  Aquaporin-1 (AQP1) is a membrane protein which is selectively permeable to
water. Due to its dumbbell shape, AQP1 can sense the size information of solute
molecules in osmosis. At the cost of consuming this information, AQP1 can move
water against its chemical potential gradient: it is able to work as one kind
of Maxwell's Demon. This effect was detected quantitatively by measuring the
water osmosis of mice red blood cells. This ability may protect the red blood
cells from the eryptosis elicited by osmotic shock when they move in the
kidney, where a large gradient of urea is required for the urine concentrating
mechanism. This finding anticipates a new beginning of inquiries into the
complicated relationships among mass, energy and information in bio-systems.
