1650 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



tribution of C* taken up in light was fast only in light. It occurred faster 

 in presence than in absence of CO2 (fig. 36.8). It was therefore con- 

 cluded that fraction B contained one or several intermediates of photosyn- 

 thesis formed only in light, and requiring light for their further transforma- 

 tion. This intermediate did not exchange its C* with ordinary CO2 in the 

 dark. 



Since the tracer entered fraction B in light and moved out of it also in light, a steady 

 C* content of this fraction was established after a certain illumination period — about 

 one hour in saturating light (fig. 36.9). (As pointed out before, this seems too long a 

 time to be required for a main-line intermediate of photosynthesis to reach stationary 

 state in strong light.) 



KRCEHt 



CO 



50 



40 



30 



20 



10 _ 



, (S7.«*) 



(49.3%) 



PWTREATMEMT: I8MIN. PHOTOSTMTreSIS WITH EXCESS c'^Oj, 



PHOSPHATE BUFFER,PH62 

 TIMES: CONTACT WITH C'*Ot 



(VALUES' AV ERASE OF 0UPUCATE8) 



SOLID LINES % WATER -EXTRACTABLE TRACER 



(9U%) 



PHOSPHOeLVCERIC AGIO (a^ CARaON A CARBOXYL) 



(£6.8%) 



(10.1%) 



K) 



CO 



X) 



40 



eo 



60 

 SECONDS 



70 



80 



SO 



100 



no 



ito 



Fig. 36.10. Percentage of tracer fixed in water soluble compounds contained in 

 phosphoglyceric and pyruvic acid, as function of length of exposure to C*02 in light 

 (after Gaffron et al. 1951). 



When fraction B was saturated with radiocarbon in light, as much as 

 50% of it was derived from tagged C*02. Fager (1949) conjectured from 

 chemical evidence that C* was present in at least two compounds in this 

 fraction; one was three or four times more abundant than the other. 



Attempts to identify these compounds with known organic substances, 

 in particular with any common respiration intermediates, were unsuccess- 

 ful. A special search was made forpolyhydroxy acids (of which the simplest 

 is glyceric acid) by isolation of barium salts of phosphate esters and oxida- 

 tion by periodic acid after hydrolysis. Both methods gave no evidence of 

 the presence of an active polyhydroxy acid or of a phosphate ester of such 

 an acid. 



