Calvin cycle 3-phosphoglycerate formation

Most of the enzymes mediating the reactions of the Calvin cycle also participate in either glycolysis (Chapter 19) or the pentose phosphate pathway (Chapter 23). The aim of the Calvin scheme is to account for hexose formation from 3-phosphoglycerate. In the course of this metabolic sequence, the NADPH and ATP produced in the light reactions are consumed, as indicated earlier in Equation (22.3). 

As stated earlier, the rate-limiting step in the Calvin cycle is the carboxylation of ribulose 1,5-bisphosphate to form two molecules of 3-phosphoglycerate. The activity of ruhisco increases markedly on illumination because light facilitates the carbamate formation necessary to enzyme activity, Tn the stroma, the pH increases from 7 to 8, and the level of Mg rises. Both effects are consequences of the light-driven pumping of protons into the thyiakoid space,... 

The difference between this route and the Calvin cycle lies in the fact that the CO2 is fixed, not into ribulose-l,5-diphosphate, but into phosphoenol pyruvate with the intermediary formation of C4 dicarboxylic acids. Hence the name, the Q dicarboxylic acid pathway. One of the C4 dicarboxylic acids transmits the CO2 further with the formation, ultimately, of 3-phosphoglyceric acid. 

By contrast, the Calvin cycle in photosynthesis illustrates a positive feed-back mechanism. The concentration of Calvin cycle intermediates persisting in photosynthetic cells in the dark is very low. When photosynthesis commences the availability of ribulose diphosphate (RuDP) may limit the rate of COj assimilation. However, the formation of phosphoglyceric acid (PGA) immediately raises the level of other intermediates in the cycle, including that of RuDP, and this in turn speeds up the rate of CO assimilation. Similarly, when cells... 

Calvin and his school believe that ribulose diphosphate is the immediate precursor of 3-phosphoglyceric add. Thus the photosynthetic cycle would begin by a carboxylation of Ru-PP with formation Cj + C = 2Ca) of two molecules of phosphoglyceric acid (PGA). Starting with PGA, all... 

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