A protein occurring in cell membranes that functions as a water channel. Aquaporins increase permeability of the membrane to water, and can thus accelerate the process of osmosis. They are integral to the plasma membranes of red blood cells and proximal convoluted tubules, and are inserted into the plasma membrane of the renal collecting ducts in response to antidiuretic hormone. In plants they occur in the plasma membrane and tonoplast, where alterations in their abundance and permeability regulate the rate of osmosis across those membranes.