Hydrogen ions, are also known as protons.

A gradient exists when there is a higher concentration of a molecule in one compartment

compared to a neighboring compartment.

This animation will demonstrate how the potential energy that results from a hydrogen ion gradient

uses ADP and inorganic phosphate,

also known as Pi, to synthesize ATP.

This process involves an enzyme complex called ATP synthase.

Gradients and the potential energy they create

are key aspects of the biological world.

A good example of the use of a gradient

occurs in the mitochondria when ATP is synthesized.

ATP is synthesized by ATP synthase,

a large complex of membrane-bound protein.

Here we see ATP synthase,

along with other membrane-bound proteins.

Notice the large difference in the number of hydrogen ions on the two sides of the membrane.

This difference is a hydrogen ion, or proton,

concentration gradient.

The energy associated with this gradient

is used to synthesize ATP from ADP and Pi.

This occurs at the ATP synthase complex.

One hydrogen ion enters the ATP synthase complex from the intermembrane space

and a second hydrogen ion leaves it on the matrix space.

The upper part of the ATP synthase complex rotates when a new hydrogen ion enters.

Once three protons have entered the matrix space,

there is enough energy in the ATP synthase complex

to synthesize one ATP.

In this way, the energy in the hydrogen ion gradient is used to make ATP.

Now let's watch the process again...

Notice how the proton enters the ATP synthase and exits into the matrix space.

Once three more hydrogen ions have crossed the membrane,

another molecule of ATP will be made.

In this example,

the hydrogen ion gradient is large enough to produce six ATP molecules.

Please watch as the remaining ATP molecules are synthesized...

The process has now completed,

and the result is an equal number of protons on each side of the inner membrane.

Without a gradient, there is no more energy available to make ATP.

In biological systems, however, a gradient is always maintained.

The mitochondrial hydrogen ion gradient

is generated as electrons pass through three membrane complexes.

That process can be seen in the mitochondrial electron transport chain animation.