Overview
This lecture covers oxidative phosphorylation, detailing how electrons move through the electron transport chain, establish a proton gradient, and drive ATP synthesis in cells.
Electron Transport Chain (ETC)
- The ETC is the final stage of aerobic respiration and uses oxygen as the final electron acceptor.
- It consists of four protein complexes (I-IV) embedded in the inner mitochondrial membrane.
- Electrons from NADH enter at complex I, which pumps four protons into the intermembrane space.
- FADH₂ donates electrons directly to complex II, bypassing complex I and resulting in fewer protons pumped.
- Ubiquinone (Q) shuttles electrons from complexes I and II to complex III.
- Complex III transfers electrons to cytochrome c and pumps more protons across the membrane.
- Complex IV receives electrons and reduces O₂ to H₂O, removing protons from the matrix, further contributing to the proton gradient.
Chemiosmosis and ATP Synthase
- ETC activity creates a high concentration of protons (H⁺) in the intermembrane space, forming an electrochemical gradient.
- Protons flow back into the mitochondrial matrix through ATP synthase, a membrane protein complex.
- The flow of protons through ATP synthase drives the conversion of ADP and inorganic phosphate to ATP.
- This process is called oxidative phosphorylation.
ATP Yield and Regulation
- The total ATP yield from glucose catabolism varies by species and tissue type.
- NADH-linked electrons produce more ATP than those from FADH₂ because of differences in proton pumping.
- Some electron carriers and intermediates are used in anabolic processes, affecting ATP yield.
- About 34% of glucose’s energy is captured as ATP; the rest is lost as heat.
Disruptions and Inhibitors
- Uncouplers like dinitrophenol (DNP) make the membrane leaky to protons, dissipating the gradient and reducing ATP production.
- Cyanide inhibits cytochrome c oxidase, blocking electron flow, reducing proton pumping, decreasing ATP synthesis, and increasing intermembrane pH.
Key Terms & Definitions
- Oxidative Phosphorylation — ATP formation using energy from electron transfer and chemiosmosis.
- Electron Transport Chain (ETC) — Series of protein complexes transferring electrons and pumping protons to form a gradient.
- Chemiosmosis — Movement of protons across a membrane that drives ATP synthesis.
- ATP Synthase — Enzyme that produces ATP using the proton gradient.
- Prosthetic Group — Non-protein molecule required for a protein's activity.
- Ubiquinone (Q) — Mobile electron carrier in the ETC.
- Cytochrome c — Protein transferring electrons between complexes in the ETC.
Action Items / Next Steps
- Review diagrams of the electron transport chain and ATP synthase.
- Answer review and critical thinking questions at the end of the section.
- Read about inhibitors such as DNP and cyanide for deeper understanding of ETC regulation.