<P> Electrons may enter an electron transport chain at the level of a mobile cytochrome or quinone carrier . For example, electrons from inorganic electron donors (nitrite, ferrous iron, etc .) enter the electron transport chain at the cytochrome level . When electrons enter at a redox level greater than NADH, the electron transport chain must operate in reverse to produce this necessary, higher - energy molecule . </P> <P> When bacteria grow in aerobic environments, the terminal electron acceptor (O) is reduced to water by an enzyme called an oxidase . When bacteria grow in anaerobic environments, the terminal electron acceptor is reduced by an enzyme called a reductase . </P> <P> In mitochondria the terminal membrane complex (Complex IV) is cytochrome oxidase . Aerobic bacteria use a number of different terminal oxidases . For example, E. coli does not have a cytochrome oxidase or a bc complex . Under aerobic conditions, it uses two different terminal quinol oxidases (both proton pumps) to reduce oxygen to water . </P> <P> Anaerobic bacteria, which do not use oxygen as a terminal electron acceptor, have terminal reductases individualized to their terminal acceptor . For example, E. coli can use fumarate reductase, nitrate reductase, nitrite reductase, DMSO reductase, or trimethylamine - N - oxide reductase, depending on the availability of these acceptors in the environment . </P>

What does it mean that the etc and atp synthesis are coupled