Xref: utzoo sci.bio:3802 sci.chem:2369 sci.physics:15201 sci.misc:4530 Path: utzoo!attcan!uunet!decwrl!wuarchive!cs.utexas.edu!news-server.csri.toronto.edu!helios.physics.utoronto.ca!alchemy.chem.utoronto.ca!mroussel From: mroussel@alchemy.chem.utoronto.ca (Marc Roussel) Newsgroups: sci.bio,sci.chem,sci.physics,sci.misc Subject: Re: Osmosis - the cause at the molecular level. Message-ID: <1990Nov2.144847.19911@alchemy.chem.utoronto.ca> Date: 2 Nov 90 14:48:47 GMT References: <4396@pkmab.se> <29046@boulder.Colorado.EDU> Organization: Department of Chemistry, University of Toronto Lines: 28 In article richard@locus.com (Richard M. Mathews) writes: >eddy@boulder.Colorado.EDU (Sean Eddy) writes: > >>Osmosis is a colligative property. That is, osmotic pressure is >>dependent on the *number* of particles in solution -- *not* >>their size, mass, etc. > > 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. In fact deviations from the ideal law of a factor of two or three are relatively easy to find with large polymers. This is the point I was attempting to make in a previous post: the truth isn't as simple as the textbooks make it out to be. Marc R. Roussel mroussel@alchemy.chem.utoronto.ca