The Electron Transport Chain (ETC) is a crucial component of cellular respiration, located in the inner mitochondrial membrane.
It is responsible for the production of ATP, the cell's energy currency, through a process known as oxidative phosphorylation. Here's an organized explanation of its mechanism:
The ETC is the terminal phase of cellular respiration and involves a series of protein complexes and small electron carriers.
It functions to transfer electrons from NADH and FADH2 (produced during earlier stages of cellular respiration) to molecular oxygen, creating a proton gradient that drives the synthesis of ATP.
The Mechanism in Steps
1. Electron Donation to the ETC
NADH and FADH2, generated through glycolysis, the citric acid cycle, and beta-oxidation of fatty acids, donate electrons to the ETC. NADH donates its electrons to Complex I, while FADH2 donates to Complex II.
2. Complex I: NADH:ubiquinone oxidoreductase
Accepts electrons from NADH, transferring them to ubiquinone (Q), and pumps four protons (H+) from the mitochondrial matrix into the intermembrane space, aiding in building the proton gradient.
3. Complex II: Succinate:ubiquinone oxidoreductase
Accepts electrons from FADH2 and transfers them to ubiquinone (Q). Complex II does not contribute to proton pumping across the membrane.
4. Ubiquinone (Q) and Complex III: Cytochrome bc1 complex
Ubiquinol (QH2), the reduced form of ubiquinone, transfers electrons to Complex III. Complex III moves electrons to cytochrome c while pumping protons across the membrane, further enhancing the proton gradient.
5. Cytochrome c and Complex IV: Cytochrome c oxidase
Cytochrome c carries electrons from Complex III to Complex IV. This complex transfers the electrons to the final electron acceptor, oxygen (O2), which combines with protons to form water (H2O). Complex IV also pumps protons, further increasing the proton gradient.
6. Proton Gradient and ATP Synthesis
The creation of a proton gradient across the inner mitochondrial membrane, known as the proton motive force, is crucial for ATP synthesis. As protons flow back into the mitochondrial matrix through ATP synthase, their movement is coupled with the conversion of ADP to ATP.