What process releases energy in the absence of oxygen?

• A. Glycolysis
• B. Fermentation
• C. Photosynthesis
• D. Oxidation

Answer: (B)

When organisms run in low or no oxygen, they still need a way to make some energy. Glycolysis can break down glucose to pyruvate and release a small amount of ATP, but without a way to recycle the electron carriers, glycolysis would quickly stall. Fermentation steps in by converting the NADH produced during glycolysis back into NAD+, allowing glycolysis to continue and keep ATP production going in the absence of oxygen. The process doesn’t produce a lot of energy by itself, but it provides the necessary bridge to keep cells alive under anaerobic conditions, yielding end products like lactic acid in animals or ethanol and CO2 in other organisms. In contrast, photosynthesis stores energy using light, and oxidation (in the sense of aerobic respiration) relies on oxygen to harvest much more energy via the electron transport chain.