Xref: utzoo sci.chem:2221 sci.bio:3685
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!sdd.hp.com!uakari.primate.wisc.edu!aplcen!aplcomm!uunet!mcsun!ukc!educ-isis!teexmmo
From: teexmmo@ioe.lon.ac.uk (Matthew Moore)
Newsgroups: sci.chem,sci.bio
Subject: Re: osmosis - how is it caused?
Message-ID: <1990Oct23.140559.23008@ioe.lon.ac.uk>
Date: 23 Oct 90 14:05:59 GMT
References: <1990Oct22.110253.2277@newcastle.ac.uk>
Reply-To: teexmmo@ioe.lon.ac.uk (Matthew Moore)
Organization: Institute of Education University of London
Lines: 40

In article <1990Oct22.110253.2277@newcastle.ac.uk> william@lorien.newcastle.ac.uk (William Coyne) writes:
>Osmosis & concentration gradients
>I have read that osmosis is caused
>by a concentration gradient with the number of water molecules building
>up on the side of the membrane which has the highest concentration of the
>salt.  Why is this?
> 
>Does it have something to do with the salt forming temporary weak bonds
>with the water molecules, so on the side of the membrane with the higher
>concentration fewer water molecules will be free to cross the membrane.

Osmosis is a particular case of diffusion.

Diffusion is simply movement of a substance from a region in which it
is higghly concentrated to regions where it is less highly
concentrated.

Osmosis occurs when mixtures of substances, of differing
concentrations, are separated by a barrier through which one can pass,
but not the other.

Diagram:
--------

                semi-permeable barrier (permeable to water, not salt)
                         |
                         |
    salt solution        |     water
                         |
                         |
                         |
                        <--movement of water

The water diffuses from the region where it is more highly
concentrated (on the right) to the region where it is diluted by
having salt dissolved in it (on the left).

This is a simple view of osmosis, presenting the essential feature.
Understanding the nature of the membrane is not necessary, and has not
been considered here. (yet :-)