Concentration gradients are a key component of the biological world. The potential energy from these gradients is often used to perform biological work.
Here we will focus on hydrogen ion concentration gradients. 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.