Some aspects of cell transport cannot be explained by simple or facilitated diffusion. The epithelial linings of the small intestine and kidney tubules, for example, move glucose from the side of lower to the side of higher concentration—from the space within the tube (lumen) to the blood. Similarly, all cells extrude Ca2+ into the extracellular environment and, by this means, maintain an in-tracellular Ca2+ concentration that is 1,000 to 10,000 times lower than the extracellular Ca2+ concentration. This steep concentration gradient sets the stage for Ca2+ to be used as a regulatory signal. The opening of plasma membrane Ca2+ channels, and the rapid diffusion of Ca2+ that results, provides a signal for neurotransmit-ter release, muscle contraction, and many other cellular activities.
Active transport is the movement of molecules and ions against their concentration gradients, from lower to higher concentrations. This transport requires the expenditure of cellular energy obtained from ATP; if a cell is poisoned with cyanide (which inhibits oxidative phosphorylation), active transport will stop. Passive transport, by contrast, can continue even if metabolic poisons kill the cell by preventing the formation of ATP.
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