Phases of Respiration: Electron Transport Chain (Electron Transfer System)

The process occurring in the mitochondria, where NADH₂ and FADH₂ release electrons to form ATP and water, producing 3 ATP from each NADH₂ and 2 ATP from each FADH₂, is called the electron transfer chain reaction.

• The electron transport chain is located on the inner mitochondrial membrane and contains a series of electron and proton carrier complexes (I–V).
• Complex I (NADH dehydrogenase) accepts electrons from NADH via FMN and Fe‑S centres, passes them to ubiquinone (Co‑Q), and pumps 4 H⁺ into the intermembrane space.
• Complex II (succinate dehydrogenase) receives electrons from succinate through FADH₂ and Fe‑S centres, passes them to Co‑Q, but does not pump protons.
• Complex III (cytochrome‑c reductase) transfers electrons from reduced Co‑Q (UQH₂) to cytochrome‑c and pumps 4 H⁺ into the intermembrane space.
• Complex IV (cytochrome‑c oxidase) passes electrons from cytochrome‑c to oxygen, reducing it to water and pumping additional H⁺ across the membrane.
• Complex V (ATP synthase) uses the proton gradient generated by complexes I, III and IV to synthesise ATP from ADP and Pi; this proton‑driven ATP formation is called chemiosmosis.
• Oxidation of each NADH yields about 3 ATP, while every FADH₂ yields about 2 ATP, though exact yields can vary with conditions and substrate.