588 LIGHT AND LIFE 



energy-rich phosphate is observed without a concomitant net produc- 

 tion of molecular oxygen or the accumulation of reduced products. 

 In "non-cyclic photophosphorylation" by chloroplasts the phosphory- 

 lation process is linked to a net reduction of added triphosphopyridine 

 nucleotide, accompanied by the simultaneous production of molecular 

 oxygen. Arnon then assumes that in bacteria "non-cyclic photophos- 

 phorylation" does not exist, i.e., that no photoxidant or photore- 

 ductant is formed, and he explains thereby the inability of photo- 

 synthetic bacteria to form molecular oxygen. In examining this 

 hypothesis, it appears premature to assume that in bacterial systems 

 light-induced phosphorylation coupled to electron flow through a 

 "non-cyclic" electron transport system cannot be demonstrated. Ex- 

 perimentally, chloroplasts have been more favorable for this type of 

 investigation because for "cyclic photophosphorylation" they require 

 a cofactor such as FMN which is not required for the "non-cyclic" 

 type in the presence of TPN or ferricyanide. The same type of ap- 

 proach has not yet been successful with bacterial chromatophores. 

 Nevertheless it is clear that a "non-cyclic" electron transport system 

 can be demonstrated in illuminated chromatophores (8, 9, 19) (Fig. 

 IB), and that phosphorylation may or may not accompany the light- 

 induced reduction of DPN (9) , just as light-induced TPN reduction 

 by chloroplasts also is not dependent on simultaneous phosphorylation 

 (II) . The reverse has not yet been demonstrated, namely, that phos- 

 phorylation in bacterial particles is coupled to pyridine nucleotide re- 

 duction, yet Smith and Baltscheffsky (16) have demonstrated that 

 there exists a coupling between phosphorylation and light-induced oxi- 

 dation of endogenous cytochrome Co in chromatophores of Rhodospiril- 

 lum rubrum. In comparing the characteristics of light-induced reac- 

 tions of chloroplasts and of chromatophores (Figs. 1, 2) there seems to 

 be no question that "cyclic photophosphorylation" must be due to a 

 flow of electrons through a "short circuit" electron transport system 

 to which one or more phosphorylation sites are coupled; "non-cyclic 

 photophosphorylation" must also be due to the flow of electrons, 

 which in this case results in the reduction of pyridine nucleotides 

 or of other suitable electron acceptors, and in the simultaneous pro- 

 duction of O. by chloroplasts, or in the simultaneous oxidation of 

 suitable electron donors by chromatopliores. It does not appear neces- 

 sary to assume that the phosphorylation sites are the same for the 

 "cyclic" and "non-cyclic" electron transport systems. However, it 

 appears difficult to envisage that the manner in which the electron 

 flow is initiated by light should be different for these two processes 

 occurring in chloroplasts. Also, there does not appear to be a good 



