Chapter V — 57 — Mechanism of Osmosis 



high energy that causes water molecules to break away from the surface 

 in the process of evaporation; it is manifested in Brownian movement; 

 it is the same high energy that causes water to enter the solution in the 

 osmometer through the differentially permeable membrane. The arbitrary 

 assignment of zero diffusion pressure to the solvent in many considerations 

 of osmotic pressure has probably been largely responsible for the erroneous 

 concept that the solvent does not enter into the energy relations of the 

 process of osmosis but simply supplies space for the dispersion of the solute 

 molecules. It should be evident from the above discussion that the excess 

 energy of the pure solvent molecules over that of the solvent molecules in 

 solution is the cause of osmosis. And after osmosis has occurred and the 

 solution has become more dilute, the only way to return to the original state 

 is to put energy back into the system either as heat to evaporate water 

 from the solution until it is at its original concentration, or as mechanical 

 energy to press pure water from the solution through the differentially 

 permeable membrane. 



Although this discussion of the mechanism of osmosis may be long and 

 involved, it seems to be necessary to clarify certain aspects. Osmosis, as it 

 relates to cell turgor, is of paramount importance for an understanding 

 of water relations. The ability of the cell to absorb and retain water is 

 critical in its function in the plant. Upon this ability depends its turgidity, 

 its form and size, its relation to the mineral nutrients supplied by the en- 

 vironment, its capacity to expand and grow, and, in the final analysis, its 

 power to compete for water and so to survive. From this standpoint the 

 biologist needs to understand the mechanics of osmosis, the function of 

 membranes, and the energy relations of aqueous osmotic systems as re- 

 lated to living cells. 



Summary: — A simple gas osmometer is a convenient mechanism to illustrate the 

 principles of osmosis. When a gas osmometer is in equilibrium across the membrane 

 with the pure gas to which the membrane is permeable, it seems evident that the partial 

 pressure of that gas on the two sides of the membrane is equal and that the excess 

 pressure of the confined gas is due to its partial pressure. 



In the liquid state when a solute is added to a solvent, the diffusion pressure of 

 both is reduced. If the solution is now placed in an osmometer and the latter sur- 

 rounded by the liquid to which the membrane is permeable (the solvent), this liquid 

 moves in until the diffusion pressure of solvent on the inside attains its initial value, 

 that of the pure liquid outside. 



The difference in pressure between the solution inside and the pure solvent outside 

 is commonly termed the osmotic pressure of the solution. Turgor pressure at water 

 equilibrium is probably a better designation. Haldane has shown that the reduction 

 in diffusion pressure of the solvent resulting from the addition of a solute also equals 

 the osmotic pressure of the solution. This difference is more correctly termed the 

 diffusion pressure deficit of the solvent in the solution. A more valuable definition of 

 osmotic pressure is contained in the following equation: 



OP = DPD + TP 



Only when TP = O does DPD = OP, and when DPD = O does TP = OP. At 

 all intermediate stages the three terms are needed to describe an osmotic system. 



The "solvent" and "solute" pressure theories concern the two extreme states of 

 the osmometer, namely when TP = O and when DPD = O. A complete consideration 

 of osmosis must include all intermediate states as well. 



The energy responsible for entry of water into an osmometer is resident in the 

 molecules of the pure solvent. The force expressed by the turgor pressure is due to 

 energy of the solute molecules, energy transmitted to them from the entering solvent 

 molecules. 



Arbitrary reference levels for measuring changes in diffusion pressure of both 

 solute and solvent must be established. A customary practice is to set the level of 

 diffusion pressure of the solvent under atmospheric pressure at zero; that of the solute 



