Which ion concentration is higher inside the cell under resting conditions?

• A. Na+
• B. K+
• C. Ca++
• D. Cl-

Answer: (B)

Potassium is higher inside the cell at rest. This large intracellular K+ pool is actively maintained by the Na+/K+ ATPase, which pumps K+ into the cell (and Na+ out), creating a strong gradient. The resting membrane is more permeable to K+ than to other ions, so K+ tends to leak out, but the pump keeps a high intracellular concentration. This K+ gradient largely establishes the negative resting membrane potential, with the potassium equilibrium potential around -90 mV, pulling the membrane toward that value. By contrast, sodium is much more abundant outside the cell, calcium is kept very low inside by pumps and exchangers, and chloride is typically higher outside or variable, so none of those have a higher intracellular concentration at rest.