886 LIGHT AND LIFE 



ing oxygen evolution or TPNH2 accumulation, i.e., without inter- 

 rupting the non-cyclic phase of photophosphorylation. Moreover, 

 the larger quantities of FMN or vitamin K which suppress oxygen 

 evolution also suppress the fixation of CO2 in carbohydrates, but the 

 very minute concentrations which are sufficient to enhance the forma- 

 tion of ATP actually increase the reduction of COo to carbohydrate. 

 Presumably the cell maintains a well-regulated balance of cyclic 

 and non-cyclic electron flow. 



To summarize the distinctive features of Arnon's present theory, 

 one may say, first, that the primary photochemical event is seen as 

 an activation of chlorophyll that leads to the emission of an elec- 

 tron instead of the photolytic splitting of water; second, cytochromes 

 are regarded as sufficiently oxidized to be able to withdraw elec- 

 trons from hydroxy 1 ions, and in turn to reduce the excited chloro- 

 phyll ion [Chl]+; third, the essential products of the light phase of 

 photosynthesis are ATP and TPNHg, and the fixation of COo is a 

 reaction capable of proceeding in the dark and without chlorophyll 

 provided ATP, reduced TPN, and the non-green portion of the 

 chloroplast extract are present. Both cyclic and non-cyclic photo- 

 phosphorylation are regarded as essential since the ATP formed in 

 non-cyclic photophosphorylation alone is insufficient for the fixation 

 of CO2 in the form of carbohydrates. The cyclic system demonstrably 

 participates in COo assimilation when ATP and TPNHo are present 

 in catalytic amounts and must be constantly regenerated through 

 utilization of the energy of light. When either type of photophos- 

 phorylation is functioning alone, only phosphoglycerate is formed; 

 sugar phosphates appear as products only when both cyclic and non- 

 cyclic photophosphorylation are functioning in balance. 



Arnon suggests that these observations and postulates suggest a 

 course of biochemical evolution of photosynthesis rather different 

 from that generally held. The beginnings of photosynthesis per- 

 haps lay in the origin of a porphyrin that could absorb light and 

 use tiic energy of these quanta in anaerobic cyclic photophosphoryla 

 tion. If so, there would have been initially no evolution of oxy- 

 gen and no reduction of carbon dioxide. These reactions would not 

 have been necessary in a "primeval soup" containing an abundance 

 of preformed carbon compounds. This stage in the evolution of 

 photosynthesis is today still exemplified by the photoassimilation 

 of acetate by the obligate phototrophic anaerobe, Chromatium. 

 Formation of ATP by this mechanism woidd have had gieat ad- 

 vantages over use of the fermentation process, the alternative means 



