Microbial Metabolism and Energy Conservation: Study Notes for BIOL 2500

Microbial Metabolism

Introduction to Microbial Metabolism

Microbial metabolism encompasses all chemical reactions that occur within microorganisms, enabling them to grow, reproduce, and maintain cellular functions. These reactions are essential for energy production, biosynthesis, and cellular maintenance.

• Metabolism is divided into two main types: catabolism (breakdown of molecules to release energy) and anabolism (synthesis of new cellular components).

• Microorganisms require specific nutrients and energy sources to carry out metabolic processes.

Composition of a Living Cell

Cells are composed of various macromolecules, each built from specific monomers and containing major elements necessary for life.

Macromolecule	Monomer	Major Elements
Protein	Amino acid	C, O, N, H, S
Carbohydrate	Simple sugar (e.g., Glucose)	C, O, H
Lipid	Fatty acid	C, O, H
Nucleic Acid	Nucleotide	C, O, N, H, P

Essential Requirements for Cellular Metabolism

Cells require specific inputs to sustain metabolism and growth:

• Water: Solvent and medium for biochemical reactions.

• Nutrients: Carbon, nitrogen, phosphorus, sulfur, and trace elements.

• Energy: Required for building cellular structures and performing cellular work.

• Reducing Power: Electron donors like NADH or NADPH for biosynthesis.

Types of Metabolism

Catabolism and Anabolism

Metabolic pathways are classified based on their function:

• Catabolism: Breakdown of molecules to release energy and produce precursors for biosynthesis.

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy input.

These pathways are fundamentally linked, as energy released from catabolism is used to drive anabolic reactions.

Energy Conservation and ATP

Cells conserve energy primarily in the form of adenosine triphosphate (ATP), which serves as the main energy carrier.

• ATP contains high-energy phosphate bonds; breaking these bonds releases energy for cellular work.

• ATP is ideal for short-term energy storage and transfer.

ATP Hydrolysis Equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{H}^+$

Standard free energy change: $\Delta G^\circ' = -31.8\ \text{kJ/mol}$

Mechanisms of ATP Synthesis

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate (e.g., glycolysis).

• Oxidative phosphorylation: Electron transport chain generates a proton motive force used to synthesize ATP (e.g., aerobic and anaerobic respiration).

• Photophosphorylation: Light energy is used to generate a proton motive force for ATP synthesis (e.g., photosynthesis).

Free Energy and Redox Reactions

Free Energy Change ($\Delta G$)

Free energy change determines whether a reaction is spontaneous:

• Exergonic reactions: $\Delta G^\circ' < 0$; release free energy; spontaneous.

• Endergonic reactions: $\Delta G^\circ' > 0$; require energy input; non-spontaneous.

Standard Free Energy Change Equation:

$\Delta G^\circ' = -2.303RT \log K_{eq}$

Where $R$ is the gas constant and $T$ is temperature in Kelvin.

Redox Reactions and Electron Flow

Oxidation-reduction (redox) reactions involve the transfer of electrons between molecules:

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Electron donor: Molecule that loses electrons.

• Electron acceptor: Molecule that gains electrons.

OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

Redox Potential ($E_0'$)

Redox potential measures a molecule's tendency to accept or donate electrons, expressed in volts (V):

• More negative $E_0'$: Better electron donor.

• More positive $E_0'$: Better electron acceptor.

Electron flow from donors with lower $E_0'$ to acceptors with higher $E_0'$ releases energy.

Electron Carriers

Electron carriers such as NAD+ and NADH shuttle electrons between metabolic reactions:

• NAD+ + 2e- + 2H+ → NADH + H+

• These carriers are recycled and not consumed in the reaction.

Other carriers include NADP+/NADPH, FAD/FADH2, and quinones.

Metabolic Diversity in Microorganisms

Classification by Energy Source

Microorganisms are classified based on how they obtain energy:

• Phototrophs: Obtain energy from light (photosynthesis).

• Chemotrophs: Obtain energy from chemical reactions.

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

  • Chemolithotrophs: Use inorganic compounds (e.g., H2, Fe2+).

Respiration can be aerobic (O2 as electron acceptor) or anaerobic (other acceptors such as nitrate or sulfate).

Central Metabolism

Central metabolic pathways process substrates to generate energy and biosynthetic precursors:

• Glycolysis: Converts glucose to pyruvate, generating ATP and NADH.

• Tricarboxylic Acid Cycle (TCA/CAC/Krebs cycle): Oxidizes acetyl-CoA to CO2, producing NADH, FADH2, and ATP.

• Pentose Phosphate Pathway: Generates NADPH and ribose-5-phosphate for biosynthesis.

These pathways are interconnected and provide both energy and building blocks for cell growth.

Summary Table: Metabolic Classes of Microorganisms

Class	Energy Source	Electron Donor	Electron Acceptor	Example
Phototroph	Light	Water or other molecules	CO2 (photosynthesis)	Cyanobacteria
Chemoorganotroph	Organic chemicals	Glucose	O2 (aerobic) or NO3- (anaerobic)	Escherichia coli
Chemolithotroph	Inorganic chemicals	H2, Fe2+, NH3	O2 or other acceptors	Thiobacillus

Key Concepts in Microbial Metabolism

• All cells obey the laws of thermodynamics.

• Energy is conserved as ATP and reducing power (NADH, NADPH).

• Metabolic reactions are organized into pathways, each catalyzed by specific enzymes.

• Pathways are regulated to meet cellular needs.

Example: During aerobic respiration, Escherichia coli oxidizes glucose to CO2 and H2O, generating ATP and NADH for cellular processes.

Additional info: Some context and explanations have been expanded for clarity and completeness, including definitions, equations, and examples relevant to college-level microbiology.