Basic Features
Ion channels regulate the flow of ions across the membrane in all cells. Ion channels are integral membrane proteins; or, more typically, an assembly of several proteins. They are present on all membranes of cell (plasma membrane) and intracellular organelles (nucleus, mitochondria, endoplasmic reticulum, golgi apparatus and so on). Such "multi-subunit" assemblies usually involve a circular arrangement of identical or homologous proteins closely packed around a water-filled pore through the plane of the membrane or lipid bilayer. For most voltage-gated ion channels, the pore-forming subunit(s) are called the α subunit, while the auxiliary subunits are denoted β, γ, and so on. Some channels permit the passage of ions based solely on their charge of positive (cation) or negative (anion). However, the archetypal channel pore is just one or two atoms wide at its narrowest point and is selective for specific species of ion, such as sodium or potassium. These ions move through the channel pore single file nearly as quickly as the ions move through free fluid. In some ion channels, passage through the pore is governed by a "gate," which may be opened or closed by chemical or electrical signals, temperature, or mechanical force, depending on the variety of channel.
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