Carbon Fixation

In corporation of CO2 into organic material is called carbon fixation. A5- Carbon compound (RUBP) combines with CO2 to form a highly unstable 6- Carbon compound. This 6-Carbon compound then breaks into two molecules of PGA or Phosphoglyceric acid (3-carbon compound).

Since the product is a three-carbon compound, so the Calvin cycle is also known as C3 pathway.

Reduction

PGA is then reduced to a 3-Carbon Carbohydrate by the addition of a phosphate group (1, 3 biphosphoglycerate) in the presence of ATP. This is further reduced into G3P (Glyceraldehydes 3-phosphate) in presence of NADPH. This is then further processed to manufacture glucose.

Regeneration of CO2 acceptor (RUBP)

Through a complex series of reaction, the carbon skeleton of five molecules of three carbon G3P are rearranged into three molecules of five carbon ribulose phosphate (RuP).

Each RuP is phosphorylated to ribulose biphosphate (RuBP), the five-carbon CO2 acceptor with which the cycle started. Again more molecule of ATP of light reactions are used for this phosphorylation of three RUP molecules.