Chapter V 

 THE MECHANISM OF OSMOSIS 



Introduction: — In an accurate description of a physical mechanism, 

 critical definitions of the processes involved are necessary. 



A large number of plant physiologists are agreed on the terminology 

 of the Physical Methods Committee of the American Society of Plant 

 Physiologists as presented by Meyer (1945). This terminology is used 

 here and the following definitions are given for use in the discussion that 

 follows : 



Osmosis "may be conveniently, but not rigidly, defined as the diffusion 

 of a solvent across a differentially permeable membrane" (Meyer, 1945). 

 It is a result of the kinetic energy of the molecules. 



Osmotic pressure. — "Historically .... osmotic pressure has been 

 used to designate the maximum pressure which develops within a solution 

 under ideal conditions .... the solution must be confined within a mem- 

 brane permeable only to the solvent, .... the membrane [must] be im- 

 mersed in the pure solvent, .... and the construction of the osmometer and 

 attached pressure measuring apparatus [must] be such as to permit the 

 development of a pressure equilibrium without any appreciable dilution of 

 the solution. This concept of osmotic pressure is not a very useful 

 one " 



"The term 'osmotic pressure' can be more usefully employed as an 

 index of certain physical properties of a solution. Osmotic pressure is an 

 evaluation of the potential maximum turgor pressure which will develop in 

 a solution if it is permitted to come to equilibrium with pure water in an 

 ideal osmotic system. . , ." 



"Osmotic pressure is an index indicating quantitatively the amount by 

 which the diffusion pressure of the water in the solution is less than that of 

 pure water at the same temperature and under atmospheric pressure" 

 (Meyer, 1945). 



Diffusion is the net movement of the molecules in one direction result- 

 ing from their inherent thermal or kinetic energy and a difference in activi- 

 ties. The direction of diffusion is from a region of greater partial pressure 

 to a region of lesser partial pressure. 



Diffusion pressure. — Meyer defines diffusion pressure as "that physi- 

 cal property of a substance which is responsible for its diffusion whenever 

 other prevailing conditions permit the occurrence of this process." He 

 gives an extended discussion of the relation of diffusion pressure to diffu- 

 sion, osmosis, and the development of osmotic pressure. 



We offer the following definition. Diffusion pressure is a function of 

 the free energy of a substance. The term may be applied to both solute 

 and solvent of a solution. Ideally, the diffusion pressure of the solute is an 

 expression of the driving force with which its molecules will diffuse into 

 the pure solvent. Diffusion pressure of the solvent in pure solvent or in a 

 solution is a measure of the driving force with which its molecules will 

 diffuse into pure solute. 



For example, if pure water were separated from pure glycerine by a 

 membrane permeable only to water, the diffusion pressure of the water 



