The cell uses the charge differences that build up across the inner mitochondrial membrane during cellular respiration to the oxidation of pyruvic acid to produce carbon dioxide and water, and to create adenosine triphosphate, or ATP.
This happens in two processes.
The first is in the Krebs Cycle where in the charge differences allow for pyruvic acid to be oxidized. Oxygen atoms bond with carbon atoms in the pyruvic acid to form carbon dioxide. In addition, electrons are removed and transferred to NAD+ (Nicotinamide adenine dinucleotide, a coenzyme present in the membrane), producing NADH + (a variant of the molecule) and the positive charged Hydrogen atoms, H+; also a pair of electrons is removed from succinic acid(another substance found in the mitochondria) and reduces flavin adenine dinucleotide (FAD, another coenzyme) to FADH2(a variant of the molecule).
The second is in the Electron Transport Chain, the next step in the cellular respiration process in the mitochondria, specifically in the membranes.
There, the charge differences allow for protons, in the H+ and electrons, in the NADH and FADH2, to undergo series of transfers from one molecule to another. Energy is released from this and is used in ATP synthase process, which turns phosphates to ATP. As a by-product, oxygen bonds with hydrogen molecules to form water.
View the full process of Cellular Respiration here.