Q.(a) Explain the steps involved in Dark reactions in a detailed manner. OR

The dark reaction (Calvin cycle) fixes CO2 through three phases - carboxylation, reduction, and regeneration - using the ATP and NADPH produced in the light reaction to eventually make glucose.

The dark reaction (also called the light-independent reaction, or the Calvin cycle/C3 cycle, worked out by Melvin Calvin) takes place in the stroma of the chloroplast and does not directly require light, though it depends on the ATP and NADPH supplied by the light reaction. It proceeds in three main steps, occurring in a repeating cycle:

1. Carboxylation: This is the fixation of atmospheric CO2 into an organic molecule. The enzyme RuBisCO (ribulose bisphosphate carboxylase-oxygenase, the most abundant enzyme on Earth) catalyses the combination of one molecule of CO2 with the 5-carbon acceptor molecule RuBP (ribulose 1,5-bisphosphate). This forms an unstable 6-carbon intermediate that immediately splits into two molecules of the 3-carbon compound 3-phosphoglyceric acid (3-PGA) - this is why the pathway is called the C3 pathway, and 3-PGA is the first stable product of the dark reaction.

2. Reduction: Each molecule of 3-PGA is then reduced to glyceraldehyde-3-phosphate (G3P), a process requiring energy input in the form of ATP (for phosphorylation, forming 1,3-bisphosphoglycerate) and reducing power in the form of NADPH (which reduces it to G3P), both of which are supplied by the light reaction.

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