Microbial Metabolism — Glycolysis, Fermentation, Respiration & Biosynthesis Explained | Chapter 3 of Brock Biology of Microorganisms

Welcome to Last Minute Lecture, where we break down complex textbooks into clear and concise summaries. In this post, we’ll explore Chapter 3 of Brock Biology of Microorganisms, which introduces the fascinating and diverse metabolic strategies used by microorganisms. From energy production and redox chemistry to biosynthesis and nitrogen fixation, this chapter reveals how microbes power life in virtually every environment on Earth.

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This post expands on all the key metabolic concepts, making it perfect for exam prep, academic enrichment, or just a deeper understanding of microbial physiology.

Core Concepts of Microbial Metabolism

Metabolism encompasses all chemical processes that occur within a cell, divided into two major branches:

• Catabolism: Breaks down molecules to release energy (e.g., glycolysis, respiration)
• Anabolism: Uses energy to synthesize cellular components (e.g., amino acids, nucleotides)

ATP (adenosine triphosphate) is the key energy currency that links energy-releasing and energy-consuming reactions. All microbes require water, energy sources, electron donors, and nutrients to function.

Bioenergetics and Redox Reactions

Free energy change (ΔG′) determines whether a reaction occurs spontaneously:

• Exergonic: ΔG′ < 0 (energy-releasing)
• Endergonic: ΔG′ > 0 (energy-consuming)

Redox (reduction-oxidation) reactions transfer electrons from donors to acceptors. The greater the difference in reduction potential (ΔE′), the more energy is released. Microbes rely on reducing equivalents like NADH and NADPH to drive biosynthesis.

Microbial Nutritional Classification

Microbes are classified based on their energy and carbon sources:

• Phototrophs: Use light energy
• Chemotrophs: Use chemical compounds
  • Chemoorganotrophs: Use organic molecules
  • Chemolithotrophs: Use inorganic substances
• Heterotrophs: Require organic carbon
• Autotrophs: Use CO₂ as their carbon source

How Microbes Produce ATP

Three main mechanisms generate ATP:

• Substrate-level phosphorylation: Direct transfer of phosphate during glycolysis or fermentation
• Oxidative phosphorylation: Uses electron transport chain (ETC) and proton motive force (pmf)
• Photophosphorylation: Light-driven ATP synthesis in phototrophs

Enzymes and Electron Carriers

Enzymes speed up metabolic reactions by lowering activation energy. They have specific active sites and often require cofactors:

• Coenzymes: Loosely bound (e.g., NAD⁺/NADH)
• Prosthetic groups: Tightly bound (e.g., iron-sulfur clusters)

Electron carriers in the ETC include NADH dehydrogenase, cytochromes, flavoproteins, and quinones.

Catabolism in Chemoorganotrophs

Glycolysis (Embden-Meyerhof-Parnas Pathway)

• Converts glucose → 2 pyruvate + 2 ATP + 2 NADH

Citric Acid Cycle (TCA/CAC)

• Oxidizes acetyl-CoA → CO₂ + ATP + NADH + FADH₂

Fermentation

• Anaerobic process where the electron donor and acceptor are organic
• Produces ethanol, lactate, and other products

Glyoxylate Cycle

• Modified CAC that allows growth on 2-carbon compounds like acetate

Electron Transport Chain and Respiration

In respiration, electrons flow from donors to acceptors, establishing a proton gradient used by ATP synthase:

• ATP Synthase: F₀ translocates protons, F₁ synthesizes ATP
• Aerobic respiration: Oxygen as final electron acceptor
• Anaerobic respiration: Uses nitrate, sulfate, or sulfur

Alternate Metabolisms: Chemolithotrophy & Phototrophy

Chemolithotrophy

• Oxidizes inorganic compounds like H₂, NH₄⁺, Fe²⁺
• Often autotrophic and may use reverse electron flow to generate NADH

Phototrophy

• Oxygenic: H₂O is donor; produces O₂ (e.g., cyanobacteria)
• Anoxygenic: Uses H₂S or other donors; no O₂ produced
• ATP generated by photophosphorylation and pmf

Carbon and Nitrogen Fixation

Calvin Cycle

• Fixes CO₂ using enzyme RuBisCO
• Consumes ATP and NADPH

Nitrogen Fixation

• Converts atmospheric N₂ to NH₃ using nitrogenase
• ATP-intensive and O₂-sensitive
• Protective adaptations: slime layers (Azotobacter), heterocysts (Anabaena)

Biosynthesis of Macromolecules

• Polysaccharides: Synthesized from UDP- or ADP-glucose
• Gluconeogenesis: Forms glucose from non-sugar substrates
• Pentose Phosphate Pathway: Produces ribose and NADPH
• Amino Acids: Made from glycolysis and CAC intermediates
• Nucleotides: Purines from inosinic acid; pyrimidines from uridylate
• Fatty Acids: Synthesized via ACP-linked units
• Archaeal lipids: Use isoprenoids rather than fatty acids

Glossary Highlights

• ATP Synthase: Enzyme that synthesizes ATP using pmf
• Fermentation: Anaerobic ATP generation via substrate-level phosphorylation
• Chemolithotroph: Derives energy from inorganic compounds
• Photophosphorylation: Light-driven ATP production
• Calvin Cycle: CO₂ fixation into organic compounds
• Nitrogenase: Reduces N₂ to NH₃
• Reverse Electron Transport: Generates NADH against redox gradient
• Gluconeogenesis: Synthesizes glucose from non-carbohydrates
• Pentose Phosphate Pathway: Provides NADPH and ribose sugars

Conclusion

Chapter 3 of Brock Biology of Microorganisms reveals how microbes master energy and biosynthesis through a variety of elegant and efficient pathways. Understanding microbial metabolism offers insights into ecology, biotechnology, and the origins of life itself. Whether you’re reviewing for a test or exploring life’s biochemical engines, this chapter delivers essential concepts.

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