<P> Just as there are a number of different electron donors (organic matter in organotrophs, inorganic matter in lithotrophs), there are a number of different electron acceptors, both organic and inorganic . If oxygen is available, it is invariably used as the terminal electron acceptor, because it generates the greatest Gibbs free energy change and produces the most energy . </P> <P> In anaerobic environments, different electron acceptors are used, including nitrate, nitrite, ferric iron, sulfate, carbon dioxide, and small organic molecules such as fumarate . </P> <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>

Where are the proteins of the electron transport chain involved in cellular respiration located