PHOTOSYNTHESIS BY ISOLATED CHLOROPLASTS 299 



tion proceeds in several successive steps which together constitute an 

 "electron ladder" analogous to that discussed for respiration by Lip- 

 mann (6). The generation of ATP during photosjnithesis by a phos- 

 phorylation linked to the recombination of the products of photode- 

 composition of water was first postulated on theoretical grounds by 

 Ruben and has since occupied a position of varying prominence in 

 several schemes of photosynthesis (see review, 7). 



Discussion 



Strehler: Apropos of j'our last sentence, there is some evidence in studies on 

 whole cells which is consistent in the main with the conclusions that you have 

 made. When we measured ATP formation in illuminated Chlorella, one of the 

 things that concerned us most was whether this was merely an oxidative phos- 

 phorylation through the mitochondria, or whether it was directly tied to the 

 photochemical process. We came to the conclusion that at least a goodly portion 

 of the ATP that was formed as the result of illumination was not due to respira- 

 tory activity. The conclusion was based upon temperature studies in which we 

 showed (1) ii oxygen was admitted to an anaerobic preparation of Chlorella and 

 the rate of ATP formation was measured at two temperatures, 25°C. and 4°C., 

 there was a large temperature dependence of the reaction. (2) On the other hand, 

 if one measured the light-induced ATP formation in the same preparation, one 

 observed a much lower dependence on temperature. 



Moreover, as the rate of light-induced phosphorylation at 25° C. was consider- 

 ably lower than that of the oxygen-induced ATP formation, I think it would be 

 wise to be cautious about the assumption that the only site of important ATP 

 formation for photosynthetic purposes is in the chloroplasts, inasmuch as the rate 

 induced by oxygen at 25°C. is about three or four times that produced by light. 



Amon : As for the last part of your comment, we are inclined to the conclusion 

 that photosynthetic phosphorylation is the dominant kind of phosphorylation in 

 photosynthesis. We base this conclusion on the point that photosynthesis in 

 saturating light is anywhere from 20 to 30 times as high as respiration, and so we 

 do not think that respiration could keep pace with the demands for ATP, and 

 that is the reason why we believe that the cell has a need for an aiLxiliary abundant 

 supply of ATP w^hich is independent of the respiratory activities of the cell. 



Frenkel: In comparing the characteristics of oxidative phosphorylation with 

 those of photosynthetic phosphorylation, are data for oxidative phosphorylation 

 taken from observations on chloroplast fragments or on animal mitochrondria? 



Amon : Our antimycin data for chloroplasts are compared with those by Lehn- 

 inger, Chance, and others, on the effect of antimycin on animal mitochondria. All 

 the other information, insofar as it pertains to photosynthetic phosphorylation, 

 is our own. Insofar as it pertains to oxidative phosphorylation, it is taken from 

 the literature with the one exception on the comparison of oxidative and photo- 

 synthetic phosphorylation between mitochondria and chloroplasts from the same 

 leaves. 



