# Explain why photosynthesis in a C3 plant eventually stops when a plant is kept in complete darkness for an extended period.

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

## Mark scheme (4 marks)

1. In darkness, the light-dependent reactions cannot occur / no light energy is absorbed by photosystems
2. ATP and NADPH are not produced / are depleted
3. Without ATP and NADPH, the Calvin cycle cannot proceed / GP cannot be reduced to triose phosphate (G3P)
4. RuBP is not regenerated, so CO2 fixation cannot continue / the cycle cannot be sustained

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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