active transport

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Movement of molecules or ions across a cell membrane using energy provided by respiration. Examples of substances that can be actively transported across membranes are sodium ions and glucose.

Energy is needed because the movement occurs against a concentration gradient, with substances being moved from an area of low concentration to an area where there is a higher concentration. Active transport is therefore quite different from diffusion, which requires no input of energy. In diffusion the movement is in the opposite direction – from an area of high concentration to an area where the concentration is low. An example of diffusion is the movement of oxygen into the blood vessels of the lungs.

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