BIO Exam 1: Ch 4

Eukaryotic Cells

General Characteristics

Eukaryotic cells are complex cells characterized by the presence of a true nucleus and various membrane-bound organelles. They are fundamental to the structure and function of all multicellular organisms and many unicellular organisms.

• Defined Nucleus: Contains the cell's genetic material (DNA) enclosed within a nuclear envelope.

• Membrane-bound Organelles: Includes mitochondria, chloroplasts (in plants and algae), endoplasmic reticulum, Golgi apparatus, lysosomes, and more.

• Evolution: Eukaryotic cells evolved approximately 2.5 billion years ago, likely through endosymbiotic events where ancestral prokaryotic cells merged with other cells.

• Endosymbiotic Theory: Suggests that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotes.

Example: Human, plant, fungal, and protozoan cells are all eukaryotic.

Cell Division

Eukaryotic cells can reproduce both sexually and asexually, involving complex processes to ensure genetic material is accurately passed to offspring.

• Mitosis: A process that produces two genetically identical daughter cells from a single parent cell. The chromosome number remains the same as the parent.

• Meiosis: Involved in sexual reproduction, consisting of two sequential cell divisions. One parent cell produces four gametes (daughter cells), each with half the chromosome number (haploid) of the parent. Crossing over during meiosis allows for genetic recombination.

Definition: Haploid cells contain a single set of chromosomes (n), while diploid cells have two sets (2n).

Cell Transport Mechanisms

Eukaryotic cells use various mechanisms to import and export substances across their membranes.

• Endocytosis: The process of importing substances into the cell by engulfing them in vesicles.

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

  • Phagocytosis: "Cell eating"; uptake of large, undissolved particles. Specialized immune cells called phagocytes engulf targets and fuse with lysosomes containing hydrolytic enzymes for digestion.

  • Receptor-mediated endocytosis: Specific uptake of molecules after binding to cell surface receptors.

• Exocytosis: The process of exporting substances out of the cell via vesicles that fuse with the plasma membrane.

Note: The prefix "cyto-" refers to "cell".

Eukaryotic Glycocalyx

Most eukaryotic cells possess a glycocalyx, a sticky extracellular layer composed of polysaccharides, glycoproteins, and glycolipids.

• Functions as a protective barrier, aids in cell recognition, and can serve as a signal receptor.

• May appear as a network of fibers, a slime layer, or a capsule.

Flagella Structure

Eukaryotic flagella are long, whip-like structures used for movement.

• Composed of the protein tubulin arranged in a characteristic "9+2" microtubule pattern (nine pairs surrounding two central microtubules).

Major Groups of Eukaryotic Microorganisms

Overview

Eukaryotic microorganisms include fungi, algae, protozoa, and helminths. These groups display diverse structures, life cycles, and ecological roles.

Fungi

Fungi are a diverse group of eukaryotic organisms that include molds, yeasts, and mushrooms. They play essential roles in decomposition, nutrient cycling, and as pathogens.

• Estimated 600,000 species; most are multicellular (molds), but yeasts are unicellular.

• Nutrition: Absorb nutrients from the environment; do not perform photosynthesis.

• Growth Form: Grow as a collection of tubular structures called hyphae.

  • Septate hyphae: Have cross-walls (septa) dividing the filament into individual cells.

  • Aseptate hyphae: Lack septa, forming a continuous multinucleated cell.

• Dimorphic fungi: Can exist as either yeast or mold forms depending on environmental conditions.

• Reproduction: Asexual spores (from mitosis) and sexual spores (from meiosis).

• Pathogenicity: About 300 species can cause human disease, including community-acquired, hospital-associated, and opportunistic infections.

Example: Candida albicans (yeast infections), Aspergillus (mold infections).

Protozoa

Protozoa are unicellular, mostly motile eukaryotes found in water and soil. While most are harmless, some are significant human pathogens.

• Four main groups based on movement:

  • Amoebas: Move using pseudopods ("false feet").

  • Flagellates: Move using flagella.

  • Ciliates: Move using cilia.

  • Apicomplexa: Non-motile, often parasitic (e.g., Plasmodium).

• Structure includes ectoplasm (outer layer) and endoplasm (inner layer).

• Nutrition: Heterotrophic; require complex organic food sources.

• Life Cycle: Alternates between active, feeding trophozoite stage and dormant cyst stage.

• Transmission can occur via the fecal-oral route.

Example: Entamoeba histolytica (amoebic dysentery), Giardia lamblia (giardiasis).

Helminths

Helminths are multicellular, worm-like parasites that infect humans and animals. They are visible to the naked eye in their adult forms.

• Two main groups:

  • Flatworms: Includes cestodes (tapeworms) and trematodes (flukes).

  • Roundworms: Includes nematodes (e.g., ascarids, pinworms).

• Reproduction can be hermaphroditic or bisexual; high reproductive capacity (can lay up to 25 million eggs per day).

• Life cycle includes fertilized egg, larval, and adult stages.

• Eggs are highly resistant and adapted for survival.

• About 50 species cause human diseases, especially in tropical, moist environments.

Example: Enterobius vermicularis (pinworm), Schistosoma (blood fluke).

Protists

Protists are a diverse group of eukaryotic organisms that are not classified as plants, animals, or fungi. They can be unicellular, multicellular, or multinucleated masses.

• Can be autotrophic (photosynthetic) or heterotrophic.

• Reproduce sexually and asexually.

• May have a cell wall or lack one.

• Examples include algae, slime molds, protozoans, and kelp.

Note: Protists serve as a link between simple and complex life forms.

Algae

Algae are photosynthetic, aquatic eukaryotes that play a crucial role in global oxygen production and aquatic food webs.

• Unicellular or multicellular; lack true roots, stems, leaves, and specialized reproductive structures of plants.

• Produce about half of Earth's atmospheric oxygen.

• Serve as the base of aquatic food chains and help remove CO2 from the atmosphere.

Example: Chlorella (unicellular green algae), diatoms.

Table: Comparison of Major Eukaryotic Microorganisms

Group	Cellularity	Nutrition	Reproduction	Movement	Examples
Fungi	Mostly multicellular (molds), unicellular (yeasts)	Heterotrophic (absorption)	Asexual and sexual (spores)	Non-motile	Candida, Aspergillus
Protozoa	Unicellular	Heterotrophic	Asexual and sexual	Pseudopods, flagella, cilia	Giardia, Plasmodium
Helminths	Multicellular	Heterotrophic (parasitic)	Sexual (hermaphroditic or bisexual)	Motile (larval stages)	Pinworm, tapeworm
Algae	Unicellular or multicellular	Autotrophic (photosynthetic)	Asexual and sexual	Some motile (flagella)	Chlorella, diatoms