THE CHEMICAL BASIS OP THE CELL. 21 



later in connection with ionization. Water has also a very great 

 surface tension. It is this which determines the height to which 

 it will rise in plants and in the soil, and which no doubt plays a 

 role in the processes of absorption going on in various parts of 

 the animal body. 



PROTEINS. The great importance of proteins in animal life is 

 attested by the fact that they are absolutely indispensable in- 

 gredients of food. An animal fed on food containing no protein 

 will die nearly as soon as if food had been withheld altogether. 

 Proteins are complex bodies composed of carbon, hydrogen, oxy- 

 gen, nitrogen, and, in nearly all cases, sulphur. Some may con- 

 tain in addition phosphorus, iron, iodine, or certain other 

 elements. The proportions in which the above elements are 

 found in different proteins do not vary so much as the differences 

 in the chemical behavior of the proteins would lead us to expect. 

 In general the percentage composition by weight is : 



Carbon 53 per cent 



Hydrogen 7 per cent 



Oxygen 22 per cent 



Nitrogen . , 16 per cent 



Sulphur Ito2 per cent 



The essential differences in the structure of the molecules of 

 different proteins have been brought to light by studies of the 

 products obtained by partially splitting up the molecule. We 

 are able to do this by subjecting protein to the action of super- 

 heated steam, or by boiling with acids or alkalies in various con- 

 centrations, or by the action of the ferments of digestive juices 

 or by bacteria. The cleavage produced by ferments or bacteria 

 is much more discriminate than that brought about by strong 

 chemical reagents ; that is to say, the chemical groupings are not 

 so roughly torn asunder by the biological as by the chemical 

 agencies. 



At first the proteins break up into compounds still possessing 

 many of the features of the protein molecule. These are the 

 proteases and peptones, and they consist of aggregates of smaller 



