Introduction to Eukaryotic Cells

Endosymbiotic Theory and Evolution of Eukaryotes

The endosymbiotic theory explains the origin of eukaryotic cells as a result of a symbiotic relationship between ancestral prokaryotic cells. This theory is supported by several lines of evidence, particularly regarding mitochondria and chloroplasts.

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

• Eukaryotic cells likely arose from a series of cell-merging events between an ancient eukaryotic ancestor and certain prokaryotes.

• Evidence: Mitochondria and chloroplasts have their own circular DNA, 70S ribosomes, double-membrane structures, similar size to bacteria, replicate by binary fission, and possess genes resembling bacterial genes.

Basic Description of Eukaryotic Cells and Comparison with Prokaryotes

Eukaryotic cells are structurally more complex than prokaryotic cells and possess membrane-bound organelles, including a nucleus.

• Eukaryotes: Unicellular (protists, yeast) or multicellular (animals, plants, fungi).

• Prokaryotes: Unicellular archaea and bacteria.

• Eukaryotic cells are usually larger and contain a nucleus, while prokaryotic cells lack a nucleus and most membrane-bound organelles.

Classification and Diversity of Eukaryotes

The Four Kingdoms of Eukaryotes

Eukaryotic organisms are classified into four kingdoms: Animalia, Fungi, Protista, and Plantae. Each kingdom has unique characteristics regarding cell structure, reproduction, and ecological roles.

• Animalia: Multicellular, do not carry out photosynthesis, obtain organic carbon from nutrients, include parasitic worms (helminths) and arthropods.

• Fungi: Yeasts, molds, mushrooms; unicellular or multicellular; cell walls contain chitin; absorb nutrients from the environment.

• Protista: Diverse group including algae, protozoa, slime molds; unicellular or multicellular; some have cell walls made of cellulose or other materials.

• Plantae: Multicellular, photosynthetic, cell walls contain cellulose (not detailed in the provided materials).

Animals and Helminths

Animals are multicellular, chemoheterotrophic organisms that do not perform photosynthesis. Helminths are parasitic worms with complex life cycles and organ systems.

• Helminths include roundworms (nematodes), tapeworms (cestodes), and flukes (trematodes).

• They have complex organ systems and life cycles involving eggs, larvae, and adult worms.

• Helminth infections are common worldwide, especially in areas with poor sanitation.

Fungi: Structure and Reproduction

Fungal Morphologies

Fungi exist in two main morphologies: yeast (unicellular) and molds (multicellular, composed of hyphae). Some fungi are dimorphic, exhibiting both forms depending on environmental conditions.

• Yeast: Unicellular, reproduce by budding.

• Molds: Multicellular, composed of hyphae (branching filaments).

• Dimorphic fungi: Can switch between yeast and mold forms; many are pathogenic.

Hyphae Structure

Hyphae are tubular structures that make up the body of multicellular fungi. They can be septate (with cross-walls) or aseptate (without cross-walls).

Fungal Spores

• Asexual spores:

  • Conidiospores: Chains of spores not enclosed in a sac (e.g., Penicillium, Aspergillus).

  • Sporangiospores: Spores enclosed in a sporangium (sac) (e.g., Absidia).

• Sexual spores:

  • Zygospores

  • Ascospores

  • Basidiospores

Mycoses

Mycoses are fungal diseases. Most are opportunistic, affecting immunocompromised individuals or those with disrupted microbiota. Some fungi are true pathogens (e.g., dermatophytes causing ringworm).

Protists and Protozoans

Protista: The Catchall Kingdom

Protista is a diverse kingdom that includes organisms with various means of motility and nutritional strategies. It is sometimes called a "catchall" kingdom due to its diversity.

Protozoan Classification

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

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

• Ciliated: Move using cilia (e.g., Balantidium coli).

• Spore-forming: Intracellular, form spores (e.g., Plasmodium species causing malaria).

Eukaryotic Cell Structure and Function

Plasma Membrane

The eukaryotic plasma membrane is a phospholipid bilayer with embedded proteins and sterols (e.g., cholesterol) that stabilize the membrane. It is selectively permeable and contains proteins that act as receptors and transporters.

• Unlike prokaryotes, ATP synthesis does not occur at the plasma membrane in eukaryotes.

Cell Walls in Eukaryotes

Not all eukaryotes have cell walls. When present, cell walls are external to the plasma membrane, help maintain cell shape, and protect against stress. Composition varies:

• Fungi: Chitin

• Plants: Cellulose

• Some protists: Cellulose or other substances

• Never contain peptidoglycan

Transport Mechanisms

Eukaryotic cells use passive (diffusion, osmosis) and active (energy-requiring) transport. Unique to eukaryotes are endocytosis and exocytosis, which move bulk materials across the membrane.

• Endocytosis: Uptake of substances into the cell; includes phagocytosis (solids), pinocytosis (liquids), and receptor-mediated endocytosis.

• Exocytosis: Release of substances from the cell.

Glycocalyx

The glycocalyx is a sticky extracellular layer composed of carbohydrates, glycoproteins, and glycolipids. It promotes or prevents adhesion, aids in biofilm formation, and facilitates cell communication.

Flagella and Cilia

• Flagella: Used for motility, built from protein, move in a wave-like motion. Eukaryotic flagella are structurally different from prokaryotic flagella.

• Cilia: Similar structure to flagella but shorter and more numerous; function in movement and moving substances along cell surfaces.

Ribosomes

Eukaryotic ribosomes are larger (80S) than prokaryotic ribosomes (70S) and are found free in the cytoplasm or bound to the rough endoplasmic reticulum. They are the site of protein synthesis.

Cytoskeleton

The cytoskeleton is composed of microtubules, intermediate filaments, and microfilaments. It maintains cell shape, facilitates movement, protects against external forces, directs transport, and coordinates cell division.

Nucleus

The nucleus is the control center of the cell, housing DNA. It consists of:

• Chromatin: DNA and associated proteins

• Nucleolus: Site of ribosome synthesis

• Nuclear envelope: Double membrane surrounding the nucleus

• Nuclear pores: Openings that regulate traffic in and out of the nucleus

Endoplasmic Reticulum (ER)

• Rough ER (RER): Studded with ribosomes; modifies proteins.

• Smooth ER: Lacks ribosomes; involved in lipid production, detoxification, and storage.

Golgi Apparatus

The Golgi apparatus is a stack of flattened sacs that receives vesicles from the ER, modifies proteins, builds lipids, sorts, and distributes cellular products.

Vesicles and Vacuoles

• Vesicles: Membrane-bound sacs for transport and storage.

• Lysosomes: Contain digestive enzymes.

• Peroxisomes: Contain enzymes for breaking down fats and amino acids.

• Vacuoles: Large sacs, mainly for water storage; common in plants, fungi, and some protists.

Mitochondria

Mitochondria are the "powerhouse" of the cell, generating ATP and regulating cell division. They have a double membrane, their own DNA, and are present in variable numbers depending on cell type.

Summary of Eukaryotic Cell Structures

Tables

Table: Eukaryotic versus Prokaryotic Cells

Table: Summary of Eukaryotic Organisms

Table: Medical and Structural Features of Eukaryotic Kingdoms

Table: Groups of Parasitic Worms