# Explain how the absorption of light by photosystems I and II leads to the reduction of NADP⁺ to 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 pigments in photosystem II, causing excitation/boosting of electrons to a higher energy level
2. Excited electrons from PSII pass along an electron transport chain, losing energy used to produce ATP (by photophosphorylation)
3. Electrons from the transport chain reach PSI, where light energy is absorbed again, re-energising/boosting the electrons to an even higher energy level
4. The re-energised electrons from PSI are used to reduce NADP⁺ to NADPH (with the addition of H⁺/protons)

## Key terms

- [NADP⁺](https://www.gradenine.co.uk/glossary/nadp)
- [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-the-absorption-of-light-817abab7) · Published by Druglandscape Ltd.