# Explain why the rate of photosynthesis in a C3 plant does not continue to increase indefinitely as carbon dioxide concentration rises, even when light intensity and temperature are kept constant at optimal levels.

> OCR A-Level Biology A (H420) — 5.6 Photosynthesis · Explain · 4 marks

> Researchers studying crop yield noticed that supplying extra carbon dioxide to a greenhouse increased photosynthesis rates up to a point, after which no further increase was observed despite continued carbon dioxide enrichment.

## Mark scheme (4 marks)

1. RuBisCO becomes saturated with carbon dioxide / all active sites of RuBisCO are occupied
2. The supply of RuBP becomes limiting / RuBP is regenerated at a fixed maximum rate
3. ATP and reduced NADP from the light-dependent reactions are being supplied at a fixed maximum rate, limiting the Calvin cycle
4. Therefore a further increase in CO2 cannot increase the rate of carbon fixation / rate of the Calvin cycle, so overall photosynthesis rate plateaus

## Key terms

- [carbon dioxide concentration](https://www.gradenine.co.uk/glossary/carbon-dioxide-concentration)
- [rate of photosynthesis](https://www.gradenine.co.uk/glossary/rate-of-photosynthesis)
- [light intensity](https://www.gradenine.co.uk/glossary/light-intensity)
- [C3 plant](https://www.gradenine.co.uk/glossary/c3-plant)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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/explain-why-the-rate-of-photosynthesis-fe124486) · Published by Druglandscape Ltd.