BENTLEY GLASS 



885 



ticipation of oxygen in the system is wasteful of energy and is less 

 efficient than anaerobic cyclic photophosphorylation. Perhaps it 

 increases the over-all rate only when light is abundant and the con- 

 centrations of the cofactors are limiting. A scheme for this postulated 

 cyclic oxygen-catalyzed photophosphorylation is presented in Fig. 12. 

 The electron flow is comprised of two parts. In the first, the elec- 

 trons from excited chlorophyll pass to oxygen and, with added pro- 

 tons, make water. In the second, the electrons are replaced in flow 

 by donation from OH~ ions, and oxygen is evolved. There is in 

 consequence, as the isotope-labeling experiments require, an ex- 

 change between molecular oxygen and the oxygen of water. These 





cofo^^=i cof 



red. 



i 



Oz^i-HaO 



\ 



Chi l: >; t l^L^^ 



cytox.,r— Cytred. 



J 



LIGHT ~P 



ADP 



-KATP 



O^-catolyzed cylic photophosphorylation 

 Fig. 12. Scheme for oxygen-catalyzed cyclic photophosphorylation in chloroplasts. 



findings are taken to mean that oxygen, when present with FMN 

 or vitamin K, is able to compete effectively for the electrons of 

 cyclic photophosphorylation. The cycle can therefore be maintained 

 only by a release of electrons in the oxygen-evolving reaction. 



The ability of chloroplasts to carry out both the cyclic and non- 

 cyclic forms of photophosphorylation raises the question of the 

 normal relation of these two processes. In isolated chloroplasts the 

 cofactors of the cyclic process, FMN and vitamin K, are shown ex- 

 perimentally to increase strongly the formation of ATP, and to 

 abolish the accumulation of reduced pyridine nucleotide, and the 

 evolution of oxygen. But in very much smaller, less than millimi- 

 cromolar, quantities they will increase ATP formation without affect- 



