Em is known as what?

• A. Membrane potential
• B. Equilibrium potential
• C. Resting potential
• D. Reversal potential

Answer: (A)

Membrane potential is the electrical voltage across the cell membrane, created by ion concentration differences and the membrane’s selective permeability. Em is the symbol used to denote this overall membrane potential. It represents the general voltage difference across the membrane, not tied to a specific ion. An equilibrium potential, by contrast, is the voltage at which a particular ion would have no net current through the membrane, and the reversal potential refers to the voltage where the direction of that ion’s current would reverse. Resting potential is simply a specific instance of membrane potential observed when the cell is at rest. So Em is best described as the membrane potential—the overall electrical state across the membrane.