Explain why an enzyme works best at its optimum pH and why changing the pH above or below this value reduces the rate of reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Enzymes are biological catalysts that speed up chemical reactions in living organisms. Each enzyme has an active site with a specific shape that is complementary to its substrate.
Model answer (4 marks)
At the optimum pH the active site has the correct shape to fit the substrate.
If the pH is too high or too low the shape of the active site changes.
This happens because the altered pH changes the ionisation of amino‑acid side chains, breaking or altering the bonds that maintain the enzyme’s shape.
The substrate can no longer bind properly, so fewer enzyme‑substrate complexes form and the reaction rate falls.
If the pH is too high or too low the shape of the active site changes.
This happens because the altered pH changes the ionisation of amino‑acid side chains, breaking or altering the bonds that maintain the enzyme’s shape.
The substrate can no longer bind properly, so fewer enzyme‑substrate complexes form and the reaction rate falls.
Examiner tips
- Use the word "optimum" and link pH to enzyme shape. Show that pH changes ionisation, affecting bonds. Explain the consequence for substrate binding and reaction rate. Keep each point concise and use exam terminology.
Common mistakes
- Confusing pH with temperature. Failing to mention ionisation of amino‑acid side chains. Using vague phrases like "enzyme is damaged" instead of "shape changes".
Mark scheme (4 marks)
- At the optimum pH, the active site has the correct/complementary shape to fit the substrate
- Changing the pH above or below the optimum alters the shape of the active site
- This is because the bonds (holding the enzyme's shape) are broken/changed by the altered pH
- The substrate can no longer fit / bind to the active site, so fewer enzyme-substrate complexes form and the rate of reaction decreases
Key terms in this question
Related
- All Edexcel GCSE Biology (1BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Enzymes questions
- Explain why two different enzymes, such as amylase and lipase, cannot catalyse e…
- Explain why enzymes can speed up reactions in living cells without being used up…
- Explain why enzymes are described as specific, and describe what happens to enzy…
- Explain why enzymes stop working when the temperature is increased above their o…