A student is investigating the effect of substrate concentration on the rate of enzyme activity using lipase and a lipid solution. The student wants to ensure their investigation is valid and produces reliable results. Describe how the student should control variables and improve the reliability of their results in this investigation.

OCR A-Level Biology B — Advancing Biology (H422) — 1.2 Practical skills assessed in the practical endorsement · Describe · 5 marks · View as Markdown

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

Lipase is a biological catalyst that breaks down lipids into fatty acids and glycerol. A student measures the time taken for a lipid emulsion to clear (become transparent) as an indication of the rate of reaction, at five different concentrations of lipid solution.

Model answer (5 marks)

1. Keep the temperature constant, e.g. by performing all trials in a water bath at the same temperature.
2. Use the same volume (or concentration) of lipase enzyme in every experiment.
3. Maintain a constant pH, for example by adding a suitable buffer to each reaction mixture.
4. Repeat each lipid concentration at least three times and calculate a mean rate.
5. Identify and exclude any anomalous results or outliers before calculating the mean.

Examiner tips

  • Use bullet points or short numbered sentences to show each control clearly.
  • Mention the specific method (water bath, buffer) to demonstrate understanding of how to control each variable.
  • Show that you know why each control is important – e.g. temperature affects enzyme kinetics.

Common mistakes

  • Failing to mention the need for a water bath or buffer, or not specifying how to keep temperature/pH constant.
  • Not repeating trials or calculating a mean, or including outliers in the mean calculation.

Mark scheme (5 marks)

  1. Control temperature throughout the investigation (e.g. using a water bath)
  2. Keep the volume (or concentration) of lipase enzyme the same in each experiment
  3. Keep the pH the same / constant throughout (e.g. using a buffer solution)
  4. Repeat each concentration at least three times and calculate a mean
  5. Identify and exclude anomalous results / outliers when calculating the mean

Key terms in this question

control variable · reliability · enzyme

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