- 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: O2 is the terminal electron acceptor of the ETC, reduced to water at Complex IV (cytochrome c oxidase). Without O2 as an acceptor, the chain backs up and aerobic respiration halts.
Concept context
Glycolysis, the Krebs cycle, the electron transport chain, fermentation, and the respiratory quotient. The cellular energy-release counterpart to photosynthesis.