Functional Anatomy of Prokaryotic and Eukaryotic Cells: Study Notes

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview

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

Prokaryotic Cells

General Characteristics

• Genetic Material: Usually contain one circular chromosome.

• Nuclear Region: DNA is not enclosed in a membrane-bound nucleus.

• Organelles: Lack membrane-bound organelles.

• Cell Wall: Bacterial cell walls contain peptidoglycan.

• Reproduction: Reproduce by binary fission (asexual division).

• Examples: Escherichia coli, Staphylococcus aureus

Eukaryotic Cells

General Characteristics

• Genetic Material: DNA enclosed within a nuclear membrane (nucleus).

• Organelles: Contain membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

• Size: Larger and more complex than prokaryotes.

• Division: Divide by mitosis (and sometimes meiosis).

• Examples: Fungi, Protozoa, Algae, Plants, Animals

Bacterial Shapes and Arrangements

Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical-shaped

• Spiral Forms:

  • Vibrio: Comma-shaped

  • Spirillum: Rigid spiral

  • Spirochete: Flexible spiral

Arrangements

• Pairs: Diplococci, Diplobacilli

• Chains: Streptococci, Streptobacilli

• Clusters: Staphylococci

• Groups of Four: Tetrads

Glycocalyx

Structure and Types

• Located outside the cell wall.

• Composition: Polysaccharides and/or polypeptides.

• Types:

  • Capsule: Firmly attached; helps prevent phagocytosis.

  • Slime Layer: Loose and unorganized; helps cells attach to surfaces.

Functions

• Increases virulence (disease-causing ability).

• Helps form biofilms.

• Protects cells from drying out (desiccation).

Flagella

Structure and Function

• Function: Provide bacterial motility.

• Structure: Made of the protein flagellin.

Arrangement Types

• Monotrichous: Single flagellum at one end.

• Lophotrichous: Tufts of flagella at one or both ends.

• Amphitrichous: Single flagellum at both ends.

• Peritrichous: Flagella distributed over the entire cell.

Types of Movement

• Run: Movement in one direction.

• Tumble: Random direction changes.

• Chemotaxis: Movement toward or away from chemical stimuli.

Axial Filaments

• Also called endoflagella.

• Found in spirochete bacteria.

• Cause corkscrew-like movement.

Fimbriae and Pili

Fimbriae

• Help bacteria attach to surfaces.

• Important in biofilm formation.

Pili

• Functions:

  • DNA transfer during conjugation.

  • Twitching motility.

Cell Wall

Functions

• Prevents osmotic lysis.

• Protects the plasma membrane.

• Helps determine cell shape.

• Site of action for some antibiotics (e.g., penicillin).

Gram-Positive Cell Walls

• Thick peptidoglycan layer.

• Contain teichoic acids (lipoteichoic and wall teichoic acids).

• Carry a negative charge.

• Lipoteichoic acid: Links cell wall to plasma membrane.

• Wall teichoic acid: Attached to peptidoglycan.

Gram-Negative Cell Walls

• Thin peptidoglycan layer.

• Contain an outer membrane.

• Have a periplasmic space.

• Outer membrane contains:

  • Lipopolysaccharide (LPS)

  • Porins (protein channels)

  • Phospholipids

• LPS Parts:

  • Polysaccharide: Functions as an antigen (O antigen).

  • Lipid A: Functions as an endotoxin.

Atypical Cell Walls

• Acid-Fast Bacteria: Contain waxy mycolic acid (e.g., Mycobacterium, Nocardia).

• Mycoplasmas: Lack cell walls; contain sterols in plasma membrane.

Plasma Membrane

Structure and Function

• Made mainly of phospholipids and proteins.

• Controls movement of materials into and out of the cell.

Membrane Transport

Passive Processes

• Move substances from high concentration to low concentration.

• Simple Diffusion: No energy or transport proteins required.

• Facilitated Diffusion: Requires membrane transport proteins.

Osmosis

• Movement of water across a selectively permeable membrane.

• Isotonic Solution: Water movement is at equilibrium.

• Hypotonic Solution: Water moves into the cell.

• Hypertonic Solution: Water moves out of the cell.

Active Processes

• Active Transport: Requires energy (ATP) and transport proteins; moves substances from low to high concentration.

• Group Translocation: Requires transport proteins and energy; substance is chemically modified during transport.

Cytoplasm

• Mostly made of water.

• Contains DNA, ribosomes, and inclusions (storage granules).

Nucleoid

• Contains the bacterial chromosome.

• Characteristics:

  • Circular DNA molecule.

  • Not enclosed by a nuclear membrane.

Plasmids

• Small circles of DNA.

• May carry genes for antibiotic resistance or toxin production.

Ribosomes

• Function: Site of protein synthesis.

• Prokaryotic Ribosomes: 70S

• Eukaryotic Ribosomes: 80S

Endospores

• Produced when nutrients are depleted.

• Functions:

  • Survival mechanism

  • Resistant to heat, chemicals, radiation, desiccation

• Important Genera: Bacillus, Clostridium

Eukaryotic Organelles

Nucleus

• Contains the cell’s DNA.

• DNA is combined with histone proteins.

Endoplasmic Reticulum (ER)

• Rough ER: Contains attached ribosomes; function is protein synthesis and processing.

• Smooth ER: Function is lipid synthesis and detoxification.

Golgi Complex

• Modifies proteins.

• Packages materials into vesicles for transport.

Lysosomes

• Contain digestive enzymes for intracellular digestion.

Mitochondria

• Site of ATP production (cellular respiration).

• Contain circular DNA and 70S ribosomes (similar to prokaryotes).

Peroxisomes

• Function: Break down fatty acids and detoxify harmful substances (e.g., hydrogen peroxide).

Cytoskeleton

• Maintains cell shape.

• Helps with cell movement.

• Assists movement of organelles.

Flagella and Cilia in Eukaryotes

• Flagella: Long projections; few in number.

• Cilia: Short projections; numerous.

• Both contain microtubules arranged in a 9 + 2 pattern.

Summary Table: Key Differences Between Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Cell Wall Composition	Peptidoglycan (bacteria)	Cellulose (plants), chitin (fungi), or absent
Ribosome Size	70S	80S (cytoplasm), 70S (mitochondria/chloroplasts)
Cell Division	Binary fission	Mitosis/meiosis
Size	Usually 0.2–2.0 μm	Usually 10–100 μm

Additional info: Some details (e.g., specific examples, inferred arrangement types, and functions) were logically filled in based on standard microbiology textbooks to ensure completeness and clarity.