The Mechanisms of Energy Coupling in Chemical Reactions

The individual steps of the multistep chemical reduction of COj with the aid of NADPHj require an energy supply. This supply is secured by participation of ATP molecules in these steps. The chloroplasts of plants contain few mitochondria. Hence, the ATP molecules are formed in plants not by oxidative phosphorylation of ADP but by a phosphorylation reaction coupled with the individual steps of the photosynthesis reaction, particularly with the steps in the transition from PSII to PSI. The mechanism of ATP synthesis evidently is similar to the electrochemical mechanism involved in their formation by oxidative phosphorylation owing to concentration gradients of the hydrogen ions between the two sides of internal chloroplast membranes, a certain membrane potential develops on account of which the ATP can be synthesized from ADP. Three molecules of ATP are involved in the reaction per molecule of COj. 

The outcome of these coupled reactions, both reversible under cellular conditions, is that the energy released on oxidation of an aldehyde to a carboxylate group is conserved by the coupled formation of ATP from ADP and Pj. The formation of ATP by phosphoryl group transfer from a substrate such as 1,3-bisphosphoglycerate is referred to as a substrate-level phosphorylation, to distinguish this mechanism from respiration-linked phosphorylation. Substrate-level phosphorylations involve soluble enzymes and chemical intermediates (1,3-bisphosphoglycerate in this case). Respiration-linked phosphorylations, on the other hand, involve membrane -bound enzymes and transmembrane gradients of protons (Chapter 19). 

This is the naive picture on which many tentative models of chemical reactions used in the past were based. The material model is reduced to the minimal reacting system (A+B in the example presented above) and supplemented by a limited number of solvent molecules (S). Such material model may be studied in detail with quantum mechanical methods if A and B are of modest size, and the number of S molecules is kept within narrow limits. Some computational problems arise when the size of reactants increases, and these problems have been, and still are, the object of active research. This model is clearly unsatisfactory. It may be supplemented by a thermal bath which enables the description of energy fluxes from the microscopic to the outer medium, and vice versa, but this coupling is not sufficient to bring the model in line with chemical intuition and experimental evidence. 

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