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A clearcut definition of Potential please

Michael Edelman mje at mich.com
Mon Feb 8 11:08:59 EST 1999



Richard Norman wrote:

> Wookie wrote in message <36bb94b0.44628370 at news.casema.net>...
> >I know that a difference in charge between intracellular and
> >extracellular is responsible for the membrane potential.
> >But I just can't grasp the idea of a potential, the only thing that
> >you can measure is a difference.
> >As I understand correctly the membrane potential is the force
> >experienced by a charged particle that's within this electric field.
> >Can please someone give a better definition
> >
> You have it, basically.  The most important idea is that the electric
> potential is a measure of one of the forces that tends to make ions
> move, the other force being diffusion measured by  the concentration
> gradient (or difference).   What I tell students is that it potential
> is
> the number you get when you stick a voltmeter across the membrane.
> For more details, take physics.

Right. Think of it this way: You're standing on a large plateau at
10,000'. You hold in your hand a 1 lb cannonball, 6' off the ground.

You drop the cannonball.

In a sense, you've dropped it from an altitude of 10,006 feet, but the
actual potential energy that you could extract was only the difference
between where it started (+10,006'), and where it ended up (+10,000').

Similar thought experiment:

Now you're standing at sea level, holding that same canonball. next to a
gorge that extends to 100 below sea level. If you drop the ball it goes
from +6' to -100'. The energy released is a function of the difference- a
total of 106'.

Whenever you talk about a voltage, you're talking about a difference. If
I say that the voltage in the wall sockets in your house is 120 volts,
I'm telling you the difference between the hot leg and the neutral leg is
120 volts.

Voltage is also referred to as electrical potential, as in potential
work. The potential is the difference between two states.

So in a membrane you've got a pump that moves one ion one way, and the
other, the other way. It's sorting ions of different charges on different
sides of the membrane. There's a charge that attracts these ions, and so
to sort them out you have to do work. How much work? Well, what's the
difference in the total charge between the ions? I.e., what's the
potential difference?

Any clearer?

-- mike




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