A student is investigating the effect of temperature on the rate of reaction of the enzyme amylase. The student adds amylase solution and a starch solution to a test tube and places it in a water bath at 20 °C. Every 30 seconds, the student removes a drop of the mixture and adds it to a drop of iodine solution on a spotting tile. When starch is present, iodine solution turns blue-black. When starch is no longer detected, the student records the time taken. The student repeats the investigation at 40 °C and 60 °C. Explain why the rate of the amylase reaction differs between 40 °C and 60 °C.

OCR A-Level Biology B — Advancing Biology (H422) — 1.1 Practical skills assessed in a written examination · Explain · 5 marks · View as Markdown

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

Amylase is an enzyme that breaks down starch into simple sugars. The enzyme has an active site which is complementary in shape to its substrate. At high temperatures, the shape of the active site can be permanently changed.

Model answer (5 marks)

At 40 °C the enzyme is still in its native conformation, so the active site remains complementary to starch and the enzyme–substrate complex forms efficiently. Consequently the rate of starch hydrolysis is high.

At 60 °C the temperature is above the optimum for amylase. The high kinetic energy causes the protein to unfold and the active site changes shape permanently – the enzyme is denatured. Because the active site is no longer complementary, starch cannot bind and the enzyme–substrate complex cannot form.

Therefore the reaction rate at 60 °C is greatly reduced or zero, whereas at 40 °C the rate is much higher.

Examiner tips

  • Use the term ‘denatured’ and explain the loss of complementary shape; link this to loss of complex formation; show the contrast in reaction rate between 40 °C and 60 °C.
  • Include the word ‘optimum’ to show understanding of temperature effect on enzymes.
  • Mention that the rate is ‘greatly reduced or zero’ to cover the full mark.
  • Keep the answer concise – 5 points can be covered in 3–4 sentences.

Common mistakes

  • Saying the enzyme is simply ‘hot’ without mentioning denaturation; "high temperature causes more collisions" – ignores loss of structure.
  • Using vague terms like ‘slow’ or ‘fast’ without linking to active site shape or complex formation.
  • Failing to state that the reaction rate at 60 °C is zero or greatly reduced.”

Mark scheme (5 marks)

  1. At 40 °C the enzyme amylase has a higher rate of reaction / breaks down starch faster than at 60 °C
  2. At 60 °C the enzyme has been denatured
  3. At 60 °C the active site changes shape permanently / is no longer complementary to the starch substrate
  4. Because the active site is changed, the substrate (starch) can no longer bind to the active site / the enzyme–substrate complex cannot form
  5. Therefore at 60 °C the starch is not broken down / the rate of reaction is greatly reduced or zero at 60 °C

Key terms in this question

amylase

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