Functional Anatomy of Prokaryotic and Eukaryotic Cells

Two Kinds of Cells

Cells are classified into two fundamental types: prokaryotes and eukaryotes. This distinction is foundational in microbiology, as it determines cellular structure, function, and classification of organisms.

• Prokaryotes: Include Bacteria and Archaea. No known prokaryotic macroorganisms.

• Eukaryotes: Include Animals, Plants, Algae, Fungi, and Protozoa. Both macroorganisms and microorganisms exist in this group.

Cell Structure Comparison

Prokaryotic and eukaryotic cells differ in their internal organization and complexity. Understanding these differences is essential for identifying and studying microorganisms.

• Prokaryotes: DNA not enclosed in a nuclear membrane, usually a single circular chromosome, no membrane-bound organelles, complex cell wall if present, divide by binary fission, smaller ribosomes, no cytoskeleton.

• Eukaryotes: DNA enclosed in a nuclear membrane, multiple chromosomes, associated with histones and non-histones, membrane-bound organelles (e.g., Golgi complex, mitochondria), simple cell wall if present, divide by mitosis, larger ribosomes, cytoskeleton present.

Origin of Terms

The terms prokaryote and eukaryote are derived from Greek:

• Prokaryote: "prenucleus"

• Eukaryote: "true nucleus"

The Prokaryotes

Prokaryotes include Bacteria and Archaea. They are distinguished by their cellular structure and metabolic diversity.

• Some bacteria are photosynthetic (e.g., Cyanobacteria).

• Species are differentiated by:

  • Morphology (shape)

  • Chemical composition (detected by staining)

  • Nutritional requirements

  • Biochemical activities

  • Source of energy (sunlight or chemicals)

Basic Shapes of Bacteria

Bacteria exhibit a variety of shapes, which are important for identification and classification.

• Bacillus: Rod-shaped

• Coccus: Spherical

• Spiral: Includes Spirillum, Vibrio, and Spirochete

Shapes of Bacterial Cells: Coccus

Cocci are round, spherical, oval, or elongated. Their arrangements are key to identification.

• Single

• Diplococci: Pairs

• Tetrad: Groups of four

• Sarcinae: Cube-like groups of eight

• Streptococci: Chain-like pattern

• Staphylococci: Grape-like clusters

Shapes of Bacterial Cells: Bacillus and Spiral

Bacilli are rod-shaped, and spiral bacteria have distinctive helical forms.

• Bacillus: Single, diplobacilli, streptobacilli, coccobacilli

• Spiral:

  • Vibrios: Curved rods

  • Spirilla: Helical shape

  • Spirochetes: Helical and flexible

Unusual Bacterial Shapes

Some bacteria exhibit unusual shapes, such as star-shaped (Stella) and square (Haloarcula).

Monomorphic and Pleomorphic Bacteria

Bacterial shape is determined by heredity. Most bacteria are monomorphic (maintain a single shape), but some are pleomorphic (can have more than one genetically controlled shape).

• Monomorphic: Environmental factors may change shape, complicating identification.

• Pleomorphic: Examples include Rhizobium and Corynebacterium.

Prokaryotic Cell Diagram

The structure of a typical prokaryotic cell includes the plasma membrane, cell wall, capsule, cytoplasm, ribosomes, nucleoid, and flagella.

Structures External to the Cell Wall: Glycocalyx

The glycocalyx is a sugar coat on the surface of many cells, providing protection and aiding in attachment.

• Bacterial glycocalyx is viscous (sticky).

• Gelatinous polymer of polysaccharide and/or polypeptide.

• Made inside and secreted to the cell surface.

• Called capsule if organized and firmly attached; slime layer if unorganized and loose.

Structures External to the Cell Wall: Capsule

The capsule is an important virulence factor for pathogenic bacteria.

• Provides protection from phagocytosis (e.g., Bacillus anthracis).

• Allows attachment to surfaces (e.g., Klebsiella colonization).

• Can serve as a source of nutrition when energy stores are low.

• Protects against dehydration.

• Capsules are antigenic (contain antigens).

Structures External to the Cell Wall: Flagella

Flagella are long filamentous appendages that propel bacteria, contributing to motility and classification.

• Flagellar protein (H antigen) is used to distinguish among serovars.

• Four arrangements:

  • Monotrichous: Single polar flagellum

  • Amphitrichous: Tuft of flagella on both ends

  • Lophotrichous: Two or more flagella at one pole

  • Peritrichous: Flagella distributed over entire cell

Structure of a Prokaryotic Flagellum

A flagellum consists of three basic parts: filament, hook, and basal body.

• Filament: Long, outermost region; composed of flagellin protein arranged in intertwining chains forming a helix around a hollow core.

• Hook: Filament attached to it; wider than filament; composed of different protein.

• Basal body: Anchors flagellum to cell wall and plasma membrane (not shown in the extracted text but standard academic context).

Additional info: The basal body is a critical component for flagellar function, anchoring the flagellum and enabling rotation for motility.