A student is investigating the effect of temperature on the rate of enzyme activity using a colorimeter. Explain why the student should use a control cuvette containing the reaction mixture but with a boiled enzyme solution, and describe how this control should be used during the investigation.

OCR A-Level Biology A (H420) — 1.2 Practical skills assessed in the practical endorsement · Explain · 4 marks · View as Markdown

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

A student sets up an experiment to measure the rate of an enzyme-controlled reaction at different temperatures. They use a colorimeter to measure the absorbance of a coloured product formed as the reaction proceeds. The student prepares several cuvettes, each containing the substrate and buffer solution at a set temperature.

Model answer (4 marks)

The boiled enzyme is denatured and therefore inactive, so the control contains no enzyme activity.
The control is used to zero or blank the colourimeter, removing any absorbance that comes from the substrate, buffer or other components of the reaction mixture.
This ensures that any change in absorbance recorded in the experimental cuvettes is due solely to the enzyme‑catalysed reaction and product formation.
The control cuvette should be prepared at each temperature tested so that the blank accounts for any temperature‑dependent changes in absorbance of the reaction mixture.

Examiner tips

  • Use the exact phrase ‘boiled enzyme is denatured and inactive’ to gain full credit.
  • Show that the control is used to blank the instrument, not just to check enzyme activity.
  • Mention that a separate control is needed for each temperature to correct for temperature effects.

Common mistakes

  • Assuming the control is only to check enzyme activity and ignoring the blanking function.
  • Preparing the control once for all temperatures, missing temperature‑dependent absorbance changes.
  • Using a non‑boiled enzyme in the control, which would give a false blank.

Mark scheme (4 marks)

  1. The boiled enzyme is denatured and therefore inactive, so the control contains no enzyme activity
  2. The control is used to zero / blank the colorimeter, accounting for any absorbance due to the reaction mixture itself rather than the enzyme-produced colour change
  3. This ensures that any change in absorbance recorded in experimental cuvettes is due solely to the enzyme-catalysed reaction / product formation
  4. The control cuvette should be prepared at each temperature tested so that the blank accounts for any temperature-dependent changes in absorbance of the reaction mixture

Key terms in this question

colorimeter · control cuvette

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