Explain how substrate concentration affects the rate of an enzyme-controlled reaction. Include in your answer what happens to the rate when substrate concentration continues to increase beyond a certain point.

AQA A-Level Biology (7402) — 3.1.4 Proteins and Enzymes · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

As substrate concentration increases, the rate of reaction increases because more substrate molecules are available to collide with and bind to the active sites of the enzymes.
When the substrate concentration continues to rise beyond a certain point, the rate levels off because all active sites are occupied and all enzymes are working at maximum capacity. Adding more substrate cannot increase the rate further as there are no free active sites available.

Examiner tips

  • Use the phrase ‘rate increases because more substrate molecules are available to bind’ to hit the first two points. Mention ‘rate levels off’ and ‘all active sites are occupied’ to cover the last three points. Keep the answer concise – 5 marks can be earned with two short sentences. Use UK spelling (e.g. ‘rate’ not ‘rate’).

Common mistakes

  • Forgetting to explain why the rate levels off (e.g. ‘all enzymes are saturated’). Using vague terms like ‘more enzyme’ instead of ‘more substrate’. Over‑expanding the answer with unnecessary detail.

Mark scheme (5 marks)

  1. As substrate concentration increases, the rate of reaction increases
  2. Because there are more substrate molecules available to collide with / bind to the active site of the enzyme
  3. Above a certain substrate concentration, the rate levels off / plateaus / stays constant
  4. Because all active sites are occupied / all enzymes are working at maximum capacity
  5. Adding more substrate cannot increase the rate further because there are no free active sites available

Key terms in this question

substrate concentration

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