Explain how the structure of an enzyme allows it to speed up a chemical reaction, and describe what happens to the enzyme if the temperature becomes too high.

OCR GCSE Biology A: Gateway Science (J247) — B1.2 What happens in cells (and what do cells need) · Explain · 4 marks · View as Markdown

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

Amylase is an enzyme produced in the salivary glands. It breaks down carbohydrates into simple sugars during digestion.

Model answer (4 marks)

The enzyme has an active site that is shaped to fit the carbohydrate substrate. When the substrate binds, an enzyme‑substrate complex is formed, lowering the activation energy and speeding the reaction. If the temperature rises too high, the active site’s shape changes permanently – the enzyme is denatured and can no longer function normally.

Examiner tips

  • Use the phrase "active site" and "substrate"; mention "lowering activation energy" to show speed‑up.
  • Explain denaturation as a permanent shape change at high temperature.
  • Keep answer concise – 4 marks only.

Common mistakes

  • Confusing enzyme with substrate; writing that the enzyme is destroyed rather than denatured.
  • Using vague terms like "changes shape" without linking to loss of function.
  • Including unnecessary details about other enzymes or digestion steps.

Mark scheme (4 marks)

  1. The enzyme has an active site with a complementary shape to the substrate (carbohydrate/starch)
  2. The substrate binds to the active site, forming an enzyme-substrate complex, which lowers the energy needed for the reaction (speeds up the reaction)
  3. At very high temperatures the shape of the active site changes (permanently)
  4. This process is called denaturation and the enzyme can no longer function normally

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