membrane potentials as electrochemical gradients

Meaning: A membrane potential (Vm) is the voltage across a cell membrane, inside relative to outside. It arises from uneven ion distribution and selective permeability. Each ion feels an electrochemical gradient: a chemical part (diffusion down its concentration gradient) plus an electrical part (pull or push from Vm). At rest, K⁺ leaks out through channels, making the inside negative, until the inward electrical pull balances the outward concentration push.

Equation: Eion = (RT/zF) ln([ion]out/[ion]in), where Eion is the equilibrium (Nernst) potential in V, R = 8.31 J/(mol·K), T is temperature in K, z is the ion's charge, F = 9.65 × 10⁴ C/mol, and concentrations in mol/L. Driving force = Vm − Eion (V).

On the MCAT: Given concentrations, predict which way an ion moves when its channel opens, or the sign of Eion. A cation moves outward if Vm is more positive than its Eion, inward if less. Raising extracellular K⁺ makes EK less negative and depolarizes the cell.

Common confusion: At Eion the net flux is zero, though ions still cross in both directions. Resting Vm sits near EK, not at it, because some Na⁺ leaks in; the pump's direct electrical effect is small.