Overview of Eukaryotes

Endosymbiotic Theory and Evolution of Eukaryotes

The endosymbiotic theory explains the origin of eukaryotic cells as a result of a series of symbiotic relationships between ancestral prokaryotes. This theory proposes that mitochondria and chloroplasts originated from free-living bacteria that were engulfed by a proto-eukaryotic cell, leading to a mutually beneficial relationship.

• Prokaryotes evolved approximately 3.5 billion years ago, while eukaryotes appeared around 2.5 billion years ago.

• Mitochondria are believed to have evolved from engulfed nonphotosynthetic prokaryotes (possibly Rickettsia species).

• Chloroplasts evolved from engulfed photosynthetic prokaryotes (such as cyanobacteria).

• Evidence supporting this theory includes the presence of circular DNA, 70S ribosomes, double membranes, and binary fission-like replication in mitochondria and chloroplasts.

Basic Description of Eukaryotic Cells and Comparison with Prokaryotes

Eukaryotic cells are structurally more complex and larger than prokaryotic cells. They possess a defined nucleus and various membrane-bound organelles.

• Found in plants, animals, fungi, and protists.

• Contain multiple linear chromosomes and larger genomes.

• Prokaryotic cells lack a nucleus and most membrane-bound organelles.

Mitosis vs. Meiosis

Eukaryotic cells can reproduce asexually (mitosis) or sexually (meiosis).

• Mitosis: Produces two genetically identical daughter cells, maintaining the chromosome number of the parent cell.

• Meiosis: Involves two sequential divisions, resulting in four haploid gametes with genetic variation due to crossing over.

Main Transport Mechanisms in Eukaryotic Cells

Eukaryotic cells use specialized mechanisms for importing and exporting substances:

• Endocytosis: Uptake of materials via vesicle formation. Includes pinocytosis (liquids), phagocytosis (solids), and receptor-mediated endocytosis (specific ligands).

• Exocytosis: Export of materials by vesicle fusion with the plasma membrane.

Classification of Eukaryotes

The Four Kingdoms of Eukaryotes

Eukaryotic organisms are classified into four kingdoms: Animals, Plants, Fungi, and Protists.

• Animals: Multicellular, non-photosynthetic, obtain nutrients from organic sources. Includes parasitic worms (helminths) and arthropods.

• Plants: Multicellular, photosynthetic, produce organic carbon from light energy.

• Fungi: Mostly multicellular (except yeasts), non-photosynthetic, absorb nutrients, include pathogens and saprobes.

• Protists: Diverse group, can be unicellular, multicellular, or multinucleated; includes algae, slime molds, and protozoans.

Parasitic Helminths

Helminths are parasitic worms with complex life cycles, often infecting humans in microscopic forms. Major groups include roundworms (nematodes) and flatworms (cestodes and trematodes).

Fungi: Structure and Classification

Fungi are classified based on their hyphal structure and spore type.

• Hyphae: Tubular structures; can be septate (with divisions) or aseptate (without divisions).

• Dimorphic fungi: Can switch between hyphal and yeast-like forms.

• Spores: Asexual (conidiospores, sporangiospores) or sexual (zygospores, ascospores, basidiospores).

Fungal Diseases (Mycoses)

Mycoses are diseases caused by fungi, often affecting immunocompromised individuals or those with disrupted microbiota. Dermatophytes cause skin, hair, and nail infections (tinea).

Protists and Protozoans

Protists are a diverse group, including algae, slime molds, and protozoans. Protozoans are classified by their motility:

• Amoeboid: Move by pseudopods (e.g., Entamoeba histolytica).

• Flagellated: Move by flagella (e.g., Giardia lamblia).

• Ciliated: Move by cilia (e.g., Tetrahymena thermophila).

• Spore-forming (Apicomplexa): Move by gliding; complex life cycles (e.g., Plasmodium species).

Extracellular Structures of Eukaryotic Cells

Plasma Membrane

All eukaryotic cells have a plasma membrane composed of a phospholipid bilayer with embedded sterols, serving as a selective barrier.

Cell Wall

Some eukaryotes (plants, fungi, certain protists) possess a cell wall external to the plasma membrane, providing structural support and protection. Eukaryotic cell walls lack peptidoglycan.

Glycocalyx

The glycocalyx is a sticky extracellular layer composed of carbohydrates, glycoproteins, and glycolipids, involved in protection, adhesion, and communication.

Flagella and Cilia

Eukaryotic flagella are long, whip-like structures made of tubulin in a 9+2 arrangement, anchored by a basal body, and move in a wave-like motion. Cilia are shorter, more numerous, and move in an oar-like fashion.

Intracellular Structures of Eukaryotic Cells

Ribosomes

Eukaryotic ribosomes (80S) are composed of a 40S small subunit and a 60S large subunit. They can be free in the cytoplasm or bound to the endoplasmic reticulum, synthesizing proteins for various cellular destinations.

Cytoskeleton

The cytoskeleton is a network of protein fibers (microtubules, intermediate filaments, microfilaments) that maintains cell shape, enables movement, and organizes intracellular transport.

• Microtubules: Hollow tubes of tubulin, form the spindle apparatus during cell division, arise from the centrosome.

• Intermediate filaments: Rope-like, provide tensile strength.

• Microfilaments: Thin actin fibers, involved in muscle contraction and cell movement.

Nucleus

The nucleus houses DNA, organized as chromatin, and contains the nucleolus (site of ribosome assembly). It is surrounded by a double-membrane nuclear envelope with pores for molecular traffic.

Endoplasmic Reticulum (ER)

The ER is a network of membranes continuous with the nuclear envelope. The rough ER is studded with ribosomes and modifies proteins, while the smooth ER is involved in lipid synthesis and detoxification.

Golgi Apparatus

The Golgi apparatus consists of stacked cisternae that modify, sort, and package proteins and lipids for delivery to various destinations.

Vesicles and Vacuoles

Vesicles are small, membrane-bound sacs for transport, secretion, and digestion. Lysosomes contain hydrolytic enzymes, peroxisomes detoxify substances, and vacuoles (large in plants and fungi) store water and solutes.

Mitochondria and Chloroplasts

Mitochondria generate ATP, regulate cell death, and have bacterial-like features (double membrane, 70S ribosomes, circular DNA). Chloroplasts, found in photosynthetic cells, convert light energy to chemical energy and have a unique internal structure of thylakoids and grana.

Visual Summary: Introduction to Eukaryotic Cells