The light-independent reactions (Calvin cycle)

The Calvin cycle

1. Carbon fixation: CO2 combines with ribulose bisphosphate (RuBP), a 5-carbon compound. The enzyme RuBisCO catalyses this. The unstable 6-carbon product splits at once into two molecules of glycerate 3-phosphate (GP), a 3-carbon compound.

2. Reduction: GP is reduced to glyceraldehyde 3-phosphate (GALP), using hydrogen from reduced NADP and energy from ATP.

3. Regeneration: most GALP (5 of every 6 molecules) is used to regenerate RuBP, which needs more ATP. The rest is used to make other molecules.

Counting molecules

Each turn of the cycle fixes one CO2. Glucose has 6 carbons, so 6 turns are needed per glucose.

Per glucose the cycle uses 18 ATP (12 for reduction, 6 for regeneration) and 12 reduced NADP: 3 ATP and 2 reduced NADP for each CO2 fixed.

What GALP is used for

GALP is used to make glucose, which can be converted to sucrose, starch and cellulose. GALP can also make glycerol and, with fatty acids, lipids; and with nitrates, amino acids.

If light is removed, ATP and reduced NADP run out: GP builds up and RuBP falls. If CO2 is removed, RuBP builds up and GP falls.

Worked example

How many ATP and reduced NADP molecules does the Calvin cycle use to make 2 glucose molecules?

1. One glucose needs 18 ATP and 12 reduced NADP.
2. 2 x 18 = 36 ATP; 2 x 12 = 24 reduced NADP

Answer: 36 ATP and 24 reduced NADP.

Check yourself

1. Which enzyme catalyses the fixation of carbon dioxide in the Calvin cycle?
   • ATP synthase
   • DNA ligase
   • Amylase
   • RuBisCO
   Show answer

   RuBisCO. RuBisCO combines CO2 with RuBP.

2. What is formed when CO2 combines with RuBP?
   • One molecule of GALP
   • An unstable 6-carbon compound that splits into two GP
   • Glucose directly
   • Reduced NADP
   Show answer

   An unstable 6-carbon compound that splits into two GP. The 6C intermediate splits at once into two 3C GP molecules.

3. How many turns of the Calvin cycle are needed to make 3 molecules of glucose?
   Show answer

   18. 6 turns per glucose; 3 x 6 = 18.

4. How many molecules of GP are made when 6 molecules of CO2 are fixed?
   Show answer

   12. Each CO2 fixed gives two GP: 12.

5. How many ATP molecules does the Calvin cycle use to fix 9 molecules of CO2?
   Show answer

   27. 3 ATP per CO2 fixed: 9 x 3 = 27.

6. A plant is moved from light into darkness. What happens to the levels of GP and RuBP?
   • GP rises and RuBP falls
   • GP falls and RuBP rises
   • GP and RuBP stay the same
   • GP falls and RuBP falls
   Show answer

   GP rises and RuBP falls. Without ATP and reduced NADP, GP is not reduced and RuBP is not regenerated, but CO2 fixation continues for a while.

7. Which molecule can GALP be used to make, together with nitrate ions?
   • Starch
   • Cellulose
   • Amino acids
   • Glycerol
   Show answer

   Amino acids. Nitrate supplies the nitrogen needed for amino groups.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)
• OpenStax Concepts of Biology: The Calvin Cycle

Last updated 7 November 2026. Spotted a mistake? Tell us.