PHYSIOLOGY OF CELLS 47 



protoplasm of cells shares in this general characteristic. Cell mem- 

 branes act differently, in this regard, from the deeper protoplasm of 

 the cell. An animal or plant tissue being composed of cells is thus made 

 up of a multitude of minute semi-permeable membranes which, function- 

 ing alike, serve in the aggregate as a large membrane. Through the 

 membranes of the body osmotic interchange of substances between 

 solutions of various sorts is continually taking place. 



Semi-permeable membranes composed of lifeless material (dead animal 

 tissues or collodion films, for example) must always act in the same man- 

 ner under a given set of external conditions. Living cell membranes, 

 however, do not always act thus, nor do the cell membranes of various 

 sorts of cells act alike. A membrane of living cells at one time permits 

 the passage of certain substances and prevents the passage of others 

 which would pass through dead membranes without difficulty. At 

 other times some of the substances which formerly were denied passage 

 are permitted to pass. It is evident that the cell membrane is capable 

 of selecting (not consciously, of course) what substances shall pass 

 through and what shall be denied passage, and furthermore that this 

 selection may be varied. What is the cause of this variation in selection? 

 The usual answer is that there is a change in permeability. The change 

 in permeability is due to some change in the colloidal matter of the 

 protoplasm. In some cases many have thought that some of the 

 lipoids are removed from the surface layer and this induces greater 

 permeability. The character of these changes and their causes are still 

 subjects for investigation. It is known that in living protoplasm many 

 of the colloids may be changed repeatedly from a liquid to a semi-solid 

 condition and back to liquid again. This reversibility of coagulation of 

 protoplasmic colloids may be of great importance in the chemical 

 processes of the cell. 



Metabolism. — The protoplasm of the cell carries on all the general x 

 processes of the living body. Within the cell occurs a multitude of 

 complex chemical reactions by which the protoplasm maintains and 

 renews itself, and produces more protoplasm. Protoplasm digests food 

 and for this process secretes various chemical substances. When food 

 is broken down into simpler substances during digestion it is absorbed 

 and assimilated, built up into the living substance itseK or perhaps is 

 combined with oxygen for the production of heat and motion. Proto- 

 plasm also respires, gets rid of waste materials by the process of excretion, 

 grows, is capable of movement, and responds to changes in external 

 conditions, or exhibits irritability. The chemical processes involved in 

 all these activities of protoplasm are included under the one term X, 

 metabolism. 



Metabolism may be defined as the sum of all the chemical processes 

 . carried on within the organism. Perhaps also the contributory physical 



