<P> Since electron transport chains are redox processes, they can be described as the sum of two redox pairs . For example, the mitochondrial electron transport chain can be described as the sum of the NAD / NADH redox pair and the O / H O redox pair . NADH is the electron donor and O is the electron acceptor . </P> <P> Not every donor - acceptor combination is thermodynamically possible . The redox potential of the acceptor must be more positive than the redox potential of the donor . Furthermore, actual environmental conditions may be far different from standard conditions (1 molar concentrations, 1 atm partial pressures, pH = 7), which apply to standard redox potentials . For example, hydrogen - evolving bacteria grow at an ambient partial pressure of hydrogen gas of 10 atm . The associated redox reaction, which is thermodynamically favorable in nature, is thermodynamic impossible under "standard" conditions . </P> <P> Bacterial electron transport pathways are, in general, inducible . Depending on their environment, bacteria can synthesize different transmembrane complexes and produce different electron transport chains in their cell membranes . Bacteria select their electron transport chains from a DNA library containing multiple possible dehydrogenases, terminal oxidases and terminal reductases . The situation is often summarized by saying that electron transport chains in bacteria are branched, modular, and inducible . </P> <P> In oxidative phosphorylation, electrons are transferred from a low - energy electron donor (e.g., NADH) to an acceptor (e.g., O) through an electron transport chain . In photophosphorylation, the energy of sunlight is used to create a high - energy electron donor and an electron acceptor . Electrons are then transferred from the donor to the acceptor through another electron transport chain . </P>

Where does electron transport chain occur in a eukaryotic cell