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Daniel I. Arnon 



that required for the photoreduction of TPN (8) . 



The aim of this article is to summarize the work in our labor- 

 atory since 1961 which extends and supports the electron flow 

 concept of photophosphorylation in chlorop lasts . The discussion 

 will include experiments which led to: (a) The identification of 

 ferredoxin as the most electronegative electron carrier isolated 

 so far from chloroplasts and the elucidation of its role in 

 photosynthetic electron transport. This followed a series of ex- 

 periments on photoreduction of methyl viologen and photoproductLon 

 of hydrogen gas by chloroplasts. (b) Correlations between pig- 

 ment function and photochemical activity of spinach chloroplasts 

 and algal chromatophores, and (c) identification of the position 

 of plastoquinone in the noncyclic electron transport chain of 

 chloroplasts . 



Some of the material given here was published in more detail 

 elsewhere (9-1^); other reports are in preparation. Extensive 

 reviews of earlier work from this and other laboratories are 

 available (6,l4). 



Position of plastoquinone in the noncyclic electron transport 

 chain. Crane (15) found that plastoquinone is localized in 

 chloroplasts and Bishop (l6) and Krogmann (1?) have shown that 

 it is required for the photoreduction of ferricyanide or 2,6- 

 dichlorophenol indophenol by isolated chloroplasts. Fig. 1 

 shows that, after extraction of plastoquinone, chloroplasts lost 

 the ability to photoreduce TPN when water (0H~) was the electron 

 donor but not when the ascorbate-DPIP couple replaced water as 

 the electron donor system (l8). 



Plastoquinone thus appears to be required in that portion of 

 the photosynthetic electron transport chain in chloroplasts 

 which is concerned with the photooxidation of water to molecular 

 oxygen and which has been identified by Losada et al. (7) as the 

 first of the two light reactions that jointly bring about the 

 transfer of electrons from water to TPN. Plastoquinone may thus 

 be the endogenous chloroplast factor which occupies the position 

 marked as "A" in the noncyclic electron flow scheme of 1961 (Fig. 

 3 in ref. 7). Similar conclusions about the position of plasto- 

 quinone in the chloroplast electron transport chain were reached 

 by Witt et al. (19). 



Photoreduction of methyl viologen . According to the electron 

 flow hypothesis, the primary electron carrier, common to the 

 cyclic and noncyclic electron flow, must be able to reduce not 



