Anaerobic Routes and the ATP Yield Compared
Without oxygen the chain backs up, so cells reduce pyruvate to lactate or to ethanol purely to regenerate NAD. The ATP arithmetic shows how much a cell gives up by doing it.
What a learner can do afterwards
- Explains that the anaerobic pathways exist to regenerate NAD rather than to make ATP.
- Compares the ATP yield per glucose for the aerobic and the anaerobic route.
- Names the anaerobic product in a muscle cell and in a yeast cell.
1 · Read
With oxygen, one glucose pays about 36 to 38 ATP through the full chain to oxygen. Without oxygen the chain backs up, because carriers like NADH have nowhere to unload. Glycolysis then stalls too, since it needs fresh NAD+.
Fermentation fixes exactly one problem: it dumps electrons onto pyruvate to free NAD+ again. It makes no extra ATP. The anaerobic route nets only 2 ATP per glucose, the same small amount glycolysis gives alone.
Your muscle cells turn pyruvate into lactate during a hard sprint. Yeast turns it into ethanol plus carbon dioxide, which raises bread and fizzes beer. Same problem, two patches, and both are dead ends rather than extra energy.
When asked why a cell bothers making lactate, answer with NAD, not ATP. The point is keeping glycolysis alive at the price of most of the ATP yield.
Without oxygen cells ferment pyruvate only to free NAD+, settling for 2 ATP per glucose instead of about 36 to 38.
2 · Watch
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Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.