<P> Some prokaryotes can use inorganic matter as an energy source . Such an organism is called a lithotroph ("rock - eater"). Inorganic electron donors include hydrogen, carbon monoxide, ammonia, nitrite, sulfur, sulfide, manganese oxide, and ferrous iron . Lithotrophs have been found growing in rock formations thousands of meters below the surface of Earth . Because of their volume of distribution, lithotrophs may actually outnumber organotrophs and phototrophs in our biosphere . </P> <P> The use of inorganic electron donors as an energy source is of particular interest in the study of evolution . This type of metabolism must logically have preceded the use of organic molecules as an energy source . </P> <P> Bacteria can use a number of different electron donors . When organic matter is the energy source, the donor may be NADH or succinate, in which case electrons enter the electron transport chain via NADH dehydrogenase (similar to Complex I in mitochondria) or succinate dehydrogenase (similar to Complex II). Other dehydrogenases may be used to process different energy sources: formate dehydrogenase, lactate dehydrogenase, glyceraldehyde - 3 - phosphate dehydrogenase, H dehydrogenase (hydrogenase), etc . Some dehydrogenases are also proton pumps; others funnel electrons into the quinone pool . Most dehydrogenases show induced expression in the bacterial cell in response to metabolic needs triggered by the environment in which the cells grow . </P> <P> Quinones are mobile, lipid - soluble carriers that shuttle electrons (and protons) between large, relatively immobile macromolecular complexes embedded in the membrane . Bacteria use ubiquinone (the same quinone that mitochondria use) and related quinones such as menaquinone . Another name for ubiquinone is Coenzyme Q10 . </P>

Where does the electron transport chain occur in prokaryotes and eukaryotes