Transport Across Cell Membranes

Structure and Function of Na+/K+ Pump

The sodium-potassium pump maintains a steep concentration gradient of Na+ across the cell membrane, with a greater concentration outside of the cell.

Operation of the sodium-potassium (Na+/K+) pump, a transmembrane ATPase that pumps sodium ions out of the cell and potassium ions into the cell, accounts for about one-third of a typical animal cell's ATP requirement, suggesting the importance of the gradients it maintains. The Na+ gradient established by this pump is important in regulating cell volume. Because the interior of a cell has a higher concentration of solutes than the exterior, osmotic pressure would tend to drive water into the cell and eventually cause it to burst. The Na+/K+ pump actively pumps ions out of the cell, compensating for the concentration of other solutes. In addition, the Na+ gradient established by the Na+/K+ pump is used to provide energy for transporting other molecules, such as sugars and amino acids, into the cell.

The concentration of Na+ is greater outside the cell than inside it, while the concentration of K+ is greater in the cytosol. The Na+/K+ pump is responsible for pumping both types of ions against their concentration gradients. When the cytoplasmic side of the pump is open, three binding sites with a high affinity for Na+ are exposed. When Na+ ions are bound in those three sites, ATP is hydrolyzed, broken down into another form through the addition of water, phosphorylating the pump. This addition of a phosphate group to the molecule changes the conformation of the pump so that the three Na+ ions are exposed to the extracellular environment and their binding affinity to the pump is decreased. The Na+ ions are released outside the cell. The same conformational change exposes two high-affinity binding sites for K+. When two K+ ions from the extracellular medium have bound to those sites, the dephosphorylation of the pump is triggered. This removal of a phosphate group changes the shape of the molecule, resulting in the reopening of the pump toward the cytoplasm, releasing the two K+ ions.

Sodium-Potassium ATPase

Powered by ATP, the Na+/K+ pump moves Na+ and K+ ions in opposite directions, each against its concentration gradient. Three Na+ ions are removed from and two K+ ions are imported into the cell with each cycle.