Anaerobic respiration: lactate and yeastSpec C1.2.9, C1.2.10
In short
In anaerobic respiration, reduced NAD from glycolysis cannot pass hydrogen to the electron transport chain. In humans, pyruvate accepts the hydrogen and becomes lactate, regenerating NAD so glycolysis continues, with a net yield of 2 ATP per glucose. Yeast uses the same pathway except that pyruvate becomes ethanol and carbon dioxide, which are used in brewing and baking.
Glycolysis needs a supply of NAD to accept hydrogen. Without oxygen, reduced NAD cannot be oxidised by the electron transport chain, so the cell's NAD would soon all be reduced and glycolysis would stop. Anaerobic respiration solves this by using pyruvate as the hydrogen acceptor.
Humans: pyruvate to lactate
Regeneration of NAD allows glycolysis to continue, with a net yield of two ATP molecules per molecule of glucose. No carbon dioxide is produced. The reaction happens in the cytoplasm.
Yeast
The pathways of anaerobic respiration are the same in humans and yeasts apart from the regeneration of NAD using pyruvate, and therefore the final products. In yeast, pyruvate is decarboxylated (CO₂ removed) to ethanal, which is then reduced by reduced NAD to ethanol, regenerating NAD.
| Process | Useful product | What happens to the other product |
|---|---|---|
| Brewing (beer, wine) | Ethanol | CO₂ escapes, or is kept to make the drink fizzy |
| Baking (bread) | Carbon dioxide, which makes the dough rise | Ethanol evaporates during baking |
Lactate formation does not produce extra ATP. Its purpose is to regenerate NAD so glycolysis can keep making its 2 ATP.
Written and checked against the IB Biology HL specification · Updated October 2026