REACTIONS OF CARBON DIOXIDE I09 



while part would be utilized in a process providing the 

 two-carbon atom precursor. 



Various suggestions as to the nature of the two-carbon 

 compound were made in attempts to include those reactions 

 known to result in carbon dioxide fixation in heterotrophs, 

 such as the Wood-Werkman reaction. The evidence now 

 indicates that only one port of entry for carbon dioxide is 

 probable and that this results in the formation of PGiV. 

 Suggestions as to the nature of this reaction have been 

 modified by the recent finding that among the compounds 

 which become radioactive during very brief periods of illu- 

 mination are sedoheptulose phosphate and ribulose phos- 

 phate. After 5 seconds illumination these compounds are 

 respectively labelled in their 3, 4, 5, and 3 with some but 

 less in 4 and 5 C atoms. This is consistent with the following 

 sequence of reactions. 



3C5 + 3CO2 6 PGA (C1C2C3*) 



light 

 6 PGA ^ 6 triose phosphate 



2 triose phosphate ^ Hexose (CiC2C3*C4*C5C6) 



Hexose + 2 triose ^ 



(CiC2C3*C4*C5C6) (CiC2C3*C3*C2Ci) 



Pentose + Sedoheptulose 

 (CiC^Ca^C.Cs) (CiC2C3*C4*C5*C6C7) 

 Sedoheptulose + Triose ^ 2 Pentose 



(CiCoC3*C4C5 + CiC2C3*C4*C5*) 



The reactions can be illustrated by the following diagram: 



+ CO2 



Further evidence for this mechanism has been obtained by 

 suddenly decreasing the concentration of carbon dioxide 

 when the labelling of the 5 C compound increases while 



