MECHANISM OF PHOTOSYNTHESIS I37 



almost complete removal of inorganic phosphate is supplied, 

 both steps may be reversed and starch is formed again. This 

 was accomplished in vitro by Cori by having barium ions 

 present, the barium phosphate being sufficiently insoluble. 

 The increase in free energy with reference to the carbo- 

 hydrate is at the expense of an irreversible reaction, the 

 precipitation of barium phosphate. In the living cell there 

 is a very efficient way of removing inorganic phosphate, 

 namely, oxidative phosphorylation. The significance of this 

 in the present case depends in part on respiration being 

 regarded as an irreversible change with reference to the 

 starch hexose phosphate system. Therefore in order to 

 reverse any reaction the energy supplied for removing a 

 product is at the expense of an irreversible reaction in 

 another system starting far from its position of equilibrium. 

 This then gives a mechanism of direct reversal of an enzyme 

 catalysed reaction but only relatively small energy steps may 

 be reversed in this way. Another example of a reverse 

 reaction is the formation of starch from glucose. Starch is 

 broken down to glucose by alpha- and beta-amylase to- 

 gether with maltase. Here the action is hydrolytic and it 

 would be difficult to conceive of a direct reversal in dilute 

 aqueous solution. The reaction may be reversed in water 

 by an indirect mechanism. This possibility depends on the 

 resistance of organic phosphate compounds to spontaneous 

 hydrolysis and on the direct transfer of the phosphate 

 group. The glucose in presence of hexokinase and adeneno- 

 sinetriphosphate (ATP) is converted into hexose phosphate. 



ATP+glucose -^ glucose-6-phosphate+ADP (3) 



Then starch may be formed by the reversal of the reactions 

 (2) and (i) as previously described. The process of oxidative 

 phosphorylation both removes the phosphate and regener- 

 ates the ATP and the energy is derived from respiration. 



As a third example of reversal of a process, the reduction 

 of phosphoglyceric acid to triose phosphate introduces 

 another principle. The potential of the system triose 

 phosphate/phosphoglyceric acid at pH 7 would be about 

 -0-48 volt. When phosphoglyceric acid reacts with ATP in 

 10 



