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Daniel I. Arnon 



in the chain must be at least partly oxidized. If they are kept 

 in a reduced form they cannot accept electrons from reduced fer- 

 redoxin. 



Our hypothesis proposes that molecular oxygen acts as a redox 

 "buffer" for the electron transport chain involved in cyclic 

 photophosphorylation by chloroplasts . In the presence of oxygen, 

 the electrons from water cannot overreduce the electron carriers 

 in the electron transport chain. Without this "buffering" effect 

 of oxygen, the flow of electrons from water (through photoreac- 

 tion B) overreduces the components of the electron transport 

 chain in chloroplasts and the endogenous cyclic photophosphory- 

 lation via ferredoxin cannot proceed. 



The hypothesis just presented is supported by experiments 

 with two beams of light (13), as illustrated in Fig. 10. Fig. 10 

 shows that, under anaerobic conditions, cyclic photophosphoryla- 

 tion at 708 mn is inhibited by the addition of a second monochro- 

 matic beam of light at 663 m^. This chromatic inhibition, which 

 we attribute to overreduction by the 663 mn beam, occurred imme- 

 diately when illumination by the combined 7O8 and 663 mpi beams 

 was preceded by preillumination (under Ng) at 663 m[x (bottom 

 curve. Fig. 10). Without an anaerobic preillumination treatment, 

 the inhibitory effect of the 663 mji beam, added to the 708 m^ 

 beam, was observed only after k min. Evidently an interval of 

 time was needed to bring about a sufficient state of overreduc- 

 tion by the 663 m^ beam. 



Photochemical activity of subcellular preparations of blue - 

 green algae . In blue-green algae, phycobilins and not chlorophyll 

 b constitute the "accessory" pigment system for chlorophyll a. 

 The phycobilin pigments are water soluble and can be readily 

 separated from chlorophyll a. Thus, cell-free preparations of 

 blue-green algae offer attractive possibilities for testing the 

 view that the photoproduction of oxygen depends on the accessory 

 pigment system and can be experimentally separated from photo- 

 phosphorylation and TPN reduction. 



Thomas and DeRover (58) have already reported that a loss of 

 phycocyanin is associated with a loss of oxygen evolution by 

 cell macerates of blue-green algae. Petrack and Lipmann (23) 

 found that fragments of Anabaena cells which lost phycocyanin and 

 the capacity for oxygen evolution still retained a capacity for 

 cyclic photophosphorylation. Black et al. (59) showed that cell- 

 free preparations of blue-green algae contain ferredoxin and are 

 able to photoreduce TPN with either water or ascorbate-DPIP as 



