🚀 go-pugleaf

>Why does the concentrations tend to even out? There will always be a lot
>of water molecules that are passing through the membrane in both directions,
>since nothing is stopping them from doing so. However, on the side with
>higher salt concentration, there will be fewer water molecules (and more
>salt ions) adjacent to a given membrane surface area, and therefore a
>slightly lower flow of water from that side to the other than in the other
>direction.

Indeed, that's the explanation I was taught, and in turn taught to
undergrads here at Boulder. It was intellectually satisfying to
me until a couple of weeks ago, when my complacency was thrashed by
a fellow grad student. We started arguing, and reading, and arguing
some more, and pretty soon the debate was spreading through the
department... but we're just a bunch o' biologists, so I'll
relate the problem to you guys. 

Osmosis is a colligative property. That is, osmotic pressure is
dependent on the *number* of particles in solution -- *not*
their size, mass, etc. 

This is a highly useful property, of course; we can measure
the molecular weights of things by finding the osmotic pressure of
(and hence the number of particles in) a given weight of stuff.

But trying to figure out how osmotic pressure isn't affected
by the size of the solute is the problem. Let me make it
clearer: the osmotic pressure of a DNA solution, avg. MW
say 10^8, length of the molecules damn near in the visible range, is
the *same* osmotic pressure as, say, a sugar solution
of the same concentration. I *cannot* rationalize this
in my brain using the "more solute, proportionally less
access to membrane for solvent" model. Big molecules would
obscure more of the membrane. (Furthermore, cut those DNA
molecules in half... now they have twice the osmotic pressure.
Yeah, right :) )

See my problem? P-chem texts have been unhelpful and generally
quite circular in their logic on this point, it's seemed ("it
just *is*").

What is the detailed rationale for osmotic pressure being
dependent upon only the number of solute molecules, not
their size, shape, and mass?

- Sean Eddy
- Dept. of Molecular, Cellular, Developmental Biology
- U. of Colorado at Boulder
- eddy@boulder.colorado.edu