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What happens in the electron transport chain of a mitochondrion?

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Fluorescent confocal image of HeLa cells with green microtubules outlining cell shapes, red mitochondria, and blue nuclei.
mitochondrion Mitochondria (red) are found throughout the cytoplasm of almost all eukaryotic cells (cell nucleus is shown in blue; cytoskeleton is shown in yellow).

The electron transport chain (ETC) is a series of protein complexes embedded in the cristae membrane of the mitochondrion that facilitates the production of adenosine triphosphate (ATP) through oxidative phosphorylation.12

The process functions as follows:

  • Electron Transfer: The process begins when electron donors, specifically NADH and FADH2, deposit electrons into the chain. These electrons are passed through a series of protein complexes, including NADH dehydrogenase, the b-c1 complex, and cytochrome oxidase.12
  • Proton Pumping: As electrons move down the chain, the energy released is used to pump protons (H+) from the mitochondrial matrix into the intermembrane space. This creates an electrochemical gradient across the inner membrane.13
  • ATP Synthesis: Because the inner membrane is impermeable to protons, they must flow back into the matrix through a specialized enzyme called F1F0ATPase. This flow provides the energy required to synthesize ATP from ADP and inorganic phosphate.13
  • Water Formation: In the final step, cytochrome oxidase transfers the electrons to oxygen atoms, which then combine with protons to form water.12