Introduction to Eukaryotic Cells: Structure, Function, and Diversity

Chapter 4 – Introduction to Eukaryotic Cells

I. The Endosymbiotic Theory: Evolution of Eukaryotes

The endosymbiotic theory explains the evolutionary origin of eukaryotic cells from ancestral prokaryotic organisms. This theory is supported by multiple lines of evidence and is fundamental to understanding the complexity of eukaryotic cells.

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

• The term "endosymbiotic" refers to a symbiotic relationship where one organism lives inside another.

• Eukaryotic cells are thought to have arisen from a series of cell-merging events between ancestral eukaryotes and certain prokaryotes.

• Mitochondria are believed to have evolved from engulfed non-photosynthetic prokaryotes.

• Chloroplasts are believed to have evolved from engulfed photosynthetic prokaryotes, such as cyanobacteria.

Evidence supporting the endosymbiotic theory:

• Mitochondria and chloroplasts possess their own circular DNA.

• They contain 70S ribosomes, similar to those found in bacteria.

• Both organelles have double-membrane structures.

• They are similar in size to bacteria.

• They replicate independently of the cell, using a process similar to binary fission.

• Their genes closely resemble those of certain bacteria.

II. Eukaryotic Cell Structures, Division, and Transport

Eukaryotic cells are structurally and functionally more complex than prokaryotic cells, with distinct features in their organelles, genetic material, and cellular processes.

A. General Features of Eukaryotic Cells

• Include plants, animals, protists, and fungi.

• Larger and more complex than prokaryotic cells.

• Contain multiple, linear chromosomes (prokaryotes have a single circular chromosome).

• Possess a defined nucleus and various membrane-bound organelles (e.g., mitochondria, chloroplasts).

B. Cell Division in Eukaryotes

• Eukaryotic cells can reproduce sexually and/or asexually.

• Cell division is more complex and time-consuming due to larger genomes and organelles.

• Prokaryotes divide by binary fission; eukaryotes use mitosis and meiosis:

  • Mitosis: Produces two genetically identical daughter cells, each with the same chromosome number as the parent cell.

  • Meiosis: Involved in sexual reproduction; produces four genetically diverse gametes (haploid cells) through two successive divisions. Crossing over during meiosis allows for genetic recombination.

C. Cellular Transport: Endocytosis and Exocytosis

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

  • Pinocytosis: Uptake of dissolved substances (small solutes) in small vesicles.

  • Phagocytosis: Uptake of large particles or cells (e.g., bacteria); performed by specialized cells called phagocytes.

  • Receptor-mediated endocytosis: Specific ligands (e.g., hormones, nutrients, pathogens) bind to cell-surface receptors to trigger uptake.

• Exocytosis: Export of substances from the cell via vesicles fusing with the plasma membrane.

III. The Four Kingdoms of Eukaryotes

Eukaryotic organisms are classified into four major kingdoms: Animals, Plants, Fungi, and Protists. Each kingdom exhibits unique characteristics and biological roles.

A. Animals

• Multicellular, heterotrophic organisms (obtain organic carbon from nutrients).

• Do not perform photosynthesis.

• Include parasitic worms (helminths) and arthropods.

• Helminths: Parasitic worms (e.g., roundworms, flatworms) with complex life cycles; often spread in microscopic forms. WHO estimates that half the world's population is infected with some type of helminth.

B. Plants

• Multicellular, autotrophic organisms (perform photosynthesis using chloroplasts).

• Over 290,000 species.

• Produce organic molecules from light energy.

• Vegetation can serve as a vehicle for infectious pathogens.

C. Fungi

• Over 600,000 species; most are multicellular, but yeasts are unicellular.

• Heterotrophic; absorb nutrients from their environment.

• Include pathogens and saprobes (organisms that digest dead matter).

• Fungi grow as tubular structures called hyphae:

  • Septate hyphae: Have divisions between cells.

  • Aseptate (coenocytic) hyphae: Lack divisions, forming continuous chains with many nuclei.

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

• Fungal spores: Reproductive structures used for classification; can be asexual (from mitosis, e.g., conidiospores, sporangiospores) or sexual (from meiosis, e.g., zygospores, ascospores, basidiospores).

• Fungal diseases (mycoses): Often affect immunocompromised individuals or those with disrupted microbiota (e.g., Candida infections). Some fungi are true pathogens (e.g., Histoplasma, Coccidioides).

• Dermatophytes: Infect skin, hair, and nails; cause "tinea" infections.

• Some fungi produce mycotoxins (e.g., ergot toxin from Claviceps purpurea), which can cause severe health effects.

D. Protists

• Diverse group; can be unicellular, multicellular, or multinucleated masses.

• May be autotrophic or heterotrophic; reproduce sexually or asexually; may or may not have cell walls.

• Examples include:

  • Algae: Plant-like protists.

  • Slime molds: Soil-dwelling amoebae, originally classified as fungi.

  • Protozoans: Animal-like protists; include many pathogens.

• Protozoans are classified by their mode of motility:

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

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

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

  • Spore-forming (Apicomplexa): Move by gliding; obligate intracellular parasites (e.g., Plasmodium causes malaria, Toxoplasma gondii causes toxoplasmosis).

IV. Eukaryotic Cell Structure – External Structures

Eukaryotic cells possess various external structures that provide protection, facilitate movement, and mediate interactions with the environment.

A. Plasma Membrane

• All eukaryotes have a plasma membrane composed of a phospholipid bilayer.

• Acts as a selective, semipermeable barrier.

• Contains sterols (e.g., cholesterol) for membrane stability.

B. Cell Wall

• Present in fungi, plants, and some protists; absent in animals.

• Located external to the plasma membrane; provides shape and protection against mechanical and osmotic stress.

• Composition varies:

  • Fungi: Chitin (polysaccharide)

  • Plants: Cellulose

  • Protists: Variable

• Note: Eukaryotic cell walls lack peptidoglycan (found only in bacteria).

C. Structures for Protection, Adhesion, and Movement

• Glycocalyx: Sticky extracellular layer composed of carbohydrates, glycoproteins, and glycolipids; aids in protection and adhesion.

• Flagella: Long, whip-like structures for motility; move with a wavelike motion.

• Cilia: Short, numerous projections for movement and feeding; structurally similar to flagella but shorter and more abundant.

V. Intracellular Structures of Eukaryotic Cells

Eukaryotic cells contain a variety of specialized organelles that perform essential cellular functions.

1. Ribosomes

• Sites of protein synthesis; composed of protein and ribosomal RNA (rRNA).

• Eukaryotic ribosomes are 80S (40S small subunit + 60S large subunit); prokaryotic ribosomes are 70S.

• Can be free in the cytoplasm (produce cytosolic proteins) or bound to the endoplasmic reticulum (produce secreted or membrane proteins).

• Mitochondria and chloroplasts contain 70S ribosomes, supporting the endosymbiotic theory.

2. Cytoskeleton

• Dynamic network of protein fibers that maintains cell shape, facilitates movement, protects against external forces, directs transport, and coordinates cell division.

• Composed of three main types of fibers:

  • Microtubules

  • Intermediate filaments

  • Microfilaments

3. Nucleus

• Houses the cell's DNA, organized as chromatin in the nucleoplasm.

• Contains the nucleolus, where ribosomal subunits are assembled.

• Enclosed by a double-membrane nuclear envelope with nuclear pores for molecular transport.

• Endoplasmic reticulum is continuous with the nuclear envelope at certain sites.

4. Endoplasmic Reticulum (ER)

• Network of membranes originating from the nuclear envelope; involved in protein and lipid synthesis.

• Rough ER: Studded with ribosomes; synthesizes and modifies proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies substances.

5. Golgi Apparatus

• Stack of flattened sacs (cisternae); modifies, sorts, and distributes proteins and lipids received from the ER.

6. Vesicles and Vacuoles

• Vesicles: Small, membrane-bound sacs for transport, secretion, and digestion.

• Types of vesicles:

  • Transport vesicles: Move substances within the cell.

  • Secretory vesicles: Deliver materials to the cell surface for exocytosis.

  • Lysosomes: Contain hydrolytic enzymes for breaking down macromolecules and damaged organelles.

  • Peroxisomes: Contain enzymes for oxidation of fats and amino acids; detoxify harmful substances.

• Vacuoles: Large, membrane-bound sacs (formed from merged vesicles); store water, nutrients, and waste; common in plants and fungi. Some protists have contractile vacuoles for osmoregulation.

7. Mitochondria and Chloroplasts

• Mitochondria: "Powerhouse" of the cell; generate ATP, synthesize amino acids and vitamins, regulate cell division and apoptosis, and play roles in disease and aging.

• Structure includes a smooth outer membrane, inner membrane with cristae, and matrix.

• Number of mitochondria varies by cell type; active cells have more mitochondria.

• Most eukaryotic cells have mitochondria; only photosynthetic cells have chloroplasts.

• Chloroplasts: Found in photosynthetic cells; harvest energy from sunlight using pigments. Structure includes inner and outer membranes, thylakoids (stacked into grana), and stroma.

Table: Comparison of Eukaryotic and Prokaryotic Cells

Feature	Eukaryotic Cells	Prokaryotic Cells
Nucleus	Present (membrane-bound)	Absent
Chromosomes	Multiple, linear	Single, circular
Membrane-bound organelles	Present (e.g., mitochondria, ER, Golgi)	Absent
Ribosome size	80S (cytoplasm), 70S (mitochondria/chloroplasts)	70S
Cell division	Mitosis and meiosis	Binary fission
Cell wall composition	Cellulose (plants), chitin (fungi), variable (protists), absent (animals)	Peptidoglycan (bacteria)

Key Terms and Definitions

• Endosymbiosis: A symbiotic relationship where one organism lives inside another.

• Heterotroph: Organism that obtains carbon from organic molecules.

• Autotroph: Organism that produces its own food from inorganic sources (e.g., via photosynthesis).

• Mycoses: Diseases caused by fungi.

• Dimorphic: Ability to exist in two different forms (e.g., yeast and hyphal forms in fungi).

• Phagocytosis: Cellular process of engulfing large particles or cells.

• Osmoregulation: Regulation of water and solute balance within a cell.

Example: Plasmodium species, the causative agents of malaria, are spore-forming protozoans (Apicomplexa) that require a mosquito vector and human host to complete their life cycle.

Additional info: The presence of 70S ribosomes in mitochondria and chloroplasts, as well as their independent replication, provides strong molecular evidence for the endosymbiotic origin of these organelles.