CARBOHYDRATES 35 



Similarly, for a ketohexose, which contains three asymmetric 

 carbon atoms, there are eight possible arrangements. Three 

 sugars of this type are known, only one (fructose) being common in 

 plants; the others are of only theoretical interest. 



CHEMICAL CONSTITUTION OF MONOSACCHARIDES 



The term " monosaccharides," as commonly used, refers to 

 hexoses. It applies equally well, however, to any other sugar-like 

 substance which either occurs naturally or results from the decom- 

 position of more complex carbohydrates, and which cannot be 

 further broken down without destroying its characteristic alde- 

 hyde-alcohol groups and sugar-like properties. 



All such monosaccharides, being alcohol-aldehydes, can easily 

 be reduced to the corresponding polyatomic alcohols, containing 

 the same number of carbon atoms as the original monosaccharides, 

 each with one OH group attached to it. All aldose monosac- 

 charides are converted, by gentle oxidation, into the corresponding 

 monobasic acid, having a COOH group in the place of the original 

 CHO group. Further ' oxidation either changes the alcoholic 

 groups into COOH groups, producing polybasic acids, or breaks 

 up the chain. When ketose monosaccharides are submitted to 

 similar oxidation processes, they are broken down into shorter 

 chain compounds. 



The various monosaccharides which have thus far been found 

 as constituents of plant tissues, or as parts of other more complex 

 compounds which occur in plants, are shown in the following table: 



Trioses (C 3 H 6 3 ) Tetroses (C 4 H 8 O 4 ) 



Aldose Glyceric aldehyde, Aldoses d- and Z-Erythrose, 



or glycerose Z-Threose 



Ketose Dioxyacetone 



Pentoses (C 5 Hi 5 ) Methyl Pentoses (C 6 Hi 2 5 ) 



Aldoses d- and Z-Arabinose Aldoses Rhamnose 

 d- and Z-Xylose Fucose 



/-Ribose Rhodeose 



Z-Lyxose Chinovose 



