Membrane
boundary of a cell, organism, organization, system
A membrane is a Layer of material which serves as a selective barrier between two phases and remains impermeable to specific particles or group of particles or substances when exposed to the action of a driving force. Some components are allowed passage by the membrane into a permeate stream, whereas others are retained by it and accumulate in the retentate stream. http://en.wikipedia.org/wiki/Membrane
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