CH. XXV.] CARBOHYDRATES 387 



ever, found in or formed by the animal organism. The most important 

 of these are glycogen, or animal starch ; dextrose ; and lactose, or milk 

 sugar. 



The carbohydrates may be conveniently denned as compounds of 

 carbon, hydrogen, and oxygen, the two last named elements being in 

 the proportion in which they occur in water. But this definition is 

 only a rough one, and if pushed too far would include many substances 

 like acetic acid, lactic acid, an$ inosite, which are not carbohydrates. 

 Kesearch has shown that the chemical constitution of the simplest 

 carbohydrates is that of an aldehyde, or a ketone, and that the more 

 complex carbohydrates are condensation products of the simple ones. 

 In order, therefore, that we may understand the constitution of these 

 substances, it is first necessary that we should understand what is 

 meant by the terms aldehyde and ketone. 



A primary alcohol is one in which the hydroxyl (OH) is attached 

 to the last carbon atom of the chain ; its end group is CH 2 OH. Thus 

 the formula for common alcohol (primary ethyl alcohol) is 



CH 3 .CH 2 OH. 



The formula for the next alcohol of the same series (primary 

 propyl alcohol) is 



CH 3 . CH 2 . CH 2 OH. 



If a primary alcohol is oxidised, the first oxidation product is 

 called an aldehyde ; thus ethyl alcohol yields acetic aldehyde : 



CH 3 .CH 2 OH + O = CH 3 .CHO + H 2 O. 



[Ethyl alcohol.] [Acetic aldehyde.] 



The typical end-group CHO of the aldehyde is not stable, but is 

 easily oxidisable to form the group COOH, and the compound so formed 

 is called an acid ; in this way acetic aldehyde forms acetic acid : 



CH 3 .CHO + O = CH 3 .COOH. 



[Acetic aldehyde.] [Acetic acid.] 



The majority of the simple sugars are aldehydes of more complex 

 alcohols than this ; they are spoken of as aldoses. The readiness with 

 which aldehydes are oxidisable renders them powerful reducing agents, 

 and this furnishes us with some of the tests for the sugars. 



Let us now turn to the case of the ketones. A secondary alcohol 

 is one in which the OH group is attached to a central carbon atom ; 

 thus secondary propyl alcohol has the formula 



CH, . CHOH . CH r 



o o 



Its typical group is therefore CHOH. When this is oxidised, the 

 first oxidation product is called a ketone, thus : 



CH 3 .CHOH.CH 3 + O - CH 3 .CO.CH 3 + H 2 O, 



[Secondary propyl alcohol.] [Propyl ketone.] 



