Functional Anatomy of Prokaryotic and Eukaryotic Cells: Cell Structures and Their Functions

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Axial Filaments

Axial filaments are specialized structures found in certain bacteria, particularly spirochetes, that enable unique motility.

• Definition: Axial filaments, also known as endoflagella, are bundles of fibrils that spiral around the cell beneath an outer sheath.

• Location: Found in spirochetes such as Leptospira.

• Function: Provide a corkscrew motion, allowing the bacterium to move efficiently through viscous environments like mucus.

• Structure: Composed of protein filaments anchored at one end of the cell, running between the cell wall and outer sheath.

• Example: The photomicrograph shows Leptospira with a visible axial filament.

Fimbriae

Fimbriae are hair-like appendages found on the surface of many Gram-negative and some Gram-positive bacteria.

• Definition: Short, thin, proteinaceous projections that extend from the bacterial cell surface.

• Function: Enable bacteria to adhere to surfaces, host tissues, and each other, which is important for colonization and biofilm formation.

• Pathogenicity: Fimbriae contribute to the ability of bacteria to cause disease by facilitating attachment to host cells.

• Example: The image shows a bacterium with numerous fimbriae radiating from its surface.

Bacterial Cell Wall

The bacterial cell wall is a rigid structure that surrounds the cell membrane, providing shape, protection, and support.

• Prevents osmotic lysis: Protects the cell from bursting in hypotonic environments.

• Composition: Contains peptidoglycan (unique to bacteria), a polymer of sugars and amino acids.

• Role in Pathogenicity: The structure of the cell wall can influence a bacterium's ability to cause disease.

• Antibiotic Target: Many antibiotics, such as penicillin, target the synthesis of peptidoglycan, weakening the cell wall.

• Taxonomic Importance: Differences in cell wall composition are used to differentiate major groups of bacteria (e.g., Gram-positive vs. Gram-negative).

Composition and Characteristics of Bacterial Cell Walls

Bacterial cell walls are primarily composed of peptidoglycan, a complex macromolecule unique to bacteria.

• Peptidoglycan Structure: Consists of repeating units of two sugars:

  • N-acetylglucosamine (NAG)

  • N-acetylmuramic acid (NAM)

• These sugars are linked in long chains and cross-linked by short peptides, forming a strong, mesh-like layer.

• Diagram: The provided image shows the chemical structure of NAG and NAM, highlighting their functional groups and the glycosidic bond between them.

• Function: Provides structural integrity and shape to the cell, and is essential for survival.

Summary Table: Key Components of Bacterial Cell Walls

Component	Description	Function
Peptidoglycan	Polymer of NAG and NAM with peptide cross-bridges	Structural support, shape, protection from lysis
Axial Filaments	Internal flagella-like structures in spirochetes	Motility (corkscrew movement)
Fimbriae	Short, hair-like protein appendages	Attachment to surfaces and host tissues

Additional info: The notes above are based on the provided images and text, with expanded academic context for clarity and completeness. Further details on Gram-positive and Gram-negative differences, as well as antibiotic mechanisms, would be covered in subsequent sections.