Chapter 5: Eukaryotic Cells in Microbiology – Study Guide and Key Concepts

Chapter 5: Eukaryotic Cells in Microbiology

Introduction to Eukaryotic Cells

Eukaryotic cells are complex cells characterized by membrane-bound organelles and a defined nucleus. They are found in organisms such as fungi, protozoa, algae, and animals. Understanding their structure and function is essential in microbiology, especially for distinguishing them from prokaryotic cells.

Structural Features of Eukaryotic Cells

• Flagella: Eukaryotic flagella differ from prokaryotic flagella in their structure and movement. Eukaryotic flagella are sheathed cylinders containing microtubules arranged in a 9+2 pattern and are covered by an extension of the cell membrane. They are thicker and move in a whip-like fashion, while prokaryotic flagella rotate.

• Cilia: Cilia are short, hair-like structures that function in motility, feeding, and filtering. They are found on protozoa and animal cells, such as those lining the human respiratory tract. Cilia move in coordinated power and recovery strokes.

• Glycocalyx: The glycocalyx, also called the extracellular matrix, is the outermost boundary that contacts the environment. It is composed of polysaccharides and appears as fibers, slime layers, or capsules. It functions in adherence, protection, and signal reception. In multicellular organisms, the glycocalyx is critical for cell-to-cell communication and tissue integrity.

• Cell Wall: Fungi and most algae possess a thick, rigid cell wall for structural support and shape. The wall is composed of polysaccharides such as cellulose, pectin, mannans, silicon dioxide, and calcium carbonate. Protozoa, a few algae, and all animal cells lack a cell wall and have only a plasma membrane.

• Plasma Membrane: The plasma membrane is a phospholipid bilayer with embedded proteins, providing selective permeability and stability. Sterols in the membrane (e.g., cholesterol) confer additional stability.

Internal Structures and Organelles

• Nucleus: The nucleus is the genetic center, containing DNA and surrounded by a double nuclear envelope with pores for transport. The nucleolus within the nucleus is the site of ribosomal RNA synthesis and ribosome assembly.

• Endoplasmic Reticulum (ER):

  • Rough ER (RER): Studded with ribosomes, the RER synthesizes proteins for secretion or membrane insertion.

  • Smooth ER (SER): Lacks ribosomes and is involved in lipid synthesis, nutrient processing, and storage.

• Ribosomes: Eukaryotic ribosomes are 80S, composed of 60S and 40S subunits. They are responsible for protein synthesis and are found free in the cytoplasm or attached to the RER.

• Golgi Apparatus: The Golgi apparatus modifies, packages, and sorts proteins received from the ER. It consists of stacked cisternae and produces vesicles for transport.

• Vesicles: Membrane-bound sacs that transport materials within the cell. Types include secretory vesicles, lysosomes, and vacuoles.

• Lysosomes: Contain digestive enzymes for intracellular digestion and protection against pathogens.

• Mitochondria: The site of ATP production via aerobic respiration. The inner membrane is highly folded into cristae to increase surface area for energy production.

• Chloroplasts: Found in algae and plant cells, chloroplasts convert sunlight into chemical energy through photosynthesis.

• Cytoskeleton: Composed of microfilaments, intermediate filaments, and microtubules, the cytoskeleton provides structural support, movement, and rapid adaptation for specialized cell functions.

Gene Expression and Protein Secretion Pathway

To express and secrete a protein, the following organelles are involved:

1. Nucleus: DNA is transcribed to mRNA.

2. Ribosomes: mRNA is translated into a polypeptide.

3. Rough ER: Polypeptide enters the RER for folding and modification.

4. Golgi Apparatus: Protein is further modified, sorted, and packaged.

5. Secretory Vesicles: Transport the protein to the plasma membrane for exocytosis.

Equation:

$\text{Nucleus} \rightarrow \text{Ribosome} \rightarrow \text{RER} \rightarrow \text{Golgi} \rightarrow \text{Secretory Vesicle} \rightarrow \text{Exocytosis}$

Fungi: Structure, Function, and Importance

• Yeasts vs. Molds:

  • Yeasts: Unicellular, round/ovoid, reproduce asexually by budding, lack locomotor structures.

  • Molds: Multicellular, filamentous hyphae, reproduce by spores, form cottony or hairy colonies.

• Fungi in the Food Industry: Fungi can spoil food and produce toxins, but also have beneficial uses in fermentation and biotechnology.

• Dermatophytes: Fungi that infect keratinized tissues (skin, hair, nails), causing dermatophytosis (e.g., ringworm).

• Benefits of Fungi:

  • Source of antibiotics (e.g., Penicillium produces penicillin).

  • Used in food production (e.g., Saccharomyces in bread and wine).

Algae and Protists

• Algae: Eukaryotic microorganisms with cell walls, capable of photosynthesis. Examples include green algae, red algae, and diatoms.

• Red Tide: Caused by dinoflagellate blooms, producing toxins that can harm marine life and humans (paralytic shellfish poisoning).

• Bioluminescent Dinoflagellates: Some dinoflagellates emit light due to chemical reactions, making them appear pretty at night.

Protozoa: Diversity and Pathogenicity

• General Features: Unicellular, lack cell walls, heterotrophic, motile by flagella, cilia, or pseudopodia.

• Pathogenic Protozoa:

  • Entamoeba histolytica: Causes amebic dysentery.

  • Naegleria fowleri: 'Brain-eating amoeba', contracted from warm freshwater.

  • Trypanosoma brucei: African sleeping sickness.

  • Trypanosoma cruzi: Chagas disease.

  • Giardia lamblia: Giardiasis (gastroenteritis).

  • Trichomonas vaginalis: Trichomoniasis (sexually transmitted).

  • Plasmodium falciparum: Malaria.

  • Toxoplasma gondii: Toxoplasmosis.

  • Cryptosporidium: Cryptosporidiosis.

• Neglected Tropical Diseases: Diseases caused by protozoa and helminths that affect billions globally but are under-researched due to low prevalence in developed countries.

Parasitic Helminths

• General Features: Multicellular, specialized for parasitism, reduced digestive and nervous systems, complex reproduction.

• Classification:

  • Platyhelminthes (Flatworms):

    • Cestodes (Tapeworms): Segmented, absorb nutrients through body wall, transmitted via undercooked meat.

    • Trematodes (Flukes): Leaf-shaped, have oral suckers, transmitted via snails or contaminated water.

  • Nematodes (Roundworms): Cylindrical, complete digestive system, can be free-living or parasitic. Examples: Ascaris lumbricoides (ascariasis), Trichuris trichiura (whipworm), Enterobius vermicularis (pinworm).

• Transmission: Ingestion of eggs/larvae, contaminated water, insect vectors.

Arthropods and Disease Transmission

• Arthropods: Animals with segmented bodies and exoskeletons, including insects and arachnids.

• Mechanical Transmission: Pathogen is carried externally (e.g., flies).

• Biological Transmission: Pathogen multiplies within the vector (e.g., mosquitoes for malaria).

• Definitive Host: The vector is part of the pathogen's life cycle and sexual reproduction occurs within it.

Table: Comparison of Eukaryotic Cell Structures

Structure	Function	Present in
Flagella	Motility	Protozoa, some animal cells
Cilia	Motility, feeding, filtering	Protozoa, animal cells
Cell Wall	Structural support	Fungi, algae
Plasma Membrane	Selective barrier	All eukaryotes
Nucleus	Genetic material storage	All eukaryotes
RER	Protein synthesis	All eukaryotes
SER	Lipid synthesis	All eukaryotes
Golgi Apparatus	Protein modification/packaging	All eukaryotes
Lysosome	Intracellular digestion	Animal cells, some protozoa
Mitochondria	ATP production	All eukaryotes
Chloroplast	Photosynthesis	Algae, plants

Additional info:

• Children are often reservoirs for infectious diseases due to close contact and developing immune systems.

• Fungi are adapted to hostile environments, growing at low pH, high osmotic pressure, and low moisture.

• Neglected tropical diseases include those caused by helminths and protozoa, with high global prevalence but limited research.