DANIEL I. ARNON bbl 



anaerobic environment, with a much more efficient niechanisni than 

 fermentation lor the formation of ATP, which is the universal cellu- 

 hir "energy currency" needed in the transformation of existing car- 

 bon compomuls into fats, carbohychates, proteins, etc. CycHc photo- 

 jjhosplioryhition gave the anaerobic photosynthetic cell a mechanism 

 which in efficiency of ATP formation is comj)arable with the process 

 of oxidative pliosphorylation in aerobic cells, that followed it later 

 in the evolutionary scale. 



From the point of view of biochemical evolution, one of the most 

 interesting findings in the study of cell-free photosynthesis was that 

 liigher aerobic jjlants have retained to this day the anaerobic cyclic 

 j)hotophosphorylation as a mechanism for making ATP, while shar- 

 ing with other organisms the acquisition of the process of oxidative 

 ])hosphorylation by mitochondria. 



We can next envisage that the photoassimilation of organic com- 

 poimds was gratlually replaced by the assimilation of COo, which 

 gained in importance as the supply of organic compounds in the en- 

 vironment diminished. Here the main contribution of light was still 

 the formation of ATP by cyclic photophosphorylation. The reduc- 

 tant, it is reasonable to suppose, was at first the molecular hydrogen 

 of the surrounding atmosphere. Chromatium, for example, is still 

 capable of using molecular hydrogen for reducing (in the dark) the 

 pyridine nucleotide that is needed for CO2 assimilation. 



Next, the photosynthetic cell acquired the enzymatic apparatus 

 for photochemically generating electrons with a reducing potential 

 equal to molecular hydrogen, from more oxidized electron donors, 

 such as succinate or thiosulfate. In organisms which contain, or can 

 adaptively form, hydrogenase (photosynthetic bacteria and algae) , 

 this phase of photosynthesis can also be observed today as a photo- 

 production of molecular hydrogen. Light energy now served a dual 

 purpose. It supplied ATP by cyclic photophosphorylation and it 

 provided electrons for reducing pyridine nucleotides (Fig. 17) . 



Finally, in the most advanced type of photosynthesis, found in 

 green plants, COo became the sole carbon source and water became 

 the electron donor. Here the function of light was not only to provide 

 ATP, but also to raise the electrons from water to an energy level 

 high enough for the reduction of TPN. Only in the last case, when 

 water became the electron donor, did oxygen evolution form an 

 inseparable part of photosynthesis, as is indicated in the proposed 

 mechanism for non-cyclic photophosphorylation in chloroplasts (Fig. 

 19). 



