<P> In an antiporter, one substrate is transported in one direction across the membrane while another is cotransported in the opposite direction . In a symporter, two substrates are transported in the same direction across the membrane . Antiport and symport processes are associated with secondary active transport, meaning that one of the two substances is transported against its concentration gradient, utilizing the energy derived from the transport of another ion (mostly Na, K or H ions) down its concentration gradient . </P> <P> If substrate molecules are moving from areas of lower concentration to areas of higher concentration (i.e., in the opposite direction as, or against the concentration gradient), specific transmembrane carrier proteins are required . These proteins have receptors that bind to specific molecules (e.g., glucose) and transport them across the cell membrane . Because energy is required in this process, it is known as' active' transport . Examples of active transport include the transportation of sodium out of the cell and potassium into the cell by the sodium - potassium pump . Active transport often takes place in the internal lining of the small intestine . </P> <P> Plants need to absorb mineral salts from the soil or other sources, but these salts exist in very dilute solution . Active transport enables these cells to take up salts from this dilute solution against the direction of the concentration gradient . For example, the molecules chlorine (Cl ^ -) and nitrate NO3 - exist in the cytosol of plant cells, and need to be transported into the vacuole . While the vacuole has channels for these ions, transportation of them is against the concentration gradient, and thus movement of these ions is driven by hydrogen pumps, or proton pumps </P> <P> Primary active transport, also called direct active transport, directly uses metabolic energy to transport molecules across a membrane . Substances that are transported across the cell membrane by primary active transport include metal ions, such as Na, K, Mg, and Ca . These charged particles require ion pumps or ion channels to cross membranes and distribute through the body . </P>

Where does active transport occur in the human body