Ion channels are present in the membranes that surround all biological cells. By conducting and controlling the flow of ions, these pore-forming enzymes help establish the small negative voltage that all cells possess at rest (see cell potential). An ion channel is an integral membrane protein or more typically an assembly of several proteins. Such "multi-subunit" assemblies usually involve a circular arrangement of identical or related proteins closely packed around a water-filled pore. Ions move through the pore single file--nearly as fast as the ions move through free fluid. Access to the pore is governed by "gates," which may be opened or closed by chemical or electrical signals, or mechanical force, depending on the variety of channel.