Why should potassium ions leak from neurons?

sodium.dioxid

In terms of neurons, the outside is more positive than the inside. Thus, a potassium ion trying to make an escape (due to it's concentration gradient) should be deflected/repelled back into the neuron. Those ions should be bounced back in when they reach close to the surface. But since not, this leads me to think that the outward acting "force" of concentration gradient is more powerful than the inward acting force of the electric gradient.

So, why is the outward acting "force" of the concentration gradient more powerful than the counteracting inward force of the electric gradient?

Reptillian

I'm not much of a biologist, but I'm guessing it's all about osmosis. I'd suppose the process makes use of the kinetic energy of the particles due to their temperature, and outside cation and anion concentrations in the solution. Probably a neuron membrane that becomes more or less permeable depending upon outside concentrations.

Pythagorean

Potassium/sodium pumps throughout the membrane are always working to take potassium in and push sodium out. The result is a much higher concentration of potassium inside the neuron than outside. The calculation of the force balance between the electricomagnetic force and the concentration gradient is known as the nernst equation.

sodium.dioxid

But how can concentration gradient counteract electrical force? Concentration gradient is not really a force; it is only random collisions leading ions from higher concentration to lower concentration by probability alone. How can random movement oppose electrical pull? This is the concept I am having trouble with.

Pythagorean

The energy supplying the force is thermal energy. The molecules bounce around from the thermal energy. Particles of the same size and charge will hit and repel each other. Because the collisions have no uniform scattering angle, the net force willl be outward. If you confine such particles, they will produce a pressure on the inside of the walls of the confinement barrier as the particles knock against it. All the while, you can continue to provide energy to the system by heating it. The pressure on the inside of the container is a significant, measurable force (per area).

sodium.dioxid

So, what you are saying is that thermal energy is the driving force of potassium ions against the electrical gradient. Correct?

Edit: The problem with the container explanation is that it is not applicable to cells, I think. I would assume that the concentration of OVERALL molecules inside a cell and outside the cell are the same, even though there may be more of a particular molecule on one side. If I had a container with a non-permeable membrane and concentrated each side with different molecules, there would still be no net pressure.

Pythagorean

Overall concentration should not be considered. Only like particles interact.

The Goldman equation becomes important for determining the final resting potential of a membrane containing a variety of particles, but diffusion is still isolated for each flavor of molecule. If this is hard to grasp intuitively, try working through the derivation yourself a couple times:

http://en.wikipedia.org/wiki/Goldman...ion#Derivation

sodium.dioxid

Does that mean water molecules don't contribute thermally to the diffusion of ions?

Pythagorean

Water is the "medium" of diffusion; as a medium, it contributes a lot but in a different way.

sodium.dioxid

Thank you very much, pythagorean. Now I perfectly understand this!