Question

Give an account on energetics of Calvin cycle.

Answer 1

Calvin cycle is a complete biosynthetic pathway that is taking place in the stroma of chloroplasts of the organisms conducting photosynthesis or it is also known as the ${C_3}$ cycle. It is named after its discoverer Melvin Calvin.

Complete answer:
After studies, Melvin Calvin found that the first $C{O_2}$ fixation product was a 3-carbon organic acid. Hence this is called a Calvin cycle. The first product of the process was 3-phosphoglyceric acid or PGA.
The Calvin cycle can be described through three stages-
Carboxylation: in this, there is the fixation of the $C{O_2}$ into an intermediate organic compound. Here $C{O_2}$ is utilized for the RuBP carboxylation with the help of enzyme RuBP carboxylase. 3-PGA is the product.
Reduction: In this step, the carbohydrate (glucose) is formed by the utilization of ATP and NADPH.
Regeneration: in this step, there is the formation of RuBP for the cycle to move uninterrupted. This step also requires the utilization of the ATP molecule.
Hence, in this cycle for every $C{O_2}$ molecule entering in the Calvin cycle, there is the requirement of 3 ATP molecules and 2 NADPH molecules.

RuBP- ribulose bisphosphate is the $C{O_2}$ acceptor.
ATP- adenosine triphosphate
NADPH- nicotinamide adenine dinucleotide phosphate.

Note: The plants in which ${C_3}$ acid (PGA) is the first product of $C{O_2}$2 fixation, such plants are called ${C_3}$ plants e.g. wheat, rice, etc.
Those plants in which ${C_4}$ acid (Oxaloacetic acid) is the first product of $C{O_2}$ fixation, such plants are called ${C_4}$ plants e.g. corn, sugarcane, etc.