470 



FURTHER EVOLUTION 



organisms, in higher organisms it has become standardised 

 to some extent."* There is less reason to suppose that the 

 same is true of the second system, which is devoted to the 

 oxidation to water of the hydrogen which is obtained during 



GLYCOLYTIC 



Pructose-6 -phosphate 

 (ATP) 



Fructose- 1, b-diphosphate 



Dihydroxyacetone ■ phosphate 



+ 

 Clyceraldehyde -3 - phosphote 



OXIDATIVE 



Glucose - b-phosphote 

 (TPN) 

 6-Phosphoqluconate 



A 



(TPN) 



f 



Pentose -5-phosphate + CO 



{s mo/es) 



(ThPP) 



^ Sedoheptulose-?- phosphate 

 ^ GlycerQlclehyde-3-phosphaf e 



(DPN)- 



Y blocked by I Ac ^ 

 ^ or F" / 



Phosphopyruvote 



Acetate-f CO? or Lactate 



yio Krebs 

 Cycle 



CO2 



Fig. 42. Glycolytic and oxidative pathways for the 

 breakdown of glucose (after Dickens). 



dehydrogenation, by means of atmospheric oxygen with the 

 formation of high-energy compounds at the expense of the 

 energy liberated by this reaction. 



As is well known, the combination of gaseous oxygen and 

 hydrogen has such a high energy of activation that, at ordin- 

 ary temperatures, it hardly occurs at all. Organisms overcome 

 this energy barrier, breaking it down into a number of steps 

 so that the hydrogen is transferred successively through a 

 system of mediators with the help of a series of specific 



