Microbial Metabolism I & II: Nutrients, Pathways, and Energy Conservation

Microbial Metabolism: Overview

Introduction to Microbial Metabolism

Microbial metabolism encompasses all the chemical reactions that occur within a microbial cell, enabling growth, energy production, and maintenance. These reactions are organized into metabolic pathways, which are catalyzed by enzymes and allow microbes to convert nutrients into cellular components and energy.

• Metabolism is the sum of all biochemical reactions in a cell.

• Pathways are regulated by enzymes and can be catabolic (breaking down molecules) or anabolic (building molecules).

• Metabolic intermediates connect different pathways, allowing flexibility in nutrient usage.

Microbial Nutritional Requirements

Macronutrients and Micronutrients

Microbes require a variety of nutrients for growth and survival, which are classified as macronutrients (needed in large amounts) and micronutrients (needed in small amounts).

• Macronutrients: Carbon (C), Nitrogen (N), Oxygen (O), Hydrogen (H), Phosphorus (P), Sulfur (S), Potassium (K), Calcium (Ca), Magnesium (Mg)

• Micronutrients: Iron (Fe), vitamins, trace metals (e.g., zinc, copper, manganese)

These nutrients are used to build cellular macromolecules such as proteins, nucleic acids, lipids, and polysaccharides.

Cellular Composition of Microbes

The major macromolecular components of a typical microbial cell are:

• Protein: ~15% of cell mass

• RNA: ~6%

• Phospholipids: ~2%

• DNA: ~1%

• Polysaccharides: ~5%

• Small molecules: ~4%

• Water: ~70% of total cell mass

Essential Elements and Their Functions

Each element plays a specific role in cellular function:

• Carbon: Backbone of all organic molecules

• Nitrogen: Component of amino acids, nucleic acids

• Phosphorus: Found in nucleic acids, ATP, phospholipids

• Sulfur: Present in some amino acids and vitamins

• Iron: Essential for electron transport and enzyme function

Table: Macromolecular Composition of a Microbial Cell

Macromolecule	Percent of Dry Weight
Protein	55.0
Lipid	9.1
Polysaccharide	5.0
Lipopolysaccharide	3.4
DNA	3.1
RNA	20.5

Classification of Microbes by Metabolic Type

Energy and Carbon Sources

Microbes are classified based on their sources of energy and carbon:

• Energy Sources:

  • Chemotrophs: Obtain energy from chemicals

  • Phototrophs: Obtain energy from light

  • Chemoorganotrophs: Use organic chemicals (e.g., glucose)

  • Chemolithotrophs: Use inorganic chemicals (e.g., H2, NH3)

• Carbon Sources:

  • Autotrophs: Use CO2 as carbon source

  • Heterotrophs: Use organic carbon

  • Mixotrophs: Can use both inorganic and organic sources

Example: Escherichia coli is a chemoorganotroph and heterotroph; Thiobacillus is a chemolithotroph and autotroph.

Metabolic Pathways: Catabolism and Anabolism

Catabolic Pathways

Catabolism involves the breakdown of complex molecules into simpler ones, releasing energy that can be conserved as ATP.

• Exergonic reactions: Energy-releasing

• Examples: Glycolysis, fermentation, respiration

Anabolic Pathways

Anabolism uses energy and building blocks to synthesize complex molecules required for cell growth and maintenance.

• Endergonic reactions: Energy-consuming

• Examples: Protein synthesis, DNA replication

Bioenergetics and Energy Conservation

High-Energy Bonds and ATP

Cells store energy in high-energy bonds, most notably in ATP (adenosine triphosphate). Hydrolysis of these bonds releases energy for cellular work.

• High-energy bonds: Phosphoanhydride bonds in ATP

• ATP is generated by substrate-level phosphorylation, oxidative phosphorylation, or photophosphorylation

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Mechanisms of Energy Conservation

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP from a high-energy substrate

• Oxidative phosphorylation: Electron transport chain generates a proton motive force used to synthesize ATP

• Photophosphorylation: Light energy drives ATP synthesis in photosynthetic organisms

Redox Reactions and Electron Transport

Redox Potential and Electron Flow

Redox reactions involve the transfer of electrons from electron donors to electron acceptors. The tendency of a molecule to gain or lose electrons is measured as redox potential (E0).

• Electron donors: Have negative redox potential, lose electrons

• Electron acceptors: Have positive redox potential, gain electrons

• Energy is released when electrons flow from donors to acceptors down the redox tower

Equation:

$\Delta G^0' = -nF\Delta E^0'$

Where $\Delta G^0'$ is the change in free energy, $n$ is the number of electrons transferred, $F$ is Faraday's constant, and $\Delta E^0'$ is the change in redox potential.

Electron Transport Chains (ETC)

ETCs are membrane-associated systems that transfer electrons through a series of carriers, generating a proton gradient (proton motive force) used to synthesize ATP via ATP synthase.

• Electron carriers are arranged in order of increasing redox potential

• Terminal electron acceptor is often oxygen (aerobic respiration) or other molecules (anaerobic respiration)

Fermentation and Respiration

Fermentation

Fermentation is an anaerobic process that generates energy by substrate-level phosphorylation, using organic molecules as both electron donors and acceptors.

• Does not require oxygen

• Does not use the Krebs cycle or ETC

• Produces small amounts of ATP

• Common products: lactic acid, ethanol, CO2

Example: Saccharomyces cerevisiae (yeast) ferments glucose to ethanol and CO2.

Respiration

Respiration is a process where electrons from organic or inorganic molecules are transferred through an ETC to a terminal electron acceptor, generating a proton motive force and ATP.

• Can be aerobic (O2 as acceptor) or anaerobic (other acceptors)

• Produces more ATP than fermentation

Glycolysis: The Embden-Meyerhof-Parnas Pathway

Overview of Glycolysis

Glycolysis is a central catabolic pathway that converts glucose to pyruvate, generating ATP and NADH.

• Inputs: Glucose, 2 NAD+, 2 ADP, 2 Pi

• Outputs: 2 Pyruvate, 2 NADH, 2 ATP

Equation:

$\text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \rightarrow 2\text{Pyruvate} + 2\text{NADH} + 2\text{ATP} + 2\text{H}_2\text{O}$

ATP Synthase and Proton Motive Force

ATP Synthase Function

ATP synthase is a reversible enzyme complex that uses the proton motive force generated by the ETC to synthesize ATP from ADP and inorganic phosphate.

• Protons flow through ATP synthase, driving the phosphorylation of ADP

• Can also hydrolyze ATP to pump protons in reverse under certain conditions

Summary Table: Comparison of Fermentation and Respiration

Feature	Fermentation	Respiration
Oxygen Requirement	Not required	Required (aerobic) or not (anaerobic)
ATP Yield	Low	High
Electron Transport Chain	Absent	Present
End Products	Organic acids, alcohols, gases	CO2, H2O (aerobic)

Additional info:

• Some details on the periodic table and macromolecular composition were inferred from standard microbiology textbooks.

• Examples of metabolic types and representative organisms were added for clarity.