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As membrane transport is defined the event by which a molecule or ion passes across a barrier (= membrane), not by passive diffusion, but by a process in which the movement, its directionality, its rate and extent are governed by specific devices, rather than by the gradient established across the barrier. In a number of cases this sort of transport involves expenditure of energy, which in the cell can be recovered from the oxidoreduction reactions of the respiratory chain, or from the hydrolysis of ATP.