The Calvin cycle for A-level Biology

The Calvin cycle is the light-independent reaction of photosynthesis. In the stroma of the chloroplast, carbon dioxide is fixed onto a five-carbon sugar, RuBP, by the enzyme rubisco. The product, GP, is reduced to triose phosphate (TP), powered by ATP and reduced NADP made in the light-dependent stage. Most of the TP regenerates RuBP so the cycle keeps turning; the rest is made into glucose and other organic substances.

Where the Calvin cycle fits in photosynthesis

Photosynthesis has two linked stages. The light-dependent reaction, on the thylakoid membranes, uses light energy to make ATP and reduced NADP and splits water, releasing oxygen. The light-independent reaction, the Calvin cycle, takes place in the stroma and uses that ATP and reduced NADP to turn carbon dioxide into sugar. AQA puts it this way: reduced NADP is used to form a simple sugar, and hydrolysis of ATP provides the additional energy.

"Light-independent" means the cycle doesn't use light directly. It still stops soon after the light goes off, because it runs out of ATP and reduced NADP. For the whole process at GCSE level, see photosynthesis, and for the stroma and thylakoids, chloroplast structure.

The Calvin cycle diagram

The 3 stages of the Calvin cycle

1. Carbon fixation

Carbon dioxide reacts with ribulose bisphosphate (RuBP), a five-carbon compound. The reaction is catalysed by the enzyme rubisco. Each CO₂ fixed gives two molecules of glycerate 3-phosphate (GP), each with three carbons. One carbon from CO₂ plus five from RuBP gives six, shared between two GP.

2. Reduction

GP is reduced to triose phosphate (TP), using the ATP and reduced NADP that the light-dependent stage made. Reduced NADP supplies the hydrogen (it is oxidised back to NADP), and hydrolysis of ATP supplies the energy. Both NADP and ADP go back to the thylakoids to be reloaded.

3. Regeneration of RuBP

Most of the TP is used to make RuBP again, which needs more ATP. This is what makes it a cycle: without fresh RuBP, no more carbon dioxide could be fixed. The rest of the TP leaves the cycle and is converted into useful organic substances.

Inputs and outputs of the Calvin cycle

OpenStax and some other books use different names for the same molecules: 3-PGA (3-phosphoglycerate) for GP, G3P (glyceraldehyde 3-phosphate) for TP, and NADPH for reduced NADP. Use whichever your exam board uses.

What triose phosphate is used for

AQA says some of the triose phosphate is converted to useful organic substances. OpenStax describes the exported TP being made into glucose: two three-carbon TP give one six-carbon hexose. From glucose the plant makes sucrose to carry in the phloem, starch to store and cellulose for its cell walls.

Worked example: counting carbons, ATP and reduced NADP

How many CO₂ molecules, ATP and reduced NADP does the Calvin cycle need to make one glucose? OpenStax describes the cycle in sets of three turns: each turn fixes one CO₂.

1. Three turns fix 3 CO₂ onto 3 RuBP: 3 + 3 × 5 = 18 carbon atoms, in 6 GP.
2. Reducing the 6 GP to 6 TP uses 6 ATP and 6 reduced NADP.
3. Five TP (15 carbons) regenerate the 3 RuBP (15 carbons), using 3 more ATP.
4. So one TP leaves for every three turns, at a cost of 6 + 3 = 9 ATP and 6 reduced NADP.
5. Glucose has six carbons, so it needs two TP: 6 CO₂, 18 ATP and 12 reduced NADP, from six turns of the cycle.

Limiting factors and the Calvin cycle

AQA asks you to identify environmental factors that limit the rate of photosynthesis. Each one hits the Calvin cycle in a different place, and you can predict what happens to the amounts of GP, TP and RuBP:

Work the light row through. With little ATP and reduced NADP, GP can't be turned into TP, so GP builds up. Less TP means less RuBP is made, but rubisco carries on using what RuBP is left, so RuBP falls. The CO₂ row is the mirror image. Temperature matters too, because rubisco and the other enzymes of the cycle work more slowly when it is cold.

Common mistakes

• Saying CO₂ reacts with GP. CO₂ joins RuBP; GP is what forms.
• Writing one GP per CO₂. Each CO₂ fixed gives two molecules of GP.
• Saying ATP is made in the Calvin cycle. It is used there, in reduction and regeneration, and made in the light-dependent reaction.
• Saying the cycle happens in the dark. It doesn't need light directly, but in the dark it soon stops for lack of ATP and reduced NADP.
• Putting it on the thylakoids. The Calvin cycle is in the stroma.
• Saying glucose is made in the cycle. TP is the product; glucose is made from TP that leaves the cycle.

Photosynthesis is A-level only in AQA's specification, so practise it on today's A-level Biology: Year 2 short timed paper. Respiration runs alongside it in the same topic: see cellular respiration.

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