Microbial Cell Structure, Metabolism, and Genetics: Study Notes

Chapter 5: Prokaryotic Cell Structure

Prokaryotic vs. Eukaryotic Cells

Prokaryotes are unicellular organisms that lack a true nucleus, distinguishing them from eukaryotes. Their cellular structures and external features are adapted for survival in diverse environments.

• Prokaryotes do not have a membrane-bound nucleus; eukaryotes do.

• Some prokaryotes possess external structures such as flagella (for motility) and capsules (for adherence and immune evasion).

• Capsules are usually made of polysaccharides and help bacteria adhere to surfaces and evade immune responses.

Cell Wall Structure

The bacterial cell wall is a rigid structure that protects cells from lysis and provides shape. The composition of the cell wall is a key factor in bacterial classification and antibiotic susceptibility.

• Peptidoglycan: A mesh-like polymer of sugars and amino acids forming the main component of most bacterial cell walls.

• Peptidoglycan consists of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by short peptide chains.

Gram-Positive Bacteria

• Thick peptidoglycan layer reinforced by teichoic acids.

• Retains crystal violet stain (appears purple in Gram stain).

• Example: Staphylococcus aureus

Gram-Negative Bacteria

• Thin peptidoglycan layer, surrounded by an outer membrane containing lipopolysaccharide (LPS).

• LPS acts as an endotoxin and can trigger strong immune responses.

• Contains lipid A (toxic component) and polysaccharide chains.

• Does not retain crystal violet stain (appears pink/red in Gram stain).

• Example: Escherichia coli

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Thickness	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
LPS	Absent	Present
Stain Color	Purple	Pink/Red

Acid-Fast Bacteria

• Cell wall contains peptidoglycan and mycolic acid, making it resistant to staining.

• Requires special "acid-fast" staining techniques.

• Example: Mycobacterium tuberculosis

Mollicutes

• Bacteria lacking a true cell wall.

• Example: Mycoplasma species

Capsule

• Gel-like layer outside the cell wall.

• Protects against desiccation and phagocytosis; aids in surface attachment.

Chapter 7: Microbial Metabolism

Overview of Microbial Metabolism

Microbial metabolism encompasses all biochemical reactions that allow cells to obtain energy and build cellular components. Organisms are classified by how they obtain energy and carbon.

• Autotrophs: Use CO2 as a carbon source to produce organic compounds.

• Heterotrophs: Obtain carbon from organic molecules.

• Phototrophs: Use light as an energy source.

• Chemotrophs: Obtain energy from chemical compounds via oxidation-reduction reactions.

Energy Transfers in Biology

• Redox reactions (oxidation-reduction) transfer electrons and energy between molecules.

• Electron donors (reductants) lose electrons; electron acceptors (oxidants) gain electrons.

• Common electron carriers: NAD+, FAD, NADP+.

• One molecule of NADH carries about three times as much energy as one molecule of ATP.

Central Metabolic Pathways

Three main pathways oxidize glucose to CO2 and generate energy:

• Glycolysis: Converts glucose (6C) to two pyruvates (3C), producing ATP and NADH.

• Pentose Phosphate Pathway: Generates NADPH and ribose-5-phosphate for biosynthesis.

• Tricarboxylic Acid (TCA) Cycle: Oxidizes pyruvate to CO2, generating NADH, FADH2, ATP, and precursor metabolites.

Summary Table: Energy-Yielding Metabolism

Class of Metabolism	Source of Energy	Example (Simplified)
Phototrophy	Light absorption excites electrons	Photosynthesis: Light + H2O → sugar + O2
Chemoorganotrophy	Organic molecules donate electrons	Aerobic respiration: Sugar + O2 → CO2 + H2O
Organotrophy, anaerobic	Organic molecules donate electrons to alternative acceptors	Fermentation
Lithotrophy, aerobic	Inorganic molecules donate electrons to O2	Ammonia oxidation: NH4+ + O2 → NO2- + H2O
Lithotrophy, anaerobic	Inorganic molecules donate electrons to other acceptors	Ammonia oxidation: NH4+ + NO2- → N2 + H2O

Catabolism and Anabolism

• Catabolism: Breakdown of complex molecules to release energy (ATP) and produce smaller molecules.

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy input.

Genetics: DNA, RNA, and Protein Synthesis

Genetic Material in Bacteria

• Genome: Complete set of genetic information, including chromosomes and plasmids.

• Bacteria may have a single circular chromosome and additional plasmids (small, circular DNA molecules).

Gene Expression: The Central Dogma

Genetic information flows from DNA to RNA to protein through the processes of transcription and translation.

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).

• Translation: mRNA is decoded by ribosomes to assemble amino acids into proteins.

Operons

• In bacteria, genes are often organized in operons: clusters of genes transcribed together under the control of a single promoter and operator.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Operator: DNA segment where regulatory proteins can bind to influence transcription.

Genomic Evolution

• Mutations: Random changes in the nucleotide sequence of DNA.

• Natural selection: Determines which mutations become established in a population.

• Reverse transcriptase: Enzyme from retroviruses (e.g., HIV) that synthesizes DNA from an RNA template.

Key Definitions

• Lithotroph: Organism that uses inorganic molecules as electron donors for energy generation (can be aerobic or anaerobic).

• Phototroph: Organism that uses light as an energy source.

Summary of Key Equations

• Glycolysis (simplified):

$\text{Glucose} + 2\ \text{NAD}^+ + 2\ \text{ADP} + 2\ \text{P}_i \rightarrow 2\ \text{Pyruvate} + 2\ \text{NADH} + 2\ \text{ATP} + 2\ \text{H}_2\text{O}$

• Aerobic Respiration (simplified):

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

• Photosynthesis (simplified):

$\text{Light} + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{Sugar} + \text{O}_2$

Additional info:

• Some details, such as specific examples of bacteria and the full structure of operons, were inferred for completeness.

• Tables were recreated to summarize key differences and metabolic pathways.