Which process releases relatively small amounts of energy by the breakdown of food substances in the absence of oxygen?

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

Answer: (A)

Energy can be released from food without using oxygen through anaerobic respiration. When oxygen isn’t available, cells still break down glucose, but only a small amount of energy is produced—about 2 ATP per glucose, far less than the thousands produced with oxygen in aerobic respiration. In animals, the pyruvate from glycolysis is converted to lactic acid to keep glycolysis going without oxygen; in some microbes, fermentation converts pyruvate to ethanol and carbon dioxide. The other options don’t fit: aerobic respiration needs oxygen and yields much more energy, and transpiration is a plant process involving water loss rather than energy release from food. Fermentation is related and also happens without oxygen, but the general term for this energy-releasing breakdown is anaerobic respiration.