently)
The substrate can no longer fit into the active site
Fewer / no enzyme-substrate complexes can form
Therefore the rate of reaction decreases

Key terms in this question

active site · enzyme-substrate complex