Ion concentration gradients contribute to the membrane potential primarily because which process occurs?

• A. K+, Na+, Ca++ diffusing in/out of the cell causing depolarization or repolarization
• B. Opening of ion channels
• C. Na+/K+-ATPase pumping ions against gradients
• D. Movement of chloride only

Answer: (A)

The membrane potential is set by ions moving down their electrochemical gradients through the membrane’s leak channels. Because the membrane is selectively permeable, ions such as K+, Na+, and Ca2+ diffuse in ways that carry charge across the membrane, and this diffusion creates a voltage difference as the intracellular and extracellular environments attempt to balance. At rest, the membrane is most permeable to potassium, so the resting potential sits near potassium’s equilibrium potential; openings that allow Na+ or Ca2+ to enter shift the potential toward their own equilibrium values, causing depolarization or repolarization depending on the direction of movement. The Na+/K+-ATPase maintains these gradients over time, but the actual membrane potential is primarily generated by ions diffusing down their gradients through channels. Movement of chloride alone doesn’t account for the potential, since other ions contribute as well.