THE PHYSICAL BASIS OF LIFE 21 



The average composition of the human body, including cellular 

 and intercellular material, is about as follows: 



Oxygen 65.00% 



Carbon 18.00 



Hydrogen 10.00 



Nitrogen 3.00 



Calcium 2.00 



Phosphorus 1.00 



Potassium 0.35 



Sulfur 0.25 



Sodium 0.15 



Chlorine 0.15 



Magnesium 0.05 



Iron 0.004 



At first glance there is nothing very striking about this list of 

 elements. They are all common ones with which the chemist is 

 familiar in the non-living world. The materials of Man's body are 

 worth less than one dollar! Furthermore, quantitatively the most 

 important compound is nothing more complex than water 

 (H 2 0). It composes more than two-thirds of the human body. 

 But there are combinations of the elements which are highly 

 significant and characteristic, and result from the capacity of 

 carbon, hydrogen, and oxygen, or carbon and hydrogen together, 

 to form the numerous complex compounds which in turn supply 

 the basis for intimate associations with other elements. As a matter 

 of fact, the bulk of protoplasm is composed of carbon, oxygen, 

 hydrogen, and nitrogen associated with each other in an apparently 

 infinite series of relationships, in which the carbon seems to play 

 the leading role - - the indispensable bond that links all other ele- 

 ments in organic unity. Some of these compounds are relatively 

 simple, but the majority consist of elaborate atomic arrangements 

 and not a few represent molecular complexes of hundreds and even 

 thousands of atoms. 



The compounds of carbon which are characteristic of proto- 

 plasm fall into three chief groups: proteins, carbohydrates, and 

 fats. 



Proteins invariably consist of the elements carbon, oxygen, 

 hydrogen, nitrogen, and usually sulfur, and frequently phosphorus, 

 iron, etc. Examples are albumin of the white of egg, casein of 

 milk, gliadin of wheat, and myosin of lean meat. The nitrogen par- 

 ticularly distinguishes proteins from the other compounds of the 



