chap 4 micro

Introduction to Eukaryotic Cells

Overview of Eukaryotes

Eukaryotic cells are fundamental units of life found in plants, animals, fungi, and protists. They are distinguished from prokaryotic cells by their structural complexity and the presence of membrane-bound organelles. Understanding eukaryotic cells is essential for microbiology, as many pathogens and model organisms are eukaryotes.

• Endosymbiotic Theory: Proposes that eukaryotic cells evolved from symbiotic relationships between ancestral prokaryotic cells.

• Key Features: Larger size, complex internal structure, multiple linear chromosomes, and compartmentalization via organelles.

• Examples: Homo sapiens (humans), Arabidopsis thaliana (plant), Amoeba proteus (protist), Saccharomyces cerevisiae (fungus).

The Endosymbiotic Theory

How Eukaryotes Evolved

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells. According to this theory, these organelles originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Mitochondria: Evolved from engulfed aerobic prokaryotes (e.g., proteobacteria).

• Chloroplasts: Evolved from engulfed photosynthetic prokaryotes (e.g., cyanobacteria).

• Supporting Evidence:

  • Double membranes around mitochondria and chloroplasts

  • Own circular DNA, similar to bacteria

  • 70S ribosomes (like prokaryotes)

  • Replication by binary fission

  • Genes resembling bacterial genes

Timeline: Prokaryotes evolved ~3.5 billion years ago; eukaryotes evolved ~2.5 billion years ago.

Eukaryotic Cell Structure and Comparison with Prokaryotes

Key Differences

• Eukaryotic Cells: Found in plants, animals, protists, and fungi. Larger, more complex, with multiple linear chromosomes and membrane-bound organelles.

• Prokaryotic Cells: Found in bacteria and archaea. Smaller, simpler, with a single circular chromosome and no membrane-bound organelles.

Feature	Eukaryotic Cells	Prokaryotic Cells
Nucleus	Present	Absent
Chromosomes	Multiple, linear	Single, circular
Membrane-bound organelles	Present (e.g., mitochondria, ER, Golgi)	Absent
Cell division	Mitosis and meiosis	Binary fission
Size	Larger (10–100 μm)	Smaller (0.5–5 μm)

Membrane-Bound Organelles

• Nucleus: Contains genetic material (DNA) and is surrounded by a nuclear envelope.

• Mitochondria: Site of cellular respiration and energy (ATP) production.

• Chloroplasts: Site of photosynthesis in plants and algae.

• Other Organelles: Endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes.

Cell Division in Eukaryotes

Mechanisms of Cell Division

Eukaryotic cells can reproduce sexually or asexually. Cell division is more complex and takes longer than in prokaryotes due to the need to replicate larger genomes and organelles.

• Mitosis: Produces two genetically identical daughter cells, each with the same number of chromosomes as the parent cell. Used for growth, repair, and asexual reproduction.

• Meiosis: Involved in sexual reproduction. Consists of two sequential cell divisions, resulting in four genetically unique haploid gametes. Crossing over during meiosis increases genetic diversity.

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Genetic identity	Identical to parent	Genetically unique
Chromosome number	Diploid (2n)	Haploid (n)
Function	Growth, repair, asexual reproduction	Sexual reproduction (gamete formation)

Eukaryotic Cell Transport: Endocytosis and Exocytosis

Endocytosis

Endocytosis is the process by which cells import substances by engulfing them in vesicles formed from the plasma membrane.

• Pinocytosis: "Cell drinking"; uptake of dissolved substances in small vesicles.

• Phagocytosis: "Cell eating"; uptake of large, undissolved particles or cells.

• Receptor-mediated endocytosis: Specific uptake of molecules via receptor-ligand interactions and clathrin-coated vesicles.

Phagocytosis

• Specialized immune cells (e.g., macrophages) engulf pathogens or debris.

• Engulfed material is enclosed in a phagosome, which fuses with a lysosome to form a phagolysosome.

• Hydrolytic enzymes digest the contents; waste is expelled.

• Pathogen Evasion: Some pathogens can escape or neutralize the phagolysosome, leading to infection.

Exocytosis

Exocytosis is the process by which cells export substances in vesicles that fuse with the plasma membrane, releasing their contents outside the cell.

• Removes waste, replenishes plasma membrane, and allows secretion of signaling molecules.

Classification of Eukaryotes

The Four Kingdoms of Eukaryotes

Eukaryotic organisms are classified into four major kingdoms: Animalia, Plantae, Fungi, and Protista. Each kingdom has unique characteristics and significance in microbiology.

Kingdom	Cell Type	Plasma Membrane	Cell Wall	Examples
Animalia	Multicellular	Yes	No	Animals, helminths, arthropods
Plantae	Multicellular	Yes	Yes (cellulose)	Plants, algae
Fungi	Unicellular or multicellular	Yes	Yes (chitin)	Yeasts, molds, mushrooms
Protista	Unicellular or multicellular	Yes	Some (varied composition)	Protozoa, algae, slime molds

Animals and Helminths

• Animals: Multicellular, do not perform photosynthesis, obtain organic carbon from nutrients, include ~7.5 million species.

• Helminths: Parasitic worms (e.g., roundworms, flatworms) with complex life cycles, often microscopic in some stages. WHO estimates half the world's population is infected with some type of helminth.

Overview of Parasitic Helminths

Phylum/Group	Example	Characteristics	Diseases Caused
Nematoda (Roundworms)	Ascaris lumbricoides	Cylindrical, unsegmented, complete digestive tract	Ascariasis
Trematoda (Flukes)	Schistosoma spp.	Flat, leaf-shaped, complex life cycles	Schistosomiasis
Cestoda (Tapeworms)	Taenia solium	Flat, segmented, absorb nutrients through skin	Taeniasis, cysticercosis

Additional info: Table entries inferred from standard microbiology sources for clarity.

Plants

• Multicellular organisms (~290,000 species)

• Carry out photosynthesis (contain chloroplasts)

• Produce organic carbon using light energy

• Vegetation can serve as a vehicle for infectious pathogens