Eukaryotic Cells: Structure, Function, and Diversity

Overview of Eukaryotes

Basic Description and Comparison with Prokaryotes

Eukaryotic cells are complex cells characterized by the presence of a true nucleus and membrane-bound organelles. They differ from prokaryotic cells, which lack a nucleus and most organelles. Both cell types share fundamental features such as a plasma membrane, cytosol, DNA as genetic material, and ribosomes for protein synthesis.

• Eukaryotic cells have a nucleus, multiple linear chromosomes, and organelles such as mitochondria and endoplasmic reticulum.

• Prokaryotic cells (e.g., bacteria) have a nucleoid region instead of a nucleus and generally lack internal compartmentalization.

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells. According to this theory, these organelles evolved from free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Mitochondria are thought to have originated from an engulfed non-photosynthetic prokaryote (possibly a Rickettsia species).

• Chloroplasts are believed to have evolved from an engulfed photosynthetic prokaryote (such as cyanobacteria).

Evidence supporting this theory includes the presence of double membranes, their own DNA, and similarities to bacteria in size and reproduction.

Cell Division: Mitosis, Meiosis, and Binary Fission

Eukaryotic cells divide by mitosis (asexual reproduction) or meiosis (sexual reproduction), while prokaryotes reproduce by binary fission.

• Mitosis: Produces two genetically identical diploid cells; used for growth and repair.

• Meiosis: Produces four genetically unique haploid cells; essential for sexual reproduction.

• Binary fission: Prokaryotic asexual reproduction resulting in two identical cells.

Transport Mechanisms in Eukaryotic Cells

Eukaryotic cells use various mechanisms to transport materials across their membranes:

• Endocytosis: Import of substances into the cell via vesicle formation.

• Exocytosis: Export of substances out of the cell via vesicle fusion with the plasma membrane.

• Phagocytosis: "Cell eating"—engulfment of large particles or cells.

• Pinocytosis: "Cell drinking"—uptake of dissolved substances.

• Receptor-mediated endocytosis: Specific uptake of molecules after binding to cell surface receptors.

Classification of Eukaryotes

The Four Kingdoms of Eukaryotes

Eukaryotes are classified into four kingdoms: Animalia, Plantae, Fungi, and Protista.

• Animalia: Multicellular, heterotrophic organisms; includes parasitic helminths.

• Plantae: Multicellular, autotrophic organisms with cell walls containing cellulose.

• Fungi: Absorptive heterotrophs, often with chitinous cell walls; includes molds and yeasts.

• Protista: Diverse group, often unicellular or simple multicellular; includes algae, protozoa, and slime molds.

Parasitic Helminths

Helminths are parasitic worms classified into two main groups: roundworms (nematodes) and flatworms (tapeworms and flukes).

Phylum	Subtypes	Structure	Size	Reproduction	Examples in Humans	Transmission Mechanism
Roundworms	Nematodes	Non-segmented, elongated, cylindrical	Microscopic - 1 meter	Sexual reproduction; two sexes	Pinworm, Ascaris, filarial worms, Trichinella, Strongyloides, whip worm	Fecal-oral (eggs) or contaminated food, soil, or water; larvae can enter via skin or bite into intestinal wall
Flatworms	Tapeworms (cestodes)	Segmented, flat, ribbon-like	1 millimeter - 10 meters	Sexual reproduction; hermaphroditic (male and female organs in one worm)	Tapeworms infect humans (Examples: Diphyllobothrium latum, Taenia species)	Fecal-oral through contaminated food or water; undercooked meat or fish
Flatworms	Flukes (trematodes)	Non-segmented, flattened leaf-shaped	1 millimeter - 7 centimeters	Sexual reproduction; most are hermaphroditic	Blood flukes: Schistosoma species; Lung flukes: Paragonimus species; Liver flukes: Fasciola species	Embryonated eggs from host feces or urine; larvae infect aquatic snails or fish then infect human host

Fungi: Structure and Reproduction

Fungi absorb nutrients from their environment and may be pathogenic or saprobic. Most fungi grow as hyphae, which can be septate (with cross-walls) or aseptate (without cross-walls).

• Asexual spores: Produced by mitosis; include conidiospores and sporangiospores.

• Sexual spores: Produced by meiosis; include zygospores, ascospores, and basidiospores.

Mycoses: Fungal Diseases

Mycoses are diseases caused by fungi. Dermatophytes are true pathogens that infect skin, hair, and nails, causing "tinea" infections.

• Tinea unguium: Fungal infection of the nails, leading to discoloration and brittleness.

• Tinea pedis: Athlete's foot, a common infection causing itching, burning, and scaling of the skin.

Protists and Protozoans

Protists are a diverse group of eukaryotes, including unicellular, multicellular, and multinucleated organisms. They may be autotrophic or heterotrophic, and reproduce sexually or asexually.

• Protozoans: Motile, heterotrophic protists classified by their mode of movement:

  • Amoeboid (pseudopods)

  • Flagellated (flagella)

  • Ciliated (cilia)

  • Spore-forming (non-motile in mature form; e.g., Apicomplexa)

Extracellular Structures of Eukaryotes

Plasma Membrane and Cell Wall

All eukaryotic cells have a plasma membrane, but only some (fungi, plants, certain protists) have a cell wall. Eukaryotic cell walls lack peptidoglycan, which is found in bacterial cell walls.

Glycocalyx

The glycocalyx is a sticky, carbohydrate-rich layer found on the outer surface of most eukaryotic cells. It plays roles in protection, cell recognition, and adhesion.

Flagella and Cilia

Eukaryotic flagella are composed of microtubules arranged in a 9+2 pattern and are anchored by a basal body. They move in a wave-like motion. Cilia are similar in structure but are shorter and more numerous, moving in an oar-like fashion to propel cells or move substances across cell surfaces.

Intracellular Structures of Eukaryotes

Ribosomes

Ribosomes are essential for protein synthesis and are composed of protein and ribosomal RNA (rRNA). Eukaryotic ribosomes (80S) consist of a large (60S) and a small (40S) subunit and may be free in the cytoplasm or bound to the endoplasmic reticulum.

Cytoskeleton

The cytoskeleton provides structural support, facilitates movement, and organizes cellular components. It consists of:

• Microtubules: Made of tubulin; form the spindle apparatus during cell division and serve as intracellular "roadways."

• Intermediate filaments: Provide tensile strength.

• Microfilaments: Made of actin; involved in muscle contraction, cell division, and movement of pseudopodia.

Nucleus

The nucleus houses the cell's DNA, organized as chromatin. It is surrounded by a double-membrane nuclear envelope with pores for molecular transport. The nucleolus within the nucleus is the site of ribosome synthesis.

Endoplasmic Reticulum (ER) and Golgi Apparatus

The ER is a network of membranes involved in protein and lipid synthesis. The rough ER is studded with ribosomes, while the smooth ER is not. The Golgi apparatus modifies, sorts, and packages proteins and lipids for transport.

Vesicles and Vacuoles

Vesicles are small, membrane-bound sacs that transport substances within the cell. Types include:

• Transport vesicles: Move materials within the cell.

• Secretory vesicles: Release substances outside the cell.

• Lysosomes: Contain hydrolytic enzymes for digestion.

• Peroxisomes: Break down fats and amino acids by oxidation.

Vacuoles are large, membrane-bound sacs formed by the fusion of multiple vesicles, primarily for storage.

Mitochondria and Chloroplasts

Mitochondria are the "powerhouses" of the cell, generating ATP through cellular respiration. Chloroplasts, found in plants and algae, harvest energy from sunlight via photosynthesis. Both organelles have double membranes and their own DNA, supporting the endosymbiotic theory.

$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP}$

Equation for ATP production (cellular respiration):

Equation for photosynthesis:

$\mathrm{6CO_2 + 6H_2O + light \ energy \rightarrow C_6H_{12}O_6 + 6O_2}$