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.

The electron transport chain takes place in the inner mitochondrial membrane. The four complexes that are involved in the chain, complex 1, 2, 3, and 4 as well as ATP synthase are embedded in the inner mitochondrial membrane. First, NADH produced during the Krebs cycle brings electrons to complex 1, where the electrons undergo redox reactions and produce energy. Complex 1 pumps 4 protons out of the mitochondrial matrix into the inter membrane space. The electrons are then carried over to complex 3 by coenzyme Q. FADH2 from the Krebs cycle brings electrons to complex 2, where they move through a series of redox centres, producing energy,  before being carried to complex 3 by coenzyme Q. The electrons move through additional redox centres in complex 3, which also pumps 4 protons out from the matrix to the inter membrane space. Cytochrome c carries electrons from complex 3 to complex 4. In complex four the electrons reduce oxygen, the final election accept, to produce water. Complex 4 also pumps 2 protons form the matrix to the inter membrane space. ATP synthase uses energy from the electrochemical gradient created by pumping protons out of the matrix. A proton comes back into the mitochondrial matrix and ADP is phosphorylated to produce ATP using the resulting energy.

Figure 1. Detailed steps of the electron transport chain.