PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 399 



pyridine nucleotide by cyclic and non-cyclic photophosphorylation. This 

 transformation of light into the common currency of cellular energy is 

 fundamentally independent of carbon dioxide assimilation. There is no 

 particular reason why adenosine triphosphate or a photochemically 

 generated reductant could not be used for driving endergonic cellular 

 processes other than COo assimilation. 



The photoassimilation of acetate by Chromatitim is a case of photo- 

 synthesis without either oxygen evolution or CO2 reduction [121]. So is 

 the light-dependent conversion of glucose into starch [128]. All these 

 light-driven reactions are also known to occur in the dark in non-chloro- 

 phyllous cells, but in this respect they resemble "photosynthetic" CO., 

 assimilation which occurs, by essentially the same pathway, in non-photo- 

 synthetic bacteria [32, 33]. Other manifestations of the photosvnthetic 

 process, now under active investigation, are the photofixation of nitrogen 

 and the photoproduction of hydrogen gas. Usually, these reactions would 

 be considered as being distinct from photosynthesis proper but according 

 to our present concept these examples represent photosynthetic events 

 because they are being dri\en by light energy. 



In this view of photosynthesis, CO., assimilation, although quantita- 

 tively the dominant form of photosynthesis on our planet, is funda- 

 mentally only a special case of the use and storage of light energy. COg 

 assimilation proper, in both green plants and photosynthetic bacteria, 

 consists of exclusively dark reactions that are not peculiar to photo- 

 synthesis.* The familiar accumulation of carbon compounds as carbo- 

 hydrates during photosynthesis in green plants constitutes storage of 

 trapped light energy. The first products of photosynthesis in green plants 

 [94, 35], ATP and TPNH^, are present in the cell only in catalytic amounts 

 and cannot be stored to any appreciable degree for future use, whereas 

 carbohydrates or fats can. 



The proposal that ATP formation is a fundamental event in photo- 

 synthesis has been made earlier, notably in 1944 by Emerson et al. [167], 

 but, as was recently pointed out by Umbreit, "the early experiments were 

 not adequate to demonstrate it" [167]. Without sufficient experimental 

 evidence, the theoretical proposals of Umbreit and his associates could 

 not be adequately defended against the theoretical objections levelled 

 against them (as for example by Rabinowitch [75, p. 229]), particularly 

 since later, the first experiments with ^^P to test the occurrence of light- 

 induced phosphorylation in cell-free systems led to negative results. 



* A similar conclusion was also reached by investigators of the carbon path in 

 photosynthesis [166, i66a]. Calvin [i66a] wrote recently: " The reduction of carbon 

 dioxide, we now have every reason to suppose, occurs in a series of reactions 

 which can take place entirely in the dark. In fact, all the enzyme systems that 

 we now know participate in the conversion of CO.^ to carbohydrates have been 

 found in a wide variety of organisms, many of which are not photosynthetic." 



