The electron transport chain of cellular respiration needs oxygen at its end. When oxygen runs short, as in a hard-working muscle or a sealed bowl of bread dough, cells fall back on a simpler plan.
Key idea
Without oxygen, cells keep glycolysis running by fermentation. Muscle cells (and the bacteria that make yogurt) make lactic acid; yeast make ethanol and carbon dioxide, and the CO₂ makes bread rise. Fermentation yields only 2 ATP per glucose, all from glycolysis.
| Lactic acid fermentation | Alcoholic fermentation | |
|---|---|---|
| Who | Muscle cells; some bacteria (yogurt, sauerkraut) | Yeast; some bacteria |
| Pyruvate becomes | Lactic acid (lactate) | Ethanol + CO₂ |
| ATP per glucose | 2 | 2 |
| Everyday example | Yogurt thickening | Bread dough rising |
Why it works
Glycolysis makes 2 ATP, but it also uses up NAD⁺, turning it into NADH. A cell has only a little NAD⁺, so glycolysis would stop within moments unless the NADH is turned back into NAD⁺. With oxygen, the electron transport chain does that. Without oxygen, fermentation does it: the NADH hands its electrons to pyruvate (making lactic acid) or to a molecule made from pyruvate (making ethanol). Fermentation makes no extra ATP. Its whole job is to recycle NAD⁺ so that glycolysis, with its 2 ATP, can keep going.
Worked example: How much glucose?
Aerobic respiration makes about 30 ATP per glucose; fermentation makes 2. How many glucose molecules must a cell ferment to make as much ATP as it would get from one glucose with oxygen?
- $30 \div 2 = 15$.
- The cell must ferment about 15 glucose molecules: it burns through sugar quickly.
Answer: About $15$
Worked example: Read the balloon
Yeast and sugar are mixed with warm water in a bottle with a balloon on top. The balloon fills. What gas is it, and where does it come from?
- Yeast in a closed bottle soon run low on oxygen and switch to alcoholic fermentation.
- Alcoholic fermentation turns each pyruvate into ethanol and CO₂.
- So the balloon fills with carbon dioxide.
Answer: Carbon dioxide, from alcoholic fermentation
Common mistake
Thinking fermentation makes as much ATP as aerobic respiration, or that it makes none at all. It makes 2 ATP per glucose, the 2 from glycolysis, against about 30 with oxygen. The fermentation step itself makes none; it only regenerates NAD⁺.