Fermentation

Fermentation is an anaerobic metabolic process by which cells break down sugars to generate energy without using oxygen. In fermentation, glycolysis (sugar breakdown) produces ATP and NADH, and then NADH is recycled back to NAD+ by transferring electrons to an organic molecule. This allows glycolysis to continue producing a small amount of ATP. Common end products of fermentation are lactic acid (in muscles and some bacteria) or ethanol and carbon dioxide (CO2) (in yeast).

• Fermentation occurs when oxygen is not available as the final electron acceptor. For example, your muscle cells switch to lactic acid fermentation during intense exercise when oxygen supply is insufficient, resulting in lactate buildup.
• Fermentation produces far less ATP than aerobic respiration - only the 2 ATP from glycolysis per glucose (no additional ATP from the Krebs cycle or electron transport). This lower efficiency is a key distinction: anaerobic fermentation vs aerobic respiration (~30+ ATP).
• Yeast performing alcoholic fermentation convert pyruvate (from glycolysis) into ethanol and CO2. This is exploited in brewing (ethanol in beer/wine) and baking (CO2 makes bread rise).
• A common confusion: fermentation vs anaerobic respiration. Fermentation uses an organic molecule (like pyruvate or acetaldehyde) as the final electron acceptor and involves no electron transport chain; anaerobic respiration (in some microbes) uses an electron transport chain with a non-oxygen final acceptor (like nitrate or sulfate). Most human biology questions use "fermentation" for the simple anaerobic pathway in muscle or yeast.

• If an exam question asks about how muscles continue to produce ATP during short bursts of intense exercise (when oxygen is depleted), the answer is lactic acid fermentation. You might need to identify lactate as the end product and that this regenerates NAD+ so glycolysis can continue.
• Questions may present a scenario of yeast in a sealed container producing gas - they're hinting at alcoholic fermentation (glucose -> ethanol + CO2). Recognize that CO2 from fermentation is what causes bubbles in beer or dough rising.
• A classic compare/contrast question: aerobic vs anaerobic metabolism. Know that fermentation (anaerobic) yields 2 ATP per glucose and produces lactic acid (in humans) or ethanol (yeast), whereas aerobic metabolism yields much more ATP and fully oxidizes glucose to CO2 and H2O.
• They may test recognition of fermentation pathways: e.g., "Which process can occur in human cells when mitochondria lack oxygen?" - the answer is lactic acid fermentation (conversion of pyruvate to lactate).