Eukaryotic Cells and Microorganisms: Structure and Diversity

Structures of the Eukaryotic Cell

Appendages for Moving

Eukaryotic cells possess specialized appendages for movement, including flagella and cilia, which differ structurally and functionally from those found in prokaryotes.

• Eukaryotic Flagella:

  • Structurally more complex and about ten times thicker than bacterial flagella.

  • Covered by an extension of the cell membrane.

  • Movement is typically undulating rather than rotating.

• Eukaryotic Cilia:

  • Shorter and more numerous than flagella.

  • Found in certain protozoa and animal cells.

  • Besides motility, cilia can also function in sensation.

The Cell Wall

Cell walls provide structural support and protection in certain eukaryotes, such as fungi and algae, but are absent in protozoa and helminths.

• Function: Rigid and provides structural support.

• Composition: Usually a single layer of polysaccharide (chitin or cellulose), surrounded by a mixed outer layer of glycans.

The Cytoplasmic Membrane

The cytoplasmic membrane is a selectively permeable barrier composed mainly of phospholipids, proteins, and sterols.

• Composition: Phospholipids with embedded proteins and sterols.

• Function: Selectively permeable, controlling the movement of substances in and out of the cell.

The Nucleus

The nucleus is the control center of the eukaryotic cell, containing most of the cell's genetic material.

• Separated from the cytoplasm by a double-layered nuclear envelope with nuclear pores.

• Contains the nucleolus, the site of ribosomal RNA synthesis and ribosome assembly.

Endoplasmic Reticulum (ER)

The ER is a network of membranous tubules involved in the synthesis and transport of cellular materials.

• Rough ER (RER):

  • Studded with ribosomes on its surface.

  • Transports materials from the nucleus to the cytoplasm and cell exterior.

• Smooth ER:

  • Lacks ribosomes; involved in synthesis and storage of non-protein macromolecules such as lipids.

  • Good at storing calcium.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Consists of several flattened, disc-shaped sacs called cisternae.

• Receives transitional vesicles from the ER.

• Proteins are modified by addition of polysaccharides and lipids.

• Condensing vesicles pinch off and are transported within or outside the cell.

Vesicle Type	Main Function
Lysosomes	Intracellular digestion of food particles; protection against invading microorganisms; removal of cell debris
Other vesicles	Transport and storage of materials

Mitochondria

Mitochondria are the energy-generating organelles of eukaryotic cells, often called the "powerhouses" of the cell.

• Composed of a smooth outer membrane and a highly folded inner membrane (cristae).

• Site of aerobic respiration and ATP production.

• Unique features:

  • Divide independently of the cell.

  • Contain circular DNA similar to bacteria.

  • Possess 70S ribosomes (bacterial type).

Ribosomes

Ribosomes are the sites of protein synthesis in all cells.

• Found free in the cytoplasm or attached to the rough ER.

• Also present inside mitochondria and chloroplasts.

• Composed of large and small subunits; eukaryotic ribosomes are 80S (60S + 40S), while prokaryotic ribosomes are 70S.

The Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that provides structural support, anchors organelles, and enables cell movement.

• Anchors organelles in place.

• Facilitates movement of RNA and vesicles.

• Permits shape changes and cellular movement.

Eukaryotic Organisms

The Fungi

Fungi are a diverse group of eukaryotic organisms that include yeasts and molds, with important ecological and medical roles.

• Yeasts:

  • Unicellular, round to oval shape.

  • Reproduce asexually by budding (parent cell forms a daughter cell that detaches).

  • May exist in chains called pseudohyphae.

• Molds:

  • Consist of long, threadlike cells in chains called hyphae.

  • Macroscopic growth forms a mycelium (visible mass of hyphae).

  • Types of hyphae:

    • Vegetative hyphae (involved in visible growth and nutrient absorption).

    • Reproductive/aerial hyphae (produce spores).

  • Some fungi are dimorphic (can exist as either yeast or mold forms).

Fungi and Human Disease

• Sources of fungal infections:

  • Community-acquired

  • Hospital-associated

  • Opportunistic infections (increasing due to more immunocompromised individuals and climate change)

• Some fungal cell walls release substances that cause allergies.

Benefits of Fungi

• Decompose organic matter and recycle nutrients.

• Form stable associations with plant roots (mycorrhizae), enhancing nutrient absorption.

• Produce antibiotics, alcohol, organic acids, and vitamins.

• Used in food production (e.g., bread, cheese, alcoholic beverages).

Reproductive Strategies and Spore Formation

• Fungi can reproduce by outward growth of hyphae or fragmentation.

• Spore formation is the primary reproductive mechanism; spores can be produced sexually or asexually and are easily dispersed.

The Protozoa

Protozoa are unicellular eukaryotes, mostly free-living in water or soil, with some species causing significant human diseases.

• Unicellular organisms.

• Most are harmless; some are parasitic.

• Classified by method of locomotion:

  • Amoeboid (move using pseudopods, or "false feet")

  • Ciliated

  • Flagellated

  • Non-motile

Stage	Description
Trophozoite	Motile, feeding stage; requires moisture to remain active
Cyst	Dormant, resting stage; resistant to heat, drying, and chemicals; important for transmission

• Cysts can be dispersed by air and water currents and are important in disease transmission (e.g., via contaminated food, water, or person-to-person contact).

• Not all protozoa form cysts; some exist only in the trophozoite stage.

The Helminths

Helminths are multicellular parasitic worms, including flatworms and roundworms, with complex life cycles involving multiple hosts.

• Adult forms are usually macroscopic; some life stages are microscopic.

• Many live in soil or water; parasitic forms inhabit the gastrointestinal tract of hosts.

• Flatworms (Phylum Platyhelminthes):

  • Thin, often segmented bodies.

  • Includes tapeworms and flukes.

• Roundworms (Phylum Aschelminthes):

  • Elongated, cylindrical, unsegmented bodies.

• General Morphology:

  • Multicellular animals with organ systems.

  • Reproductive system is usually the most developed.

  • Digestive, excretory, nervous, and muscular systems are reduced.

Helminth Life Cycle

• Transmission requires an infective form (egg or larva) to enter a new host.

• The host where the larva develops is the intermediate (secondary) host.

• Adulthood and mating occur in the definitive (final) host.

• Transport host: an intermediate that experiences no parasitic development.

• Common routes of infection: oral and skin.

• Sources of infection: contaminated food, soil, water, or infected animals.