Introduction to Eukaryotic Cells

Overview of Eukaryotic Cell Structure

Eukaryotic cells are characterized by their complex internal organization, including a defined nucleus and various membrane-bound organelles. These cells are found in plants, animals, fungi, and protists.

• Nucleus: Contains the cell's genetic material (DNA) and is surrounded by a nuclear envelope.

• Organelles: Includes mitochondria, lysosomes, Golgi apparatus, endoplasmic reticulum (smooth and rough), peroxisomes, and centrosomes.

• Plasma Membrane: Phospholipid bilayer that acts as a selective barrier.

• Cytoskeleton: Provides structural support and facilitates cell movement.

• Flagellum and Cilia: Structures for motility; flagella are longer and fewer, cilia are shorter and more numerous.

Endosymbiotic Theory

The endosymbiotic theory explains the evolutionary origin of mitochondria and chloroplasts in eukaryotic cells. It proposes that these organelles originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Mitochondria: Evolved from engulfed nonphotosynthetic prokaryotes.

• Chloroplasts: Evolved from engulfed photosynthetic prokaryotes (e.g., cyanobacteria).

• Evidence:

  • Own circular DNA

  • 70S ribosomes (similar to bacteria)

  • Double-membrane structures

  • Similar size to bacteria

  • Replicate by binary fission

  • Genes resembling bacterial genes

Comparison of Eukaryotic and Prokaryotic Cells

Eukaryotic cells differ from prokaryotic cells in several key aspects, including size, complexity, and genetic organization.

Membrane-Bound Organelles

Key Organelles and Their Functions

Eukaryotic cells contain specialized organelles that perform distinct cellular functions.

• Nucleus: Stores genetic information and coordinates cell activities.

• Mitochondria: Site of cellular respiration and energy production.

• Chloroplasts: Site of photosynthesis in plant cells.

• Endoplasmic Reticulum (ER):

  • Rough ER: Protein synthesis and processing.

  • Smooth ER: Lipid synthesis and detoxification.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain hydrolytic enzymes for intracellular digestion.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

Cell Cycle and Division in Eukaryotes

Cell Cycle

The eukaryotic cell cycle includes growth, DNA synthesis, and division. Eukaryotes can reproduce sexually or asexually.

• Genome Replication: DNA must be copied before cell division.

• Longer Division Time: Due to larger genome and organelles.

Mitosis

Mitosis is the process by which eukaryotic cells divide to produce two genetically identical daughter cells.

• Maintains chromosome number

• Used for growth, repair, and asexual reproduction

Meiosis

Meiosis is involved in sexual reproduction and results in four genetically unique haploid gametes.

• Consists of two cell divisions

• Crossing over allows for genetic recombination

• Gametes are haploid

Comparison Table: Mitosis, Meiosis, Binary Fission

Eukaryotic Cell Transport: Endocytosis and Exocytosis

Endocytosis

Endocytosis is the process by which cells import substances by engulfing them in vesicles.

• Pinocytosis: Uptake of dissolved substances ('cell drinking').

• Phagocytosis: Uptake of undissolved substances ('cell eating').

• Receptor-mediated endocytosis: Specific uptake via cell-surface receptors and clathrin-coated vesicles.

Phagocytosis

• Phagocytes: Specialized immune cells (e.g., macrophages).

• Engulf target to form a phagosome.

• Phagosome fuses with lysosome to form phagolysosome.

• Hydrolytic enzymes destroy contents; waste expelled.

Receptor-Mediated Endocytosis

• Ligands bind to cell-surface receptors.

• Clathrin-coated pits form and pinch off to create vesicles.

• Vesicles shed clathrin, fuse with endosomes, and sort ligands/receptors.

Exocytosis

• Vesicles deliver contents to the plasma membrane for export.

Diversity of Eukaryotic Organisms

Four Kingdoms of Eukaryotes

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

Animals and Helminths

• Animals: Multicellular, do not photosynthesize, obtain carbon from nutrients.

• Helminths: Parasitic worms (roundworms, flatworms), complex life cycles, often spread in microscopic form.

Table: Overview of Parasitic Helminths

Plants

• Multicellular, photosynthetic (contain chloroplasts).

• Produce organic carbon using light energy.

• Vegetation can transmit infectious pathogens.

Fungi

• Most are multicellular; yeasts are unicellular.

• Do not photosynthesize; absorb nutrients.

• Include pathogens and saprobes.

• Grow as hyphae (septate or aseptate).

• Dimorphic fungi can switch between hyphal and yeast forms.

• Classified by spore type: asexual (conidiospores, sporangiospores) or sexual (zygospores, ascospores, basidiospores).

Table: Fungal Spores

Fungal Diseases (Mycoses)

• Occur in immunocompromised individuals or those with disrupted microbiota.

• True pathogens infect healthy hosts (e.g., Histoplasma, Coccidioides).

• Dermatophytes infect skin, hair, nails (tinea).

• Some fungi produce mycotoxins (e.g., ergot toxin from Claviceps purpurea).

Protists

• Diverse group: unicellular, multicellular, or multinucleated masses.

• Autotrophs (e.g., algae) or heterotrophs (e.g., protozoa).

• Examples: algae, slime molds, protozoans.

Protozoan Classification by Motility

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

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

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

• Spore-forming (Apicomplexa): Move by gliding; complex life cycles (e.g., Toxoplasma gondii, malaria parasites).

Additional info:

• All eukaryotes have a plasma membrane with sterols, and some have cell walls (lacking peptidoglycan).

• Eukaryotic flagella have a 9+2 microtubule arrangement and are membrane-bound.

• Ribosomes in eukaryotes are 80S, composed of 40S and 60S subunits.