This is a single protein in the plasma membrane, with a carrier action that is tightly linked to an enzymatic one. Under normal conditions, it hydrolyzes ATP. The energy released drives sodium ions up their electrochemical gradient out of the cell. The Na+/K+ ATPase also moves potassium ions the other way, into the cytosol. For every ATP hydrolyzed, three Na+ ions are moved out and two K+ ions are moved in. Figure 12.10 summarizes the reaction in terms of energy currencies.
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