# A student is investigating the effect of light intensity on the rate of photosynthesis using pondweed. Describe how the student should carry out a fair and valid investigation, including how they would measure the rate of photosynthesis and control relevant variables.

> OCR A-Level Biology B — Advancing Biology (H422) — 1.2 Practical skills assessed in the practical endorsement · Describe · 5 marks

> Pondweed releases oxygen bubbles during photosynthesis. A student places a lamp at different distances from the pondweed and counts the oxygen bubbles produced in one minute at each distance.

## Mark scheme (5 marks)

1. Count the number of oxygen bubbles produced (per minute) as a measure of the rate of photosynthesis
2. Change the distance of the lamp from the pondweed to change light intensity
3. Control temperature throughout the investigation (e.g. use a water bath or keep the surrounding water temperature constant)
4. Control carbon dioxide concentration (e.g. add sodium hydrogen carbonate to the water to keep CO₂ levels constant)
5. Repeat the experiment at each light intensity and calculate a mean to improve reliability

## Key terms

- [photosynthesis](https://www.gradenine.co.uk/glossary/photosynthesis)
- [rate of photosynthesis](https://www.gradenine.co.uk/glossary/rate-of-photosynthesis)
- [light intensity](https://www.gradenine.co.uk/glossary/light-intensity)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-the-effect-5795ec3b) · Published by Druglandscape Ltd.