THE MECHANISM OF OSMOSIS 97 



the sj'Stem are at the same temperature and under the same external pressure : 

 Osmotic pressure = Diffusion pressure of pure water — Diffusion pres- 

 sure of water in the sokition. 



In other words the osmotic pressure of a solution is a measure of what 

 may be called the diffusion pressure deficit of the water in that solution. By 

 this term, as applied to any given solution, is meant the amount (usually 

 expressed in atmospheres) by which the diffusion pressure of the solvent in 

 that solution is less than the diffusion pressure of the pure solvent when the 

 latter is at the same temperature and under atmospheric pressure. For ex- 

 ample, if a solution has an osmotic pressure of lO atmos., this signifies that 

 the diffusion pressure of the water in that solution is just lO atmos. less than 

 the diffusion pressure of pure water under atmospheric pressure at the same 

 temperature; in other words its diffusion pressure deficit is lO atmos. The 

 osmotic pressure of a solution is a measure of the diffusion pressure deficit 

 of the water in that solution only when the solution is not under a pressure 

 or a tension (except, of course, atmospheric pressure) since these factors also 

 affect the diffusion pressure of water. In plant cells, as we shall see later, 

 pressures and tensions must constantly be taken into account in evaluating 

 the diffusion pressure deficit of the water in the cell sap. 



When two aqueous solutions, both initially subjected only to atmospheric 

 pressure, are separated by a membrane permeable only to water, diffusion 

 of water will take place towards the solution of greater osmotic pressure. 

 For example, if solution A with an osmotic pressure of 20 atmos. be enclosed 

 in a stoppered membrane permeable only to water which is immersed in solu- 

 tion B with an osmotic pressure of 12 atmos., water will diffuse inwards, 

 i.e. towards solution A. The diffusion pressure deficit of the internal solution 

 A is 20 atmos.; that of the external solution B, 12 atmos. Water always 

 moves in any osmotic system towards the region of its lesser diffusion pressure, 

 or in other w^ords towards the region of its greater diffusion pressure deficit. 

 In this particular example, disregarding dilution, the internal solution will 

 exert a turgor pressure of 8 atmos. at equilibrium. Correspondingly the mem- 

 brane will exert a wall pressure of 8 atmos, against the enclosed solution. 

 Since subjection to a pressure raises the diffusion pressure of water by the 

 amount of the imposed pressure, the diffusion pressure deficit of the water 

 in the internal solution at equilibrium will not be 20 atmos., but 12 atmos. 

 In other words, at equilibrium, the dift'usion pressure deficit of the water 

 will be equal on both sides of the membrane. 



In the preceding discussion it has been tacitly assumed that water diffuses 

 through the membrane during osmosis in the liquid state. IVIany investigators 

 consider, however, that actual diffusion of water across the membrane occurs 



