The Electron Transport Chain Reading Page
ATP synthesis is not an energetically favorable reaction: energy is needed in order for it to occur. This energy is derived from the
oxidation of NADH and FADH2 by the four protein complexes of the electron transport chain (ETC). The ten NADH that enter the
electron transport originate from each of the earlier processes of respiration: two from glycolysis, two from the transformation of
pyruvate into acetyl-CoA,
and six from the citric acid
cycle. The two FADH2
originate in the citric acid
cycle.
The events of the electron
transport chain involve
NADH and FADH, which
act as electron
transporters as they flow
through the inner
membrane space. In
complex I, electrons are
passed from NADH to the
electron transport chain,
where they flow through
the remaining complexes.
NADH is oxidized to NAD
in this process. Complex II
oxidizes FADH, garnering
still more electrons for the
chain. At complex III, no
additional electrons enter
the chain, but electrons from complexes I and II flow through it. When electrons arrive at complex IV, they are transferred to a molecule
of oxygen. Since the oxygen gains electrons, it is reduced to water.
While these oxidation and reduction reactions take place, another, connected event occurs in the electron transport chain. The
movement of electrons through complexes I-IV causes protons (hydrogen atoms) to be pumped out of the intermembrane space into
the cell cytosol. As a result, a net negative charge (from the electrons) builds up in the matrix space while a net positive charge (from
the proton pumping) builds up in the intermembrane space. This differential electrical charge establishes an electrochemical gradient. It
is this gradient that drives ATP synthesis in oxidative phosphorylation.
As a result of the electron transport chain, an electrochemical gradient is formed on either side of the inner mitochondrial membrane.
The outside of the membrane is positive while the inside is negative. The positive hydrogen ions are allowed to flow back across the
membrane through specialized channels manned by proton translocating ATP synthase, which uses the energy created by the
energetically favorable transport to synthesize ADP and
phosphate into ATP.
The transport of just two electrons through the electron
transport chain generates enough free energy in the form
of electrochemical gradient to drive the synthesis of one
molecule of ATP. The synthesis of ATP necessitates the
dissolution of the electrochemical gradient, however,
since the whole process is driven by positive hydrogen
ions (protons) flowing back into the matrix space from the
intermembrane space. The ETC maintains the
electrochemical gradient by continuing to generate
hydrogen ions.
In total, the process started through the glycolysis of one
glucose molecule yields about 32 ATP in oxidative
phosphorylation. In total, oxidative phosphorylation
accounts for around 90 percent of the body’s total ATP.