CH. V] CARBOHYDRATES 43 



The most commonly occurring sugars in plants are glucose, laevulose 

 sucrose and maltose : sucrose is hydrolyzed by the enzyme, invertase, 

 into one molecule of glucose and one molecule of laevulose : maltose by 

 the enzyme, maltase, into two molecules of glucose. Both invertase and 

 maltase are widely distributed. The connexion between various sugars 

 and photosynthesis, and their inter-relationships with each other in the 

 leaves, are reserved for another section. 



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-relationships will be considered. 



Monosaccharides. 



These are termed tetroses, pentoses or hexoses according to the number 

 of carbon atoms in the molecule. They contain primary (— CH2OH) or 

 secondary (= CHOH) alcohol groups, and either an aldehyde (- CHO) 

 group, as in glucose, or a ketone (= C = 0) 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 themselves 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 charac- 

 teristic crystalline forms, constitute, in several cases, valuable tests for the 

 presence of sugars. 



A reaction exhibited by many of the monosaccharides is that of 

 forming a coloured product when heated with a phenol in presence of a 



