membrane potential class demonstration

Matt Jones jonesmat at ohsu.edu
Wed Dec 4 13:21:35 EST 1996


In article <k-quinn-0312961325340001 at mac074.physio.nwu.edu> Kevin Quinn,
k-quinn at nwu.edu writes:
>psychology that I will be teaching (lecture only - no lab).  Ideally it
>would be accomplished with very simple materials - to make the point
about
>a semipermeable membrane and the right ion distribution really resulting
>in a voltage gradient!  If anyone has suggestions for what may have
worked

At some point in college, I took a lab course where one of the exercises
was using artificial semi-permeable membranes to illustrate the Nernst
potential. The membranes were these plastic sheets that looked like
sandwhich wrap, but were probably more like dialysis tubing (permeable to
water and small solutes, available in different mesh sizes, and
purchasable from biochemistry or medical supply houses). These membranes,
however, featured charged polymer side groups that surrounded all the
microscopic holes. They came with either positive or negative charges.
Sorry, I don't remember where you can buy them.  

The experiment was to stretch the membrane over a hole between two
chambers of a container. Then each side was filled with salt (NaCl or
KCl) solutions of different concentrations. Electrodes were placed in
each side to measure potentials with a voltmeter or 'scope. Nothing
fancy.  A Nernst potential develops rapidly between the two chambers,
because (for a membrane with negative charges) sodium ions can cross the
membrane, but chloride ions are repelled away from the mesh by the
negative charges. The membranes can't distinguish between different
monovalent cations very well, so I'm not sure what would happen with KCl
on one side and NaCl on the other.

Anyway, It's cheap and simple, and doesn't require lots of amplifiers and
'scopes and stuff.

Cheers,
Matt



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