Introduction to Prokaryotic Cells

Overview of Prokaryotic Domains

Prokaryotic cells are fundamental to life on Earth, having originated approximately 3.8 billion years ago. They are classified into two distinct domains: Bacteria and Archaea. Both domains share the characteristic of lacking a membrane-bound nucleus and organelles, but differ in their cell wall composition and membrane structure.

• Bacteria: Possess peptidoglycan in their cell walls; fatty acids in membranes are linear.

• Archaea: Possess pseudopeptidoglycan; membrane fatty acids are long-branched, and some form lipid monolayers for extreme environments.

• Similarity: Both are unicellular and lack membrane-bound organelles.

• Difference: Cell wall and membrane composition.

Basic Description and Size of Prokaryotic Cells

Prokaryotic cells are typically small, ranging from less than 1 µm to several µm in diameter. Their small size is due to reliance on diffusion for nutrient uptake, which is limited by the surface area-to-volume ratio.

• Smallest: Mycoplasma species

• Largest: Thiomargarita namibiensis

Shapes and Arrangements of Prokaryotes

Prokaryotes exhibit diverse shapes and arrangements, which are important for identification and pathogenicity.

• Bacilli: Rod-shaped

• Cocci: Spherical

• Vibrio: Comma-shaped

• Stella: Star-shaped

• Coccobacilli: Ovoid

• Spirochetes: Spiral-shaped, corkscrew motion

• Monomorphic: One shape

• Pleomorphic: Variable shapes, enhancing survival and transmission

• Diplococci: Paired cocci

• Streptococci: Chains of cocci

• Staphylococci: Grapelike clusters

• Diplobacilli: Paired bacilli

• Streptobacilli: Chains of bacilli

• Palisade: Clusters of bacilli

Example:

Streptococcus pyogenes, a Gram-positive coccus, forms chains (streptococci) and is clinically relevant in pharyngitis.

Binary Fission

Prokaryotic cells reproduce primarily by binary fission, a process that ensures rapid population growth.

• DNA is copied

• Cell grows

• Chromosomes move to opposite ends

• Septum forms at the midpoint

• Two daughter cells are separated

Extracellular Structures

Plasma Membrane Structure and Function

The plasma membrane is a thin, flexible phospholipid bilayer acting as a selective barrier. Proteins embedded in the membrane serve as transporters, anchors, receptors, and enzymes, and are crucial for metabolic reactions such as ATP synthesis.

• Selective permeability: Allows gases, water, and small noncharged substances to diffuse; larger or charged substances require transport proteins.

• Membrane fluidity: Essential for function; affected by temperature and fatty acid composition.

Comparison of Bacterial and Archaeal Membranes

• Bacteria: Linear fatty acids in phospholipids

• Archaea: Long-branched fatty acids; some form monolayers for extreme heat

Cell Wall Structure and Function

Cell walls provide rigidity and protection. Bacteria use peptidoglycan, while archaea use pseudopeptidoglycan.

Gram Staining and Clinical Relevance

Gram staining differentiates bacteria based on cell wall structure:

• Gram-positive: Thick peptidoglycan, stains purple, lacks outer membrane

• Gram-negative: Thin peptidoglycan, stains red/pink, has outer membrane with lipopolysaccharide

Gram-negative bacteria are harder to kill due to their outer membrane, which acts as a selective barrier.

Table: Comparing Gram-Negative and Gram-Positive Bacteria

Acid-Fast Staining

Acid-fast staining detects mycolic acid in cell walls. Acid-fast bacteria (e.g., Mycobacterium, Nocardia) appear red/pink and are slow-growing due to their waxy cell wall, which also impedes drug penetration.

Mycoplasma and L-Forms

• Mycoplasma: Lack cell wall, have sterol-enriched membrane, pleomorphic, often intracellular

• L-forms: Bacteria that lost their cell wall, resistant to certain stresses, may cause persistent infections

Transport Mechanisms

Passive Transport

• Diffusion: Movement from high to low concentration

• Simple diffusion: Small, noncharged molecules

• Facilitated diffusion: Uses channel or carrier proteins

Osmosis

Osmosis is the diffusion of water toward higher solute concentration.

• Isotonic: No net water movement

• Hypertonic: Cell loses water, plasmolysis

• Hypotonic: Cell gains water, possible lysis if cell wall is damaged

Active Transport

• Primary: Uses ATP

• Secondary: Uses ion gradients (symport and antiport)

• Phosphotransferase: Group translocation, transfers phosphate to substrate (e.g., glucose)

External Structures for Adhesion, Movement, and Protection

Flagella

Flagella are protein filaments used for motility. Gram-positive bacteria have two rings for anchoring, while Gram-negative have four. The run-and-tumble system allows directional movement in response to stimuli (chemotaxis, phototaxis, aerotaxis).

• Monotrichous: Single flagellum

• Lophotrichous: Tuft at one pole

• Amphitrichous: Flagella at both poles

• Peritrichous: Flagella all over

Periplasmic flagella (axial filaments) allow spirochetes to move in a corkscrew motion.

Fimbriae and Pili

• Fimbriae: Short, bristle-like, adhesive, common in Gram-negative bacteria

• Pili: Longer, fewer, used for adhesion, movement, and gene transfer (conjugation)

Glycocalyx

• Slime layer: Unorganized, loosely associated

• Capsule: Well-organized, tightly associated

Glycocalyx aids in adhesion, protection from desiccation, and resistance to antibiotics.

Intracellular Structures

Nucleoid

Prokaryotic DNA is organized in a single, circular chromosome located in the nucleoid region.

Ribosomes

Prokaryotic ribosomes (70S) consist of a 50S large and 30S small subunit. They synthesize proteins and support the endosymbiotic theory.

Cytoskeleton

Composed of protein filaments, the cytoskeleton provides structural support.

Inclusion Bodies

Inclusion bodies are storage granules for substances such as carbon-fixing enzymes (carboxysomes) and magnetic iron (magnetosomes).

Endospores

Endospores are dormant, highly resistant structures formed by certain bacteria (e.g., Bacillus, Clostridium, Clostridioides). They survive harsh conditions and pose challenges in healthcare due to their persistence.

• Sporulation: DNA is copied, packaged, surrounded by resistant layers, and released as a spore.

• Examples: Bacillus anthracis (anthrax), Clostridium tetani (tetanus), Clostridium botulinum (botulism), Clostridium perfringens (gas gangrene), Clostridioides difficile (severe diarrhea)

Clinical Case Example: Streptococcal Pharyngitis

A 5-year-old patient diagnosed with streptococcal pharyngitis caused by Streptococcus pyogenes (Gram-positive, nonmotile, encapsulated). Penicillin-family drugs are effective due to the presence of peptidoglycan. No endospores or acid-fast properties are expected. Cell structures present include capsule, cell wall, plasma membrane, fimbriae, and ribosomes; absent structures include flagella, endospores, and acid-fast cell wall.

Classification: S. pyogenes is in the domain Bacteria, not Archaea.