A cell may need to have a substance cross the plasma membrane against its concentration gradient (that is, from low to high concentration). 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. Three sodium ions inside the cell bind to the protein.The addition of a phosphate group from ATP changes the shape of the protein and the sodium is expelled. The phosphate is released and, as the protein returns to its former shape, two potassium ions are moved across the membrane. Sodium can once again bind to the protein, and the process repeats as long as there is a supply of ATP.