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Definition

noun

A kind of transport wherein ions or molecules move against a concentration gradient, which means movement in the direction opposite that of diffusion – or – movement from an area of lower concentration to an area of higher concentration. Hence, this process will require expenditure of energy, and the assistance of a type of protein called a carrier protein.

Supplement

Active transport is important so that substances can move in and out of a cell across the selectively permeable cell membrane against a concentration gradient.

Compare: passive transport, diffusion.
See also: concentration gradient.