Question

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Answer 1

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The process by which green plants converts carbon dioxide into complex organic compounds like carbohydrates is known as carbon fixation. It occurs through C3 and C4 cycle.

Melvin Calvin discovered Calvin cycle or C3 cycle while doing the radioactive study of alga. It occurs in all photosynthetic plants whether they have C3 or C4 pathway.

Calvin cycle can be described in three phases:

Carboxylation:

It is the fixation of CO2 into a stable organic intermediate. In this, CO2 is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBisCO and it results in the formation of two molecules of 3-PGA (3-Phosphoglyceric acid).

Reduction:

These reactions lead to the formation of glucose. The steps involve utilisation of two molecules of ATP for phosphorylation and two of NADPH for reduction, per molecule of CO2 fixed. The fixation of six molecules of CO2 and six turns of the cycle are required for the removal of one molecule of glucose from the pathway.

Regeneration:

For the cycle to continue uninterrupted, regeneration of the CO2 acceptor molecule is crucial. This step requires one ATP for phosphorylation to form RuBP. To make one molecule of glucose, six turns of the cycle are required. 18 ATP and 12 NADPH molecules are used to make a molecule of glucose. Hence, for every CO2 molecule entering the Calvin cycle, three molecules of ATP and two molecules of NADPH are required.