Outside Of Cell

Cell membrane.

figure 5-6

Follow what happens during one cycle of the sodium-potassium pump. O Three Na+ ions located in the cytosol bind to the carrier protein. O A phosphate group, represented by the letter P in the diagram, is removed from ATP and bound to the carrier protein. Q The binding of the phosphate group changes the shape of the carrier protein, allowing the three Na+ ions to be released into the cell's environment. O Two K+ ions located outside the cell bind to the carrier protein. G The phosphate group is released, restoring the original shape of the carrier protein. Q The two K+ ions are released into the cytosol, and the cycle is ready to repeat.

At this point, the carrier protein has the shape it needs to bind two K+ ions outside the cell, as step Q shows. When the K+ ions bind, the phosphate group is released, as indicated in step G, and the carrier protein restores its original shape. As shown in step © this time, the change in shape causes the carrier protein to release the two K+ ions inside the cell. At this point the carrier protein is ready to begin the process again. Thus, a complete cycle of the sodium-potassium pump transports three Na+ ions out of the cell and two K+ ions into the cell. At top speed, the sodium-potassium pump can transport about 450 Na+ ions and 300 K+ ions per second.

The exchange of three Na+ ions for two K+ ions creates an electrical gradient across the cell membrane. That is, the outside of the membrane becomes positively charged relative to the inside of the membrane, which becomes relatively negative. In this way, the two sides of the cell membrane are like the positive and negative terminals of a battery. This difference in charge is important for the conduction of electrical impulses along nerve cells. The sodium-potassium pump is only one example of a cell membrane pump. Other pumps work in similar ways to transport important metabolic materials across cell membranes.

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