Cellular respiration and fermentation are two pathways that follow glycolysis. Cellular respiration requires oxygen, while fermentation occurs in its absence. Cellular respiration consists of a chemical pathway named the Krebs cycle or the citric acid cycle, followed by an electron transport system.
A transition reaction occurs after glycolysis and before either respiration or fermentation begins. Pyruvate, the product of glycolysis, is processed into acetyl-CoA, a high-energy molecule. Krebs-cycle reactions process the acetyl-CoA into electron-carrying molecules ATP and NADH. The electron transport system passes those through chemical gradient membranes to create additional ATP used to power cellular processes. Fermentation converts acetyl-CoA into ethanol or lactic acid.