Microbiology Study Guide: Metabolism, Enzymes, Catabolism, Growth, Genetics, and Cell Relationships

METABOLISM BASICS

Introduction to Metabolism

Metabolism encompasses all chemical reactions occurring within a cell, divided into two main processes: anabolism (building up molecules) and catabolism (breaking down molecules). These processes are essential for energy production, growth, and cellular maintenance.

• Anabolism: Synthesis of complex molecules from simpler ones; requires energy.

• Catabolism: Breakdown of complex molecules into simpler ones; releases energy.

• Role in the cell: Provides energy and building blocks for cellular functions.

Redox Reactions

Redox (reduction-oxidation) reactions are central to metabolism, involving the transfer of electrons between molecules.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Oxidizing agent: Accepts electrons.

• Reducing agent: Donates electrons.

• Example: In cellular respiration, glucose is oxidized and oxygen is reduced.

Oxidized and Reduced Forms

Compound	Oxidized Form	Reduced Form
NAD+	NAD+	NADH
FAD	FAD	FADH2

ATP, ADP, and Phosphorylation

ATP (Adenosine Triphosphate) is the primary energy currency of the cell. ADP (Adenosine Diphosphate) is formed when ATP loses a phosphate group. Phosphorylation is the process of adding a phosphate group to a molecule.

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

• Function of NAD and FAD: Electron carriers in metabolic pathways.

ENZYMES

Enzyme Basics

Enzymes are biological catalysts that speed up chemical reactions in cells without being consumed. They are highly specific for their substrates.

• Six basic categories: Oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.

• Enzyme types: Apoenzyme (protein part), coenzyme (organic cofactor), holoenzyme (complete, active enzyme).

• Enzyme activity: Influenced by temperature, pH, substrate concentration, and inhibitors.

• Inhibitors: Non-competitive, competitive, feedback inhibition.

Example: Temperature affects enzyme activity; too high or too low can denature the enzyme.

CATABOLISM

Aerobic vs. Anaerobic Respiration and Fermentation

Catabolism includes processes that break down molecules to release energy. The main types are aerobic respiration, anaerobic respiration, and fermentation.

• Aerobic respiration: Uses oxygen as the final electron acceptor.

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

• Fermentation: Does not use an electron transport chain; produces less ATP.

Steps of Cellular Respiration

• Glycolysis: Glucose is broken down into pyruvate; ATP and NADH are produced.

• Krebs Cycle: Pyruvate is further oxidized; more NADH, FADH2, and ATP are produced.

• Electron Transport Chain: Electrons from NADH and FADH2 are transferred through a series of proteins, generating a proton gradient used to produce ATP.

Equation for aerobic respiration:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$

GROWTH REQUIREMENTS

Chemical and Energy Requirements

Microorganisms require specific nutrients and energy sources for growth. Oxygen requirements and temperature preferences are key factors.

• Oxygen forms: Aerobes, anaerobes, facultative anaerobes, microaerophiles, aerotolerant anaerobes.

• Enzymes for detoxifying oxygen: Catalase, superoxide dismutase.

• Temperature classes: Psychrophiles (cold), mesophiles (moderate), thermophiles (hot), hyperthermophiles (very hot).

• pH requirements: Neutrophiles, acidophiles, alkaliphiles.

Example: Escherichia coli is a mesophile, growing best at human body temperature.

RELATIONSHIPS BETWEEN CELLS

Symbiotic and Non-Symbiotic Relationships

Microbial cells interact in various ways, forming symbiotic (mutualistic, commensal, parasitic) or non-symbiotic relationships (synergism, antagonism).

• Mutualism: Both organisms benefit.

• Commensalism: One benefits, the other is unaffected.

• Parasitism: One benefits, the other is harmed.

• Synergism: Cooperative interaction for mutual benefit.

• Antagonism: One organism inhibits or destroys another.

Quorum sensing: Cell-to-cell communication regulating gene expression based on population density.

CULTURING MICROORGANISMS

Isolation and Media Types

Microbiologists use various techniques and media to isolate and grow microorganisms.

• Isolation techniques: Streak plate, pour plate, spread plate.

• Media types: Selective, differential, enriched, minimal.

• Application: Selective media inhibit unwanted microbes; differential media distinguish between species.

GROWTH OF MICROBIAL POPULATIONS

Binary Fission and Growth Phases

Prokaryotic cells reproduce by binary fission, resulting in exponential population growth under optimal conditions.

• Binary fission: Cell divides into two identical daughter cells.

• Growth phases: Lag, log (exponential), stationary, death.

• Generation time: Time required for a cell to divide.

Equation for exponential growth:

$N = N_0 \times 2^n$

Where $N$ is the final cell number, $N_0$ is the initial cell number, and $n$ is the number of generations.

REPLICATION OF GENOMES

DNA Replication and Genome Structure

DNA replication is the process by which cells copy their genetic material before division. Prokaryotic and eukaryotic genomes differ in structure and replication mechanisms.

• Genome: Complete set of genetic material in a cell.

• DNA structure: Double helix, base pairing (A-T, G-C), antiparallel strands.

• Replication enzymes: DNA polymerase, helicase, primase, ligase.

• Replication direction: 5' to 3' synthesis; leading and lagging strands.

Example: In prokaryotes, replication starts at a single origin; in eukaryotes, multiple origins exist.

TRANSCRIPTION AND TRANSLATION

Gene Expression

Transcription and translation are processes by which genetic information is converted into functional proteins.

• Transcription: DNA is copied into RNA by RNA polymerase.

• Translation: mRNA is decoded by ribosomes to synthesize proteins.

• tRNA: Transfers amino acids to the ribosome during translation.

• Anticodon: Sequence on tRNA complementary to mRNA codon.

Equation for transcription:

$\text{DNA} \xrightarrow{\text{RNA polymerase}} \text{mRNA}$

Equation for translation:

$\text{mRNA} \xrightarrow{\text{ribosome}} \text{Protein}$

REGULATION OF GENETIC EXPRESSION

Gene Regulation Mechanisms

Cells regulate gene expression to respond to environmental changes and conserve resources.

• Constitutive genes: Expressed continuously.

• Inducible genes: Expressed in response to specific stimuli (e.g., lac operon).

• Repressible genes: Turned off when end product is abundant (e.g., trp operon).

Example: The lac operon is induced in the presence of lactose; the trp operon is repressed when tryptophan is abundant.

MUTATION OF GENES

Types and Effects of Mutations

Mutations are changes in the DNA sequence that can affect gene function and phenotype.

• Silent mutation: No change in amino acid sequence.

• Nonsense mutation: Introduces a stop codon, truncating the protein.

• Missense mutation: Changes one amino acid in the protein.

• Frameshift mutation: Insertion or deletion shifts the reading frame.

• Mutagens: Physical (radiation) or chemical agents that increase mutation rate.

Example: UV radiation can cause thymine dimers, leading to mutations.

GENETIC RECOMBINATION

Mechanisms of Genetic Exchange

Genetic recombination introduces genetic diversity in bacteria through several mechanisms.

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA by bacteriophages.

• Conjugation: Direct transfer of DNA between cells via pilus.

• Plasmids: Small, circular DNA molecules; often carry antibiotic resistance genes.

Example: F+ cells transfer plasmids to F- cells during conjugation.

Additional info: Some explanations and examples have been expanded for clarity and completeness.