# 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

> 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.

## 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

- [lipase](https://www.gradenine.co.uk/glossary/lipase)
- [lipid](https://www.gradenine.co.uk/glossary/lipid)
- [fatty acids](https://www.gradenine.co.uk/glossary/fatty-acids)
- [glycerol](https://www.gradenine.co.uk/glossary/glycerol)
- [fair test](https://www.gradenine.co.uk/glossary/fair-test)
- [pH](https://www.gradenine.co.uk/glossary/ph)

## Related

- [Revision notes for OCR A-Level Biology B — Advancing Biology (H422)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" 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-f6f1d55b) · Published by Druglandscape Ltd.