Eukaryotic Cell Evolution, Structure, and Comparison: Study Notes for Microbiology

Evolution and Structure of Eukaryotic Cells

Endosymbiotic Theory and the Evolution of Eukaryotes

The endosymbiotic theory explains the origin of eukaryotic cells from prokaryotic organisms. It proposes that certain organelles, such as mitochondria and chloroplasts, originated as free-living bacteria that were engulfed by ancestral eukaryotic cells, leading to a symbiotic relationship.

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

• Proof 2: These organelles replicate independently of the cell cycle, through a process resembling binary fission.

• Proof 3: Double membranes surround mitochondria and chloroplasts, consistent with engulfment by a host cell.

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

• Proof 5: Phylogenetic analysis shows that mitochondrial and chloroplast genes are closely related to those of certain bacteria (e.g., alpha-proteobacteria for mitochondria, cyanobacteria for chloroplasts).

Example: The origin of mitochondria from an ancestral alpha-proteobacterium is a classic example of endosymbiosis.

Comparative Cell Biology

Comparison of Eukaryotes and Prokaryotes

Eukaryotic and prokaryotic cells differ in several fundamental ways, including their structure, genetic material, and complexity.

• Unicellular/Multicellular: Prokaryotes are almost always 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 divide by mitosis or meiosis.

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

• Cell Wall: Most prokaryotes have a cell wall (peptidoglycan in bacteria); some eukaryotes (plants, fungi) have cell walls of cellulose or chitin.

• Nucleus: Eukaryotes have a membrane-bound nucleus; prokaryotes have a nucleoid region without a membrane.

• Ribosomes: Prokaryotic ribosomes are 70S; eukaryotic cytoplasmic ribosomes are 80S.

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

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

Example: Escherichia coli (prokaryote) vs. human liver cell (eukaryote).

Classification of Eukaryotes

The Four Kingdoms of Eukarya

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

• Protista: Mostly unicellular, some multicellular; can be autotrophic or heterotrophic. Example: Amoeba.

• Fungi: Mostly multicellular (except yeasts), heterotrophic, cell walls of chitin, reproduce sexually or asexually. Example: Aspergillus.

• Plantae: Multicellular, autotrophic (photosynthetic), cell walls of cellulose, sexual/asexual reproduction. Example: Arabidopsis thaliana.

• Animalia: Multicellular, heterotrophic, no cell walls, mostly sexual reproduction. Example: Homo sapiens.

Comparison Table:

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

Pathogenic Protozoa

Examples and Disease Description

Protozoa are unicellular eukaryotic organisms, some of which are pathogenic to humans.

• Example: Plasmodium falciparum causes malaria.

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

Eukaryotic Cell Structure

Intracellular and Extracellular Structures

Eukaryotic cells have complex internal and external structures that support their functions.

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

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

• Animal vs. Plant Cells: Plant cells have cell walls, chloroplasts, and large central vacuoles; animal cells lack these but have centrioles and more prominent lysosomes.

Example: The presence of chloroplasts in plant cells enables photosynthesis, which is absent in animal cells.

The Cytoskeleton in Eukaryotic Cells

The cytoskeleton is a network of protein filaments that provides structural support, facilitates cell movement, and organizes organelles.

• Microfilaments: Composed of actin; involved in cell shape and movement.

• Intermediate Filaments: Provide mechanical strength.

• Microtubules: Composed of tubulin; involved in chromosome movement and intracellular transport.

Structure and Function of Eukaryotic Organelles

Major Eukaryotic Organelles

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

• Mitochondria: 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 for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes for breakdown of macromolecules.

• Vesicles: Small membrane-bound sacs for transport and storage.

Specialized Eukaryotic Cell Structures

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

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

• Glycocalyx: Carbohydrate-rich layer outside the plasma membrane; involved in cell recognition and protection.

• Flagella and Cilia: Motile structures composed of microtubules; flagella are longer and fewer, cilia are shorter and more numerous.

Evolutionary Developments in Eukaryotic Cells

Key Innovations Enabling Eukaryotic Cell Emergence

The evolution of eukaryotic cells involved several major innovations that distinguish them from prokaryotes.

• 1. Compartmentalization: Development of internal membrane-bound organelles (e.g., nucleus, mitochondria) allowed for specialized cellular functions.

• 2. Endosymbiosis: Acquisition of mitochondria and chloroplasts through symbiotic relationships with prokaryotes.

• 3. Cytoskeleton: Evolution of a dynamic cytoskeleton enabled complex cell shapes, movement, and intracellular transport.

Additional info: Other developments include the evolution of sexual reproduction, which increased genetic diversity, and the development of multicellularity in some lineages.