# 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

> 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.

## 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

- [lipase](https://www.gradenine.co.uk/glossary/lipase)
- [lipids](https://www.gradenine.co.uk/glossary/lipids)
- [valid](https://www.gradenine.co.uk/glossary/valid)

## 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-8756cfd6) · Published by Druglandscape Ltd.