Overview
This lecture explains the electron transport chain (ETC) and chemiosmosis, the final stages of cellular respiration, focusing on electron flow, ATP production, and the role of electronegativity.
Electron Transport Chain (ETC) Overview
- ETC is stage four of cellular respiration, occurring in the inner mitochondrial membrane.
- NADH and FADH2 donate electrons to the ETC, starting at complexes I and II respectively.
- Complex I (NADH dehydrogenase) oxidizes NADH, transferring electrons to ubiquinone (Q).
- Complex II (succinate dehydrogenase) receives electrons from FADH2, which are also transferred to ubiquinone.
- Ubiquinone (Q) is a mobile electron carrier that transfers electrons to complex III (cytochrome reductase).
- Complex III passes electrons to cytochrome c, another mobile carrier.
- Cytochrome c delivers electrons to complex IV (cytochrome oxidase), which transfers them to oxygen.
- Oxygen is the final electron acceptor, forming water by combining with electrons and protons.
Chemiosmosis and ATP Synthesis
- As electrons move through the ETC, complexes I, III, and IV pump protons into the intermembrane space.
- This creates a proton gradient (high [H⁺] in intermembrane space, low in matrix), leading to low pH in the intermembrane space.
- Protons flow back into the matrix through ATP synthase (a membrane protein), generating ATP from ADP and phosphate.
- Chemiosmosis is the process of ATP production using the diffusion of protons through ATP synthase.
- ETC plus chemiosmosis = oxidative phosphorylation (ATP production via electron transfer and phosphorylation of ADP).
Role of Electronegativity and Energy Extraction
- Electrons flow from less electronegative (carbon) to more electronegative (oxygen) atoms, releasing energy.
- Oxygen’s high electronegativity drives electron flow through the ETC.
- Complexes and mobile carriers are arranged so that electron affinity increases along the ETC.
- Analogies: Like a battery or water turbine, energy is extracted as electrons move from high to low potential.
ATP Yield and Complex Activation
- NADH donates electrons to three complexes, yielding 3 ATP per molecule.
- FADH2 donates to two complexes, yielding 2 ATP per molecule.
- The ATP yield is proportional to the number of complexes activated by each carrier.
Key Terms & Definitions
- Electron Transport Chain (ETC) — sequence of protein complexes transferring electrons and pumping protons to generate ATP.
- Chemiosmosis — ATP production via the movement of protons through ATP synthase.
- Oxidative Phosphorylation — combination of electron transfer and ATP production in the mitochondria.
- Electronegativity — an atom’s tendency to attract electrons.
- Complex I — NADH dehydrogenase, oxidizes NADH.
- Complex II — succinate dehydrogenase, oxidizes FADH2.
- Ubiquinone (Q) — mobile electron carrier within the membrane.
- Cytochrome c — mobile surface protein electron carrier.
- ATP Synthase — enzyme that produces ATP using proton flow.
- Final Electron Acceptor — oxygen in cellular respiration, forms water.
Action Items / Next Steps
- Review the net ATP calculation for one glucose molecule in cellular respiration.