Microbial Metabolism and Metabolic Diversity

Chapter Overview

  • Chapter 3: Microbial Metabolism
  • Chapter 14: Metabolic Diversity of Microorganisms

3.1 Defining the Requirements for Life

  • Microbial life requires specific nutrients and energy sources to thrive.
  • Metabolic Classes:
    • Catabolism: Energy-releasing metabolic reactions.
    • Anabolism: Biosynthesis and building of cell components.
  • Nutrient Sources: Organic and inorganic nutrients are essential for metabolic processes.

Energy Classes of Microorganisms

  • Classification based on how microorganisms obtain energy:
    • Chemotrophy: Uses chemicals (organic/inorganic) as energy sources.
    • Example: Chemoorganotrophs (e.g., Escherichia coli) use organic compounds (e.g., glucose).
    • Example: Chemolithotrophs (e.g., Thiobacillus thiooxidans) use inorganic chemicals (e.g., H2S).
    • Phototrophy: Uses light as an energy source.

3.2 Electron Transfer Reactions

  • Redox Reactions involve transfer of electrons:
    • Electron Donor: Substance that loses electrons (oxidized).
    • Electron Acceptor: Substance that gains electrons (reduced).
  • Reduction Potential (E0'): Measures tendency of a substance to gain electrons. The more negative the potential, the better it is at donating electrons.
  • Key Reactions:
    • H2 + rac{1}{2} O2
      ightarrow H_2O ext{ with } ext{ } riangle G'^{0} = -237 ext{ kJ}
    • H_2 + fumarate
      ightarrow succinate ext{ with } riangle G'^{0} = -86 ext{ kJ}

Gibbs Free Energy Calculation

  • riangle G'^{0} = - n imes F imes riangle E'^{0}
  • The energy generated from the reduction of electron acceptors can drive ATP synthesis.

3.4 Cellular Energy Conservation

  • Energy Conservation Mechanisms:
    • Substrate-Level Phosphorylation: Direct ATP synthesis in coupled reactions.
    • Electron Transport Phosphorylation: ATP synthesized via a proton motive force (pmf).
  • Long-term storage involves forming insoluble polymers that can be oxidized for energy when needed (e.g., glycogen, polyhydroxybutyrate).

Stage of Metabolism in Glycolysis

  • Glycolysis is the universal pathway for glucose oxidation:
    • Stages:
    1. Preparatory Stage: Investment phase (uses ATP).
    2. Redox Stage: Production of ATP and NADH, leading to the formation of pyruvate.
    3. Fermentation Stage: Recycling of NAD+ through various fermentation pathways (anaerobic conditions).

Types of Fermentation

  • Alcoholic Fermentation: Produces ethanol and CO2.
  • Homolactic Fermentation: Produces lactic acid with no byproducts.
  • Heterolactic Fermentation: Produces both lactic acid and other compounds (ethanol, CO2).

3.6 Principles of Respiration

  • Respiration involves complete oxidation of pyruvate via the Citric Acid Cycle (CAC), resulting in more ATP production than fermentation.
  • Respiration Types:
    • Aerobic: Oxygen serves as the final electron acceptor.
    • Anaerobic: Other molecules like nitrate or sulfate serve as electron acceptors.

Key Pathways in Respiration

  • Citric Acid Cycle (CAC): Converts pyruvate to CO2, producing NADH and FADH2 for further ATP production through oxidative phosphorylation.
  • Proton Motive Force (pmf): Generated during electron transport, used to synthesize ATP by ATP synthase.

3.8 Energetics and Efficiency of Respiration

  • Efficiency of ATP Production by NADH and FADH2 varies by organism and environmental conditions:
    • Approximately 2.5-3 ATP per NADH under optimal conditions, and lesser for FADH2.
  • Conclusion: Energy generated through respiration depends on electron carriers and terminal electron acceptors used, allowing microorganisms to adapt to different environmental conditions.

Additional Considerations

  • Microbial metabolism encompasses a wide array of mechanisms, including anaerobic respiration, chemolithotrophy, and phototrophy, emphasizing the diversity and adaptability of microorganisms in various environments.