THE MECHANISM OF PHOTOSYNTHESIS 319 



illumination (under carbon dioxide-free conditions) is omitted, the dark uptake 



of carbon dioxide was only about 0.1-0.01 of that after preillumination. The 



products were also different; i.e., 95 per cent of the total carbon fixed was in malic 



and succinic acid, alanine, and some other acids. The labeling seems to indicate 



the reversibility of normal carboxylation reactions. AccorcUng to Calvin et al. 



(1950, p. 528): "Those compounds labeled in the light and in preillumination 



experiments only are considered products of photosynthesis, while alanine, malic 



acid, and aspartic acid, labeled slowly in non-preilluminated dark experiments and 



much more rapidly in light and preilluminated dark experiments, are considered 



to be products of both photosynthesis and reversible respiration reactions." 



If the length of exposure is shortened, most C^Os is in the COOH of phospho- 



glyceric acid, which suggests that this substance is formed by carboxylation of a 



C2 compound {ibid. p. 529). The activity of malic and aspartic acids in short 



exposures is also in the carboxyl groups. Together with the early appearance of 



labeled malic acid and phosphopyruvic acid, this suggests the conversion: 



+CO2 

 Phosphoglyceric -^ phosphopyruvic > oxaloacetic -^ malic acid. 



Thus carbon dioxide reduction in the light would involve two carboxylations 

 (C2 — > C3, C3 ^ C4). It is a very interesting finding that, although at high light 

 intensities (400-10,000 ft-c) in short exposures chiefly phosphoglyceric acid is 

 formed, below 50 ft-c chiefly malic acid arises. The variation is ascribed to a 

 variation in the concentrations of the respective carbon dioxide acceptors with 

 light intensity {ibid., p. 530). The way the authors look upon the nature of the 

 C2 compound is of interest. They considered it to be a highly reduced compound. 

 "It would be formed readily in the presence of photochemically produced reduc- 

 ing power, but in the dark it would probably be formed only by reversal of the 

 C2 — * C3 carboxylation, and in the latter case subsequent rapid oxidative reactions 

 might keep its concentration at a very low level." The concentration of C3 car- 

 bon dioxide acceptor (which is probably phosphopyruvic acid) is maintained in 

 darkness by glycolysis, whereas the concentration of C2 carbon dioxide acceptor 

 is then low. This would explain why C^*02 uptake in dark results in labeled C4 

 preferably and C^^02 in light or after carbon dioxide-free preillumination results 

 in labeled C3 (phosphoglyceric acid). The same holds for a comparison of low 

 and high light intensities; with the high intensities the Co carbon dioxide acceptor 

 predominates, and phosphoglyceric acid arises as the chief labeled compound in 

 short exposures. 



These considerations show that Calvin et al. (1950) do not consider the C2 and 

 C3 carbon dioxide acceptors to be identical with the reducing power generated 

 during continuous illumination or carbon dio.xide-free preillumination; they are 

 "formed in the presence of photochemically produced reducing power" (p. 530). 

 It is unlikely, for several reasons, that the C2 compound is generated by Ci — Ci 

 condensation. Traces of formaldehyde, formic acid, and acetic acid were found 

 which, according to these workers, might well be artifacts. A good argument 

 against direct synthesis of a C2 compound is that in brief experiments no signifi- 

 cant amounts of labeled C2 compounds, as, for example, glycine and glycohc acid, 

 are found and that in phosphoglyceric acid only a few per cent of the labeling is 

 outside the carboxyl group. On the other hand, if plants, by brief illumination 

 in the presence of 0**02, have produced labeled C3 and C4 compounds (covering 



