# 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

> 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.

## 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

- [lipase](https://www.gradenine.co.uk/glossary/lipase)
- [substrate concentration](https://www.gradenine.co.uk/glossary/substrate-concentration)

## Related

- [Revision notes for OCR A-Level Biology B — Advancing Biology (H422)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-the-effect-975c1836) · Published by Druglandscape Ltd.