Antiport Mechanism

 In antiport mechanism, a cell uses movement of an ion across a membrane and down its concentration gradient to power the transport of a second substance "uphill" against its gradient. During this process, the 2 substances move across the membrane in opposite directions. An example of an antiport process is that the transport of Ca2+ ions out of heart muscle cells. Muscle cells are triggered to contract by an increase in intracellular Ca2+ concentration, so it's imperative that Ca2+ be far away from the cytoplasm in order that the muscle can relax before contracting again. This antiport system is so effective that it can maintain the cellular concentration of Ca2+ at levels 10,000 times less than the external concentration.  

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