Aquaporin membrane proteins. Artwork of a cell membrane and membrane proteins known as aquaporins (red). These structures (four red balls) form channels that help water molecules (red and blue) pass in and out of the cell. Unlike ion channels, aquaporins help prevent ions and other dissolved substances carrying electrical charge (green) from entering the cell, as they only allow lone water molecules or certain uncharged solutes to pass through. This helps to maintain the electrochemical potential of the membrane. The 1992 discovery of aquaporins led to a share of the 2003 Nobel Prize in Chemistry for US molecular biologist Peter Agre.