THE CARBOHYDRATES 131 



4. They increase the osmotic concentration of the cell sap and aid 

 in maintaining the turgidity of the plant. The soluble sugars thus 

 aid in regulating the amount of water which enters or leaves a cell. 



5. They aid in the formation of pigments. The red and blue 

 coloring matters seem to depend upon the presence of carbohy- 

 drates for their formation, but this will be discussed further in the 

 chapter on pigments. 



Classification of Carbohydrates. — The carbohydrates are di- 

 vided into two groups, — the sugars and the nonsugars. The for- 

 mer have a sweetish taste, which varies in degree according to the 

 sugar. They are given names ending in ose, and wherever in organic 

 chemistry such a name is found, one can be sure he is dealing with 

 a carbohydrate. 



I. Sugars. II. Nonsugars — polysaccharides. 



1. monosaccharides. 1. food materials — hexosans. 



a. pentoses. 2. structure materials. 



b. hexoses. a. gums. 



2. disaccharides. 1. natural gums and 



3. trisaccharides. pentosans. 



4. tetrasaccharides. 2. mucilages and 



pectins. 

 b. celluloses. 



Sugars. — Among the best known hexoses of plants are fructose, 

 glucose, mannose, galactose, and sorbose, all of which have the 

 formula C 6 Hi 2 6 . Many others have been prepared with the same 

 formula and still others are theoretically possible. How can one 

 have several compounds with the same formula? Different sub- 

 stances having the same empirical formula or ratio of different 

 atoms in the molecule are called isomers, and this property of form- 

 ing isomers is one of the striking characteristics of the sugars. These 

 isomers are of two different types, — structural and stereoisomers. 



Structural isomers may be illustrated by the formulae commonly 

 given for glucose and fructose : 



Glucose Fructose 



CH 2 OH CH 2 OH 



I I 



CHOH CHOH 



I I 



CHOH CHOH 



CHOH CHOH 



I I 



CHOH C = 



I I 



= CH CH 2 OH 



