In the absence of oxygen, some cells undergo fermentation instead of aerobic respiration.

Pyruvate serves as an electron acceptor for oxidizing NADH back to NAD⁺, which is reused in glycolysis.

Two end products formed from fermentation are ethanol with the release of CO₂ and lactic acid.

Note that fermentation. itself yields no ATP molecules; it serves to recycle NADH back to NAD⁺.