Certain biological processes—producing tears and processing urine, for instance—require cells to expel or take in water faster than it can diffuse through a cell membrane. In those cases, cells deploy aquaporins, special proteins that straddle a cell membrane and form water-permeable pores. Discovered two decades ago by Peter Agre (see Physics Today, December 2003, page 27), aquaporins are now known to provide the biomolecular plumbing for numerous species of plants, animals, and bacteria.

The proteins are highly selective gatekeepers. All of the dozens of known variants contain a narrow passageway—a so-called selectivity filter (SF), positioned near the pore’s extracellular opening—that stems the flow of large solutes. Likewise, strategically positioned charge centers embedded in the pore wall create potential barriers that block the flow of ions.

Harder to explain, however, is aquaporins’ impermeability to protons. Because protons can hop freely along a network of hydrogen-bonded water molecules, one would...

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