400 DANIEL I. ARNON 



Aronoff and Calvin, who made these experiments with spinach grana, 

 reported that "there is no direct connection between hght and the gross 

 formation of organic phosphorus compounds" [i68]. 



17. Photosynthesis and biochemical evolution 



The insight into the mechanism of photosynthesis, gained from cell- 

 free experiments with chloroplasts and chromatophores, permits us to 

 interpret, with somewhat enhanced confidence, certain aspects of bio- 

 chemical evolution which we have already discussed elsewhere [95, 121]. 



The beginning of photosynthesis may be viewed as an emergence of 

 a porphyrin that gave rise to chlorophyll and permitted the cell to use for 

 metabolic purposes the energy of sunlight. This primitive photosynthesis 

 consisted only of anaerobic cyclic photophosphorylation. No oxygen was 

 evolved and no photochemically formed reductant was required for the 

 photoassimilation of say, acetate, or for the assimilation of COg, as long as 

 hydrogen gas was present in the atmosphere. Oparin [169] and Miller and 

 Urey [170] have summarized the evidence that in the early periods of 

 evolution of life forms, the environment contained hydrogen gas and 

 simple carbon compounds such as acetate. This primitive type of photo- 

 synthesis is still seen today in photosynthetic bacteria. Chromatium, for 

 example, is capable of using molecular hydrogen for reducing, in the dark, 

 the pyridine nucleotide that is needed for CO2 assimilation, or of photo- 

 assimilating acetate without the aid of an external reductant. 



The harnessing of light energy for the synthesis of ATP was an event 

 of supreme importance to the cell. It provided the cell, in an anaerobic 

 environment, with a much more efficient mechanism than fermentation for 

 the formation of ATP that was needed for the transformation of existing 

 carbon compounds, into fats, carbohydrates, proteins, etc. Cyclic photo- 

 phosphorylation gave the anaerobic photosynthetic cell a mechanism which, 

 in efficiency of ATP formation, is comparable with the process of oxidative 

 phosphorylation 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 cell-free photosynthesis was that higher, aerobic 

 plants have retained to this day the anaerobic cyclic photophosphorylation 

 as a mechanism for making ATP while sharing with other organisms in the 

 acquisition of the process of oxidative phosphorylation by mitochondria. 



As hydrogen gas vanished from the primitive atmosphere, the photo- 

 synthetic cell became dependent on an enzymic apparatus for generating 

 photochemically a strong reductant, from such electron donors as succinate 

 or thiosulphate. Light energy now served a dual purpose. It supplied ATP 

 by cyclic photophosphorylation and it provided electrons for reducing 



