Eukaryotic Cells Overview

Overview of Eukaryotic Cells

• First primitive eukaryotes were single-celled and independent
• Over time, cells aggregated to form colonies
• Cells within colonies specialized for specific functions
• Complex multicellular organisms evolved as individual cells lost the ability to survive independently

External Cell Structures

• Flagella: Similar to cilia but longer. About 10 times thicker than bacterial flagella, more complex in structure, have a membrane extension containing regularly spaced microtubules.
• Cilia: Similar to flagella in structure but smaller and more numerous, found in certain animal and protozoa cells.

Microtubules in Flagella

• Regularly spaced microtubules extend along the entire length of the flagellum
• Nine pairs of microtubules surround a single pair
• Arrangement is known as 9 + 2

The Glycocalyx

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

Boundary Structures: Cell Walls

• Protozoa and helminths lack cell walls.
• Fungal cell walls are rigid, providing structure and shape.
• Different chemical composition from bacterial and archaeal cell walls
• Thick inner layer (chitin or cellulose) and a thin outer layer (mixed glycans)

Boundary Structures: Cell Membranes

• Composed of a phospholipid bilayer with embedded protein molecules
• Contains sterols that provide stability
• Important in cells without cell walls
• Has a similar function to bacteria and archaea membranes (selectively permeable barriers)

Internal Structures: The Nucleus

• Prominent eukaryotic organelle
• Separated from the cytoplasm by a nuclear envelope composed of 2 membranes
• Contains pores allowing macromolecule movement between cytoplasm and nucleus
• Nucleolus: site of ribosomal RNA synthesis
• Chromatin: made of linear DNA and histone proteins

Internal Structures: Endoplasmic Reticulum

• Network of membrane tunnels involved in transport and storage
• Rough ER has ribosomes attached for protein synthesis
• Smooth ER involved in lipid synthesis and nutrient processing

Internal Structures: Golgi Apparatus

• Involved in protein modification and directing proteins to their destinations
• Consists of flattened sacs (cisternae)
• Closely associated with ER
• Modifies and packages proteins using carbohydrates and lipids

Vesicles

• Lysosomes: Contain enzymes for intracellular digestion
• Bud off from the Golgi apparatus

Mitochondria

• Generate energy for the cell
• Have an outer and inner membrane with cristae forming folds
• Contain enzymes and electron carriers for aerobic respiration
• Contain circular DNA/ 70S ribosomes; hypothesized to evolve from bacteria cells

Chloroplasts

• Found in algae and plants
• Convert sunlight into chemical energy via photosynthesis
• Contain a unique set of pigments

Ribosomes

• Scattered in cytoplasm or attached to Rough ER
• Found inside mitochondria and chloroplasts
• Large and small subunits of ribonucleoprotein
• Involved in protein synthesis

The Cytoskeleton

• Three types of elements: Actin filaments, Intermediate filaments, Microtubules
• Provide support, movement, and anchoring for organelles.

Fungal Cells

• Two main forms: Yeasts (round to oval) and Hyphae (long threadlike)
• Yeast cells reproduce by budding
• Fungi in nature are either heterotrophic (absorb from organic matter) or saprobes/parasites (absorbing from dead organisms or living organisms)
• Cell walls made of chitin or cellulose

Fungi and Human Disease

• Approximately 300 fungal species can cause human disease.
• Common types include community-acquired, hospital-associated, and opportunistic infections.

Protozoa

• Single-celled eukaryotic organisms, found in water and soil environments.
• Most are harmless, some are pathogens
• Some have specialized structures (oral grooves) for feeding.
• Reproduce asexually or sexually, some have life cycles with both trophozoite (feeding) and cyst phases (dormant).
• Important cause of disease and can be transmitted in water and food.

Helminths

• Multicellular animals that reproduce sexually
• Include tapeworms, flukes, and roundworms
• Some are harmless, others lead to disease
• Often have complex life cycles requiring multiple hosts