# Explain how cyclic photophosphorylation differs from non-cyclic photophosphorylation in terms of the electron pathway and the products formed.

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

## Mark scheme (4 marks)

1. In non-cyclic photophosphorylation, electrons are sourced from the photolysis of water and passed from photosystem II through an electron transport chain to photosystem I, then on to NADP⁺; in cyclic photophosphorylation, electrons from photosystem I are cycled back to the electron transport chain without involving photosystem II or water.
2. Non-cyclic photophosphorylation produces NADPH, whereas cyclic photophosphorylation does not produce NADPH.
3. Non-cyclic photophosphorylation produces oxygen as a by-product of photolysis, whereas cyclic photophosphorylation does not produce oxygen.
4. Both pathways produce ATP via the flow of protons through ATP synthase / chemiosmosis, but cyclic photophosphorylation produces only ATP, making it useful when the cell requires additional ATP without additional NADPH.

## Key terms

- [cyclic photophosphorylation](https://www.gradenine.co.uk/glossary/cyclic-photophosphorylation)
- [non-cyclic photophosphorylation](https://www.gradenine.co.uk/glossary/non-cyclic-photophosphorylation)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-cyclic-photophosphorylation-differs-from-023972ef) · Published by Druglandscape Ltd.