Anaerobic Respiration: Process, Efficiency, and Biological Examples

Anaerobic respiration generates ATP without oxygen as the final electron acceptor. Less efficient than aerobic respiration (2 vs. ~30–32 ATP/glucose), but essential in hypoxic environments.

Glycolysis (Common First Step)

Glucose → 2 pyruvate + 2 ATP (net) + 2 NADH. Occurs in cytoplasm.

Fermentation Pathways (Regenerate NAD⁺)

Organism/Tissue	Pyruvate Fate	Products	ATP Yield
Human muscle (intense exercise)	Lactate dehydrogenase	Lactate + NAD⁺	2 ATP
Yeast (Saccharomyces)	Pyruvate decarboxylase + alcohol dehydrogenase	Ethanol + CO₂ + NAD⁺	2 ATP
Lactic acid bacteria	Lactate dehydrogenase	Lactate (+ other acids)	2 ATP
Some bacteria (e.g., Clostridium)	Mixed acid / butyric / propionic pathways	Various acids, alcohols, gases	2–3 ATP

Anaerobic Respiration (Strict Sense)

Uses alternative terminal electron acceptors (nitrate, sulfate, fumarate, CO₂) with electron transport chains → higher ATP yield than fermentation. Performed by specialized prokaryotes.

Physiological Relevance

• Muscle fatigue: Lactate/H⁺ accumulation → acidosis → impaired contraction.
• Corrie cycle: Muscle lactate → liver → gluconeogenesis → glucose.
• Food production: Yogurt, cheese, sauerkraut, beer, wine, biofuels.
• Gut microbiome: Anaerobic fermentation produces SCFAs (butyrate) vital for colon health.