At rest, the cell typically has a membrane potential of about what value due to ion distribution and the Na+/K+ pump?

• A. -90 mV
• B. -60 mV
• C. -40 mV
• D. 0 mV

Answer: (A)

At rest, the membrane potential is driven mainly by potassium leaks and the Na+/K+ pump. The membrane is most permeable to K+ when the cell is quiet, so K+ tends to diffuse out, making the inside more negative. The electrical gradient builds until it balances the outward chemical gradient for K+, which happens at roughly the potassium equilibrium potential, about -90 mV. Since the resting membrane potential tracks this potassium-dominated balance, it sits near -90 mV in many cells. The Na+/K+ pump then maintains the ion gradients by pumping Na+ out and K+ in, which keeps the inside negative over time and prevents the gradients from dissipating. So the resting value is about -90 mV.