Anaerobic ATP Synthesis and Fermentation

ATP Synthesis Under Anaerobic Conditions

  • Anaerobic respiration occurs in low oxygen conditions, utilized by bacteria and archaea.
  • These organisms use electron transport chain (ETC) with electron acceptors like sulfate, sulfur, nitrate, or carbon dioxide instead of oxygen.
  • Methanogens use sulfur, producing hydrogen sulfide (H2SH_2S), which smells like rotten eggs, as an end product.

Fermentation

  • Occurs without oxygen and the electron transport chain.
  • Glucose is converted to pyruvate via glycolysis, yielding 2 ATP and 2 NADH.
  • Pyruvate is converted to acetaldehyde, then to ethanol, regenerating NAD+NAD^+.
  • This process is seen in yeast (e.g., beer and bread making).
  • Ethanol is the end product; no oxygen or electron transport chain is involved.

Lactic Acid Fermentation

  • Glucose converts to pyruvate via glycolysis (2 net ATP).
  • Pyruvate is converted to lactate, regenerating NAD+NAD^+.
  • Critical for cells lacking mitochondria (e.g., red blood cells, muscle cells).
  • Key to making cheese and yogurt.

Facultative vs. Obligate Anaerobes

  • Facultative anaerobes: Survive and thrive with or without oxygen (e.g., yeast).
  • Obligate anaerobes: Survive only in low or no oxygen environments.

Glycolysis

  • Glycolysis is common to both fermentation and cellular respiration.
  • In the absence of oxygen, pyruvate undergoes anaerobic respiration or fermentation to produce ethanol or lactate.
  • In the presence of oxygen, pyruvate enters the mitochondria, forming acetyl coenzyme A, entering the citric acid cycle and electron transport chain for aerobic cellular respiration.

Comparison of Fermentation, Anaerobic, and Aerobic Respiration

  • Starting substrate: Glucose for all three processes.
  • Oxidizing agent: NAD+NAD^+ in all three.
  • Fermentation: Pyruvate is converted to lactic acid or ethanol; 2 ATP produced.
  • Aerobic Respiration: Electron-rich molecules go through the electron transport chain, producing 30-32 (or more) ATP.
  • Anaerobic Respiration: Electron acceptors are sulfate, sulfur, nitrate, or carbon dioxide; ATP production depends on the pathway.