Equilibrium potential is defined as the membrane potential at which an ion would not diffuse across the membrane. Which term describes this?

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

Answer: (A)

This concept describes the membrane voltage at which diffusion would stop for a specific ion because the electrical and chemical gradients are exactly balanced. That voltage is the equilibrium potential (also called the Nernst potential) for that ion. When the membrane reaches this potential, there’s no net movement of that ion across the membrane.

Remember that the actual membrane potential of a cell is determined by the combined influence of multiple ions and how permeable the membrane is to each one. Resting potential is the steady voltage of a cell at rest, shaped by all ions together. An action potential is a rapid, transient change driven by voltage-gated channels. Ion-specific equilibrium potentials help explain why these different states occur, with each ion having its own characteristic equilibrium potential (for example, potassium around -90 mV, sodium around +60 mV). The key point is that equilibrium potential is the voltage where net diffusion of that ion would cease.