MCBP 2616 LECTURE 7

The Basic Principles of Microbiology (MCBP 2616)

Microbial Metabolism Lecture 7 (Chapter 3)

1. Fundamentals of Metabolism

1.1 Defining the Requirements for Life

• Metabolism:

  • Comprises all biochemical reactions essential for sustaining life.

  • Categorized into:

    • Catabolism: Reactions that obtain energy from breakdown processes.

    • Anabolism: Reactions that synthesize cellular material.

  • Relies on electron donors directing electrons to electron acceptors.

• Energy Conservation:

  • Energy is neither created nor destroyed; it is conserved and transformed into usable forms.

  • Adenosine triphosphate (ATP) is generated to store energy and power cellular processes.

1.2 Fundamental Metabolic Requirements

• All cells require:

  • Water

  • Carbon and Nutrients

  • Free Energy: Energy available for work.

  • Reducing Power: Source of electrons.

1.3 Free Energy

• Energy Measurement:

  • Measured in kilojoules (kJ) of heat energy.

• Free Energy (G):

  • The energy available to do work in chemical reactions.

• ΔG0':

  • Change in free energy during a reaction under standard conditions (pH 7, 25°C, 1 atm).

• Reactions Types:

  • Exergonic (Catabolism): Reactions with −ΔG0' release energy.

  • Endergonic (Anabolism): Reactions with +ΔG0' require energy.

1.4 Catabolic and Anabolic Pathways

• Catabolic Pathways:

  • Generate free energy, conserved in energy-rich molecules like ATP.

  • ATP formation requires ΔG0' = −31.8 kJ/mol.

  • Example: Aerobic respiration of 1 mole of glucose can theoretically produce 91 moles of ATP.

• Anabolic Pathways:

  • Require energy for cellular synthesis, pulling energy from ATP hydrolysis.

  • Catabolism and anabolism are interconnected.

2. Electron Transfer Reactions

2.1 Role of Reducing Power

• Reducing Power:

  • Ability to donate electrons in redox reactions.

  • Energy from redox reactions is utilized to synthesize energy-rich compounds (e.g., ATP).

  • Redox Reactions:

    • Consists of two half-reactions:

      • Electron Donor: Transfers electrons (oxidized).

      • Electron Acceptor: Gains electrons (reduced).

  • Example: In aerobic respiration, glucose (electron donor) is oxidized, and O2 (electron acceptor) is reduced.

2.2 Redox Reactions Dynamics

• Half Reactions:

  • First half reaction generates electrons, consumed by the second half reaction.

• Redox Couples:

  • Various redox couples exist in nature, and the potential for donating or accepting electrons changes under different conditions.

2.3 Metabolic Classes of Microorganisms

• Phototrophs:

  • Obtain energy from light; can be oxygenic (produce O2) or anoxygenic (do not produce O2).

• Chemotrophs:

  • Obtain energy from chemical reactions; differentiate between aerobic (uses O2) and anaerobic (does not use O2) via respiration or fermentation.

• Chemoorganotrophs:

  • Utilize organic compounds (carbohydrates and proteins) for energy.

• Chemolithotrophs:

  • Obtain energy from inorganic compounds within biogeochemical cycles.

• Autotrophs:

  • Utilize CO2 as a carbon source; known as primary producers.

3. Calculating Changes in Free Energy

3.1 The Redox Tower

• Represents electron transfer levels; arranged from strong electron donors to strong acceptors.

• A greater potential difference between donor and acceptor yields more energy.

• Free Energy Calculation Formula:

  • ΔG0 = −nFΔE0' where n = number of electrons transferred and F = Faraday constant.

3.2 Electron Carriers

• NAD+/NADH Cycling:

  • NAD+ is a solvent electron carrier facilitating redox reactions and is recycled.

  • Reduction of NAD+ leads to NADH and H+.

3.3 ΔG0′ Calculations

• To find out if a reaction is exergonic or endergonic, subtract the free energies of formation of reactants from products.

• Example Calculation:

  • Calculate ΔG0’ using free energy of formation from given reactions to understand energy changes during chemical reactions.