94 OSMOSIS AND OSMOTIC PRESSURE 



the solute eventually the entire system will come to equilibrium after which 

 there is no further increase in the volume of water inside of the membrane. 



When the movement of the solvent is referred to in discussing osmosis 

 it is always the net movement which is meant. Solvent molecules will always 

 be moving across the membrane in both directions, but except when an equi- 

 librium has been attained, more molecules will move per unit of time in one 

 direction than in the other. As will be shown in the later discussion this 

 net movement is always from the region of the greater diffusion pressure of 

 the solvent molecules to the region of their lesser diffusion pressure. The 

 maintenance of the diffusion gradient of the liquid in most osmotic phenomena 

 is due to the relative impermeability of the membrane to the molecules of 

 solutes. It is the presence of this differentially permeable membrane which 

 gives to osmosis its distinctive aspect as compared with other diffusion processes. 

 If the membrane were permeable to the solute particles, they would diffuse 

 in one direction, while the molecules of the solvent were diffusing in the 

 other; this would be another example of the principle of independent diffusion. 



Terminology. — The conditions under which osmosis can occur are so 

 varied that it is impossible to characterize the process in terms of a simple 

 definition. The following statement will, however, apply to most cases: 

 Osmosis is the diffusion of a liquid through a differentially permeable mem- 

 brane into a solution in which the solvent is that same liquid or into another 

 liquid with which it is miscible. In living organisms water is the only im- 

 portant liquid which moves from cell to cell by osmosis, hence the further 

 discussion of this process will be in terms of water and aqueous solutions. 



The attainment of a satisfactory comprehension of osmosis and osmotic 

 pressure has been delayed both by confusion in theoretical interpretations as 

 well as by lack of a consistent terminology. Many obscurities have resulted 

 from the application of the term osmosis to the movement of solutes through 

 membranes. The use of the term in this sense has not only led to many 

 misunderstandings, but also to positive errors and misinterpretations of data. 

 The movement of solutes through a membrane is a diffusion process; it may 

 be simple diffusion, or it may be complicated by a number of factors which 

 do not enter the phenomenon of simple diffusion. Both from the historical 

 and theoretical standpoint it seems desirable that the term osmosis be re- 

 stricted to the sense in which it is used in this discussion. 



Osmotic pressure may be defined as the maximum pressure which can be 

 developed in a solution when separated from pure water by a rigid membrane 

 permeable only to water. In order to determine the osmotic pressure of a 

 solution it would be necessary to enclose it in a rigid membrane permeable 

 only to water, to immerse this membrane in pure water, and to exert just 



