Eukaryotes

4.1: Overview of Eukaryotes

  • Endosymbiotic Theory

    • Definition: Proposes how eukaryotes evolved through a series of cell-merging events.

    • Key Terms:

    • Endo: Refers to "inside"

    • Symbiotic: Refers to collaboration between organisms.

    • Events: Involves an ancient eukaryotic ancestor merging with specific prokaryotes.

    • Evolution of Organelles:

    • Mitochondria: Evolved from engulfed non-photosynthetic prokaryotes.

    • Chloroplast: Evolved from engulfed photosynthetic prokaryotes, such as cyanobacteria.

    • Supporting Evidence for the theory includes:

    • Presence of their own circular DNA.

    • Possession of 70S ribosomes (similar to bacteria).

    • Structure: Double-membrane organelles.

    • Size: Similar to bacteria dimensions.

    • Replication process similar to binary fission.

    • Genetic material: Genes resembling certain bacterial genes.

  • Characteristics of Eukaryotic Cells

    • Include organisms such as plants, animals, protists, and fungi.

    • Size: Generally larger and more complex than prokaryotic cells.

    • Genome: Contain larger genomes with multiple linear chromosomes (See Table 4.1).

    • Presence of Membrane-Bound Organelles: Defined nucleus and various other organelles (e.g., mitochondria, chloroplasts).

4.2: Classification of Eukaryotes

  • Eukaryotic Organisms classed into four kingdoms:

    • Animals

    • Helminths:

    • Defined as parasitic worms, including roundworms and flatworms.

    • Exhibit complex life cycles and are usually spread in microscopic forms.

    • WHO estimates that half the world's population is infected with helminths.

    • Fungi:

    • Comprises >600,000 different species.

    • Most are multicellular (e.g., molds, mushrooms); however, yeasts are unicellular.

    • Nutritional Mode: Do not perform photosynthesis and instead absorb nutrients from their environment, categorized as heterotrophs.

    • Include both pathogens and saprobes, which absorb nutrients from dead organisms.

    • Hyphae: Growth form of most fungi, which are collections of tubular/filamentous structures that may hold resistant reproductive spores.

    • Mycelia: Mass of hyphae.

    • Dimorphic Fungi: These fungi transition between a hyphal form and a yeast-like form; many pathogenic fungi belong to this category.

    • Fungal Spores: Important for classification, can be asexual or sexual in origin. Can be suspended within hyphae and may cause allergy or illness upon inhalation or ingestion.

    • Fungal Diseases:

      • Collective term for diseases caused by fungi known as mycoses.

      • Most prevalent in immunocompromised individuals (e.g., pneumocystis pneumonia in AIDS patients, disrupt normal microbiota can lead to infections like yeast infections caused by Candida).

      • Dermatophytes: Considered true pathogens that infect the skin, hair, and nails. Commonly referred to as "tinea" or "ringworm."

  • Protists:

    • A diverse group of eukaryotes that can be unicellular, multicellular, or form multinucleated masses.

    • Nutritional Modes: Can be autotrophs or heterotrophs.

    • Reproductive Modes: Capable of both asexual and sexual reproduction.

    • Cell Structure: May have a cell wall or none.

    • Examples include algae, slime molds, and protozoans.

Protozoans

  • Defined as single-celled organisms that are often free-living in water and soil but can also be responsible for numerous pathogenic infections.

  • Nutritional Mode: Feed on fluids/tissues from their hosts.

  • Life Cycles: Complex, sometimes playing roles in transmission modes.

  • Trophozoite: Refers to active, feeding stage of a protozoan.

  • Cyst: Dormant and resistant stage, protected by a proteinaceous coat.

  • Included pathogens:

    • Toxoplasma gondii

    • Trichomonas vaginalis

    • Naegleria fowleri

    • Giardia spp.

  • Group Classification: Based on means of motility in mature forms, such as amoeboid, flagellated, ciliated, and spore-forming.

Amoeboid Protozoans

  • Use pseudopods for movement.

  • Some are free-living (e.g., Amoeba proteus), while others are pathogenic.

  • Pathogenic Examples:

    • Naegleria fowleri: Causes a rare form of encephalitis.

    • Acanthamoeba: Causes corneal infections that may lead to blindness.

    • Entamoeba histolytica: Most common amoeboid infection in humans, causing amoebic dysentery through ingestion of contaminated food or water containing cysts.

Flagellated Protozoans

  • Utilize flagella for motility.

  • Some significant parasitic species:

    • Trichomonas vaginalis: An STD in humans.

    • Trypanosoma spp.: Responsible for African sleeping sickness.

    • Giardia lamblia: Causes a diarrheal disease.

Ciliated Protozoans

  • Move via cilia and are commonly found in aquatic environments.

  • Notable example:

    • Balantidium coli: The only ciliated protozoan known to cause human disease, leading to dysentery due to contaminated food or water ingestion.

Spore-Forming Protozoans

  • Movement is typically achieved by gliding or relying on an intermediate host for transmission, such as mosquitoes.

  • Most are obligate intracellular parasites with complex life cycles.

  • Notable pathogens include:

    • Toxoplasmosis gondii: Causes toxoplasmosis, presenting flu-like symptoms or may be asymptomatic, often transmitted by cats.

    • Plasmodium species: Known for causing malaria.

    • Cryptosporidium parvum: Cause of cryptosporidiosis, characterized by intestinal illness.