1984 EPILOGUE CHAP. 38 



C(ll), and the lack of ability of chloroplast fragments to use carbon di- 

 oxide as hydrogen acceptor in the Hill reaction, had made it appear likely, 

 for a while, that the cytoplasm has an essential function in the carbon 

 dioxide reduction.) However, this conclusion needs confirmation, since 

 complete separation of chloroplasts from residues of cytoplasm, which 

 may include mitochondria or similar enzyme cari'iers, is not easy. Fur- 

 thermore, it is not yet excluded that separated chloroplasts may be able 

 to continue the photochemical reduction of carbon dioxide only for a very 

 short time, before some chemical agents, normally supplied by the cyto- 

 plasm, become exhausted. 



With the help of the long-lived radioactive carbon, C(14), several im- 

 portant landmarks could be identified in the field of photosynthetic inter- 

 mediates, which at the time this book was begun still was the terra incognita 

 (and has now become merely the "dark continent") on the biochemical 

 maps. Certain paths have been identified connecting these landmarks, 

 and we can hopefully look forward to a gradual mapping-out of the whole 

 area — even if this is likely to take as much time and effort as did the un- 

 raveling of another tangled skein of enzymatic reactions — the respira- 

 tory degradation of carbohydrates in the cell. Although the mechanism 

 of the latter seemed to have been essentially clarified by the establishment 

 of the tricarboxylic acid cycle by Krebs, new intermediates and new 

 alternative paths are being added almost every year. 



The two main landmarks which the application of tracer techniciue has 

 established, as of 1955, in the domain of CXX.-reduction intermediates in 

 photosynthesis are phosphoglyceric acid (PGA) and ribulose diphosphate 

 (RDP). The role of the first one as the immediate product of C02-fL\ation 

 and substrate of photochemical reduction seems to be established beyond 

 all reasonable doubt : while the function of the second as the carbon dioxide 

 acceptor, whose carboxylation leads to PGA, seems eminently plausible, 

 and is supported by enzymatic experiments in vitro. 



Calvin and co-workers have outlined a complete mechanism of sucrose 

 synthesis incorporating, in addition to PGA and RDP, other intermediates 

 (on the reduction le\'el of carboh^'drates). Most of these — dihydroxyace- 

 tone phosphate, sedoheptulose mono- and diphosphates, glucose and fruc- 

 tose monophosphates, and uridine diphosphoglucose (but not erythrose 

 phosphate!) — have been identified among the early C(14)-tagged products 

 of photosynthesis.* This reaction sequence requires, in addition to the 



* See scheme on p. 1697. A more detailed presentation was given by Calvin and 

 Bassham in a paper at the Geneva Conference on Peaceful Uses of Atomic Energy, 

 August 1955 (cf. in particular figs. 5 and 14 of that paper). 



