Cell Structure and Function in Microbiology: Prokaryotes, Eukaryotes, and Viruses

Cell Structure and Function

Overview of Microbial Life Forms

Microorganisms are classified into three major groups: prokaryotes (Bacteria and Archaea), eukaryotes, and viruses. Each group exhibits unique structural and functional characteristics that distinguish them from one another and from multicellular life forms.

• Prokaryotes: Simple, unicellular organisms lacking a nucleus and membrane-bound organelles. Includes Bacteria and Archaea.

• Eukaryotes: More complex cells with a true nucleus and various internal organelles. Includes algae, protozoa, fungi, animals, and plants.

• Viruses: Acellular entities that lack cellular structure and metabolism, requiring host cells for replication.

Characteristics of Life in Microbes

The table below summarizes the distribution of key life characteristics among Bacteria, Archaea, Eukaryotes, and Viruses.

Characteristic	Bacteria, Archaea, Eukaryotes	Viruses
Growth	Occurs in all	Does not occur
Reproduction	Occurs in all	Host cell replicates the virus
Responsiveness	Occurs in all	Reaction to host cells seen in some viruses
Metabolism	Occurs in all	Uses host cell metabolism
Cellular structure	Present in all	Lacks cytoplasmic membrane or cellular structure

Comparing Prokaryotic and Eukaryotic Cells

Prokaryotic Cell Structure

Prokaryotic cells are characterized by their simplicity and lack of internal compartmentalization. They are typically small (about 1.0 μm in diameter) and possess the following features:

• No nucleus: DNA is located in a region called the nucleoid.

• Lack of membrane-bound organelles: Cellular processes occur in the cytoplasm or at the cell membrane.

• Cell wall: Provides structural support and shape.

• External structures: May include glycocalyces, flagella, fimbriae, and pili.

Eukaryotic Cell Structure

Eukaryotic cells are larger (10–100 μm in diameter) and more complex than prokaryotes. They contain:

• True nucleus: DNA is enclosed within a nuclear envelope.

• Membrane-bound organelles: Includes mitochondria, Golgi apparatus, endoplasmic reticulum, lysosomes, and more.

• Complex cytoskeleton: Provides structural support and facilitates intracellular transport.

Endosymbiotic Theory

Origin of Eukaryotic Organelles

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells. According to this theory:

• Mitochondria and chloroplasts originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• These organelles contain their own circular DNA and 70S ribosomes, similar to bacteria.

• They divide independently of the cell but rely on the host cell for many proteins.

Example: The mitochondria in animal cells and the chloroplasts in plant cells are believed to have evolved from aerobic and photosynthetic bacteria, respectively.

External Structures of Prokaryotic Cells

Glycocalyces

Glycocalyces are gelatinous, sticky substances that surround the outside of some prokaryotic cells. They are composed of polysaccharides, polypeptides, or both, and serve as protective and adhesive layers.

• Capsule: Organized, firmly attached to the cell surface, and protects against desiccation and host immune responses (anti-phagocytic).

• Slime layer: Loosely attached and less organized; aids in adherence to surfaces.

Flagella

Flagella are long, whip-like appendages responsible for motility in many prokaryotes. They are composed of three main parts: filament, hook, and basal body.

• Filament: Made of flagellin protein, forms a helical structure.

• Hook: Connects the filament to the basal body.

• Basal body: Anchors the flagellum to the cell wall and membrane, allowing rotation.

Arrangements of Flagella

• Monotrichous: Single flagellum at one end.

• Lophotrichous: Tuft of flagella at one end.

• Amphitrichous: One or more flagella at both ends.

• Peritrichous: Flagella distributed over the entire cell surface.

Function of Flagella

Flagella rotate to propel bacteria through their environment. Movement is powered by a proton motive force across the cell membrane. Bacteria exhibit taxis, moving toward or away from stimuli (e.g., chemotaxis, phototaxis, aerotaxis).

• Runs: Straight movement, flagella rotate counterclockwise (CCW).

• Tumbles: Random changes in direction, flagella rotate clockwise (CW).

Internal Flagella (Axial Filaments)

Some bacteria, such as spirochetes, possess internal flagella called axial filaments or endoflagella. These structures are located between the cell wall and outer membrane, enabling corkscrew-like movement.

Fimbriae and Pili

Fimbriae and pili are nonmotile, proteinaceous appendages found on many prokaryotes.

• Fimbriae: Short, bristle-like projections used for attachment to surfaces, host tissues, and in biofilm formation. Hundreds may be present per cell.

• Pili (Conjugation Pili/Sex Pili): Longer than fimbriae but shorter than flagella. Usually one or two per cell, used for DNA transfer (conjugation) and specific attachment.

Prokaryotic Cell Walls

Structure and Function

The cell wall surrounds the cytoplasmic membrane and provides structural support, shape, and protection from osmotic lysis. It also serves as a barrier to certain antimicrobial substances. Animal cells lack cell walls, making bacterial cell walls a selective target for antibiotics.

Bacteria and Archaea differ in cell wall composition and structure.

Prokaryotic Cell Morphologies and Arrangements

Common Shapes

• Coccus: Spherical

• Bacillus: Rod-shaped

• Spirillum: Rigid spiral

• Spirochete: Flexible spiral

• Vibrio: Comma-shaped

• Coccobacillus: Short rod

• Pleomorphic: Variable shape

• Star-shaped: Rare, star-like appearance

Spirillum vs. Spirochete

• Spirilla: Rigid, thicker, external flagella, do not rotate as they move (e.g., Spirillum, Campylobacter).

• Spirochetes: Flexible, thinner, internal endoflagella, rotate in corkscrew fashion (e.g., Leptospira, Borrelia, Treponema).

Arrangements of Cocci

Cocci can be arranged in various patterns depending on the plane of division and whether cells remain attached after division:

• Diplococci: Pairs

• Streptococci: Chains

• Tetrads: Groups of four

• Sarcinae: Cubes of eight

• Staphylococci: Irregular clusters

Arrangements of Bacilli

Bacilli may also form characteristic arrangements:

• Single bacillus: Individual rods

• Diplobacilli: Pairs

• Streptobacilli: Chains

• Palisade: Side-by-side arrangement

• V-shape: Angled pairs

Additional info: Understanding the structure and function of prokaryotic and eukaryotic cells, as well as the unique features of viruses, is foundational for studying microbial physiology, genetics, and pathogenesis.