CARBOHYDRATES 51 



A third, and still more complex, type of polysaccharide is made up 

 of several different kinds of simple sugar units, which may be arranged 

 either in a single chain or in u branched structure. These are called 

 heteropolysacchaiides, whereas those containing only one sugar are 

 classed as homopoly saccharides. The chemical formulas of homopoly- 

 saccharides are often written in a still more abbreviated form than that 

 of amylose given above. Since one molecule of water is taken away 

 when each glycoside linkage is formed, most of the simple sugar units 

 in the polysaccharide structure (in fact all except those at chain ends) 

 must have the composition of the simple sugar concerned, less one 

 oxygen and two hydrogen atoms. Thus the formula of a pentosan 

 (polysaccharide made up of pentose units) may be written as (C5H804)a, 

 and that of a hexosan as (CeHioOy)^;. These formulas are commonly 

 used because they are compact and easy to write, but they are not 

 precisely correct. 



Another important feature of polysaccharide structure is the glycosidic 

 linkage between the monosaccharide residues. This linkage always 

 extends from the reducing group of one simple sugar unit to one of the 

 other carbons of the next unit. This second unit is attached through 

 its reducing group to a third, and so on. Thus no uncombined reducing 

 groups are present in the polysaccharide molecule except the one at the 

 end of the chain (see formula for amylose above). Even branched poly- 

 saccharides like glycogen have only one reducing group per molecule. 

 Consequently, polysaccharides as a rule have practically no reducing 

 power. 



As a class the polysaccharides are noncrystalline, white solids, which 

 are insoluble or only slightly soluble in water. Probably as a result 

 of this limited solubility they have no appreciable sweetening power. 

 On boiling with dilute solutions of strong acids they are all hydrolyzed, 

 although at greatly differing rates, into the component monosaccharides. 



Pentosans iC5H804)j. 



Polysaccharides giving D-xylose or L-arabinose on hydrolysis, that is, 

 the pentosans, are very common in nature, especially in the plant kingdom. 

 Most of them, however, arc not comjiosed exclusively of pentose residues, 

 but also contain various hexoses, or hexuronic acids, or both, atid thus 

 belong to the mixed type of polysacciiaridcs. 



The total amount of pentosans contained in various plants is shown 

 in Table 3-5. It will be noted that the largest percentages are found 

 in two main types of plant materials, the plant gums, and the woody 

 or fibrous tissues. Xylan occurs chiefly in wood, straw, leaves, seeds, 

 and vegetables, whereas araban is commonly found in gums and mu- 

 cilaginous materials. Xylan is frequently associated with glucose in 



