Atp

FIGURE 20 The possible operation of the Na+ - K+ pump, (Na+, K+)-ATPase.

(E-P) stages back to the original E stage requires the simultaneous presence of Na+ in the intracellular compartment and K+ in the extracellular compartment. Digitalis glycosides (such as ouabain) appear to bind tightly with (E-P) at or near the site where K+ interacts with the enzyme and thereby prevent the conversion of (E-P) to E, which aborts the cycle. This sequence of events is shown in Fig. 20.

mediated by the pumps are precisely balanced by the oppositely directed flows of these ions through their leak pathways.

This pump-leak system is present in all cells in higher animals and is responsible for maintaining the low cell Na+ and high cell K+ concentrations characteristic of those cells. As discussed previously, the resulting Na+ gradient (or concentration difference)

The Pump-Leak Model

The plasma membranes surrounding cells of higher animals not only contain (Na+-K+ ) pumps but are also traversed by channels that permit the diffusional flows (leaks) of Na+ and K+ across those barriers. The interaction between these pumps and leaks is illustrated in Fig. 21. Briefly, the (Na+-K+) pumps extrude Na+ from the cells and simultaneously propel K+ into the cells at the expense of metabolic energy (ATP). This results in a low intracellular Na+ concentration and a high intracellular K+ concentration, which in turn sets the stage for Na+ diffusion into the cell and K+ diffusion out of the cell through their respective leak pathways. The final result of the interactions between the pump and leaks is a time-independent or steady-state condition, where the movements of Na+ and K+

FIGURE 21 Model of a cell containing an ATP-dependent (Na+ -K+) exchange pump, leak pathways for Na+ and K+, and a pathway for Cl- diffusion across the membrane; Pr- denotes negatively charged intracellular macromolecules (mainly proteins). Reasonable values for intracellular and extracellular K+ are 140 and 5 mmol/L, respectively; for intracellular and extracellular Na+, 15 and 140 mmol/L, respectively.

FIGURE 21 Model of a cell containing an ATP-dependent (Na+ -K+) exchange pump, leak pathways for Na+ and K+, and a pathway for Cl- diffusion across the membrane; Pr- denotes negatively charged intracellular macromolecules (mainly proteins). Reasonable values for intracellular and extracellular K+ are 140 and 5 mmol/L, respectively; for intracellular and extracellular Na+, 15 and 140 mmol/L, respectively.

across the plasma membrane can serve to energize a number of secondary active transport processes, such as the Na+-coupled accumulation of amino acids (cotransport), the extrusion of H+ produced by metabolic processes via the Na+-proton countertransport mechanism, and the regulation of cell Ca2+ by the Na+-Ca2+ countertransport mechanism. In the final analysis, all of these secondary active transport processes derive their energy from the ATP hydrolyzed by the (Na+-K+) pump—a truly remarkable design.

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