THE CARBOHYDRATES 185 



Tetroses. * 



The tetroses contain 4 atoms of carbon. An aldose or a ketose 

 are theoretically possible and several stereoisomers, e.g. the aldotetroses, 

 d- and /-erythreose, threose. 



Pentoses. 



Five carbon atoms are present in the molecule of a pentose, 

 and isomers, an aldose and 2 ketoses, are possible. Three asym- 

 metric carbon atoms are present : 2 3 or 8 stereoisomeric aldoses can 

 exist. All but one are known, but all do not occur in nature. 



df-ribose is contained in the nucleic acid of plants from which it is 

 obtained by hydrolysis. 



/-arabinose is contained in the polysaccharides cherry gum, gum 

 arabic, peach gum. It is obtained by the hydrolysis of these substances 

 with dilute sulphuric acid. Arabinose crystallises in prisms, has a 

 sweet taste, is dextrorotatory, though termed /-arabinose on account of 

 its stereochemical relation to glucose ; it melts at 1 60. 



Xylose is obtained by the hydrolysis of wood gum or xylane, straw, 

 and various forms of cellulose. .It is optically inactive and melts at 

 144-145. 



A pentose, as yet not definitely identified, but probably arabinose 

 or ribose, is excreted in the urine in certain diseases. 



Methyl Pentoses. 



Rhamnose, C 6 H 12 O 5 , is obtained by the hydrolysis of the glucosides, 

 quercitrin, xanthorhamnin, and some saponins. Rhamnose crystallises 

 with a molecule of water, C 6 H 14 O 6 , and was formerly regarded as a 

 hexahydric alcohol isodulcitol. It melts at 93. 



Fucose in seaweed, chinovose in chinovin, and other methyl pen- 

 toses have also been prepared. 



Hexoses. 



The hexoses contain 6 atoms of carbon ; two isomeric ketoses and 

 one aldose are possible. Four asymmetric carbon atoms are present 

 in the molecule of an aldohexose ; 2 4 or 1 6 stereoisomers are possible 

 but only three are found in nature. Most of the other stereoisomers 

 have been prepared in the laboratory by Emil Fischer. Two 

 stereoisomeric ketoses are also found in nature. The formulae of the 

 natural hexoses are : 



