A student is investigating the effect of substrate concentration on the rate of reaction of the enzyme lipase. The student measures how quickly lipase breaks down lipids in milk by recording the time taken for the mixture to change from alkaline to neutral pH. Explain how the student could make this investigation a fair test, and explain what results would be expected as substrate concentration increases.

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 a biological catalyst that breaks down lipids into fatty acids and glycerol. When lipids in milk are broken down, fatty acids are released, causing the pH to drop. A pH indicator can be used to detect when the reaction is complete.

Model answer (5 marks)

To make the investigation a fair test the student should control:
1. Temperature – keep it constant, e.g. by using a water bath at the same temperature for all trials.
2. Enzyme – keep the concentration, volume and type of lipase the same in every experiment.
3. pH indicator – use the same volume and type of indicator, and keep the initial pH of the milk constant.
4. Milk – use the same volume and type of milk for each trial.

With these variables controlled, any change in the time taken for the pH to drop from alkaline to neutral can be attributed to the change in substrate concentration.

As the substrate (lipid) concentration is increased, the rate of reaction will increase because more enzyme–substrate complexes are formed and more active sites are occupied. At sufficiently high substrate concentrations the rate will level off, giving a plateau, because all active sites are occupied and the enzyme becomes the limiting factor.

Examiner tips

  • State each controlled variable and justify why it must be constant.
  • Explain the expected trend (rate up, then plateau) and link it to enzyme saturation.
  • Use the exact terms ‘substrate concentration’, ‘rate of reaction’, ‘enzyme–substrate complex’, ‘active sites’ and ‘plateau’.
  • Show the relationship between time and rate (shorter time = higher rate).

Common mistakes

  • Failing to mention temperature control or using a different temperature for each trial.
  • Confusing the effect of substrate concentration with enzyme concentration.
  • Not recognising the plateau at high substrate concentrations (saturation).

Mark scheme (5 marks)

  1. Keep the temperature constant / use a water bath at the same temperature throughout
  2. Keep the concentration / volume / type of lipase constant
  3. Keep the pH / volume of pH indicator / volume of milk constant (any one additional controlled variable stated and justified or listed)
  4. As substrate (lipid) concentration increases, the rate of reaction increases because more enzyme–substrate complexes form / more active sites are occupied
  5. At high substrate concentrations, the rate levels off / plateaus because all active sites are occupied / the enzyme becomes the limiting factor

Key terms in this question

lipase · substrate concentration

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