LIVING SUBSTANCE 



113 



not at all ; the latter, however, becomes blue in the presence of iodine 

 and sulphuric acid. 



In addition to the free carbohydrates, combinations of carbohydrates 

 exist in living substance e.g., combinations with proteids, as an 

 example of which mucin has already been mentioned. 



The most important decomposition-products of carbohydrates have 

 also been mentioned, such as lactic acid, butyric acid, carbonic acid, 

 etc., all of which are met with in living substance. 



c. Fats 



The fats likewise do not belong to the general constituents of 

 living substance, but they are wide-spread, chiefly in animal cells. 

 Like the carbohydrates, the 

 fats are non-nitrogenous, and 

 contain only the elements car- 

 bon, hydrogen and oxygen. But 

 chemically they differ funda- 

 mentally from the carbohy- 

 drates. For example, they re- 

 present the so-called compound 

 ethers, or esters i.e. compounds 

 in which an acid has combined 

 with alcohol with the loss of 

 water. The alcohol that is the 

 basis of all fats is glycerine, 

 C 3 H 5 (OH) 3 , and the acids that 

 are combined with glycerine 

 belong to the series of fatty 

 acids, whose general formula is 

 C U H 211 O 2 . Since the glycerine 

 represents a trivalent alcohol, 

 in the neutral fats three atoms 

 of fatty acid are always combined with one atom of glycerine into 

 tri-glycerides. The general formula of the fats is, therefore 



C 3 H 5 (OH) 3 + 3C n H 2n 2 -3H 2 0. 



As examples of the fatty acids there may here be mentioned 

 palmitic acid, stearic acid, butyric acid, valeric acid and capronic 

 acid. In addition to these, oleic acid, which does not belong to 

 the normal series of fatty acids, occurs in the various oils combined 

 with glycerine. 



In correspondence with their composition, the neutral fats may 

 by certain methods be split up by hydrolysis into their constituents 

 i.e., into glycerine and free fatty acids ; this process takes place in 

 the organism as the result of the action of the digestive juices. It 



I 



i it 



FIG. 40. /. Plant-cell containing starch-grains. 

 II. Starch-grains isolated a, from the po- 

 tato ; b, from the corn ; c, from the pea. 



