Xref: utzoo sci.bio:3800 sci.chem:2367 sci.physics:15197 sci.misc:4529 Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!uakari.primate.wisc.edu!sdd.hp.com!elroy.jpl.nasa.gov!turnkey!orchard.la.locus.com!fafnir.la.locus.com!fafnir.la.locus.com!richard From: richard@locus.com (Richard M. Mathews) Newsgroups: sci.bio,sci.chem,sci.physics,sci.misc Subject: Re: Osmosis - the cause at the molecular level. Message-ID: Date: 2 Nov 90 09:18:29 GMT References: <1990Oct28.115303.7221@newcastle.ac.uk> <4396@pkmab.se> <29046@boulder.Colorado.EDU> Organization: Locus Computing Corporation, Los Angeles, California Lines: 29 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 lcc!richard@seas.ucla.edu ...!{uunet|ucla-se|turnkey}!lcc!richard