COxAIPOUND CARBOHYDRATES 381 



of the mucilages is almost totally lacking; present indications are that they 

 are fundamentally similar to the gums. 



The glycosides are compounds formed by the reaction between a sugar 

 (most commonly glucose) and one or more compounds which are non-sugars. 

 All glycosides may exist in two forms, a or /3, but most of them which occur 

 in plants are of the (i type. Although of widespread occurrence in plants 

 the glycosides are never present in large quantities. They may be found in 

 almost any part of the plant. In a pure state they are mostly levorotatory, 

 cri'Stalline, colorless, bitter, and soluble in either water or alcohol. All (i- 

 glycosides can be hydrolyzed by the enzyme emulsin (Chap. XXVII) or by 

 dilute mineral acids. Several hundred different glycosides have been isolated 

 from plant tissues. Their role in the metabolism of plants, if any, is obscure 

 although it is possible that they may serve in a minor way as storage foods. 



Several representative glycosides will be discussed briefly in order to 

 indicate more clearly the general nature of these compounds. 



Salicin is found in the bark and leaves of the willow tree. Upon hydrol- 

 ysis with emulsin it yields glucose and the alcohol saligenol according to the 

 following equation: 



C13H1SO7 + H2O -^^^^> CeH^ ' + CeHioOe 



Salicin NDH Glucose 



Saligenol 



Amygdalin has probably been studied more than any other glycoside. It 

 occurs in the seeds of apples, peaches, and plums, as well as in the leaves of 

 cherries and peaches. Upon hydrolysis with emulsin it produces glucose, 

 hydrocyanic acid, and benzaldehyde : 



C20H27NO11 + 2H20-^^^2C6Hi206 + HCN -f CeHsCHO 



Amygdalin Glucose Hydrocyanic Benzaldehyde 



acid 



Domestic animals are sometimes poisoned from eating plant material such 

 as cherry leaves which contain this glycoside. The toxic action of amygdalin 

 is due to the liberation of hydrocyanic acid upon hydrolysis. The enzyme 

 (emulsin) and the glycoside apparently are kept apart in the cells since 

 usually rapid hydroljsis of amygdalin does not occur unless the cells are 

 crushed in some way. 



Sifiigrin is called the mustard oil glycoside. It is found in the black 

 mustard {Brassica nigra), and is hydrolyzed as follows: 



C10H16O9NS2K + H2O ^^^ C3H5CNS + CbHioOg + KHSO4 



Sinigrin Allyl isothio- Glucose Potassium 



cyanate ("Mus- hydrogen 



tard oil") sulfate 



