Name: Light and the Dark Reactions of Photosynthesis
Uploaded: 2019-07-19
Duration: 4 min 38 s
Description: A big plan is to show all the reactions involved in photosynthesis, the light reactions as well as the dark reactions leading to the synthesis of carbohydrates.

Question

Where does Calvin cycle take place in chloroplasts? Describe the three phases of Calvin cycle.

Accepted Answer

Calvin cycle is the light-independent phase of photosynthesis in which CO₂ is reduced to glucose. It occurs in the stroma of the chloroplast. The path of conversion of CO₂ into carbohydrates was discovered by Calvin and his co-workers. Thus, this pathway is called the Calvin cycle. The first stable product formed in this cycle is a three-carbon phosphoglyceric acid. Hence, it is also called the C₃ cycle.

The cycle involves the following three main steps:
i) Carboxylation ii) Reduction and iii) Regeneration

i) Carboxylation: In the presence of RuBisCO, Ribulose 1,5-bisphosphate reacts with water and carbon dioxide to form 3-phosphoglyceric acid (PGA). It is a three-carbon compound and the first stable product of the Calvin cycle.

RuBP + CO₂ $\to_{M g + +}^{R u B P C a r b o x y l a s e}$ 3PGA

ii) Reduction or glycolytic reversal: The Phosphoglyceric acid molecules undergo reduction through a series of reaction leading to the formation of glucose. During this step ATP and NADPH₂ are utilised. The Phosphoglyceric acid molecules reduced using NADPH₂ to produce phosphoglyceraldehyde (PGAL).
For the synthesis of one glucose molecule, six turns of the Calvin cycle are required. 1/6 part of PGAL i.e. out of 12 molecules, 2 are used for the synthesis of glucose.

iii) Regeneration: RuBP gets regenerated through several biochemical reactions. 5/6 part of PGAL i.e., out of 12 molecules, the remaining 10 molecules are used for regeneration of 6 molecules of ribulose monophosphate (RuMP) that phosphorylated to form RuBP.

Where does Calvin cycle take place in chloroplasts? Describe the three phases of Calvin cycle.

Important Questions on Photosynthesis in Higher Plants

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