is the most crucial step of the Calvin cycle where CO 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 -PGA. Since this enzyme also has an oxygenation activity it would be more correct to call it RuBP carboxylase-oxygenase or RuBisCO . .
Reduction – These are a series of reactions that lead to the formation of glucose. The steps involve utilisation of molecules of ATP for phosphorylation and two of NADPH for reduction per CO molecule fixed. The fixation of six molecules of CO and turns of the cycle are required for the formation of one molecule of glucose from the pathway. .
Regeneration – Regeneration of the CO acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. The regeneration steps require one ATP for phosphorylation to form RuBP. Ribulose- , - bisphosphate Atmosphere C0 + H O Carboxylation ADP Regeneration -phosphoglycerate Triose phosphate Reduction ATP NADPH ADP P i +NADP + Sucrose, starch ATP Figure . The Calvin cycle proceeds in three stages : ( ) carboxylation, during which CO combines with ribulose- , -bisphosphate; ( ) reduction, during which carbohydrate is formed at the expense of the photochemically made ATP and NADPH; and ( ) regeneration during which the CO acceptor ribulose- , -bisphosphate is formed again so that the cycle continues Hence for every CO molecule entering the Calvin cycle, molecules of ATP and 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 turns of the cycle are required. Work out how many ATP and NADPH molecules will be required to make one molecule of glucose through the Calvin pathway. It might help you to understand all of this if we look at what goes in and what comes out of the Calvin cycle.
In Out Six CO One glucose ATP