Functional Anatomy of Prokaryotic and Eukaryotic Cells: Structure and Function

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview: Prokaryotic vs. Eukaryotic Cells

Cells are the fundamental units of life, and in microbiology, they are classified as either prokaryotic or eukaryotic based on structural and genetic differences.

• Prokaryotes: Organisms whose cells lack a true nucleus and membrane-bound organelles. Includes Bacteria and Archaea.

• Eukaryotes: Organisms with cells containing a true nucleus and various organelles. Includes Fungi, Algae, Protozoa, and Helminths.

Feature	Prokaryote	Eukaryote
Chromosomes	One circular, not in membrane	Paired, in nuclear membrane
Histones	Absent	Present
Organelles	Absent	Present
Cell Wall	Peptidoglycan (Bacteria), Pseudomurein (Archaea)	Polysaccharide (when present)
Division	Binary fission	Mitosis

The Size, Shape, and Arrangement of Bacterial Cells

Size and Morphology

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

• Average size: 0.2–2.0 μm in diameter, 2–8 μm in length

• Monomorphic: Most bacteria have a single, consistent shape

• Pleomorphic: Some bacteria can vary in shape

Common Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical

• Spiral: Includes Vibrio (curved rods), Spirillum (rigid spirals), and Spirochete (flexible spirals)

• Star-shaped and Rectangular forms (rare)

Arrangements

• Pairs: Diplococci, diplobacilli

• Chains: Streptococci, streptobacilli

• Clusters: Staphylococci

• Groups of four: Tetrads

• Cubelike groups of eight: Sarcinae

Structure of a Prokaryotic Cell

Major Components

• Capsule (if present)

• Cell wall

• Plasma membrane

• Cytoplasm

• Nucleoid (DNA)

• Ribosomes

• Inclusions

• Flagella, fimbriae, pili (if present)

Glycocalyx

The glycocalyx is a gelatinous, sticky substance external to the cell wall, composed of polysaccharide and/or polypeptide.

• Capsule: Organized and firmly attached; protects against phagocytosis

• Slime layer: Unorganized and loosely attached

• Contributes to virulence and biofilm formation

Flagella

Flagella are long, whip-like appendages used for motility. They are composed of the protein flagellin and have three main parts: filament, hook, and basal body.

• Filament: Outermost region

• Hook: Connects filament to basal body

• Basal body: Anchors flagellum to cell wall and membrane

Flagella arrangements include monotrichous (single), lophotrichous (tuft), amphitrichous (both ends), and peritrichous (all over).

• Enable movement toward/away from stimuli (taxis)

• Flagella proteins (H antigens) are used for serotyping

Archaella and Axial Filaments

• Archaella: Motility structures in Archaea, composed of archaellins

• Axial filaments (endoflagella): Found in spirochetes, anchored at one end, cause corkscrew movement

Fimbriae and Pili

• Fimbriae: Hairlike appendages for attachment to surfaces

• Pili: Involved in motility (gliding, twitching) and DNA transfer (conjugation pili)

The Cell Wall

Functions and Composition

The bacterial cell wall provides structural support, prevents osmotic lysis, and contributes to pathogenicity. It is primarily composed of peptidoglycan in bacteria.

• Peptidoglycan: Polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by polypeptides

Gram-Positive Cell Walls

• Thick peptidoglycan layer

• Teichoic acids (lipoteichoic and wall teichoic acids) link cell wall to plasma membrane and regulate cation movement

• 2 rings in basal body of flagella

• Produce exotoxins; high susceptibility to penicillin; disrupted by lysozyme

Gram-Negative Cell Walls

• Thin peptidoglycan layer

• Outer membrane contains lipopolysaccharide (LPS), lipoproteins, and phospholipids

• Periplasmic space between outer and plasma membranes

• LPS contains O polysaccharide (antigen) and Lipid A (endotoxin)

• Porins form channels through the membrane

• 4 rings in basal body of flagella; produce endotoxins and exotoxins; low susceptibility to penicillin

Gram Stain Mechanism

• Gram-positive: Alcohol dehydrates peptidoglycan; crystal violet-iodine (CV-I) complexes remain

• Gram-negative: Alcohol dissolves outer membrane, CV-I washes out; safranin counterstain colors cells red

Atypical Cell Walls

• Acid-fast cell walls: Like gram-positive but with mycolic acid (waxy lipid); e.g., Mycobacterium, Nocardia

• Mycoplasmas: Lack cell walls; have sterols in plasma membrane

• Archaea: May lack cell walls or have walls of pseudomurein (lack NAM and D-amino acids)

Damage to the Cell Wall

• Lysozyme: Hydrolyzes bonds in peptidoglycan

• Penicillin: Inhibits peptide bridges in peptidoglycan

• Protoplast: Wall-less gram-positive cell

• Spheroplast: Wall-less gram-negative cell

• L forms: Wall-less cells that swell into irregular shapes

The Plasma (Cytoplasmic) Membrane

Structure and Function

• Phospholipid bilayer enclosing cytoplasm

• Contains peripheral, integral, and transmembrane proteins

• Described by the fluid mosaic model: proteins move freely, membrane is self-sealing

• Selective permeability, contains enzymes for ATP production, and may have photosynthetic pigments (chromatophores)

Movement of Materials Across Membranes

• Passive processes: No energy required; substances move from high to low concentration

• Active processes: Energy required; substances move from low to high concentration

Passive Processes

• Simple diffusion: Movement of solute down its concentration gradient

• Facilitated diffusion: Solute combines with transporter protein

• Osmosis: Movement of water across a selectively permeable membrane

• Osmotic pressure: Pressure needed to stop water movement

• Isotonic, hypotonic, hypertonic solutions: Affect water movement and cell volume

Active Processes

• Active transport: Uses transporter protein and ATP

• Group translocation: Substance is chemically altered during transport (requires PEP)

Cytoplasm and Internal Structures

Cytoplasm

• Substance inside plasma membrane; 80% water, plus proteins, carbohydrates, lipids, ions

• Contains cytoskeleton

Nucleoid

• Bacterial chromosome: Circular DNA containing genetic information

• Plasmids: Extrachromosomal DNA; carry non-essential genes (e.g., antibiotic resistance)

Ribosomes

• Sites of protein synthesis

• Composed of protein and rRNA

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

Inclusions

• Metachromatic granules: Phosphate reserves

• Polysaccharide granules, lipid inclusions, sulfur granules: Energy reserves

• Carboxysomes: Contain RuBisCO for CO2 fixation

• Gas vacuoles: Maintain buoyancy

• Magnetosomes: Iron oxide inclusions; destroy H2O2

Endospores

• Resting, highly resistant cells formed when nutrients are depleted

• Resistant to desiccation, heat, chemicals, and radiation

• Produced by Bacillus and Clostridium

• Sporulation: Endospore formation

• Germination: Endospore returns to vegetative state

Additional info: This summary covers the essential structural and functional features of prokaryotic cells, with emphasis on bacterial morphology, cell wall composition, and specialized structures. Understanding these features is foundational for topics such as microbial metabolism, genetics, and pathogenesis.