found. Some of these (the sugar phosphates) are not sepa- 

 rated from each other by the first chromatography and must 

 be subjected to further analysis. When the sugar mono- 

 phosphates are hydrolyzed to remove the phosphate groups 

 and rechromatographed, separate spots are found of triose 

 (dihydroxyacetone), tetrose, pentoses (ribulose, xylulose, and 

 ribose), hexoses (glucose and fructose), and heptose (sedo- 

 heptulose). The radioactive sugar diphosphates area gives 

 free ribulose, fructose, glucose, and sedoheptulose. 



After periods of photosynthesis with C^* of less than 5 

 seconds, 3-phosphoglyceric acid (PGA) was found to be the 

 predominant radioactive product. Chemical degradation of 

 this compound showed that the radioactivity first appears in 

 the carboxyl carbon (14). Later kinetic studies showed that 

 the rate of incorporation of C^^ into PGA at very short 

 times was much greater than the rate of labeling of any 

 other compound (18,1). Therefore, it was concluded that 

 PGA is the first stable product of carbon dioxide fixation 

 during photosynthesis, and, furthermore, that carbon dioxide 

 first enters the carboxyl group of PGA, presumably via a 

 carboxylation reaction. 



Further reactions in the photosynthetic sequence were 



^^^ MALIC ACIO 

 ALANINE 



ASPARTIC ACID 





TRIOSE PHOSPHATE ^^V PGA 



10 SEC PHOTOSYNTHESIS *ITH C'«0, ^^^ PHOSPHATES 



CHLO/KLLA g^ 



SUGAR DIPHOSPHATES 



Figure 3. Radioautograph of two-dimensional paper chromato- 

 gram. Alcoholic extract of Chlorella pyrenoidosa after 10 seconds 

 photosynthesis with Ci'*02. 



15 



