DANIEL I. ARNON 501 



formation by chloroplasts to distinguish it from oxidative (respira- 

 tory) phosphorylation by mitochondria and the anaerobic phosphory- 

 lations at substrate level that occur in glycolysis. In both of these 

 processes ATP formation occins at the expense of energy liberated 

 by the oxidation of a chemical substrate, whereas the only "substrate" 

 which is being consumed in photosynthetic phosphorylation is light. 



6. Photosynthetic; Phosphorylation and Molecular Oxygen 



Although there was no net consumption of molecular oxygen in 

 photosynthetic phosphorylation (as measured by manometric pressure 

 change) , the process, when first discovered, proceeded at a sustained 

 rate only in the presence of oxygen (11, Fig. 2b) . We noted at the 

 time that "the overall reaction of photosynthetic phosphorylation 

 differs from the conventional Hill reaction in that there is no net 

 O., evolution"; and that "photosynthetic phosphorylation also differs 

 from oxidative phosphorylation of the type investigated by Lehninger. 

 In oxidative phosphorylation oxygen is consumed, while in photo- 

 synthetic phosphorylation there is no evidence of utilization of at- 

 mospheric oxygen" (11). Oxygen thus acted as a catalyst in photo- 

 synthetic phosphorylation, not as a substrate, as it does in oxidative 

 phosphorylation. A decisive difference between photosynthetic and 

 oxidative phosphorylation was the inability of chloroplasts to form 

 ATP in the dark by oxidizing hydrogen donors of oxidative phos- 

 phorylation with molecular oxygen (12). 



Further investigation of photosynthetic phosphorylation by spinach 

 chloroplasts soon resulted in the identification of flavin mononu- 

 cleotide (FMN) and vitamin K as catalysts in the process (15, p. 

 6326; 16, 170). At optimal (but still catalytic) concentrations of 

 either FMN (170) or vitamin K (Fig. 3), photosynthetic phosphoryla- 

 tion became independent of external oxygen and proceeded vigorously 

 in an atmosphere of nitrogen or argon. At a much lower, "micro- 

 catalytic," concentration of the added cofactors, photosynthetic phos- 

 phorylation remained dependent on oxygen, although still showing 

 no net oxygen consumption. 



The effects of added FMN and vitamin K were thus compatible with 

 two hypotheses that were advanced earlier to account for the absence 

 of a net oxygen consumption in photosynthetic phosphorylation. 

 When no cofactors were added (11) or at low concentrations of FMN 

 and vitamin K (Fig. 3) , the dependence on external oxygen was con- 

 sistent with the hypothesis that under these conditions oxygen was 

 acting catalytically and sustaining photosynthetic phosphorylation by 



