DANIEL I. ARNON 559 



The photoassiniilation of acetate by Chronuttiutn is a case of photo- 

 synthesis witliout cither oxygen evohition or CO^ reduction (97). 

 So is the light-clc])cndent conversion of gkicose into starch (100). 

 In the conventional view of photosynthesis this state of affairs would 

 be a contradiction of terms, but according to our present concept 

 these examples represent photosynthesis because they involve bio- 

 chemical syntheses that are being driven by light energy after light 

 has been converted by the cell into ATP. 



In this view of photosynthesis, COo assimilation, although quanti- 

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

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

 COo assimilation proper, in both green plants and photosynthetic 

 bacteria, consists of exclusively dark reactions that are not peculiar 

 to photosynthesis. The familiar accumulation of carbon compounds 

 as carbohydrates during photosynthesis in green plants constitutes 

 storage of trapped light energy. The first products of photosynthesis 

 in green plants (9, 96) , ATP and TPNHo, 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 the main event in photosyn- 

 thesis has been made earlier, notably in 1944 by Emerson, Stauffer, 

 and Umbreit (45) , who suggested that the "sole function of light 

 energy in photosynthesis is the formation of 'energy-rich' phosphate 

 bonds." They included in this generalization both green plants and 

 photosynthetic bacteria, but made no provision for a photochemical 

 generation of a reductant for CO2 assimilation by green plants. The 

 main reason, however, why their proposal met with little acceptance 

 in the ensuing years, and had little influence on subsequent research 

 in photosynthesis, was the lack of substantial experimental evidence 

 for the participation of phosphorus in their experiments with intact 

 Chlorella cells (cf. 124, p. 228; 176) . The theoretical proposals of 

 Emerson et al. could not be adequately defended against the theoreti- 

 cal objections levelled against them [as for example by Rabinowitch 

 (124, p. 229)], particularly since the first experiments with V^- to 

 test the occurrence of light-induced phosphorylation in cell-free sys- 

 tems led to negative results. Aronoff and Calvin, who made these ex- 

 periments with spinach grana, reported that "there is no direct con- 

 nection between light and the gross formation of organic phosphorus 

 compounds" (19) . 



As the history of photosynthesis approaches the end of its second 

 century, one finds fresh relevance in the admonition of Priestley, 



