B3 1.2 Active Transport

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Cells need to move substances in and out. Water often moves across the cell boundaries by osmosis. Dissolved substances also need to move in and out of cells. There are two main ways in which this happens. Substances move by diffusion, along with a concentration gradient. This must be in the right direction to be useful to the cells. However, sometimes the substances needed by a cell have to be moved against a concentration gradient, or across a partially permeable membrane. This needs a special process called active transport.

Moving subtances by active transport

Active transport allows cells to move substances from an area of low concentration to an area of high concentration. This movement is against the concentration gradient. As a result, cells can absorb ions from very dilute solutions. It also enables them to move substances, such


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