CH. V.] 



MONOSACCHARIDES. 



101 



The simple sugars contain from two to nine carbon 

 atoms, and are called biases, trioses, tetroses, pentoses, 

 hexoses, etc., depending on the number of carbon atoms 

 they contain.* 



The pentoses, C 5 H 10 O 5 , are widely distributed in nature. 

 In plants they are found both in the free state and in the 

 form of condensation products, known as pentosans 

 (C 5 H 8 O 4 ) n . The most important of the pentoses are the 

 aldoses, arabinose and xylose, best obtained from the 

 corresponding pentosans in gum arabic and beech sawdust 

 respectively. 



Ribose is a constituent part of the molecule of yeast 

 nucleic acid. 



Pentoses are occasionally found in human urine (see 

 P- 312). 



The hexoses, C 6 H 12 O 6 , are of great physiological import- 

 ance. Of the many that have been synthesised in the 

 laboratory only the following are found in nature, and are 

 of physiological interest : 



* This is not strictly true, for there exist substituted sugars in which one 

 or more H atom is replaced by a methyl group. A methyl pentose thus con- 

 tains six carbon atoms. 



