🚀 go-pugleaf

eddy@boulder.Colorado.EDU (Sean Eddy) writes:

>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....

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

I have a thought on an intuitive answer.  Think of a large particle in
the solution as a perfectly rigid, lumpy blob.  Think of the membrane
as a perfectly rigid, lumpy surface.  In general when you bring 2 lumpy
surfaces together they will touch at no more than 3 points.  When the
particle touches at just one point, that might be enough to start
bouncing it away.  If the gaps between the particle and the membrane
are large enough for water molecules to fit (which isn't very big) then
we find that each particle is only capable of blocking water in 1 to 3
spots.  At most we have to account for a factor of 3 difference between
a monatomic particle and a huge molecule rather than a factor based on the
ratio of surface areas.  If the assumption of perfect rigidity is wrong,
the factor could be a bit larger; but in the time scale of a collision,
I would expect rigidity to be a halfway decent approximation.

Richard M. Mathews			D efend
Locus Computing Corporation		 E stonian-Latvian-Lithuanian
richard@locus.com			  I ndependence
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