# Explain why the rate of photosynthesis in a C3 plant does not continue to increase indefinitely as atmospheric CO₂ concentration rises, even when light intensity and temperature are kept constant at optimum levels.

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

> Researchers growing crops in controlled glasshouses found that doubling CO₂ concentration from 400 ppm to 800 ppm increased the rate of photosynthesis significantly, but a further doubling to 1600 ppm produced only a small additional increase, and beyond this point no measurable increase was observed.

## Mark scheme (4 marks)

1. CO₂ is fixed by RuBisCO / combines with RuBP in the Calvin cycle to form GP
2. As CO₂ rises, RuBisCO becomes saturated / all active sites on RuBisCO are occupied
3. The supply of RuBP becomes limiting because regeneration of RuBP depends on ATP and NADPH from the light-dependent reactions
4. Therefore CO₂ concentration is no longer the limiting factor / another factor (enzyme concentration / RuBP availability) becomes the limiting factor

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
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