A student investigates the effect of temperature on the rate of enzyme activity using a colorimeter to measure the absorbance of a solution over time. Explain why the student should use a water bath rather than placing the reaction tubes directly in a beaker of water on a hot plate, and describe one additional step the student should take to ensure the results are reliable.

OCR A-Level Biology A (H420) — 1.1 Practical skills assessed in a written examination · Explain · 4 marks · View as Markdown

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

A student is measuring the rate of an enzyme-controlled reaction at five different temperatures (20°C, 30°C, 40°C, 50°C, 60°C). The enzyme breaks down a coloured substrate, causing the solution to become less coloured over time. Absorbance is recorded every 30 seconds using a colorimeter.

Model answer (4 marks)

A water bath gives a uniform, stable temperature throughout the reaction tube, so the enzyme works at the intended temperature.
A hot plate creates a temperature gradient; the tube surface may be hotter than the solution inside, giving an inaccurate reaction temperature.
To ensure reliable results the student should pre‑incubate the enzyme and substrate solutions separately in the water bath at the set temperature before mixing them, and then repeat the experiment at each temperature to obtain a mean value and identify any outliers.

Examiner tips

  • Use the word ‘uniform’ or ‘stable’ to show understanding of temperature control. Mention that the reaction mixture must be at the set temperature before the reaction starts. Show the two steps (water bath and pre‑incubation) clearly to hit all 4 marks.

Common mistakes

  • Assuming the beaker of water gives the same temperature as the reaction tube. Omitting the pre‑incubation step or not recognising the need to repeat the experiment for a mean value. Using vague terms like ‘good temperature’ instead of ‘uniform, stable temperature’.

Mark scheme (4 marks)

  1. A water bath provides a more stable / uniform temperature throughout the tube
  2. A hot plate creates a temperature gradient / uneven heating, so the temperature at the reaction site may not match the set temperature
  3. Pre-incubate / equilibrate the enzyme and substrate solutions separately in the water bath at the required temperature before mixing
  4. Repeat the experiment at each temperature and calculate a mean / identify and exclude anomalous results

Key terms in this question

colorimeter · water bath · absorbance · reliable

Related

More Practical skills assessed in a written examination questions

▶ Try answering this question with AI marking (free) →