Co-transport, also known as coupled transport, refers to the simultaneous or sequential passive transfer of molecules or ions across biological membranes in a fixed ratio. Permitting one ion or molecule to move from the side where it is more concentrated to that where it is less concentrated increases entropy and can serve as a source of energy for metabolism (e.g. in ATP synthase). In cotransporters, it is used to force the transport of another ion or molecule (usually) from the side where it is less concentrated to that where it is more concentrated. Cotransporters can be classified as symporters and antiporters depending on whether the substances move in the same or opposite directions.
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