What is fermentation?

• A. A process that uses oxygen to produce ATP.
• B. A metabolic process that oxidizes NADH to NAD+ in the absence of oxygen.
• C. A pathway that converts NAD+ to NADH.
• D. A light-dependent reaction that generates ATP.

Answer: (B)

Fermentation is the way cells regenerate NAD+ from NADH when no oxygen is available, so glycolysis can keep producing ATP. During glycolysis, NAD+ is reduced to NADH. If oxygen isn’t present to accept those electrons in the mitochondria, the cell recycles NADH back to NAD+ by transferring electrons to an organic molecule, like pyruvate or acetaldehyde. This allows glycolysis to continue, providing a small amount of ATP, even under anaerobic conditions. Different fermentation outcomes occur in different organisms, such as lactate formation from pyruvate or ethanol formation from acetaldehyde. The other options don’t fit because fermentation doesn’t use oxygen, doesn’t convert NAD+ to NADH, and isn’t a light-driven process.