THE CARBOHYDRATES 65 



CH 2 OH 



CO 

 the ketone (CHOH) 3 . These two oxidation products of the polyatomic 



CH 2 OH 



alcohols are known as aldoses and ketoses respectively. All these 

 compounds are distinguished by the termination ' ose.' It is con- 

 venient to call those compounds containing six carbon atoms the 

 sugars, because it is to this group that the natural sugars belong. 



Stereoisomerism in the Sugars. It will be noticed that of the six 

 carbon atoms contained in the sugar molecule, e.g. the aldose 

 CH 2 OH 



(CHOH) 4 , four are asymmetric, i.e. their four combining affinities are 



COH 



saturated with groups of different kinds, viz. several carbon atoms, 

 one H atom, and one OH group : 



C 



H- C OH 



C 



They must therefore present many stereoisomeric forms. If n repre- 

 sent the number of asymmetric carbon atoms in a compound, the 

 possible number of stereoisomers is 2 n . Thus an aldehexose with four 

 asymmetric carbon atoms (CHOH) 4 must present 2 4 isomers, i.e. sixteen 

 isomeric compounds, so that there must be sixteen sugars all possessing 

 the formula CH 2 OH(CHOH) 4 COH, in addition to the inactive sugars 

 obtained by a mixture of two oppositely active members of this group. 

 Of the sixteen possible sugars of this formula, as many as twelve 

 have been found or have been artificially prepared. Only a small 

 number are, however, of any physiological importance. These in- 

 clude the aldoses, glucose, mannose, and galactose, and the ketrse, 

 fructose or levulose. All the other sugars are unassimilable by the 

 animal cell and are not manufactured by plants. 



Since these sugars can be divided into the optically active and the 

 inactive varieties, an obvious mode of designation would be to repre- 

 sent them as d-, 1-, and i- varieties respectively, i.e. dextro-rotatory, 

 laevo-rotatory, and inactive. On Fischer's suggestion, however, this 

 mode of nomenclature has been altered in favour of representing, by the 



5 



