IMAT Biology · Bioenergetics and Metabolism

Fermentation: Respiration Without Oxygen

Without oxygen, cells keep glycolysis running by using pyruvate to reoxidise NADH, making lactate in muscle or ethanol and CO₂ in yeast, for only 2 ATP per glucose.

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Why this matters

The 2025 paper asked what happens to pyruvate in lactic fermentation, and 2023 asked for a ranking of pathways by the ATP they make. Older papers asked where both kinds of fermentation happen, which end products belong to muscle and to yeast, and what a muscle produces during vigorous exercise.

Concept 1 of 2: Lactic and alcoholic fermentation compared

Without oxygen the respiratory chain stops, so NADH cannot be reoxidised. Glycolysis would then stop too, because it needs a supply of

\mathrm{NAD^+}

. Fermentation solves this: pyruvate, or a molecule made from it, takes the electrons back from NADH, so

\mathrm{NAD^+}

is regenerated and glycolysis can keep making its 2 ATP.

Definition

Both kinds happen in the cytoplasm. Mitochondria are not involved.
Lactic fermentation (muscle during vigorous exercise, red blood cells, lactic acid bacteria in yoghurt): pyruvate + NADH $\rightarrow$ lactate + $\mathrm{NAD^+}$ , catalysed by lactate dehydrogenase. Pyruvate is reduced. No $\mathrm{CO_2}$ is released.
Alcoholic fermentation (yeast, plant roots in waterlogged soil): pyruvate is first decarboxylated to ethanal (acetaldehyde), releasing $\mathrm{CO_2}$ ; ethanal is then reduced by NADH to ethanol.
Yield: 2 ATP per glucose, all from glycolysis. The fermentation step itself makes no ATP; its job is to regenerate $\mathrm{NAD^+}$ .
In humans, lactate travels in the blood to the liver, which turns it back into pyruvate and glucose. The extra oxygen taken in after exercise (the oxygen debt) pays for this.
Strictly, anaerobic respiration uses an electron transport chain with a final acceptor other than oxygen (some bacteria use nitrate or sulfate), while fermentation uses no chain. School books often use the two names for the same thing.

Feature	Lactic fermentation	Alcoholic fermentation
Where it occurs	Animal muscle, red blood cells, lactic acid bacteria	Yeast, some plant cells
Site in the cell	Cytoplasm	Cytoplasm
End products	Lactate	Ethanol and CO₂
CO₂ released	No	Yes, one per pyruvate
What happens to pyruvate	Reduced directly to lactate	Decarboxylated to ethanal, which is then reduced
ATP per glucose	2	2
Everyday use	Yoghurt and cheese making	Bread rising, beer and wine

Practice this conceptself-check · 4 quick reps

Fermentation regenerates NAD⁺; it makes no extra ATP

All the ATP of fermentation, 2 per glucose, comes from glycolysis. The step from pyruvate to lactate or ethanol makes no ATP; it only reoxidises NADH so that glycolysis can continue.

Only alcoholic fermentation releases CO₂

Lactic fermentation keeps all 3 carbons of pyruvate in lactate. Alcoholic fermentation removes one as

\mathrm{CO_2}

. Exercising muscle does still release

\mathrm{CO_2}

, but that comes from its aerobic respiration running at the same time, not from lactic fermentation.

Both fermentations happen in the cytoplasm

Neither kind uses mitochondria. Options placing fermentation in the matrix or on the cristae are wrong for yeast and for muscle alike.

Concept 2 of 2: Comparing ATP yields: fermentation against aerobic respiration

Fermentation stops after glycolysis, so most of the energy of glucose stays locked in lactate or ethanol. Aerobic respiration oxidises glucose all the way to

\mathrm{CO_2}

and water and gets about 15 times as much ATP from each glucose. A fermenting cell must therefore use glucose much faster to keep up.

Definition

Fermentation: 2 ATP per glucose. Aerobic respiration: about 30 to 32 ATP per glucose. The ratio is about 15 to 16.
Glycolysis alone gives the same net 2 ATP per glucose, whatever happens to the pyruvate afterwards.
The respiratory chain stage (oxidative phosphorylation) gives about 26 to 28 ATP per glucose: more than any other stage.
To compare pathways that use different amounts of glucose, compare totals: number of glucose multiplied by ATP per glucose.

Total ATP from a pathway

\text{total ATP} = (\text{number of glucose}) \times (\text{ATP per glucose for that pathway})

$2$ ATP per glucose for glycolysis alone, or glycolysis + fermentation
$30 \text{ to } 32$ ATP per glucose for complete aerobic respiration (approximate)

Worked example

During a hard 400 m race, suppose a leg muscle breaks down 50 glucose molecules by lactic fermentation and 5 by aerobic respiration (take 30 ATP per glucose). What fraction of its ATP comes from fermentation?

Practice this conceptself-check · 3 quick reps

Compare totals, not yields per glucose

Per glucose, aerobic respiration always beats fermentation. But a ranking question may give different numbers of glucose for each pathway; then multiply first. Many fermented glucose molecules can out-produce one glucose respired aerobically.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Comparing ATP yields: fermentation against aerobic respiration
Total ATP from a pathway
$\text{total ATP} = (\text{number of glucose}) \times (\text{ATP per glucose for that pathway})$

Reference tables (1)

Lactic and alcoholic fermentation compared7 rows

Feature	Lactic fermentation	Alcoholic fermentation
Where it occurs	Animal muscle, red blood cells, lactic acid bacteria	Yeast, some plant cells
Site in the cell	Cytoplasm	Cytoplasm
End products	Lactate	Ethanol and CO₂
CO₂ released	No	Yes, one per pyruvate
What happens to pyruvate	Reduced directly to lactate	Decarboxylated to ethanal, which is then reduced
ATP per glucose	2	2
Everyday use	Yoghurt and cheese making	Bread rising, beer and wine

Watch out for (4)

Fermentation regenerates NAD⁺; it makes no extra ATP
→ Lactic and alcoholic fermentation compared
Only alcoholic fermentation releases CO₂
→ Lactic and alcoholic fermentation compared
Both fermentations happen in the cytoplasm
→ Lactic and alcoholic fermentation compared
Compare totals, not yields per glucose
→ Comparing ATP yields: fermentation against aerobic respiration

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