8 Anaerobic Respiration

Anaerobic Pathways of Cellular Respiration

1. Overview of Anaerobic Cellular Respiration

  • Definition: Anaerobic cellular respiration refers to ATP production without oxygen.

  • Context of Occurrence:

    • Common in organisms in oxygen-poor environments.

    • Eukaryotic organisms (e.g., humans) utilize this when oxygen is scarce.

  • Productivity: Generally low biological productivity due to lower energy yields compared to aerobic respiration.

2. Types of Anaerobic Processes

2.1 Fermentation

  • Mechanism: Uses an organic molecule as the final electron acceptor and does not involve an electron transport chain (ETC).

  • Key Points:

    • Not classified as true respiration due to the absence of an ETC.

2.2 Anaerobic Respiration

  • Mechanism: Utilizes an ETC that employs inorganic substances (not oxygen) and a terminal electron acceptor.

3. Fermentation Process

  • Conditions: Occurs when oxygen is low but glucose is available.

  • Role of NAD+: After glycolysis, NADH needs to be oxidized back to NAD+ to allow glycolysis to continue.

  • Process: Fermentation regenerates NAD+ without an ETC, allowing glycolysis to proceed.

4. Types of Fermentation

4.1 Alcohol Fermentation

  • Occurrence: Found in many bacteria and yeast.

  • Process:

    • Glycolysis converts glucose to 2 pyruvate, yielding 2 NADH.

    • Pyruvate is decarboxylated to form acetaldehyde and CO2.

    • Acetaldehyde oxidizes NADH to NAD+, producing ethanol.

  • Outcome: Produces 2 ATP and releases 2 ethanol and 2 CO2 molecules as waste products; much less energy is generated compared to aerobic respiration (38 ATP).

  • Applications: Used in baking, brewing, and winemaking.

4.2 Lactate Fermentation

  • Occurrence: Used by some bacteria and in human muscle cells during strenuous activities when oxygen supply is inadequate.

  • Process:

    • Glycolysis yields 2 ATP per glucose.

    • Excess pyruvate is converted into lactate (lactic acid).

    • Regeneration of NAD+ maintains glycolysis and increases ATP production.

  • Recovery: Lactate builds up during intense exercise but is converted back to pyruvate when oxygen levels normalize, allowing recovery of NADH for the Krebs cycle and ETC.

  • Duration of Lactate: Lactate levels return to normal typically within an hour post-exercise.