12 FOUNDATIONS OF BIOLOGY 



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. But it is the 

 combination of the elements which is significant, and this 

 results 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 inti- 

 mate 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. Some of these com- 

 pounds are relatively simple, such as water (H 2 O) which is 

 quantitatively the most important constituent of all proto- 

 plasm, but the majority consist of elaborate atomic arrange- 

 ments and not a few represent molecular complexes of hun- 

 dreds and even thousands of atoms. 



The compounds of carbon which are characteristic of 

 protoplasm fall into three chief groups: proteins, carbohy- 

 drates, and fats. 



PROTEINS invariably consist of the elements carbon, 

 oxygen, hydrogen, nitrogen, and sulfur, and frequently 

 phosphorus and iron; Examples are albumin of the white of 

 egg, casein of milk, gluten of cereals, and myosin of lean 

 meat. The nitrogen particularly distinguishes proteins 

 from the other compounds of the living complex and, as we 

 shall see later when considering the chemical processes in 

 animals and plants, is largely responsible for their command- 

 ing position as "the chemical nucleus or pivot around which 

 revolve a multitude of reactions characteristic of biological 

 phenomena." Study of the relationship of nitrogen to the 

 other chemical elements of proteins long since revealed the 

 fact that the protein molecule is a huge complex of linked 



