THE EXPERIMENTS OF THE CHICAGO GROUP 1653 



and Fager and Rosenberg (1950) described the isolation of phosphogly eerie 

 acid from tagged products of photosynthesis in Scenedesmus. An analyti- 

 cally pure sample of barium glycerate was prepared from this material by 

 an ion exchange separation method. The proportion of total activity found 

 in phosphoglycerate decreased with increased length of an exposure (10 to 

 120 sec.) from 58 to 38%; while that in pyruvic acid (a second tagged 

 compound also identified in these experiments) declined from 10 to 7%. 

 Within the phosphoglyceric acid molecule, the amount of tagging in the 

 a position grew from to 11% (of the total C* taken up) ; probably, that 

 of tagged /3-carbon increased in the same way (fig. 3G. 10). After 2-min. ex- 

 posure, activity was more or less uniform in all three carbon atoms (com- 

 pare table 36. V). 



The ratio of C* in phosphoglyceric acid to that in pyruvic acid remained 

 the same (5-6) over the whole range of exposures; it dropped to 4 if after 

 exposure the cells were left in the dark for 20 sec, before killing. This was 

 interpreted as an indication that if pyruvic acid is involved in the carbon 

 dioxide reduction in photosynthesis, its place in the reaction sequence is 

 after and not before the phosphoglyceric acid. 



Most of the above results were in good agreement with observations of 

 the Berkeley group. Gaffron et al. (1950, 1951) disagreed, however, with 

 Calvin's interpretation of C* uptake in the dark after preillumination. As 

 mentioned in section 2 Calvin saw in this uptake proof of photochemical 

 formation of both a CO2 acceptor ("C2 compound"), and of a comparatively 

 long-lived, powerful reductant ("reducing power"). Fager and Gaffron 

 doubted the second conclusion, referring to the observation of Mehler 

 (chapter 35, page 1615) that adding dyes (known to act as Hill oxidants in 

 light) to preilluminated chloroplasts produces no reduction, indicating 

 that no "reducing power" survives the illumination period.* 



In further support of the assumption that what survives in the dark 

 after illumination is only the CO2 acceptor, Gaffron and co-workers quoted 

 their observation (fig. 36.11) that practically only phosphoglyceric acid 

 and pyruvic acid were tagged in 60-sec. exposure to C*02 in the dark after 

 15 min. preillumination in CO2 and O2 free gas (bar A) ; absence of O2 is re- 

 quired to avoid CO2 formation by respiration. (In other words, they found, 

 under these conditions, no tagged products whose formation would require 

 both carboxylation and reduction.) If preilluminated cells were exposed to 

 C*02 in light for 10 sec, the percentage of C* fixed in phosphoglyceric 

 and pyruvic acid was found by Gaffron et al. to be much smaller than with- 

 out preillumination (bar B), indicating that much of the tracer has pene- 



* This conclusion is supported, to some degree, by Spruit's (1953) measurements of 

 the redox potential in Chlorella suspensions. We can merely refer here to this interest- 

 ing study, as well as to the earlier one by Wassink (1947) on suspensions of Chromaiium. 



