NATURE OF LIFE AND LIVING MATERIAL 15 



compounds that may be formed by the nineteen difFerent amino- 

 acids is likewise almost infinite. It should now be clear that it is 

 possible for two quite dissimilar organisms to contain some proteins 

 that are identical and that it is possible for two quite similar organ- 

 isms to contain some proteins that are different. Similarly, in the 

 same animal different parts of the body contain quite dissimilar 

 proteins. 



The general physical properties of the proteins are similar to 

 those of gelatin and of egg white, which are in fact proteins. Pro- 

 teins constitute approximately 15 per cent of protoplasm and in 

 quantity exceed all other types of substances except water. Thus 

 many of the physical properties of protoplasm, its fluidity and 

 viscosity, its tendency to coagulate when exposed to high tempera- 

 ture, its changes with time, its responses to chemical agents, may 

 be ascribed to the behavior of the proteins of which it is composed. 



The FATS form the second group of organic constituents. All fats 

 are much more complex than inorganic molecules and contain 

 carbon, oxygen, and hydrogen, but not in the same arrangement:, 

 as in the proteins. Three types occur in the living organism; fats; 

 containing no nitrogen nor phosphorus; fats containing nitrogen 

 but no phosphorus, and fats containing both nitrogen and phos- 

 phorus. They are sparingly soluble in water and in protoplasm are 

 in general found associated with or composing various boundaries 

 and surfaces. 



The CARBOHYDRATES are the third organic group found in the liv- 

 ing system. These always contain carbon and also hydrogen and 

 oxygen in the same proportions as in water. For example, muscle 

 sugar, also known as glycogen, is considered as having the follow- 

 ing formula: (CGHio05)n- The n represents an unknown number 

 of the CgHioOs groups that make up the molecule. Other carbo- 

 hydrates with which all are familiar are the starches, sugars, and 

 cellulose, the last being the substance of which paper is composed. 



The chemical structure and chemical properties of all these sub- 



