INTRODUCTION 13 



The free-living animal cell takes something from its en- 

 vironment and returns something to it. It takes into it- 

 self a variety of organic substances together with small 

 quantities of mineral salts. These constitute its food. 

 It receives also a supply of oxygen. This is not ordinarily 

 reckoned as a food and for a good reason. The term food 

 is best restricted to material which can serve constructive 

 purposes or at least be stored in the cell. The function 

 of oxygen is not to promote constructive processes, but to 

 release energy, a process of decomposition in which the 

 stores of the cell are sacrificed. The process in which 

 oxygen reacts with substances within the cell, giving rise 

 to simple oxidized products in place of complex material 

 rich in potential energy, is called respiration. (The word 

 is, indeed, frequently used as a synonym for breathing, 

 but we shall use it in its chemical sense.) Respiration 

 is often compared with combustion, and while the two 

 are not identical in all their stages, the fact remains that 

 the initial and the final conditions are essentially the same 

 for both. The release of energy is generally just as great 

 in the physiologic change as in the actual burning of like 

 quantities of the cell constituents. 



The free-living animal cell is thus an accumulator of 

 fuel and a furnace in which it is burnt. But this is a very 

 imperfect comparison, for it has in addition the property 

 of self-repair, and under favorable circumstances capacity 

 for growth and reproduction. Cells multiply by cleaving 

 into two similar parts, and the tendency to do this after 

 a certain increase in size usually limits very definitely the 

 dimensions to which a single cell may attain. When 

 growth is taking place it is evident that not all the food 

 is serving as fuel; a certain portion is becoming incor- 

 porated with the more permanent substance of the cell 

 and is so changed as to become entirely typical protoplasm. 

 The process through which food becomes an integral part 

 of the cell is called assimilation. The word emphasizes 

 through its root-meaning the attainment of likeness to 

 the material of the cell and indirectly implies that the food 



