ANDRE T. J AGENDO RF AND GIORGIO FORT I 577 



are under active consideration at the present time. However, no 

 matter what compounds X and Y turn out to be, and no matter 

 whether Avater is an early reactant or not, the general characteristics 

 of equation / have important experimental consequences in our 

 reaction systems. X and Y will have to be catalysts, rather than sub- 

 strates, and bound firmly to the chloroplast structure. It seems likely 

 that at any one active center in the chloroplast there will be only one, 

 or at most a few, of either X or Y. It is obvious, then, that at the 

 active center the component XH2 must be oxidized back to X and 

 YO reduced back to Y in order for reaction 1 to occur a second time; 

 and a continuous flow of electrons away from XHo and to YO is a 

 prerequisite for a measurable reaction. 



The oxidation of XHo is indicated in equation 2, by transfer to 

 an added redox dye or pyridine nucleotides; any possible electron ac- 

 ceptor being designated as A. We must point out that equation 2, 

 as the rest, is almost certain to be more complex than shown here. 

 In particular, it seems likely that equation 2 hides an electron trans- 

 port chain prior to the reduction of the added oxidant A; and that 

 electron transport along this chain generates ATP. There is an alter- 

 native possibility that ATP is formed on the side of the photo- 

 oxidant (1) , but at the present time there does not seem to be suffi- 

 cient evidence to decide betw^een the two possible areas. 



The reduction of 70 to Y may occur by two alternative means: 

 reactions 3 or •/. In the Hill reaction YO is reduced by discharging 

 oxygen (equation 5) ; in cyclic electron transport YO is reduced by 

 the generated reductant AHo (reaction 4). Thus the Hill reaction 

 consists of the reaction sequence 1, 2, 3; cyclic electron transport of 

 I, 2, and 4. 



During the past year Vennesland and her coworkers Krogmann and 

 Nakamoto have drawn our attention forcibly to the existence of re- 

 action 5, the oxidation of the added redox compound by molecular 

 oxygen (19, 22, 23) . It is evident that when AYi^ is oxidized by 

 molecular oxygen in equation 5, it is not available to reduce 70 in 

 equation 4. Thus when free molecular oxygen is involved, YO must 

 be reduced via reaction 5. The net result is that oxygen is both con- 

 sumed (reaction 5) and evolved (reaction 3) ; and as written, the 

 gas exchanges will cancel out. The overall sequence, under these con- 

 ditions, will be reactions I, 2, 3 and 5. This sequence can be defined 

 as an oxygen exchange reaction, since it predicts that O atoms from 

 water w411 be liberated into the atmosphere, and vice versa. 



There are two major criteria for the existence of the oxygen ex- 



