Explain why enzymes can speed up reactions in living cells without being used up themselves.

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).

Enzymes are biological catalysts found in all living organisms. A single enzyme molecule can be used repeatedly to catalyse the same reaction.

Model answer (4 marks)

Enzymes are biological catalysts that lower the activation energy required for a reaction. The substrate binds to the enzyme’s active site, forming an enzyme‑substrate complex. The reaction then occurs, producing the products, which are released from the active site. The enzyme itself is unchanged – its active site remains intact – so it can bind another substrate molecule and repeat the process.

Examiner tips

  • Use the phrase ‘lower the activation energy’ to show understanding of catalysis.
  • Mention the enzyme‑substrate complex and the release of products.
  • Show that the enzyme is unchanged after the reaction to justify reuse.
  • Keep the answer concise – 4 marks can be earned with 4 short points.

Common mistakes

  • Confusing the enzyme with the substrate as being used up.
  • Failing to state that the enzyme’s active site remains unchanged.
  • Using vague terms like ‘speed up’ without linking to activation energy.

Mark scheme (4 marks)

  1. Enzymes are biological catalysts / they lower the activation energy needed for a reaction
  2. The substrate binds to the active site of the enzyme to form an enzyme-substrate complex
  3. After the reaction, the products are released from the active site
  4. The enzyme is unchanged / the active site is unchanged after the reaction, so the same enzyme can catalyse the same reaction again

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