12.2.9—Mitochondria structure-function relationship

Anaerobic respiration allows glycolysis to continue when oxygen is unavailable. It gives a small ATP yield because glycolysis continues, while fermentation reoxidises reduced NAD to regenerate the oxidised NAD needed for glycolysis.

1. Oxygen unavailable: oxygen cannot act as the final electron acceptor, so the electron transport chain and oxidative phosphorylation stop. Reduced coenzymes cannot be reoxidised through that route, so the Krebs cycle also stops.
2. Glycolysis continues: glucose is converted to pyruvate in the cytoplasm, producing a small amount of ATP and reduced NAD.
3. NAD is regenerated: fermentation transfers hydrogen from reduced NAD to an acceptor. This restores oxidised NAD, allowing the dehydrogenation step in glycolysis to continue.
4. Mammalian muscle / lactate route: pyruvate accepts hydrogen from reduced NAD and is converted to lactate. This route does not release carbon dioxide.
5. Yeast and some plants / ethanol route: pyruvate is decarboxylated to ethanal, releasing carbon dioxide; ethanal then accepts hydrogen from reduced NAD and is converted to ethanol. Ethanol is a waste product.

Anaerobic respiration has a low energy yield because only glycolysis supplies ATP; it does not sustain the mitochondrial ATP-producing stages. Fermentation is a temporary or condition-dependent way to regenerate NAD, not a higher-yield replacement for aerobic respiration. Do not infer rice adaptations, temperature effects or substrate-concentration effects from this card.