In every living cell, there are membranes, and in every membrane there are proteins, each of which acts as a chemical gatekeeper. Rather than passively letting ions pass in and out of the cell, these biochemical bouncers throw the door wide or shut it as needed. They let more life-sustaining materials like potassium or sodium through the cells' biological ion channels when the cell needs them, but shut off the flow before the chemical concentration gets too high.
