Explain the process of the biosynthetic phase of photosynthesis occurring in the chloroplasts.
The biosynthetic phase of photosynthesis : It occurs in the stroma of chloroplasts. These reactions reduce the carbon dioxide into carbohydrates, making use of the ATP and NADPH2 produced in the photochemical reactions. The reactions are also called as Calvin cycle. The three phases of the Calvin cycle are as follows: (i) Carboxylation Six molecules of Ribulose 1,5 bisphosphate react with six molecules of carbon dioxide to form six molecules of a short-lived 6C- compound. The reaction is catalysed by RuBP carboxylase (RuBisCo). The six molecules of the 6C-intermediate break into 12 molecules of 3- phosphoglyceric acid (3-PGA), an SC- compound. It is through this step that carbon dioxide is fixed in the plant. (ii) Reduction 12 molecules of 3-phosphoglyceric acid are converted into 12 molecules of 1, 3 diphosphate-glyceric acid, utilising 12 molecules of ATP and then reduced to 3- phosphoglyceraldehyde making use of 12 molecules of NADPH. Two molecules of phosphoglyceraldehyde react to form one molecule of glucose. It is in this step that there is an actual reduction of carbon dioxide leading to sugar formation. (iii) Regeneration of RuBP 10 molecules of phosphoglyceraldehyde, by a series of complex enzyme-catalyzed reactions, are converted into six molecules of ribulose 1,5-bisphosphate; six molecules of ATP are needed for this step. This step of ‘ regeneration of RuBP is important for the cycle to continue
