PHOTOS YNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 355 



has returned to the ion" [96]. In cycHc photophosphorylation the electron 

 expelled from chlorophyll is visualized as being transferred [98] to the 

 first intermediary acceptor in the photosynthetic electron transport chain 

 and thus initiating the electron transfer process that makes cyclic photo- 

 phosphorylation possible [94]. 



To summarize, then, the simplest experimentally demonstrable case of 

 conversion of light energy into chemical energy, a case that is common to 

 all chlorophvU-containing particles, is cyclic photophosphorylation. We 

 visualize that in cyclic photophosphorylation electrons flow from chloro- 

 phyll that becomes excited by light, to a cofactor (Figs. 4 and 5), from the 

 cofactor to cytochromes and from cytochromes back to chlorophyll. 

 During this cyclic flow of electrons the cofactor and cytochromes present 

 in the photosvnthetic particles undergo oxidation-reductions which are 

 believed to be coupled to phosphorylation reactions that produce ATP. 



The proposed mechanism for this process may be divided into three 

 phases : (a) the primary photochemical act that results in the generation by 

 the excited chlorophvU molecule of a high energy electron and of the 

 ultimate electron acceptor [Chi "^], (b) transport of the high energy electron 

 through a photosynthetic electron transport system, and (r) phosphorylation 

 reactions coupled to electron transport. Phases (b) and (c) are analogous 

 and possibly identical in some respects with their counterparts in oxidative 

 phosphorylation, whereas phase (a) is peculiar to photosynthetic phos- 

 phorylation. 



7. Evidence for electron flow mechanism in cyclic 

 photophosphorylation 



The validitv of the proposed mechanisms for cyclic photophosphoryla- 

 tion is supported bv several lines of evidence. These include recent 

 experiments on the eflfect of chloride and ferricyanide on photosynthetic 

 phosphorylation in isolated chloroplasts and chromatophores, and experi- 

 ments on the effect of light and vitamin K on cytochromes of chloro- 

 phvllous particles. This evidence will now be discussed in more detail. 



EFFECT OF CHLORIDE 



The role of chloride in photosynthesis was discovered by Warburg [99], 

 who found that chloride, replaceable by bromide but not by other anions, 

 was essential for oxvgen evolution by isolated chloroplasts. This discovery 

 was fully confirmed by Arnon and Whatley [100], but they were dis- 

 inclined to accept, at that time, Warburg's conclusion that chloride is a 

 coenzvme of photosvnthesis, because this conclusion would have conferred 

 on chloride the then unwarranted status of an essential element for green 



