60 PHYSIOLOGY 



CH 2 OH 



COH CO 



(CHOH) 4 or the ketone (CHOH) 3 . These two oxidation products of the 



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CH 2 OH CH 2 OH 



polyatomic alcohols are known as aldoses and ketoses respectively. All these 

 compounds are distinguished by the termination ' ose.' It is convenient 

 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 



CH 2 OH 



atoms contained in the sugar molecule, e.g. the aldose (CHOH) 4 , four 



COH 



are asymmetric, i.e. their four combining affinities are saturated with groups 

 of different kinds, viz. several carbon atoms, one H atom, and one OH 

 group : c 



H C OH 



They must therefore present many stereoisomeric forms. If n represent 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 fourteen have been found or have been artificially prepared. 

 Only a small number are, however, of any physiological importance. These 

 include the aldoses, glucose, mannose, and galactose, and the ketose, 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 represent them sis 

 d-, 1-, and i- varieties respectively, i.e. dextro-rotatory, Isevo-rotatory, and 

 inactive. On Fischer's suggestion, however, this mode of nomenclature has 

 been altered in favour of representing, by the letter prefixed, not the optical 

 qualities of the substance in question, but its relation to other substances, 

 especially glucose. Thus, d-fructose means that fructose is the ketose corre- 

 sponding to the dextro-rotatory glucose, d-fructose itself being lee vo- rotatory, 

 though its active asymmetric C atoms are identically arranged with those in 



