The aqua (glycero) porins conduct water (and glycerol) across cell membranes. The structure of these channels reveals a tripathic channel that supports a hydrophobic surface and, opposite to this, a line of eight hydrogen-bond acceptors and four hydrogen-bond donors. The eight carbonyls act as acceptors for water (or glycerol OH) molecules. The central water molecule in the channel is oriented to polarize hydrogen atoms outward from the center. This arrangement suggests how the structure prevents the potentially lethal conduction of protons across the membrane. The structure also suggests the mechanism behind the selectivity of aquaglyceroporins for glycerol, the basis for enantioselectivity among alditols, and the basis for the prevention of any leakage of the electrochemical gradient.