Introduction
The electron transport system is the place in the cell
where electrons generated by oxidation are transferred.
Passage of the electrons through the system generates
potential energy that is used to make ATP in oxidative
phosphorylation.
Components of electron transport chain
Components of ETC are arranged in the inner mitochondrial membrane
involved in the process of electron transport. In biological system,
fuel molecules are oxidized by losing electrons, which are
transferred through the carrier molecules like NAD and FAD. Finally
electrons are donated to oxygen to reduce it to a water molecule.
Coenzyme Q is a respiratory electron carrier. Cytochromes are a
group of heme containing proteins. The major respiratory cytochromes
are b, c and c1 and a or a3.
Electron Transport � Carriers and
arrangement of carriers into complexes
Integral membrane proteins embedded in the inner mitochondrial
membrane are arranged into complex I, II, III and IV. They accept or
donate electrons from preceding carrier to the following in
sequence.
Pathway of Electron Transfer through the
Carriers
I NADH dehydrogenase, II Succinate dehydrogenase, III CoQ �
Cytochrome c oxidoreductase, and IV Cytochrome oxidase are the
four complexes. Electrons are transferred through these complexes.
Through complex I, III and IV, protons are pumped out of
mitochondrial matrix creating proton gradient.
Proton Motive Force
Coupling of ATP synthesis to electron transfer via an
electrochemical H+ gradient across a membrane is called
chemiosmosis. Energy from electron transport drives an active
transport system which pumps protons out of the mitochondrial matrix
into inter- membrane space. An electrochemical gradient of protons
is created, with a lower pH value outside the inner mitochondrial
membrane than inside. The protons on the outside have a
thermodynamic tendency to flow back in, so as to equalize pH on both
sides of the membrane.
ATP Synthesis
When protons from inter-membrane space do flow back into the
matrix, the free energy arising from the gradient is used to drive
the synthesis of ATP. ATP synthase also called complex V is the site
where ADP is phosphorylated to ATP.
