52 CHEMISTRY OF PLANT LIFE 



Isomaltose is a synthetic sugar, obtained by Fischer, by con- 

 densing two molecules of glucose. Its properties are quite similar 

 to those of maltose, but it yields a slightly different osazone and is 

 not fermentable by yeast. These differences are explained by the 

 assumption that this sugar is a glucose-/3-glucoside, while normal 

 maltose is a glucose-a-glucoside. 



Gentiobiose is a disaccharide which results from the partial 

 hydrolysis of the trisaccharide gentianose (see page 53). It is 

 very similar in its general properties to isomaltose. Cellobiose is a 

 disaccharide which results from the hydrolysis of cellulose. It is a 

 reducing sugar, forms an osazone, and resembles maltose. 



Maltose, isomaltose, gentiobiose, and cellobiose, are all glu- 

 cose-glucosides, the difference between them being undoubtedly 

 due to linkage being between different alcoholic groups in the glu- 

 cose molecules. 



The disaccharide lactose is a glucose-galactoside. It is the 

 sugar which is present in the milk of all mammals. It has never 

 been found in plants. Melibiose, which is the corresponding vege- 

 table glucose-galactoside, may be obtained by the partial hydrolysis 

 of the trisaccharide raffinose (see below). It is a reducing sugar; 

 forms a characteristic osazone ; and exhibits mutarotation. It is 

 not fermented by ordinary top-yeasts, but is first hydrolyzed and 

 then fermented by the enzymes present in bottom-yeasts. 



TRISACCHARIDES 



Trisaccharides, as the name indicates, consist of three hexoses 

 (or monosaccharides) linked together by the dropping out of two 

 molecules of water. Their formula is CisH^OiB. When com- 

 pletely hydrolyzed, they yield three molecules of monosaccharides; 

 when partially hydrolyzed, one each of a disaccharide and a mono- 

 saccharide. 



One trisaccharide of the reducing sugar type, namely rhamnose, 

 exists in plants as a constituent of the glucoside xanthorhamnin. 

 It is composed of one molecule of glucose united to two molecules 

 of rhamnose (methyl pentose, CeH^Os). It is of interest only in 

 connection with the properties of the glucoside in which it is present 

 (see page 84). 



Three trisaccharides whicn are non-reducing sugars are found 

 in plants; namely, raffinose, gentianose, and melizitose. 



