EVOLUTION OF CARBOHYDRATES 55 



The mono-saccharides are aldehydes or ketones 

 of polyhydric alcohols. The first are aldoses, e.g. 

 dextrose, C 6 H 12 6 , and the second are ketoses, e.g. 

 laevulose, which is also C 6 H 12 6 . The difference 

 between them is shown by the ionic linking or 

 structural formula. 



Dextrose = 

 CH 2 (OH).CH(OH).CH(OH).CH(OH).CH(OH).CHO 



Lsevulose = 

 CH 2 (OH).CH(OH).CH(OH).CH(OH).CO-CH 2 (OH). 



The mono-saccharides are convertible into corre- 

 sponding alcohols by nascent hydrogen, i.e. by H 

 ions ; thus dextrose and Isevulose become sorbite, 

 mannite, and dulcite : C 6 H 14 6 . Inversely the corre- 

 sponding sugars may be derived from the alcohols 

 through oxidation by OH ions.* The pentoses, 

 C 5 H 10 5 , are formed from more complex carbo- 

 hydrates by hydrolytic splitting to form pentosanes. 

 These are of great importance in the vegetable king- 

 dom as building material, and consequently as food 

 for the herbivorous animals. The chief pentoses are 

 arabinose and xylose, the latter of which is widely 

 distributed throughout the vegetable kingdom. 



One of the derivatives of the glucoses (dextrose or 

 mannose) is glucosamine, C 6 H 13 N0 5 or CH 2 (OH)- 

 CH(OH)-CH(OH).CH(OH).CH(NH 2 ).CHO. This is 

 an intermediary member between the carbohydrates 

 (hexoses) and the amino-acids obtainable from 

 proteids, and is therefore to be regarded as a bridge 

 between the carbohydrates and the proteids.f 



* Hammarten's " Physiological Chemistry," p. 85. 

 t Ibid., p. 98. 



