Interactive visualisation of the pyruvate fork: oxygen sends pyruvate into the mitochondrion for about 30 ATP, while no oxygen sends it to fermentation in the cytosol, ending in lactic acid in animals or ethanol and carbon dioxide in yeast
Fermentation makes no ATP. It makes NAD⁺
Shut off oxygen and the electron transport chain stalls, yet glycolysis keeps going because fermentation keeps handing back NAD⁺. Cut the oxygen and watch where the pyruvate goes, then switch the cell to yeast and watch what changes at the end of the left branch, and what does not.
Pyruvate fork
Fully aerobic
Cut the oxygen. Watch the fork, then change the cell type.
Cell type
100% O₂ · ≈32 ATP · 0% fermented
Oxygen available100%
0%50%100%
ATP per glucose
≈32
Pyruvate fermented
0%
Which branch
Know This
In both animals and yeast, fermentation in the cytosol regenerates NAD⁺ so glycolysis keeps making its 2 ATP; only the product differs: lactic acid in animals, ethanol and CO₂ in yeast.