5. During the process of oxidative phosphorylation, which molecule acts as the terminal electron
acceptor.
a. Hydrogen
b. Phosphate
c. Water
d. Oxygen
Oxygen acts as the terminal electron acceptor in oxidative phosphorylation, accepting electrons at Complex IV (cytochrome c oxidase) and combining with protons to form water. This reaction sustains electron transport chain (ETC) flow, enabling proton gradient formation for ATP synthesis. Without oxygen, ETC stalls, halting aerobic respiration.
Electron Transport Chain Pathway
NADH/FADH₂ donate electrons to Complexes I/II, passing through ubiquinone, Complex III, cytochrome c, to Complex IV. Complex IV catalyzes: 4e⁻ + 4H⁺ + O₂ → 2H₂O, pumping additional protons. This yields ~30 ATP/glucose via chemiosmosis.
Detailed Option Analysis
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a. Hydrogen: Incorrect. Protons (H⁺) form the electrochemical gradient but do not accept electrons; consumed in water production.
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b. Phosphate: Wrong. Inorganic phosphate (Pi) phosphorylates ADP to ATP at ATP synthase, post-ETC.
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c. Water: Incorrect. Water is product of oxygen reduction, not electron acceptor.
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d. Oxygen: Correct. High electronegativity and abundance make O₂ ideal terminal acceptor.
| Stage | Electron Carrier | Proton Pumping | Role |
|---|---|---|---|
| Complex I | NADH → Q | 4H⁺ | Entry point |
| Complex II | FADH₂ → Q | 0H⁺ | Succinate oxidation |
| Complex III | QH₂ → cyt c | 4H⁺ | Q-cycle |
| Complex IV | cyt c → O₂ | 2H⁺ | Terminal acceptor → H₂O |
