320 RADIATION BIOLOGY 



practically the total of radioactivity) and then are illuminated in the absence of 

 carbon dioxide, labeling in C3 and C4 compounds decreases in favor of the forma- 

 tion of labeled gly colic acid and glycine. This not only raises another argument 

 against a Ci — Ci condensation as a source of C2 compounds but favors the 

 assumption of a C2 — C3 — C4 — C2 cycle, as mentioned earUer, in which C4 -^ C2, 

 since generation of C2 from C3 would be likely to yield products such as formic 

 acid or formaldehyde and these were not observed. Calvin et at. carried out 

 experiments to see whether malic acid is an intermediate in this cycle. They 

 used malonate-treated Scenedesmus, which, after suspension in malonate-free 

 buffer, was actively photosynthesizing, and exposed it to C^^02 over a short 

 period. Whereas radiocarbon in malic acid was greatly decreased as compared 

 with normal cells, the other short-exposure products were practically unchanged. 

 The labehng of a. and jS carbon atoms of glyceric acid was the same. They con- 

 cluded that, if phosphoglyceric acid is an intermediate in photosynthetic carbo- 

 hydrate S3^nthesis by reversing glycolysis, malic acid is not an intermediate but 

 rather a carbon reservoir that is readily derived from some intermediate in photo- 

 synthesis. Neither fumaric nor succinic acid showed any special relation to 

 photosynthesis. Oxaloacetic acid is thought to be a possible intermediate for 

 the formation of the C2 carbon dioxide acceptor. 



Late reports (Benson, 1951; Benson et al., 1951) indicate the finding 

 of sedoheptulosephosphate and ribulosediphosphate in a very early stage 

 of C^^02 fixation in photosynthesizing plants. Tentatively these pro- 

 ducts are suggested as participating in the system for regeneration of 

 the C2 carbon dioxide acceptor. A critical survey of the photosynthesis 

 research with 0^*02 was given by Utter and Wood (1951). 



It is interesting that, if the external atmosphere contains 20 per cent 

 oxygen, about ten times more labeled glycolic acid is formed than if there 

 is only 1 per cent oxygen. Calvin et al. see two possible explanations: 

 (1) the oxaloacetic acid is oxidized before splitting, or (2) a glycolic acid 

 is formed by oxidation of a more reduced C2 compound. This action of 

 oxygen can be considered to be an interaction of respiration and photo- 

 synthesis. It is too early to judge whether this may be looked upon as 

 a competition of oxygen and carbon dioxide for certain hydrogen donors, 

 although this may well be the case. Weigl et al. (1951) found not only 

 that light decreased the respiratory evolution of carbon dioxide but also 

 that intermediates from 0^*02 were not respired while the light was on 

 but were as soon as the light was turned off. In a sense this is in har- 

 mony with the earlier statements of Brown et al. (1948) concerning some 

 respiration-stable special photosynthetic intermediate. In the whole of 

 knowledge now gained and in accordance with the present general con- 

 cept of photosynthesis as outlined in Sect. 2, it is more plausible to look 

 upon this again as a competitive activity of oxygen vs. carbon dioxide 

 for these intermediates. The question arises, of course, as to why this 

 competition is different in the light and in darkness if the same sub- 

 stances are involved. The preliminary answer would seem to be that 



