The Electron Transport Chain (ETC), located in the inner mitochondrial membrane, is the final phase of cellular respiration and a key step in ATP production through oxidative phosphorylation.
It involves a series of protein complexes and electron carriers that transfer electrons from NADH and FADH2 to molecular oxygen, creating a proton gradient that drives ATP synthesis.
Steps of the ETC Mechanism
1) Electron Donation
NADH and FADH2 (from glycolysis, the citric acid cycle, and beta-oxidation) donate electrons to the ETC.
NADH transfers electrons to Complex I, while FADH2 transfers to Complex II.
2) Complex I: NADH oxidoreductase
Accepts electrons from NADH, transferring them to ubiquinone (Q).
Pumps 4 protons (H+) into the intermembrane space, contributing to the proton gradient.
3) Complex II: Succinate oxidoreductase
Accepts electrons from FADH2 and transfers them to ubiquinone.
Does not pump protons.
4) Ubiquinone (Q) and Complex III: Cytochrome bc1
Ubiquinol (QH2) transfers electrons to Complex III, which passes them to cytochrome c.
Pumps additional protons into the intermembrane space.
5) Cytochrome c and Complex IV: Cytochrome c oxidase
Cytochrome c transfers electrons from Complex III to Complex IV, which transfers them to oxygen (O2), forming water (H2O).
Complex IV also pumps protons, increasing the proton gradient.
6) Proton Gradient and ATP Synthesis
The proton gradient (proton motive force) drives protons back into the mitochondrial matrix through ATP synthase, coupling their movement with the conversion of ADP to ATP.
In summary, the ETC transfers electrons from NADH and FADH2 to oxygen, generating a proton gradient that powers ATP synthesis.
