# Light reactions

The first stage of photosynthesis, in the thylakoid membranes of chloroplasts. Light energy splits water, releases oxygen, and makes ATP and NADPH.

Biology (university) · Part 03: Respiration and photosynthesis · Card 7 of 8 · Last updated 2026-10-19

## Analogy

Solar panels charging batteries.

## In practice

The oxygen we breathe comes from the splitting of water in these reactions.

## Practice

### Flashcards

- **What do the light reactions produce?** ATP, NADPH and oxygen.

### Interview questions

**Compare the light reactions and the Calvin cycle of photosynthesis.**

- Light reactions in thylakoids make ATP, NADPH and oxygen
- Calvin cycle in stroma uses ATP and NADPH
- Calvin cycle fixes CO2 into sugar using rubisco

## Questions people ask

### What are photosystems?

Photosystems are clusters of pigment molecules, including chlorophyll, and proteins in the thylakoid membrane. They catch light energy and use it to excite electrons. Photosystem II splits water to replace its lost electrons. Photosystem I re-energises them again so that NADPH can be made.

### Why are plants green?

Chlorophyll absorbs mostly red and blue light and reflects green. So the light that reaches our eyes from leaves is green. Plants also hold other pigments, such as carotenoids, that absorb other colours. In autumn, when chlorophyll breaks down, those yellow and orange pigments become visible.

## Related concepts

- [Calvin cycle](https://studyvisually.com/learn/science/biology-university/biology-university-part-3/biology-university-part-3-concept-8)

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