Which process describes energy production in the absence of oxygen?

• A. Aerobic metabolism
• B. Fermentation
• C. Anaerobic metabolism
• D. Photosynthesis

Answer: (C)

When oxygen isn’t available, cells rely on anaerobic metabolism to make energy. Without oxygen, the electron transport chain can’t operate, so cells must regenerate NAD+ to keep glycolysis going and produce ATP through anaerobic pathways. Fermentation is a common example of these pathways, converting pyruvate into lactate or ethanol and CO2 to allow glycolysis to continue, but anaerobic metabolism is the broader term that covers all energy-producing processes that don’t require oxygen. In contrast, aerobic metabolism needs oxygen and yields most of the ATP, while photosynthesis is a separate process that uses light energy to build sugars. So the best description for energy production without oxygen is anaerobic metabolism.