34 PHYSIOLOGICAL CHEMISTRY 



to four different chemical groupings. Each of these four car- 

 bon atoms is thus asymmetric, and confers the property of opti- 

 cal activity upon the compound. Around each asymmetric 

 atom either of two groupings may exist, one of which rotates 

 the plane of polarized light to the right (dextro-rotatory), the 

 other an equal distance to the left (levo-rotatory). These two 

 compounds are called optical isomers. It has been found that 

 the number of optical isomers possible when a compound con- 

 tains more than one asymmetric carbon atom, is represented 

 by the value of 2 n where n is the number of such carbon atoms. 

 We will thus expect to find 2 4 =16 different sugars, all isomers 

 of glucose. Twelve of these sixteen actually have been prepared. 

 Classification of Carbohydrates. The carbohydrates are 

 divided into three great classes. 



These are subdivided as indicated below. 

 Monosaccharides 



Bioses 



Trioses 



Tetroses 



Pentoses arabinose, xylose, ribose 



Hexoses glucose, fructose, galactose, mannose 



Heptoses 



Octoses 



Nonoses 

 Disaccharides 



Saccharose (sucrose, or cane sugar) 



Maltose (malt sugar) 



Lactose (milk sugar) 

 Polysaccharides 



Dextrins 



Starches 



Glycogen 



Inulin 



Cellulose 



Gums and Mucilages. 



