Chapter V — 53 — Mechanism of Osmosis 



of water. And when the tendency for water to enter the osmometer 

 (DPD) is at a maximum ("state A") and therefore numericaUy equal to 

 osmotic pressure, there is no turgor pressure, and hence no excess pressure 

 (above the diffusion pressure of pure solvent) resulting from the diffusion 

 of solute molecules. 



Some confusion results from the search for the source of energy re- 

 sponsible for the expression of turgor. When an osmotic system in the 

 initial stage ("state A," Figure 15) is allowed to come to water equilib- 

 rium ("state B") by the absorption of water, the energy required to ac- 

 count for the osmotic work that the system can perform comes from the 

 water molecules that enter. This does not prove that the turgor pressure 

 against the membrane results solely from diffusion of water molecules. In 

 the first place, the membrane is permeable to water and will not sustain a 

 static pressure from water molecules. And, secondly, the diffusion pressure 

 of water in the solution has arisen from an initially low value. With the 

 absorption of water to attain water equilibrium, all molecules in the osmom- 

 eter are raised in energy value. Water molecues in the solution attain 

 a new energy level equal to those outside the membrane. The solute mole- 

 cules pass through an equivalent increase in energy. Being the molecules 

 that cannot pass through the membrane, these are responsible for a static 

 pressure equal to the turgor pressure in the solution. Whether their in- 

 crease in energy comes from the water molecules entering the osmometer, 

 or from work done in compressing the solution by means of a piston, the 

 end result is the same, namely, an increase in the diffusion pressure of the 

 solute. This increase in the diffusion pressure of the solute equals the 

 turgor pressure which in turn equals in value the diffusion pressure deficit 

 of the water in the solution at the same concentration and under only at- 

 mospheric pressure. 



Controversy still exists over the "solute pressure" and "solvent pres- 

 sure" theories of osmotic pressure (Beck, 1928; Meyer and Anderson, 

 1939). Early physicists, following van't Hoff defined osmotic pressure 

 as the hydrostatic pressure in an ideal osmometer at water equilibrium. 

 They attributed the pressure to bombardment by solute molecules. Op- 

 ponents of this view define it in terms of the lowered vapor pressure, 

 diffusion pressure, or activity of the solvent. This viewpoint was carried to 

 the extreme by Bousfield (1917) who stated, "It has also been shown that 

 the osmotic phenomena may be interpreted as resulting from the activity of 

 the steam (monohydrol) molecules in the molecular interspaces of the solu- 

 tion, which when subjected to external pressure approximately obey the 

 gas law. This leaves us free to conclude that osmotic pressure has no real 

 existence as an expansive force in the interior of a solution, attributable 

 to the molecules of a solute behaving like an enclosed gas." In view of the 

 permeability of the membrane to water this interpretation is difficult to 

 understand. 



Study of the literature indicates that the controversy has been based on 

 confusion of the two definitions that have been discussed. Advocates of 

 the "solute pressure" mechanism have pointed out that a static pressure 

 such as those measured by Morse, et al., Berkeley and Hartley, and 

 others, could not be caused by bombardment of the membrane by water 

 molecules because the membranes are permeable to water. Those support- 

 ing the "solvent pressure" hypothesis insist that it is the water entering 

 the osmometer that brings about the pressure. Both groups have failed to 



