Functional Anatomy of Prokaryotic Cells

Functional Anatomy of Prokaryotic Cells

Introduction

Prokaryotes, which include the domains Bacteria and Archaea, are unicellular organisms characterized by their simple cellular organization. This chapter explores the structural features, diversity, and specialized adaptations of prokaryotic cells, with a focus on their comparison to eukaryotic cells, cell wall structure, external appendages, and internal components.

Major Differences Between Prokaryotic and Eukaryotic Cells

Cellular Organization

• Prokaryotic cells lack a membrane-bound nucleus and organelles. Their genetic material is located in a nucleoid region.

• Eukaryotic cells possess a true nucleus enclosed by a nuclear membrane and contain various membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

Genetic Material

• Prokaryotes: Single, circular chromosome; not enclosed by a membrane.

• Eukaryotes: Multiple, linear chromosomes within a nuclear envelope.

Cell Division

• Prokaryotes: Divide by binary fission (asexual reproduction).

• Eukaryotes: Divide by mitosis (asexual) and meiosis (sexual reproduction).

Cell Wall Composition

• Prokaryotes: Most have a cell wall containing peptidoglycan (bacteria) or pseudopeptidoglycan (archaea).

• Eukaryotes: Plants and fungi have cell walls made of cellulose or chitin; animal cells lack cell walls.

Other Features

• Prokaryotes: No extensive cytoskeleton; smaller ribosomes (70S).

• Eukaryotes: Extensive cytoskeleton; larger ribosomes (80S).

Basic Shapes and Arrangements of Bacteria

Common Morphologies

• Coccus (spherical): e.g., Streptococcus, Staphylococcus

• Bacillus (rod-shaped): e.g., Bacillus anthracis

• Vibrio (comma-shaped): e.g., Vibrio cholerae

• Spirillum (rigid spiral): e.g., Spirillum volutans

• Spirochete (flexible spiral): e.g., Borrelia

• Pleomorphic (variable shape): e.g., Corynebacterium diphtheriae

Cellular Arrangements

• Diplococci: Pairs (e.g., Neisseria meningitidis)

• Streptococci: Chains

• Staphylococci: Clusters

• Streptobacilli: Chains of rods

External Structures of Prokaryotic Cells

Glycocalyx

• General term for extracellular polysaccharide/protein layer outside the cell wall.

• Capsule: Organized, firmly attached; protects against phagocytosis and desiccation; can be a virulence factor.

• Slime layer: Loosely attached, less organized; aids in adherence and biofilm formation.

• Biofilm: Community of microorganisms encased in a self-produced matrix, often attached to surfaces.

Flagella

• Long, whip-like appendages for motility; rotate like propellers.

• Composed of flagellin protein; anchored by a basal body.

• Arrangements: monotrichous (single), lophotrichous (tuft), amphitrichous (both ends), peritrichous (all over).

• Enable chemotaxis (movement toward/away from chemical stimuli).

Axial Filaments (Endoflagella)

• Unique to spirochetes (e.g., Borrelia); located in periplasmic space.

• Produce corkscrew motion for movement through viscous environments.

Fimbriae and Pili

• Short, hair-like structures made of pilin protein.

• Fimbriae: Numerous; for attachment to surfaces and biofilm formation.

• Pili: Usually longer; involved in DNA transfer (conjugation) and specialized motility (twitching, gliding).

Bacterial Cell Wall Structure

Peptidoglycan

• Unique to bacteria; provides rigidity and protection against osmotic pressure.

• Composed of repeating disaccharide units: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), cross-linked by peptide bridges.

• Target for antibiotics (e.g., penicillin inhibits peptide cross-bridge formation).

Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick, multilayered	Thin, single/few layers
Teichoic acids	Present (wall & lipoteichoic acids)	Absent
Outer membrane	Absent	Present (contains lipopolysaccharide, LPS)
Periplasmic space	Minimal	Extensive
Lipid A (endotoxin)	Absent	Present (part of LPS)

Atypical Cell Walls

• Mycobacteria: Contain mycolic acids (waxy lipids) in addition to peptidoglycan; acid-fast staining required.

• Mycoplasma: Lack cell wall; have sterols in plasma membrane for stability.

• Archaea: May lack peptidoglycan; have pseudopeptidoglycan or proteinaceous cell walls.

Protoplasts, Spheroplasts, and L-forms

• Protoplast: Gram-positive cell with cell wall removed.

• Spheroplast: Gram-negative cell with partial cell wall loss.

• L-forms: Bacteria that have lost their cell wall and can survive in this state.

• Lysozyme and certain antibiotics can induce these forms by degrading peptidoglycan.

Plasma Membrane Structure and Function

Composition and Permeability

• Phospholipid bilayer with embedded proteins (integral and peripheral).

• Selective permeability: small, nonpolar molecules diffuse freely; ions and large molecules require transport proteins.

Transport Mechanisms

• Simple diffusion: Passive movement down concentration gradient.

• Facilitated diffusion: Passive, but requires transport proteins.

• Osmosis: Diffusion of water across membrane; may involve aquaporins.

• Active transport: Requires energy (usually ATP) to move substances against their concentration gradient.

Equation for diffusion rate:

$J = -D \frac{dC}{dx}$

where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Internal Structures of Prokaryotic Cells

Cytoplasm

• Aqueous solution containing water, proteins, ions, sugars, and other molecules.

• Contains nucleoid, ribosomes, and inclusions.

Nucleoid

• Region containing the bacterial chromosome (single, circular, double-stranded DNA).

• May also contain plasmids: small, circular DNA molecules carrying nonessential genes (e.g., antibiotic resistance).

Ribosomes

• Sites of protein synthesis; composed of rRNA and proteins.

• Prokaryotic ribosomes are 70S (50S large + 30S small subunits).

• Target for many antibiotics (e.g., tetracycline, streptomycin).

Inclusions

• Reserve deposits for nutrients, energy, or building blocks.

• Types include:

  • Metachromatic granules (volutin): phosphate storage.

  • Polysaccharide granules: glycogen, starch.

  • Lipid inclusions: polyhydroxybutyrate (PHB).

  • Sulfur granules: energy reserves in sulfur bacteria.

  • Magnetosomes: iron oxide crystals for orientation along magnetic fields.

  • Carboxysomes: contain enzymes for CO2 fixation in autotrophs.

  • Gas vacuoles: provide buoyancy in aquatic bacteria.

Endospores

Structure and Function

• Dormant, highly resistant structures formed by certain Gram-positive bacteria (e.g., Bacillus, Clostridium).

• Formed in response to environmental stress (e.g., nutrient depletion).

• Contain dipicolinic acid and Ca2+ for DNA protection; low water content.

• Resistant to heat, desiccation, chemicals, and radiation.

• Germinate into vegetative cells when conditions improve.

Endospore Formation (Sporulation)

1. DNA replication

2. Septum formation

3. Forespore development

4. Spore coat synthesis

5. Release of mature endospore

Medical Importance

• Endospores can survive improper sterilization (e.g., in food industry).

• Diseases: botulism (Clostridium botulinum), tetanus (Clostridium tetani).

Endosymbiotic Theory

Origin of Eukaryotic Organelles

• Suggests mitochondria and chloroplasts originated from free-living bacteria engulfed by ancestral eukaryotic cells.

• Evidence:

  • Similar size to bacteria

  • Own circular DNA

  • 70S ribosomes (like bacteria)

  • Replicate independently of host cell

  • Susceptible to antibiotics that affect bacteria

Summary Table: Key Features of Prokaryotic Cell Structures

Structure	Function	Example/Notes
Glycocalyx	Protection, adherence, biofilm formation	Capsule in Streptococcus pneumoniae
Flagella	Motility, chemotaxis	Peritrichous in Escherichia coli
Fimbriae	Attachment to surfaces	Biofilm formation
Pili	DNA transfer (conjugation), motility	Sex pilus in Escherichia coli
Cell wall	Shape, protection, osmotic stability	Peptidoglycan in bacteria
Plasma membrane	Selective permeability, transport, energy generation	Phospholipid bilayer
Nucleoid	Genetic information storage	Circular DNA
Ribosomes	Protein synthesis	70S ribosomes
Inclusions	Storage of nutrients/energy	PHB, glycogen, sulfur granules
Endospores	Dormancy, survival	Bacillus, Clostridium