Light-independent reactions
Biology· Unit 14: Photosynthesis· 15 min read
1. Carbon Fixation★★☆☆☆⏱ 4 min
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.
A plant is exposed to radioactive ¹⁴CO₂ for 10 seconds. Where would you expect radioactivity to first accumulate, and why?
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Step 1: Identify the first stable product of carbon fixation
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After CO₂ reacts with RuBP, the resulting 6-carbon intermediate breaks down almost instantly into two 3-carbon GP molecules.
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Step 2: Explain the result
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Unreacted ¹⁴CO₂ is inorganic, so radioactivity will first accumulate in GP molecules before the reaction proceeds further to form TP.
2. Reduction and RuBP Regeneration★★★☆☆⏱ 5 min
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.
Explain why only 1 net molecule of TP is produced for every 3 CO₂ molecules that enter the Calvin cycle.
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Step 1: Calculate total TP produced
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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.
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Step 2: Account for RuBP regeneration
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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.
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Step 3: Find net output
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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.
3. Fate of Triose Phosphate★★☆☆☆⏱ 3 min
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
Predict the effect of cutting off light on the concentration of RuBP in chloroplast stroma, and explain your answer.
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Step 1: Identify what stops when light is removed
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Light-dependent reactions stop immediately, so production of ATP and reduced NADP ceases.
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Step 2: Trace the effect on the Calvin cycle
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GP cannot be converted to TP, so no TP is available to regenerate RuBP.
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Step 3: Final effect on RuBP concentration
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Existing RuBP continues to react with residual CO₂ to form GP, but no new RuBP is produced. So RuBP concentration decreases rapidly.
4. Common Pitfalls
Wrong move:
Claiming the Calvin cycle does not require light at all (calling it the "dark reaction")
Why:
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 move:
Refer to it as light-independent, and state it relies on products from the light-dependent stage
Wrong move:
Stating that all TP produced by the cycle is used to make glucose
Why:
5 out of every 6 TP molecules are used to regenerate RuBP to keep the cycle running, only 1 is net output
Correct move:
Explain that most TP is used for RuBP regeneration, with 1/6 used for organic molecule synthesis
Wrong move:
Confusing TP and GP as the first product of carbon fixation
Why:
GP is the first stable product formed immediately after CO₂ fixation, TP is produced later in the cycle
Correct move:
Memorise: CO₂ → GP → TP, so GP is the first stable product
Wrong move:
Claiming RuBisCO only catalyses carboxylation of RuBP
Why:
RuBisCO can also bind oxygen, leading to photorespiration that reduces photosynthetic efficiency, a common exam topic
Correct move:
Recall RuBisCO has affinity for both CO₂ and oxygen, with carboxylation being the desired reaction for the Calvin cycle
5. Quick Reference 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 |
6. Frequently Asked
Does the Calvin cycle only run in the dark?
No. It is called light-independent because it does not directly use light energy, but it relies entirely on ATP and reduced NADP from the light-dependent reactions, so it stops quickly in prolonged darkness.
What is the difference between light-dependent and light-independent reactions?
Light-dependent reactions occur on the thylakoid membrane, require light, and produce ATP and reduced NADP. Light-independent reactions occur in the chloroplast stroma, use ATP/reduced NADP to fix CO2 into organic sugars.
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 · 22
Describe steps of the Calvin cycle
- 2023 · 12
Outline roles of triose phosphate
- 2021 · 23
Effect of low CO2 on Calvin cycle
Going deeper
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.