1676 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



appreciable assimilation. In the dark, under anaerobic conditions, the 

 label was incorporated in glycine and serine. In light, tagged products 

 similar to those obtained from C*02 were observed. The PGA obtained 

 in this way was labelled uniformly in a and /3 positions, irrespective of the 

 position of the label in glycolic acid. 



In section 12, we will refer to Calvin and Benson's speculations (1949) 

 that the C2 acceptor in photosynthesis may be vinyl phosphate, CH= 

 CHOPO3H2, a dehydration product of glycol phosphate (c/. equation 

 36.9). These speculations have been superseded by the above-mentioned 

 hypothesis of direct carboxylation of a C5 sugar diphosphate to form two 

 molecules of PGA. 



9. Kinetic Studies 



Most conclusions presented so far in this chapter are based on qualita- 

 tive kinetic information — the sequence in which different tagged com- 

 pounds appear when photosynthesizing cells are offered tagged carbon di- 

 oxide. On a few occasions, we have referred to more quantitative data. 



SLUCOSE MONOPHOSPHATE 



Fig. 36.19. C*02 incorporation in hexose phosphates as function of time of 

 photosynthesis. 15° C, 10 /xl. Scenedesmus cells. "Unidentified glucose phos- 

 phate" later identified as uridine diphosphate glucose + related compounds 

 (after Benson et al. 1952). 



e. g., to the changes in the ratio of labels in phosphoglyceric and pyruvic 

 acid with time {cf. fig. 36.10). 



More recently, the absolute and relative amounts of labelling on dif- 

 ferent compounds as function of time have been studied more systematically 

 by Calvin and co-workers (Benson, Kawaguchi, Hayes and Calvin 1952; 

 Calvin and Massini 1952; Benson et al. 1954). 



