A cell may need to have a substance cross the plasma membrane against its concentration gradient. This requires energy and is accomplished by active transport. The sodium-potassium pump is an example of this process. A large protein in the plasma membrane provides the doorway through which sodium and potassium ions can move. ATP is the energy source. Sodium inside the cell binds to the protein. The addition of a phosphate group from ATP changes the shape of the protein and the sodium is expelled. Now potassium binds to the protein. The phosphate is released and, as the protein returns to its former shape, potassium is moved into the cell. Sodium can once again bind to the protein and the process repeats as long as there is a supply of ATP.