PRIMING REACTIONS 



191 



the production, first noted by Pasteur, of small amounts of glycerol during 

 alcoholic fermentation. But acetaldehyde is more strongly oxidizing than 

 phosphodihydroxyacetone. As long as it is present it will take hydrogen 

 from the DPNH to form ethanol. 



Glycogen (or starch) 



phospliorylju- 



Galactose 



+ ATP 

 gjl.ictokimse 



Fructose 



_ _, gjliciowjlJinuc 



G— 1— P — Galactose— 1—? 



UDPG 



G-1,6 — FP 



Hcxose menophosphate shur.t 



dipliosphofriictosophosphat.ise 



F— 1,6— PP 



I*hosphodih\'clrox)'acetonc 



Pl-.ospho^lyceraldchydc 



Fig. 37 — Preludes to glycolysis. 



B. Preludes to Glycolysis 



Glycolysis is essentially the passage of F — 1,6 — PP to pyruvic acid by 

 the intermediary of a splitting into two triosephosphates, two oxido- 

 reductions and two transfers of phosphate groups. It is an anaerobic 

 oxido-reduction of trioses, at substrate level, with formation of two energy- 

 rich bonds per molecule of triose. 



Glycolysis, defined as above, is very common in the biosphere. There 

 are numerous variants of the beginning of the process. The entry into 

 the pathway varies from one carbohydrate to another, as Fig. 37 

 sufficiently illustrates. 



C. The Hexosemonophosphate Shunt 

 (Pentose Cycle) 

 Although it is true that the Embden-Meyerhof scheme traces the most 

 general form of the start of carbohydrate catabolism, there exists an alter- 

 native route, oxidative in nature, which with a fragment of the glycolysis 



