96 OSMOSIS AND OSMOTIC PRESSURE 



pressures developed in plant cells. Since these are largely of osmotic origin 

 the name turgor pressure seems equally appropriate for physical systems. 

 Actually the turgor pressure developed in a solution enclosed vt^ithin a mem- 

 brane may be increasing while its osmotic pressure is decreasing, as shown in 

 the last example mentioned above. This indicates clearly that each of these 

 terms — osmotic pressure and turgor pressure— refers to a different physical 

 quantity. In an osmometer set up as just described the maximum turgor 

 pressure which could be developed will never exceed the osmotic pressure. 



The Mechanism of Osmosis. — Like a gas, water or any other liquid may 

 be considered to possess a diffusion pressure (Haldane, 1918). The only 

 two factors which influence the diffusion pressure of a pure liquid are pres- 

 sure and temperature. The imposition of pressure from an external source 

 will raise the diffusion pressure of a liquid. For example, if a pressure of 

 10 atmos. be exerted upon water in a closed vessel by means of a leak- 

 proof piston, the diffusion pressure of that water will be increased by 10 atmos. 

 Under certain conditions "negative pressures" or tensions may develop in 

 liquids. These result in a reduction in the diffusion pressure of the liquid 

 in an amount equal to the magnitude of the tension. 



Although the diffusion pressure of water is also influenced by temperature 

 no interpretation of the influence of this factor will be undertaken. In order 

 to simplify the following discussion it will be consistently assumed that all 

 parts of every osmotic system considered are at the same temperature. 



However, when a substance is dissolved in water the diffusion pressure 

 of the water in the resulting solution is decreased as compared with that 

 of pure water at the same temperature and pressure. This diminution in 

 diffusion pressure is proportional, within a wide range of solution concentra- 

 tions, to the number of solute particles present in a given volume {i.e. a given 

 number of molecules) of the solvent. In a solution, therefore, the diffusion 

 pressure of the solvent may be influenced by the three factors of (i) tem- 

 perature, (2) pressure, and (3) the ratio of solute particles to solvent 

 molecules. 



When a solution is confined within a membrane permeable only to water 

 and that membrane is immersed in water there will be a net movement of 

 water through the membrane into the solution, because of the excess diffusion 

 pressure of the water on the water side of the membrane. The passage of 

 water through the membrane results in the development of a turgor pressure 

 on the solution side of the membrane. Since the maximum possible pressure 

 (= osmotic pressure) which can develop in the solution is equal to the excess 

 of the diffusion pressure of pure water over the diffusion pressure of the 

 water in the solution, the following relation holds, providing all parts of 



