120 PROBLEMS OF PHOTOSYNTHESIS 



CO2 CO2 CO2 



/ / / 



Ci C, Ci C4 C2 C2 C3 C4 



-I— I I I I I 



Ca C2 C3 Cs C3 C3 Ci Cs 



J. ^ TK I J. TA I r TK ' '^ 



C3 + C3 fc.Ci + C6 fc. Ci + C4 ►O2 + Cs 



III III 



C4 C4 C2 C« C5 C3 



III I I 



Cs Cs C3 Cs C4 



I ' ' A 



Ce C4 C6 C5 



I 



Cs 



Fig. 46. The fate of the C atoms of gIucose-6-phosphate (Racker). 



the other transketolase reaction condensing pentose phosphate with erythrose- 

 4-phosphate and producing fructose-6-phosphate and triose phosphate. 

 Oxidative decarboxylation of the fructose-6-phosphate produced again gives 

 ribose-5-phosphate which can condense with a further molecule of xylulose- 

 5-phosphate derived from glucose-6-phosphate. Thus, we have a cyclic proc- 

 ess in which ribose-5-phosphate acts as a catalyst, like oxaloacetic acid in 

 the tricarboxylic acid cycle. 



In a comparison of the possible pathways of carbohydrate degradation the 

 question arises as to which factors induce the cells to follow a particular path- 

 way. It is probable that the quantitative distribution of the enzymes is of 

 utmost importance. Glycolysis is nearly non-existent in various micro- 

 organisms, but is predominant in muscle cells. Although the enzymes of the 

 pentose phosphate pathway are present in yeast, they barely participate in 

 carbohydrate breakdown, as the enzymes of glycolysis are present in much 

 higher concentrations. In respiration of higher plants the pentose phosphate 

 pathway may play a role since its enzymes have been found to be present. 

 However, at present, no exact information on the importance of the pentose 

 phosphate pathway is available (1, 3). 



REFERENCES 



1. AxELROD, B. and Beevers, H.: Ann. Rev. Plant Physiol., 7:267, 1956. 



2. Badin, E. J. and Calvin, M.: J. Amer. Chem. Soc, 72:5266, 1950. 



3. Beevers, H. and Gibbs, M. : Plant Physiol., 29:322, 1954. 



4. Cohen, S. S. : Other Pathways of Carbohydrate Metabolism. In Chemical Path- 



ways of Metabolism I, Academic Press, New York, 1954. 



5. CoRi, O. and Lipmann, F.: J. Biol. Chem., 194:417, 1952. 



6. Dickens, F. : Bnt. Med. Bull., 9:105, 1953. 



7. Green, D. E. : Science, 7/5:661, 1952. 



8. Green, D. E.: Phosphorus Metabolism, 7:330, 1951. 



9. GuNSALUS, I. C: Fed. Proc, 73:715, 1954. 



10. HiFT, H., OuELLET, L., LiTTLEFiELD, J. W. and Sanadi, D. R. : J. Biol. Chem.. 



204:565, 1953. 



11. HoRECKER, B. L.: J. Cell. Comp. Physiol., -^7:Suppl. I, 137, 1953. 



