<P> Note that electrons can enter the chain at three levels: at the level of a dehydrogenase, at the level of the quinone pool, or at the level of a mobile cytochrome electron carrier . These levels correspond to successively more positive redox potentials, or to successively decreased potential differences relative to the terminal electron acceptor . In other words, they correspond to successively smaller Gibbs free energy changes for the overall redox reaction Donor → Acceptor . </P> <P> Individual bacteria use multiple electron transport chains, often simultaneously . Bacteria can use a number of different electron donors, a number of different dehydrogenases, a number of different oxidases and reductases, and a number of different electron acceptors . For example, E. coli (when growing aerobically using glucose as an energy source) uses two different NADH dehydrogenases and two different quinol oxidases, for a total of four different electron transport chains operating simultaneously . </P> <P> A common feature of all electron transport chains is the presence of a proton pump to create a transmembrane proton gradient . Bacterial electron transport chains may contain as many as three proton pumps, like mitochondria, or they may contain only one or two . They always contain at least one proton pump . </P> <P> In the present day biosphere, the most common electron donors are organic molecules . Organisms that use organic molecules as an energy source are called organotrophs . Organotrophs (animals, fungi, protists) and phototrophs (plants and algae) constitute the vast majority of all familiar life forms . </P>

​which of the following is a feature of the electron transport chain