Evolution and Classification of Eukaryotes

Endosymbiotic Theory

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 cell cycle, through a process similar to binary fission.

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

• Proof 4: Ribosomes within these organelles resemble prokaryotic ribosomes (70S) rather than eukaryotic ribosomes (80S).

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

Example: The presence of mitochondrial DNA mutations inherited maternally supports the endosymbiotic origin of mitochondria.

Classification: 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 made of chitin. Example: Mushroom (Agaricus)

• Plantae: Multicellular, autotrophic, cell walls made of cellulose, contain chloroplasts. Example: Oak tree (Quercus)

• Animalia: Multicellular, heterotrophic, lack cell walls, complex organ systems. Example: Human (Homo sapiens)

Comparison Table:

Comparison of Eukaryotes and Prokaryotes

Key Differences

• Cellularity: Prokaryotes are 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.

• Organelles: Eukaryotes have membrane-bound organelles (nucleus, mitochondria, etc.); prokaryotes do not.

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

• Ribosomes: Prokaryotic ribosomes are 70S; eukaryotic ribosomes are 80S (except in mitochondria and chloroplasts).

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

Pathogenic Protozoa

Examples and Disease

• Plasmodium spp.: Causes malaria in humans. Transmitted by Anopheles mosquitoes. Symptoms include fever, chills, and anemia.

• Trypanosoma brucei: Causes African sleeping sickness. Transmitted by tsetse flies. Symptoms include fever, headaches, and neurological disorders.

• Giardia lamblia: Causes giardiasis, a diarrheal disease. Transmitted via contaminated water.

Example: Plasmodium falciparum is responsible for the most severe form of malaria.

Structure of Eukaryotic Cells

Intracellular and Extracellular Structures

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

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

• Extracellular structures: Plasma membrane, cell wall (in plants and 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 lysosomes.

Example: The extracellular matrix in animal cells provides structural support and mediates cell signaling.

Cytoskeleton in Eukaryotic Cells

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

• Microtubules: Hollow tubes made of tubulin; involved in cell division, intracellular transport, and cilia/flagella movement.

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

• Intermediate filaments: Provide mechanical strength to cells.

Example: The mitotic spindle, composed of microtubules, separates chromosomes during cell division.

Structure and Function of Eukaryotic Organelles

Major 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 and detoxifies chemicals.

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

• Lysosomes: Contain hydrolytic enzymes for intracellular digestion.

• Vacuole: Stores nutrients and waste products; large central vacuole in plant cells maintains turgor pressure.

Example: Mitochondria are often called the "powerhouse of the cell" due to their role in energy production.

Structure and Function of Eukaryotic Cell Structures

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

• Cell Wall: Provides structural support and protection; found in plants (cellulose), fungi (chitin), and some protists.

• Glycocalyx: Carbohydrate-rich layer on the cell surface; involved in cell recognition and protection.

• Flagella: Long, whip-like structures for cell movement; composed of microtubules in a 9+2 arrangement.

• Cilia: Short, hair-like structures for movement or moving substances along the cell surface.

Example: The flagella of Euglena enable it to swim toward light sources.

Evolutionary Developments in Eukaryotic Cells

Key Innovations

• Compartmentalization: Development of membrane-bound organelles allowed for specialized cellular functions.

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

• Cytoskeleton: Provided structural support and enabled complex cell shapes and movements.

Example: The evolution of the nucleus separated transcription from translation, allowing for more complex gene regulation.

Additional info: Some details, such as the specific structure of the cytoskeleton and the comparison table, were expanded for academic completeness.