Functional Anatomy of Prokaryotic and Eukaryotic Cells

Terminology & Evolutionary Background

• Prokaryote derives from Greek roots meaning “before nucleus” (literally “before kernel”).

• Eukaryote translates as “true nucleus.”

• Serial Endosymbiosis Theory (SET)

  • Eukaryotic cells likely emerged from symbioses among multiple prokaryotes > 2 \text{ billion years} ago.

  • Evidence: Molecular systematics show mitochondria ≈ "purple" bacteria, chloroplasts ≈ cyanobacteria, flagella ≈ spirochetes.

  • Membrane-infolding model explains origin of endomembrane system (ER, nuclear envelope).

Comparative Overview: Prokaryote vs. Eukaryote

• Genetic material

  • Prokaryote: One circular chromosome, NOT membrane-bound, no histones.

  • Eukaryote: Paired linear chromosomes inside nuclear envelope, histone proteins present.

• Internal structures

  • Prokaryote: No membrane-bound organelles.

  • Eukaryote: Extensive organelles (ER, Golgi, mitochondria, etc.).

• Cell division

  • Prokaryote: Binary fission.

  • Eukaryote: Mitotic spindle → mitosis.

• Cell wall composition

  • Prokaryote: Peptidoglycan (bacteria) or pseudomurein (some Archaea).

  • Eukaryote: Cellulose (plants/algae), chitin (fungi), glucan/mannan (yeast) or none (animals—with glycocalyx instead).

Size, Shape & Arrangement of Bacterial Cells

• Average dimensions: 0.2{-}1.0\, \mu m \times 2{-}8\, \mu m.

• Basic morphologies

  • Cocci (spherical), Bacilli (rod-shaped), Spirals (vibrio, spirillum, spirochaete).

• Unusual forms

  • Stella (star-shaped), Haloarcula (square archaeon).

  • Most species are monomorphic; a few are pleomorphic.

• Typical arrangements

  • Pairs: diplococci, diplobacilli.

  • Chains: streptococci, streptobacilli.

  • Clusters: staphylococci.

Glycocalyx

• Sticky layer external to cell wall; polymer is usually polysaccharide (extracellular polysaccharide, EPS).

• Capsule: Neatly organized, firmly attached → immune evasion (anti-phagocytic).

• Slime layer: Loose, unorganized, facilitates surface attachment (biofilms).

Flagella (Prokaryotic)

• Structure

  • Filament (flagellin chains) + Hook + Basal body (anchors through wall and membrane).

• Arrangements

  • Monotrichous (single), Amphitrichous (one at each pole), Lophotrichous (tufts at a pole), Peritrichous (all over).

• Motility mechanics

  • Rotation → “run” (straight) or “tumble” (re-orient).

  • Chemotaxis & phototaxis = movement toward/away from stimuli.

  • Flagellar proteins serve as H antigens (e.g., E.\ coli\,O157{:}H7).

Axial Filaments (Endoflagella)

• Found in spirochetes; anchored at one end within periplasmic space.

• Rotation of the filament generates corkscrew motion through viscous media.

Fimbriae & Pili

• Fimbriae: Numerous short appendages; key for adhesion to surfaces/host tissues.

• Pili (sex pili): Longer; mediate DNA transfer between cells via conjugation.

Peptidoglycan & Cell Wall Architecture

• Peptidoglycan = Polymer of alternating \text{NAG} & \text{NAM} disaccharides cross-linked by short peptides.

• Functions: Structural rigidity & protection against osmotic lysis.

Gram-Positive Wall

• Thick peptidoglycan layer.

• Teichoic acids

  • Wall teichoic acid ↔ peptidoglycan.

  • Lipoteichoic acid ↔ plasma membrane.

  • Regulate cation flow & confer antigenic specificity.

• Acid-fast bacteria (e.g., Mycobacterium) embed mycolic acid within wall.

Gram-Negative Wall

• Thin peptidoglycan + outer membrane.

• Outer membrane components

  • Lipopolysaccharide (LPS): Lipid A (endotoxin) + O-polysaccharide antigen.

  • Porins: Protein channels for small molecules.

  • Periplasmic space houses enzymes, transport proteins.

• Provides resistance to phagocytosis, complement, some antibiotics.

Gram Stain Mechanism

1. Crystal violet + iodine forms CV-I complex.

2. Alcohol

   • Gram +: Dehydrates thick wall → pores close; dye retained.

   • Gram −: Dissolves outer membrane & leaves gaps; dye washed out.

Atypical Walls & Wall Damage

• Mycoplasma: No cell wall; sterols in plasma membrane.

• Archaea: Wall-less or pseudomurein (lacks NAM, D-amino acids).

• Agents of wall damage

  • Lysozyme cleaves \beta(1\rightarrow4) linkage (NAG–NAM).

  • Penicillin blocks peptide cross-bridges.

• Resulting forms

  • Protoplast (wall-less Gram +), spheroplast (wall-less Gram −), L-forms (unstable wall-deficient cells) – all susceptible to osmotic lysis.

Plasma (Cytoplasmic) Membrane: Prokaryotes

• Fluid-mosaic phospholipid bilayer (viscosity ≈ olive oil).

• Protein types: Peripheral & integral (transmembrane) – migrate laterally to function.

• Functions

  • Selective permeability.

  • Electron transport & ATP synthesis enzymes.

  • Pigmented infoldings (chromatophores/thylakoids) in photosynthetic species.

• Damage agents: Alcohols, quaternary ammonium detergents, polymyxin antibiotics ⇒ leakage.

Membrane Transport Processes

• Passive

  • Simple diffusion (down gradient).

  • Facilitated diffusion (transporter protein).

  • Osmosis (water movement); quantified by osmotic pressure.

  • Isotonic: No net H$_2$O.

  • Hypotonic: Water influx → osmotic lysis.

  • Hypertonic: Water efflux → plasmolysis.

• Active

  • Primary active transport: Transporter + ATP.

  • Group translocation: Substrate chemically modified during import (requires \text{PEP}).

Prokaryotic Internal Features

• Cytoplasm: Aqueous matrix inside membrane.

• Nucleoid: Irregular region containing circular DNA; NOT a true nucleus.

• Ribosomes: 70S = 30S + 50S; site of translation.

• Inclusions (reserve deposits)

  • Metachromatic granules (volutin) – PO_4^{3-} storage.

  • Polysaccharide, lipid, sulfur granules – energy reserves/waste.

  • Carboxysomes – \text{RuBP}\, carboxylase for CO_2 fixation.

  • Gas vacuoles – buoyancy.

  • Magnetosomes – Fe3O4; orient to Earth’s field & detoxify H2O2.

• Endospores (Bacillus, Clostridium)

  • Dormant, highly resistant to desiccation, heat, chemicals.

  • Sporulation = formation; germination = return to vegetative state.

Eukaryotic Cell Surface Structures

• Flagella & Cilia

  • Core of microtubules (tubulin) arranged 9 + 2.

  • Movement via ATP-dependent dynein motors.

• Cell walls (when present)

  • Plants/algae: Cellulose.

  • Fungi: Chitin.

  • Yeast: Glucan/mannan.

• Glycocalyx in animal cells: Carbohydrate layer bonded to lipids/proteins; cell-to-cell recognition.

Eukaryotic Plasma Membrane

• Similar bilayer but with sterols (cholesterol/ergosterol) for rigidity.

• Transport repertoire

  • Passive: Simple/facilitated diffusion, osmosis.

  • Active transport (carrier + ATP).

  • Endocytosis

  • Phagocytosis (pseudopods engulf solids).

  • Pinocytosis (membrane invaginates fluid).

Eukaryotic Cytoplasmic Components

• Cytosol + Cytoskeleton (microfilaments, intermediate filaments, microtubules).

• Cytoplasmic streaming distributes nutrients, organelles.

Membrane-Bound Organelles

• Nucleus: Enclosed by double membrane with nuclear pores; contains DNA & nucleolus.

• Endoplasmic Reticulum (ER)

  • Rough ER: Ribosome-studded; protein synthesis & packaging.

  • Smooth ER: Lipid synthesis, detox.

• Golgi Complex: Stack of cisternae; modifies, sorts, secretes proteins/lipids (vesicular traffic).

• Lysosomes: Hydrolytic enzymes for intracellular digestion.

• Vacuoles: Large in plants (turgor, storage); small food vacuoles in protozoa/animals.

• Mitochondria: Double membrane; inner folds = cristae, matrix contains enzymes for cellular respiration & own 70S ribosomes.

• Chloroplasts: Thylakoids (photosynthetic membranes) stacked into grana; own DNA & 70S ribosomes.

• Peroxisomes: Oxidize fatty acids; catalase breaks down H2O2.

Non-Membranous Structures

• Ribosomes

  • 80S (\approx 60S + 40S) in cytoplasm or bound to ER.

  • 70S remain within mitochondria & chloroplasts (supports SET).

• Centrosome & Centrioles: Microtubule-organizing centers; build mitotic spindle.

Domains of Life (Three-Domain System)

• Bacteria: Includes Gram-positive/negative, cyanobacteria, Thermotoga, etc.

• Archaea: Methanogens, extreme halophiles, hyperthermophiles; ancestral to Eukarya.

• Eukarya: Protists (Chromista, ciliates, slime molds, etc.), plants, fungi, animals.

• Universal ancestor diverged into these domains.

Clinical & Practical Connections

• Gram stain guides antibiotic therapy; Lipid A (endotoxin) triggers septic shock.

• Capsule presence relates to pathogenicity (e.g., Streptococcus pneumoniae).

• Mycoplasma sterols target of specific antimicrobials.

• Lysozyme (saliva, tears) is a natural antibacterial.

• Endospore resistance necessitates stringent sterilization (autoclaving).

• Antibiotics (penicillin, polymyxin) exploit structural differences in walls/membranes.

• SET explains mitochondrial disorders (maternal inheritance) due to prokaryotic ancestry.