CHAPTER IV. 



THE CARBOHYDRATES: THE DISACCHARIDES, POLY- 

 SACCHARIDES AND GLUCOSIDES. 



THE DISACCHARIDES. 



The disaccharides are carbohydrates which contain twelve carbon 

 atoms, and are formed from two molecules of hexose with the elimi- 

 nation of water in accordance with the equation: 



C 6 Hi 2 O 6 + C 6 H 12 O 6 = Ci 2 H 2 2On + H 2 O. 



The majority of the disaccharides reduce Fehling's solution (i. e., 

 cupric oxide in alkaline solution), react with phenylhydrazine to form 

 hydrazones and osazones, and exhibit mutarotation in solution. They 

 therefore contain a potentially aldehyde or ketone group or groups, 

 and an oxide linkage analogous to that in glucose. Certain of them 

 are exceptions to this rule, however, one of the most marked excep- 

 tions being cane-sugar, which is formed by the union of one molecule 

 of glucose with one of fructose (levulose), and which does not reduce 

 Fehling's solution nor react with phenylhydrazine, nor display muta- 

 rotation in solution. 



The disaccharides are merely special instances of a very large group 

 of compounds which are generically termed Glucosides, or compounds 

 of sugars with other bodies, the point of union being the aldehyde 

 group of the sugar. A typical glucoside, for example, is Amygdalin, 

 found in cherry-stones and in almonds, which on hydrolysis yields 

 glucose, hydrocyanic acid and benzaldehyde. The nucleic acids are 

 glucosides. Glucosides which yield galactose on hydrolysis are found 

 in the tissues of the brain. The disaccharides are glucosides in which 

 both constituents of the molecule are sugars. 



The most important disaccharides from the point of view of animal 

 biochemistry are cane-sugar or Sucrose, Maltose, Isomaltose, Lactose 

 and Isolactose. All of these excepting sucrose contain one potentially 

 active aldehyde group; that is, they reduce Fehling's solution, form 

 osazones and exhibit mutarotation. 



Cane-sugar is the ordinary sugar of commerce and occurs widely 

 distributed in the vegetable kingdom, where it acts as a reserve-material, 

 that is, as a store of nutriment to be broken up into utilizable material 

 and consumed when needed. It occurs especially in the sugar-cane, 

 in the sap of certain palms and of the sugar maple, the birch and the 

 carob tree. Ripe fruits and many leaves contain considerable amounts 



