A student carried out an investigation into the effect of pH on the activity of amylase. The student tested amylase at pH 2, pH 7 and pH 12. Explain why amylase works best at its optimum pH and why activity decreases at pH 2 and pH 12.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Amylase is an enzyme that breaks down carbohydrates into simple sugars. It is produced in the pancreas and salivary glands and has an optimum pH of around 7.
Model answer (5 marks)
At the optimum pH the active site of amylase has the correct shape and charge to bind starch, so enzyme‑substrate complexes form readily and the reaction rate is high.
At pH 2 the excess H⁺ ions protonate amino‑acid residues in the active site, altering its shape and charge; the substrate can no longer fit, so complexes do not form and the rate falls.
At pH 12 the excess OH⁻ ions deprotonate residues, again changing the active‑site structure; the substrate cannot bind, so activity decreases.
At pH 2 the excess H⁺ ions protonate amino‑acid residues in the active site, altering its shape and charge; the substrate can no longer fit, so complexes do not form and the rate falls.
At pH 12 the excess OH⁻ ions deprotonate residues, again changing the active‑site structure; the substrate cannot bind, so activity decreases.
Examiner tips
- Use the phrase ‘optimum pH’ and link it to the shape of the active site. Explain that at extreme pH the enzyme is denatured, preventing complex formation. Show the sequence: optimum → good binding → high rate; non‑optimum → shape change → no binding → low rate.
Common mistakes
- Confusing denaturation with inhibition; not mentioning the loss of complementary shape. Using vague terms like ‘bad conditions’ instead of ‘excess H⁺ or OH⁻ ions’. Failing to state that the rate decreases because complexes cannot form.
Mark scheme (5 marks)
- At the optimum pH, the active site has a complementary shape to the substrate (starch/carbohydrate)
- Enzyme-substrate complexes can form readily at the optimum pH, increasing the rate of reaction
- At pH 2 and pH 12 the pH moves away from the optimum, causing the shape of the active site to change (denature)
- The substrate can no longer fit into the active site because the shapes are no longer complementary
- No enzyme-substrate complexes can form, so the rate of reaction decreases
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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