66 



CARBOHYDRATES 



or alkali. Since no body enzymes can digest alginic acid or pectin, they 

 have no food value. 



HETEROPOLYSACCHARIDES 



Most of the carbohydrates in this group are too complex and too im- 

 perfectly known to be included in an elementary book. Examples of 

 several types have been given in connection with the classification of 

 carbohydrates (p. 21), and several others have been mentioned briefly 

 in the sections on wood (p. 61), pentosans (p. 52), and galactans (p. 

 63). A few heteropolysaccharides of special importance are discussed 

 in more detail below. 



Heteropolysaccharides from plants 



The hemicelluloses are one of the most important subgroups of this 

 large, rather poorly-defined, class of carbohydrates. As indicated on 

 p. 61, they are present in fibrous and woody plant tissues, where they 

 are combined with cellulose and lignin to form the cell walls. The hemi- 

 celluloses are distinguished from cellulose by the facts that they are 

 acidic substances and are made up quite largely of D-xylose units, although 

 other sugars (D-galactose, L-arabinose, D-glucose, D-mannose) may also 

 be present in smaller amounts. Their acidic properties arise from the 

 presence of a hexuronic acid, probably D-glucuronic acid, which is also 

 one of the component units. Ordinary wood pulp contains considerable 

 amounts of hemicelluloses. 



The plant gums such as cherry gum, mesquite gum, gum arabic, and 

 gum tragacanth are neutral salts of complex polysaccharide acids com- 

 posed of residues of hexoses, pentoses, methyl pentoses and uronic acids. 

 The uronic acid in nearly all plant gums is D-glucuronic acid, and the 

 sugars commonly present include D-galactose, D-mannose, L-arabinose, 

 D-xylose, L-rhamnose, and L-fucose. The complete structures have not 

 been worked out. Plants produce such gums when they are injured, no 

 doubt as a protective mechanism. 



Another group of mixed-type polysaccharides, widely distributed in 

 plants, form viscous, colloidal solutions in water and hence are called 

 mucilages. These are roughly divided into neutral, acidic, and sulfate- 

 containing groups. An example of a neutral mucilage is gum ghatti, 

 which on hydrolysis gives rise to 16 per cent of D-galactose and 84 per cent 

 of D-mannose. The majority of the mucilages in seeds are of the acidic 

 type, with the acidity being due in all cases to D-galacturonic acid resi- 

 dues. Sea weeds contain a large number of mucilages, nearly all of 

 which contain sulfate groups {i.e., some of the hydroxyl groups of the 

 sugar residues present are esterified with sulfuric acid). D-Galactose 



