Functional Anatomy of Prokaryotic and Eukaryotic Cells: Structure and Function

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview: Prokaryotic vs. Eukaryotic Cells

Understanding the differences between prokaryotic and eukaryotic cells is fundamental in microbiology. These differences influence cellular structure, function, and classification.

• Prokaryotes: Organisms without a true nucleus; genetic material is not enclosed in a membrane.

• Eukaryotes: Organisms with a true nucleus; genetic material is enclosed within a nuclear membrane.

Feature	Prokaryote	Eukaryote
Chromosomes	One circular, not in a 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

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

• 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 have variable shapes

Common Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical

• Spiral: Includes Vibrio (comma-shaped), Spirillum (rigid spiral), and Spirochete (flexible spiral)

• Star-shaped and Rectangular: Rare forms

Arrangements

• Pairs: Diplococci, diplobacilli

• Chains: Streptococci, streptobacilli

• Clusters: Staphylococci

• Groups of four: Tetrads

• Cubelike groups of eight: Sarcinae

Structure of a Prokaryotic Cell

Prokaryotic cells have several key structures that contribute to their survival and function.

Glycocalyx

• External to the cell wall; viscous and gelatinous

• Composed of polysaccharide and/or polypeptide

• Types: Capsule (organized, firmly attached) and Slime layer (unorganized, loose)

• Functions: Contributes to virulence by preventing phagocytosis and aiding in biofilm formation

Flagella

• Filamentous appendages for motility, composed of flagellin protein

• Three parts: Filament, Hook, Basal body

• Enable movement toward/away from stimuli (taxis); movement by rotation (run/tumble)

• Flagellar proteins serve as H antigens for serotyping

Archaella and Axial Filaments

• Archaella: Motility structures in Archaea, composed of archaellins, rotate like flagella

• Axial filaments (endoflagella): Found in spirochetes, anchored at one end, rotation causes 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

The bacterial cell wall provides structural support, prevents osmotic lysis, and contributes to pathogenicity. Its composition is a key factor in Gram staining and antibiotic susceptibility.

• Composed mainly of peptidoglycan in bacteria

• Peptidoglycan: Polymer of repeating disaccharides 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 peptidoglycan

• 2 rings in basal body of flagella

• Produce exotoxins; highly susceptible 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

• 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 stains cells red

Atypical Cell Walls

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

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

• Archaea: Wall-less or 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

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins. It is essential for cellular homeostasis and energy production.

• Fluid mosaic model: Membrane is dynamic, proteins and phospholipids move laterally

• Contains enzymes for ATP production

• Some bacteria have photosynthetic pigments on infoldings called 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: Equal solute concentrations

• Hypotonic: Lower solute outside; water enters cell

• Hypertonic: Higher solute outside; water leaves cell

Active Processes

• Active transport: Requires transporter protein and ATP; moves substances against gradient

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

Cytoplasm

The cytoplasm is the substance inside the plasma membrane, consisting mainly of water, proteins, carbohydrates, lipids, and ions. It contains the cytoskeleton for structural support.

The Nucleoid

• Bacterial chromosome: Circular DNA containing genetic information

• Plasmids: Extrachromosomal DNA elements; often carry antibiotic resistance or toxin genes

Ribosomes

Ribosomes are the sites of protein synthesis, composed of protein and ribosomal RNA. Prokaryotic ribosomes are 70S (50S + 30S subunits).

Inclusions

Inclusions are reserve deposits found in prokaryotic cells, serving as storage for nutrients and other substances.

• Metachromatic granules: Phosphate reserves

• Polysaccharide granules: Energy reserves

• Lipid inclusions: Energy reserves

• Sulfur granules: Energy reserves

• Carboxysomes: Contain RuBisCO for CO2 fixation

• Gas vacuoles: Maintain buoyancy

• Magnetosomes: Iron oxide inclusions; destroy H2O2

Endospores

Endospores are highly resistant, dormant structures formed by certain bacteria (e.g., Bacillus, Clostridium) when nutrients are depleted. They are resistant to desiccation, heat, chemicals, and radiation.

• Sporulation: Process of endospore formation

• Germination: Endospore returns to vegetative state

Additional info: This guide 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 critical for interpreting microbial physiology, taxonomy, and the mechanisms of antimicrobial action.