The Calvin Benson Cycle

The Calvin-Benson cycle is the second phase of photosynthesis, where plants use ATP and NADPH—the end products of the light reactions—to form sugar.

This series of reactions is subdivided into three stages.

During the carbon fixation stage, ribulose-1,5-bisphosphate carboxylase/oxygenase or RuBisCo catalyzes the addition of CO₂ to a five-carbon sugar, ribulose 1,5-bisphosphate or RuBP, therefore fixing inorganic CO₂ into an organic molecule. 

 This reaction generates an unstable six-carbon intermediate, which is further cleaved into two small three-carbon molecules called 3-phosphoglycerate or 3-PGA. 

In the reduction stage, 3-phosphoglycerate kinase adds a phosphate to the carboxyl group of 3-PGA, yielding 1,3-bisphosphoglycerate.

Then, glyceraldehyde 3-phosphate dehydrogenase transfers electrons from NADPH to 1,3-bisphosphoglycerate, producing two molecules of glyceraldehyde-3-phosphate or G3P.

One G3P leaves the Calvin-Benson cycle to form essential plant metabolites, and the other undergoes a complex set of reactions along with ATP to regenerate RuBP. 

Overall, it takes 6 CO₂ molecules in the fixation stage, 12 ATP and 12 NADPH in the reduction stage, and 6 ATP in the regeneration stage to produce one six-carbon sugar.