Also known as the sodium/potassium pump, the Na+/K+ pump is the most common primary active transport mechanism in human cells. It helps maintain the electrochemical gradient in cells. Although Na+ and K+ ions have equal charge, the pump expells 3 Na+ ions for each 2 K+ ions that it takes in. This reaction requires energy from the breakdown of adenosine triphosphate (ATP). 40% (if I remember correctly) of a cell's energy is used for these pumps. They are especially prevalent in nerve and muscular cells due to their method of transmission of impulses, both action potentials and graded potentials.