Electron Transport Pathway
Electrons from sugars are captured in the glycolytic pathway and become part of NADH. When fatty acids are digested by β -oxidation, their high-energy electrons become part of NADH and FADH2. Similarly, in the TCA cycle, electrons end up in either NADH or FADH2.
Electrons in NADH are in orbitals of higher energy than those in FADH2.
Complex I
NADH donates two electrons to Complex I. Within this complex, the electrons lose a good deal of their energy. Complex I uses this energy to pump hydrogen ions (H+) out of the mitochondrial matrix into the inter-membrane space.
Complex I donates 1-2 electrons to ubiquinone.
Complex II
FADH2 donates 2 electrons to Complex II. Little energy is lost in Complex II.
Complex II donates 1-2 electrons to ubiquinone.
Complex III
Ubiquinone (coenzyme Q10) moves toward Complex III and donates 2 electrons to it. Within this complex, the electrons lose another substantial amount of their energy, which is used to pump more hydrogen ions into the inter-membrane space.
Complex III donates one electron to cytochrome c.
Complex IV
Cytochrome c moves toward Complex IV and donates one electron to it. Within this complex, electrons lose the rest of their energy and are deposited one at a time into a molecule of oxygen (O2), which then decomposes into water. This decomposition of oxygen into water requires four electrons. Once again, the energy lost by the electrons is used to pump hydrogen ions from the matrix into the inter-membrane space.
At the end of electron transport, the electrons are part of water molecules and a pH gradient has been formed across the inner mitochondrial membrane.
Complex V
The hydrogen ions flow from the inter-membrane space back into the mitochondrial matrix through Complex V, also called ATP synthase.
