Eukaryotic Cell Biology and Evolution: Study Notes for Microbiology

Evolution and Structure of Eukaryotic Cells

Endosymbiotic Theory and Eukaryotic Evolution

The endosymbiotic theory explains the origin of eukaryotic cells from prokaryotic ancestors, proposing that certain organelles originated as symbiotic bacteria engulfed by a host cell. This theory is central to understanding the evolution of complex cellular life.

• Proof 1: Mitochondria and chloroplasts have their own circular DNA, similar to bacterial genomes.

• Proof 2: These organelles replicate independently of the cell cycle, resembling binary fission in bacteria.

• Proof 3: Double membranes around mitochondria and chloroplasts suggest engulfment by a host cell.

• Proof 4: Ribosomes within mitochondria and chloroplasts are more similar to prokaryotic ribosomes (70S) than to eukaryotic ribosomes (80S).

• Proof 5: Phylogenetic analysis shows close genetic relationships between these organelles and certain bacteria (e.g., mitochondria and α-proteobacteria).

• Example: The origin of mitochondria from aerobic bacteria and chloroplasts from cyanobacteria.

Comparison of Eukaryotes and Prokaryotes

Eukaryotes and prokaryotes differ in cellular organization, complexity, and genetic structure.

• Unicellular/Multicellular: Prokaryotes are typically unicellular; eukaryotes can be unicellular or multicellular.

• Cell Size: Eukaryotic cells are generally larger (10–100 μm) than prokaryotic cells (0.1–5 μm).

• Cell Division: Prokaryotes divide by binary fission; eukaryotes use mitosis and meiosis.

• Plasma Membrane: Both have plasma membranes, but eukaryotes may have additional internal membranes.

• Cell Wall: Present in most prokaryotes (peptidoglycan); in eukaryotes, cell walls are found in plants (cellulose) and fungi (chitin).

• Nucleus: Eukaryotes have a membrane-bound nucleus; prokaryotes have a nucleoid region.

• Ribosomes: Prokaryotes have 70S ribosomes; eukaryotes have 80S ribosomes in the cytoplasm.

• Genetic Material: Prokaryotes have circular DNA; eukaryotes have linear chromosomes.

• Membrane-bound Organelles: Present only in eukaryotes (e.g., mitochondria, ER, Golgi apparatus).

Classification: The Four Kingdoms of Eukarya

Eukarya is divided into four major kingdoms, each with unique characteristics and representative species.

• Protista: Mostly unicellular, some multicellular; examples: Amoeba, Paramecium.

• Fungi: Mostly multicellular (except yeasts); examples: Aspergillus, Saccharomyces.

• Plantae: Multicellular, photosynthetic; examples: Arabidopsis thaliana, Zea mays.

• Animalia: Multicellular, heterotrophic; examples: Homo sapiens, Mus musculus.

Kingdom	Cellularity	Reproduction	Cell Wall	Chloroplasts	Mitochondria
Protista	Unicellular/Multicellular	Asexual/Sexual	Variable	Some	Yes
Fungi	Multicellular (except yeast)	Asexual/Sexual	Chitin	No	Yes
Plantae	Multicellular	Sexual/Asexual	Cellulose	Yes	Yes
Animalia	Multicellular	Sexual	No	No	Yes

Pathogenic Protozoa

Protozoa are unicellular eukaryotes, some of which cause diseases in humans and animals.

• Example: Plasmodium species cause malaria.

• Disease Description: Malaria is transmitted by Anopheles mosquitoes, leading to fever, chills, and anemia due to destruction of red blood cells.

Structure of Eukaryotic Cells: Intracellular and Extracellular Components

Eukaryotic cells have complex structures with distinct intracellular and extracellular features.

• Intracellular Structures: Nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, cytoskeleton.

• Extracellular Structures: Plasma membrane, cell wall (in plants/fungi), extracellular matrix.

• Animal vs. Plant Cells: Plant cells have cell walls and chloroplasts; animal cells lack these but have centrioles.

Cytoskeleton in Eukaryotic Cells

The cytoskeleton provides structural support, facilitates movement, and organizes cellular components.

• Microtubules: Hollow tubes made of tubulin; involved in cell shape, transport, and division.

• Microfilaments: Thin filaments of actin; support cell shape and enable movement.

• Intermediate Filaments: Provide mechanical strength.

• Function: Maintains cell integrity, enables intracellular transport, and forms structures like cilia and flagella.

Structure and Function of Eukaryotic Organelles

Eukaryotic cells contain specialized organelles, each with distinct roles.

• Nucleus: Contains genetic material; site of DNA replication and transcription.

• Mitochondria: Powerhouse of the cell; site of aerobic respiration and ATP production.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes for intracellular digestion.

• Vesicles: Transport materials within the cell.

Eukaryotic Cell Structures: Plasma Membrane, Cell Wall, Glycocalyx, Flagella, Cilia

These structures contribute to cell protection, communication, and motility.

• Plasma Membrane: Phospholipid bilayer controlling entry and exit of substances.

• Cell Wall: Provides structural support (cellulose in plants, chitin in fungi).

• Glycocalyx: Carbohydrate-rich layer for cell recognition and protection.

• Flagella: Long, whip-like structures for movement (e.g., sperm cells).

• Cilia: Short, hair-like projections for movement or fluid transport (e.g., respiratory tract).

Evolutionary Developments Enabling Eukaryotic Cells

Several key innovations allowed the emergence and success of eukaryotic cells.

• 1. Endosymbiosis: Acquisition of mitochondria and chloroplasts enabled efficient energy production and photosynthesis.

• 2. Compartmentalization: Internal membranes and organelles allowed specialized functions and increased complexity.

• 3. Cytoskeleton: Provided structural support and enabled dynamic cell movement and division.

Example: The evolution of multicellularity and complex tissues in plants and animals.

Additional info: These notes expand on the brief question prompts to provide a comprehensive overview suitable for exam preparation in a college-level microbiology course.