Cell Metabolism Without Oxygen

Overview

This lecture covers how cells metabolize energy without oxygen, focusing on anaerobic cellular respiration and fermentation, including lactic acid and alcohol fermentation processes.

Aerobic vs Anaerobic Metabolism

• Aerobic respiration uses oxygen as the final electron acceptor to produce ATP via the electron transport chain.
• When oxygen is absent, cells must regenerate NAD+ from NADH for glycolysis to continue.
• Anaerobic metabolism occurs through either anaerobic cellular respiration or fermentation.

Anaerobic Cellular Respiration

• Some prokaryotes (Bacteria and Archaea) use inorganic molecules (not O₂) as final electron acceptors in their electron transport chains.
• Methanogens reduce carbon dioxide to methane to oxidize NADH.
• Sulfate-reducing bacteria reduce sulfate to hydrogen sulfide to regenerate NAD+.

Fermentation

• Fermentation uses an organic molecule as the final electron acceptor to regenerate NAD+ from NADH.
• Fermentation enables energy production without oxygen by allowing glycolysis to continue.

Lactic Acid Fermentation

• Used by animals (in muscles and red blood cells) and bacteria like those in yogurt.
• Occurs when oxygen is insufficient, converting pyruvic acid and NADH to lactic acid and NAD+.
• The enzyme lactate dehydrogenase catalyzes this reaction.
• Lactic acid buildup in muscles is removed by blood and processed by the liver.
• This pathway is important when rapid ATP production is needed but oxygen is limited.

Alcohol Fermentation

• Performed by yeasts and some bacteria, producing ethanol and CO₂ as byproducts.
• Pyruvic acid is converted to acetaldehyde (by pyruvate decarboxylase) and then to ethanol (by alcohol dehydrogenase), regenerating NAD+.
• This process is responsible for alcohol production and CO₂ in fermentation industries.

Other Types and Features of Fermentation

• Some bacteria are facultative anaerobes, switching between aerobic respiration and fermentation depending on oxygen.
• Obligate anaerobes only grow in absence of oxygen, which can be toxic to them.
• Most fermentation types (except lactic acid) produce gas, which helps identify bacteria in labs.
• Fermentation pathways ensure NAD+ supply for glycolysis, maintaining ATP production.

Key Terms & Definitions

• Fermentation — an anaerobic process that regenerates NAD+ by using an organic molecule as the final electron acceptor.
• Anaerobic cellular respiration — an energy-conversion process using an inorganic molecule (other than O₂) as the final electron acceptor.
• Methanogen — an organism that produces methane by reducing carbon dioxide.
• Lactic acid fermentation — pathway converting pyruvate to lactic acid in absence of oxygen.
• Alcohol fermentation — process converting pyruvate to ethanol and CO₂, regenerating NAD+.
• Obligate anaerobe — an organism that cannot survive in presence of oxygen.
• Facultative anaerobe — an organism that can switch between aerobic respiration and fermentation.

Action Items / Next Steps

• Review glycolysis and the role of NAD+/NADH in metabolism.
• Be able to compare and contrast lactic acid and alcohol fermentation pathways.
• Understand when and why cells switch from aerobic to anaerobic metabolism.