Functional Anatomy of Prokaryotic and Eukaryotic Cells: Study Notes

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview of Prokaryotic and Eukaryotic Cell Structure

Both prokaryotic and eukaryotic cells are composed of similar chemical substances and utilize comparable chemical reactions. However, they differ significantly in their structural organization and complexity.

• Prokaryotic Cells:

  • Genetic material (DNA) is not enclosed within a nuclear membrane.

  • DNA is typically a single, circular chromosome not associated with histone proteins.

  • Lack membrane-bound organelles.

  • Cell walls contain peptidoglycan.

  • Reproduce by binary fission.

• Eukaryotic Cells:

  • Genetic material is enclosed within a nuclear membrane.

  • DNA is linear, associated with histone proteins, and organized into multiple chromosomes.

  • Contain membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).

  • Cell walls (if present) do not contain peptidoglycan.

  • Reproduce by mitosis (and meiosis in sexually reproducing organisms).

Shapes of Bacteria

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

• Coccus: Spherical-shaped bacteria.

• Bacillus: Rod-shaped bacteria.

• Spiral: Includes vibrio, spirillum, and spirochete forms.

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

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

Structures External to the Cell Wall

Bacteria possess various external structures that contribute to their survival and pathogenicity.

• Glycocalyx: A viscous, gelatinous layer external to the cell wall, composed of polysaccharide and/or polypeptide.

  • Capsule: Firmly attached; protects against phagocytosis, increases virulence.

  • Slime Layer: Loosely attached; aids in attachment and protection from dehydration.

• Flagella: Long, filamentous appendages for motility.

  • Arrangements:

    • Monotrichous: Single flagellum at one pole.

    • Amphitrichous: Tuft of flagella at both poles.

    • Lophotrichous: Two or more flagella at one pole.

    • Peritrichous: Flagella distributed over the entire cell.

  • Movement is powered by rotation from the basal body; enables taxis (movement toward/away from stimuli).

  • Chemotaxis: Movement in response to chemicals.

  • Phototaxis: Movement in response to light.

• Axial Filaments: Found in spirochetes; enable corkscrew motion.

• Fimbriae: Short, hairlike structures for attachment to surfaces; numerous per cell.

• Pili: Longer than fimbriae, usually 1–2 per cell; involved in DNA transfer (conjugation).

Bacterial Cell Wall

The bacterial cell wall provides structural support, maintains shape, and prevents cell lysis due to osmotic pressure. Its composition and structure are key to bacterial classification and antibiotic susceptibility.

• Peptidoglycan: Main component; consists of repeating disaccharides (N-acetylglucosamine and N-acetylmuramic acid) cross-linked by polypeptides.

• Gram-Positive Bacteria: Thick, multilayered peptidoglycan cell wall.

• Gram-Negative Bacteria: Thin peptidoglycan layer plus an outer membrane containing lipoproteins, lipopolysaccharides (LPS), and phospholipids.

  • Lipid A: Endotoxin component of LPS; causes fever and shock.

  • Outer membrane provides resistance to certain antibiotics and digestive enzymes.

• Mycoplasmas: Lack a cell wall; smallest known bacteria.

• Antibacterial Agents:

  • Lysozyme: Breaks glycosidic bonds in peptidoglycan; effective against gram-positive bacteria.

  • Penicillin: Inhibits peptide cross-linking; causes cell lysis.

  • Gram-negative bacteria are less susceptible due to their outer membrane.

Bacterial Cell Membrane

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins. It regulates the movement of substances into and out of the cell and is involved in energy generation.

• Structure: Phospholipid bilayer with hydrophilic heads and hydrophobic tails; proteins are peripheral or integral.

• Function: Selective permeability, nutrient breakdown, ATP production.

• Disruption: Alcohols, quaternary ammonium compounds, and some antibiotics can damage the membrane.

Transport Mechanisms Across the Membrane

Cells utilize various mechanisms to transport substances across the plasma membrane, classified as passive or active processes.

• Simple Diffusion: Movement from high to low concentration until equilibrium is reached; no energy required.

• Facilitated Diffusion: Movement from high to low concentration via carrier proteins; no energy required.

• Osmosis: Net movement of water across a selectively permeable membrane from high to low water concentration.

  • Isotonic Solution: Equal solute concentration inside and outside the cell.

  • Hypotonic Solution: Lower solute concentration outside; water enters cell, may cause lysis.

  • Hypertonic Solution: Higher solute concentration outside; water leaves cell, causing shrinkage.

• Active Transport: Movement against the concentration gradient using ATP.

• Group Translocation: Unique to prokaryotes; substance is chemically modified during transport (e.g., glucose phosphorylation using PEP).

Cytoplasm and Its Contents

The cytoplasm is the internal matrix of the cell, containing water, enzymes, nutrients, and various cellular structures.

• Cytoplasm: 80% water, contains proteins, carbohydrates, lipids, inorganic ions, and small molecules.

• Nuclear Area (Nucleoid): Contains the bacterial chromosome (single, circular DNA molecule); no nuclear membrane or histones.

• Plasmids: Small, circular, double-stranded DNA molecules; carry non-essential but advantageous genes (e.g., antibiotic resistance).

• Ribosomes: Sites of protein synthesis; composed of protein and rRNA; prokaryotic ribosomes (70S) are smaller than eukaryotic ribosomes (80S).

• Inclusions: Reserve deposits for nutrients and other substances (e.g., metachromatic granules for phosphate, polysaccharide granules, lipid inclusions, sulfur granules).

Endospores, Sporulation, and Germination

Certain bacteria form endospores to survive adverse conditions. Endospores are highly resistant, dormant structures.

• Endospores: Resting cells formed by some gram-positive bacteria during nutrient depletion; highly resistant to heat, desiccation, chemicals, and radiation.

• Sporulation: Process of endospore formation; involves septum formation, peptidoglycan deposition, and protein coat development.

• Germination: Return of endospore to vegetative state upon exposure to favorable conditions; triggered by physical or chemical damage to the spore coat.

Eukaryotic Cells: Structure and Differences from Prokaryotes

Eukaryotic cells are structurally more complex and larger than prokaryotic cells, with compartmentalization of functions into organelles.

• Size: 10–100 μm in diameter.

• Nucleus: Surrounded by nuclear membrane; contains DNA associated with histones, organized into multiple chromosomes.

• Flagella: Composed of microtubules; move in a wave-like motion (unlike rotary prokaryotic flagella).

• Cell Walls:

  • Algae/Plants: Cellulose.

  • Fungi: Chitin.

  • Yeasts: Glucan.

  • Protozoa: No cell wall.

  • Animals: Glycocalyx for cell-cell interactions.

  • Note: Eukaryotes do not have peptidoglycan.

• Organelles:

  • Endoplasmic Reticulum (ER): Rough ER (with ribosomes) for protein synthesis; smooth ER for lipid synthesis.

  • Golgi Complex: Modifies, packages, and delivers proteins.

  • Mitochondria: Site of aerobic respiration and ATP production.

  • Chloroplasts: Present in algae and plants; site of photosynthesis.

  • Lysosomes: Contain digestive enzymes.

  • Vacuoles: Storage of nutrients and waste products.

• Cytoskeleton: Provides structural support; absent in prokaryotes.

• Cell Division: Mitosis for somatic cells; meiosis for gamete formation in sexually reproducing organisms.

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present (with nuclear membrane)
DNA	Single, circular, no histones	Multiple, linear, with histones
Organelles	Absent	Present (mitochondria, ER, etc.)
Cell Wall	Peptidoglycan (most)	Cellulose (plants/algae), chitin (fungi), glucan (yeasts), absent in animals/protozoa
Ribosomes	70S	80S
Cell Division	Binary fission	Mitosis (meiosis for gametes)
Size	0.2–2.0 μm	10–100 μm

Example: Antibiotic Action

Penicillin targets the synthesis of peptidoglycan, making it effective against gram-positive bacteria but less so against gram-negative bacteria due to their protective outer membrane.

Additional info: The above notes expand on the original content by providing definitions, examples, and a comparison table for clarity and completeness.