Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why should potassium ions leak from neurons?

  1. Jul 2, 2012 #1
    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?
     
  2. jcsd
  3. Jul 2, 2012 #2
    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.
     
  4. Jul 3, 2012 #3

    Pythagorean

    User Avatar
    Gold Member

  5. Jul 3, 2012 #4
    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.
     
  6. Jul 3, 2012 #5

    Pythagorean

    User Avatar
    Gold Member

    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).
     
  7. Jul 3, 2012 #6
    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.
     
    Last edited: Jul 3, 2012
  8. Jul 3, 2012 #7

    Pythagorean

    User Avatar
    Gold Member

    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_equation#Derivation
     
  9. Jul 3, 2012 #8
    Does that mean water molecules don't contribute thermally to the diffusion of ions?
     
  10. Jul 3, 2012 #9

    Pythagorean

    User Avatar
    Gold Member

    Water is the "medium" of diffusion; as a medium, it contributes a lot but in a different way.
     
  11. Jul 5, 2012 #10
    Thank you very much, pythagorean. Now I perfectly understand this!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook