Answer: Oxygen, reduced to water.
- A NAD+, regenerated for further use in glycolysis
- B FAD, embedded in Complex II of the inner membrane
- C Oxygen, reduced to water
- D Pyruvate, shuttled into the mitochondrial matrix
Correct answer: C. Oxygen, reduced to water
Explanation: O<sub>2</sub> is the terminal electron acceptor of the ETC, reduced to water at Complex IV (cytochrome c oxidase). Without O<sub>2</sub> as an acceptor, the chain backs up and aerobic respiration halts.
Aerobic respiration unfolds in three locations: glycolysis in the cytoplasm yields a small amount of ATP directly, the Krebs cycle in the mitochondrial matrix harvests electron carriers (NADH, FADH2), and the electron transport chain in the inner mitochondrial membrane uses those carriers to generate the bulk of the ATP, with O2 needed only at this final stage.
Concept context
Glycolysis, the Krebs cycle, the electron transport chain, fermentation, and the respiratory quotient. The cellular energy-release counterpart to photosynthesis.