# Calvin cycle

The second stage of photosynthesis, in the chloroplast stroma. It uses ATP and NADPH to fix carbon dioxide into sugar. The key enzyme is rubisco.

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

## Analogy

A workshop that uses the charged batteries to build sugar.

## Example

**Rubisco**

CO2 + RuBP (5 carbons)
makes 2 x 3PGA (3 carbons).
This is carbon fixation.

## In practice

Three turns of the cycle fix 3 carbon dioxide and yield one 3-carbon sugar for the plant to use.

## Practice

### Flashcards

- **What does rubisco do?** It attaches carbon dioxide to RuBP, the first step of the Calvin cycle.

### 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 is carbon fixation?

Carbon fixation is turning carbon from inorganic carbon dioxide into an organic molecule. In the Calvin cycle, rubisco attaches carbon dioxide to a 5-carbon sugar, RuBP, giving two 3-carbon molecules. These are then converted, using ATP and NADPH, into sugar that the plant can build with.

### Why is it sometimes called the dark reactions?

Because the Calvin cycle does not use light directly. But the name is misleading. It depends on ATP and NADPH from the light reactions, so in practice it runs mainly in daylight. Several of its enzymes are also switched on by light, so it slows or stops in the dark.

## Related concepts

- [Light reactions](https://studyvisually.com/learn/science/biology-university/biology-university-part-3/biology-university-part-3-concept-7)

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