In biochemistry, the movement of molecules and particles across a cell membrane, a selective barrier that allows some substances (fat-soluble molecules and some small molecules) to pass and blocks others (ions and large, water-soluble molecules). Transport of these vital substances occurs via several systems. Open channels allow diffusion (passive transport) of ions directly into cells; facilitators use a chemical change to help substances diffuse past the membrane; pumps force dilute substances through even when their concentration on the other side is higher (a form of active transport). Primary active transport is powered directly by energy released in cell metabolism (seeATP, adenosine triphosphate). In secondary active transport, a molecule is linked to a different molecule that carries it across the membrane (cotransport) or is exchanged for a different molecule crossing in the other direction (countertransport). The membrane itself opens and closes to let larger particles in or out.