The Calvin Cycle

The Calvin cycle is the series of enzyme-controlled reactions in the stroma that fixes carbon dioxide into carbohydrate.1. Carbon fixation: CO₂ combines with ribulose bisphosphate (RuBP), a 5-carbon compound, catalysed by RUBISCO. The unstable 6-carbon product immediately splits into two molecules of glycerate 3-phosphate (GP), a 3-carbon compound.
2. Reduction: GP is reduced to glyceraldehyde 3-phosphate (GALP), a 3-carbon sugar phosphate (triose phosphate), using hydrogen from reduced NADP and energy from ATP. The NADP and ADP return to the thylakoids to be reused.
3. Regeneration of RuBP: five out of every six GALP molecules are rearranged, using more ATP, to regenerate RuBP. The sixth GALP leaves the cycle and is used to make other organic molecules.Worked example: one turn fixes one CO₂. For three turns:
• 3 CO₂ + 3 RuBP → 6 GP
• 6 GP → 6 GALP, using 6 ATP and 6 reduced NADP
• 5 GALP → 3 RuBP, using 3 ATP
• net: 1 GALP made, using 9 ATP and 6 reduced NADPA glucose molecule contains 6 carbons, so it needs six turns of the cycle (two GALP): 18 ATP and 12 reduced NADP.Calvin worked out the pathway by supplying the alga Chlorella with radioactive ¹⁴CO₂ for different short times and identifying which compounds became labelled. After only a few seconds, GP was the first labelled compound.