LIFE AND LIVING THINGS 29 



arises. This ability to change continually yet be essentially 

 the same is perhaps the most striking quality of protoplasm. 

 (3) The most important and characteristic chemical com- 

 pounds in protoplasm are colloidal i.e.j they consist of 

 enormous molecules, which mix readily with water but 

 never completely dissolve, as many crystalline substances 

 do. The most important colloidal substances are the 

 proteins. These contain carbon, oxygen, hydrogen, and 

 nitrogen and possess various unique properties. They are 

 made up of enormous molecules which may be readily 



H S W, CH a 



/VHCCCCCOOH HC - CCOOH. 



C H H H H C t H s H 



NH* 



Jso/euc/ne 

 Arginine 





CH 3 /V/i A/H 2 H H H 



HCCKCCOOH. H-kCCC CCOOH 



CH 3 H H H H H H 



Leucine Lys/ne 



FIG. 19. Some of the chemical compounds formed when protoplasm breaks 

 down. In these diagrams the significance of the letters is as follows: c, carbon; 

 h, hydrogen; n, nitrogen; o, oxygen. 



modified by adding or eliminating whole groups of atoms. 

 If we compare an atom to a soldier, we may liken a protein 

 molecule to an army corps in which a regiment of infantry, 

 or a hospital unit, or some other part is continually being 

 transferred from one position to another. One like the 

 other has a certain recognizable unity yet the internal 

 arrangements are never continuously the same. 



The chemical compounds which make up protoplasm are 

 not all organic. Inorganic compounds are present in con- 

 siderable quantity. Water is an universal constituent of 

 protoplasm and there are always mineral salts, such as the 



