Electrochemical Equilibrium

So, when we put this all together, we find that if potassium and chloride are permeable, they will both be in equilibrium if and only if

1. There is a greater concentration of chloride in the extracellular fluid compared to the intracellular concentration (thus making a diffusion gradient pushing chloride in).
2. There is a greater concentration of potassium in the intracellular fluid compared to the extracellular concentration (thus making a diffusion gradient pushing potassium out).
3. The inside of the cell has a net negative charge (thus forming the electrical field pushing chloride out and potassium in). See the following figure.

The electric field sets up a membrane potential, an inbalance of charges inside and outside the cell. This membrane potential is measured in mV (millivolts) and is actually no different than the voltage found in our electrical sockets in our homes (just much smaller in magnitude). By agreement of all scientists, the memgrane potential is always measured on the inside of the cell. Thus a membrane potential of -70 mV means that the inside of the cell is negative compared to the outside (the outside of the cell is always of the opposite charge of what is inside).

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