Aquaporins also called water channels, are integral membrane proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells. The cell membranes of a variety of different bacteria, fungi, animal and plant cells contain aquaporins through which water can flow more rapidly inside the cell than by diffusing through the phospholipid bilayer.

The 2003 Nobel Prize in Chemistry was awarded jointly to Peter Agre for the discovery of aquaporins, and Roderick MacKinnon for his work on the structure and mechanism of potassium channels.

Genetic defects involving aquaporin genes have been associated with several human diseases including nephrogenic diabetes insipidus and neuromyelitis optica.

The mechanism of facilitated water transport and the probable existence of water pores has attracted researchers since 1957. In most cells, water moves in and out by osmosis through the lipid component of cell membranes. Due to the relatively high water permeability of some epithelial cells, it was long suspected that some additional mechanism for water transport across membranes must exist. Solomon and his co-workers performed pioneering work on water permeability across the cell membrane in the late 1950s. In the mid-1960s an alternative hypothesis (the "partition–diffusion model") sought to establish that the water molecules partitioned between the water phase and the lipid phase and then diffused through the membrane, crossing it until the next interphase where they left the lipid and returned to an aqueous phase. Studies by Parisi, Edelman,Carvounis et al. accented not only the importance of the presence of water channels but also the possibility to regulate their permeability properties. In 1990, Verkman's experiments demonstrated functional expression of water channels, indicating that water channels are effectively proteins.

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