Photosynthesis: Light-Independent Reactions (Calvin Cycle) Revision Notes | A Level AQA Biology

1. The Problem the Calvin Cycle Solves

The light-dependent reactions produce ATP and reduced NADP — but neither of those is glucose. The plant still needs to take atmospheric $CO_2$ and build it into something useful. That is exactly what the light-independent reactions, collectively called the Calvin cycle, accomplish.

The Calvin cycle runs in the stroma of the chloroplast. Unlike the light-dependent reactions, it does not require light directly — but it cannot run without the ATP and reduced NADP that the light-dependent reactions supply. Cut off the light and both stages grind to a halt.

2. Carbon Fixation: $CO_2$ Meets RuBP

Picture a five-carbon molecule — ribulose bisphosphate, or RuBP — waiting in the stroma.

When a molecule of $CO_2$ diffuses in from the air spaces, it collides with RuBP and the enzyme rubisco catalyses their combination. The result is an unstable six-carbon intermediate that immediately splits into two molecules of glycerate 3-phosphate (GP), each carrying three carbons.

Rubisco is the critical enzyme here — and it is worth knowing its full name: ribulose bisphosphate carboxylase/oxygenase. AQA examiners expect you to state that rubisco catalyses this specific reaction. GP itself is not yet a sugar; it is a carboxylic acid derivative that must be reduced before it becomes useful.

3. Reduction of GP to Triose Phosphate

GP on its own cannot be used to build sugars or regenerate RuBP. It must first be reduced — and this is where the products of the light-dependent reactions become essential. ATP provides the energy, and reduced NADP provides the hydrogen (electrons and protons) needed to convert GP into triose phosphate (TP).

In this reduction step, ATP is hydrolysed to ADP and inorganic phosphate, releasing energy. Reduced NADP is oxidised back to NADP, which returns to the light-dependent reactions to be reduced again. This recycling of NADP and ADP links the two stages of photosynthesis into one continuous system.

Triose phosphate is a three-carbon sugar phosphate containing an aldehyde group — it is the first true sugar produced in the Calvin cycle. This distinguishes it from GP, which is a 3-carbon carboxylic acid derivative and not a sugar. From here, the cycle branches: some TP is used to regenerate RuBP, and some exits the cycle to be converted into other organic molecules.

4. The Two Fates of Triose Phosphate

For every six turns of the Calvin cycle, six molecules of $CO_2$ are fixed and twelve molecules of TP are produced. Ten of those twelve TP molecules are used to regenerate six molecules of RuBP — this regeneration also requires ATP from the light-dependent reactions. Only two TP molecules exit the cycle to be used for biosynthesis.

• Regeneration of RuBP: five-sixths of TP is phosphorylated using ATP and rearranged to reform RuBP, keeping the cycle running
• Synthesis of organic compounds: the remaining TP is converted into glucose (and then sucrose or starch), glycerol, fatty acids, and amino acids (requiring nitrogen from nitrate ions absorbed by roots)

The ratio matters for exam questions: the majority of TP goes back into the cycle to sustain carbon fixation. Only a small fraction leaves to build the organic molecules the plant actually needs. Without continuous RuBP regeneration, the cycle would stop and no further $CO_2$ could be fixed.

5. The Full Cycle: Seeing It as One System

It helps to see the Calvin cycle as a loop rather than a straight line. RuBP accepts $CO_2$ to form GP; GP is reduced to TP using ATP and reduced NADP; most TP is phosphorylated using more ATP to regenerate RuBP; and the cycle continues. The two molecules of TP that leave the cycle represent the net gain of fixed carbon per two turns.

Worked example: Tracing One Turn of the Calvin Cycle

Describe what happens to one molecule of CO₂ as it passes through one turn of the Calvin cycle, identifying the molecules involved at each stage.

1. $CO_2$ (1C) + RuBP (5C) → unstable 6C intermediate — Rubisco catalyses this carboxylation reaction in the stroma. The six-carbon intermediate is too unstable to isolate.
2. Unstable 6C intermediate → 2 × GP (3C each) — Two molecules of glycerate 3-phosphate are formed. Each carries one phosphate group and a carboxyl group.
3. 2 GP + 2 ATP + 2 reduced NADP → 2 TP + 2 ADP + 2 NADP + 2 Pᵢ — ATP is hydrolysed to provide energy; reduced NADP donates hydrogen. NADP and ADP are released back to the light-dependent reactions.
4. Most TP → RuBP (requires ATP); small fraction → organic compounds — RuBP regeneration uses ATP from the light-dependent reactions. The TP that exits the cycle can form glucose, lipids, or amino acids.

Answer: CO₂ is fixed onto RuBP by rubisco to form GP; GP is reduced to TP using ATP and reduced NADP; most TP regenerates RuBP (using ATP) and a small fraction is used to synthesise organic compounds.

Notice how ATP appears twice in the cycle: once to reduce GP to TP, and again to phosphorylate TP back into RuBP. This means the light-dependent reactions must supply a continuous stream of both ATP and reduced NADP for the Calvin cycle to keep turning.

6. Required Practical 7 — Chromatography of Leaf Pigments

Photosynthetic pigments absorb different wavelengths of light, and plants vary in which pigments they carry. Chromatography separates these pigments based on their solubility in a solvent and their affinity for the stationary phase, allowing you to identify and compare pigment profiles from different leaves.

The Rf value is a ratio, so it has no units and always falls between 0 and 1. In standard chromatography solvents, carotene typically travels furthest (highest Rf), followed by xanthophylls, then chlorophyll a, and finally chlorophyll b (lowest Rf). This order reflects their polarity and solubility; carotene is the most non-polar and thus travels furthest.

7. Exam Technique and Common Mistakes

AQA questions on the Calvin cycle tend to target three areas: naming molecules correctly, explaining the role of ATP and reduced NADP precisely, and understanding what happens when light is removed. Each of these has a characteristic trap.

Six-mark questions on photosynthesis frequently ask you to link the two stages. A strong answer will name the specific products of the light-dependent reactions (ATP and reduced NADP), state precisely where they are used in the Calvin cycle (reduction of GP; regeneration of RuBP), and explain what happens to NADP and ADP after they are used — they return to the light-dependent reactions to be recycled.

The Calvin cycle is the mechanism by which inorganic carbon enters the organic world — every carbon atom in glucose, lipids, and proteins in a plant passed through this cycle. Understanding it as a continuous loop, powered by the light-dependent reactions, is the key to answering any AQA question on photosynthesis at A Level.