A student is investigating the effect of pH on the rate at which lipase breaks down lipids. The student uses full-fat milk as a source of lipids and measures the time taken for the milk to change from white to colourless. Explain how the student should plan this investigation to make the results valid and how they would know when the reaction is complete.

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).

Lipase is an enzyme that breaks down lipids into fatty acids and glycerol. When lipids in milk are fully broken down, the solution turns from white to colourless. A student wants to investigate whether changing pH affects the rate of this reaction.

Model answer (5 marks)

1. Keep all other variables constant – temperature, volume of milk, concentration of lipase and volume of the reaction mixture should be the same for every trial.
2. Change only the pH – prepare a series of buffer solutions (e.g. pH 4, 5, 6, 7, 8) and add the same amount of each to separate samples.
3. Measure the time taken for the milk to turn colourless – record the time from the start of the reaction until the mixture is completely colourless.
4. Repeat each pH condition several times and calculate the mean time – this gives a reliable estimate of the rate.
5. The reaction is complete when the mixture is fully colourless, indicating that all lipids have been hydrolysed into fatty acids and glycerol.

Examiner tips

  • Use the command word ‘explain’ – describe each step and why it is important. Mention that the dependent variable is time and the independent variable is pH. Show that repeating trials improves reliability and allows calculation of a mean.
  • Use the exact terminology: ‘control variables’, ‘independent variable’, ‘dependent variable’, ‘mean’, ‘reaction complete’.
  • Include the reason for colour change – loss of white turbidity when lipids are hydrolysed.

Common mistakes

  • Not recognising that temperature must be kept constant. Failing to repeat trials and therefore not calculating a mean. Thinking the reaction is complete when it is merely cloudy, not fully colourless.

Mark scheme (5 marks)

  1. Keep all other variables constant / control variables (e.g. temperature, volume/concentration of lipase, volume of milk)
  2. Change only the pH (independent variable) using different buffer solutions
  3. Measure the time taken for the milk to turn colourless / time is the dependent variable
  4. Repeat the experiment at each pH and calculate a mean / repeat for reliability
  5. The reaction is complete when the mixture is fully colourless / no white colour remains, because all the lipids have been broken down into fatty acids and glycerol

Key terms in this question

lipase · lipids · valid

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