C*0! 



C 



2-carhon 

 CO2 acceptor 



PRINCIPLES OF TRACER METHODOLOGY 

 ICH2OPO3H2 



43 



2CH0H 

 »C*OOH 



phosphoglyceric acid 



i 



2H 



ICH2OPO3H2 



2CH0H 

 »C*HO 



1CH2OPO3H2 



I 

 - 2c=0 



3C*H20H 



dihydroxj-acetoiic 

 phosphate 



fructose diphosphate 



With this sequence carbon atoms 3 and 4 of the hexose must come from 

 carbon atom 3 of the glyceric acid. This is in direct agreement with the 

 15-sec values from Table 1-8, where carbon atom 3 of the acid was 56 per 

 cent labeled as compared with a 52 per cent label for carbon atoms 3 and 

 4 of the hexose. Likewise all the other carbon atoms in both compounds 

 were labeled at a level of about 21 to 25 per cent. 



It was next necessary to postulate a mechanism to account for a two- 

 carbon acceptor that becomes labeled at a slow rate and for the various 

 specific intermediates that have been observed. As already mentioned, 

 in addition to the hexose, the five- and seven-carbon keto sugars, ribulose 

 diphosphate and sedoheptulose monophosphate, were found to acquire a 

 C^^ label rapidly during short-term photosynthesis tracer experiments 

 with C^'*02. This indicated that they were in the path of carbon during 

 photosynthesis and probably function in regenerating the two-carbon 

 acceptor complex. The exact mechanism for this pathway has not yet 

 been elucidated, but by trace studies ribulose diphosphate has been shown 



