The electron transport chain consists of 4 protein complexes (labelled I to IV), 2 mobile electron carriers, ubiquinone and cytochrome C (labelled Q and Cyt C) and ATP synthase.
Complex I and II accept electrons from the electron carriers, NADH and FADH2, produced during glycolysis and the Krebs cycle. The electrons are transported by ubiquinone (Q) to Complex III and eventually cytochrome C transfers the electrons to Complex IV. Complex IV catalyzes the reduction of molecular oxygen to produce a water molecule.
The energy released through the oxidation of NADH and FADH2 by Complex I and II enables the formation of an electrochemical proton gradient across the inner mitochondrial membrane. Complex IV also contributes to this electrochemical gradient. This electrochemical gradient is utilized by ATP synthase to produce ATP via chemiosmosis.
Figure 1. Detailed steps of the electron transport chain.