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The transport of substances, usually polar molecules or ions, across a membrane against a concentration gradient. Active transport is energy requiring and is mediated by specific carrier proteins or translocases, which selectively bind a substrate and transport it across the membrane. The most common example of an active transport system is the sodium/potassium system, which actively pumps sodium ions out of the cell at the expense of ATP, while at the same time carrying potassium ions into the cell. The resulting concentration gradient can be very high; in the freshwater alga Nitella clavata the ratio of potassium inside the cell to that outside is 1065:1.
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