aquaporin
/ AH-kwuh-POR-in /
Water can trickle slowly across a plain membrane on its own, but some cells need water to move much, much faster — your kidney, for instance, recovers liters of water a day, and a slow leak through fat would never keep up. The answer is a dedicated water-only doorway: a protein called an aquaporin, literally a water pore.
An aquaporin is a transmembrane channel with a pore so cleverly shaped that water molecules slide through single file, billions per second, while almost everything else — even tiny charged protons — is turned away. It does not push water; like all channels it is passive, simply giving osmosis a fast lane to follow. Where a cell studs its membrane with aquaporins, water rushes across; where it removes them, the same membrane becomes nearly waterproof. So a cell controls its water flow not by changing the water but by adding or hiding these channels.
Aquaporins quietly run a lot of biology. Your kidneys insert them to concentrate urine and save water; the hormone that does this (and whose absence causes a disease of constant thirst and torrents of urine) works precisely by telling kidney cells to put more aquaporins in their membranes. Plant roots, red blood cells, sweat glands, and your eyes all rely on these tiny water gates. Their discovery earned a Nobel Prize because they solved an old mystery: how water crosses membranes so fast.
When you are dehydrated, a hormone tells your kidney cells to push more aquaporins into their membranes, so more water is reclaimed and your urine turns dark and scant.
Cells dial water flow up or down by adding or removing aquaporins.
An aquaporin lets water through but firmly blocks ions and even protons — if it leaked charged particles too, it would wreck the cell's carefully kept electrical balance.