Making lactate or ethanol recycles NAD+ and makes no ATP
- The idea
- Predict
- See it
- Why
- Check
The idea
“When there's no oxygen, turning pyruvate into lactate or ethanol is what makes the ATP.”
Fermentation kicks in right when oxygen runs out, so it's easy to read it as a backup way to make ATP.
The short version
The mistake: When there's no oxygen, turning pyruvate into lactate or ethanol is what makes the ATP.
What's actually true: The fermentation step makes no ATP. It turns NADH back into NAD+ so glycolysis, which makes the ATP, can keep running.
- Glycolysis makes a net 2 ATP per glucose. It also hands electrons to NAD+, making NADH. A cell has only a small supply of NAD+.
- With oxygen, the electron transport chain takes those electrons and NAD+ comes back. Without oxygen, NADH piles up.
- Fermentation gives the electrons to pyruvate or a molecule made from it, which restores NAD+. Muscle makes lactate and yeast makes ethanol. Neither step makes ATP.
On the exam: Follow the NAD+. If nothing turns NADH back into NAD+, glycolysis stops.