Unlike bacteria, most eukaryotic cells do not have an H+ electrochemical gradient across their plasma membranes. Rather, it is sodium ions that are more concentrated outside the cell than inside (Fig. 12.3). Typically, the sodium concentration in the cytosol is about 10 mmol liter-1 while the concentration in the extracellular medium is about 150 mmol liter-1. This chemical gradient is supplemented by a voltage gradient. The cytosol is between 70 and 90 mV more negative than the extracellular medium, that is, the transmembrane voltage of the plasma membrane is between -70 and -90 mV. There is therefore a large inward electrochemical gradient for sodium ions. If sodium ions are allowed to rush down this gradient, they release energy—approximately 15 kJ for every mole of Na+ entering the cytosol.
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