electrochemical gradient and membrane potential

Why does a charged particle move across a membrane, and which way? For ordinary uncharged molecules, only one thing matters: they drift from crowded to sparse. But ions carry an electrical charge, so they feel two pulls at once — the usual pull toward where there are fewer of them, and an electrical pull toward the opposite charge. The combined push of these two together is called the electrochemical gradient.

Here electro means the voltage difference across the membrane and chemical means the concentration difference. The two can agree or fight. Sodium is both crowded outside and pulled in by the cell's negative inside, so both forces push it inward — strongly. Potassium is crowded inside (chemical force pushing it out) but also pulled back in by the negative inside (electrical force) — the two partly cancel, so its net urge to leave is gentle. An ion always flows along the sum of these forces, not just along concentration.

The voltage side of this story has its own name: membrane potential, the small electrical charge difference across a resting cell's membrane, typically the inside being about 70 thousandths of a volt negative. It exists because pumps and channels distribute ions unevenly. This stored electrical tension is a battery the cell can discharge: nerves fire and muscles contract by suddenly letting ions rush along their electrochemical gradients, and the energy in the sodium gradient also powers secondary transport. Membrane potential is, quite literally, the spark of thought and movement.