Electron transport chain
The electron transport chain in the mitochon-
drion is the site of oxidative phosphorylation
in eukaryotes. The NADH and succinate gen-
erated in the citric acid cycle is oxidized,
providing energy to power ATP synthase.
Photosynthetic electron transport chain of
the thylakoid membrane.
An electron transport chain 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 bio-
chemical reactions. These H+ ions are used
to produce adenosine triphosphate (ATP),
the main energy intermediate in living organ-
isms, as they move back across the mem-
brane. Electron transport chains are used for
extracting
energy
from
sunlight
(photosynthesis) and from redox reactions
such as the oxidation of sugars (respiration).
In chloroplasts, light drives the conversion
of water to oxygen and NADP+ to NADPH
and a transfer of H+ ions. NADPH is used as
an electron donor for carbon fixation. In mi-
tochondria, it is the conversion of oxygen to
water, NADH to NAD+ and succinate to fu-
marate that drives the transfer of H+ ions.
While some bacteria have electron transport
chains similar to those in chloroplasts or mi-
tochondria, other bacteria use different elec-
tron donors and acceptors. Both the respirat-
ory and photosynthetic electron transport
chains are major sites of premature electron
leakage to oxygen, thus being major sites of
superoxide production and drivers of oxidat-
ive stress.
Background
The electron transport chain is also called
the ETC. An enzyme called ATP synthase
catalyzes a reaction to generate ATP. The
structure of this enzyme and its underlying
genetic code is remarkably conserved in all
known forms of life.
ATP synthase is powered by a transmem-
brane electrochemical potential gradient usu-
ally in the form of a proton gradient. The
function of the electron transport chain is to
produce this gradient. In all living organisms,
a series of redox reactions is used to p