Biochemistry
Fermentation
Anaerobic energy production — much less efficient than aerobic respiration
Fermentation is the anaerobic breakdown of glucose for energy — without oxygen, without electron transport chain. Glycolysis only; pyruvate not entering Krebs. Yields just 2 ATP per glucose (vs ~30 aerobic). Function: regenerate NAD⁺ from NADH so glycolysis continues. Two main types: (1) Lactic acid fermentation — pyruvate → lactate (animals during intense exercise; some bacteria). (2) Alcoholic fermentation — pyruvate → ethanol + CO₂ (yeast, some bacteria). Industrial: bread, beer, wine, cheese, yogurt. Used: when O₂ unavailable or fast ATP needed.
- DefinitionAnaerobic glucose breakdown
- ATP yield2 per glucose (vs ~30 aerobic)
- PurposeRegenerate NAD⁺; enable continued glycolysis
- Lactic acid fermentationPyruvate → lactate (animal muscle, bacteria)
- Alcoholic fermentationPyruvate → ethanol + CO₂ (yeast)
- IndustrialBread, beer, wine, cheese, yogurt
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Why fermentation matters
- Industrial biotech. Bread, beer, wine, etc.
- Food preservation. Lactic acid kills pathogens.
- Anaerobic biology. Many microbes ferment.
- Energy. Bioethanol from fermentation.
- Medicine. Penicillin, drugs from fermentation.
- Sports physiology. Muscle behavior.
- Evolution. Ancient process; precursor to aerobic.
Common misconceptions
- Fermentation = aerobic. Anaerobic.
- Lactic acid burn from CO₂. From lactate (and H⁺).
- Yeast only ferments. Aerobic too when O₂ available.
- Fermentation efficient. 15× less ATP than aerobic.
- Fermentation only sugars. Many substrates.
- All bacteria ferment. Some only aerobic; some only anaerobic.
Frequently asked questions
How does fermentation work?
Glycolysis converts glucose → 2 pyruvate (+ 2 ATP, 2 NADH). Without O₂: ETC can't run; NADH accumulates; glycolysis would stop without NAD⁺. Solution: convert pyruvate to other product, regenerating NAD⁺. Lactic acid fermentation: pyruvate + NADH → lactate + NAD⁺. Alcoholic: pyruvate → CO₂ + acetaldehyde; acetaldehyde + NADH → ethanol + NAD⁺. NAD⁺ recycled; glycolysis continues.
What's lactic acid fermentation?
Common in: animal muscle during intense exercise (when O₂ supply can't keep up), some bacteria (e.g., Lactobacillus). Reaction: pyruvate + NADH → lactate + NAD⁺. Net per glucose: 2 ATP, 2 lactate. Lactate buildup contributes to muscle fatigue/burn. Industrial: yogurt (Lactobacillus ferments milk lactose); cheese; sauerkraut.
What's alcoholic fermentation?
Yeast and some bacteria. Reaction: pyruvate → CO₂ + acetaldehyde (via pyruvate decarboxylase); acetaldehyde + NADH → ethanol + NAD⁺ (via alcohol dehydrogenase). Net per glucose: 2 ATP, 2 ethanol, 2 CO₂. Industrial: beer, wine (yeast ferments sugars), bread (CO₂ makes dough rise; ethanol evaporates during baking), bioethanol fuel.
Why so much less ATP?
Aerobic: full oxidation to CO₂. NADH and FADH₂ feed ETC → makes ATP. Yield: ~30 ATP per glucose. Fermentation: glucose only partially broken down. Energy locked in lactate or ethanol (still has C-C and C-H bonds). Fermentation captures only ~5% of glucose energy; aerobic captures ~40%. Difference: factor of 15x.
When does fermentation occur?
When O₂ unavailable or insufficient. (1) Anaerobic environments: deep mud, sealed containers, GI tract. (2) Brief O₂ shortage: intense exercise; cells switch to anaerobic. (3) Obligate anaerobes: bacteria that can't tolerate O₂ (Clostridium). (4) Facultative: prefer aerobic; can switch (yeast, E. coli). Fast ATP when needed.
How is fermentation industrial?
Major economic activity. (1) Yeast for bread (CO₂), beer/wine (ethanol). (2) Lactic acid bacteria for yogurt, cheese, kimchi, sauerkraut. (3) Acetic acid bacteria for vinegar. (4) Industrial ethanol production for fuel. (5) Pharmaceutical: penicillin, citric acid, amino acids. Fermentation is one of oldest biotechnologies — thousands of years.
Why does muscle burn during exercise?
Lactic acid buildup. Intense exercise: O₂ delivery can't keep up with demand. Muscles switch to anaerobic glycolysis + lactic acid fermentation. Lactate accumulates; pH drops slightly. Combined with H⁺ accumulation: muscle fatigue, "burn" sensation. After exercise: Cori cycle — liver converts lactate back to glucose. Recovery requires O₂.