Calvin cycle

The Calvin cycle (also known as Calvin-Benson cycle) is a series of biochemical reactions taking place in the chloroplasts of photosynthetic organisms. It was discovered by Melvin Calvin and Andy Benson at the University of California, Berkeley. It is one of the light-independent reactions and occurs in the stroma.

During photosynthesis, light energy has been converted into chemical energy stored in ATP and NADPH. The light-independent Calvin cycle uses the energy from these short-lived energy carriers to convert carbon dioxide into organic compounds that can be used by the organism. This set of reactions is also called carbon fixation. The key enzyme of the cycle is called RubisCO.

The total sum of reactions in the Calvin cycle is the following:

6 CO₂ + 12 NADPH + 12 H₂O + 18 ATP → C₆H₁₂O₆ + 12 NADP⁺ + 18 ADP + 18 P_i

Reactions in the Calvin Cycle

Ribulose 1,5 bisphosphate (RuBP) + carbon dioxide → 3-phosphoroglycerate (catalysed by RubisCO)

3 C₅H₈P₂O₁₁ + 3 CO₂ → 6 C₃H₃P₁O₆ + H₂O

3-phosphoroglycerate + ATP → 1,3 bisphosphoroglycerate + ADP

6 C₃H₃P₁O₆ + 6 ATP → 6 C₃H₃P₂O₁₀ + 6 ADP

1,3 bisphosphoroglycerate + NADPH → 3-phosphoglyceraldehyde + NADP⁺ + P_i

6 C₃H₃P₂O₁₀ + 6 NADPH → 6 C₃H₅P₁O₆ + 6 NADP⁺ + 6 P_i

3-phosphoglyceraldehyde → ribulose 5-phosphate + P_i

5 C₃H₅P₁O₆ → 3 C₅H₈P₁O₇ + 2 P_i

ribulose 5-phosphate + ACP→ ribulose 1,5 bisphosphate + ADP

3 C₅H₈P₁O₇ + 3 ATP → 3 C₅H₈P₂O₁₁ + 3 ADP

The missing 3-phosphoglyceraldehyde molecule is converted into glucose.

The known Calvin cycle as of 1997 is as follows:

- Ribulose 5-Phosphate → (3 ATP → 3ADP) → Ribulose 1,5-bisphosphate.

- Ribulose 1,5-bisphosphate + 3 Carbon Dioxide → 3-Phosphoglycerate.

- 3-Phosphoglycerate → (6 ATP → 6 ADP) → 1,3-Bisphosphoglycerate.

- 1,3-Bisphosphoglycerate → (6 NADPH → 6 NADP⁺) → 3 GAP + 3 DHAP + 6 P_i

- GAP + DHAP → Fructose 1,6-bisphosphate.

- Fructose 1,6-bisphosphate → (H₂O → P_i) → Fructose 6-phosphate.

- GAP + Fructose 6-phosphate → Erythrose 4-phosphate + Xylulose 5-phosphate.

- DHAP + Erythrose 4-phosphate → Sedoheptulose 1,7-bisphosphate.

- Sedoheptulose 1,7-bisphosphate → (H₂O → P_i) → Sedoheptulose 7-phosphate.

- Sedoheptuose 7-phosphate + GAP → Ribose 5-phosphate + Xylulose 5-phosphate.

- Xylulose 5-phosphate → (spontaneous degradation) → Ribulose 5-phosphate.

- Ribose 5-phosphate → (spontaneous degradation) → Ribulose 5-Phosphate.