CARBOHYDRATES 69 



cellulose supplies needed bulk to the digestive tract and aids in tlie 

 elimination of food residues. 



In woody tissues cellulose is associated with another material of 

 great molecular size called lignin. Dried wood is about 65 per cent cel- 

 lulose, 30 per cent lignin, and 1 to 2 per cent pentosans. In the manu- 

 facture of paper, wood is cooked with water, lime, and sulfur dioxide 

 to remove the lignin, leaving the cellulose. These fibers are then 

 spread into mats and rolled into sheets which may be treated in various 

 ways for waterproofing or coated to provide a smooth writing surface. 

 When certain grades of heavy paper are treated with sulfuric acid, 

 translucent parchment paper is obtained. 



Cellulose is insoluble in most solvents but does dissolve in solutions 

 of cupric ammoniimi hydroxide or zinc chloride. Cotton treated with 

 alkali becomes somewhat translucent, acquires a silky luster, and is 

 called mercerized cotton. Rayon is made from cellulose by dissolving 

 the material in sodium hydroxide and carbon disidfide. This viscous 

 material is forced through small jets into an acidic solution of sodium 

 bisulfite, regenerating the cellulose in small threads. 



Esters of cellulose and nitric acid form in the presence of sulfuric 

 acid. These nitrates are used in explosives, lacquers, celluloid, and 

 collodion. Much of the former market for cellulose nitrates has been 

 captured by cellulose acetate, much less flammable and widely used 

 in lacquers, photographic film, and packaging materials. Celluloid 

 has been replaced by various newer plastics which possess a greater 

 diversity of properties and do not burn so readily. 



Another group of plant materials called hemicelluloses is somewhat 

 like ordinary cellulose. These substances may serve a structinal func- 

 tion in some species and also be available as reserve carbohydrates for 

 metabolic needs. Hydrolysis yields glucose, galactose, mannose, and 

 glucuronic acid, although all may not and at least in some cases do not 

 occur in the same hemicellulose. Little is known of linkages between 

 the monosaccharide units. 



Inulin 



This polysaccharide serves as the carbohydrate reserve in a number 

 of plants, dahlia and artichoke for example. Inidin forms a colloidal 

 suspension in hot water, is non-reducing, and consists entirely of 

 D-fructose units in the furanose form. It might be inferred, then, that 

 the linkages probably are 2-6. Certain bacteria produce polysac- 

 charides based on D-fructose, although the products apparently are 

 not identical to inulin. 



