Overview of the Eukaryotic Cell

Overview of the Eukaryotic Cell

• First primitive eukaryotes were likely single-celled and independent
• Cells aggregated over time, forming colonies
• Cells within colonies specialized to perform specific functions
• Complex multicellular organisms evolved as individual cells lost the ability to survive separately from the intact colony

External Structures

• Flagella: Different from bacterial flagella, about ten times thicker, structurally more complex, covered by a cell membrane extension, and contain regularly spaced microtubules.
• Cilia: Similar to flagella in structure, but smaller and more numerous, found in protozoa and certain animal cells; microtubules are regularly spaced along the entire length.

Microtubules in Flagella

• Regularly spaced microtubules extend along the entire length
• Nine pairs of microtubules surround a single pair (9+2 arrangement)
• Also present in cilia

The Glycocalyx

• An outermost layer that directly contacts the environment
• Composed of polysaccharides
• Appears as a network of fibers

Boundary Structures: Cell Wall

• Protozoa and helminths do not have cell walls
• Fungal cell walls provide rigidity and shape
• Chemical composition differs from bacterial and archaeal cell walls
• Thick inner layer of polysaccharide fibers (chitin or cellulose)
• Thin outer layer of mixed glycans

Boundary Structures: The Cell Membrane

• Typical bilayer of phospholipids with embedded protein molecules
• Contains sterols for membrane rigidity and stability
• Important for cells lacking cell walls
• Eukaryotic cytoplasmic membranes function similarly to bacterial and archaeal membranes, acting as selectively permeable barriers.

Internal Structures: The Nucleus

• Most prominent eukaryotic organelle
• Separated from the cytoplasm by the nuclear envelope (double membrane)
• Perforated with pores for macromolecule migration
• Includes the nucleolus (ribosomal RNA synthesis), and chromatin (DNA and histone proteins)

Internal Structures: Endoplasmic Reticulum

• Series of membrane tunnels for transport and storage
• Rough ER: Ribosomes attached, responsible for protein transport from nucleus to cytoplasm/exterior
• Smooth ER: Tubular, no ribosomes, involved in nutrient processing and synthesis/storage of non-protein macromolecules (lipids)

Internal Structures: Golgi Apparatus

• Flattened disc-shaped sacs (cisternae) associated with the endoplasmic reticulum
• Modifies and processes proteins, adding polysaccharides and lipids
• Packages modified proteins into vesicles for transport to other parts of the cell or exterior

Internal Structures: Vesicles (Lysosomes)

• Bud off the Golgi apparatus
• Contain enzymes for intracellular digestion of particles
• Provide protection against invading organisms

Internal Structures: Mitochondria

• Generate energy for the cell
• Composed of an outer and inner membrane (cristae)
• Extract chemical energy from nutrients and store it as ATP
• Contain circular DNA and 70S ribosomes (similar to bacteria)
• Independent division

Internal Structures: Chloroplasts

• Found in algae and plants
• Convert light energy to chemical energy through photosynthesis
• Produce oxygen
• Possess their own DNA and ribosomes
• Larger, more varied in shape, and possess pigments than mitochondria

Internal Structures: Ribosomes

• Scattered, or attached to the rough ER
• Involved in protein synthesis
• Composed of large and small subunits of ribonucleoprotein

The Cytoskeleton

• Three major components: actin filaments, intermediate filaments, microtubules
• Roles include anchoring organelles, moving materials, and enabling shape changes/movement

Fungal Cells

• Two basic forms: yeasts (round/oval) and hyphae (long/thread-like)
• Yeast reproduce by budding; hyphae by fragmentation/spores
• Asexual Spores: Sporangiospores (in a sac) and conidiospores (not in a sac)

Fungal Disease

• 300 species cause human disease (community-acquired, hospital-associated, opportunistic)