After glycolysis, pyruvate oxidation, and the citric acid cycle, glucose has been completely oxidized into CO₂.
Very little of the energy released by glucose oxidation has been used to make ATP so far. The remaining energy has been transferred to electron carriers. To harvest this energy, electrons must be released from these carriers and flow through a series of mitochondrial proteins. This process requires a very electronegative molecule, usually , that can accept low-energy electrons.