and the complex that makes ATP,

In addition to these complexes, two mobile carriers are also involved:

Other key components in this process are

and the electrons from it,

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

transfered 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 cytchrome 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 transfered.

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 sythase 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.