A student is investigating the effect of pH on the rate of lipase activity. They use a lipid solution and lipase enzyme, and measure how quickly the pH of the solution changes as the lipase breaks down the lipid into fatty acids and glycerol. Describe how the student should carry out this investigation, ensuring it is a fair test, and explain how they would know when the reaction is complete.

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 an enzyme that breaks down lipids into fatty acids and glycerol. As fatty acids are produced, the pH of the reaction mixture decreases. A pH probe or indicator can be used to monitor the progress of the reaction.

Model answer (5 marks)

1. Prepare a series of buffer solutions at the desired pH values (e.g. pH 4, 6, 7, 8, 10) and keep the buffer concentration the same for all.
2. Measure and record the volume and concentration of the lipase solution and the lipid solution; use the same amounts for every trial.
3. Keep the temperature constant for all experiments, for example by performing the tests in a water bath set to 37 °C.
4. Add the same volume of lipase to each lipid–buffer mixture and immediately start recording the pH with a calibrated pH probe or a universal indicator.
5. Record the pH at regular time intervals (e.g. every 30 s) until the pH no longer changes.
6. Repeat the whole procedure for each pH value at least twice and calculate the mean time or mean rate of pH change for each pH.
7. The reaction is complete when the pH stabilises – i.e. the pH readings are constant over successive measurements, indicating that no further fatty acids are being produced and the lipid substrate has been exhausted.

Examiner tips

  • Use the command word ‘describe’ – list the steps in order and explain why each step ensures a fair test.
  • Show that you keep all variables other than pH constant – volume, concentration, temperature, and repeat trials for reliability.
  • State clearly that the reaction is complete when the pH stops decreasing, not when it reaches a particular value.

Common mistakes

  • Failing to keep temperature constant, which affects enzyme activity.
  • Not repeating trials or calculating a mean, giving unreliable data.
  • Stopping the experiment before the pH stabilises, mistaking a temporary plateau for completion.

Mark scheme (5 marks)

  1. Prepare a range of buffer solutions at different pH values (e.g. pH 4, 6, 7, 8, 10) to test different pH conditions
  2. Keep all other variables constant (e.g. volume/concentration of lipase, volume/concentration of lipid solution, temperature) to make it a fair test
  3. Add lipase to each lipid-buffer solution and use a pH probe or universal indicator to monitor the pH change
  4. Repeat the experiment at each pH value and calculate a mean time / mean rate to improve reliability
  5. The reaction is complete when the pH stops decreasing / remains constant, because no more fatty acids are being produced as all the lipid substrate has been broken down

Key terms in this question

lipase · lipid · fatty acids · glycerol · fair test · pH

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