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

• A. Na+
• B. K+
• C. Ca++
• D. Cl-

Answer: (A)

The key idea is how ion gradients across the cell membrane are established and maintained when the cell is at rest. The Na+/K+ ATPase pump continuously moves sodium out of the cell and potassium into the cell, creating and sustaining a low intracellular Na+ concentration and a high extracellular Na+ concentration. The membrane is relatively more permeable to potassium, so K+ tends to leak out, which helps set the resting membrane potential negative inside. With these mechanisms, sodium remains much higher outside than inside, while potassium is higher inside than outside. Typical values illustrate this: extracellular sodium is around 140–145 mM, whereas intracellular sodium is only about 10–15 mM. Calcium is kept at very low levels inside, and chloride distribution is also different, but the standout resting-state difference is the high outside concentration of sodium.