The electron transport chain is the powerhouse of cellular energy production. It's a series of protein complexes in the mitochondrial inner membrane that transfer electrons from NADH and FADH2 to oxygen, creating a proton gradient in the process.
This gradient drives ATP synthesis, the cell's energy currency. The chain's efficiency and organization are crucial for maximizing energy output, linking cellular respiration to ATP production in oxidative phosphorylation.
Electron Transport Chain Complexes
NADH Dehydrogenase and Succinate Dehydrogenase
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- NADH dehydrogenase (Complex I) oxidizes NADH to NAD+
- Transfers electrons from NADH to ubiquinone
- Pumps 4 protons from matrix to intermembrane space
- Largest complex in the electron transport chain, containing 45 subunits
- Succinate dehydrogenase (Complex II) oxidizes succinate to fumarate
- Transfers electrons from FADH2 to ubiquinone
- Does not pump protons across the membrane
- Only complex that participates in both the citric acid cycle and electron transport chain
Cytochrome Complexes
- Cytochrome bc1 complex (Complex III) transfers electrons from ubiquinol to cytochrome c
- Employs the Q-cycle mechanism to pump protons
- Pumps 4 protons from matrix to intermembrane space
- Contains heme groups and iron-sulfur clusters
- Cytochrome c oxidase (Complex IV) reduces molecular oxygen to water
- Transfers electrons from cytochrome c to oxygen
- Pumps 2 protons from matrix to intermembrane space
- Terminal complex in the electron transport chain
Electron Carriers
Mobile Electron Carriers
- Ubiquinone (Coenzyme Q) shuttles electrons between complexes I/II and III
- Lipid-soluble molecule that diffuses freely in the inner mitochondrial membrane
- Exists in three redox states: fully oxidized (Q), semiquinone (Q•-), and fully reduced (QH2)
- Plays a crucial role in the Q-cycle of Complex III
- Cytochrome c transfers electrons between complexes III and IV
- Small, water-soluble protein located in the intermembrane space
- Contains a heme group that alternates between reduced (Fe2+) and oxidized (Fe3+) states
- Loosely associated with the outer surface of the inner mitochondrial membrane
Other Electron Carriers
- Electron carriers within complexes facilitate electron transfer
- Iron-sulfur clusters found in complexes I, II, and III
- Heme groups present in complexes II, III, and IV
- Copper ions in complex IV
- Flavin mononucleotide (FMN) serves as the initial electron acceptor in Complex I
- Flavin adenine dinucleotide (FAD) acts as the electron acceptor in Complex II
Mitochondrial Structure and Energy Generation
Mitochondrial Membrane Organization
- Mitochondrial inner membrane forms cristae to increase surface area
- Cristae house the electron transport chain complexes
- Folded structure maximizes ATP production capacity
- Outer membrane contains porins for small molecule transport
- Intermembrane space accumulates protons pumped by the electron transport chain
- Matrix contains enzymes for the citric acid cycle and fatty acid oxidation
Proton Gradient and ATP Synthesis
- Proton gradient generated by electron transport chain complexes
- Complexes I, III, and IV pump protons from matrix to intermembrane space
- Creates an electrochemical gradient across the inner membrane
- Gradient consists of a pH difference (ΔpH) and an electrical potential (Δψ)
- ATP synthase (Complex V) utilizes the proton gradient to produce ATP
- Protons flow back into the matrix through ATP synthase
- Rotational catalysis mechanism couples proton flow to ATP synthesis
- Produces approximately 2.7 ATP molecules per 10 protons
Redox Potential and Electron Flow
- Redox potential measures the tendency of a molecule to accept or donate electrons
- More positive redox potential indicates a stronger electron acceptor
- Electron flow occurs from molecules with more negative to more positive redox potentials
- Electron transport chain arranged in order of increasing redox potential
- NADH (-320 mV) → Complex I → Ubiquinone → Complex III → Cytochrome c → Complex IV → O2 (+820 mV)
- Energy released during electron transfer drives proton pumping
- Larger redox potential differences between complexes allow for more proton pumping
- Efficiency of energy conversion from electron transfer to proton gradient approximately 40-50%