Explain the process of the biosynthetic phase of photosynthesis occurring in the chloroplast.
Biosynthetic phase (Dark Reaction): It is the carbon fixation in plants by which carbon - dioxide is converted to carbohydrates during the biosynthetic phase (Dark Reaction). Carbon is fixed in the stroma of chloroplasts via a series of enzyme-catalyzed processes. C3 pathway: It is referred to as the Calvin cycle. Melvin Calvin traced the pathway of carbon in the dark reaction using autoradiography and 14C and therefore, this pathway is known as the Calvin cycle. Calvin Cycle Consist of Three Phases: (i) Carboxylation: Six molecules of Ribulose 1, 5 bisphosphates combine with six molecules of carbon dioxide to produce six molecules of carbon dioxide and six molecules of a short-lived 6C – compound. The process is catalyzed by RuBP – carboxylase (Rubisco). Six molecules of the 6C – compound combines to form 12 molecules of 3-phosphoglyceric acid (PGA), the first stable chemical in this pathway. (ii) Reduction: Using ATP and NADPH molecules, 12 molecules of phosphoglyceric acid are transformed into 12 molecules of 1,3 diphosphoglycerate, which is subsequently reduced to phosphoryl acetaldehyde (PGAL). Two molecules of PGAL are diverted for sugar production and then into starch. (iii) Regeneration of RuBP – The principal acceptor of carbon dioxide, i.e., RuBP, must be regenerated by converting 10 molecules of PGAL into 6 molecules of 5C – chemical, RuBP, via a series of complicated reactions. Six ATP molecules are required for the formation of six RuBP molecules.
