DANIEL I. A UN ON 513 



days, but ai only sligluly higher temperatures the color disappears. 

 Then presumably the electron has returned to the ion" (91) . In 

 cyclic pliotoj)hosphorylation the electron expelled from chlorophyll 

 is visualized as reaching the first intermediary acceptor in the electron 

 transport chain either by resonance transfer or by a semi-conductor 

 mechanism (I22a), and thus initiating the electron transfer process 

 that makes cyclic photophosphorylation possible (9) . 



To summarize, then, the simplest experimentally demonstrable 

 case of conversion of light energy into chemical energy, a case that 

 is common to all chlorophyll-containing particles, is cyclic phos- 

 phorylation. In cyclic photophosphorylation electrons flow from 

 chlorophyll 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 cytochromes present in the photosynthetic 

 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 light-induced generation of a high-energy elec- 

 tron and the ultimate electron acceptor [Chl]+; (b) electron trans- 

 port; and (c) phosphorylation reactions coupled to electron trans- 

 port. 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 phosphorylation. 



9. Evidence eor Electron Flow Mechanism in Cyclic 

 Photophosphorylation 



The validity of the proposed mechanisms for cyclic photophosphory- 

 lation is supported by several lines of evidence. These include recent 

 experiments on the effect of chloride and ferricyanide on photo- 

 synthetic phosphorylation in isolated chloroplasts and chromatophores, 

 and experiments on the effect of light and vitamin K on cytochromes 

 of chlorophyllous particles. This evidence will now be discussed in 

 more detail. 



Effect of Chloride 



The role of chloride in photosynthesis was discovered by Warburg 

 (164) , who found that chloride, replaceable by bromide but not 

 by other anions, was essential for oxygen evolution by isolated chloro- 

 plasts. This discovery was fully confirmed by Arnon and Whatley 

 (14), but they were disinclined to accept, at that time, Warburg's 

 conclusion that chloride is a coenzyme of photosynthesis, because this 



