Which process produces energy in the absence of oxygen, often via fermentation?

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

Answer: (C)

When oxygen isn’t available, cells still need to harvest energy by keeping glycolysis going. Fermentation does just this by regenerating NAD+ from NADH after glycolysis, so glycolysis can continue to run and produce a small amount of ATP. The pyruvate from glycolysis is converted into lactate in animals or into ethanol and CO2 in yeast, with the end products being organic molecules rather than carbon dioxide if oxygen were present. Because there’s no oxygen to drive the full mitochondrial electron transport chain, the total ATP yield is only about 2 per glucose, which is why fermentation is the typical energy-producing pathway under these conditions. In contrast, aerobic respiration requires oxygen and yields much more ATP via the mitochondria; anaerobic respiration also lacks oxygen but uses other electron acceptors and can have different yields; photosynthesis is a different process that builds sugars using light energy, not a direct source of energy during oxygen absence.