In anaerobic conditions (absence of oxygen), some cells undergo fermentation instead of aerobic respiration.

After glycolysis, pyruvate is broken down by fermentation in the cytosol via two catabolic pathways:

• ethanol

• lactic acid

Note that fermentation itself yields no ATP molecules; it serves to recycle NADH back to NAD⁺, which is reused in glycolysis.

Fermentation review: