Microbial Nutrition and Metabolism: Study Notes for Microbiology

Chapter 7: Microbial Nutrition and Growth

Microbial Nutrition

Microbial nutrition refers to the process by which microorganisms obtain and utilize nutrients necessary for growth, metabolism, and survival.

• Organic nutrients: Compounds containing carbon and hydrogen, such as carbohydrates, proteins, lipids, and nucleic acids. These are typically derived from living organisms.

• Inorganic nutrients: Simple molecules that do not contain both carbon and hydrogen, such as water, salts, oxygen, and minerals (e.g., magnesium, iron).

• Heterotrophs: Microorganisms that obtain their carbon from organic compounds.

• Autotrophs: Microorganisms that use inorganic CO2 as their carbon source.

• Growth factor: An organic compound (such as a vitamin or amino acid) that a microbe cannot synthesize and must obtain from its environment.

Example: Escherichia coli requires certain vitamins as growth factors in minimal media.

Transport Mechanisms

Microbes use various mechanisms to transport nutrients across their cell membranes.

• Passive transport: Movement of substances down their concentration gradient without energy input (e.g., diffusion, osmosis).

• Active transport: Movement of substances against their concentration gradient, requiring energy (ATP).

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Isotonic: Environment where solute concentration is equal inside and outside the cell; no net water movement.

• Hypotonic: Environment with lower solute concentration outside the cell; water moves into the cell, possibly causing lysis.

• Hypertonic: Environment with higher solute concentration outside the cell; water moves out, causing cell shrinkage (plasmolysis).

Endocytosis

Endocytosis is a process by which cells engulf external substances, forming vesicles.

• Phagocytosis: "Cell eating"; engulfment of large particles or cells.

• Pinocytosis: "Cell drinking"; uptake of small droplets of liquid.

Environmental Factors Affecting Microbial Growth

Microbial growth is influenced by several environmental factors:

• Temperature: Microbes are classified by their temperature preferences (e.g., psychrophiles, mesophiles, thermophiles).

• Gas requirements: Oxygen requirements vary (obligate aerobes, obligate anaerobes, facultative anaerobes, microaerophiles).

• Other factors: pH, osmotic pressure, and presence of inhibitory substances.

Example: Thermus aquaticus thrives at high temperatures in hot springs.

Microbial Growth and Division

Microbial populations grow by binary fission, leading to exponential increase in cell numbers.

• Binary fission: Asexual reproduction where a cell divides into two identical daughter cells.

• Exponential growth: Population doubles at regular intervals.

• Population growth curve: Graphical representation of microbial growth phases: lag, log (exponential), stationary, and death.

Chapter 8: Introduction to Microbial Metabolism

Metabolism of Microbes

Metabolism encompasses all chemical and physical processes in a cell, divided into two main types:

• Catabolism: Degradative reactions that break down large molecules into smaller ones, releasing energy.

• Anabolism: Biosynthetic reactions that build macromolecules from smaller units, requiring energy input.

Example: Catabolism of glucose during glycolysis; anabolism during protein synthesis.

Enzymes and Their Functions

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

• Simple enzymes: Consist of protein alone.

• Conjugated enzymes (holoenzymes): Contain a protein portion (apoenzyme) and a nonprotein cofactor (metal ions or organic coenzymes).

• Active site: Region on the enzyme where substrate binds and reaction occurs.

• Enzyme specificity: Each enzyme acts on a specific substrate due to the shape of its active site.

Enzyme-Substrate Interactions

Enzyme and substrate form a temporary complex, leading to product formation and release.

• Induced fit: Enzyme changes shape slightly to accommodate substrate.

Cofactors and Coenzymes

Cofactors are nonprotein components required for enzyme activity.

• Metal ions: Iron, copper, magnesium; help bring substrate and enzyme together.

• Coenzymes: Organic molecules (often vitamins) that serve as carriers for chemical groups or electrons.

Classification of Enzyme Functions

Enzymes are classified by their action, substrate, and site of activity.

Enzyme Name	Substrate	Function
Lactase	Lactose	Breaks lactose into glucose and galactose
Penicillinase	Penicillin	Degrades penicillin
DNA polymerase	DNA nucleotides	Synthesizes DNA strand
Pyrvuate dehydrogenase	Pyruvic acid	Converts pyruvic acid
Reductase	Various	Catalyzes reduction reactions

Types of Enzymes

• Exoenzymes: Secreted outside the cell to break down large molecules.

• Endoenzymes: Function inside the cell.

• Constitutive enzymes: Always present and produced at constant rates.

• Inducible enzymes: Produced only when substrate is present.

Synthesis and Hydrolysis Reactions

• Condensation (dehydration) reactions: Anabolic; join molecules by removing water.

• Hydrolysis reactions: Catabolic; break molecules by adding water.

Enzyme Regulation

Enzyme activity is regulated to maximize efficiency and respond to environmental changes.

• Competitive inhibition: Molecule similar to substrate competes for active site.

• Allosteric inhibition: Inhibitor binds to regulatory site, changing enzyme shape and reducing activity.

• Enzyme repression: Suppression of enzyme synthesis when end product accumulates.

• Enzyme induction: Synthesis of enzymes in response to substrate presence.

Bioenergetics and Energy Management

Cells manage energy through chemical reactions that make or break bonds and transfer electrons.

• Endergonic reactions: Consume energy.

• Exergonic reactions: Release energy.

• Redox reactions: Involve electron transfer between donor and acceptor; always occur in pairs.

Example equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$

Electron Carriers

• NAD+, FAD, coenzyme Q: Accept and transfer electrons during metabolic reactions.

• Oxygen: Final electron acceptor in aerobic respiration.

Adenosine Triphosphate (ATP)

ATP is the main energy currency of the cell.

• Composed of adenine (base), ribose (sugar), and three phosphate groups.

• Removal of terminal phosphate releases energy.

ATP hydrolysis equation:

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

ATP Synthesis Mechanisms

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

• Oxidative phosphorylation: ATP generated via electron transport chain and chemiosmosis.

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

Major Metabolic Pathways

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

• Krebs cycle (TCA cycle): Processes pyruvate, generating CO2, NADH, and FADH2.

• Electron transport chain (ETC): Transfers electrons, produces ATP via oxidative phosphorylation.

Respiration Types

• Aerobic respiration: Uses oxygen as final electron acceptor; yields maximum ATP.

• Anaerobic respiration: Uses other inorganic molecules (e.g., nitrate, sulfate) as final electron acceptors.

• Fermentation: Organic compounds serve as electron acceptors; yields less ATP.

Fermentation Products

• Alcoholic fermentation: Produces ethanol and CO2 (e.g., by yeasts).

• Acidic fermentation: Produces acids, gases, and other products (e.g., lactic acid by bacteria).

Amphibolic Pathways

Metabolic pathways are often bidirectional, allowing intermediates to be used for both catabolism and anabolism.

• Pyruvic acid can be converted to amino acids (amination).

• Amino acids can be deaminated for energy production.

• Glyceraldehyde-3-phosphate can be used for synthesis of amino acids, carbohydrates, and fats.

Photosynthesis in Microbes

Overview

Photosynthesis is the process by which light energy is converted to chemical energy, providing the ultimate source of energy for most cells.

General equation:

$6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Occurs in two stages: light-dependent and light-independent reactions.

• Major photosynthesizers: plants, algae, cyanobacteria.

Light-Dependent Reactions

• Photons are absorbed by pigments (chlorophyll, carotenoids, phycobilins).

• Water is split (photolysis), releasing O2 and electrons.

• ATP and NADPH are synthesized.

Light-Independent Reactions (Calvin Cycle)

• ATP and NADPH are used to fix CO2 into organic molecules (e.g., glucose).

• Key step: CO2 is attached to ribulose-1,5-bisphosphate and converted to glucose.

Additional info: These notes expand on brief points and fill in missing context for a comprehensive review suitable for college-level microbiology students.