Light-Independent Stage: Calvin Cycle

The Calvin cycle — also called the light-independent stage — is where carbon is actually fixed into organic molecules. OCR specification module 5.2.1(f)(ii) requires you to describe the three-phase cycle involving RuBP, GP and TP, the role of RuBisCO, and how the products of the light-dependent stage (ATP and reduced NADP) are used. This is the step that turns light energy into a real, countable molecule of sugar.

Key Definitions:

• Light-independent stage (Calvin cycle) — the cyclic series of reactions in the stroma in which CO₂ is reduced to carbohydrate using ATP and reduced NADP.
• RuBP (ribulose bisphosphate) — a 5-carbon sugar that accepts CO₂ (the "CO₂ acceptor").
• RuBisCO (ribulose bisphosphate carboxylase/oxygenase) — the enzyme that catalyses the combination of CO₂ and RuBP; the most abundant enzyme on Earth.
• GP (glycerate 3-phosphate, also G3P or 3-PGA) — the 3-carbon intermediate formed by carboxylation of RuBP.
• TP (triose phosphate) — the 3-carbon sugar produced when GP is reduced; the first true carbohydrate of the cycle.

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Why "Light-Independent"?

The reactions of the Calvin cycle do not directly use light — the enzymes work in darkness as long as ATP and reduced NADP are available. However, ATP and reduced NADP are made by the light-dependent reactions, so without light the cycle very quickly grinds to a halt. That is why it is called light-independent rather than "dark reactions" — it is more accurate to say it does not use photons directly.

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The Three Phases

flowchart LR
    CO2[CO2 from atmosphere] -->|Rubisco| C6[Unstable 6C intermediate]
    RuBP[RuBP - 5C] --> C6
    C6 --> GP1[2 x GP - 3C]
    GP1 -->|ATP + reduced NADP| TP1[2 x TP - 3C]
    TP1 -->|1 in 6| GLU[To glucose, starch, sucrose, etc.]
    TP1 -->|5 in 6| REG[Regeneration of RuBP]
    REG -->|ATP| RuBP

Phase 1: Carboxylation (CO₂ Fixation)

• CO₂ diffuses into the stroma from the atmosphere (via the stomata and mesophyll airspaces).
• It combines with ribulose bisphosphate (RuBP, 5C) in a reaction catalysed by RuBisCO.
• The immediate product is a very unstable 6-carbon intermediate, which splits spontaneously into two molecules of glycerate 3-phosphate (GP, 3C).

$\text{RuBP (5C)} + \text{CO}_2 \xrightarrow{\text{RuBisCO}} \text{2 GP (3C)}$RuBP (5C)+CO2​RuBisCO​2 GP (3C)

This is the actual moment of carbon fixation: an inorganic carbon atom has now become part of an organic molecule. Every organic carbon atom in your body was fixed at some point in this reaction (or in one catalysed by a closely related enzyme).

Phase 2: Reduction (GP → TP)

• The GP molecules are phosphorylated using ATP (becoming 1,3-bisphosphoglycerate) and then reduced by reduced NADP to form triose phosphate (TP, 3C).
• Both ATP and reduced NADP (both products of the light-dependent stage) are consumed here.
• The reduced NADP donates two electrons and a proton; the ATP supplies the extra phosphate energy needed for reduction.

$\text{2 GP} + \text{2 ATP} + \text{2 reduced NADP} \rightarrow \text{2 TP} + \text{2 ADP} + \text{2 Pi} + \text{2 NADP}$2 GP+2 ATP+2 reduced NADP→2 TP+2 ADP+2 Pi+2 NADP

Phase 3: Regeneration of RuBP

• For the cycle to continue, RuBP must be regenerated — otherwise the CO₂ acceptor would be exhausted.
• For every 6 TP molecules produced, 5 are used to regenerate 3 RuBP (this rearrangement uses 3 more ATP).
• The remaining 1 in 6 TP molecules is the net "product" of the cycle and is used to synthesise glucose and other biomolecules.

Net Accounting (per CO₂ fixed)

	GP	TP	RuBP
Carbons in	1 (CO₂)
Per turn	2 GP made	2 TP made	1 RuBP consumed, regenerated
ATP used	1 (reduction)	1 (regeneration)
Reduced NADP used	1

To produce one molecule of glucose (C₆H₁₂O₆), the cycle must turn 6 times, consuming:

• 6 CO₂
• 18 ATP (12 for reduction + 6 for regeneration)
• 12 reduced NADP
• Producing 12 TP, of which 2 are exported as glucose precursors and 10 regenerate RuBP.

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What Happens to Triose Phosphate?

Triose phosphate is the central metabolic hub of the chloroplast. It can be used to make: