^Osmosis and Other Mechanisms 

 by Which Cells Take In and 

 Give Off Materials 



THE METABOLISM of each living cell re- 

 quires a ceaseless supply of new materials 

 and produces a steady stream of waste prod- 

 ucts. These substances must be passed in and 

 out between the protoplasm and the sur- 

 rounding medium through the living cell 

 surface, or plasma membrane. The metabo- 

 lism itself may provide energy to keep the 

 traffic moving, and extraneous cell coatings 

 (p. 33) may influence the speed of flow, but 

 the dominant role is played by the plasma 

 membrane (Fig. 6-1). 



PASSIVE vs. ACTIVE TRANSPORT 

 MECHANISMS 



In some respects the cell may be compared 

 to a very small cellophane balloon, tilled with 

 a sugar solution and immersed in water. 

 Owing to the tendency of various molecules 

 and ions to disperse themselves evenly 

 throughout any system (p. 113), water mole- 



1 19 



cules would continue to enter the balloon 

 and sugar molecules woidd leave it, until an 

 equilibrium was reached. In like fashion 

 water frequenth tends to enter the cell and 

 solute molecules tend to leave it. fn such 

 cases, however, the energy that drives the 

 transport mechanism is not generated by the 

 cell. It is the kinetic or thermal movement 

 of molecules, which is characteristic of all 

 matter — except when the temperature falls 

 to absolute zero (— 273.2 ; C). No energy is 

 expended by the cell in forcing the sub- 

 stances to enter or leave, and accordingly 

 such exchanges are referred to as passive 

 transport. 



Frequently it is found, however, that the 

 cell may accumulate a dissolved substance, 

 so that the concentration is far greater in- 

 side the cell than outside: or the cell may ex- 

 clude the entrance of a substance present in 

 very high concentration in the outside me- 

 dium. Generally speaking, for example, the 



