THE CARBOHYDRATES 13'/ 



large amounts during the germination of barley and other grains. 

 The action may be represented as follows: 



(C 6 Hio0 5 )n+H 2 = Ci 2 H 22 0n+ (C 6 Hi O 5 )n— ,. 



Maltose is white, soluble in water and alcohol, dextrorotary, and 

 crystallizes in the form of slender needles. When hydrolyzed it 

 yields two molecules of glucose. 



Isomaltose is thought to be an isomer of maltose, but its proper- 

 ties have not been well determined. Cellobiose is a disaccharide 

 formed in the hydrolysis of cellulose. Gentiobiose results from 

 the hydrolysis of the trisaccharide, gentianose, which is found 

 in the yellow gentian (G. lutea). It is also the sugar found in the 

 glucoside amygdalin (Chap. XV). 



Lactose or milk sugar is found only in animals. The correspond- 

 ing plant sugar is melibiose, which is produced by hydrolyzing 

 raffinose. Both these sugars yield glucose and galactose when 

 hydrolyzed. 



Nonreducing Sugars. — Sucrose, which is also called cane sugar, 

 beet sugar, and saccharose, is the most common of all the sugars. 

 It is the ordinary sugar of commerce and is found especially in 

 quantities which can be commercially exploited in maple sap, 

 cane and sorghum juice, and the root of the sugar beet. Sugar 

 beets contain 15-20% of sugar, and individual beets have been 

 known to contain 25-30%. The only common sugar which is 

 sweeter is fructose. Sucrose is formed from one molecule of glucose 

 and one of levulose, and when hydrolyzed breaks down into these 

 two sugars. As previously mentioned, it is dextrorotary, but the 

 resulting mixture of levulose and dextrose is levorotary and is 

 hence known as invert sugar. Sucrose is the sugar commonly 

 found in nectaries, but the bees change it to dextrose and levulose, 

 probably by the means of formic acid or secreted enzymes, with 

 the result that honey contains monosaccharides but little disac- 

 charide. 



Trehalose, which is dextrorotary and nonreducing, seems to serve 

 the same function in plants lacking chlorophyll, e. g., the fungi, 

 that sucrose does in the autotrophic plants. It resembles sucrose 

 in that it is not directly fermentable by yeast, but differs from 

 it in that it yields only glucose when hydrolyzed. 



Trisaccharides and Tetrasaccharides— As the name indicates, 

 the trisaccharides (formula Ci 8 H 32 0i 6 ) are built up of three mole- 



