---
title: "The Calvin Cycle and Carbon Fixation"
description: "Rubisco joins carbon dioxide to RuBP, and the ATP and reduced NADP from the light reactions turn the product into triose phosphate. Most of that triose phosphate is spent regenerating RuBP."
canonical: https://lightmysky.com/learn/science/the-calvin-cycle-and-carbon-fixation-mt_zGOCx9mNrx
source: https://lightmysky.com/learn/science/the-calvin-cycle-and-carbon-fixation-mt_zGOCx9mNrx.md
retrieved: 2026-09-12
---

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# The Calvin Cycle and Carbon Fixation

Rubisco joins carbon dioxide to RuBP, and the ATP and reduced NADP from the light reactions turn the product into triose phosphate. Most of that triose phosphate is spent regenerating RuBP.

Subject: Science · Area: Organisms & Life Processes · Ages 16 to 18
Page: https://lightmysky.com/learn/science/the-calvin-cycle-and-carbon-fixation-mt_zGOCx9mNrx

## Ready when they can

- Traces one carbon atom from carbon dioxide into a triose phosphate.
- Explains what the ATP and the reduced NADP are used for inside the cycle.
- Says why the cycle stalls quickly once the light-dependent stage stops.

## Lesson: Turning thin air into sugar

The Calvin cycle fixes carbon in the stroma, starting the sugar that feeds the plant and the animals that eat plants. Its enzyme Rubisco grabs carbon dioxide and bolts it onto RuBP, a five-carbon acceptor. The product splits into two molecules of GP.

ATP and reduced NADP from the light reactions then reduce GP into triose phosphate. Most of that triose is ploughed back into rebuilding RuBP, at the cost of more ATP. Only a small spare leaves the cycle, so three turns net a single extra triose. Kill the light and the shipped stocks drain within moments, so the cycle stalls.

**Example.** Follow one carbon atom. It enters as carbon dioxide gas, joins RuBP, becomes part of GP, and is reduced into triose phosphate. From there it either cycles back into RuBP or leaves inside exported sugar. No photon is trapped here; the cycle only spends carriers shipped from the thylakoids.

**Tip.** Light independent does not mean light irrelevant. Kill the light and the shipped stocks of ATP and reduced NADP drain within moments, so reduction halts and RuBP cannot be rebuilt.

**Recap.** Fix carbon onto RuBP, reduce it with ATP and reduced NADP, and rebuild RuBP with most of the product.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Light-Dependent Reactions in the Thylakoid Membrane](https://lightmysky.com/learn/science/light-dependent-reactions-in-the-thylakoid-membrane-mt_3Ooq3BfmpX)

## Opens up

- [Glycolysis and the Link Reaction](https://lightmysky.com/learn/science/glycolysis-and-the-link-reaction-mt_crbadbBvqB)
- [Flux Control and Following Atoms with Labelled Substrates](https://lightmysky.com/learn/science/flux-control-and-following-atoms-with-labelled-substrates-mt_PzhsDwD_aW)