Osmosis and Other Mechanisms - 113 



concentration of potassium ions (K+) in the 

 protoplasm is often 30 times greater than in 

 the medium surrounding the cell; and the 

 level of sodium ions (Na+) outside may be 

 10 times greater than inside. Spontaneously, 

 therefore, potassium ions are under pressure 

 to leave the cell and sodium ions have a great 

 tendency to enter. Under such circumstances 

 the cell must generate energy to overcome 

 the kinetic forces that are driving the sub- 

 stances to enter or leave. In short, whenever 

 the cell expends energy to foster the entrance 

 or exit of a substance, an active transport 

 mechanism is involved. Much remains to be 

 learned about the cellular mechanisms of ac- 

 tive transport; they are very important in the 

 life of the cell and currently they are a focal 

 area of considerable research. 



* ■ 311 



IHBELLUIAR SPACE 



TWO CELL .MEMBRANES*'.: 



CELL 2 



PASSIVE TRANSPORT: 

 OSMOSIS 



DIFFUSION AND 



The surrounding medium of the cell gen- 

 erally consists of some kind of aqueous solu- 

 tion. This solution may be the fresh or salt 

 water in which the organism lives, or it may 

 be a body fluid such as the blood and lymph 

 of animals, or the tissue sap of plants. To 

 enter the cell passively, the penetrating sub- 

 stance must display at least a minimum sol- 

 ubility in the fluid surrounding the cell and 

 in the protoplasm of the cell itself. Ordinar- 

 ily only dissolved substances can pass spon- 

 taneously through the plasma membrane. 



Not all dissolved substances can penetrate 

 the plasma membrane with equal facility. 

 The molecules or ions may be too big, or 

 they may encounter a variety of other inter- 

 fering factors (p. 116). Thus the plasma mem- 

 brane is a semipermeable membrane — per- 

 meable to some substances, but not to all. 



Whenever two different solutions are sepa- 

 rated by a semipermeable membrane, an 

 osmotic system is established. Each typical 

 cell, therefore, represents an osmotic unit, 

 since the semipermeable plasma membrane 

 always intervenes between the protoplasm, on 

 the one hand, and the external solution on 





Fig. 6-1. Boundary membranes of two contiguous 

 cells: electron micrograph of section of a nerve 

 (mouse). The double membrane of each cell measures 

 about 80 Angstroms across; whereas the intercellular 

 material is about twice as thick. Compare this photo 

 with the Danielli diagram of the cell membrane (Fig. 

 6-5). (Photo courtesy of J. David Robertson, Depart- 

 ment of Neuropathology, Harvard Medical School.) 



the other. Large quantities of water and lesser 

 amounts of dissolved substances are con- 

 stantly passing into or out of the cell across 

 the plasma membrane, and these exchanges 

 are very important in the life of every cell. 



Diffusion. The spontaneous migration of 

 molecules and ions within the limits of a 

 single solution must be considered before we 

 deal with the more complex process of os- 

 mosis. If any dissolved substance is concen- 

 trated more heavily in one part of a solution, 

 this substance will spread gradually until its 

 molecules (or ions) are evenly distributed 

 throughout the whole solution. This process 

 is called diffusion. It is caused by the random 

 movements of all the particles (solute as well 

 as solvent) that make up the entire solution. 

 Essentially these random movements are a 

 manifestation of the molecular kinetic energy 

 (heat). 



The direction that a given particle will 



