Lecture Notes: Cellular Respiration

Overview

• Cellular respiration is split into two main types:
  • Aerobic Respiration: Involves glycolysis, citric acid cycle, and oxidative phosphorylation.
  • Anaerobic Respiration and Fermentation: Occurs without oxygen.

Aerobic Respiration

• Oxidative Phosphorylation: Requires oxygen as the electron acceptor.
  • Electrons move through the electron transport chain (ETC), losing energy.
  • Oxygen acts as the final electron acceptor, forming water.
  • Most ATP is generated in this step.

Anaerobic Respiration

• Uses ETC without oxygen.
• Final electron acceptor is another electronegative molecule (e.g., sulfate in sulfate-reducing bacteria).
  • Example: Sulfate ions are used, and hydrogen sulfide is produced.

Fermentation

• Does not use ETC; relies on glycolysis.
• Glycolysis plus reactions to regenerate NAD+ by transferring electrons from NADH to pyruvate.
• Two main types:
  • Alcohol Fermentation:
    • Pyruvate is converted to ethanol in two steps.
    • Releases CO2 and regenerates NAD+.
    • Used by yeasts in brewing, winemaking, and baking.
  • Lactic Acid Fermentation:
    • Pyruvate is reduced to lactate by NADH.
    • No CO2 is produced.
    • Occurs in human muscles during low oxygen conditions, producing ATP.

Fermentation vs Anaerobic vs Aerobic Respiration

• Commonalities:
  • All use glycolysis, producing pyruvate and net 2 ATP.
  • Utilize NAD+ as an oxidizing agent.
• Differences:
  • Fermentation: Final electron acceptor is an organic molecule (e.g., pyruvate or acetaldehyde).
  • Anaerobic/Aerobic Respiration: Electrons go through ETC to an electronegative molecule like oxygen or sulfate.
  • ATP Production:
    • Aerobic: Up to 32 ATP per glucose.
    • Fermentation: 2 ATP per glucose.

Organisms and Respiration

• Obligate Anaerobes: Can only perform fermentation/anaerobic respiration; oxygen is toxic.
  • Example: Bacteria causing gingivitis and botulism.
• Obligate Aerobes: Perform only aerobic respiration (e.g., brain neurons).
• Facultative Anaerobes: Can switch between aerobic respiration and fermentation (e.g., human muscle cells).

Evolutionary Significance

• Glycolysis predates oxygen in the atmosphere (~3.5 billion years ago).
• Oxygen levels rose ~2.7 billion years ago, owing to cyanobacteria.
• Evolution from prokaryotic cells utilizing glycolysis to eukaryotic cells with more complex respiration.

Metabolic Pathways

• Glycolysis and the citric acid cycle integrate various metabolic pathways, not limited to glucose:
  • Carbohydrates: Glycogen and sucrose can enter the cycle as glucose.
  • Proteins: Must be broken down into amino acids; enter glycolysis as intermediaries.
  • Fats: Glycerol enters glycolysis; fatty acids undergo beta-oxidation and enter as acetyl CoA.

Regulation

• Inhibition:
  • Excess ATP or citrate inhibits phosphofructokinase, slowing glycolysis.
• Activation:
  • AMP stimulates phosphofructokinase, enhancing glycolysis.

This concludes the lecture on cellular respiration, covering both aerobic and anaerobic methods along with fermentation.