Explain why increasing the concentration of an enzyme increases the rate of a reaction, and describe what happens to the rate when the substrate concentration becomes the limiting factor.

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).

A student investigates how enzyme concentration affects the breakdown of hydrogen peroxide by catalase. She keeps substrate concentration constant throughout.

Model answer (5 marks)

Increasing the concentration of enzyme gives more active sites for the substrate to bind to. With more active sites, substrate molecules collide with and bind to them more frequently, so the number of catalytic events per unit time rises and the reaction rate increases.
When the substrate concentration becomes the limiting factor, all active sites are occupied and the enzyme is saturated. Adding more enzyme no longer increases the number of reactions per unit time, so the rate levels off and remains constant.

Examiner tips

  • Use the phrase ‘more active sites’ and ‘substrate binding’ to show understanding of the mechanism.
  • Explain the saturation point clearly – all sites occupied – to justify the rate plateau.
  • Show the logical link: more enzyme → more sites → more collisions → higher rate.
  • Mention that the plateau occurs because the enzyme is saturated, not because the substrate is exhausted.

Common mistakes

  • Confusing substrate concentration with enzyme concentration; students may say ‘more substrate gives a higher rate’. Failing to mention that the rate plateaus when all active sites are occupied. Using vague terms like ‘more enzyme means faster’ without explaining the active‑site mechanism.

Mark scheme (5 marks)

  1. More enzyme molecules means more active sites available
  2. Substrate molecules collide with / bind to active sites more frequently / faster
  3. So the rate of reaction increases (as enzyme concentration increases)
  4. When substrate concentration is limiting, all active sites are occupied / enzyme molecules are saturated
  5. Rate no longer increases / rate levels off / rate stays constant (because adding more enzyme has no effect)

Key terms in this question

substrate concentration · limiting factor

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