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Aquaporin

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Biological cells are batteries charged by a voltage across their cell membrane. The voltage is maintained through a gradient of protons; the latter push back into the cell driving key cellular processes, e.g., the synthesis of ATP (see March 2004 highlight). But leakage of protons back into the cell is lethal. This generates a problem for water channels that exist in cell walls, e.g., in human tissues and plants (see October 2003 highlight), and conduct each about 10 million water molecules a second. Since protons are found in physiological solutions at a fraction of 1/10 million each channel should pass along with the water a proton per second, draining the cellular battery. The cells' water channels amazingly prevent this leakage. The underlying mechanism was suspected to be connected with an orientation pattern of water in the channel enforced by an internal electrostatic field (see May 2002 highlight) that neither allows protons to hop through the channel, overtaking the water transport speedwise, nor permits protons to flow along with the water. Now two collaborative studies (1, 2) have confirmed the suggestion, finding an energy barrier high enough to reduce the chance of proton passage to less than a proton per day. (more)