Microbial Metabolism: Energetics and Energy Transfer

Metabolism Overview

Metabolism Definition

  • Metabolism: All chemical reactions in the cell divided into:
    • Catabolism:
    • Fuels and conserves energy
    • Provides a source of electrons (reducing power)
    • Generates precursors for biosynthesis
    • Anabolism:
    • Synthesis of complex molecules from simpler ones
    • Requires energy (usually as ATP) and electrons stored in reducing power

Amphibolic Pathways

  • Amphibolic pathways: Reversible pathways used in both catabolic and anabolic processes.

Energy Acquisition in Microbes

Energy Sources for Microorganisms

  • Photo-: Light absorption excites electrons.
  • Chemo-: Chemical electron donors are oxidized.
  • Litho-: Inorganic molecules donate electrons.
  • Organo-: Organic molecules donate electrons.

Carbon Sources for Biomass

  • Auto-: CO2 is fixed into organic molecules.
  • Hetero-: Preformed organic molecules are acquired from the environment.

Importance of Energy

  • All living organisms require matter and energy for growth.
  • Energy: Capacity to perform work.

Types of Work Done by Microbial Cells

  • Chemical Work: Synthesis of complex molecules.
  • Transport Work: Nutrient uptake, waste elimination, and ion balance maintenance.
  • Mechanical Work: Cell motility and internal structure movement.

Thermodynamics and Energy Interchange

First Law of Thermodynamics

  • Energy cannot be created or destroyed; it can only change forms.

Second Law of Thermodynamics

  • Entropy (disorder) of the universe increases over time.

Free Energy (Gibbs-Helmholtz Equation)

  • Equation: ΔG = ΔH - TΔS
    • ΔG: Change in Gibbs Free Energy (in joules/mol)
    • ΔH: Enthalpy (heat change in joules)
    • ΔS: Entropy (disorder measurement in joules/°K)
    • T: Temperature in °K

Gibbs Free Energy Implications

  • ΔG Negative: Reaction proceeds spontaneously.
  • ΔG Positive: Reaction is non-spontaneous.

Factors Affecting ΔG

Changes in Concentration
  • High reactant concentration favors forward reaction (more negative ΔG).
  • High product concentration favors reverse reaction (more positive ΔG).

Energy Carriers

  • Energy carriers: Molecules that gain/release small energy amounts in reversible reactions.
  • Examples: ATP, NADH, NADPH, FADH2.

ATP Utilization

  • ATP Hydrolysis: Releases energy used in coupled anabolic reactions.
  • Phosphorylation: ATP can phosphorylate organic molecules.

Redox Reactions

Definition

  • Involves electron transfer:
    • Oxidation: Loss of electrons.
    • Reduction: Gain of electrons.

Redox Pair Example

  • NADH + H+ (electron donor) => NAD+ (electron acceptor)
  • Electron transfer can release energy, which is conserved to form ATP.

Standard Electron Potential (ΔE0’)

  • Measures the tendency to lose electrons (oxidation potential).
  • Lower potential means a better electron donor, higher means a better acceptor.

Relationship Between Redox Reactions and Free Energy

Nernst Equation

  • Relationship between change in electron potential (ΔE°’) and free energy (ΔG°’):
    • ΔG°’ = -nFΔE°’
    • n: Number of electrons transferred
    • F: Faraday constant (96,480 joules/mol volt)

Important Terms

  • Catabolism: Breakdown of molecules for energy.
  • Anabolism: Building of complex molecules.
  • Redox Reaction: Electron transfer reactions.
  • Gibbs Free Energy (ΔG): Indicates the energy available for work.
  • Enzymes: Catalysts for reactions in biological processes.
  • Energy Carriers: Molecules like ATP, NADH, etc. that transport energy and/or electrons.