Question

Explain Calvin cycle.

Answer 1

Hint:  Calvin cycle is the path of carbon assimilation and it was given by Calvin, Benson, and Bassham. They got the Nobel Prize for this work in 1961. It is also known as $C_3$ cycle. It occurs in all photosynthetic plants.

Complete answer:

- In this cycle, the carbon dioxide acceptor molecule is RuBP and RuDP (i.e. Ribuose 1, 5- bisphosphate).

- This cycle has many steps that are known as glycolytic reversal or formation of sugar and takes place between reduction and regeneration.

It has following three major steps:

> Carboxylation- It is the most crucial step of this cycle that involves the fixation of carbon dioxide in the form of carboxylation of RuBP. It is catalyzed by RuBP carboxylase. This finally leads to the formation of two molecules of 3 phosphoglyceric acid (PGA).

> Reduction- In this, the reduction of PGA takes place through a series of reactions leading to the formation of glucose. In this step, two molecules of ATP and 2 molecules of NADPH are utilized for the phosphorylation and for the reduction of carbon dioxide respectively. 

> Regeneration- This step is important for the continuous and uninterrupted functioning of the Calvin cycle. And for this, the regular supply of ATP and NADPH and sufficient amount of RuBP are required. The regeneration of RuBP is a complex step. It involves many types of sugars starting from triose to heptose. The regeneration step requires one molecule of ATP for phosphorylation. Hence for every carbon dioxide molecule that enters the Calvin cycle, three molecules of ATP and two molecules of NADPH are required. 

Note: The site for the Calvin cycle is the stroma of the chloroplast. The cyclic phosphorylation takes place in order to meet the difference in the number of ATP and NADPH used in the dark reaction.