42 CARBOHYDRATES AND THEIR [CH. 



Of the polysaccharides, cellulose is universally distributed in higher 

 plants and constitutes the greater part of the cell-walls. The pentosans, 

 galactans and mannans also, but to a lesser degree, are components of 

 their structure. Starch, in addition, is very widely distributed : it is 

 converted by the enzyme, diastase, into dextrin and maltose, and possibly 

 the same enzyme also controls its synthesis. In some plants no starch 

 is formed, and its place in metabolism is taken by inulin or cane-sugar. 



The various carbohydrates will first be dealt with in detail, and later 

 their inter-relatioriships will be considered. 



MONOSACCHARIDES. 



These are termed tetroses, pentoses or hexoses according to the 

 number of carbon atoms in the molecule. They contain primary 

 ( CH 2 OH) or secondary (= CHOH) alcohol groups, and either an 

 aldehyde ( CHO) group, as in glucose, or a ketone (= C = O) group, as 

 in laevulose. They are, as a class, white crystalline substances, soluble 

 in water and aqueous alcohol, but insoluble in ether, acetone and many 

 other organic solvents. They are capable of certain characteristic 

 chemical reactions which form a basis for their detection and estimation. 

 One of the most important is that connected with the aldehyde and 

 ketone groups, owing to which they act as reducing agents, being them- 

 selves oxidized. The reducing action usually employed is that which 

 takes place with copper salts in hot alkaline solution, whereby cuprous 

 oxide is formed. Hence they are termed "reducing" sugars. Another 

 important reaction is the formation of crystalline osazones (only in the 

 case of sugars with aldehyde or ketone groups), which, by virtue of 

 their melting points and characteristic crystalline forms, constitute, in 

 several cases, valuable tests for the presence of sugars. 



PENTOSES. 



These sugars contain five carbon atoms, and have the general formula 

 C 5 H 10 O 5 . They are said to be present in the free state to some extent 

 in leaves (Davis and Sawyer, 10). In plants they occur chiefly, however, 

 as condensation products formed with elimination of water. These 

 products are termed the pentosans, and are widely distributed ; on 

 hydrolysis they yield pentoses again. The various gums found in plants 

 consist largely of pentosans, and the pectins also contain pentose groups; 

 both consequently yield pentoses on hydrolysis (see pp. 61 and 64). 



