84 METABOLIC PATHWAYS IN MICROORGANISMS 



tually exclusively one following glycolysis and the Krebs 

 cycle, which is maintained throughout post-paitum life. 

 It is not understood why phosphogluconate cleavage dis- 

 appears altogether in the late foetal stages; the need for 

 rapid synthesis is much reduced at this point as compared 

 to the situation in the earlier foetus, and it seems even pos- 

 sible that the synthesis observed in a young animal is so 

 small (relative to oxidation) as to escape measurement by 

 the methods used. 



Finally, as suggested in the review by Horecker and Hiatt 

 (70) a possible role of the pentose cycle (here operating 

 clockwise, or in the forward direction) may be to supply 

 TPNH to the system. In many species, the pentose cycle 

 is TPN-specific; glycolysis and Krebs cycle oxidations are, 

 on the other hand, largely DPN-requiring (a possible ex- 

 ception in some systems is isocitric dehydrogenase) so that 

 processes such as fatty acid synthesis may derive significant 

 amounts of needed TPNH from phosphogluconate cleavage. 



CONCLUSION 



Our ideas about carbohydrate metabolism have under- 

 gone considerable shifting during the past quarter century, 

 as glycolysis, the Krebs cycle, and the pentose cycle have 

 successively emerged as major pathways. Each of these 

 appears to contribute to the breakdown of carbohydrates; 

 in selected organisms, major use appears to be made 

 of one or another of these pathways. In many organisms, 

 however, a strong case appears to be developing for the 

 employment of the pentose cycle largely as a synthetic ap- 

 paratus. Time and the diligence of researchers in this 

 area will determine whether overlapping may occur with 



