Chapter 4 micro

Chapter 4: Survey of Prokaryotic Cells

Characteristics of Cells and Life

  • All living things, whether single or multicellular, are composed of cells that share several fundamental characteristics:

    • Basic shapes: spherical, cubical, cylindrical

    • Internal content includes:

      • Cytoplasm: Jelly-like substance inside the cell

      • DNA chromosomes: Genetic material of the cell

      • Ribosomes: Sites of protein synthesis

      • Metabolic capabilities: Ability to perform chemical reactions necessary for life

  • Two basic cell types exist:

    • Eukaryotic cells: Contain a nucleus and membrane-bound organelles.

    • Prokaryotic cells: Lack a nucleus and membrane-bound organelles.

Characteristics of Life

  • Reproduction and heredity:

    • Genetic material composed of DNA organized in chromosomes; reproduction can be either sexual or asexual.

  • Growth and development: Increase in size and complexity over time.

  • Metabolism: Encompasses all chemical and physical processes that support life.

  • Movement and/or irritability: Ability to respond to internal and external stimuli; many organisms exhibit self-propulsion.

  • Cell support, protection, and storage:

    • Mechanisms include cell walls, vacuoles, and granules.

  • Transport of nutrients and waste: Essential processes for maintaining homeostasis.

Structures of a Bacterial Cell

  • Major components of a bacterial cell include:

    • Cell wall: Provides structure and protection.

    • Cell membrane: Controls the entry and exit of substances.

    • Cytoplasmic matrix: Contains ribosomes and genetic material.

    • Chromosome (DNA): Circular and located in the nucleoid region.

    • Appendages:

      • Flagellum: For motility.

      • Fimbriae: Hair-like structures for adhesion.

      • Pili: Tubular structures for DNA transfer.

External Structures of Prokaryotic Cells

  • Appendages categorized into:

    • Motility structures: Flagella and axial filaments (e.g., periplasmic flagella).

    • Attachment structures: Fimbriae and pili (important for adhesion).

  • Glycocalyx: Surface coating that can function as a capsule or slime layer.

Flagellar Arrangements

  • Monotrichous: Single flagellum at one end of the cell.

  • Lophotrichous: Multiple flagella at one end.

  • Amphitrichous: Flagella at both ends.

  • Peritrichous: Flagella covering the entire surface of the cell.

Fimbriae and Pili

  • Fimbriae:

    • Fine, hair-like structures that aid in adhesion to surfaces.

  • Pili:

    • Rigid structures made of pilin protein, unique to Gram-negative bacteria, used for conjugation (DNA transfer).

Biofilms

  • Definition: Communities of bacteria encased in a protective matrix often found on medical devices (e.g., catheters).

The Cell Envelope

  • Composed of two layers:

    • Cell wall: Structure varies between Gram-positive (thick peptidoglycan) and Gram-negative (thin peptidoglycan and outer membrane).

    • Cell membrane: Inner layer that maintains cell integrity.

Structure of Cell Walls

  • Peptidoglycan: A macromolecule providing structural support.

    • Consists of a framework of glycan chains linked by peptide cross-bridges.

  • Gram-positive bacteria have a thicker layer of peptidoglycan compared to Gram-negative bacteria.

Characteristics of Gram-Positive and Gram-Negative Cell Walls

  • Gram-positive: Thick layer (20-80 nm) of peptidoglycan, contains teichoic acids, and responds differently to antibiotics.

  • Gram-negative: Thin peptidoglycan layer (8-11 nm), outer membrane contains lipopolysaccharides (LPS) which can be toxic when released.

Nontypical Cell Walls

  • Some bacteria do not conform to typical structures, exhibiting significant variations:

    • Mycobacterium: Contains mycolic acid, making it resistant to certain dyes and chemicals.

    • Mycoplasma: Lack cell walls; stability is provided by sterols.

Cell Membrane Structure

  • Fluid Mosaic Model: Phospholipid bilayer with embedded proteins retaining fluidity and flexibility.

    • Functions in energy reactions, nutrient processing, and waste discharge.

    • Selectively permeable to molecules.

Inside the Bacterial Cell

  • Cytoplasm: A dense environment containing cell components, primarily water (70-80%) and various dissolved substances crucial for cellular functions.

  • Nucleoid: Contains circular, double-stranded DNA (chromosome) and plasmids useful in genetic engineering and not essential for growth, but advantageous in stress scenarios.

Bacterial Ribosome

  • Composed of ribosomal RNA and protein, exists as two subunits (70S).

  • Site of protein synthesis, differing from eukaryotic ribosomes.

Internal Structures of Bacteria

  • Inclusions and Granules: Storage granules varying in type and content utilized when nutrients are scarce.

  • Cytoskeleton: Network of protein polymers supporting the cell wall.

  • Endospores: Dormant structures capable of withstanding extreme environmental conditions without reproduction.

  • Germination: Returning from endospore state to active cell.

Bacterial Shapes and Arrangements

  • Basic Bacterial Shapes:

    • Coccus: Spherical

    • Bacillus: Rod-shaped

    • Vibrio: Curved rod

    • Spirillum: Helical

    • Spirochete: Spring-like shape

  • Arrangements depend on cell division patterns: single, pairs (diplococci), clusters, chains, or packets.

Size of Bacteria

  • Size varies among species, typically measured in micrometers (1 µm = 10^-6 m).

Prokaryotes with Unusual Characteristics

  • Includes free-living nonpathogenic bacteria, photosynthetic organisms (e.g., cyanobacteria) which charge nutrients while utilizing sunlight without oxygen production.

Cyanobacteria

  • Characterized by Gram-negative walls, thylakoids with photosynthetic pigments, can form significant ecosystems known for their nitrogen-fixing abilities.

Unusual Forms of Medically Significant Bacteria

  • Rickettsias and Chlamydias: Obligate intracellular parasites often linked to severe human diseases and infections.

Archaea: The Other Prokaryotes

  • Constitutes the third domain, distinct from bacteria, demonstrating a unique genetic makeup and adaptability to extreme conditions.

    • Includes extremophiles adept at surviving high temperatures, salinity, and acidity.