Microbial Metabolism and Energy Conservation: Study Notes for BIOL 2500

Microbial Metabolism

Introduction to Microbial Metabolism

Microbial metabolism encompasses all the chemical reactions that occur within microorganisms, enabling them to grow, reproduce, and respond to their environment. 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).

• Microbial cells require specific inputs for metabolism, including water (as a solvent), nutrients, and energy sources.

• Energy is needed for cellular work such as active transport, biosynthesis, and motility.

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

Types of Metabolism

Catabolism and Anabolism

Metabolic pathways are classified based on their function and energy requirements.

• Catabolism: Breaks down molecules to release energy and reducing power (e.g., ATP, NADH).

• Anabolism: Uses energy and reducing power to build new cellular components (e.g., proteins, DNA, cell wall).

• Catabolic and anabolic pathways are fundamentally linked; energy released from catabolism is used to drive anabolism.

Energy Conservation and ATP

Cells conserve energy primarily in the form of ATP, which is the universal energy currency.

• Adenosine Triphosphate (ATP): Contains high-energy phosphate bonds; hydrolysis releases energy for cellular work.

• ATP hydrolysis equation: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{H}^+$ Standard free energy change: $\Delta G^\circ' = -31.8 \text{ kJ/mol}$

• ATP is ideal for short-term energy storage and is used in biosynthesis, active transport, and movement.

Mechanisms of ATP Synthesis

Microbial cells generate ATP through three main mechanisms:

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

• Oxidative phosphorylation: Electron transport chain creates a proton motive force used by ATP synthase (e.g., aerobic and anaerobic respiration).

• Photophosphorylation: Light energy is used to generate a proton motive force (e.g., photosynthetic bacteria).

Free Energy and Redox Reactions

Free Energy Change ($\Delta G$)

Free energy change determines whether a reaction is spontaneous or requires energy input.

• Exergonic reactions: $\Delta G^\circ$ is negative; energy is released and the reaction is spontaneous.

• Endergonic reactions: $\Delta G^\circ$ is positive; energy is required and the reaction is 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 are central to energy conservation in cells.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• OIL RIG: "Oxidation Is Loss, Reduction Is Gain" of electrons.

• Redox reactions involve electron donors and acceptors, forming conjugate redox pairs.

• Standard reduction potential ($E_0'$) measures a molecule's tendency to donate or accept electrons (in volts).

• Electron carriers such as NAD+ and NADH shuttle electrons between reactions: $\text{NAD}^+ + 2e^- + 2H^+ \rightarrow \text{NADH} + H^+$

Electron Donors and Acceptors

Microorganisms use a variety of electron donors and acceptors for metabolism.

• Electron donors: Can be organic (e.g., glucose) or inorganic (e.g., H2, NH3).

• Electron acceptors: Can be oxygen (aerobic respiration), nitrate, sulfate, or other compounds (anaerobic respiration).

• Energy yield depends on the difference in reduction potential between donor and acceptor.

Metabolic Diversity in Microorganisms

Classification by Energy Source

Microorganisms are classified based on how they obtain energy and carbon.

Type	Energy Source	Electron Source	Example
Phototrophs	Light	Water, H2S	Cyanobacteria
Chemotrophs	Chemicals	Organic or inorganic compounds	Escherichia coli, Thiobacillus
Chemoorganotrophs	Organic chemicals	Organic compounds	Most bacteria, fungi
Chemolithotrophs	Inorganic chemicals	Inorganic compounds	Nitrifying bacteria

Central Metabolism

Overview of Central Metabolic Pathways

Central metabolism refers to the core pathways that process substrates and generate energy and precursors for biosynthesis.

• 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.

• Substrates from proteins, lipids, and carbohydrates are funneled into these pathways for energy conservation and production of anabolic intermediates.

Regulation and Organization of Metabolic Pathways

Metabolic pathways are highly regulated and organized to ensure efficient energy use and biosynthesis.

• Each step is catalyzed by a specific enzyme or ribozyme.

• Pathways are regulated by feedback inhibition, gene expression, and allosteric control.

• Metabolic diversity allows microorganisms to adapt to various environments and utilize different energy sources.

Additional info: Some content inferred from context and standard microbiology curriculum, including definitions and examples for metabolic classes and central metabolism pathways.