Functional Anatomy of Prokaryotic and Eukaryotic Cells: Structure, Classification, and Cell Wall Differences

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview of Prokaryotic and Eukaryotic Cells

This section introduces the fundamental differences between prokaryotic and eukaryotic cells, which are the two primary cell types in microbiology. Understanding these differences is essential for classifying microorganisms and understanding their biology.

• Prokaryote: Derived from Greek for 'prenucleus.' These cells lack a true nucleus and membrane-bound organelles.

• Eukaryote: Derived from Greek for 'true nucleus.' These cells possess a nucleus enclosed by a membrane and various organelles.

Comparative Features of Prokaryotic and Eukaryotic Cells

Feature	Prokaryote	Eukaryote
Chromosomes	One circular chromosome, not in a membrane	Paired chromosomes, in nuclear membrane
Histones	Absent	Present
Organelles	Absent	Present
Cell Wall	Bacteria: peptidoglycan; Archaea: pseudomurein	Polysaccharide (when present)
Cell Division	Binary fission	Mitosis

The Size, Shape, and Arrangement of Bacterial Cells

General Characteristics

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

• Monomorphic: Most bacteria have a single, consistent shape.

• Pleomorphic: Some bacteria can vary in shape.

Common Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical-shaped

• Spiral: Includes vibrio (comma-shaped), spirillum (rigid spiral), and spirochete (flexible spiral)

• Star-shaped and Rectangular forms (rare)

Arrangements of Bacterial Cells

• Pairs: diplococci, diplobacilli

• Clusters: staphylococci

• Chains: streptococci, streptobacilli

• Groups of four: tetrads

• Cubelike groups of eight: sarcinae

Example: Bacillus anthracis is a rod-shaped bacterium that forms chains.

External Structures of Prokaryotic Cells

Glycocalyx

The glycocalyx is a viscous, gelatinous layer external to the cell wall, composed of polysaccharide and/or polypeptide. It exists in two forms:

• Capsule: Neatly organized and firmly attached; prevents phagocytosis, contributing to virulence.

• Slime layer: Unorganized and loosely attached; helps form biofilms.

Flagella

Flagella are long, filamentous appendages that propel bacteria. They are made of the protein flagellin and consist of three parts:

• Filament: Outermost region

• Hook: Connects filament to basal body

• Basal body: Anchors flagellum to cell wall and membrane

Flagella enable motility (movement toward or away from stimuli, called taxis) and can be used to distinguish bacterial serovars (e.g., Escherichia coli O157:H7).

Other Motility Structures

• Archaella: Motility structures in Archaea, made of glycoproteins, rotate like flagella.

• Axial filaments (endoflagella): Found in spirochetes, anchored at one end, cause corkscrew motion.

Fimbriae and Pili

• Fimbriae: Hairlike appendages for attachment to surfaces.

• Pili: Involved in motility (gliding, twitching) and DNA transfer (conjugation pili).

The Prokaryotic Cell Wall

Structure and Function

• Prevents osmotic lysis and protects the cell membrane.

• Composed of peptidoglycan in bacteria, which is a polymer of repeating disaccharides (N-acetylglucosamine [NAG] and N-acetylmuramic acid [NAM]) linked by polypeptides.

Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic acids	Present	Absent
Outer membrane	Absent	Present
Periplasmic space	Absent	Present
Susceptibility to penicillin	High	Low
Flagella basal body rings	2	4
Toxins produced	Exotoxins	Endotoxins and exotoxins

Gram Stain Mechanism

• Gram-positive: Alcohol dehydrates peptidoglycan, trapping crystal violet-iodine (CV-I) complexes; cells remain purple.

• Gram-negative: Alcohol dissolves outer membrane and leaves holes in peptidoglycan; CV-I washes out, cells are colorless until counterstained with safranin (appear pink/red).

Special Cell Wall Types

• Acid-fast cell walls: Like gram-positive but with waxy mycolic acid (e.g., Mycobacterium).

• Mycoplasmas: Lack cell walls; have sterols in plasma membrane.

• Archaea: May lack cell walls or have walls of pseudomurein (lack NAM and D-amino acids).

Damage to the Cell Wall

• Lysozyme: Hydrolyzes bonds in peptidoglycan.

• Penicillin: Inhibits peptide bridges in peptidoglycan.

• Protoplast: Wall-less gram-positive cell.

• Spheroplast: Wall-less gram-negative cell.

• Both are susceptible to osmotic lysis.

Summary Table: Key Differences Between Prokaryotic and Eukaryotic Cells

Characteristic	Prokaryotic Cell	Eukaryotic Cell
Nucleus	No	Yes
Membrane-bound organelles	No	Yes
Cell wall composition	Peptidoglycan (Bacteria), Pseudomurein (Archaea)	Polysaccharides (when present)
Cell division	Binary fission	Mitosis

Additional info: The notes above are based on textbook slides and include expanded academic context for clarity and completeness. For further study, students should review diagrams and micrographs of cell structures and arrangements.