Question

Explain Calvin cycle.

Accepted Answer

The reaction pathway that occurred during the dark phase of photosynthesis in most of the plants in which 3C phosphoglyceric acid is the first stable product is called C₃ cycle. The plants in which this cycle takes place are called C₃ plants. In C₃ plants, the primary carbon dioxide acceptor is a 5C compound, RuBP that undergo carboxylation to form two molecules of 3C phosphoglyceric acid. It was first investigated by Melvin Calvin in 1957, hence, it is called Calvin cycle.

It is divided into three parts:

i) Carboxylation: It is the first step of C₃ cycle. The primary carbon dioxide acceptor is a 5C compound, RuBP that fix carbon dioxide to form two molecules of 3C phosphoglyceric acid (PGA). It takes place in the presence of an enzyme, RuBisCO. Six CO₂ molecules are required for the production of one molecule of glucose. Hence, 12 molecules of PGA are formed.

Reduction: The next step is the phosphoglyceric acid is combined with one ATP molecule to form 1, 3 - biphosphoglycerate. This reaction is catalysed by the triose phosphate dehydrogenase enzyme. 1.3-bisphosphoglyceric acid accepts H⁺ ions and electrons from NADPH₂ and reduced to form phosphoglyceraldehyde (glyceraldehyde 3-phosphate). Out of 12 molecules of 3-Phosphoglyceraldehyde, only two molecules form one glucose molecule.

Regeneration of RuBP: Out of 12 molecules of 3-Phosphoglyceraldehyde, 10 molecules form 5 molecules of Ribulose 5-phosphate (RuMP) by different biochemical reactions. It undergoes phosphorylated to form RuBP.

Calvin Cycle

Explain Calvin cycle.

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