depict the stages in the biosysnthetic phase of photosynthesis with a neat diagram
Products of Light reactions are ATP, NADPH,O2 . O2 gets diffused out of chloroplast.
Biosynthetic phase of photosynthesis: where ATP and NADPH is used for synthesizing sugars.
The Calvin cycle can be described under three stages: carboxylation, reduction and regeneration.This phase do not depend on light but on the products of light reactions.Melvin Calvin used racdioactive14C in the algal photosynthesis and discovered that the first product of carbon fixation is a 3 carbon organic acid called 3 phosphoglyceric acid or PGA.
The whole biosynthetic pathway was explained by Calvin hence it is named after him .
1. Carboxylation – Carboxylation :In this step fixation of CO2 into a stable organic intermediate takes place. Carboxylation is the most crucial step of the Calvin cycle where CO2 is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA. Since this enzyme also has an oxygenation activity it would be more correct to call it RuBP carboxylase-oxygenase or RuBisCO.
2. Reduction – These are a series of reactions that lead to the formation of glucose. The steps involve utilization of 2 molecules of ATP for phosphorylation and 2 NADPH molecules for reduction per CO2 molecule fixed. The fixation of six molecules of CO2 and 6 turns of the cycle are required for the removal of one molecule of glucose from the pathway.
3. Regeneration – Regeneration of the CO2 acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. The regeneration steps require one ATP for phosphorylation to form RuBP.
Hence for every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required. It is probably to meet this difference in number of ATP and NADPH used in the dark reaction that the cyclic phosphorylation takes place.
To make one molecule of glucose 6 turns of the cycle are required.
