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Concentration gradients are a key component of the biological world.

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The potential energy from these gradients is often used to perform biological work.

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Here we will focus on hydrogen ion concentration gradients.

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Hydrogen ions are also known as protons.

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A gradient exists when there is a higher concentration of a molecule in one compartment compared to a neighboring compartment.

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

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uses ADP and inorganic phosphate, also known as PI, to synthesize ATP.

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This process involves an enzyme complex called ATP synthase.

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Gradients and the potential energy they create are key aspects of the biological world.

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A good example of the use of a gradient occurs in the mitochondria when ATP is synthesized.

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ATP is synthesized by ATP synthase, a large complex of membrane-bound protein.

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Here we see ATP synthase, along with other membrane-bound proteins.

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

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This difference is a hydrogen ion, or proton, concentration gradient.

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The energy associated with this gradient is used to synthesize ATP from ADP and PI.

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This occurs at the ATP synthase complex.

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One hydrogen ion enters the ATP synthase complex from the intermembrane space, and a second

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hydrogen ion leaves it on the matrix space.

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

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Once three protons have entered the matrix space, there is enough energy in the ATP synthase

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complex to synthesize one ATP.

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

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Now let's watch the process again.

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

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Once three more hydrogen ions have crossed the membrane, another molecule of ATP will

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be made.

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In this example, the hydrogen ion gradient is large enough to produce six ATP molecules.

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Please watch as the remaining ATP molecules are synthesized.

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The process has now completed and the result is an equal number of protons on each side

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of the inner membrane.

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

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

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The mitochondrial hydrogen ion gradient is generated as electrons pass through three

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membrane complexes.

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