Functional Anatomy of Prokaryotic and Eukaryotic Cells: Microbiology Study Notes

Comparing Prokaryotic and Eukaryotic Cells

Cell Structure Overview

Prokaryotic and eukaryotic cells differ fundamentally in their structural organization, genetic material, and cellular processes. Understanding these differences is essential for microbiology students.

• Prokaryotes: Characterized by the absence of a membrane-bound nucleus and organelles. Their genetic material is a single circular chromosome located in the nucleoid.

• Eukaryotes: Possess a true nucleus surrounded by a nuclear membrane and various membrane-bound organelles. Their genetic material is organized into paired chromosomes.

Feature	Prokaryote	Eukaryote
Nucleus	No (nucleoid)	Yes (membrane-bound)
Chromosomes	One circular	Paired, linear
Organelles	Absent	Present
Cell Wall	Peptidoglycan (Bacteria), Pseudomurein (Archaea)	Polysaccharides (plants, fungi, some protists)
Division	Binary fission	Mitosis

The Size, Shape, and Arrangement of Bacterial Cells

Bacterial Morphology

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

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

• Monomorphic: Most bacteria have a single, consistent shape

• Pleomorphic: Some bacteria can vary in shape

Basic Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical-shaped

• Spiral: Includes vibrio (comma-shaped), spirillum (rigid), and spirochete (flexible)

• Star-shaped and Rectangular: Rare morphologies

Arrangements

• Pairs: Diplococci, diplobacilli

• Chains: Streptococci, streptobacilli

• Clusters: Staphylococci

• Groups of four: Tetrads

• Cubelike groups of eight: Sarcinae

Structure of a Prokaryotic Cell

Cell Components

Prokaryotic cells contain several key structures, each with specific functions.

• Capsule: External polysaccharide layer contributing to virulence

• Cell wall: Provides shape and protection

• Plasma membrane: Selective permeability and metabolic functions

• Cytoplasm: Contains ribosomes, nucleoid, and inclusions

• Flagella, fimbriae, pili: Appendages for movement and attachment

Structures External to the Cell Wall

Glycocalyx

The glycocalyx is a viscous, gelatinous layer external to the cell wall, composed of polysaccharides and/or polypeptides.

• Capsule: Organized, firmly attached; prevents phagocytosis

• Slime layer: Unorganized, loosely attached

• Biofilms: Glycocalyx helps bacteria adhere to surfaces and form biofilms

Flagella

Flagella are filamentous appendages used for motility.

• Structure: Composed of filament, hook, and basal body

• Basal body: Anchors flagellum; arrangement differs in gram-positive (2 rings) and gram-negative (4 rings) bacteria

• Function: Enables movement via "run" and "tumble"; taxis (movement toward/away from stimuli)

Flagella Arrangements

• Monotrichous: Single flagellum at one end

• Lophotrichous: Tuft of flagella at one end

• Amphitrichous: Flagella at both ends

• Peritrichous: Flagella all over the cell

Axial Filaments

Axial filaments, or endoflagella, are found in spirochetes and enable corkscrew movement.

• Structure: Anchored at one end, wraps around cell

• Function: Rotation causes movement

Fimbriae and Pili

• Fimbriae: Hairlike appendages for attachment to surfaces and other cells

• Pili: Longer, involved in motility and DNA transfer (conjugation)

The Cell Wall

Composition and Function

The cell wall is a critical structure for bacterial shape, protection, and classification.

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

• Function: Prevents osmotic lysis, anchors flagella, site of antibiotic action

Gram-Positive vs. Gram-Negative Cell Walls

• Gram-Positive: Thick peptidoglycan, teichoic acids, no outer membrane

• Gram-Negative: Thin peptidoglycan, outer membrane with lipopolysaccharides (LPS), periplasmic space

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic acids	Present	Absent
Outer membrane	Absent	Present
LPS	None	High
Basal body rings	2	4
Toxins	Exotoxins	Endotoxins & Exotoxins
Antibiotic resistance	Low	High

Gram Stain Mechanism

The Gram stain differentiates bacteria based on cell wall structure.

• Crystal violet: Primary stain

• Iodine: Mordant

• Alcohol: Decolorizer

• Safranin: Counterstain

• Gram-positive: Retain purple color

• Gram-negative: Stain pink

Atypical Cell Walls

• Acid-fast: Waxy mycolic acid layer (e.g., Mycobacterium)

• Mycoplasmas: Lack cell walls, contain sterols

• Archaea: Wall-less or pseudomurein walls

Structures Internal to the Cell Wall

Plasma Membrane

The plasma membrane is a phospholipid bilayer with embedded proteins, responsible for selective permeability and metabolic functions.

• Fluid mosaic model: Proteins and lipids move freely

• Damage: Alcohols, detergents, and antibiotics can disrupt membrane integrity

Movement of Materials Across Membranes

• Passive processes: Simple diffusion, facilitated diffusion, osmosis

• Active processes: Active transport (requires ATP), group translocation (unique to prokaryotes)

Cytoplasm, Nucleoid, Ribosomes, and Inclusions

• Cytoplasm: 80% water, contains proteins, carbohydrates, lipids, ions

• Nucleoid: Contains bacterial chromosome (DNA)

• Plasmids: Extrachromosomal DNA, often carry antibiotic resistance genes

• Ribosomes: 70S (prokaryotes), site of protein synthesis

• Inclusions: Reserve deposits (e.g., phosphate, polysaccharide, lipid, sulfur granules, gas vacuoles, magnetosomes)

Endospores

Endospores are highly resistant, dormant structures formed by certain bacteria (e.g., Bacillus, Clostridium) under adverse conditions.

• Sporulation: Formation of endospore

• Germination: Return to vegetative state

• Resistant to: Heat, desiccation, chemicals, radiation

Eukaryotic Cell Structures

Organelles and Functions

Eukaryotic cells contain specialized organelles, each with distinct functions.

• Nucleus: Contains DNA, surrounded by nuclear envelope

• Endoplasmic reticulum: Rough (protein synthesis), smooth (lipid synthesis)

• Golgi complex: Modifies and transports proteins

• Lysosomes: Digestive enzymes

• Vacuoles: Storage and shape

• Mitochondria: ATP production

• Chloroplasts: Photosynthesis

• Peroxisomes: Oxidation of fatty acids

• Centrosomes: Cell division

Flagella and Cilia

• Flagella: Long, few; movement

• Cilia: Short, numerous; movement and substance transport

• Structure: 9+2 arrangement of microtubules (tubulin)

Cell Wall and Glycocalyx

• Cell wall: Plants (cellulose), fungi (chitin), yeasts (glucan, mannan)

• Glycocalyx: Carbohydrate layer for attachment and recognition (animal cells)

Plasma Membrane

• Contains sterols: Complex lipids

• Contains carbohydrates: Attachment and cell recognition

• Endocytosis: Phagocytosis and pinocytosis (not in prokaryotes)

Cytoplasm and Ribosomes

• Cytoskeleton: Microfilaments and intermediate filaments

• Cytoplasmic streaming: Movement of cytoplasm (eukaryotes only)

• Ribosomes: 80S (cytoplasm), 70S (mitochondria and chloroplasts)

The Evolution of Eukaryotes

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotic cells as a result of symbiosis between different prokaryotic cells.

• Mitochondria: Derived from aerobic bacteria

• Chloroplasts: Derived from photosynthetic bacteria

Example: Evidence includes similarities in DNA, ribosomes, and reproduction between mitochondria/chloroplasts and bacteria. Additional info: These notes expand on brief points with academic context, definitions, and examples for clarity and completeness.