CARBOHYDRATES 



57 



CHO CHO 



H C OH HO C H 



HO C H HO C H 



H C OH 



H C OH 



CH 2 OH 



Glucose 



CH 2 OH 



HO--C H 



CHOH 

 4-OH 

 HO C H 



CHoOH 



Galactoae 



CH 2 OH 



Talose 



CH 2 OH 



Tagatose 



It will be noted that in the case of glucose, mannose, and fructose, 

 the configuration is identical at every point except at the aldehyde 

 end of the chain, and that here the two groups readily arrange 

 themselves into the same enolic form for the three sugars. Galac- 

 tose differs from these three sugars only in the arrangement of the 

 H and OH groups attached to one of the other carbon atoms (the 

 third from the alcoholic end); the difficulty of its fermentation 

 indicates that some molecular rearrangement to bring this group 

 into its proper configuration must precede the fermentation process. 

 The fact that it is the third HCOH group which thus undergoes 

 rearrangement is significant because of the participation of these 

 parts of molecules in groups of threes in many biological processes, 

 as will be mentioned elsewhere. Talose is unfermentable, even 

 though the arrangement of its upper three groups is the same as in 

 the galactose and the lower three the same as in mannose. 



If further proof that fermentability depends upon molecular 

 configuration were needed, it is furnished by the fact that no 

 pentose is fermentible, even though the stereo-arrangement of 



