Without oxygen, regenerate NAD⁺ or glycolysis stops.
0 ATP here: the product is NAD⁺, which keeps glycolysis’s 2 ATP flowing
The point
Fermentation regenerates NAD⁺; the ATP came from glycolysis
Cori cycle accounting
2 ATP made in muscle − 6 spent in liver = net −4: it buys time at an energy cost
Glycolysis step 6 consumes NAD⁺, and the cytosolic pool is tiny. With no O₂ to reoxidize NADH through the electron transport chain, fermentation dumps the electrons onto pyruvate instead. It makes no ATP of its own; its entire job is keeping the NAD⁺ books balanced.
1
PyruvateLactate
Lactate dehydrogenaseNADH → NAD⁺
Fully reversible: the liver runs it backwards in the Cori cycle. Red blood cells, having no mitochondria, do this even when oxygen is everywhere. Lactate is a fuel in transit, wrongly maligned as a poison; the soreness myth refuses to die.
2
PyruvateAcetaldehyde
Pyruvate decarboxylaseCO₂ released
A yeast pathway: humans lack this enzyme. Requires thiamine (TPP), like every decarboxylation of an α-keto acid. The CO₂ leavens bread and carbonates champagne.
3
AcetaldehydeEthanol
Alcohol dehydrogenaseNADH → NAD⁺
Same job as lactate dehydrogenase: electrons off NADH, NAD⁺ back to work. Your liver runs this enzyme in reverse on Saturday nights, which is also why alcohol metabolism floods the liver with NADH.