# Explain how oxygen is produced as a by-product during the light-dependent reactions of photosynthesis.

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

## Mark scheme (4 marks)

1. Water molecules are split (photolysed) in the light-dependent reactions
2. Photolysis occurs in the thylakoid / at Photosystem II, using light energy absorbed by chlorophyll
3. Electrons (and protons/H⁺) released from water replace those lost by oxidised P680 / Photosystem II
4. Oxygen is released as a waste product when the water molecule is split: 2H₂O → 4H⁺ + 4e⁻ + O₂

## Related

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