THE CARBOHYDRATES 



197 



THE DISACCHARIDES. 



The disaccharides consist of two units of monosaccharide combined 

 together with loss of water. Theoretically any two monosaccharides 

 can be thus combined, but actually only a few disaccharides are known. 

 Most of these are natural compounds, but some have been obtained by 

 synthesis. Their composition is shown by hydrolysis by acids or 

 enzymes when they are converted into their constituents, e.g. in the 

 case of a bihexose : 



C 12 H 22 U + H 2 = C 6 H 12 6 + C 6 H 12 6 . 



The natural disaccharides are of two kinds, those which reduce 

 Fehling's solution and those which do not, and they are usually class- 

 ified accordingly : 



Non-Reducing. 



Reducing. 

 Lactose fGlucose 



or 

 Milk Sugar [Galactose 



TI/T i'i (Glucose 

 Mehbiose \ ~ . 



{ Galactose 



,-. ,, fGlucose 



Maltose 



[Glucose 



~ ... fGlucose 

 Gentiobu>se j Glucose 



CeHobiose 



Turanose 



Isolacose{gS se 



Isomaltose{ - r 



Glucose 

 Arabinose 

 Glucose 

 Xylose 



Galacto- f Galactose 

 arabinose 1 Arabinose 



Vicianose 



Gluco- 

 xylose 



It is not yet definitely known how the two units are combined to- 

 gether. In the non-reducing members the two functional aldehyde or 

 ketone groupings will be, combined ; in the reducing members the 

 aldehyde or ketone grouping of the one unit will be united to one of 

 the hydroxyl groups of the other unit. The following are the prob- 

 able formula for 



Cane o ^ 



CH QH fructose residue 



GH 2 OH (CH.QHV CH CH 2 



CH (GHO.H) C 



GKQH CHgOH gbieo.se residue 



Trehalose 



CH 2 OH CHOH CH (CHOH) 2 uu.^ 

 CHOH CH (CHOHJ2 CH X 



