The cell is constantly working, using energy and creating wastes. Mechanisms are in place for the transport of substances across the cellular membrane; some are active and others, such as osmosis, diffusion, or filtration, are passive. Active transport involves the use of energy to move molecules from an area of lower to an area of higher concentration and is divided into primary and secondary active transport. Primary active transport uses the initial source of energy to carry the substance. Secondary active transport harnesses the energy obtained from the primary active transport and uses it as a cotransporter of a secondary substance.