What metabolic pathway do many facultative anaerobes switch to when oxygen runs out?

• A. Fermentation or anaerobic respiration
• B. Aerobic respiration
• C. Photosynthesis
• D. Krebs cycle only

Answer: (A)

When oxygen is not available, facultative anaerobes rely on pathways that can generate ATP without needing oxygen and, just as important, regenerate NAD+ so glycolysis can keep running. The best choice is fermentation or anaerobic respiration. Fermentation lets pyruvate be converted into lactate or ethanol, reoxidizing NADH to NAD+ so glycolysis can continue to produce ATP. Anaerobic respiration uses alternative electron acceptors (like nitrate or sulfate) in its electron transport chain to keep ATP production going without oxygen. Both options yield less ATP than aerobic respiration, but they keep energy production going when oxygen is absent.

Aerobic respiration requires oxygen, so it isn’t available under these conditions. Photosynthesis isn’t the energy strategy for most facultative anaerobes in low-oxygen environments. The Krebs cycle and oxidative phosphorylation depend on oxygen to drive the electron transport chain and indirectly regenerate NAD+, so relying on the cycle alone wouldn’t sustain energy production without oxygen.