THE MECHANISM OF PHOTOSYNTHESIS 



317 



for the respiratory mechanism in the dark. This compound was found 

 to be different from any known respiratory intermediate and could be 

 transformed further only in light reactions (Brown et al., 1948). It would 

 thus have many of the properties of a "stabilized photoproduct," as 

 required by the Franck-Herzfeld theory. 



Quite different results were obtained by the group associated with 

 Benson and Calvin. In an earlj^ stage of their investigation they made 

 the previously discussed observations on the increased carbon dioxide 

 uptake by carbon dioxide-free preilluminated suspensions of Chlorella. 



C2 occeptor [ch=ch-o -(?)?] 



C'O; 



OH 0-0 



+6H 



I 



-^-CHj-CH-Cl^'OgH 



2-phosphoglyceric acid 

 -HgO 





 H02C'^-CH2-C-C"'02H-«- 



oxoloocetic acid 



c'-'o. 



->- 3-phosphoglycerate 

 +2H 



trlosephosphotes 



-® 



HOjC'^-CHg-CHNHg-C'^OgH 



osportic acid 



— CH2=C— C'^OjH 

 phosphopyruvic acid 



I 



CHj-CHNHj-C"* O2H 



lonme 



t 



fructose-i,6-diphosphote 



1 



I — fructose-6-phosphate 



\ 

 g I ucose-6-phosphate 



^ i 



glucose- 1 -phosphote 



i 

 starches 



■ [sucrose phosphote] 



I 



sucrose 

 Fig. 5-14. The assumed path of CO2 reduction in photosynthesis. {From Benson 

 and Calvin, 1950.) 



This led them to the assumption that reducing power is generated by 

 the light and brought them surprisingly near to the conclusions and views 

 derived earlier from fluorescence work by Wassink and Katz (1939). 

 Moreover their assumption was consistent with Van Niel's deductions 

 from comparative biochemistry. An important development of the work 

 of Benson and Calvin was the discovery that, if cells in stationary con- 

 ditions of photosynthesis (with C^'Oo) were allowed to take up Ci^02 

 for a few seconds, radioactivity is predominant in a few compounds, 

 especially in 2- and 3-phosphoglyceric acid (Calvin, 1949). Phospho- 

 glyceric acid was identified as the earliest labeled intermediate also in 

 more recent work of the Chicago group (Fager ei al., 1950). The distribu- 

 tion of radioactivity in increased exposures to C^'*02 in the light led to 

 the suggestion that the "path of carbon in photosynthesis" is chiefly a 

 reversal of respiration. Benson and Calvin (1950) drew up a scheme 

 (Fig. 5-14), the essence of which is a dicarboxylic cycle in which a two- 

 carbon "acceptor" molecule is converted to a four-carbon compound by 

 two successive carboxylations. Fat, protein, and carbohydrate syntheses 

 use intermediates of photosynthesis from this cycle. The two-carbon 



