R

Functional Anatomy of Prokaryotic Cells

Chapter 4 Study Guide: Functional Anatomy of Prokaryotic Cells

Overview of Cellular Classification

  • Living Cell Groups: All living cells can be classified into two groups: prokaryotes and eukaryotes.

    • Prokaryotes are characterized as having simpler and smaller structures compared to eukaryotes.

    • DNA Structure: Prokaryotic DNA is typically a single, circular chromosome that is not surrounded by a membrane, whereas eukaryotic DNA is organized into multiple chromosomes enclosed within a membrane-bound nucleus.

    • Organism Examples: In the microbial world, examples of prokaryotes include bacteria and archaea, while plants and animals are composed of eukaryotic cells.

Comparing Prokaryotic and Eukaryotic Cells: An Overview

  • Commonalities: Both prokaryotes and eukaryotes contain nucleic acids, proteins, lipids, and carbohydrates; they utilize similar chemical reactions for metabolism, protein synthesis, and energy storage.

  • Differences: Major distinctions include:

    • Cell Structure: The structural differences in cell walls and ribosomes, as well as the absence of organelles in prokaryotes, differentiate the two groups.

Characteristics of Prokaryotes

  1. DNA Characteristics: The DNA of prokaryotes is typically not enclosed by a membrane and is usually a single circular chromosome.

  2. Histone Association: Prokaryotic DNA is not associated with histones.

  3. Lack of Organelles: Prokaryotes generally lack organelles such as nuclei, mitochondria, and chloroplasts.

  4. Cell Wall Composition: Bacterial cell walls often contain the complex polysaccharide peptidoglycan.

  5. Reproduction: They typically divide via binary fission, a process wherein DNA is copied followed by cell division into two cells.

Characteristics of Eukaryotes

  1. DNA Organization: Eukaryotic DNA is located in the nucleus, separated from the cytoplasm by a nuclear membrane, and organized into multiple chromosomes.

  2. Histone Interaction: The DNA of eukaryotes is consistently associated with chromosomal proteins known as histones.

  3. Presence of Organelles: Eukaryotic cells possess membrane-enclosed organelles, such as mitochondria, endoplasmic reticulum, Golgi complex, lysosomes, and in some cases, chloroplasts.

  4. Cell Wall Composition: When present, eukaryotic cell walls are chemically simpler than those of prokaryotes.

  5. Cell Division Method: Eukaryotes typically undergo cell division via mitosis, where chromosomes replicate perfectly for distribution to the daughter nuclei.

Diversity of Prokaryotic Cells

  • Cell Types: Prokaryotes encompass a vast group of small unicellular organisms, primarily bacteria and archaea.

  • Bacterial Species Differentiation: Bacterial species can be distinguished based on factors such as morphology, chemical composition, nutritional needs, biochemical activities, and energy sources.

Bacterial Morphology

  • Shapes of Bacteria: Bacteria display various shapes, categorized as:

    • **Spherical shaped (Coccus)

    • Rod-shaped (Bacillus)

    • Spiral shaped (Vibrious, Spirilla, Spirochetes)**

Cocci (Spherical Shape)

  • Cocci can appear as round, oval, elongated, or flattened shapes.

  • When cocci divide, they may remain attached to each other, leading to specific group classifications:

    • Diplococci: Pairs of cocci

    • Streptococci: Chains of cocci

    • Tetrads: Groups of four cocci

    • Sarcinae: Cube-like groups of eight

    • Staphylococci: Grape-like clusters or broad sheets

Bacilli (Rod Shape)

  • Bacilli only divide across their short axis, resulting in fewer group configurations, such as:

    • Single Bacilli: Individual rod shapes

    • Diplobacilli: Paired bacilli

    • Streptobacilli: Chains of bacilli

    • Variants: Some bacilli may resemble straws, cigars, or have oval shapes (coccobacilli).

Spiral Bacteria

  • Spiral bacteria have one or more twists and are never straight.

    • Vibrious: Curved rods

    • Spirilla: Helical and rigid

    • Spirochetes: Helical and flexible, moving via axial filaments enclosed within a sheath.

Glycocalyx

  • Definition & Function: Many prokaryotes secrete a surface layer known as glycocalyx (Greek for sugar coat), which surrounds cells. It comprises polysaccharide, polypeptide, or both.

    • Capsule: Organized, firmly attached to the cell wall.

    • Slime Layer: Unorganized and loosely attached to the cell wall.

  • Clinical Importance: Capsules provide protection for pathogenic bacteria against phagocytosis by host cells.

Flagella

  • Function: Organelles for bacterial locomotion, consisting of three parts:

    1. Filament: The outermost region, made of globular protein flagellin, arranged in helical chains with a hollow core.

    2. Hook: A slightly wider structure made of a different protein.

    3. Basal Body: Anchors the flagellum to the cell membrane.

Fimbriae and Pili

  • Fimbriae: Hairlike appendages found in many gram-negative bacteria, involved in adhesion and biofilm formation. They can number from a few to several hundred and can be located at the poles or spread across the cell's surface.

  • Pili: Typically longer than fimbriae and present in lower numbers (one or two per cell); facilitate motility and DNA transfer through conjugation.

Cell Wall Structure

  • Purpose & Composition: The bacterial cell wall is a complex, semirigid structure that defines cell shape and protects the plasma membrane from environmental changes. It helps to prevent cell rupture under high osmotic pressure.

  • Clinical Relevance: The cell wall is crucial for the pathogenicity of certain bacterial species and is a target for various antibiotics.

Gram-Positive vs. Gram-Negative Cell Walls

  • Gram Staining Mechanism: The Gram stain identifies differences in cell wall structure, revealing distinct reactions to reagents:

    1. Primary Stain (Crystal Violet): Stains both types purple by binding to peptidoglycan.

    2. Iodine Application: Forms insoluble complexes with crystal violet.

    3. Alcohol Wash: Disrupts the outer membrane of gram-negative cells, causing loss of the crystal violet-iodine complex.

    4. Counterstain (Safranin): Stains gram-negative cells pink/red while remaining masked in gram-positive cells by the absorbed purple dye.

Plasma Membrane

  • Structure & Function: Located inside the cell wall; the plasma membrane is selectively permeable, primarily composed of phospholipids and proteins.

    • Lack of sterols makes prokaryotic membranes less rigid than eukaryotic.

  • Nucleoid: Contains the bacterial chromosome (a single, continuous strand of double-stranded DNA).

  • Plasmids: Small, circular DNA molecules carrying extra genes that can provide advantages such as antibiotic resistance and can be transferred through conjugation.

Ribosomes

  • Presence in Cells: All cells contain ribosomes for protein synthesis, with prokaryotic cells having large numbers (tens of thousands), giving cytoplasm a granular appearance.

Endospores

  • Formation: Certain gram-positive bacteria can form endospores when nutrients are scarce. These structures are specialized, highly durable, and capable of surviving extreme conditions including heat, dehydration, and exposure to toxic substances.

Summary of Key Structures

  • A) Flagella

  • B) Plasma Membrane

  • C) Cell Wall

  • D) Capsule

  • E) Plasmid

  • AB) Fimbriae / Pili

  • AC) Nucleoid / Chromosome

  • AD) Ribosomes