fermentation
Summary
Fermentation is the anaerobic regeneration of NAD⁺ from NADH by passing electrons to pyruvate (giving lactate) or to acetaldehyde (giving ethanol in yeast). It lets glycolysis keep making its net 2 ATP per glucose without oxygen; the fermentation step itself makes no ATP.
Detail
Meaning: Fermentation is a set of anaerobic pathways that regenerate NAD⁺ from NADH so glycolysis can continue without oxygen. Glycolysis needs NAD⁺ at the glyceraldehyde-3-phosphate dehydrogenase step. Without a working electron transport chain, NADH would build up and glycolysis would stall. Fermentation transfers electrons from NADH to pyruvate or a pyruvate derivative. Animal cells make lactate. Yeast decarboxylate pyruvate to acetaldehyde, then reduce it to ethanol. The ATP yield is the net 2 ATP per glucose from substrate-level phosphorylation in glycolysis.
Equation: Pyruvate + NADH + H⁺ → lactate + NAD⁺ (catalyzed by lactate dehydrogenase). Each term is a chemical species counted in moles (mol).
On the MCAT: Passages use hard-working muscle, red blood cells (which lack mitochondria) or brewing yeast. Expect to identify what is oxidized (NADH) and what is reduced (pyruvate or acetaldehyde), to predict lactate buildup when oxygen or the electron transport chain is limited, or to compare yields: 2 ATP per glucose versus roughly 30 to 32 with oxidative phosphorylation.
Common confusion: Fermentation does not itself make ATP. The ATP comes from glycolysis, and fermentation only recycles NAD⁺. It also differs from anaerobic respiration, which uses an electron transport chain with a final electron acceptor other than O₂.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.