Microbial Metabolism and Energy Generation: Study Notes

Microbial Metabolism

Introduction to Metabolism

Metabolism refers to the sum of all chemical reactions that occur within a living organism, enabling it to grow, reproduce, maintain its structures, and respond to environmental changes. Microbial metabolism is diverse and underpins many ecological and industrial processes.

• Anabolism: Biosynthetic reactions that build complex molecules from simpler ones, requiring energy input.

• Catabolism: Degradative reactions that break down complex molecules into simpler ones, releasing energy.

• Energy Involvement: Catabolic reactions provide energy (often in the form of ATP) for anabolic processes.

Classification of Microorganisms by Metabolic Type

Microorganisms are classified based on their energy and carbon sources, which determines their ecological niche and metabolic capabilities.

• Autotrophs: Use inorganic carbon (CO2) as their carbon source.

• Heterotrophs: Use organic carbon compounds as their carbon source.

• Phototrophs: Obtain energy from light.

• Chemotrophs: Obtain energy from chemical compounds.

These categories can be combined (e.g., photoautotrophs, chemoheterotrophs) to describe specific metabolic strategies.

Organism Classification Table

Energy Source	Electron Source	Carbon Source	Example
Light (Phototroph)	Inorganic (Lithotroph)	CO2 (Autotroph)	Cyanobacteria
Chemicals (Chemotroph)	Organic (Organotroph)	Organic (Heterotroph)	Escherichia coli
Light (Phototroph)	Organic (Organotroph)	Organic (Heterotroph)	Rhodobacter species
Chemicals (Chemotroph)	Inorganic (Lithotroph)	CO2 (Autotroph)	Nitrosomonas species

Energy Generation in Microbes

ATP Synthesis and Energy Transfer

ATP (adenosine triphosphate) is the universal energy currency in cells. Microbes generate ATP through substrate-level phosphorylation, oxidative phosphorylation, and photophosphorylation.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate.

• Oxidative phosphorylation: ATP synthesis driven by electron transport chains and chemiosmosis.

• Photophosphorylation: ATP synthesis using light energy (in phototrophs).

Central Metabolic Pathways

Microbes utilize several key metabolic pathways to extract energy from nutrients.

• Glycolysis (Embden-Meyerhof-Parnas pathway): Converts glucose to pyruvate, generating ATP and NADH.

• Entner-Doudoroff pathway: Alternative to glycolysis, found in some bacteria; yields ATP, NADH, and NADPH.

• Pentose Phosphate Pathway (PPP): Generates NADPH and pentoses for biosynthesis.

• Pyruvate Oxidation: Pyruvate is converted to acetyl-CoA, entering the TCA (Krebs) cycle.

• TCA Cycle (Citric Acid Cycle): Completes oxidation of organic molecules, producing NADH, FADH2, and ATP.

Fermentation

Fermentation is an anaerobic process that allows ATP generation in the absence of oxygen by regenerating NAD+ from NADH.

• Purpose: To continue glycolysis by recycling NAD+.

• Products: Organic acids, alcohols, gases (e.g., lactic acid, ethanol, CO2).

• ATP Yield: Lower than aerobic respiration (typically 2 ATP per glucose).

Aerobic Respiration

Aerobic respiration uses oxygen as the terminal electron acceptor in the electron transport chain (ETC), generating more ATP than fermentation.

• ATP Yield: 30-38 ATP per glucose molecule.

• Key Steps: Glycolysis, pyruvate oxidation, TCA cycle, ETC.

• ETC: Electrons flow from NADH/FADH2 to oxygen, generating a proton gradient used for ATP synthesis.

Electron Transport Chain (ETC)

The ETC is a series of protein complexes that transfer electrons from donors to acceptors, coupled to proton translocation across a membrane.

• Components: NADH dehydrogenase, cytochromes, quinones, terminal oxidases.

• Proton Motive Force (PMF): The electrochemical gradient generated by proton translocation drives ATP synthesis.

• Equation:

$\text{NADH} + \text{H}^+ + 1/2\text{O}_2 \rightarrow \text{NAD}^+ + \text{H}_2\text{O}$

ATP Synthase and Chemiosmosis

ATP synthase is an enzyme that synthesizes ATP using the energy stored in the proton motive force.

• Mechanism: Protons flow through ATP synthase, driving the phosphorylation of ADP to ATP.

• Equation:

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

Microbial Growth

Growth Factors and Requirements

Microbial growth depends on the availability of nutrients, energy sources, and environmental conditions.

• Essential Nutrients: Carbon, nitrogen, sulfur, phosphorus, trace elements, vitamins.

• Growth Factors: Organic compounds required in small amounts (e.g., amino acids, nucleotides).

• Oxygen Requirements: Aerobes, anaerobes, facultative anaerobes, microaerophiles.

Microbial Growth Curve

The growth of a microbial population in batch culture follows a characteristic curve with distinct phases.

Phase	Description
Lag Phase	Cells adapt to new environment; little to no cell division.
Log (Exponential) Phase	Rapid cell division; population increases exponentially.
Stationary Phase	Growth rate slows; nutrient depletion and waste accumulation.
Death Phase	Cells die at an exponential rate due to unfavorable conditions.

Microbial Growth Equation

Microbial growth can be mathematically described by the exponential growth equation:

$N_t = N_0 e^{\mu t}$

• Nt: Number of cells at time t

• N0: Initial number of cells

• μ: Specific growth rate

• t: Time

Biogeochemical Cycles and Microbes

Role of Microbes in Carbon Cycle

Microorganisms play a crucial role in the cycling of carbon through ecosystems, mediating processes such as carbon fixation, decomposition, and methanogenesis.

• Carbon Fixation: Conversion of inorganic CO2 to organic compounds by autotrophs.

• Methanogenesis: Production of methane by archaea in anaerobic environments.

• Decomposition: Breakdown of organic matter by heterotrophic microbes.

Key Microbes in the Carbon Cycle

• Cyanobacteria: Photoautotrophs that fix CO2 via photosynthesis.

• Methanogens: Archaea that produce methane from CO2 and H2.

• Chemoheterotrophs: Decompose organic matter, releasing CO2.

Interconnectedness of Biogeochemical Cycles

Biogeochemical cycles (carbon, nitrogen, sulfur, etc.) do not occur independently; they are interconnected through microbial activity and environmental processes.

• Microbes: Link cycles by transforming elements between different chemical forms.

• Environmental Factors: Influence the rate and direction of cycling.

Additional info:

• Some context and terminology were inferred based on standard microbiology curriculum and the structure of the handwritten notes.

• Equations and tables were expanded for clarity and completeness.