Functional Anatomy of Prokaryotic and Eukaryotic Cells: Study Notes

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Comparison 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.

• Genetic Material: Prokaryotes have DNA not enclosed within a nuclear membrane; eukaryotes have DNA within a nucleus surrounded by a nuclear envelope.

• Chromosome Structure: Prokaryotic DNA is typically a single, circular chromosome not associated with histone proteins. Eukaryotic DNA is linear, associated with histones, and organized into multiple chromosomes.

• Organelles: Prokaryotes lack membrane-bound organelles; eukaryotes possess various organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus.

• Cell Wall Composition: Prokaryotic cell walls contain peptidoglycan; eukaryotic cell walls (when present) are composed of cellulose (plants, algae), chitin (fungi), or glucan (yeasts).

• Reproduction: Prokaryotes reproduce by binary fission; eukaryotes divide by mitosis and, in sexually reproducing organisms, meiosis.

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 (comma-shaped), spirillum (rigid spiral), and spirochete (flexible spiral).

• Monomorphic: Most bacteria maintain a single shape.

• Pleomorphic: Some bacteria can alter their shape in response to environmental conditions.

External Structures of Prokaryotic Cells

Several structures external to the cell wall contribute to bacterial survival, motility, and pathogenicity.

• Glycocalyx: A viscous, gelatinous layer external to the cell wall, composed of polysaccharide and/or polypeptide. If firmly attached, it is called a capsule; if loosely attached, a slime layer. Capsules enhance virulence by protecting bacteria from phagocytosis and dehydration.

• Flagella: Long, filamentous appendages for motility. Arrangements include:

  • 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 surface.

  Flagella rotate from a basal body and enable movement toward or away from stimuli (taxis), such as chemotaxis (chemicals) and phototaxis (light).

• Axial Filaments: Found in spirochetes, these structures enable corkscrew motion.

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

• Pili: Longer than fimbriae, usually 1-2 per cell, used for DNA transfer between bacteria (conjugation).

Bacterial Cell Wall

The cell wall provides structural support, maintains shape, and prevents osmotic lysis. Its composition is a key factor in bacterial classification and antibiotic susceptibility.

• Peptidoglycan: A lattice of repeating disaccharides (N-acetylglucosamine and N-acetylmuramic acid) cross-linked by polypeptides.

• Gram-Positive Cell Walls: Thick layers of peptidoglycan.

• Gram-Negative Cell Walls: Thin peptidoglycan layer and an outer membrane containing lipoproteins, lipopolysaccharides (LPS), and phospholipids. The LPS contains:

  • Polysaccharide portion: Used for bacterial typing.

  • Lipid A: Endotoxin causing fever and shock.

• Mycoplasmas: Bacteria lacking a cell wall; smallest known bacteria.

• Antibacterial Agents: Lysozyme breaks glycosidic bonds in peptidoglycan (effective against gram-positive bacteria). Penicillin inhibits peptide cross-linking, leading to cell lysis. Gram-negative bacteria are more resistant 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, and ATP production.

• Disruption: Some alcohols, quaternary ammonium compounds, and antibiotics can damage the membrane.

Transport Mechanisms Across the Membrane

Cells use 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.

• 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).

Osmotic Conditions:

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

• Hypotonic: Lower solute concentration outside; water enters cell, risk of lysis.

• Hypertonic: Higher solute concentration outside; water leaves cell, cell shrinks.

Cytoplasm and Its Contents

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

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

• Plasmids: Small, circular, double-stranded DNA molecules; replicate independently and often carry 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), allowing selective antibiotic targeting.

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

Endospores, Sporulation, and Germination

Some bacteria form endospores to survive harsh conditions. Endospores are highly resistant, dormant structures.

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

• Sporulation: Process of endospore formation involving septum formation, peptidoglycan deposition, and protein coat synthesis.

• Germination: Return of endospore to vegetative state upon exposure to favorable conditions; triggered by physical or chemical damage to the spore coat, followed by water uptake and resumption of metabolism.

Eukaryotic Cells: Structure and Differences from Prokaryotes

Eukaryotic cells are structurally more complex and larger than prokaryotic cells, with specialized organelles and compartmentalization.

• Size: 10–100 μm in diameter.

• Nucleus: Surrounded by a 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: Composition varies:

  • Plants and algae: Cellulose

  • Fungi: Chitin

  • Yeasts: Glucan

  • Protozoa: No cell wall

  • Animals: Glycocalyx (for cell-cell interactions)

  Eukaryotes do not contain peptidoglycan.

• Organelles: Membrane-bound structures such as mitochondria (ATP production), chloroplasts (photosynthesis in plants and algae), endoplasmic reticulum (protein and lipid synthesis), Golgi complex (protein modification and transport), lysosomes (digestive enzymes), and vacuoles (storage).

• Cytoskeleton: Network of protein filaments providing structural support and facilitating movement.

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

Table: Key Differences Between Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
DNA	Single, circular, no histones	Multiple, linear, with histones
Organelles	Absent	Present (membrane-bound)
Cell Wall	Peptidoglycan (most)	Cellulose, chitin, glucan, or absent
Ribosomes	70S	80S
Reproduction	Binary fission	Mitosis/meiosis
Size	0.2–2.0 μm	10–100 μm

Example: Escherichia coli is a prokaryote with a single circular chromosome, no nucleus, and a cell wall containing peptidoglycan. Saccharomyces cerevisiae (yeast) is a eukaryote with a nucleus, multiple linear chromosomes, and a cell wall containing glucan.

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