Final answer:
Aerobic respiration requires oxygen as it is the final electron acceptor in respiration. Without oxygen, organisms use fermentation to continue glycolysis and regenerate NAD+.
Step-by-step explanation:
Aerobic respiration requires oxygen because it is the terminal electron acceptor in respiration. During aerobic respiration, pyruvate from glycolysis enters the Krebs cycle, where additional energy is extracted and electrons are transferred to receptors, including NAD+ and FAD. The NADH and FADH2 then transfer electrons to the electron transport chain, and oxygen is the final electron acceptor, creating water. Without oxygen, organisms use fermentation to regenerate NAD+ and continue glycolysis, producing end products such as alcohol or lactic acid.