Cell Membrane Pumps

Ion channels and carrier proteins not only assist in passive transport but also help with some types of active transport. The carrier proteins that serve in active transport are often called cell membrane "pumps" because they move substances from lower to higher concentrations.

Carrier proteins involved in facilitated diffusion and those involved in active transport are very similar. In both, the molecule first binds to a specific kind of carrier protein on one side of the cell membrane. Once it is bound to the molecule, the protein changes shape, shielding the molecule from the hydrophobic interior of the phospholipid bilayer. The protein then transports the molecule through the membrane and releases it on the other side. However, cell membrane pumps require energy. Most often the energy needed for active transport is supplied directly or indirectly by ATP.

Sodium-Potassium Pump

One example of active transport in animal cells involves a carrier protein known as the sodium-potassium pump. As its name suggests, this protein transports Na+ ions and K+ ions up their concentration gradients. To function normally, some animal cells must have a higher concentration of Na+ ions outside the cell and a higher concentration of K+ ions inside the cell. The sodium-potassium pump maintains these concentration differences.

Follow the steps in Figure 5-6 to see how the sodium-potassium pump operates. First, three Na+ ions bind to the carrier protein on the cytosol side of the membrane, as shown in step Q. At the same time, the carrier protein removes a phosphate group from a molecule of ATP. As you can see in step ©, the phosphate group from the ATP molecule binds to the carrier protein. Step © shows how the removal of the phosphate group from ATP supplies the energy needed to change the shape of the carrier protein. With its new shape, the protein carries the three Na+ ions through the membrane and then forces the Na+ ions outside the cell where the Na+ concentration must remain high.

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