THE CARBOHYDRATES 135 



Rhamnose (CH3C5H9O5) is a pentose in which one of the hydrogen 

 atoms has been replaced by a methyl group. 



Hexoses. — The hexoses contain six carbon atoms and have the 

 formula C 6 Hi 2 6 . Unlike the pentoses, all except galactose can 

 readily be fermented by the enzymes of yeast, which break them 

 down to alcohol and carbon dioxide according to the equation 



C 6 H 12 06 = 2C2H 5 OH+C02 



(Chap. XVI and XXV). Glucose and fructose are the only hexoses 

 found as such in plants, but mannose, sorbose, and galactose 

 occur in various complexes. 



Since the common form of glucose is dextrorotary, it is also 

 called dextrose. Grape sugar is still another name so that these 

 three different terms are names for the same sugar. It is found 

 in juices of ripening fruits and in honey, and is widely distributed 

 throughout plants in roots, stems, and leaves. It is also produced 

 by the hydrolysis of cane sugar, maltose, lactose, and cellulose, 

 as well as from many other di- and poly-saccharides, all of which 

 yield some glucose. 



Dextrose is a white crystalline substance readily soluble in 

 water and is doubtless one of the first products of photosynthesis, 

 as discussed in Chapter VI. It is produced for the market from 

 hydrolysis of cornstarch, and is the chief constituent of "corn 

 syrup." In the U. S. A., 100-200,000 tons of dextrose are manu- 

 factured annually from corn. It is only about 60% as sweet as 

 cane sugar, which, although soluble, must be broken down in the 

 body to glucose before it can be utilized. For this reason patients 

 with weak digestive powers are given glucose instead of sucrose, 

 and in some sanitaria it is the form of sugar ordinarily served. 

 It is also possible to buy candy and other sweets which contain 

 glucose instead of sucrose. 



Fructose, the other hexose found as such in plants, is also widely 

 distributed in honey, fruits, and other plant organs. It rotates 

 the beam of polarized light to the left and is hence called levulose. 

 Another common name is fruit sugar. It is generally found along 

 with dextrose, and both result in equal amounts from the hy- 

 drolysis of sucrose. Levulose turns the beam of light to the left 

 (93° in a 10% solution at 20° C.) more than dextrose turns it to the 

 right (52.5°). Since the original sucrose is dextrorotary (66.5°) 

 and the mixture of glucose and fructose which results is levo- 



