Osmosis

Osmosis refers to the flow or displacement of volume across a barrier due to the movements of matter in response to concentration differences. Although, in principle, any substance (solutes as well as solvents) may contribute to the volume of matter displaced during osmotic flow, the term osmosis has come to have a much more restricted meaning when applied to biologic systems. Because biologic fluids are relatively dilute aqueous solutions in which water comprises more than 95% of the volume, osmotic flow across biologic membranes has come to imply the displacement of volume resulting from the flow of water from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a more concentrated solution).

Osmosis and the diffusion of uncharged solutes are closely related phenomena. Both are spontaneous processes that involve the flow of matter from a region of higher concentration to one of lower concentration; both are the results of random molecular movements and, hence, are dependent only on the thermal forces inherent in any system; and the end result of both processes is the abolition of concentration differences.

How do diffusion and osmosis differ? As we shall see, the answer to this question carries with it the key to understanding osmosis. Let us start by reexamining the definitions of these two processes. Osmosis refers to the flow of matter that results in a displacement of volume, and diffusion refers to a flow of matter in which displacements of volume are not involved. As it turns out, the key to understanding osmosis is the answer to the question, "Why is there no displacement of volume in diffusion?''

The answer to this question emerges when we carefully reconsider the molecular events involved in the diffusion of uncharged solutes. Referring once more to Fig. 3, when we say that the concentration of an uncharged solute in compartment o is greater than that in compartment i, we are, at the same time, implying that the concentration of water (or solvent) in compartment i is greater than that in compartment o. When there is a concentration difference across a membrane for one component of a binary solution (one solute and one solvent), there must also be a concentration difference for the other component. Thus, there are two concentration differences, two driving forces, and two diffusional flows: solute diffuses from compartment o to compartment i and water diffuses from compartment i to compartment o. In short, when we say that there is diffusion of a solute down a concentration difference we are describing only one half of the mixing process and are overlooking the fact that diffusion (or mixing) in the closed system illustrated in Fig. 3 is, in fact, interdiffusion. Clearly, in the closed system illustrated in Fig. 3, if the membrane is rigid, mixing or interdiffusion of solute and solvent must take place without any change in the volumes of compartments o or i.

To appreciate how diffusion can result in the displacement of volume, let us consider the system illustrated in Fig. 13. Compartment o is open to the atmosphere and contains pure water. Compartment i is closed by a movable piston and contains an aqueous solution of some uncharged solute. The membrane separating the two compartments is assumed to be freely permeable to water but impermeable to the solute (i.e., a semipermeable membrane). Although there are two concentration differences, there can be only one flow. Water will flow from compartment o to compartment i, driven by its concentration difference. The volume of water associated with this flow, however, cannot be counterbalanced by a flow of solute, so there will be a net displacement of volume; the volume of compartment o will decrease, whereas that of compartment i will increase. This volume flow, referred to as osmosis or osmotic flow, arises because the properties of the barrier or membrane are such as to prevent interdiffusion; mixing, which is the end to which all spontaneous processes are directed, can now only come about as the result of one flow rather than two flows.

FIGURE 13 Apparatus for determining osmotic pressure and flow.

Now, we can prevent the flow of volume from compartment o to compartment i by applying a sufficient pressure on the piston. The pressure that must be applied to prevent the flow of volume is defined as the osmotic pressure. When the solutions on both sides of the membrane are relatively dilute (as in the case of biologic fluids) and when the membrane is absolutely impermeable to the solute (i.e., interdiffusion is completely prevented), the osmotic pressure is given by the following expression, which was derived by the Dutch Nobel Laureate Jacobus Henricus van't Hoff (1852-1911):

Ax = RTACj

FIGURE 13 Apparatus for determining osmotic pressure and flow.

where An denotes the osmotic pressure; R is the gas constant; T is the absolute temperature; and AQ, is the concentration difference of the impermeant, uncharged solute across the membrane in moles per liter. At 37°C (T = 310K), this expression reduces to:

Thus, if the concentration difference across the membrane is 1 mol/L, the pressure that must be applied on the piston to prevent osmotic flow of water is 25.4 atm.

At this point, we digress briefly to consider the full meaning of the term ACi in Eq. 19. The ability of a solution of any solute to exert an osmotic pressure across a semipermeable membrane (i.e., a membrane permeable to the solvent [water] but ideally impermeable to the solute) is a colligative property of the solution that is dependent on the concentration of individual solute particles (other such colligative properties are the vapor-pressure depression, boiling-point elevation, and freezing-point depression of solutions). We must therefore introduce a new unit of concentration that is a measure of the number of free particles in the solution and thus reflects the osmotic effectiveness of a dissolved solute. For a nondissociable solute such as urea (molecular weight, 60), when we dissolve 60 g (i.e., 1 g weight) of this solute in a sufficient amount of water to yield a total volume of 1 L, we have a 1-M solution of urea that contains about 6 x 1023 individual particles (Avogadro's number). However, when we dissolve 58 g of NaCl (molecular weight, 58) in a sufficient amount of water to yield a total volume of 1 L, we obtain a 1-M solution of NaCl, but the number of individual particles in solution will be twice that of a 1-M solution of urea. Thus, one must distinguish between the molarity (the number of gram weights of a solute in a liter of aqueous solution) and the osmolarity (the number of individual particles per liter that results from dissolving 1 g weight of a solute in that volume). A 0.15-M solution of NaCl that consists of 0.15 mol/L Na+ particles and 0.15 mol/L Cl— particles has approximately the same osmolarity as a 0.3-M sucrose solution (i.e., 300 mOsm/L). Indeed, historically, this observation provided the crucial evidence for the Arrhenius theory of the dissociation of salts in solution into their constituent ions.

In short, the osmolarity or the osmotically effective concentration of a solution of a dissociable salt will be n times its molarity, where n represents the number of individual ions (particles) resulting from the dissociation of the salt.

Finally, before considering osmotic flow across biologic membranes, it is important to gain deeper insight into the nature of osmotic pressure. When, as in the example given above, sufficient pressure is applied to the piston to prevent volume flow, the system is in equilibrium. There will be no flow of water despite the presence of a concentration difference for water across the membrane. This is because the pressure applied to compartment i exerts the same driving force for the flow of water as does the difference in water concentration, but in the opposite direction. Therefore, when the osmotic pressure is applied, there is no longer a net driving force for the movement of water, and volume flow ceases.

One may find this easier to understand by arbitrarily dividing the water flows into two hypothetical streams. One stream is directed from compartment o into compartment i and derives its driving force from the difference in the concentration of water across the membrane. The second stream is directed from compartment i to compartment o and is driven by the pressure applied by the piston. Flow ceases when these two oppositely directed streams are of equal magnitude; the pressure needed to achieve this equilibrium state is given by van't Hoffs law (Eq. 19). This balancing of driving forces is analogous to the condition discussed with reference to the Nernst equilibrium potential, where an electrical potential difference counterbalances the driving force arising from a concentration difference of a permeant ion when the counterion is impermeant.

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