The rate at which molecules are actively transported across a membrane is determined by the rate of transport of individual pumps and the number of pumps in the membrane TF A. Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements. Active Cell Mechanisms Movements Through Cell Membranes Much of a cells supply of metabolic energy may be spent maintaining these processes. . Active transport E exocytosis. Proteins for Active Transport. Endocytosis is a form of active transport requiring energy. The specific proteins that facilitate active transport are called transporters.