Functional Anatomy of Prokaryotic and Eukaryotic Cells

Comparing Prokaryotic and Eukaryotic Cells

• Prokaryote:

  • Origin: Greek for "prenucleus"
  • Characteristics:
  • Single circular chromosome, not enclosed by a membrane
  • Lacks histones (proteins associated with DNA)
  • No membrane-bound organelles
  • Bacteria: Peptidoglycan cell walls
  • Archaea: Pseudomurein cell walls
  • Reproduction: Binary fission

• Eukaryote:

  • Origin: Greek for "true nucleus"
  • Characteristics:
  • Paired chromosomes within a nuclear membrane
  • Histones present
  • Membrane-bound organelles
  • When present, polysaccharide cell walls
  • Reproduction: Mitosis

Size, Shape, and Arrangement of Bacterial Cells

• Average size: 0.2 to 2.0 μm in diameter; 2 to 8 μm in length

• Shape Variations:

  • Bacillus: Rod-shaped
  • Coccus: Spherical-shaped
  • Spiral Shapes:
  • Vibrio: comma-shaped
  • Spirillum: spiral-shaped
  • Spirochete: flexible spiral
  • Other Shapes:
  • Star-shaped
  • Rectangular

• Cell Arrangements:

  • Pairs: Diplococci (spherical) and Diplobacilli (rod-like)
  • Clusters: Staphylococci
  • Chains: Streptococci (spherical) and Streptobacilli (rod-like)
  • Groups of four: Tetrads
  • Cubelike groups of eight: Sarcinae

Structure of a Prokaryotic Cell

• Components:
  • Capsule: Viscous, gelatinous layer outside cell wall
  • Cell Wall: Provides structure and protection
  • Plasma Membrane: Semi-permeable barrier
  • Pilus: Hair-like appendages used for attachment
  • Cytoplasm: Gel-like substance where organelles are found
  • Nucleoid: Area containing circular DNA
  • Ribosomes: Sites of protein synthesis (70S)
  • Inclusions: Storage granules of nutrients
  • Flagella: Whip-like structures for movement

Glycocalyx

• Definition: A gelatinous coating outside the cell wall, composed of polysaccharides and/or polypeptides.
• Types:
  • Capsule: Well-organized, tightly attached.
  • Slime Layer: Loosely attached, unorganized.
• Functions:
  • Protect against phagocytosis.
  • Important for biofilm formation, enhancing virulence.

Flagella Structure and Function

• Definition: Long, filamentous appendages used for movement.

• Structure:

  • Composed of the protein flagellin.
  • Consists of three parts:
  • Filament: Longest region that extends from the cell.
  • Hook: Connects filament to basal body.
  • Basal Body: Anchors the flagellum.

• Types of Flagella Arrangements:

  • Peritrichous: distributed all around the cell.
  • Monotrichous: single flagellum at one end.
  • Lophotrichous: multiple flagella at one end.
  • Amphitrichous: flagella at both ends.

• Movement:

  • Allows bacteria to run and tumble, responding to stimuli (taxis).

Axial Filaments

• Found in spirochetes.
• Positioned internally, wrapped around the cell.
• Function: Facilitate movement through a corkscrew motion.

Fimbriae and Pili

• Fimbriae:

  • Hair-like structures allowing attachment to surfaces.
  • Important for bacterial colonization.

• Pili/Conjugation Pili:

  • Involved in gliding motility and DNA transfer between cells (conjugation).

Cell Wall Composition and Function

• Protects against osmotic lysis.

• Composed mainly of peptidoglycan (found exclusively in bacteria).

• Structure:

  • Peptidoglycan: repeating disaccharide units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

• Types of Cell Walls:

  • Gram-Positive: Thick peptidoglycan layer; contains teichoic acids.
  • Gram-Negative: Thin peptidoglycan layer; outer membrane of lipopolysaccharides (LPS).

• Gram Staining:

  • Crystal Violet: Primary stain; Iodine: Mordant; Alcohol: Decolorizer; Safranin: Counterstain.
  • Gram-positive retains crystals; Gram-negative does not.

Eukaryotic Cells

• Nucleus: Contains cell's DNA, surrounded by a nuclear envelope.
• Cytoplasmic Organelles: Include mitochondria, Golgi apparatus, endoplasmic reticulum, lysosomes.
• Cell Wall: Found in plants (cellulose), fungi (chitin), and algae; absent in animal cells.
• Plasma Membrane:
  • Similar to prokaryotes; maintains selective permeability.
  • Endocytosis mechanisms for nutrient uptake.

Endosymbiotic Theory

• Concept: First eukaryotes arose through symbiosis when larger prokaryotic cells engulfed smaller prokaryotic cells.
• Evolution of organelles:
  • Photosynthetic bacteria became chloroplasts.
  • Aerobic bacteria became mitochondria.

Conclusion

• Understanding the differences in cellular structure and function between prokaryotic and eukaryotic cells is crucial for microbiology and related biological fields. The characteristics of bacterial cells provide insights into their pathogenicity and survival mechanisms, while the architecture of eukaryotic cells elucidates their complexity and functional specialization.