Anaerobic Respiration & Fermentation
What cells do when oxygen runs out - and why the yield collapses.
The real problem: NAD⁺ supply
Glycolysis needs NAD⁺ to oxidize glyceraldehyde-3-phosphate. Cells contain only a small pool of NAD⁺, so if NADH is never re-oxidized, glycolysis halts within seconds and ATP production stops.
With oxygen present, the electron transport chain regenerates NAD⁺. Without oxygen, fermentation does the job instead - its purpose is not to make ATP, but to recycle NAD⁺ so that glycolysis's 2 net ATP keeps coming.
Two pathways
- Lactic acid: pyruvate + NADH → lactate + NAD⁺. No CO₂ released. Found in vertebrate muscle and in bacteria used for yogurt and cheese.
- Alcoholic: pyruvate → acetaldehyde + CO₂, then acetaldehyde + NADH → ethanol + NAD⁺. Found in yeast and plant roots in flooded soil; the CO₂ is what makes bread rise.
Anaerobic respiration is not fermentation
Some prokaryotes run a full electron transport chain but use sulfate, nitrate, or ferric iron as the terminal electron acceptor instead of oxygen. Because a chain and a proton gradient are still involved, this is anaerobic respiration, and its ATP yield is far higher than fermentation's - just lower than aerobic respiration's.
The universal presence of glycolysis across all domains is strong evidence that it is an ancient pathway, present in the common ancestor of all life.
Key terms
4
- Fermentation
- Anaerobic pathway that oxidizes NADH back to NAD⁺ so glycolysis can continue.
- Lactic acid fermentation
- Pyruvate is reduced to lactate, regenerating NAD⁺; occurs in muscle at low O₂.
- Alcoholic fermentation
- Pyruvate becomes acetaldehyde, releasing CO₂, then is reduced to ethanol.
- Anaerobic respiration
- Respiration using a final electron acceptor other than oxygen (e.g., sulfate, nitrate).
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