describe biosyntheic phase of photosynthesis.how is it linked to light reaction? ( 5 marks)

Biosynthetic phase refers to the phase of photosynthesis in which food in the form of glucose is synthesised by plants utilising the products of light reactions (ATP and NADPH). It occurs through pathway called Calvin cycle or C3 cycle in stroma of chloroplast . It has three stages:
1)  Carboxylation: CO2  is utilized for carboxylation of RuBP (Ribulose 1, 5-bisphosphate and this reaction is catalyzed by RuBP carboxylase-Oxygenase (RUBISCO). The product formed by this reaction is two molecules of 3-PGA.
2) Reduction: This stage contains a series of reactions as a result of which glucose is formed. Photochemically made ATP and NADPH are used for this step. Two molecules each of ATP and NADPH are required for fixing 1 molecule of CO2.
 3) Regeneration: CO2 acceptor ribulose-1,5-bisphosphate is formed again for continuation of cycle. One molecule ATP is required for this step.
For the formation of one molecule of glucose, six molecules of CO2 need to be fixed; hence, six cycles are required.
Two molecules of NADPH and 3 of ATP molecules are required for reduction of 1 CO2 molecule . So, to fix  six molecules of CO2 into one glucose molecule, total 12 molecules of NADPH  and 18 molecules of ATP are required. 

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