Re: [HAPS-L] Osmosis

[Alan Magid <amagid@xxxxxxxxx>]

Jim-

Osmosis is driven by the difference in water activity or potential between
two phases separated by a membrane which permits solvent to penetrate but
holds back solute. In essence this means, more water per unit volume in one
phase vs another. This principle applies to any solute which does not
permeate the membrane. The difference in water pressure drives the flow of
water.

As in all issues in statistical mechanics, there is no basis for talking
about non-statistical mechanics! The differences in the osmotic pressure of
different proteins only emerges at very high protein concentrations such as
seen in RBCs or lens fibers. We did a thorough study of lens crystallins and
found striking differences in osmotic pressure vs concentration. These
differences largely, but not entirely, explain the radial gradient in
refractive index in the eye lens. This gradient enables a highly curved
simple lens to function as an almost perfect lens!

We studied rotational solution behavior and protein-protein interactions
(with Dr Koenig of IBM Watson Labs in NY) among other measures but could not
define what differences in structure lead to differences in the viral
coefficients that lead to different osmotic behavior. It is not solubility,
as you suggest, since they are highly soluble. We never could get NIH
funding to carry the work forward and so the how remains an unknown.

You can retrieve these citations by searching Pubmed for "Magid
crystallins". My supply of reprints is long ago exhausted. Sorry. A good
general reference on P-Chem of proteins is the text by Eisenberg and
Carothers that I mentioned earlier.

-Alan

PS Water cohesion via hydrogen bonds is unneccessary. Polystyrene in benzene
also shows osmotic pressure and is damn non-polar!

PPS Biological membranes are less than 10 nm thick, not 1 mm. But that makes
no difference here.

On 8/20/07 3:32 PM, "Jim Murray" <tritoniadiomedea@xxxxxxx> wrote:

> Alan
> 
> Would you mind helping me to understand the forces involved when
> proteins "suck" water from one place to another?  Of course, this is
> not due to a direct attraction between the protein molecule and water
> molecules that are a millimeter away on the other side of a
> membrane.  So should I assume it is due to the cohesion of water,
> after the protein is hydrated by the water molecules that are
> immediately adjacent to the protein?  So the suction is essentially a
> chain reaction of electrostatic forces (in hydrogen bonding).
> 
> Does this mean that molecules with more hydration bonding sites
> create a higher osmotic pressure?
> 
> Is there something I should read to help me with this?
> 
> thanks
> 
> James A. Murray
> Assoc. Prof., Department of Biology
> 156 Lewis Science Center
> University of Central Arkansas
> Conway, AR  72035
> PH: 501-450-5923
> FAX: 501-450-5914
> mailto:tritoniadiomedea@xxxxxxx
> http://www.uca.edu/biology/faculty_details_comp.php?user_id=28
> 
> 
> On Aug 20, 2007, at 8:53 AM, Alan Magid wrote:
> 
>> All becomes clearer (and a chuckle follows) when I make the point
>> that osmotic pressure is really osmotic SUCK. Thus, to pick an
>> important example, plasma proteins suck water back into the
>> capillary from the interstitial space.
> 
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