Which metabolic process allows cells to release energy without oxygen?

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

Answer: (B)

Fermentation is the process that lets cells keep releasing energy when oxygen isn’t available. After glycolysis splits glucose in the cytoplasm and yields a small amount of ATP, NAD+ must be regenerated from NADH to keep glycolysis going. In the absence of oxygen, the electron transport chain can’t accept electrons, so fermentation steps in to recycle NADH back to NAD+ by transferring electrons to pyruvate, producing either lactate (in animals and some bacteria) or ethanol and CO2 (in yeast and some microbes). This regeneration keeps glycolysis running and sustains ATP production without oxygen.

Glycolysis itself doesn’t require oxygen, but without fermentation to regenerate NAD+, glycolysis would halt once NAD+ is depleted. Photosynthesis uses light to build sugars and isn’t about releasing energy without oxygen, and oxidation is a broad term for electron transfer that doesn’t specifically describe the anaerobic pathway that maintains ATP production.