Which ion is the major intracellular cation?

• A. Sodium
• B. Potassium
• C. Calcium
• D. Magnesium

Answer: (B)

The inside of cells is dominated by potassium as the major positively charged ion. This high intracellular potassium level is maintained by the Na+/K+ ATPase pump, which moves potassium into the cell while pushing sodium out. That strong potassium gradient is a key driver of the resting membrane potential and helps regulate electrical excitability in nerve and muscle cells. Sodium, in contrast, is concentrated outside the cell, calcium is kept at very low free levels inside and acts as a signaling ion when it rises, and magnesium is present intracellularly but not the primary intracellular cation. So potassium is the principal intracellular cation.