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The plasma membrane is a selectively permeable barrier built from a phospholipid bilayer studded with proteins. Small nonpolar molecules such as oxygen and carbon dioxide diffuse across it freely, moving down their concentration gradients without help.

Water, however, crosses far more slowly on its own, so most cells rely on channel proteins called aquaporins to move it quickly in and out. These narrow pores let water molecules pass in single file while blocking ions and larger solutes.

This matters most in kidney cells, where aquaporin density controls how much water is reabsorbed from urine, and in red blood cells, which must swell and shrink instantly in capillaries of shifting osmotic pressure.