Oxidative phosphorylation is a process where the energy of biological oxidation is ultimately converted to the chemical energy of ATP.

Movement of electrons throught the electron transport system (ETS) causes protons to be pumped from the mitochondrial matrix of a eukaryotic cell to the intermembrane space. The difference in potential created by movement of the charged protons as well as the concentration gradient created by the pumping provides the energy source for making ATP in the mitochondrion. This process is called oxidative phosphorylation. It occurs as a result of protons moving through Complex V (Figure 15.2b and Figure 15.15). An analogy for ETS and oxidative phosphorylation can be made to a water pumping system, which can pump water to a holding tank on a hill. When needed, the water can be released from the holding tank and be pulled by gravity down the hill. On the way down, the water can turn a turbine and generate electricity. Similarly, the ETS pumps protons out of the mitochondrial matrix to the intermembrane space. When needed, protons flow back through Complex V and turn molecular turbines (Figure 15.19) to make ATP.

The efficiency of oxidative phosphorylation is determined by the P/O ratio, which is a measure of the amount of ATP made versus the amount of oxygen consumed. Remember that the ETS donates electrons to oxygen in the last step of the process.

The chemiosmotic coupling mechanism explains how ATP is synthesized by mitochondria as a result of protons pumped during ETS. There is a considerable amount of evidence in support of this model.

The actual site in the mitochondrion where ATP is made is Complex V (also called ATP synthase or the F0F1 complex). It is located on the inner mitochondrial cristae and has the structure shown in Figure 15.14.