890 LIGHT AND LIFE 



electron transport {1,2,4) from an oxygen exchange reaction (1,2,3,5) 

 and in identifying the nature of electron flow in other cases. For 

 instance, a small amount of photophosphorylation takes place in 

 chloroplasts in the absence of a7iy soluble redox system (FMN, vitamin 

 K, PMS, etc.) This "endogenous" phosphorylation is probably not 

 attributable to cyclic electron How, for it is dependent on oxygen 

 and is inhibited by CMU. It may be regarded as a "Mehler reaction," 

 a replacement of reaction 5 in the foregoing sequence by the following 

 reaction: 



AH. + O. -» HoOo + A {5 a) 



The full reaction sequence, involving oxygen uptake as well as 

 photophosphorylation, is markedly enhanced in activity by addition 

 of the enzyme photosynthetic pyridine nucleotide reductase (PPNR) , 

 so that this enzyme must function in the reduction of oxygen as well 

 as that of TPN and cytochromes. Minute quantities of ascorbate, 

 in this enzyme system, have the effect of stimulating phosphorylation 

 without enhancing the oxygen uptake. Ascorbate therefore seems 

 to switch the reaction system back from the Mehler reaction {1,2,3,5a) 

 to the alternative oxygen exchange {1,2,3,5) . 



Jagendorf and Forti cite a number of other inhibitors or inhibitory 

 conditions that appear to act on reaction 3. These include o-phenan- 

 throline, the chloride deficiency described by Arnon in this Sym- 

 posium, and the inhibition resulting from storing bean leaves for 

 24 hours at 0°C in the dark (unpublished experiments of Margulies 

 at Johns Hopkins University) . In the same category may be placed 

 the experiments of Barbara Petrack, which show that particles from 

 blue-green algae containing chlorophyll a but lacking phycobilin pig- 

 ments can carry on cyclic electron flow reactions with PMS as cofactor, 

 but not a Hill reaction. From this observation it seems possible that 

 oxygen evolution (reaction 3) may depend specifically upon the ab- 

 sorption of light by accessory pigments. The experiments of Kok 

 and Hoch with light of wavelength 700 m^u, or longer likewise show 

 that the PMS-catalyzed cyclic electron flow, but not the Hill reaction, 

 can follow from the absorption of light by chlorophyll a alone. The 

 function of the accessory pigments again seems to be concerned with 

 reaction 3 in the over-all photosynthetic system. 



A theoretical question of importance to the interpretation of the 

 over-all nature of photosynthesis, as Warburg has emphasized, is the 

 question of whether carbon dioxide is required for the evolution of 

 oxygen in the Hill reaction (equation 3). Vennesland reported work 



