# Explain why the rate of photosynthesis in a C3 plant eventually reaches a plateau when light intensity is increased under conditions of constant, non-limiting carbon dioxide concentration and temperature.

> IB DP Biology Standard Level (2023 syllabus) — C1.3 Photosynthesis · Explain · 4 marks

## Mark scheme (4 marks)

1. As light intensity increases, the light-dependent reactions produce more ATP and NADPH (reduced NADP), so the rate of the light-dependent reactions increases.
2. The Calvin cycle (light-independent reactions) requires the ATP and NADPH produced by the light-dependent reactions to fix and reduce carbon dioxide.
3. At high light intensities, the Calvin cycle enzymes (such as RuBisCO) become saturated / the supply of CO2 or enzyme activity becomes the limiting factor, so the rate of carbon fixation cannot increase further.
4. Therefore, even though more light is available, the overall rate of photosynthesis cannot increase beyond this maximum, producing a plateau on a graph of rate versus light intensity.

## Key terms

- [plateau](https://www.gradenine.co.uk/glossary/plateau)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-rate-of-photosynthesis-934f19e7) · Published by Druglandscape Ltd.