An electron transport chain (ETC)
couples a chemical reaction between
an electron donor (such as NADH) and
an electron acceptor (such as O2) to the
transfer of H+ ions across a membrane,
through a set of mediating biochemical
reactions. These H+ ions are used to
produce adenosine triphosphate
(ATP), the main energy intermediate
in living organisms, as they move back
across the membrane. Electron
transport chains are used for extracting
energy from sunlight (photosynthesis)
and from redox reactions such as the
oxidation of sugars (respiration).
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