<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> <P> Most of the enzymes that perform this type of transport are transmembrane ATPases . A primary ATPase universal to all animal life is the sodium - potassium pump, which helps to maintain the cell potential . The sodium - potassium pump maintains the membrane potential by moving three Na+ ions out of the cell for every two K+ ions moved into the cell . Other sources of energy for Primary active transport are redox energy and photon energy (light). An example of primary active transport using Redox energy is the mitochondrial electron transport chain that uses the reduction energy of NADH to move protons across the inner mitochondrial membrane against their concentration gradient . An example of primary active transport using light energy are the proteins involved in photosynthesis that use the energy of photons to create a proton gradient across the thylakoid membrane and also to create reduction power in the form of NADPH . </P>

Which is not an example of cellular active transport