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I am reading about the effect of extracellular potassium and chloride on the membrane potential, and now a question has come to my mind about what would happen if we added some molecules that have no charge to the extracellular? (consider that only K+ and Cl− can cross the membrane.)

I think that membrane potential and volume does not change, but I'm not sure about that.

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@MCM I cannot add comment after the great answer of MCM. Thank you alot. Then this means the volume also increase? –  Fatima Nov 3 '12 at 18:38
    
I'm wondering if you don't mean increase the volume of the cell - in which case, in extreme conditions where the cell cannot maintain water and salt concentrations, the cell would lose volume as water and salts move out. In a hypotonic solution (much more water than solutes), the cell would lyse as osmotic pressure favors water moving into the cell. –  MCM Nov 3 '12 at 23:01

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The absolute answer would depend on a lot of factors, but the basics of it would be that - Yes, the volume does change and Yes, it would have an affect on the membrane potential.

By adding mass to any liquid solution, you are changing the volume. Plain and simple. Liquids are not compressible, and the only way to maintain volume while adding mass would be to increase the density of the liquid. [Edit] I'm wondering if you don't mean increase the volume of the cell - in which case, in extreme conditions where the cell cannot maintain water and salt concentrations, the cell would lose volume as water and salts move out. In a hypotonic solution (much more water than solutes), the cell would lyse as osmotic pressure favors water moving into the cell

The membrane potential would change, but how much depends on a lot of factors. It would most likely change due to the movement of ionic solvents down their concentration gradients. Osmosis would move water out of the cell and into the extracellular environment, and ultimately reduce the membrane potential as more water outside the cell would essentially decrease the molarity of the solution. The cell would have to pump more ions out in order to regain the membrane potential.

The exception might be if you added so many hydrophobic molecules outside the cell that you basically saturate the environment, which could prevent exchange of all charged or partially charged atoms and molecules.

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"The cell would have to pump more ions out in order to regain the membrane potential." So any change would be transient? –  Alan Boyd Nov 3 '12 at 16:41
    
@AlanBoyd - Correct. The cell has the ability to open or close ion channels (free exchange down the gradient), and decrease or increase ion pumps (active transport). If the solution is hypertonic, the cell can open channels until the membrane potential is regained. If the solution is hypotonic, the cell can expend energy via ion pumps to move atoms into solution. This is particularly pronounced in neurons, where signal propagation relies on the membrane potential's existence (and is a little bit higher than your average cell). Barring extreme conditions, of course. Cells can't always cope. –  MCM Nov 3 '12 at 23:07
    
A bit of an addendum: The cell almost never completely shuts down ion channels or pumps. The membrane potential is balanced between the two (and some other players). –  MCM Nov 3 '12 at 23:12

I'm no electrophysiologist, but I do know that the membrane potential is determined by the distribution of ions across the membrane: mostly Na+, K+, Ca2+ and Cl-.

An uncharged molecule will only influence membrane potential if it has an effect upon the distribution of these ions. One example of such a molecule is valinomycin, a neutral ionophore which permeabilises membranes to K+ ions. Another example is ouabain which inhibits the Na+/K+ pump that is in part responsible for establishing resting membrane potential.

But in general the addition of uncharged molecules will have no direct effect upon membrane potential.

(I expect that there will be someone along shortly to explain why it is much more complicated than this.)

edit: It turns out that MCM got here before me!

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Remember also that plenty of molecules have receptors on the membrane that can cause membrane potential to change, directly or indirectly. –  nico Nov 4 '12 at 8:11

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