Explain how a change in pH away from an enzyme's optimum affects the rate of an enzyme-controlled reaction.

WJEC A-Level Biology (Wales) — 1.4 Biological reactions and enzymes · 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 speed up metabolic reactions in living cells. Each enzyme has an active site with a specific shape that is complementary to its substrate.

Model answer (5 marks)

Enzymes have an optimum pH at which the rate of reaction is greatest. If the pH moves away from this optimum, the shape of the active site begins to change. This alteration is called denaturation, and the enzyme becomes denatured. As a result, the substrate can no longer fit into the active site because it is no longer complementary. Consequently, fewer enzyme‑substrate complexes form and the rate of reaction decreases.

Examiner tips

  • State the optimum pH and that the rate is greatest there
  • Explain that a change in pH alters the active‑site shape (denaturation)
  • Show that this prevents substrate binding and reduces the rate

Common mistakes

  • Confusing pH with temperature effects
  • Saying the enzyme is 'inactivated' without mentioning denaturation
  • Failing to link the loss of complementarity to a lower reaction rate

Mark scheme (5 marks)

  1. Enzymes have an optimum pH at which they work best / the rate of reaction is greatest at the optimum pH
  2. As pH moves away from the optimum, the shape of the active site begins to change / is altered
  3. This is called denaturation / the enzyme becomes denatured
  4. The substrate can no longer fit into the active site / the active site is no longer complementary to the substrate
  5. Therefore the rate of reaction decreases / fewer enzyme-substrate complexes form

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