DANIEL I. ARNON 491 



conclusion (fi, 7, 8) which has recently received support from the 

 work of James and Das (79) and Warburg et al. (165). 



Chloroplasts were once widely believed to be the site of complete 

 photosynthesis, but this view was not supported by critical experi- 

 mental evidence (131, 121) and was later abandoned because of 

 much evidence to the contrary (70, 33, 26, 125, 98). In 1954, Lumry 

 et al. (98) sunniied up the many investigations with isolated chloro- 

 plasts as pointing to the conclusion that the chloroplast was a "sys- 

 tem much simpler than that required for photosynthesis" and was 

 the site of only "the light-absorbing and watersplitting reactions of 

 the overall photosynthetic process" (Hill reaction) . It seemed likely, 

 in the words of Rabinowitch (124, p. 355), that "the inability of 

 isolated — even if apparently intact — chloroplasts to carry out photo- 

 synthesis is an indication that this process requires the cooperation of 

 the cytoplasm." 



The possibility has always remained that the observed failure of 

 isolated chloroplasts to fix CO2 was merely a consequence of inappro- 

 priate experimental methods that were used in different laboratories, 

 including our own (5). Further work with isolated chloroplasts has 

 shown that the difficulties were indeed methodological. In 1954, 

 another phase of chloroplast investigations began -with the discovery 

 that, under suitable experimental conditions, isolated whole chloro- 

 plasts can assimilate carbon dioxide (H)- Photosynthesis, as it is 

 usually defined (124, p. 61), was found to be independent of the 

 organization of the living cell. Reduction of carbon dioxide to car- 

 bohydrates accompanied by oxidation of water to oxygen was carried 

 out, at physiological temperatures and with no energy supply except 

 visible light, by isolated chloroplasts unaided by other cellular par- 

 ticles or enzyme systems (11, 6, 1). 



CO2 fixation by isolated whole chloroplasts was strictly light-depen- 

 dent and proceeded at an almost constant rate for at least an hour. 

 There was approximate correspondence between the oxygen evolved 

 and the COo fixed, as would be expected from the well-known photo- 

 synthetic quotient in green plants ~ 



and insoluble products resulted from the fixation of radiocarbon by 

 chloroplasts. The insoluble product of CO2 fixation was identified 

 as starch. Among the soluble products the following were identified: 

 phosphate esters of fructose, glucose, ribulose, sedoheptulose, dihy- 

 droxyacetone, and glyceric acid; glycolic, malic, and aspartic acids: 

 alanine, glycine, and free dihydroxyacetone and glucose (1) . 



= 1. Both soluble 



