PHOTOSYNTHETIC ELECTRON TRANSPORT AND PHOSPHORYLATION 



IN CHLOROPLASTS 



Daniel I. Arnon 



The type of photosynthetic phosphorylation first found in 

 chloroplasts--the type now called cyclic photophosphorylation-- 

 in which the sole product of the reaction is ATP, and in which no 

 hydrogen (electron) donor or acceptor is consumed, yielded no ex- 

 perimental evidence for a light-induced electron transport coup- 

 led with phosphorylation (1,2). Direct experimental evidence, as 

 distinguished from supposition, for a coupling between photosyn- 

 thetic phosphorylation and photosynthetic (light-driven) electron 

 transport in chloroplasts came in 1957 with the finding of what 

 we now call noncyclic photophosphorylation (3) • Here the forma- 

 tion of ATP was linked with a thermodynamically "uphill" hydrogen 

 (electron) transfer from water to TPN (or ferricyanide)--a trans- 

 fer that was accompanied by a stoichiometric oxygen evolution. 



The early hypotheses linking photophosphorylation with photo- 

 synthetic electron transport centered on the photolysis of water 

 as a common primary photochemical event in cyclic and noncyclic 

 photophosphorylation (4). But since 1959 our work has been 

 guided by an "electron flow" hypothesis which limits the photo- 

 oxidation of water and the resulting evolution of oxygen to non- 

 cyclic photophosphorylation (5). The common primary photochemi- 

 cal event (coupled with ATP formation) in both cyclic and non- 

 cyclic photophosphorylation is now envisaged as an electron 

 transfer from excited chlorophyll to a primary electron acceptor 

 molecule and thence, with the aid of appropriate enzyme systems, 

 either to TPN (noncyclic) or back to chlorophyll via the cyto- 

 chrome chain (cyclic electron flow) (5,6). 



In 1961 this hypothesis was further elaborated (7) to accom- 

 modate the experimental separation of noncyclic photophosphory- 

 lation in chloroplasts into two partial reactions: (a) ATP for- 

 mation without oxygen evolution but coupled with the photoreduc- 

 tion of TPN by the ascorbate-DPIP couple, and (b) photooxidation 

 of water to molecular oxygen (7) . With the separation of these 

 two reactions there was also preliminary evidence that photopro- 

 duction of oxygen is catalyzed by a pigment system different from 



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