THE CARBOHYDRATES AND THEIR METABOLISM 247 



Maltose reduces Fehling's solution and forms a phenyl osazona It 

 is hydrolyzed by acids forming two molecules of glucose, but is more 

 resistant to hydrolysis than sucrose. Maltose is also hydrolyzed by mal- 

 tase in the same way, but is not hydrolyzed by emulsin. Because of this 

 behavior, maltose is assumed to be a glucose-a-glucoside. 



Polysaccharides 



Those considered under this heading form colloidal solutions or are 

 insoluble in water. The more important ones are starch, glycogen, cellu- 

 lose, dextrins, inulin and gums. They are usually named from the sugar 

 they yield on hydrolysis, with the suffix "an." Thus starch is a glucosan; 

 inulin is a levulan. 



Starch is one of the polysaccharides found in plants in the form of 

 a granule with a characteristic structure, so that it is possible to identify 

 the plant from which the starch came by microscopic examination. It 

 forms the reserve food of the plant cell. It is insoluble in the ordinary 

 solvents, but if poured into boiling water the granule is disrupted and a 

 colloidal solution results. 



Upon hydrolysis with acids or enzymes, a series of simple polysaccha- 

 rides are formed, namely, soluble starch, erythrodextrin, achroodextrin, 

 and finally, maltose and glucose. It has been quite difficult to obtain any 

 knowledge of the number of hexose groups in starch and the dextrins. 



Inulin is a levulan, found in the tubers of the dahlia and Jerusalem 

 artichokes. It forms the best source of obtaining d-levulose. It is not 

 unlike starch in its chemical behavior. 



Cellulose is the main constituent of the wall of plant cells. It has a 

 more complex structure than starch. It is insoluble in all the usual sol- 

 vents, but will dissolve in ammoniacal copper salt solutions. On hydrolysis 

 with acids it yields glucose and other monosaccharides. Nitric acid with 

 cellulose forms nitrocellulose or gun cotton. Concentrated sulphuric acid 

 dissolves cellulose. Upgn diluting with water, it is again precipitated, but 

 in a different form. The resulting compound gives a blue color witli 

 iodin and is called amyloid. 



A number of cellulose-like substances, called herni-celluloses, are 

 found in seeds and young plant tissues. They probably act both as sup- 

 porting structures and as a source of reserve food. Upon acid hydrolysis 

 they yield galactose, arabinose, mannose, rhamnose and occasionally fruc- 

 tose. 



Gums are usually pentosans. They are white substances which dis- 

 solve in water, giving a thick, viscid, mucillaginous solution. Examples 

 are gum acacia (or arable) and gum tragacanth. l>>n hydrolysis they 

 yield pentoses or their derivatives, such as arabjnosc and rhamnose. Oc- 

 casionally hexoses also result from hydrolysis of some gums, such as man- 



