CHAPTER II. 

 THE PROTEINS. 



A. Definition. 



Proteins are nitrogenous compounds found in the 

 fluids and tissues of all living organisms. Chemically, they 

 are composed of a number of amino acids (see p. 67), con- 

 densed together in a characteristic way so that the whole 

 molecule is generally neither very acid nor very basic. 

 Their chemical properties are dependent on the presence of 

 these amino acids. Their physical properties are mainly 

 due to the fact that they form colloidal solutions (p. i). 

 The percentage composition varies very considerably in 

 different proteins. The following can be taken as a rough 

 average : 



C =53 per cent. 



O = 23 



N = 16 



7 )> 

 S = i 



100 



B. Classification. 



It is not possible at present to give a rational scheme, 

 for we have not sufficient data of a chemical nature by 

 means of which we can characterise the individual proteins. 



The classification adopted here is based on physical and 

 chemical properties, and closely follows the official classi- 

 fication of the American Physiological Society. Where 

 the British Society uses a different name this is indicated 



by(B). 



