Evolution and Classification of Eukaryotes

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.

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

• Proof 2: These organelles replicate independently of the host cell through a process similar to binary fission.

• Proof 3: Double membranes surround mitochondria and chloroplasts, consistent with engulfment.

• Proof 4: Ribosomes within mitochondria and chloroplasts resemble those of bacteria (70S type) rather than eukaryotic cytoplasmic ribosomes (80S type).

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

Example: The presence of mitochondrial DNA mutations inherited maternally supports the theory.

The Four Kingdoms of Eukarya

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

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

• Fungi: Mostly multicellular (except yeasts), cell walls of chitin; examples: Aspergillus, Saccharomyces cerevisiae.

• Plantae: Multicellular, cell walls of cellulose, chloroplasts for photosynthesis; examples: Arabidopsis thaliana, Zea mays (corn).

• Animalia: Multicellular, no cell walls, heterotrophic; examples: Homo sapiens, Caenorhabditis elegans.

Comparison Table:

Comparative Cell Biology

Prokaryotes vs. Eukaryotes

Prokaryotic and eukaryotic cells differ in several fundamental ways:

• Cellularity: Prokaryotes are unicellular; eukaryotes can be unicellular or multicellular.

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

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

• Cell Wall: Present in most prokaryotes (peptidoglycan in bacteria), variable in eukaryotes (cellulose in plants, chitin in fungi, absent in animals).

• 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 (e.g., mitochondria, ER, Golgi apparatus).

Example: Escherichia coli (prokaryote) vs. Saccharomyces cerevisiae (eukaryote).

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 characterized by fever, chills, and anemia, transmitted by the bite of infected Anopheles mosquitoes.

Other examples include Giardia lamblia (giardiasis) and Trypanosoma brucei (African sleeping sickness).

Cell Structure and Organization

Animal vs. Plant Cell Structure

Animal and plant cells share many features but also have distinct differences, especially in their intracellular and extracellular structures.

• Intracellular Structures: Both have nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and cytoskeleton.

• Extracellular Structures: Plant cells have a rigid cell wall (cellulose) and large central vacuole; animal cells lack a cell wall and have smaller vacuoles.

• Chloroplasts: Present in plant cells for photosynthesis; absent in animal cells.

Example: The presence of chloroplasts allows plants to perform photosynthesis, a function not found in animal cells.

Cytoskeleton in Eukaryotic Cells

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

• Components: Microtubules, microfilaments (actin filaments), and intermediate filaments.

• Functions: Maintains cell shape, enables intracellular transport, and is involved in cell division and motility.

Eukaryotic Cell Organelles

Structure and Function of Key Organelles

• Nucleus: Contains genetic material (DNA), controls gene expression and cell division.

• Mitochondria: Site of aerobic respiration and ATP production; known as the "powerhouse" of the cell.

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

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes for breaking down waste materials and cellular debris.

• Vacuole: Large central vacuole in plants stores water, nutrients, and waste products; smaller vacuoles in animal cells.

Specialized Eukaryotic Cell Structures

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

• Cell Wall: Provides structural support in plants, fungi, and some protists.

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

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

Evolutionary Developments in Eukaryotes

Key Innovations Enabling Eukaryotic Cell Emergence

The emergence of eukaryotic cells involved several evolutionary innovations:

• 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: Enabled complex cell shapes, intracellular transport, and motility.

• Additional info: Other developments include sexual reproduction (increasing genetic diversity) and multicellularity (allowing for specialized tissues and larger organisms).