Some energy released is used to synthesize ATP by substrate-level phosphorylation, as in glycolysis. Krebs cycle turns twice for each original glucose molecule. Some protein carriers are cytochrome molecules. Electrons that enter the electron transport system are carried by NADH and FADH 2 . At each sequential oxidation-reduction reaction, energy is released to form ATP molecules.
Oxygen serves as terminal electron acceptor and combines with hydrogen ions to form water.
Because O 2 must be present for system to work, it is also called oxidative phosphorylation. Once NADH delivers electrons to electron transport system, it is free to pick up more hydrogen. Electron transport system consists of three protein complexes and two protein mobile carriers that transport electrons between complexes. Electrons released from the oxidation of NADH enter the electron transport system. ATP synthase complexes are channel proteins that also serve as enzymes for ATP synthesis.