Functional Anatomy of Prokaryotic and Eukaryotic Cells: Mini-Textbook Study Notes

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Prokaryotic Cells

Prokaryotes are unicellular organisms lacking a true nucleus and membrane-bound organelles. Their cellular structure is simpler than that of eukaryotes, and they include bacteria and archaea.

• Chromosome: Single, circular DNA molecule not enclosed in a membrane.

• Histones: Absent; DNA is not associated with histone proteins.

• Organelles: Absent; no membrane-bound organelles.

• Cell Wall: Contains peptidoglycan (in bacteria).

• Plasma Membrane: Lacks sterols.

• Reproduction: Binary fission (asexual).

• Size: Typically 0.2–1.0 μm wide and 2–8 μm long.

Shapes and Arrangements

• Coccus: Spherical

• Bacillus: Rod-shaped

• Spiral: Includes spirillum, vibrio, and spirochete

• Arrangements: Diplococci (pairs), diplobacilli (pairs), clusters

• Monomorphic: One shape

• Pleomorphic: Multiple shapes

Structures External to the Cell Wall

Glycocalyx

The glycocalyx is a sticky, extracellular polysaccharide layer outside the cell wall, which can be a capsule (organized) or a slime layer (unorganized).

• Functions:

  • Adherence to surfaces

  • Protection from phagocytosis (antiphagocytic)

  • Source of nutrition

Flagella

Flagella are long, whip-like appendages used for motility.

• Structure: Composed of flagellin; consists of filament, hook, and basal body.

• Function: Movement via rotation (run/tumble); taxis (movement toward/away from stimuli).

• Antigenic Properties: Flagella proteins are H antigens (used in identification).

Axial Filaments

Axial filaments are unique to spirochetes and provide motility via corkscrew rotation.

• Examples: Treponema pallidum (syphilis), Borrelia burgdorferi (Lyme disease)

Fimbriae and Pili

• Fimbriae: Short, hairlike appendages for attachment and colonization.

• Pili: Longer than fimbriae; involved in DNA transfer (conjugation) between bacteria (horizontal gene transfer).

Structures Internal to the Cell Wall

Cell Wall

The bacterial cell wall provides structural support, maintains shape, and protects against osmotic pressure changes. It is a major target for antibiotics and aids in bacterial identification.

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

Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic Acid	Present	Absent
Outer Membrane	Absent	Present
Lipopolysaccharide (LPS)	Absent	Present (endotoxin)
Antibiotic Resistance	Lower	Higher

• Gram-Positive: Thick peptidoglycan, teichoic acids, may contain mycolic acid (acid-fast bacteria).

• Gram-Negative: Thin peptidoglycan, outer membrane with LPS, porins, more resistant to antibiotics.

Gram Stain Mechanism

• Crystal violet-iodine (CV-I) complex: Forms in both cell types.

• Gram-Positive: Alcohol dehydrates peptidoglycan, CV-I complex retained.

• Gram-Negative: Alcohol dissolves outer membrane, CV-I complex lost.

Atypical Cell Walls

• Mycoplasmas: Lack cell wall; contain sterols in plasma membrane; cause mild pneumonia.

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

• Acid-Fast Cell Walls: Like Gram-positive but with waxy mycolic acid (e.g., Mycobacterium).

Damage to Cell Walls

• Lysozyme: Digests peptidoglycan.

• Penicillin: Inhibits peptide bridge formation in peptidoglycan.

• L forms, protoplasts, spheroplasts: Wall-less cells susceptible to osmotic lysis.

Plasma Membrane and Transport Mechanisms

Fluid Mosaic Model

The plasma membrane is a dynamic structure where proteins and phospholipids move laterally, allowing flexibility and function.

• Selective Permeability: Allows passage of certain molecules.

• ATP Production: Enzymes embedded in membrane.

• Photosynthetic Pigments: Located on chromatophores/thylakoids in some bacteria.

• Damage: Alcohol, detergents, and antibiotics can disrupt membrane integrity.

Passive Transport

• Simple Diffusion: Movement of solute from high to low concentration. No energy required.

• Facilitated Diffusion: Solute moves via transporter proteins.

• Osmosis: Movement of water across a selectively permeable membrane.

Osmotic Pressure: Pressure needed to stop water movement across membrane.

Solution Type	Effect on Bacterial Cell
Isotonic	No net movement of water; equilibrium
Hypotonic	Water enters cell; may burst (osmotic lysis) if wall is weak
Hypertonic	Water leaves cell; cytoplasm shrinks (plasmolysis)

Active Transport

• Requires: Transporter protein and ATP

• Direction: Low to high concentration

Eukaryotic Cells

Eukaryotes possess a true nucleus and membrane-bound organelles. Their cells are structurally more complex than prokaryotes.

• Chromosomes: Paired, enclosed in nuclear membrane

• Histones: Associated with DNA

• Organelles: Numerous, membrane-bound

• Cell Wall: Polysaccharide-based (cellulose, chitin, mannan)

• Plasma Membrane: Contains sterols

• Mitotic Spindle: Present for cell division

Cytoplasm

• Definition: Aqueous substance inside plasma membrane containing organelles and nucleus

Cilia and Flagella

• Structure: Microtubules (tubulin), 9+2 arrangement

• Function: Motility

Glycocalyx

• Location: Animal cells; carbohydrates extend from plasma membrane

• Function: Cell-cell recognition; bonded to proteins/lipids

Plasma Membrane

• Structure: Phospholipid bilayer with peripheral, integral, and transmembrane proteins

• Sterols: Present

• Function: Cell-cell recognition

Active Transport Mechanisms

• Endocytosis: Uptake of substances by membrane invagination

• Phagocytosis: Engulfment of particles by pseudopods

• Pinocytosis: Uptake of fluids and dissolved substances

Organelles

• Nucleus: Contains DNA

• Endoplasmic Reticulum (ER): Transport network

• Golgi Complex: Membrane formation and secretion

• Lysosome: Contains digestive enzymes

• Vacuole: Storage and support

• Mitochondrion: Site of cellular respiration

• Chloroplast: Site of photosynthesis

• Peroxisome: Oxidation of molecules

Non-Membrane Bound Structures

• Ribosome: Protein synthesis (80S in cytoplasm/ER, 70S in mitochondria/chloroplasts)

• Centrosome: Protein fibers and centrioles; involved in cell division

• Centriole: Mitotic spindle formation

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotic cells from prokaryotic ancestors. It proposes that certain organelles (mitochondria, chloroplasts) were once free-living bacteria engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own DNA, 70S ribosomes in mitochondria/chloroplasts

Transport Mechanisms: Definitions and Examples

• Passive Diffusion: Movement of molecules from high to low concentration without energy input.

• Facilitated Diffusion: Movement via specific transporter proteins; no energy required.

• Osmosis: Movement of water across a selectively permeable membrane.

• Active Transport: Movement against concentration gradient using ATP and transporter proteins.

• Group Translocation: Substance is chemically modified during transport across membrane (common in prokaryotes).

Effects of Solutions on Bacterial Cells

• Isotonic: No net water movement; cell remains unchanged.

• Hypotonic: Water enters cell; risk of osmotic lysis if cell wall is weak.

• Hypertonic: Water leaves cell; cytoplasm shrinks (plasmolysis).

Summary Table: Prokaryotes vs. Eukaryotes

Feature	Prokaryotes	Eukaryotes
Nucleus	No (nucleoid)	Yes (true nucleus)
Chromosomes	Single, circular	Multiple, linear
Organelles	Absent	Present
Cell Wall	Peptidoglycan (bacteria)	Polysaccharides (cellulose, chitin, mannan)
Ribosomes	70S	80S (cytoplasm/ER), 70S (mitochondria/chloroplasts)
Reproduction	Binary fission	Mitosis/meiosis

Key Definitions

• Organelle: Specialized structure within a cell, often membrane-bound, performing specific functions.

• Mitochondria: Organelle responsible for cellular respiration and energy production.

• Golgi Body: Organelle involved in modification, sorting, and secretion of proteins.

• Lysosome: Organelle containing digestive enzymes for breakdown of cellular waste.

• Endoplasmic Reticulum (ER): Network for protein and lipid synthesis and transport.

• Endosymbiotic Theory: Theory that eukaryotic organelles originated from symbiotic prokaryotes.

Key Equations (LaTeX Format)

• Osmotic Pressure: $\Pi = iMRT$ Where: $\Pi$ = osmotic pressure, $i$ = van 't Hoff factor, $M$ = molarity, $R$ = gas constant, $T$ = temperature (K)

Examples and Applications

• Gram Stain: Used to differentiate bacteria for diagnosis and treatment selection.

• Antibiotics: Penicillin targets peptidoglycan synthesis, effective against Gram-positive bacteria.

• Endosymbiotic Theory: Mitochondria and chloroplasts have their own DNA and ribosomes, supporting their prokaryotic origin.

Additional info: Academic context and definitions have been expanded for clarity and completeness.