Anaerobic Metabolism and Fermentation

Overview

This section explains how cells metabolize energy without oxygen through anaerobic cellular respiration and fermentation, highlighting lactic acid fermentation in animals and alcohol fermentation in yeast.

Key Concepts of Anaerobic Metabolism

• In aerobic respiration, oxygen serves as the final electron acceptor, enabling ATP production through the electron transport chain.
• Without oxygen, cells must regenerate NAD+ from NADH to maintain glycolysis and ATP production.
• Anaerobic processes use either organic molecules (fermentation) or inorganic molecules (anaerobic respiration) as final electron acceptors.

Anaerobic Cellular Respiration

• Some prokaryotes, such as methanogens and sulfate-reducing bacteria, use inorganic molecules (CO₂ or sulfate) as final electron acceptors.
• Methanogens reduce carbon dioxide to methane, while sulfate-reducing bacteria convert sulfate to hydrogen sulfide.
• Anaerobic respiration occurs mainly in bacteria and archaea in environments lacking oxygen.

Fermentation: Types and Mechanisms

• Fermentation regenerates NAD+ from NADH, using organic molecules and allowing glycolysis to continue in the absence of oxygen.
• Lactic acid fermentation occurs in animal cells (e.g., muscle and red blood cells) and some bacteria.
• In lactic acid fermentation: Pyruvic acid + NADH → Lactic acid + NAD+, catalyzed by lactate dehydrogenase.
• Accumulated lactic acid in muscles is removed by blood and converted in the liver back to pyruvic acid.
• Alcohol fermentation (in yeast) produces ethanol and carbon dioxide from pyruvate via two steps: decarboxylation and reduction.
• Alcohol fermentation is catalyzed by pyruvate decarboxylase (with TPP coenzyme) and alcohol dehydrogenase.

Other Fermentation Methods and Microbial Diversity

• Many prokaryotes are facultative anaerobes, switching between aerobic respiration and fermentation.
• Obligate anaerobes, such as Clostridia, die in the presence of oxygen.
• Except for lactic acid fermentation, most fermentation pathways generate gas as a byproduct, useful for bacterial identification.

Key Terms & Definitions

• Anaerobic Respiration — Respiration using an inorganic molecule other than oxygen as the final electron acceptor.
• Fermentation — Process using an organic molecule to regenerate NAD+ from NADH, allowing glycolysis without oxygen.
• Lactic Acid Fermentation — Fermentation pathway converting pyruvate to lactic acid, regenerating NAD+.
• Alcohol Fermentation — Fermentation pathway producing ethanol and CO₂ from pyruvate in yeast.
• Facultative Anaerobe — Organism that can switch between aerobic respiration and fermentation based on oxygen availability.
• Obligate Anaerobe — Organism that cannot survive in the presence of oxygen.

Action Items / Next Steps

• Review the chemical equations for lactic acid and alcohol fermentation.
• Consider why lactate accumulation worsens symptoms after exercise in cases of tremetol poisoning.
• Read the next section on metabolic pathway connections for carbohydrates, proteins, and lipids.