Chapter 5: A Survey of Eukaryotic Cells and Microorganisms

The History and Evolution of Eukaryotes

• Metabolic Localization: In eukaryotic cells, metabolic processes are localized within specific membrane-bound organelles.

• Endosymbiotic Theory:

  • Definition: The eukaryotic cell evolved when a larger prokaryotic cell engulfed smaller prokaryotic cells.

  • The smaller cells began to live and reproduce inside the larger cell rather than being digested.

• Multicellular Eukaryotes: These organisms evolved when individual cells lost the ability to survive independently and became specialized within a larger organism.

• Key Figures: Andrew H. Knoll and Barry Chess characterize the study of these evolutionary developments.

Survey of Eukaryotic Microbes

• Taxonomic Categories in Microbiology (Table 5.1):

  • Unicellular or a Few Colonial: Protozoa.

  • May be Unicellular, Colonial, or Multicellular: Fungi and Algae.

  • Multicellular (except reproductive stages): Helminths (parasitic worms) and Arthropods (animal vectors of diseases).

Organization of the Eukaryotic Cell

• External Structures:

  • Locomotor Appendages: Flagella and Cilia.

  • Glycocalyx: Capsules and Slimes.

• Boundary of the Cell:

  • Cell wall.

  • Cell/cytoplasmic membrane.

• Internal Organelles and Components:

  • Cytoplasmic matrix.

  • Nucleus: Nuclear envelope, nucleolus, and chromosomes.

  • Organelles: Endoplasmic reticulum, Golgi complex, Mitochondria, and Chloroplasts.

  • Ribosomes.

  • Cytoskeleton: Microtubules, intermediate filaments, and actin filaments.

External Structures: Locomotor Appendages

• Flagella:

  • Structure: A long, sheathed cylinder containing microtubules in a $9+2$ arrangement.

  • Coverage: Covered by an extension of the cell membrane.

  • Comparison: Eukaryotic flagella are $10\times$ thicker than prokaryotic flagella.

  • Movement Patterns: Can whip back and forth, move in a snakelike pattern, twiddle the tip, or lash and grab the substrate to pull the cell.

• Cilia:

  • Structure: Similar to flagella but shorter and more numerous.

  • Distribution: Found only in certain protozoa and certain animal cells.

  • Function: Motility, feeding, and filtering.

External Structures: Glycocalyx and Boundary Layers

• Glycocalyx:

  • Definition: The outermost boundary that comes into direct contact with the environment.

  • Composition: Usually composed of polysaccharides.

  • Forms: Appears as a network of fibers, a slime layer, or a capsule.

  • Functions: Adherence, protection, and signal reception.

• Sub-Glycocalyx Boundaries:

  • Fungi and most algae: Possess a thick, rigid cell wall surrounding the cell membrane.

  • Protozoa, some algae, and all animal cells: Lack a cell wall and possess only a membrane.

• Cell Wall:

  • Function: Provides structural support and shape.

  • Chemical Composition (Fungi): Thick inner layer of polysaccharide fibers composed of chitin or cellulose, with a thin outer layer of mixed polysaccharides.

  • Chemical Composition (Algae): Variable; includes cellulose, pectin, mannans, silicon dioxide, and calcium carbonate.

• Cell/Cytoplasmic Membrane:

  • Composition: Typical bilayer of phospholipids and proteins.

  • Stability: Sterols confer stability to the membrane.

  • Function: Selectively permeable barrier for transport, interaction, surface adhesion, secretion, and signal transduction.

Internal Structures and Their Functions (Table 5.3)

• Nucleus: Genetic center of the cell; repository of DNA; synthesis of RNA.

• Nucleolus: Site of ribosomal RNA ($rRNA$) synthesis and ribosome construction.

• Endoplasmic Reticulum ($ER$): Transport of material and lipid synthesis.

• Golgi Apparatus: Packaging and modification of proteins prior to secretion.

• Lysosomes: Intracellular digestion.

• Vacuoles: Temporary storage and transport; digestion (food vacuoles); regulation of osmotic pressure (water vacuoles).

• Mitochondria: Energy production using the Krebs cycle, electron transport, and oxidative phosphorylation.

• Chloroplast: Conversion of sunlight into chemical energy through photosynthesis.

• Ribosomes: Protein synthesis.

• Cytoskeleton: Composed of microfilaments and microtubules; provides cell structure and movement; anchors organelles.

Detailed Organelle Analysis

• Nucleus:

  • Appearance: Large, compact sphere; the most prominent organelle.

  • Nuclear Envelope: Two parallel membranes separated by a narrow space, perforated with nuclear pores.

  • Chromosomes: Located within the nucleus.

  • Mitosis: Nuclear changes occur in stages including Interphase, Prophase, Early Metaphase, Metaphase, Early Anaphase, Late Anaphase, Early Telophase, and Telophase (resulting in daughter cells and formation of a cleavage furrow).

• Endoplasmic Reticulum ($ER$):

  • Rough ER ($RER$): Originates from the outer membrane of the nuclear envelope and extends through the cytoplasm. It is rough due to ribosomes. Proteins are synthesized here and shunted into the $ER$ for packaging and transport; it represents the first step in the secretory pathway.

  • Smooth ER ($SER$): A closed tubular network without ribosomes. Functions in nutrient processing and the synthesis and storage of lipids.

• Golgi Apparatus:

  • Structure: Consists of a stack of flattened sacs called cisternae.

  • Function: Modifies, stores, and packages proteins received from the $ER$.

• Synthesis and Transport Machine (Secretory Pathway):

  1. The nucleolus provides ribosomes that travel to the $RER$.

  2. Transport vesicles from the $ER$ containing proteins go to the Golgi apparatus for modification and maturation.

  3. Condensing vesicles transport proteins to specific organelles or into secretory vesicles to be exported (exocytosis).

• Lysosomes, Vacuoles, and Phagosomes:

  • Lysosomes: Vesicles containing enzymes originating from the Golgi; involved in intracellular digestion of food and protection against invaders.

  • Vacuoles: Membrane-bound sacs for digestion, excretion, or storage.

  • Phagosomes: Formed when a food vacuole merges with a lysosome for digestion.

• Mitochondria:

  • Structure: Spherical organelle with an outer membrane and an inner membrane folded into cristae.

  • Cristae: Hold enzymes and electron carriers for aerobic respiration.

  • Matrix: Space around cristae containing DNA and prokaryotic-type ribosomes.

  • Semiautonomous: Divide independently of the cell.

• Chloroplasts:

  • Structure: Found in algae and plant cells. Double membrane (chloroplast envelope). Inner membrane folded into sacs called thylakoids, stacked into grana.

  • Biological role: Primary producers of organic nutrients; contain chlorophyll pigments.

• Ribosomes:

  • Composition: $rRNA$ and proteins.

  • Location: Scattered in cytoplasm or associated with the $RER$.

  • Size: Larger than prokaryotic ribosomes.

• Cytoskeleton:

  • Components: Microfilaments and microtubules.

  • Functions: Anchoring organelles, cytoplasmic movement, amoeboid motion, transport, and structural support.

Comparison of Prokaryotes, Eukaryotes, and Viruses (Table 5.4)

• Genetics:

  • Nucleic Acids: Present in all three.

  • True Nucleus: Present only in Eukaryotes.

  • Nuclear Envelope: Present only in Eukaryotes.

  • Nucleoid: Present only in Prokaryotes.

• Reproduction:

  • Mitosis: Present only in Eukaryotes.

  • Production of Sex Cells: Present in Eukaryotes; some Prokaryotes ($+/-$).

  • Binary Fission: Present in Prokaryotes and Eukaryotes (cellular level).

• Biosynthesis:

  • Independent Biosynthesis: Present in Prokaryotes and Eukaryotes; Absent in Viruses.

  • Organelles (Golgi, $ER$): Present only in Eukaryotes.

  • Ribosomes: Present in Prokaryotes (structurally different) and Eukaryotes.

• Respiration:

  • Mitochondria: Present only in Eukaryotes.

• Photosynthesis:

  • Chloroplasts: Present in some Eukaryotes.

• Motility:

  • Flagella: Present in some Prokaryotes (structurally different) and some Eukaryotes.

  • Cilia: Present only in some Eukaryotes.

• General Size:

  • Prokaryotic Cells: $0.5-3\,\mu m$.

  • Eukaryotic Cells: $2-100\,\mu m$.

  • Viruses: <0.2\,\mu m.

Phylogenetic Relationships (Domain Eukarya)

• Classification Groups:

  • Archaeplastids: Includes Red algae and Chlorophytes.

  • SAR: Includes Stramenopiles (Diatoms, Brown algae), Alveolates (Apicomplexans, Dinoflagellates, Ciliates), and Rhizaria (Foraminiferans, Radiolarians).

  • Excavates: Includes Diplomonads, Parabasalids, Euglenids, and Kinetoplastids.

  • Amoebozoans: Includes Amoeboids.

  • Opisthokonts: Includes Animals and Fungi.

Kingdom Fungi

• Characteristics:

  • Approximately $100,000$ species.

  • Majority are unicellular or colonial.

• Groupings:

  • Macroscopic Fungi: Mushrooms, puffballs, gill fungi.

  • Microscopic Fungi: Mold (hyphae) and Yeasts.

• Microscopic Morphology:

  • Hyphae: Long, threadlike cells found in filamentous fungi or molds. Can be septate (divided by cross walls) or nonseptate.

  • Mycelium: Woven, intertwining mass of hyphae.

  • Yeast: Round to oval shape; reproduces asexually by budding. Some form pseudohyphae (chains of buds).

  • Dimorphic: Some fungi can exist in either hyphal or yeast form; often characteristic of pathogens.

• Fungal Nutrition:

  • All are heterotrophic.

  • Saprobes: Majority are harmless, living off dead matter.

  • Parasites: Live on tissues of other organisms; cause Mycoses (fungal infections).

• Reproduction:

  • Primarily via spores formed on reproductive hyphae.

  • Asexual Spores:

    • Sporangiospores: Formed within a saclike head called a sporangium.

    • Conidiospores (Conidia): Free spores not enclosed by a sac (e.g., Arthrospores, Chlamydospores, Blastospores, Phialospores, Porospores).

  • Sexual Spores: Formed following the fusion of two mating strains ($+$ and $-$).

    • Types: Zygospores, Ascospores, Basidiospores.

Fungal Classification (Table 5.5)

• Phylum I: Zygomycota:

  • Sexual: Zygospores.

  • Asexual: Mostly sporangiospores.

  • Hyphae: Usually nonseptate.

  • Examples: Rhizopus (bread mold), Circinella, Mucor, Absidia.

• Phylum II: Ascomycota:

  • Sexual: Ascospores in asci.

  • Asexual: Conidia.

  • Hyphae: Porous septa.

  • Examples: Penicillium (antibiotics), Aspergillus (respiratory infection), Saccharomyces (bread/beer yeast), Candida albicans (yeast infections), Histoplasma (Ohio Valley fever), Trichophyton (ringworm), Coccidioides immitis (Valley fever), Pneumocystis jiroveci (AIDS pathogen).

• Phylum III: Basidiomycota:

  • Sexual: Basidia and basidiospores.

  • Asexual: Conidia.

  • Examples: Mushrooms, puffballs, bracket fungi, rusts, and smuts. Human pathogen: Cryptococcus neoformans (systemic infection).

• Phylum IV: Chytridimycota:

  • Primitive fungi (chytrids). Characterized by flagellated spores (zoospores) and gametes. Parasitic to frogs and plants.

Major Fungal Infections of Humans (Table 5.6)

• Superficial (Outer epidermis): Tinea versicolor (Malassezia furfur).

• Cutaneous (Epidermis, hair, dermis): Dermatophytosis (Tinea/Ringworm) caused by Microsporum, Trichophyton, and Epidermophyton.

• Mucous Membranes/Skin: Candidiasis (Candida albicans).

• Systemic (Deep/Lung):

  • Coccidioidomycosis (Coccidioides immitis).

  • North American blastomycosis (Blastomyces dermatitidis).

  • Histoplasmosis (Histoplasma capsulatum).

  • Cryptococcosis (Cryptococcus neoformans).

  • Paracoccidioidomycosis (Paracoccidioides brasiliensis).

The Protists: Algae

• Characteristics: Eukaryotic, unicellular or colonial, photosynthetic with chlorophyll a.

• Structure: Microscopic (unicellular, filamentous) and Macroscopic (colonial, multicellular). Contain chloroplasts and cell walls.

• Importance: Form the plankton that is the basis of the food web; produce large amounts of atmospheric $O_2$.

• Toxicity: Dinoflagellates can cause "red tides" and release toxins causing food poisoning with neurological symptoms.

• Algal Classifications (Table 5.7):

  • Euglenophyta: Unicellular, flagellated, no cell wall (pellicle instead).

  • Pyrrophyta (Dinoflagellates): Unicellular, dual flagella, cellulose wall; cause red tide.

  • Diatoms: Unicellular, silicon dioxide wall; component of diatomaceous earth.

  • Pheophyta (Brown Algae/Kelps): Multicellular, source of alginate.

  • Rhodophyta (Red Seaweeds): Multicellular, source of agar and carrageenan.

  • Chlorophyta (Green Algae): Unicellular to multicellular, precursor of higher plants.

The Protists: Protozoa

• Characteristics: $65,000$ species; lack cell walls; most are unicellular and heterotrophic.

• Cytoplasm: Divided into ectoplasm and endoplasm.

• Lifecycle Stages:

  • Trophozoite: Active, motile feeding stage.

  • Cyst: Dormant, resting stage formed during unfavorable conditions (encystment); reactivated when moisture and nutrients are restored (excystment).

• Pathogenic Flagellates (Trypanosomes):

  • Trypanosoma brucei: African sleeping sickness.

  • Trypanosoma cruzi: Chagas disease.

  • Transmission: Blood-sucking vectors.

• Infective Amoebas (Entamoeba):

  • Entamoeba histolytica: Amebic dysentery. Cysts are ingested in food/water, germinate in the small intestine, and trophozoites migrate to the large intestine.

Parasitic Helminths

• General Features: Multicellular animals with organs for reproduction, digestion, and movement. Possess mouthparts for attachment. Produce eggs and sperm; undergo a larval period.

• Major Groups:

  • Flatworms: Flat, no definite body cavity, simple systems. Includes Cestodes (tapeworms) and Trematodes (flukes).

  • Roundworms (Nematodes): Round, complete digestive tract, protective cuticle, spines/hooks on mouth.

• Identification: Based on shape, size, organ development, presence of hooks/suckers, reproduction mode, and egg appearance.

• Pinworm Lifecycle: Self-infection (autoinoculation) or cross-infection occurs when fertile eggs are ingested; larvae hatch in the intestine and female worms emerge from the anus to deposit eggs.