Just outside the city’s walls—the cell membrane—lay the extracellular world. K+ had heard legends about it from a grizzled old sodium ion: “Out there, kid, the concentration of us potassium folk is low. Really low. You’d be special. You’d be needed.”
But then he understood.
First, it reached inside the cell and grabbed three sodium ions, dragging them out against their chemical wishes. The sodium ions screamed—they hated the low-salt outside world—but the gatekeeper used one precious ATP to wrench them through. role of active transport