Calvin cycle is divided into three distinct stages. They are:
1. Carboxylation
2. Glycolytic reversal
3. Regeneration of RuBP
Carboxylation:
Carboxylation is the fixation of carbon dioxide, RuBP which is a five carbon compound accepts the carbon dioxide. As a result a six carbon compound is produced which is unstable. The carboxylation reaction is catalyzed by the enzyme Ribulose biphosphate carboxylase (Rubisco) and it is the most abundant protein on earth. Carboxylation can be defined as the reaction in which carbon dioxide reacts with RuBP to produce an unstable six carbon compound. This six carbon compound will immediately breakdown into two molecules of 3-phosphoglyceric acid (3-PGA). This is the first stable compound that is produced during the photosynthesis, so Calvin cycle also known as C3 cycle.
Glycolytic reversal:
In this phase of Calvin cycle the two molecules of PGA is first reduced to two molecules of 1,3- biphospho glyceric acid. This reaction takes place in the presence of the enzyme PGA kinase where two molecules of ATP are used up. In the second step of this phase two molecules of 1,3-biphosphoglyceric acid are reduced to glyceraldehyde-3-phosphate. This reaction takes place in the presence of enzyme glyceraldehyde-3-phosphate dehydrogenase and two molecules of NADPH are required. So for fixing one molecules of CO2, two ATP and two NADPH which are produced during light reaction are utilized.
Regeneration of RuBP:
The glyceraldehyde phosphate is converted to RuBP by utilization of one molecule of ATP. This phase is said to be regeneration phase since the RuBP is regenerated from the glyceraldehyde phosphate by phosphorylation. Regeneration of RuBP is necessary for the cycle to continue.
For every carbon dioxide molecule that is fixing through Calvin cycle requires 3 molecules of ATP and 2 molecules of NADPH. Six turns of Calvin cycle is required to produce one molecule of glucose from six molecules of carbon dioxide. 18 ATP and 12 NADPH are required to produce one molecule of glucose.
