Sometimes substances must move from regions of lower concentrations to a region of higher concentration-against the passive movement from higher to lower concentration. This movement of substances across the plasma membrane against a concentration gradient requires energy; therefore, it is called active transport.
Active transport occurs with the aid of carrier proteins, commonly called pumps. Some pumps move one type of substance in only one direction, while others move two substances either across the membrane in the same direction or in opposite directions. Because of active transport, the cell maintains the proper balance of substances it needs. Active transport helps maintain homeostasis.
