52 ELEMENTARY BIOLOGY 



78. Summary. We may summarize the materials required 

 by a living cell in this way : 



1 . Water. The chemical changes that distinguish living protoplasm 



can take place only in the presence of water. 



2. Protein. Out of this, new protoplasm is constructed, resulting 



either in the growth of the cell or in the replacement of 

 protoplasm that may have been destroyed. 



3. Fuel foods. In addition to the protein oxidized in the cell there 



is usually some other material that is oxidized. Two classes 

 of compounds commonly furnish this fuel : namely, (a) carbo- 

 hydrates ; (b) fats. 



4. Salts. Various mineral, or inorganic, compounds are necessary 



for maintaining the activities of protoplasm. These are of 

 many kinds, although certain of the elements contained in 

 these salts are used by all living protoplasm (see p. 31). 



5. Hormones. In the bodies of human beings and of other animals 



(and possibly also of certain plants) peculiar substances are 

 produced that have a direct influence upon the activities of 

 cells. These substances, which are called /lormones, are just 

 coming to be understood through experiments. It is sufhcient 

 for the present to note that they do affect protoplasm activity, 

 and that some of them are necessary for the life of certain 

 kinds of cells in the body. 



6. Oxygen. Although this is not usually regarded as part of the 



food, it is an essential part of the income of every cell. It 

 is the chemical union of oxygen with other substances that 

 sets free the energy by which the protoplasm does all of 

 its work. 



