Microbial Metabolism: Pathways, Energy, and Diversity

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

CH 5: Microbial Metabolism

Introduction to Microbial Metabolism

Microbial metabolism encompasses all chemical reactions occurring within a microorganism, enabling growth, energy production, and biosynthesis. These reactions are fundamental to the survival and proliferation of microbes such as Escherichia coli (E. coli) in laboratory cultures.

Growth Medium: Supplies nutrients (carbon, nitrogen, energy sources) for microbial growth.
Metabolic Reactions: Include catabolism (breakdown of molecules for energy) and anabolism (biosynthesis of cellular components).
Key Question: What fuels microbial growth? The answer lies in metabolic reactions powered by nutrients and energy sources.

Learning Objectives

Core Concepts in Microbial Metabolism

Students should be able to:

Define metabolism and distinguish between catabolism and anabolism.
Explain the role of ATP as an energy intermediary.
Describe oxidation-reduction (redox) reactions and their significance.
Identify and provide examples of substrate-level, oxidative, and photophosphorylation.
Summarize the function and organization of metabolic pathways.
Describe glycolysis and its products.
Explain the pentose phosphate and Entner-Doudoroff pathways.
List the products of the Krebs cycle.
Describe the chemiosmotic model for ATP generation.
Compare aerobic and anaerobic respiration.
Describe fermentation and its products.
Contrast cyclic and noncyclic photophosphorylation.
Compare light-dependent and light-independent reactions of photosynthesis.
Contrast oxidative phosphorylation and photophosphorylation.
Summarize energy production in cells.
Categorize nutritional patterns by carbon source and ATP generation mechanisms.

Metabolism

Definition and Organization

Metabolism is the sum total of all chemical reactions in an organism. These reactions are catalyzed by enzymes and organized into regulated pathways, often depicted as a sequence of enzyme-mediated steps from a starting molecule to a final product.

Catabolic Reactions: Energy-releasing (exergonic); break down macromolecules into simpler molecules.
Anabolic Reactions: Energy-requiring (endergonic); build up macromolecules from simpler molecules.
Enzyme-Catalyzed Pathways: Each step is facilitated by a specific enzyme, ensuring efficiency and regulation.

ATP: The Energy Currency

Structure and Function

Adenosine triphosphate (ATP) is the primary energy carrier in cells, driving most cellular work. It consists of adenine, ribose, and three phosphate groups.

ATP Hydrolysis: Releases energy for cellular processes (catabolic).
ATP Formation: Requires energy input (anabolic).

Chemical Reactions Involving ATP/ADP:

ATP Hydrolysis (energy-releasing):
ATP Formation (energy-requiring):

ATP acts as an intermediate, linking energy-releasing catabolic reactions to energy-requiring anabolic reactions.

Catabolism vs. Anabolism

Fundamental Differences

Catabolism: Breaks down complex molecules (e.g., glucose) to release energy, often producing ATP and reducing equivalents (NADH, FADH2).
Anabolism: Uses energy (ATP) and reducing power to synthesize macromolecules (proteins, nucleic acids, lipids).
Example: Catabolism of glucose via glycolysis; anabolism of amino acids into proteins.

Redox Reactions in Metabolism

Oxidation-Reduction (Redox) Processes

Redox reactions are central to energy extraction in metabolism. Oxidation is the loss of electrons, while reduction is the gain of electrons. Electron carriers such as NAD+ and FAD capture and transfer electrons during metabolic processes.

Biological Oxidations: Often involve transfer of hydrogen atoms (2H = 2 electrons + 2 protons).
Electron Carriers: NAD+ + 2H → NADH; FAD + 2H → FADH2

ATP Generation Mechanisms

Phosphorylation Types

ATP is generated by phosphorylation of ADP through three main mechanisms:

Substrate-Level Phosphorylation: Direct transfer of phosphate from a substrate to ADP (occurs in glycolysis and fermentation).
Oxidative Phosphorylation: Uses electron transport chain and chemiosmosis (proton gradient) to drive ATP synthesis (aerobic and anaerobic respiration).
Photophosphorylation: Light-driven ATP synthesis in photosynthetic organisms.

Carbohydrate Catabolism

Overview and Pathways

Microorganisms primarily oxidize carbohydrates for energy, using two main processes:

Respiration: Complete oxidation of glucose using electron transport chain; can be aerobic (O2 as terminal electron acceptor) or anaerobic (other acceptors).
Fermentation: Incomplete oxidation of carbohydrates; occurs without an electron transport chain.

Glycolysis (Embden-Meyerhof Pathway)

Glycolysis is the oxidation of glucose to pyruvate, yielding ATP and NADH.

Energy Investment Phase: Consumes ATP to phosphorylate glucose.
Energy Harvest Phase: Produces ATP via substrate-level phosphorylation and NADH.
Net Gain: 2 ATP, 2 NADH per glucose molecule.

Alternative Pathways

Pentose Phosphate Pathway: Generates NADPH and pentoses for biosynthesis.
Entner-Doudoroff Pathway: Found in some Gram-negative bacteria; yields ATP, NADPH, and pyruvate.

Krebs Cycle (Citric Acid Cycle)

Function and Products

The Krebs cycle oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP (or GTP).

Key Products per Glucose: 6 NADH, 2 FADH2, 2 ATP (from two turns of the cycle).
Intermediates: Used for biosynthetic pathways (amphibolic).

Electron Transport Chain & Chemiosmosis

Mechanism of ATP Generation

Electrons from NADH and FADH2 are transferred through a series of carriers (flavoproteins, cytochromes, ubiquinones), releasing energy to pump protons across the membrane, creating a proton motive force.

ATP Synthase: Uses the proton gradient to synthesize ATP from ADP and Pi.
Oxidative Phosphorylation: Main source of ATP in aerobic respiration.

Aerobic vs. Anaerobic Respiration

Comparison

Feature	Aerobic Respiration	Anaerobic Respiration
Terminal Electron Acceptor	O2	Inorganic (NO3-, SO42-, etc.)
ATP Yield	~38 ATP/glucose	Less than aerobic
Products	CO2, H2O	Varies (nitrate, sulfate, etc.)

Fermentation

Process and Products

Fermentation is an anaerobic process where NADH is oxidized by transferring electrons to organic molecules, producing acids, alcohols, and gases.

Lactic Acid Fermentation: Produces lactic acid (e.g., Streptococcus, Lactobacillus).
Alcoholic Fermentation: Produces ethanol and CO2 (e.g., yeasts).

Photosynthesis

Light-Dependent and Light-Independent Reactions

Photosynthetic microbes convert light energy to chemical energy, fixing CO2 into organic compounds.

Light-Dependent Reactions: Use pigments (chlorophyll, bacteriochlorophylls) to generate ATP and NADPH.
Light-Independent Reactions (Calvin Cycle): Use ATP and NADPH to fix CO2 into sugars.
Oxygenic Photosynthesis: Produces O2 (plants, algae, cyanobacteria).
Anoxygenic Photosynthesis: Uses other electron donors (H2S) and does not produce O2 (purple/green sulfur bacteria).

Metabolic Diversity and Nutritional Patterns

Classification by Energy and Carbon Source

Type	Energy Source	Carbon Source
Photoautotroph	Light	CO2
Photoheterotroph	Light	Organic compounds
Chemoautotroph	Chemicals (inorganic)	CO2
Chemoheterotroph	Chemicals (organic)	Organic compounds

Summary

Metabolism: Enzyme-catalyzed reactions, both exergonic and endergonic.
Redox Reactions: Central to energy extraction; involve electron donors, carriers, and acceptors.
ATP Synthesis: Occurs via substrate-level, oxidative, and photophosphorylation.
Glycolysis: Converts glucose to pyruvate, yielding ATP and NADH.
Respiration: Includes Krebs cycle and electron transport chain; produces ATP via chemiosmosis.
Fermentation: Anaerobic process yielding organic acids and alcohols.
Photosynthesis: Oxygenic and anoxygenic types; light-independent reactions fix CO2.
Metabolic Types: Classified by energy and carbon sources.