Lesson 4 of 5 / Respiration and ATP production
Regenerate NAD without oxygen
What does fermentation keep running?
In this lesson: Explain low ATP yield, ethanol formation in yeast and lactate formation in muscle.
About 7 min
The key ideaFermentation reoxidises reduced NAD so glycolysis can continue; it does not add ATP beyond glycolysis in these pathways.
Explore the idea
Recycle NAD so glycolysis can continue
Muscle reduces three-carbon pyruvate to three-carbon lactate. No carbon is removed as CO2 in this step; NADH is oxidised back to NAD.
Each glucose produces two pyruvate and therefore two molecules of the stated final product. The diagram follows one pyruvate branch. The net two ATP come from glycolysis; the fermentation branch enables this to continue by regenerating NAD.
Explanation
Without adequate aerobic electron transport, glycolysis would exhaust oxidised NAD. In mammalian muscle, pyruvate accepts reducing power from reduced NAD to form lactate, regenerating NAD. Lactate formation itself does not release CO2 in this pathway.
In yeast, pyruvate is first decarboxylated to ethanal, releasing CO2. Ethanal is reduced to ethanol using reduced NAD, regenerating NAD. Carbon dioxide production distinguishes this pathway from lactate formation, but oxygen conditions and controls must be known before interpreting a gas measurement.
Regenerated NAD returns to glycolysis, allowing its net two ATP per glucose to continue. Much chemical energy remains in lactate or ethanol, so glucose is not fully oxidised as it is in aerobic respiration. These fermentation steps add no further ATP in the standard syllabus accounting.
Lactate can later be metabolised when conditions allow; it is not simply permanent useless waste. Avoid the outdated claim that lactate alone causes all muscle fatigue or soreness. The required mechanism is carrier recycling and the limited ATP yield, not a clinical explanation of every exercise symptom.
Step by step
- 1
Start with the carrier problem
Reduced NAD must be reoxidised.
- 2
Choose the organic acceptor
Pyruvate in muscle or ethanal in yeast.
- 3
Account for ATP and carbon
No extra fermentation ATP; CO2 only in the stated yeast branch.
Worked example
Work through the evidence
Why can a yeast culture produce ATP while releasing ethanol, even without oxygen?
One way to explain it
Fermentation regenerates NAD, enabling glycolysis and its substrate-level ATP production to continue. Ethanol formation is not itself the ATP-generating step.
Why this answer works
- Separate NAD recycling from ATP synthesis.
- The low yield reflects incomplete oxidation of glucose.
Is this true? "Lactate formation makes an additional large supply of ATP."
It regenerates NAD; the standard anaerobic ATP yield comes from glycolysis.