Yeast anaerobic respiration converts glucose or pyruvate to ethanol and carbon dioxide, supporting fermentation in bread, beer and bioethanol, supporting usable cellular work and measurable biological activity.
Coverage 2013–2024 · Updated 15 Jul 2026
When oxygen is unavailable, human and yeast cells regenerate NAD using pyruvate-derived reactions so glycolysis can continue with a net yield of two ATP per glucose.
In humans, pyruvate accepts hydrogen/electrons from reduced NAD and becomes lactate, regenerating NAD. In yeast, pyruvate loses carbon dioxide and is reduced to ethanol, also regenerating NAD.
The pathways are the same through glycolysis; they differ in the reaction that regenerates NAD and therefore in final products: human cells form lactate, whereas yeast forms ethanol + carbon dioxide.
Carbon dioxide released by yeast expands bread dough, while ethanol production is used in brewing. Neither fermentation route adds ATP beyond the two net ATP made in glycolysis.
The purpose of reducing pyruvate-derived molecules is NAD regeneration, not a high ATP yield. Human lactate formation does not release carbon dioxide.
This objective is assessed through essay response, commonly using Explain / Identify.
Explain / Identify
Build the answer around this relationship: Yeast fermentation produces ethanol and carbon dioxide.
Representative question
Explain the use of yeast in the production of bread and beer.
In HL respiration, carbon and hydrogen are followed separately. Glycolysis turns glucose into pyruvate, net ATP, and reduced NAD. Without oxygen, pyruvate becomes lactate in humans or ethanol and carbon dioxide in yeast to regenerate NAD. With oxygen, pyruvate enters the link reaction, forming acetyl-CoA, carbon dioxide, and reduced NAD. The Krebs cycle releases more carbon dioxide and reduced coenzymes. Electron transport uses reduced NAD/FAD to pump protons, chemiosmosis through ATP synthase makes ATP, and oxygen accepts electrons and protons to form water. Substrate comparisons depend on ATP yield, oxygen demand, water production, and speed. Regenerated NAD allows glycolysis to continue. Fermentation regenerates NAD for glycolysis and is used in baking and brewing. Electron transfers release energy while coenzymes are reoxidized.
