Editing Electron transport chain (section)

=== Electron donors ===
In the current biosphere, the most common electron donors are organic molecules. Organisms that use organic molecules as an electron source are called ''[[organotroph]]s''. Chemoorganotrophs (animals, fungi, protists) and ''[[phototrophs|photolithotrophs]]'' (plants and algae) constitute the vast majority of all familiar life forms.

Some prokaryotes can use inorganic matter as an electron source. Such an organism is called a ''[[lithotroph|(chemo)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 [[organotroph]]s and [[phototroph]]s in our [[biosphere]].

The use of inorganic electron donors such as [[Methanogenesis|hydrogen 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 and oxygen as an energy source.

==== Dehydrogenases: equivalents to complexes I and II ====
Bacteria can use several different electron donors. When organic matter is the electron 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{{sub|2}} dehydrogenase ([[hydrogenase]]), electron transport chain. Some dehydrogenases are also proton pumps, while 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. In the case of [[lactate dehydrogenase]] in ''[[E. coli]]'', the enzyme is used aerobically and in combination with other dehydrogenases. It is inducible and is expressed when the concentration of DL-lactate in the cell is high.{{citation needed|date=August 2020}}