Draw and describe Calvin cycle
How does it depend on light reaction

• The path of carbon in the dark reaction was traced by Melvin Calvin using radioactive carbon (¹⁴C). The Calvin pathway occurs in all photosynthetic plants, irrespective of whether they have C₃ or C₄ pathway.
• The primary acceptor of CO₂ was found to be a 5-carbon ketose sugar called Ribulose bisphosphate (RuBP). This was surprising as it was believed that for the formation of a 3-carbon product, CO₂ would have to be accepted by a 2-carbon compound.
• RuBP is used in a cyclic manner (regenerated) and a sugar is synthesised.
• 3 stages of Calvin cycle: Carboxylation, Reduction and Regeneration.

Carboxylation 

• Ribulose 1, 5-bisphosphate combines with CO₂, and fixes it to a stable organic intermediate (3-phosphoglycerate – 2 molecules).
• Reaction catalysed by the enzyme RuBisCO (RuBP Carboxylase-Oxygenase)
• 3 PGA is the first stable product of this cycle.

Reduction 

• Here, 2 molecules each of ATP and NADPH are required for fixing 1 molecule of CO₂.
• This stage contains a series of reactions.
• Glucose is formed as a result of this series of reactions.

Regeneration

• RuBP regenerates to enable the cycle to continue uninterrupted.
• 1 ATP molecule is required.

• Photosynthesis depends on an interaction between two sets of reactions: the light reactions and the Calvin cycle.
• Chlorophyll and the other molecules responsible for the light reactions are built into the thylakoid membranes.
• The enzymes that catalyze the Calvin cycle are located in the stroma.
• Beginning with the absorption of light by chlorophyll, the light reactions convert light energy into chemical energy in the form of ATP and NADPH.
• The ATP provides the energy, and the NADPH supplies the electrons for the Calvin cycle, which converts carbon dioxide to sugar.
• The ADP and NADP+ that result from the Calvin cycle shuttle back to the light reactions, which regenerate ATP and NADPH.