Na⁺ - K⁺ pump
I. Needs energy (ATP) to work
II. Expels 3 Na⁺ for every 2K⁺ ions imported
III. Works against a concentration gradient
IV. Maintains resting potential

Ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium pump which transports 3 Na⁺ outwards for 2 K⁺ into the cell.