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transport (trans´port) movement of materials in biologic systems, particularly across the cell membrane into and out of cells or across epithelial layers. active transport
the movement of ions or molecules across cell membranes and epithelial layers, usually against a concentration gradient with expenditure of metabolic energy. Under normal circumstances more potassium ions are present inside living cells and
more sodium ions are present in the extracellular fluid. The maintaining of these normal differences in electrolytic composition
is done through active transport. Other types of transport, such as passive transport, simple diffusion, and osmosis do not require significant amounts of metabolic energy.
(A) Passive transport exemplified by diffusion of potassium ions across the plasma membrane, through specific ion channels, down a concentration gradient; (B) active transport exemplified by the cellular sodium pump, which uses ATP hydrolysis to create gradients of sodium and potassium across the plasma membrane. oxygen transport
the carrying of oxygen through the bloodstream bound to hemoglobin (see oxyhemoglobin). passive transport
the movement of materials, usually across cell membranes, by processes not requiring expenditure of metabolic energy. See also active transport.
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