I08 PHOTOSYNTHESIS 



lengthened still further. These compounds may be con- 

 sidered as formed second in a catenary sequence of reac- 

 tions. The third group such as sucrose, representing the 

 final products of the reaction sequence, show a progressive 

 increase in tracer from the shortest to the longest periods. 

 Thus it appears that PGA is the first photosynthetic inter- 

 mediate which during further illumination is in part trans- 

 formed to compounds of the second class and then finally 

 to ultimate products. 



In other experiments a different procedure was used, the 

 cells being first illuminated in absence of carbon dioxide, 

 darkened and tracer added at the same instant, and then 

 killed and analysed after a known period of darkness. This 

 'pre-illumination' resulted in a marked increase in tracer 

 uptake during the dark. Subsequent to cessation of illumina- 

 tion this increased ability for fixation of tracer decays with 

 a half time of the order of one minute. Tracer fixed after 

 *pre-illumination' is mainly confined to PGA and phospho- 

 pyruvic acid in spite of the relatively long times during 

 which fixation occurs. These experiments are interpreted as 

 showing that fixation after 'pre-illumination' represents the 

 combination of carbon dioxide with acceptor molecules 

 generated in the light and persisting in the dark; further 

 reaction of the carbon dioxide acceptor compound does not 

 occur in the dark. 



Initially this acid is labelled only in the carboxyl group; 

 the hexose derivatives first formed being predominantly 

 labelled in the 3- and 4-positions (the central carbon atoms). 

 This suggests that carbon dioxide is added to some two- 

 carbon precursor forming a new carboxyl group and that 

 two molecules of the three-carbon acid so formed condense, 

 the two carboxyl ends joining together, to give the hexose 

 molecule (p. 90). During longer periods of photosynthesis 

 the labelling of all the carbon atoms in the three-carbon acid 

 tends to equality; similarly with the hexose formed. Thus 

 the hypothetical two-carbon precursor must also be derived 

 from the carbon dioxide assimilated. For this reason Calvin 

 postulated that part of the phosphoglyceric acid presumed 

 to be formed would give rise to hexose and carbohydrate 



