GENERAL STRUCTURE AND COMPOSITION 11 



tion to the conjugated proteins whose molecules contain protein 

 in combination with a non-protein substance. 



Each of them contains carbon, hydrogen, oxygen, and nitrogen; 

 most of them contain sulphur also, and a few phosphorus in ad- 

 dition. These elements are united into very complex molecules, 

 and although different members of the group of simple proteins 

 differ from one another in minor points they all agree in their 

 broad features. The common body proteins have a similar per- 

 centage composition, falling within the Jimits given in the follow- 

 ing table: 



Carbon 50 to 55 per cent. 



Hydrogen 6.5 to 7.3 " " 



Oxygen , . 19 to 24 " " 



Nitrogen 15 to 17.6 " " 



Sulphur 0.3 to 2.4 " " 



In addition a small quantity of ash is usually left when a protein 

 is burned, showing that some inorganic salts are held in combina- 

 tion with it. 



Recent chemical investigation has shown that the protein 

 molecule is a complex, made up of a number of simpler molecules 

 joined together. When a protein is boiled with a dilute acid its 

 molecules are decomposed, and the resulting solution is found 

 when examined to contain a mixture of the substances whose in- 

 dividual molecules were formerly parts of the complex protein 

 molecules. Eighteen such substances have been obtained from de- 

 composed proteins; they all contain nitrogen, and they all belong 

 chemically to the group of amino acids. Some proteins contain 

 all of them; others only a few. The characteristics of different 

 proteins are supposed to depend on which of these amino acids 

 are present in the molecules and also on their arrangement or 

 grouping therein. 



There are a number of chemical tests that may be used in detecting the 

 presence of proteins; but only a few of them apply to the entire group. Of 

 these the so-called biuret reaction is the most easily and most commonly 

 used. It consists in making the protein solution strongly alkaline with caustic 

 soda or potash and adding a small amount of very dilute solution of copper 

 sulphate. A distinct purple color is evidence of the presence of protein. The 

 common proteins of the body may also be recognized by the following char- 

 acters: 



