526 LIGHT AND LIFE 



TABLE 5 

 Cv'CLic Photophosphorylation by Purified Grana with and without 

 Added Chloroplast Extract (Illumination 35,000 Lux.) 

 (MiJLLER, Steere, and Arnon, 110) 



Treatment QC^^i * 



Phenazine methosulfate 157 



Phenazine methosulfate + chloroplast extract 145 



Vitamin K3 39 



Vitamin K3 + chloroplast extract 78 



FMN 46 



FMN + chloroplast extract 69 



* Micromoles orthophosphate esterified per mg chlorophyll per hour. 



lation. The enzymes responsible for COo assimilation are easily dis- 

 sociable from grana in the case of chloroplasts (13, 173, 153) , and are 

 not even structurally joined together in the case of bacterial chromato- 

 phores (3, 97) . It seems reasonable to conclude, therefore, that in 

 the course of biochemical evolution, photosynthesis first emerged 

 as a process for converting light energy into ATP and this "primitive" 

 photosynthesis only later became a process linked to COo reduction. 



12. Cyclic Photophosphorylation as Primitive Photosynthesis 



In the conventional view of photosynthesis, the chemical energy 

 formed by the conversion of absorbed light is always used for the 

 reduction of COo. The case of cyclic photophosphorylation as being 

 a "primitive" photosynthesis, in the evolutionary sense, would there- 

 fore be strengthened if an example could be found today of a photo- 

 synthetic organism in which the only contribution of light to carbon 

 assimilation was the formation of ATP. 



Two such cases of photosynthesis in Chromatium have recently 

 been described by Losada et al. (97) . In one case the sole source 

 of carbon was acetate, and in the other, COo. The photoassimilation 

 of acetate occurred in the absence of an external hydrogen donor, 

 whereas in the photoassimilation of COo the reductant was exogenous 

 hydrogen gas. The sole contribution of light in both cases was the 

 formation of ATP. 



In the photoassimilation of CO2, ATP was required for the forma- 

 tion of an activated intermediate (ribulose diphosphate, phosphoenol- 

 pyruvate, or l,3-chph()sphoglycerate) for a subsequent carboxylation 

 or reduction, whereas in the photoassimilation of acetate, ATP was 

 required for the activation of the carbon source itself, by forming 



