CLASSIFICATION TJ 



significance of the various types of carbohydrates in the plant, 

 these substances are all closely related chemically, being 

 composed of the same elements — carbon, hydrogen, and 

 oxygen — united together in a similar fashion. 



The term carbohydrate originated through the erroneous 

 conception that these substances were compounds of carbon 

 with water, since the proportion of hydrogen to oxygen in 

 most of them is the same as in water, as may be seen from 

 the formula for grape sugar, which is CgHioOe, but which 

 might be written Cg . 6H2O. 



The discovery of methyl pentoses of the formula CgHiaOj 

 shows, however, that the maintenance of this proportion of 

 hydrogen to oxygen is not an essential characteristic of this 

 group of compounds. 



CLASSIFICATION OF CARBOHYDRATES. 



On purely physical grounds such as appearance, solubility 

 in water, taste, etc., the carbohydrates may be roughly divided 

 into sugars and non-sugars ; the systematic classification of 

 the carbohydrates is, however, based upon their behaviour 

 towards hydrolytic agents, such as mineral acids or enzymes. 

 Thus there are a considerable number of naturally occurring 

 sugars containing five and six carbon atoms which cannot be 

 hydrolysed ; such sugars form a group known as monosacchar- 

 ides. On the other hand, many sugars are known which on 

 hydrolysis break up into two molecules of monosaccharide 

 according to the equation — 



C12H22O11 + H3O - 2CeHi20g 



Such sugars are known as disaccharides. 



Similarly sugars which on hydrolysis give three molecules 

 of monosaccharide as follows — 



C18H32O18 + 2HjO = 3C«H,20s 



are termed trisaccharides. 



Finally, carbohydrates, such as starch and cellulose, which 

 on hydrolysis yield an unknown number of molecules of mono- 

 saccharides are classed as polysaccharides. 



The nomenclature of the monosaccharides is based on 



