Functional Anatomy of Prokaryotic and Eukaryotic Cells: Structure, Function, and Comparison

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Introduction

This chapter explores the structural and functional differences between prokaryotic and eukaryotic cells, focusing on their cellular components, arrangements, and adaptations. Understanding these differences is fundamental to microbiology, as it underpins microbial classification, physiology, and pathogenicity.

Comparing Prokaryotic and Eukaryotic Cells

Basic Definitions and Overview

• Prokaryote: Organisms whose cells lack a true nucleus and membrane-bound organelles. The term derives from Greek for "prenucleus." Includes Bacteria and Archaea.

• Eukaryote: Organisms with cells containing a true nucleus enclosed by a nuclear membrane and various membrane-bound organelles. The term means "true nucleus." Includes Fungi, Algae, Protozoa, plants, and animals.

Feature	Prokaryote	Eukaryote
Chromosomes	Usually 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

Size and Morphology

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

• Most bacteria are monomorphic (single shape); some are pleomorphic (variable shapes).

Common Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical

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

• Other: Star-shaped, rectangular

Arrangements

• Pairs: Diplococci, diplobacilli

• Chains: Streptococci, streptobacilli

• Clusters: Staphylococci

• Groups of four: Tetrads

• Cubelike groups of eight: Sarcinae

Structure of a Prokaryotic Cell

Generalized Structure

Prokaryotic cells contain a cell wall, plasma membrane, cytoplasm, nucleoid, ribosomes, and various external structures such as flagella, fimbriae, and pili.

Glycocalyx

Structure and Function

• External to the cell wall; viscous and gelatinous.

• Composed of polysaccharide and/or polypeptide.

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

• Functions: Increases virulence by preventing phagocytosis, aids in adherence to surfaces, and forms biofilms.

Examples: Bacillus anthracis, Streptococcus pneumoniae, Klebsiella pneumoniae (capsule); Streptococcus mutans, Vibrio cholerae (biofilm formation).

Flagella

Structure and Function

• Filamentous appendages for motility, composed of flagellin protein.

• Three parts: Filament (outermost), Hook (attaches filament), Basal body (anchors to cell wall and membrane).

Arrangements

• Monotrichous: Single flagellum

• Lophotrichous: Tuft at one end

• Amphitrichous: One or more at both ends

• Peritrichous: Distributed over entire cell

Function in Motility and Identification

• Enable movement toward/away from stimuli (taxis).

• Flagella proteins (H antigens) are used to distinguish serovars (e.g., E. coli O157:H7).

Other Surface Structures

Fimbriae and Pili

• Fimbriae: Hairlike appendages for attachment and biofilm formation (e.g., Neisseria gonorrhoeae, E. coli O157).

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

The Cell Wall

Composition and Function

• Prevents osmotic lysis, protects membrane, and contributes to pathogenicity.

• Composed of peptidoglycan in bacteria: polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by polypeptides.

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

Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick, many layers	Thin, single layer
Teichoic acids	Present	Absent
Outer membrane	Absent	Present (contains LPS)
Flagella basal body	2 rings	4 rings
Toxins	Exotoxins	Endotoxins & Exotoxins
Penicillin susceptibility	High	Low

Gram Stain Mechanism

• Crystal violet-iodine complex forms inside cells.

• Alcohol dehydrates peptidoglycan in gram-positive cells (retains dye).

• Alcohol dissolves outer membrane in gram-negative cells (dye washes out, safranin counterstain applied).

Atypical Cell Walls

• Acid-fast bacteria: Thick peptidoglycan, waxy mycolic acid (e.g., Mycobacterium).

• Mycoplasmas: Lack cell walls, contain sterols in membrane.

• Archaea: Wall-less or walls of pseudomurein (lack NAM and D-amino acids).

Damage to the Cell Wall

• Lysozyme: Hydrolyzes peptidoglycan bonds (especially in gram-positive bacteria).

• Penicillin: Inhibits peptide bridge formation in peptidoglycan.

• Protoplast: Wall-less gram-positive cell; Spheroplast: Wall-less gram-negative cell; L forms: Irregular, wall-less cells.

The Plasma (Cytoplasmic) Membrane

Structure

• Phospholipid bilayer with embedded proteins (integral, peripheral, transmembrane).

• Some proteins and lipids have attached carbohydrates (glycoproteins, glycolipids).

Fluid Mosaic Model

• Membrane is fluid, allowing lateral movement of proteins and lipids.

• Self-sealing and selectively permeable.

Functions

• Selective permeability: controls entry/exit of substances.

• Contains enzymes for ATP production.

• Photosynthetic pigments (chromatophores) in some bacteria.

Destruction by Antimicrobial Agents

• Alcohols, detergents, and antibiotics (e.g., polymyxin) can damage the membrane, causing leakage of cell contents.

Movement of Materials Across Membranes

Passive Processes

• Simple diffusion: Movement from high to low concentration until equilibrium is reached.

• Facilitated diffusion: Transport proteins help move substances down their concentration gradient.

• Osmosis: Net movement of water across a selectively permeable membrane.

• Osmotic pressure: Pressure needed to stop water movement.

• Isotonic: Equal solute concentrations; Hypotonic: Lower outside; Hypertonic: Higher outside.

Active Processes

• Active transport: Uses ATP and transporter proteins to move substances against their gradient.

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

Cytoplasm and Internal Structures

Cytoplasm

• Thick, aqueous, elastic substance inside the plasma membrane.

• Contains DNA (nucleoid), ribosomes, inclusions, and cytoskeleton.

Nucleoid

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

• No nuclear envelope or histones.

• Plasmids: Small, extrachromosomal DNA circles carrying nonessential genes (e.g., antibiotic resistance).

Ribosomes

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

• Prokaryotic ribosomes: 70S (50S + 30S subunits).

• Target for antibiotics (e.g., streptomycin, gentamicin).

Inclusions

• Reserve deposits for nutrients (e.g., phosphate, polysaccharides, lipids, sulfur).

• Specialized inclusions: Carboxysomes (photosynthesis), gas vacuoles (buoyancy), magnetosomes (iron oxide).

Endospores

• Resting, highly resistant cells formed by Bacillus and Clostridium when nutrients are scarce.

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

• Sporulation: Formation of endospores; Germination: Return to vegetative state.

Eukaryotic Cell Structures

Flagella and Cilia

• Projections for locomotion or moving substances along the cell surface.

• Flagella: Long, few; Cilia: Short, numerous.

• Both have a "9+2" arrangement of microtubules (tubulin protein).

• Movement is wavelike (unlike rotary movement in prokaryotes).

Cell Wall and Glycocalyx

• Cell wall: Present in plants (cellulose), fungi (chitin), algae (various polysaccharides).

• Glycocalyx: Carbohydrates bonded to proteins/lipids; found in animal cells, aids in cell recognition and attachment.

Plasma Membrane

• Similar phospholipid bilayer structure as prokaryotes.

• Contains sterols (complex lipids) and carbohydrates (for attachment and recognition).

• Functions: Selective permeability, transport, endocytosis (phagocytosis, pinocytosis, receptor-mediated).

Cytoplasm and Organelles

• Cytosol: Fluid portion; Cytoskeleton: Microfilaments, intermediate filaments, microtubules.

• Cytoplasmic streaming: Movement of cytoplasm within the cell.

Ribosomes

• 80S (60S + 40S subunits) in cytoplasm and on rough ER; 70S in mitochondria and chloroplasts.

Nucleus

• Double membrane (nuclear envelope) encloses DNA complexed with histones (chromatin).

• Chromatin condenses into chromosomes during cell division.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes, site of protein synthesis.

• Smooth ER: Synthesizes membranes, fats, hormones.

Golgi Complex

• Modifies, sorts, and packages proteins from the ER for transport.

Lysosomes and Vacuoles

• Lysosomes: Contain digestive enzymes.

• Vacuoles: Storage, shape, and food intake.

Mitochondria and Chloroplasts

• Mitochondria: Site of ATP production; double membrane, inner folds (cristae), own DNA and 70S ribosomes.

• Chloroplasts: Site of photosynthesis; thylakoids with chlorophyll, own DNA and 70S ribosomes.

Other Organelles

• Peroxisomes: Oxidize fatty acids, destroy hydrogen peroxide.

• Centrosomes: Organize mitotic spindle, contain centrioles.

The Evolution of Eukaryotes

Endosymbiotic Theory

• Proposes that eukaryotes evolved when larger prokaryotic cells engulfed smaller ones, which became organelles (mitochondria, chloroplasts).

• Evidence: Mitochondria and chloroplasts resemble bacteria in size/shape, have circular DNA, reproduce independently, have 70S ribosomes, and double membranes.