Introduction to Eukaryotic Cells and Their Diversity

Endosymbiotic Theory

Origin of Eukaryotic Cells

The endosymbiotic theory explains how eukaryotic cells evolved from prokaryotic ancestors through symbiotic relationships. This theory is central to understanding the complexity and diversity of modern eukaryotes.

• Prokaryotes appeared approximately 3.5 billion years ago; eukaryotic cells emerged around 2.5 billion years ago.

• The term "endosymbiotic" combines "endo" (inside) and "symbiotic" (living together for mutual benefit).

• Mitochondria are believed to have originated from engulfed nonphotosynthetic prokaryotes.

• Chloroplasts likely evolved from engulfed photosynthetic prokaryotes.

Evidence supporting the endosymbiotic theory:

• Mitochondria and chloroplasts contain circular DNA, similar to bacterial genomes.

• They possess 70S ribosomes, like those found in bacteria.

• They are similar in size to bacteria.

• They replicate by binary fission, not mitosis.

• Their genes closely resemble those of certain bacteria.

Basics of Eukaryotic Cells

General Characteristics

Eukaryotic cells are structurally more complex and larger than prokaryotic cells. They form the basis of plants, animals, fungi, and protists.

• Possess a defined nucleus surrounded by a nuclear envelope.

• Contain membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).

• Have multiple linear chromosomes and larger genomes.

Cell Division in Eukaryotes

Eukaryotic cells reproduce by mitosis (asexual) and/or meiosis (sexual). Cell division is more complex in eukaryotes due to their larger genomes and organelles.

• Mitosis: Produces two genetically identical diploid cells from one diploid parent cell. Involves five phases and maintains chromosome number.

• Meiosis: Involves two sequential divisions, resulting in four genetically diverse haploid gametes from one diploid germ cell. Includes crossing over for genetic recombination.

Key stages of meiosis:

• Starts with a diploid germ cell (2n).

• Meiosis I (reductional division): Homologous chromosomes separate, reducing chromosome number.

• Meiosis II (equational division): Sister chromatids separate, maintaining ploidy level.

• DNA replication occurs during interphase before meiosis begins.

Formulas:

• Mitosis: $\text{1 diploid cell} \rightarrow \text{2 diploid cells}$

• Meiosis: $\text{1 diploid cell} \rightarrow \text{4 haploid cells}$

Eukaryotic Cell Transport

Endocytosis and Exocytosis

Eukaryotic cells transport large biomolecules across their membranes using specialized processes.

• Endocytosis: Imports substances into the cell by engulfing them with the plasma membrane.

• Exocytosis: Exports substances out of the cell via vesicles that fuse with the plasma membrane.

Types of Endocytosis:

• Pinocytosis ("cell drinking"): Uptake of dissolved substances in small vesicles.

• Phagocytosis ("cell eating"): Uptake of large, undissolved particles.

• Receptor-mediated endocytosis: Specific uptake of molecules via receptor proteins on the cell surface.

Exocytosis: Vesicles deliver their contents to the plasma membrane for release outside the cell.

Types of Eukaryotes

Animals

• Multicellular, non-photosynthetic organisms.

• Obtain organic carbon from nutrients.

• Include parasitic worms (helminths) and arthropods.

• Estimated ~7.5 million animal species.

Helminths

• Parasitic worms (e.g., roundworms, flatworms).

• Complex life cycles; often spread in microscopic form.

• WHO estimates half the world’s population is infected with some type of helminth.

Plants

• Multicellular organisms with chloroplasts for photosynthesis.

• Over 290,000 species.

• Vegetation can serve as a vehicle for infectious pathogens.

Fungi

• Over 600,000 species; includes molds (multicellular) and yeasts (unicellular).

• Do not perform photosynthesis; absorb nutrients from the environment.

• Include pathogens and saprobes (decomposers).

• Most fungi grow as tubular structures called hyphae.

• Dimorphic fungi can switch between hyphal and yeast-like forms; many are pathogenic.

Fungal Spores

• Asexual spores: Produced by mitosis (no genetic variation); includes conidiospores and sporangiospores.

• Sexual spores: Produced by meiosis (genetic variation); includes zygospores, ascospores, and basidiospores.

Diseases caused by fungi are called mycoses. Most occur in immunocompromised individuals or those with disrupted microbiota (e.g., Candida infections). Some fungi are true pathogens (e.g., Histoplasma, Coccidioides).

• Dermatophytes: True pathogens infecting skin, hair, and nails; cause "tinea" infections.

• Some fungi produce allergens or mycotoxins (e.g., Claviceps purpurea produces ergot toxin, a potent neurotoxin).

Protists

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

• Can be autotrophs or heterotrophs; reproduce sexually or asexually.

• Includes algae, slime molds, and protozoans.

Protozoans

• Single-celled eukaryotes; many are human and animal pathogens.

• Grouped by motility:

  • Amoeboid: Move via pseudopods (e.g., Naegleria fowleri, Acanthamoeba, Entamoeba histolytica).

  • Flagellated: Move via one or more flagella (e.g., Trichomonas vaginalis, Trypanosoma spp., Giardia lamblia).

  • Ciliated: Move via cilia (not detailed in the notes).

  • Spore-forming (Apicomplexa): Move by gliding; most are obligate intracellular parasites with complex life cycles (e.g., Toxoplasma, Plasmodium [malaria], Cryptosporidium).

Apicomplexan life cycle stages:

• Merogony: Asexual reproduction producing merozoites.

• Gamogony: Sexual phase producing gametes.

• Sporogony: Zygote divides to form sporozoites.

Eukaryotic Cell Structure

Plasma Membrane and Cell Wall

• All cells have a plasma membrane (phospholipid bilayer) acting as a selective barrier.

• Contains sterols (hydrophobic, ring-structured compounds) for membrane stability.

• Animal cells: main sterol is cholesterol.

• Plant cells: contain phytosterols and low levels of cholesterol.

• Some eukaryotes (fungi, plants, certain protists) have a cell wall (no peptidoglycan) for shape and protection.

• Most eukaryotes have a glycocalyx (outermost sticky layer of carbohydrates, glycoproteins, glycolipids).

Motility Structures

• Flagella: Long, tail-like structures made of tubulin (9+2 arrangement), anchored by a basal body; move in a wavelike motion.

• Cilia: Shorter, more numerous than flagella; similar structure and function.

Table: Comparison of Eukaryotic Cell Features

Feature	Eukaryotes	Prokaryotes
Nucleus	Present	Absent
Membrane-bound organelles	Present	Absent
Chromosomes	Multiple, linear	Single, circular
Cell division	Mitosis and meiosis	Binary fission
Cell wall composition	Cellulose (plants), chitin (fungi), various (protists); no peptidoglycan	Peptidoglycan (bacteria)
Ribosome size	80S (cytoplasm), 70S (mitochondria/chloroplasts)	70S

Example: Plasmodium (malaria parasite) is an apicomplexan protozoan with a complex life cycle involving both asexual and sexual reproduction in different hosts.

Additional info: Some content (e.g., ciliated protozoans, details of certain life cycles) was inferred or expanded for completeness.