After the substances needed are made by the light reaction, the next thing to be done is the Calvin cycle, which is also known as the dark reaction. This process does not need direct light from the sun in order to work, but needs the products, the energy that came from the sun (solar energy) that was changed into chemical energy which the plant may use by the earlier reaction.

             There are three main phases for the Calvin cycle. They are the following:

1. Carbon fixation -->  named for the compound carbon dioxide (CO2), the cycle takes each carbon dioxide molecule and attaches it to a five carbon sugar (abbr. 5-C sugar), and therefore  producing a six carbon compound (6-C) that is so highly unstable, it automatically  splits into 2 molecules of another substance, called  3-phosphoglycerate.

2. Reduction --> here, the product made by the carbon fixation, the 2 molecules of 3-phosphoglycerate (we'll call it 3-phos. for our convenience), will each receive an additional phosphate group from an ATP molecule, and thus become 1,3 bisphosphoglycerate as a product. next, a pair of electrons from donated from NADPH reduce 1,3 bisphosphoglycerate to glyceraldehyde 3-phosphate (G3P), which stores more potential energy. G3P is a sugar. the cycle began  with 15 carbons' worth of carbohydrate in the form of 3 molecules of the 5-C sugar RuBP. One molecule exits the cycle to be used by the plant cell, but the other 5 molecules must be  recycled to regenerate the 3 molecules of RuBP.
 

3. Regeneration  of CO2 acceptor (RuBP) --> in this last step of the Calvin cycle, the carbon skeletons of 5 molecules of G3P are rearranged into 3 molecules of ATP. In order to accomplish it, the cycle spends 3 more molecules of  ATP. The  RuBP is now ready to receive CO2 again, and the cycle continues.
 
 
 
 
 

GO BACK