Eukaryotic Cell Evolution, Structure, and Comparison: 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. It proposes that certain organelles, such as mitochondria and chloroplasts, originated as free-living bacteria that were engulfed by ancestral eukaryotic cells.

• 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 surround mitochondria and chloroplasts, consistent with engulfment.

• Proof 4: Ribosomes within these organelles 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 bacterial groups (e.g., alpha-proteobacteria for mitochondria).

Comparative Cell Biology

Prokaryotes vs. Eukaryotes

Prokaryotic and eukaryotic cells differ in complexity, structure, and genetic organization.

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

• Size: Prokaryotes are generally smaller (0.1–5 μm) than eukaryotes (10–100 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) and some eukaryotes (cellulose in plants, chitin in fungi).

• Nucleus: Prokaryotes lack a true nucleus; eukaryotes have a membrane-bound nucleus.

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

• Genetic Material: Prokaryotes have a single, circular chromosome; eukaryotes have multiple, linear chromosomes.

• Membrane-bound Organelles: Absent in prokaryotes; present in eukaryotes (mitochondria, ER, Golgi, etc.).

Classification of Eukaryotes

The Four Kingdoms of Eukarya

Eukaryotes are classified into four major kingdoms, each with unique characteristics.

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

• Fungi: Mostly multicellular (except yeasts); cell walls of chitin; examples: Mushrooms, Yeast.

• Plantae: Multicellular; cell walls of cellulose; photosynthetic; examples: Oak tree, Algae.

• Animalia: Multicellular; no cell wall; examples: Human, Dog.

Comparison Table:

Kingdom	Cellularity	Reproduction	Cell Wall	Chloroplasts	Mitochondria
Protista	Unicellular/Multicellular	Asexual/Sexual	Varies	Some	Yes
Fungi	Mostly Multicellular	Asexual/Sexual	Chitin	No	Yes
Plantae	Multicellular	Sexual/Asexual	Cellulose	Yes	Yes
Animalia	Multicellular	Sexual	None	No	Yes

Pathogenic Protozoa

Examples and Disease Description

Protozoa are unicellular eukaryotes, some of which cause human diseases.

• Example: Plasmodium species cause malaria.

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

Structure of Eukaryotic Cells

Intracellular vs. Extracellular Structures

Eukaryotic cells contain various structures inside and outside the plasma membrane.

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

• Extracellular Structures: Cell wall (in plants/fungi), extracellular matrix, cilia, flagella.

• Animal vs. Plant Cells: Plant cells have cell walls and chloroplasts; animal cells do not.

Cytoskeleton in Eukaryotic Cells

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

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

• Microfilaments: Actin filaments; support cell shape and movement.

• Intermediate Filaments: Provide mechanical strength.

Eukaryotic Cell Organelles

Structure and Function of Key Organelles

• 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 breakdown of macromolecules.

• Vesicles: Transport materials within the cell.

Plasma Membrane and Associated Structures

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

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

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

• Flagella: Long, whip-like structures for movement.

• Cilia: Short, hair-like structures for movement or fluid transport.

Evolutionary Developments in Eukaryotes

Key Innovations Enabling Eukaryotic Cells

The emergence of eukaryotic cells involved several evolutionary advancements.

• 1. Endomembrane System: Development of internal membranes (nucleus, ER, Golgi) allowed compartmentalization of cellular functions.

• 2. Acquisition of Mitochondria and Chloroplasts: Endosymbiosis provided efficient energy production and photosynthesis.

• 3. Cytoskeleton: Enabled complex cell shapes, movement, and division.

Example: The evolution of multicellularity in eukaryotes allowed for specialized tissues and greater organismal complexity.

Additional info: These notes expand on the brief question prompts by providing definitions, examples, and comparative tables for clarity and completeness.