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K+ is more abundant inside the cell and thus has a propensity to move outwards. It has an equilibrium potential of -80 mV. Na+ is more abundant extracellularly and thus has a propensity to move inwards. It has an equilibrium potential of 58 mV. The resting membrane potential of a cell is about -70 mV. So, K+ moves out (more towards its equilibrium potential) and Na+ moves in (more towards its equilibrium potential). The pumps throw out the Na+ and bring back in K+ thus maintaining their concentration and hence the membrane potential.

Now if I suddenly remove the pump from the cell, what would happen? Will the cell moves towards K+'s equilibrium potential or towards Na+'s equilibrium potential? I know that the answer is that the cell will discharge and have a membrane potential of 0 mv, but I don't understand why.

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Both Na+ and K+ will move according to the electrochemical gradient. Given that K+ leaky channels are ever present, the cell is already around its K+ equilibrium. If you introduce leaky Na+ channel and remove the pumps, eventually the cell will be nothing more than a passive membrane and it will adopt the same concentrations of ions as the extracellular space, i.e., Na+ will pour in along its electrochemical gradient, after which K+ (formerly already at equilibrium) will start pouring out alongside its chemical gradient, due to the reduced electrical gradient because of Na+ coming in.

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  • $\begingroup$ Why will K+'s electrical gradient reduce due to Na+ coming in? And say, if I remove the pump but do not introduce Na+ leaky channels, then what will happen? Will the K+ still pour out? $\endgroup$ – Black Dagger May 3 '16 at 10:26
  • $\begingroup$ @BlackDagger - 1) inside less negative 2) more slowly - eventually every battery discharges $\endgroup$ – AliceD May 3 '16 at 11:19
  • $\begingroup$ Why will it discharge eventually --- I'm asking from a theoretical point of view. $\endgroup$ – Black Dagger May 3 '16 at 13:09
  • $\begingroup$ @BlackDagger stochastics; channels are never fully closed. A cell is not a sealed container. Even batteries discharge spontaneously. $\endgroup$ – AliceD May 3 '16 at 13:10
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    $\begingroup$ cause Na+ goes in $\endgroup$ – AliceD May 3 '16 at 13:42

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