3IG RADIATION BIOLOGY 



process producing a reducing agent, and it is tempting to assume that, 

 in intervals between the impact of successive quanta on the same spots, 

 dark reactions tend to reverse the situation, so that the system acts as 

 a reversible redox system. The reaction cycle may be represented as 

 follows: 



EH ^ E + H. 



dark 



The H, which may well be attached to some large group, may be con- 

 sidered active in the process of carbon dioxide reduction. The statement 

 by Van Niel quoted in Sect. 4-1 is again pertinent. 



An early isotope experiment that is still of interest is that of Pratt 

 and Trelease (1938). They investigated the photosynthesis of Chlorella 

 in deuterium oxide and found that the light-saturation value was much 

 lower than in ordinary water, whereas no difference was observed in the 

 hght-limiting range. They concluded that water enters into a dark 

 reaction. This is in agreement with results of the study of photosyn- 

 thesis and bacteriochlorophyll fluorescence in Chromatium, which led to 

 the conclusion that the hydrogen donor is connected with the transfer of 

 energy by a dark process. The experiment of Pratt and Trelease might 

 suggest a similar situation with regard to water and energy transfer in 

 Chlorella. It should be remarked, however, that, as long as correlative 

 fluorescence measurements are lacking, no definite conclusion can he 

 drawn because of the great variety of effects that replacement of water 

 by deuterium oxide may have on cellular metabolism. 



At this point a brief discussion of the work on the path of carbon in 

 photosynthesis carried out with tracer carbon, especiallj^ since 1946, is 

 pertinent. Only those points which appear of direct interest for the 

 general discussion of the mechanism of photosynthesis will be considered. 

 Study of photosynthesis with the aid of carbon isotopes was started in 

 1938 by Ruben and his associates (Kamen, 1949). They were able at 

 first to use only the short-lived isotope C^^ From this work Ruben et al. 

 concluded that a primary carboxylation reaction led to the formation of 

 RCOOH, followed by a photochemical reduction in which carbon dioxide 

 was assimilated with regeneration of the carbon dioxide acceptor RH. 

 The formation of RCOOH from carbon dioxide and RH was assumed to 

 occur in the dark. The presumed compound RCOOH was actually iso- 

 lated later by Benson and Calvin (1947), when C'"* had become available, 

 and was found to be chiefly succinic acid. 



Studies with C^* were carried on independently by Gaffron's group 

 (e.g.. Eager et al., 1950) and by the group associated with Calvin and 

 Benson (e.g., Calvin et al., 1950). The former group, who were in close 

 contact with proponents for the Franck-Herzfeld theory of photosynthe- 

 sis, claimed to have evidence for a product of the primary action of the 

 light on a carbon dioxide-containing compound which was unavailable 



