VerifiedHint: The product of the light reaction is essential for the assimilation of carbohydrate, ATP, and NADPH from $CO_2$ come under the biosynthetic phase of photosynthesis. These reactions take place in the stroma of chloroplasts where all enzymes that are required are present.
Complete answer:
Melvin Calvin used radioactive 14C and discovered that the $CO_2$ fixation product was a three- carbon organic acid. Thus, the cycle is named as Calvin cycle. The first stable product identified was 3-Phosphoglyceric acid (PGA) thus also known as the $C_3$ pathway. The primary acceptor molecule during the $C_3$ cycle is a 5- carbon Ketose sugar- Ribulose Bisphosphate (RuBP). The enzyme for $CO_2$ fixation is RuBisCO (Ribulose Bisphosphate carboxylase oxygenase). There are three stages of the Calvin cycle.
Carboxylation: It is the fixation of $CO_2$ into an organic intermediate. In this $CO_2$ is utilized for the carboxylation of RuBP. This reaction is catalyzed by the enzyme RuBisCO and it results in the formation of two molecules of 3- PGA.
Reduction: This reaction leads to the formation of glucose. This step involves the utilization of two molecules of ATP for phosphorylation and two molecules of NADPH for reduction, per molecule of $CO_2$ fixed. The fixation of six molecules of $CO_2$ and six turns of the cycle is required for the removal of 1 molecule of glucose from the pathway.
Regeneration: For the cycle to continue uninterrupted, regeneration of $CO_2$ acceptor molecule is important. This step requires 1 ATP and for phosphorylation to form RuBP. The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins. In stroma, enzymatic reactions synthesize sugar, which in turn forms starch. It also has small, double stranded circular DNA molecules and ribosomes.
So, the answer is C, ‘Both A and B.’
Note: The reaction for assimilation of $CO_2$ to carbohydrate does not directly depend on the presence of light but is dependent on the products of the light reaction i.e. ATP and NADPH. - When the light becomes unavailable, this biosynthetic process continues for some time and then stops because the products of the light reaction are all used up. But if then, light is made available again. The synthesis starts again. Hence calling the biosynthetic phase as the dark reaction is a misnomer. - The Calvin cycle though is known as the $C_3$ pathway, it occurs in all photosynthetic plants whether they have a $C_3$ or $C_4$ pathway.
