AP Biology: Fermentation – Lactic Acid and Alcohol Fermentation, Anaerobic Respiration

Fermentation – Lactic Acid and Alcohol Fermentation, Anaerobic Respiration

Concept Summary

• Fermentation: Anaerobic process regenerating NAD? to sustain glycolysis when O? is absent, producing small ATP yields (2 ATP per glucose).
• Lactic acid fermentation: Pyruvate reduced to lactate by NADH in muscle cells and bacteria, enabling continued ATP production during intense exercise.
• Alcohol fermentation: Pyruvate decarboxylated to acetaldehyde, then reduced to ethanol by NADH in yeast and some bacteria, used in brewing/baking.
• Anaerobic respiration: Electron transport chain (ETC) uses non-O? final electron acceptors (e.g., sulfate, nitrate) in prokaryotes, yielding more ATP than fermentation but less than aerobic respiration.
• Obligate vs. facultative anaerobes: Obligate anaerobes cannot survive with O?; facultative anaerobes (e.g., yeast, muscle cells) switch between aerobic/anaerobic pathways based on O? availability.

Core Questions

WHAT (definitional)

Q: What is fermentation? A: Metabolic pathway regenerating NAD? from NADH by reducing pyruvate (or its derivatives) under anaerobic conditions, allowing glycolysis to continue. Trap/Clarification: Fermentation does not produce ATP directly—it only recycles NAD? to sustain glycolysis (which nets 2 ATP).

Q: What distinguishes anaerobic respiration from fermentation? A: Anaerobic respiration uses an ETC with a non-O? final electron acceptor (e.g., NO), producing more ATP (e.g., 2–36 ATP/glucose); fermentation lacks an ETC and yields only 2 ATP from glycolysis. Trap/Clarification: Both occur without O?, but only anaerobic respiration involves an ETC—fermentation is glycolysis + NAD? regeneration.

WHY (causal/explanatory)

Q: Why is NAD? regeneration critical in fermentation? A: Without NAD?, glycolysis halts (step 6 requires NAD? to oxidize G3P), stopping ATP production; fermentation restores NAD? by oxidizing NADH. Trap/Clarification: NAD? is not a direct energy source—it’s a coenzyme enabling substrate-level phosphorylation in glycolysis.

Q: Why do muscle cells switch to lactic acid fermentation during intense exercise? A: O? demand exceeds supply, forcing cells to rely on glycolysis + fermentation for rapid ATP (though less efficient); lactate buildup causes muscle fatigue. Trap/Clarification: Lactate does not cause soreness (microtears do)—it’s later converted back to pyruvate in the liver (Cori cycle).

HOW (process/application)

Q: How does alcohol fermentation proceed after glycolysis? A: 1) Pyruvate-CO? + acetaldehyde (via pyruvate decarboxylase); 2) Acetaldehyde + NADH-ethanol + NAD? (via alcohol dehydrogenase). Trap/Clarification: CO? release (step 1) is why bread rises, but ethanol is the final product (not lactate).

Q: How is ATP yield calculated in anaerobic pathways? A: Fermentation: 2 ATP (glycolysis only). Anaerobic respiration: 2 ATP (glycolysis) + variable ATP from ETC (e.g., 2–34 ATP, depending on the final electron acceptor). Trap/Clarification: ETC efficiency in anaerobes is lower than aerobic respiration (e.g., sulfate reduction yields ~2 ATP vs. 30–32 ATP with O?).

CAN (conditions/possibilities)

Q: Can facultative anaerobes perform both fermentation and aerobic respiration? A: Yes—if O? is present, they use aerobic respiration (higher ATP yield); if absent, they switch to fermentation or anaerobic respiration. Trap/Clarification: Facultative anaerobes prefer aerobic respiration (e.g., yeast in dough ferments only when O? is depleted).

Q: Under what conditions does lactic acid fermentation occur in humans? A: During anaerobic exercise (e.g., sprinting) when O? delivery to muscles is insufficient to meet ATP demand via oxidative phosphorylation. Trap/Clarification: Lactic acid fermentation is not the primary cause of muscle fatigue—ATP depletion and ion imbalances play larger roles.

Quick Facts & Traps

• Fact: Fermentation nets 2 ATP/glucose (from glycolysis only); anaerobic respiration nets 2–36 ATP/glucose (depending on the final electron acceptor).
• Trap: "Fermentation produces CO?."-Reality: Only alcohol fermentation releases CO?; lactic acid fermentation does not.
• Fact: Obligate anaerobes (e.g., Clostridium) die in O? because they lack enzymes (e.g., superoxide dismutase) to neutralize reactive oxygen species.
• Trap: "All anaerobes use fermentation."-Reality: Anaerobic respirers (e.g., Desulfovibrio) use an ETC with sulfate/nitrate, not fermentation.
• Fact: Cori cycle recycles lactate: muscle lactate-liver-pyruvate-glucose (via gluconeogenesis), then back to muscles.
• Trap: "Yeast only ferments."-Reality: Yeast are facultative anaerobes—they respire aerobically when O? is available (e.g., in wine production, O? is excluded to force fermentation).

Rapid-Fire True/False

• Statement: Fermentation is less efficient than anaerobic respiration because it lacks an electron transport chain. Answer: TRUE Why the common mistake happens: Students conflate "anaerobic" with "fermentation," forgetting anaerobic respiration does use an ETC.

• Statement: Lactic acid fermentation produces CO? as a byproduct. Answer: FALSE Why the common mistake happens: Confusion with alcohol fermentation (which releases CO?) or misattributing muscle "burn" to CO? buildup.

• Statement: Obligate anaerobes can survive in oxygen-rich environments if they perform fermentation. Answer: FALSE Why the common mistake happens: Overlooking that obligate anaerobes lack detoxifying enzymes (e.g., catalase) to handle O? toxicity, regardless of metabolic pathway.