Carbon reduction cycle stoichiometry

FIGURE 20-4 The three stages of C02 assimilation in photosynthetic organisms. Stoichiometries of three key intermediates (numbers in parentheses) reveal the fate of carbon atoms entering and leaving the cycle. As shown here, three C02 are fixed for the net synthesis of one molecule of glyceraldehyde 3-phosphate. This cycle is the photosynthetic carbon reduction cycle, or the Calvin cycle. 

According to this reaction scheme the absorption of four pairs of quanta (8 in all) results in the transfer of four electrons from water to NADP, the concomitant synthesis of two molecules of ATP and the evolution of one molecule of oxygen. Qualitatively then the requirements of the carbon reduction cycle are fulfilled. However, only one molecule of ATP is produced per NADP reduced whereas the carbon reduction cycle in its present form requires 3 molecules of ATP per 2 molecules of NADP reduced. Non-cyclic photophosphorylation, as the above mechanism of ATP synthesis is termed, cannot quantitatively satisfy the needs of the carbon reduction cycle. However, by the addition of suitable co-factors isolated chloroplast preparations can be induced to synthesise ATP without the transfer of electrons from water to NADP. The electron path is short-circuited and the electron ejected from chlorophyll P,oo in system I returns eventually to chlorophyll Ptoo- The return route, shown by a dotted line in Fig. 5.10, involves the synthesis of ATP. In this process, usually termed cyclic photophosphorylation, the only measurable product is ATP, oxygen is not evolved and NADP is not reduced. Cyclic photophosphorylation may balance the ATP-NADP stoichiometry for the operation of the Calvin cycle and could also supply ATP for other purposes such as the synthesis of polysaccharides. 

The resultant NADPH and ATP provide the reducing power and free energy to drive the reduction of carbon dioxide (the dark reactions) via the pentose phosphate pathway (or Calvin cycle l7>) and lead ultimately to the synthesis of glucose according to the overall stoichiometry below. 

The acids of the rTCA cycle are low-energy forms at their respective hydrogen reduction stoichiometries. For instance, formaldehyde has higher free energy of formation per carbon than acetate, which has the same chemical composition as two formaldehyde molecules. Thus, in any relaxation pathway for full reduction of ( Ot. likely intermediate states are the rTCA compounds. 

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