The electron transport chain is a series of protein complexes
embedded in the mitochondrial membrane.
Electrons captured from donor molecules are transferred through these complexes.
Coupled with this transfer is the pumping of hydrogen ions.
This pumping generates the gradient used by the ATP synthase complex to synthesize ATP.
The following complexes are found in the electron transport chain:
NADH dehydrogenase,
cytochrome b-c1,
cytochrome oxidase,
and the complex that makes ATP, ATP synthase.
In addition to these complexes, two mobile carriers are also involved,
ubiquinone,
and cytochrome c.
Other key components in this process,
are NADH and the electrons from it,
hydrogen ions,
molecular oxygen,
water,
and ADP and Pi,
which combine to form ATP.
At the start of the electron transport chain,
two electrons are passed from NADH into the NADH dehydrogenase complex.
Coupled with this transfer is the pumping of one hydrogen ion for each electron
Next, the two electrons are transferred to ubiquinone.
Ubiquinone is called a mobile transfer molecule
because it moves the electrons to the cytochrome b-c1 complex.
Each electron is then passed from the cytochrome b-c1 complex
to cytochrome c.
Cytochrome c accepts each electron one at a time.
One hydrogen ion is pumped through the complex as each electron is transferred.
The next major step occurs in the cytochrome oxidase complex.
This step requires four electrons.
These four electrons interact with a molecular oxygen molecule
and eight hydrogen ions.
The four electrons, four of the hydrogen ions,
and the molecular oxygen,
are used to form two water molecules.
The other four hydrogen ions are pumped across the membrane.
This series of hydrogen pumping steps creates a gradient.
The potential energy in this gradient is used by ATP synthase
to make ATP from ADP and inorganic phosphate.
The ATP synthesis steps you see here
are discussed in greater detail in the ATP synthase gradients animation.
This animation illustrates two full cycles of electron donation.
In biological systems, however,
many electron transport cycles occur simultaneously
helping to ensure that the proton gradient is always maintained.
