Fermentation in Introductory Biology

Objectives

• Understand how cells obtain energy from glucose without oxygen.

Introduction

• Aerobic Respiration: Utilizes O2 as the final electron acceptor to produce ATP.

  • NADH and FADH2 carry high-energy electrons to the electron transport chain.

• Anaerobic Respiration: Occurs when oxygen is absent.

  • NADH must be reoxidized to NAD+ for glycolysis to continue.
  • Uses organic molecules as final electron acceptors (fermentation).
  • Some organisms use inorganic molecules (anaerobic respiration).

• Purpose of Fermentation:

  • Regenerate NAD+ from NADH to continue glycolysis.

Key Question

• What is the main purpose of fermentation in anaerobic respiration?
  • Answer: To regenerate NAD+ from NADH.

Anaerobic Cellular Respiration

• Certain prokaryotes (e.g., methanogens, sulfate-reducing bacteria) use anaerobic respiration.
• Methanogens reduce CO2 to methane, while sulfate-reducing bacteria reduce sulfate to hydrogen sulfide to regenerate NAD+.

Lactic Acid Fermentation

• Used by: Animals and some bacteria (e.g., yogurt bacteria).
• Occurs in red blood cells and muscle cells when oxygen is insufficient.
• Process:
  • Glucose is oxidized to pyruvate.
  • Pyruvate is reduced to lactate as NADH is oxidized to NAD+.
  • Lactate is transported to the liver for further metabolism.
• Enzyme: Lactate dehydrogenase.

Lactic Acid Fermentation Details

• End Product: Lactic acid.
• Note: Accumulation in muscles was believed to cause stiffness and soreness.
• Tremetol: A toxin found in white snakeroot, affects lactate metabolism.

Alcohol Fermentation

• Used by: Yeast to produce ethanol in alcoholic beverages.
• Process:
  • Pyruvic acid decarboxylated to acetaldehyde and CO2.
  • Acetaldehyde reduced to ethanol while NADH is oxidized to NAD+.

Alcohol Fermentation Details

• Enzymes:
  • Pyruvate decarboxylase catalyzes the release of CO2.
  • Alcohol dehydrogenase catalyzes reduction of acetaldehyde to ethanol.
• End Product: Ethanol.

Other Types of Fermentation

• Many bacteria are facultatively anaerobic, switching between aerobic respiration and fermentation.
• Obligate anaerobes, such as Clostridia, only survive without oxygen.
• Other fermentation methods produce gas, used to identify bacterial fermentation of specific carbohydrates.

Key Question

• Which is NOT a purpose of fermentation?
  • Answer: Generating ATP efficiently in the presence of oxygen.

Acknowledgements

• Adapted from Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Link