374 CARBOHYDRATE METABOLISM 



rived from a gamma fructose molecule, the left hand portion from an 

 ^/-glucose molecule. 



Sucrose is dextrorotatory, its specific rotatory power being + 66.5°. 

 When hydrolyzed by acids or the enzyme sucrase the resulting mixture, the 

 so-called "invert sugar", is levorotatory. Although glucose and fructose are 

 present in the resulting mixture in equimolar quantities, the latter is more 

 strongly levorotatory than the former is dextrorotatory. 



Maltose is widely distributed in plants, but is seldom present in more 

 than small amounts. It is produced by the condensation of two molecules of 

 glucose, apparently under the influence of the enzyme maltase, as follows: 



Wl 3 lt_3.^6 



CeHiaOo + CeHi.Oe > CioHsoOii + H2O 



Glucose Glucose Maltose 



Maltase also accomplishes the reverse reaction (digestion) of maltose. This 

 sugar is an intermediate product in the synthesis and digestion of starch as 

 described later. Maltose is a reducing sugar and is strongly dextrorotatory 

 (specific rotatory power + 137°). 



Cellobiose is a reducing dissacharide which is produced from cellulose by 

 the action of the enzyme cellulase. Upon hydrolj'sis it yields two molecules 

 of glucose. Cellobiose differs from maltose in that it is formed by the con- 

 densation of two molecules of /3 ^-glucose, while a molecule of maltose results 

 from the condensation of two molecules of a ^-glucose. 



Other disaccharides found in plants are listed in Table 35. 



Tri- and Tetrasaccharides. — Trisaccharides are sugars with the molec- 

 ular formula C18H32O16. Raffinose is a non-reducing trisaccharide present 

 in small quantities in many of the higher plants and in fungi. It is found in 

 appreciable quantities in cotton seeds and sugar beets. Upon complete hydrol- 

 ysis it yields one molecule each of galactose, glucose, and fructose. Partial 

 hj'drolysis may yield either fructose or melibiose, or galactose and sucrose 

 depending upon the catalyst used. 



The trisaccharide gentianose has been found in the roots of the yellow 

 gentian (Gentiana lutea). Upon partial hydrolysis it yields one molecule 

 each of fructose and gentiobiose; upon complete hydrolysis one molecule of 

 fructose and two of glucose. Melezitose is a very sweet trisaccharide which 

 has been found in the sap of several conifers (European larch, Douglas fir. 

 Scrub pine). The drops of sweet sap ("honey dew") exuded by such species 

 after attacks of sucking insects are often rich in this carbohydrate. Upon 

 complete hydrolysis this sugar yields three molecules of glucose. 



Tetrasaccharides are represented by the molecular formula C24H42O21. 

 An example is stachyose which has been isolated from the roots of the hedge 



