Chemiosmotic Coupling & Proton Motive Force

In order to understand how ATP is produced by ATP synthase, it’s important to first understand the concept of chemiosmotic coupling. Chemiosmotic coupling is the idea that the proton concentration gradient across the inner mitochondrial membrane, which is generated by the electron transport chain, is ultimately what drives ATP production via ATP synthase. 

We discussed the electron transport chain and the creation of the proton gradient in a previous post. However, just to refresh your memory, recall that in the electron transport chain, enzyme complexes oxidize electron carriers like NADH and FADH2 and couple this reaction with the pumping of protons from the mitochondrial matrix to the intermembrane space. This pumping of protons creates a high proton concentration in the intermembrane space and a low proton concentration in the mitochondrial matrix, and this proton gradient acts as a store of energy. The energy stored in the proton concentration difference is also called the proton motive force. This proton motive force is what will be used to produce ATP. (Figure 1).

The reason why proton motive force can power so much ATP production is because it is a store of both chemical potential energy and electrical potential energy. The chemical potential energy is due to the concentration difference across the membrane (recall that chemicals naturally flow from a higher concentration to a lower concentration) (Figure 1) and the electrical potential energy refers to the charge difference between the mitochondrial matrix and the intermembrane space. By pumping positive charges into the intermembrane space, the mitochondrial matrix carries a net negative charge in comparison to the intermembrane space. In this way, the mitochondrial matrix will be able to attract positive charges as a result of stored up electrical potential energy, motivating protons to move back into the matrix.