Functional Anatomy of Prokaryotic and Eukaryotic Cells: Structure and Function

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview: Prokaryotic vs. Eukaryotic Cells

This section compares 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 predicting their cellular functions.

• Prokaryotes: Characterized by a single, circular chromosome not enclosed in a membrane, absence of histones and organelles, peptidoglycan cell walls (in bacteria), and division by binary fission.

• Eukaryotes: Possess paired chromosomes within a nuclear membrane, contain histones and organelles, may have polysaccharide cell walls, and divide by mitosis.

Shape and Arrangement of Bacterial Cells

Common Bacterial Shapes

Bacteria exhibit a variety of shapes, which are important for identification and classification. The main shapes include:

• Bacillus: Rod-shaped

• Coccus: Spherical-shaped

• Spiral Forms: Includes vibrio (curved rods), spirillum (rigid spirals), and spirochete (flexible spirals)

• Unusual Shapes: Star-shaped and rectangular bacteria are less common but notable for their unique morphology.

Arrangements of Bacterial Cells

Bacterial cells can be arranged in characteristic patterns due to their division planes:

• Pairs: Diplococci, diplobacilli

• Chains: Streptococci, streptobacilli

• Clusters: Staphylococci

• Groups of Four: Tetrads

• Cubelike Groups of Eight: Sarcinae

Structure of a Prokaryotic Cell

Generalized Structure

Prokaryotic cells have a simple structure but contain all the essential components for life. Key features include the cell wall, plasma membrane, cytoplasm, nucleoid, ribosomes, and various surface structures.

Surface 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 to the cell wall.

• Slime Layer: Unorganized and loosely attached.

Functions include protection from phagocytosis, contribution to virulence, and facilitation of biofilm formation.

Flagella

Flagella are long, filamentous appendages used for motility. They are composed of the protein flagellin and have three main parts:

• Filament: Outermost region

• Hook: Connects filament to basal body

• Basal Body: Anchors flagellum to cell wall and membrane; structure differs between Gram-positive and Gram-negative bacteria

Flagella arrangements include monotrichous (single), lophotrichous (tufts), amphitrichous (both ends), and peritrichous (all over surface).

Flagella enable movement toward or away from stimuli (taxis) and are important antigens (H antigens).

Fimbriae and Pili

Fimbriae are short, hairlike appendages that allow for attachment to surfaces. Pili are longer, fewer in number, and involved in motility (gliding, twitching) and DNA transfer (conjugation).

The Bacterial Cell Wall

Functions and Composition

The bacterial cell wall provides structural support, prevents osmotic lysis, and contributes to pathogenicity. It is primarily composed of peptidoglycan, a polymer of repeating disaccharides (N-acetylglucosamine [NAG] and N-acetylmuramic acid [NAM]) linked by polypeptides.

Gram-Positive Cell Walls

Gram-positive bacteria have thick peptidoglycan layers and teichoic acids, which link the cell wall to the plasma membrane and provide antigenic specificity. The structure includes peptide cross-bridges and tetrapeptide side chains.

Teichoic acids carry a negative charge and regulate movement of cations.

Gram-Negative Cell Walls

Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane composed of lipopolysaccharides (LPS), lipoproteins, and phospholipids. The LPS contains:

• O polysaccharide: Functions as an antigen

• Lipid A: Acts as an endotoxin

• Porins: Proteins that form channels through the membrane

The outer membrane protects against phagocytes, complement, and antibiotics.

Gram Stain Mechanism

The Gram stain differentiates bacteria based on cell wall structure:

• Gram-positive: Alcohol dehydrates peptidoglycan, trapping crystal violet-iodine complexes (cells appear purple).

• Gram-negative: Alcohol dissolves outer membrane and leaves holes in peptidoglycan, allowing dye to wash out (cells appear colorless until counterstained with safranin, then appear pink/red).

Atypical Cell Walls

Some bacteria, such as Mycobacterium and Nocardia, have acid-fast cell walls containing mycolic acid, a waxy lipid bound to peptidoglycan. These stain with carbolfuchsin.

The Plasma (Cytoplasmic) Membrane

Structure and Function

The plasma membrane is a phospholipid bilayer with embedded proteins, enclosing the cytoplasm. It is selectively permeable, allowing passage of certain molecules while restricting others. The membrane contains enzymes for ATP production and, in some bacteria, photosynthetic pigments on infoldings called chromatophores.

• Damage to the membrane by alcohols, detergents, or antibiotics can cause leakage of cell contents and cell death.

Summary Table: Comparison of Gram-Positive and Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
Lipopolysaccharide (LPS)	Absent	Present
Sensitivity to Penicillin	High	Low
Gram Stain Color	Purple	Pink/Red