# Light-independent reactions

> Biology · CIE A-Level 9700
> Source: https://www.owlsprep.com/study/cie-9700-u14-light-independent-reactions/

This module covers the light-independent reactions of photosynthesis (the Calvin cycle), which fix inorganic carbon dioxide into organic sugars in the chloroplast stroma. We cover key stages, molecule roles, and common exam questions.

**Prerequisites:** [Light-dependent reactions of photosynthesis](https://www.owlsprep.com/study/cie-9700-u14-light-dependent-reactions/); [Chloroplast structure](https://www.owlsprep.com/study/cie-9700-u13-chloroplast-structure/)

## Learning objectives

- Describe the three core stages of the Calvin cycle
- Explain the roles of key molecules: RuBP, RuBisCO, GP and TP
- Outline the biological uses of net triose phosphate produced
- Predict changes in molecule concentration under limiting conditions
- Interpret standard diagrams of the light-independent reactions

## Carbon Fixation

Carbon fixation is the first stage of the light-independent reactions, occurring in the stroma of chloroplasts. This step incorporates inorganic carbon from carbon dioxide into stable organic molecules, the starting point for all biological carbon pools.

**Carbon Fixation** — The process of converting inorganic carbon dioxide into organic carbon compounds, the first committed step of the Calvin cycle.

The 5-carbon compound ribulose bisphosphate (RuBP) acts as the carbon dioxide acceptor. The enzyme RuBisCO (ribulose bisphosphate carboxylase-oxygenase) catalyses the reaction between RuBP and CO₂. This produces an unstable 6-carbon intermediate that immediately splits into two molecules of 3-carbon glycerate 3-phosphate (GP), the first stable product of the Calvin cycle.

**Worked example:** A plant is exposed to radioactive ¹⁴CO₂ for 10 seconds. Where would you expect radioactivity to first accumulate, and why?

1. Step 1: Identify the first stable product of carbon fixation
2. After CO₂ reacts with RuBP, the resulting 6-carbon intermediate breaks down almost instantly into two 3-carbon GP molecules.
3. Step 2: Explain the result
4. Unreacted ¹⁴CO₂ is inorganic, so radioactivity will first accumulate in GP molecules before the reaction proceeds further to form TP.

## Reduction and RuBP Regeneration

After carbon fixation produces GP, the next stages use products from the light-dependent reactions to produce carbohydrates and regenerate RuBP to keep the cycle running.

**Calvin Cycle** — The cyclic sequence of light-independent reactions that produces organic sugars from carbon dioxide, named for its continuous regeneration of the starting RuBP molecule.

GP is first reduced and phosphorylated using ATP and reduced NADP from the light-dependent reactions. This converts GP into triose phosphate (TP, also called glyceraldehyde 3-phosphate, G3P). The cycle is cyclic because most TP is used to rebuild RuBP, rather than being used for sugar synthesis.

**Worked example:** Explain why only 1 net molecule of TP is produced for every 3 CO₂ molecules that enter the Calvin cycle.

1. Step 1: Calculate total TP produced
2. 3 CO₂ molecules react with 3 RuBP molecules, producing 6 GP molecules, which are fully reduced to 6 TP molecules, for a total of 18 carbons.
3. Step 2: Account for RuBP regeneration
4. To keep the cycle running, 3 RuBP molecules (5 carbons each = 15 carbons total) must be regenerated. 5 out of 6 TP molecules (total 15 carbons) are rearranged and phosphorylated to reform 3 RuBP.
5. Step 3: Find net output
6. Only 1 out of 6 TP molecules (3 carbons total) remains as net output for the plant to use for organic synthesis.

Regeneration of RuBP requires an input of ATP to phosphorylate the rearranged carbon intermediates, so the cycle cannot continue without ATP from the light-dependent reactions.

## Fate of Triose Phosphate

The net TP produced by the Calvin cycle has multiple critical roles in plant metabolism:

- Two TP molecules combine to form a 6-carbon hexose sugar (glucose or fructose)
- Hexose sugars are linked to form sucrose (transport), starch (energy storage), or cellulose (cell wall structure)
- TP is used as a precursor for amino acid synthesis (with added nitrogen), lipids, and nucleotides
- Small amounts remain in the cycle to adjust for changing rates of carbon fixation

**Worked example:** Predict the effect of cutting off light on the concentration of RuBP in chloroplast stroma, and explain your answer.

1. Step 1: Identify what stops when light is removed
2. Light-dependent reactions stop immediately, so production of ATP and reduced NADP ceases.
3. Step 2: Trace the effect on the Calvin cycle
4. GP cannot be converted to TP, so no TP is available to regenerate RuBP.
5. Step 3: Final effect on RuBP concentration
6. Existing RuBP continues to react with residual CO₂ to form GP, but no new RuBP is produced. So RuBP concentration decreases rapidly.

> **tip**
>
> For concentration change questions: if the step that produces a molecule is blocked, its concentration falls. If the step that uses a molecule is blocked, its concentration rises.

## Common pitfalls

- **Wrong:** Claiming the Calvin cycle does not require light at all (calling it the "dark reaction")
  - Why it fails: It only does not require light directly, but depends entirely on ATP and reduced NADP from the light-dependent reactions, so it stops in prolonged darkness
  - Correct: Refer to it as light-independent, and state it relies on products from the light-dependent stage
- **Wrong:** Stating that all TP produced by the cycle is used to make glucose
  - Why it fails: 5 out of every 6 TP molecules are used to regenerate RuBP to keep the cycle running, only 1 is net output
  - Correct: Explain that most TP is used for RuBP regeneration, with 1/6 used for organic molecule synthesis
- **Wrong:** Confusing TP and GP as the first product of carbon fixation
  - Why it fails: GP is the first stable product formed immediately after CO₂ fixation, TP is produced later in the cycle
  - Correct: Memorise: CO₂ → GP → TP, so GP is the first stable product
- **Wrong:** Claiming RuBisCO only catalyses carboxylation of RuBP
  - Why it fails: RuBisCO can also bind oxygen, leading to photorespiration that reduces photosynthetic efficiency, a common exam topic
  - Correct: Recall RuBisCO has affinity for both CO₂ and oxygen, with carboxylation being the desired reaction for the Calvin cycle

## Cheatsheet

| Component | Location | Core Role |
| --- | --- | --- |
| RuBP | Stroma | 5-carbon CO₂ acceptor |
| RuBisCO | Stroma | Enzyme catalysing carbon fixation |
| GP | Stroma | First stable product of CO₂ fixation |
| TP | Stroma | Product of GP reduction; used for RuBP regeneration and sugar synthesis |
| ATP | Stroma | Provides energy and phosphate for GP reduction and RuBP regeneration |
| Reduced NADP | Stroma | Provides reducing power to convert GP to TP |

## What's next

The light-independent reactions are the core of global biological carbon fixation, turning inorganic CO₂ into the organic molecules that support almost all life on Earth. Understanding how the cycle works allows you to explain limiting factors on photosynthesis, the effect of changing environmental conditions on plant growth, and specialised adaptations like C4 photosynthesis that improve efficiency in hot, dry climates. This topic connects directly to broader concepts of plant metabolism, ecosystem energy flow, and climate change carbon cycles.

- [Light-dependent Reactions](https://www.owlsprep.com/study/cie-9700-u14-light-dependent-reactions/)
- [Limiting factors of photosynthesis](https://www.owlsprep.com/study/cie-9700-u14-limiting-factors-of-photosynthesis/)
- [Chloroplast structure](https://www.owlsprep.com/study/cie-9700-u14-chloroplast-structure/)

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