Overview
This lecture explains the Electron Transport Chain (ETC), its steps, components, and how it generates ATP during aerobic respiration, summarizing the total ATP yield from glucose metabolism.
Review of Glucose Metabolism
- Glucose (6 carbons) undergoes glycolysis, producing 2 NADH, 2 ATP (net), and 2 pyruvate (3 carbons each).
- In the presence of oxygen, pyruvate enters mitochondria, becomes acetyl-CoA, generating NADH and CO₂.
- Krebs cycle (citric acid cycle) produces 6 NADH, 2 FADH₂, 2 ATP, and 4 CO₂ per glucose.
Electron Transport Chain (ETC) Steps
- ETC is located on the inner mitochondrial membrane (cristae).
- Complex I: NADH donates electrons, converted to NAD⁺, protons are pumped to the intermembrane space.
- Complex II: FADH₂ donates electrons, converted to FAD, but no protons are pumped.
- Coenzyme Q (ubiquinone): Mobile electron carrier between Complex I/II and Complex III.
- Complex III: Accepts electrons, pumps protons, passes electrons to cytochrome c.
- Cytochrome c: Mobile protein that transfers electrons from Complex III to IV.
- Complex IV (cytochrome oxidase): Transfers electrons to O₂, forming water; pumps protons.
Chemiosmosis and ATP Synthesis
- Proton gradient builds in intermembrane space, creating high proton concentration.
- ATP synthase (Complex V): Protons flow back to the matrix, spinning the rotor and rod, catalyzing ADP + Pi → ATP (oxidative phosphorylation).
- Chemiosmotic theory: ATP synthase uses potential energy from proton flow to produce ATP.
ATP Yield Summary
- 1 NADH results in 3 protons pumped, producing ~3 ATP.
- 1 FADH₂ results in 2 protons pumped, producing ~2 ATP.
- Per glucose: Krebs (6 NADH x 3 ATP = 18), FADH₂ (2 x 2 = 4 ATP), Krebs ATP (2), transition step NADH (2 x 3 = 6), glycolysis NADH (2 x 3 = 6).
- Total aerobic ATP: 36; add 2 ATP from anaerobic glycolysis for a total of 38 ATP per glucose.
Key Terms & Definitions
- Electron Transport Chain (ETC) — Series of protein complexes transferring electrons and pumping protons to generate ATP.
- NADH/FADH₂ — Electron carriers produced during glycolysis and Krebs cycle.
- ATP Synthase — Enzyme complex synthesizing ATP as protons flow from high to low concentration.
- Chemiosmosis — Movement of protons across a membrane to drive ATP production.
- Oxidative phosphorylation — ATP synthesis linked to electron transfer and oxygen reduction.
- Substrate-level phosphorylation — ATP formation directly from chemical reactions in glycolysis/Krebs cycle.
Action Items / Next Steps
- Review glycolysis, Krebs cycle, and ETC diagrams.
- Practice calculating ATP yield from glucose under aerobic and anaerobic conditions.
- Prepare for questions on ETC components and functions.