william@lorien.newcastle.ac.uk (William Coyne) (10/22/90)
Osmosis & concentration gradients I have read that osmosis is caused by a concentration gradient with the number of water molecules building up on the side of the membrane which has the highest concentration of the salt. Why is this? Does it have something to do with the salt forming temporary weak bonds with the water molecules, so on the side of the membrane with the higher concentration fewer water molecules will be free to cross the membrane. Replies can be sent by email to - JANET: W.P.Coyne@uk.ac.newcastle UUCP : ...!ukc!newcastle.ac.uk!W.P.Coyne ARPA : W.P.Coyne%newcastle.ac.uk@nss.cs.ucl.ac.uk
teexmmo@ioe.lon.ac.uk (Matthew Moore) (10/23/90)
In article <1990Oct22.110253.2277@newcastle.ac.uk> william@lorien.newcastle.ac.uk (William Coyne) writes: >Osmosis & concentration gradients >I have read that osmosis is caused >by a concentration gradient with the number of water molecules building >up on the side of the membrane which has the highest concentration of the >salt. Why is this? > >Does it have something to do with the salt forming temporary weak bonds >with the water molecules, so on the side of the membrane with the higher >concentration fewer water molecules will be free to cross the membrane. Osmosis is a particular case of diffusion. Diffusion is simply movement of a substance from a region in which it is higghly concentrated to regions where it is less highly concentrated. Osmosis occurs when mixtures of substances, of differing concentrations, are separated by a barrier through which one can pass, but not the other. Diagram: -------- semi-permeable barrier (permeable to water, not salt) | | salt solution | water | | | <--movement of water The water diffuses from the region where it is more highly concentrated (on the right) to the region where it is diluted by having salt dissolved in it (on the left). This is a simple view of osmosis, presenting the essential feature. Understanding the nature of the membrane is not necessary, and has not been considered here. (yet :-)
larry@kitty.UUCP (Larry Lippman) (10/27/90)
In article <1990Oct22.110253.2277@newcastle.ac.uk>, william@lorien.newcastle.ac.uk (William Coyne) writes: > Osmosis & concentration gradients > I have read that osmosis is caused > by a concentration gradient with the number of water molecules building > up on the side of the membrane which has the highest concentration of the > salt. Why is this? My personal preference is to explain osmosis using a thermodynamic approach. If one considers "chemical potential", as defined by the Gibbs equilibrium theory, then it is simple to remember that in diffusional transport (and chemical reactions in general, for that matter) chemical substances move from higher to lower chemical potential. Osmosis represents a case where a solvent is common to both sides of a semipermiable membrane. The chemical potential of such a pure solvent would therefore be equal across such a membrane, and no transport would occur. However, the chemical potential of a solvent containing a solute is *less* than that of the solvent alone due to entropy (the solute being dispersed in a random fashion within the solvent). Therefore, from a purely thermodynamic standpoint, the pure solvent has a tendency to flow across the membrane into the side containing the solvent and solute. Unless, of course, a pressure is exerted on the side containing the solvent and solute which opposes the osmotic pressure developed in that side. > Does it have something to do with the salt forming temporary weak bonds > with the water molecules, so on the side of the membrane with the higher > concentration fewer water molecules will be free to cross the membrane. No. It is important to understand that osmosis is a colligative property of solutions in that the determining factors pertain solely to the number of molecules of solute in solvent (and thermodyamic factors), and are *independent* of actual chemical composition. Therefore, osmosis can occur with alcohol/water solutions just as well as salt/water solutions. It is interesting to note that osmosis was discovered by Nollet in the mid 1700's. Nollet filled animal bladders (*real* bladders, btw) with brandy and placed them in containers of water. Osmosis transport occurs across the membrane of the bladder resulting in water flowing into the bladder containing the brandy, in many instances causing it to burst. I won't speculate as to what possessed Nollet to begin filling animal bladders with brandy and then placing them in conainers of water... :-) Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 {utzoo, uunet}!/ \aerion!larry
chi9@quads.uchicago.edu (Lucius Chiaraviglio) (10/27/90)
In article <4137@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes: >In article <1990Oct22.110253.2277@newcastle.ac.uk>, >william@lorien.newcastle.ac.uk (William Coyne) writes: >> Does it have something to do with the salt forming temporary weak bonds >> with the water molecules, so on the side of the membrane with the higher >> concentration fewer water molecules will be free to cross the membrane. > > No. It is important to understand that osmosis is a colligative >property of solutions in that the determining factors pertain solely to >the number of molecules of solute in solvent (and thermodyamic factors), >and are *independent* of actual chemical composition. I know what you're saying -- that hydration of ions is not required to achieve osmotic effects -- but wouldn't charge-dipole interactions between ions and water molecules enhance the osmotic effects by lowering the concentration of free water to an extent beyond what they could accomplish by simple displacement? -- | Lucius Chiaraviglio | Internet: chi9@midway.uchicago.edu
larry@kitty.UUCP (Larry Lippman) (10/29/90)
In article <1990Oct27.035951.27558@midway.uchicago.edu>, chi9@quads.uchicago.edu (Lucius Chiaraviglio) writes: > > No. It is important to understand that osmosis is a colligative > >property of solutions in that the determining factors pertain solely to > >the number of molecules of solute in solvent (and thermodyamic factors), > >and are *independent* of actual chemical composition. > > I know what you're saying -- that hydration of ions is not required to > achieve osmotic effects -- but wouldn't charge-dipole interactions between > ions and water molecules enhance the osmotic effects by lowering the > concentration of free water to an extent beyond what they could accomplish by > simple displacement? I"m a little confused at the wording of your question, especially the use of the word "enhance", but I believe you might be alluding to what is usually called the "salting-out" effect, as often seen in biochemical applications. While osmosis per se is colligative in nature and not dependent upon the presence or concentration of ions, the resultant ionic strength of ions that are present may *modify* solubility and osmotic pressure. Let us consider a situation where a neutral ionic solute, like a salt, is dissolved in water. Since each salt anion and cation is surrounded by water dipoles in a hydration shell, the overall structure of the water is changed, therefore diminishing its solvent properties for a *second* solute, such as a protein. There is, in effect, less water (solvent) available for the second solute. In the case of osmosis in biological systems involving proteins as the solute, the presence of salt ions as mentioned above will have an effect upon osmotic pressure. A quantitative determination of the effect of such salt ions may be made through use of the Gibbs-Donnan equilibrium. It follows from the above that in the case of proteins the pH of the solution will also be a factor. The effect of salt ion concentration upon osmotic pressure will be maximum at the isoelectric point of the protein (where the net charge of the protein molecule is zero). While the effect of salting-out is commonly encountered with proteins in biological systems, it is by no means limited to proteins. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 {utzoo, uunet}!/ \aerion!larry