# Explain how light energy is used to produce ATP and NADPH during the light-dependent reactions of photosynthesis.

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

## Mark scheme (4 marks)

1. Light energy is absorbed by photosystems (I and/or II) / by photosynthetic pigments such as chlorophyll
2. Excited / high-energy electrons are released from chlorophyll and passed along the electron transport chain
3. Energy released from electrons moving along the electron transport chain drives the pumping of protons / H⁺ ions, establishing a proton gradient that drives ATP synthase to produce ATP (by photophosphorylation)
4. At photosystem I, re-energised electrons are used to reduce NADP⁺ to NADPH (with H⁺ from the stroma)

## Key terms

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

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](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-how-light-energy-is-used-91589abd) · Published by Druglandscape Ltd.