Anaerobic respiration, fermentation and comparison
Bioenergetics • Respiration
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Word equations for anaerobic respiration
Anaerobic respiration in yeast and many plant cells uses glucose to produce ethanol and carbon dioxide. The word equation is: glucose → ethanol + carbon dioxide. This reaction is the basis of fermentation in yeast . Muscle cells carry out a different anaerobic pathway when oxygen supply is insufficient. The word equation for muscles is: glucose → lactic acid. Lactic acid accumulates in muscle tissue during vigorous exercise and is later oxidised when oxygen returns .
Definition and economic importance of fermentation
Fermentation is anaerobic respiration carried out by microorganisms such as yeast; it specifically refers to the breakdown of glucose into ethanol and carbon dioxide in yeast cells. Fermentation produces ethanol for alcoholic drinks and carbon dioxide that helps dough rise in breadmaking. Bread baking removes or evaporates ethanol and kills the yeast so the finished product does not remain alcoholic; different fermented drinks contain different ethanol percentages (for example, beer ≈4%, wine ≈12%, spirits ≈40%) .
Why anaerobic respiration releases less energy
Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration because glucose undergoes an incomplete breakdown without oxygen as the final electron acceptor. Controlled multi-step oxidation in mitochondria during aerobic respiration extracts most of the energy; anaerobic reactions stop earlier in the pathway, so a large portion of the chemical energy remains in products such as lactic acid or ethanol and is not released as useful cellular energy . Textbook summaries state that only a small fraction of the total available energy is released during anaerobic respiration compared with aerobic respiration (the remaining energy can be released later when lactic acid is oxidised after oxygen returns) .
Comparison: aerobic versus anaerobic respiration
Oxygen requirement: Aerobic respiration requires oxygen; anaerobic respiration occurs when oxygen is absent or in short supply. Products: Aerobic respiration produces carbon dioxide and water from glucose and oxygen; anaerobic respiration produces different products depending on organism: ethanol and carbon dioxide in yeast/plant cells, lactic acid in animal muscle cells. Energy transferred: Aerobic respiration transfers much more energy per glucose molecule because the complete oxidation of glucose to CO2 and H2O extracts most chemical energy; anaerobic respiration transfers only a small fraction of that energy because glucose is only partially broken down .
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