32 ELEMENTS OF BIOLOGY 



its living has formed into types cliemically different in nature with 

 special duties to perform; in other words, specialization of both 

 structure and function has taken place. The study of the function 

 of the cell and its parts is termed cell physiology. The exact func- 

 tions of many observable parts of the cell are not completely known, 

 but as evidence accumulates through careful observation and ex- 

 periment, the general functions of some of the cell structures be- 

 come clear. 



The Cell Boundary. The limiting boundary of the cell, the 

 plasma membrane, being the area of contact between the cell and 

 its surroundings, serves as a regulator of admission and escape of 

 materials. This function it exercises by reason of its properties as a 

 membrane; it must not be thought of as possessing any power 

 resembling an ability to select substances needed within the cell by 

 any sort of intelligence. Its importance in the life of the cell directs 

 our attention to the properties of membranes. 



Membrane Permeability. Membranes may be of many sorts 

 and are classified with regard to their relative permeability. Thus 

 a membrane may be impermeable to small particles but permeable 

 to molecules. Other membranes may be impermeable to large mole- 

 cules but permeable to small molecules. Still other membranes may 

 be impermeable to molecules but permeable to ions. Living mem- 

 branes are usually considered as semi-permeable; in general, they 

 are impermeable to large molecules but may be permeable to small 

 molecules, for example, H2O, and rather freely permeable to some 

 ions but not to all. The plasma membrane of the cell is regarded 

 as a membrane of this type. 



Semi-permeable membranes such as the plasma membrane may 

 be thought of as sieves, the pores of which determine the size of the 

 substances that will pass through, as small oranges may be sorted 

 from large ones by being passed over a sorting board with small 

 holes. But the size of the molecule is not the sole condition deter- 

 mining whether or not a substance will pass a cell boundary. In 



