Fermentation

Glycolysis splits glucose into two pyruvate molecules and nets 2 ATP, but it only keeps running if there's enough NAD+ around to accept electrons. Normally that NAD+ gets recycled when NADH hands its electrons off to the electron transport chain, which needs oxygen as the final acceptor. Cut off the oxygen, and NADH piles up while NAD+ runs out. Glycolysis stalls. Fermentation is the patch: it takes the electrons back off NADH and dumps them onto pyruvate (or something made from pyruvate), turning NADH back into NAD+. That's the entire job. No ATP synthase, no proton gradient, no substrate-level phosphorylation happens in fermentation itself.

Animal cells, including your muscle cells during a hard sprint, run lactic acid fermentation: pyruvate accepts the electrons directly and becomes lactate, in one step. Yeast and some bacteria run alcoholic fermentation instead: pyruvate first loses a carbon as CO2, becoming acetaldehyde, and then acetaldehyde accepts the electrons from NADH and becomes ethanol. Both pathways regenerate the same NAD+ that glycolysis needs, just with different final products.

This sits right next to aerobic respiration in the unit: same starting point, glycolysis, but a different ending depending on whether oxygen is present to keep the electron transport chain running. Fermentation is slower to generate ATP per glucose but doesn't depend on oxygen at all, which is why a sprinting muscle, starved for oxygen, leans on it, while a resting, well-oxygenated muscle doesn't need to.