MONOSACCHARIDES. 193 



Correspondingly the carbohydrates can be divided into three chief 

 groups, namely, 1. Simple sugars or monosaccharides, 2. Complex sugars 

 or disaccharides, trisaccharides and crystalline polysaccharides, and 3. 

 Non-crystalline or colloid polysaccharides. Of these groups the mono- 

 saccharides, disaccharides and colloid polysaccharides are of special 

 physiological importance. 



Our knowledge of the carbohydrates and their structural relation- 

 ships has been very much extended by the pioneering investigations of 

 KILLIANI l and especially those of E. FiscHER. 2 



As the carbohydrates occur chiefly in the plant kingdom it is naturally 

 not the place here to give a complete discussion of the numerous carbo- 

 hydrates known up to the present time. According to the plan of this 

 work it is only possible to give a short review of those carbohydrates 

 which occur in the animal kingdom or are of special importance as food 

 for man and animals. 



i. Monosaccharides. 



All varieties of sugars are characterized by the termination " ose," 

 to which a root is added signifying their origin or other relations. Accord- 

 ing to the number of carbon atoms, or more correctly oxygen atoms, 

 contained in the molecule .the monosaccharides are divided into, trioses, 

 tetroses, pentoses, hexoses, heptoses, and so on. 



All monosaccharides are either aldehydes or ketones of polyhydric 

 alcohols. The former are termed aldoses and the latter ketoses. Ordinary 

 dextrose is an aldose, while ordinary fruit sugar (levulose) is a ketose. 

 The difference may be shown by the structural formulae of these two 

 varieties of sugar: 



Dextrose = CH 2 (OH).CH(OH).CH(OH).CH(OH).CH(OH).CHO; 

 Levulose = CH 2 (OH) .CH (OH) .CH (OH) .CH (OH) .CO.CH 2 (OH) . 



A difference is also observed on oxidation. The aldoses can be con- 

 verted into oxyacids having the same quantity of carbon, while the ketoses 

 yield acids having less carbon. On mild oxidation the aldoses yield 

 monobasic oxyacids and dibasic acids on more energetic oxidation. Thus 

 ordinary dextrose yields gluconic acid in the first case and saccharic 

 acid in the second. 



1 Ber. d. deutsch. chem. Gesellsch., 18, 19, and 20. 



2 See E. Fischer's lecture, Synthesen in der Zuckergruppe, Ber, d. deutsch. chem. 

 Gesellsch., 23, 2114. Excellent works on carbohydrates are Tollens' Kurzes Hand- 

 buch der Kohlehydrate, Breslau, 2, 1895, and 1, 2. Auflage, 1898, which gives a 

 complete review of the literature, and E. O. v. Lippmann, Die Chernie der Zucker- 

 arten, Braunschweig, 1904. 



