THE ELECTRON TRANSPORT CHAIN

Figure
Electron transport system in mitochondrion
I-II: NADH dehydrogenase complex - pumps out H⁺
Q: Ubiquinone - mobile electron carrier
III: Cytochrome b-c complex - pumps out H⁺
C: Cytochrome c - mobile electron carrier
IV: Cytochrome oxidase complex - pumps out H⁺
V: ATP synthase - uses potential energy from the H⁺ gradient to produce ATP

ROS leakage

Molecular oxygen (O₂) is reduced to water in the electron transport chain, involving four one-electron reductions. Partially reduced ROS intermediates are produced.
A small percentage of these ROS may escape the electron transport chain.

See overview of how the electron transport chain is coupled to the Krebs cycle in mitochondria: menu item Electron transport chain reactions.