Now, let us see what happens when we now let potassium move. (Let us assume that chloride is in equilibium, remember that means there is and higher concentration of chloride outside the cell and that there is a negative charge inside.) When we last left potassium, there were equal concentrations of potassium inside and out, so there is no net diffusion. However, the negative electrical charge inside the cell (due to the chloride) will attract the positively charged potassium inside the cell. See the red arrow in the following figure.
If we let enough potassium enter the cell so that they positive charges of potassium negates the negative charges inside the cell (so that there is no electrical charge) then the concentration of potassium inside the cell will be greater than that outside, setting up a diffusion gradient moving potassium back out (see the following figure).
Well, potassium will now move out. Since it is positively charged, this will make the inside negative and the outside positive (just like before), reinstituting the electrical field. So once again, potassium will move inward due to this electrical field. Just like chloride, whenever we reach a state in which the concentration gradient is removed, there is an electical force still present. Whenever we reach a state in which the electical force is removed, there is a concentration gradient. Equilibrium will occur when the electical force pushing potassium in is equal to the diffusion force pushing the potassium out. This will occur when
There must be a greater concentration of potassium in the intracellular fluid compared to the extracellular concentration (thus making a diffusion gradient pushing potassium out).
The inside of the cell must have a net negative charge (thus forming the electrical field pushing potassium in). See the following figure.
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