Introduction to Eukaryotic Cells and the Endosymbiotic Theory

The Endosymbiotic Theory and Eukaryotic Evolution

Prokaryotes evolved approximately $3.5$ billion years ago.
Eukaryotic cells evolved approximately $2.5$ billion years ago.
The Endosymbiotic Theory (from "endo" meaning inside and "symbiotic" meaning collaboration between organisms) describes a series of sequential, cell-merging events between an ancient eukaryotic ancestor and certain prokaryotes.
Mitochondria evolved from the engulfment of a nonphotosynthetic prokaryote, possibly a Rickettsia species.
Chloroplasts evolved from the engulfment of a photosynthetic prokaryote, such as a Cyanobacterium.
Primary Endosymbiosis involves a heterotrophic eukaryote engulfing a prokaryote.
Secondary Endosymbiosis occurs when a eukaryote engulfs another eukaryote that has already undergone primary endosymbiosis, sometimes resulting in a plastid with multiple membranes and a vestigial nucleus.
Evidence supporting the endosymbiotic theory includes mitochondria and chloroplasts possessing:
- Their own circular DNA, similar to bacterial DNA.
- $70S$ ribosomes, which are characteristic of bacteria.
- Double-membrane structures.
- Dimensions and sizes similar to bacteria.
- The ability to replicate via a process similar to binary fission.
- Genes that closely resemble specific bacterial genes.

Eukaryotic versus Prokaryotic Cell Characteristics

Eukaryotes comprise plants, animals, protists, and fungi. They are generally larger and more complex than prokaryotes, featuring larger genomes and multiple linear chromosomes.
Organisms: Eukaryotes include unicellular protists and yeast, and multicellular animals, plants, and most fungi. Prokaryotes consist of unicellular archaea and bacteria.
Cell Size: Eukaryotes are usually much larger than prokaryotes; prokaryotes are usually much smaller than eukaryotes.
Cell Division: Eukaryotes use asexual (mitosis) and sexual (meiosis) reproduction. Prokaryotes use asexual binary fission.
Plasma Membrane: Eukaryotic membranes often contain sterols; prokaryotic membranes rarely contain sterols.
Cell Wall: Present only in plants, fungi, and certain protists for eukaryotes. Present in most prokaryotes except Mycoplasma and L-forms.
Nucleus: Present in eukaryotes; absent in prokaryotes.
Ribosomes: Eukaryotes have $80S$ ribosomes in the cytoplasm and rough endoplasmic reticulum, and $70S$ ribosomes in mitochondria and chloroplasts. Prokaryotes possess only $70S$ ribosomes.
Genetic Material: Both use DNA. Eukaryotes have multiple linear chromosomes; prokaryotes usually have a single circular chromosome.
Membrane-Bound Organelles: Present in eukaryotes; absent in prokaryotes (though they may have membranous inclusions).

Categorization of Eukaryotic Organisms into Four Kingdoms

Kingdom Animalia: Includes birds, helminths, reptiles, mammals, fish, amphibians, sponges, and arthropods. They are multicellular, practice sexual and asexual reproduction, and lack a cell wall. Animals. paracites, ticks, helminths, and other organisms that can cause harm or disease in their hosts. helminths prefer to grow in our stomach. helminths lay eggs. best way to find helminths are stool tests to find eggs
Helminths
- Parasitic worms (eg roundworms and flatworms)
- Complex life cycles
- usually spread in a microscopic form
- WHO estimates half the world’s population is infected with some type of helminths.
Liver fluke- Fasciola hepatica. Liver fluke infections cuase serious medical and veterinary disease. fasciolosis of sheep, goats and cattle, is the major cause of economic losses in dairy and meat industry.
Kingdom Plantae: Comprised of plants. They are multicellular, practice sexual and asexual reproduction, and possess a cell wall. Plants.
Kingdom Fungi: Includes yeasts (unicellular), molds, and mushrooms (mostly multicellular). They practice sexual and asexual reproduction and possess a cell wall. Have chitin .Fungi. certain species that effect human health of yeast and mold (egot), and aspergillum. produce spores. tiny microscopism structures. porangiospores and conidiospores two primary types of asexual (mitosis) spores produced by fungi.
>600,000 different species. Most are multicellular, years are unicellular, do not carry out photosynthesis, absorb nutrients from their environment. Include pathogens and saprobes.
Meiosis (sexual): Zygospores, Ascospore,Basidiospores
Kingdom Protista: Includes Euglena, diatoms, amoebas, paramecia, algae, and slime molds. Algae. have celluoluse. Amoebae and paramecium are problematic parasites
- Animal-like protists (protozoans) are unicellular.
- Plant-like protists can be unicellular or multicellular.
- Fungus-like protists can be unicellular or multicellular.
- Reproduction can be sexual or asexual.
- Cell walls are present in some, such as algae and slime molds at specific life stages.
Plantae, fungi, and protista have cell walls
Protozoans include a number of pathogens. trypanosoma gambiense cause african sleeping disease, transmitted by mosquitoes.

Eukaryotic Cell Division: Mitosis and Meiosis

General Process: Before division, a cell must copy its genetic material. Division is longer in eukaryotes due to larger genomes and the need to replicate organelles.
Mitosis:
- Generates two genetically identical offspring from one parent cell.
- Offspring maintain the same number of chromosomes as the parent (diploid daughter cells).
- Primarily used for asexual reproduction, growth, and tissue repair.
- offspring end up with same amount of chromosomes
Meiosis:
- Involved in sexual reproduction.
- Consists of two distinct cell division stages.
- One parent cell produces four gametes (daughter cells).
- Gametes are haploid (unpaired chromosomes).
- Crossing over allows for genetic recombination.
- 4 Haploid
Binary Fission Reference: Mitochondria and chloroplasts replicate using a process that resembles the binary fission used by prokaryotes to produce two genetically identical cells.
Prokaryotic cells divide by binary fission.
Eukaryotic cells
- Vegetative: mitosis
- Reproductive: gametes & meiosis

The Nucleus: The Cell's Command Center

The nucleus contains most of the cell's DNA and genes.
Chromatin: A complex of DNA double helixes and histone proteins; it condenses into discrete chromosomes during cell division preparation.
Nucleolus: A dense region within the nucleus enriched with RNA; it is the site of ribosomal RNA ( $rRNA$ ) synthesis.
Nuclear Envelope: A double membrane (inner and outer) that encloses the nucleus, separating it from the cytoplasm.
Nuclear Pores: Openings in the envelope regulated by pore complexes ( $TEM$ scale approximately $0.25 \, \mu m$ to $1 \, \mu m$ ) that allow movement of materials.
Note on Red Blood Cells (RBCs): Most eukaryotic cells have a nucleus, but mature human RBCs lose their nuclei during maturation. Consequently, their lifespan is limited to approximately $120$ days because they lack DNA to direct protein synthesis.

The Endomembrane System: Endoplasmic Reticulum and Golgi Apparatus

Endoplasmic Reticulum (ER):
- Rough ER: Studded with ribosomes on the surface. It is primarily involved in protein production, modification, and folding. It secretes glycoproteins (proteins covalently bonded to carbohydrates) and distributes transport vesicles. It serves as the cell's membrane factory.
- Smooth ER: Lacks ribosomes. Functions include lipid synthesis, carbohydrate metabolism, detoxification of drugs and poisons, and storage of calcium ions ( $Ca^{2+}$ ).
Golgi Apparatus: Known as the "Shipping and Receiving Center."
- Consists of flattened membranous sacs called cisternae.
- Modifies products coming from the ER.
- Manufactures certain macromolecules.
- Sorts and packages materials into transport vesicles for distribution.
- Take every object (lipid or protein) coat with layer of sugar, packed into circulr substances known as vesicles. Vesicles carry everything near cell membrane, then secrete it out.

Energy-Converting Organelles: Mitochondria and Chloroplasts

Mitochondria: The "powerhouse" of the cell.
- Site of cellular respiration, using oxygen to generate $ATP$ .
- Features a smooth outer membrane and an inner membrane folded into cristae.
- Cristae provide a large surface area for $ATP$ -synthesizing enzymes.
- Contains two compartments: the intermembrane space and the mitochondrial matrix.
- Possesses its own circular genome and $70S$ ribosomes.
Chloroplasts: Found in photosynthetic eukaryotes (plants and algae).
- Site of photosynthesis, converting light energy into food.
- Contains the green pigment chlorophyll, enzymes, and ribosomes.
- Structure includes thylakoids (membranous sacs), grana (stacks of thylakoids), and stroma (internal fluid).
- Possesses its own circular genome and $70S$ ribosomes.

Specialized Intracellular Structures and the Cytoskeleton

Ribosomes: In eukaryotes, $80S$ ribosomes are either membrane-bound (on the Rough ER) or free in the cytoplasm. $70S$ ribosomes are sequestered in mitochondria and chloroplasts. They function to build proteins.
Cytoskeleton: A network of fibers extending throughout the cytoplasm that organizes structures and activities.
- Microtubules: Involved in shape, movement, and transport.
- Intermediate Filaments: Provide mechanical strength and anchor organelles.
- Microfilaments: Aid in movement and cell shape.
Lysosomes: Membranous sacs of hydrolytic enzymes that digest macromolecules (proteins, fats, polysaccharides, nucleic acids). They function best in the acidic environment found inside the lysosome.
Vesicles and Vacuoles: Membranous structures used for storage and transport. Examples include lysosomes and peroxisomes.

Protein Synthesis and Secretion Pathway

$mRNA$ is transcribed from genes in the DNA within the nucleus.
$mRNA$ exits the nucleus via nuclear pores and attaches to cytosolic ribosomes.
Some proteins are released into the cytosol or targeted to specific organelles.
Ribosomes attached to the Rough ER direct proteins into the ER lumen.
Proteins are modified as they pass through the ER lumen.
Transport vesicles move proteins from the ER to the Golgi apparatus.
Golgi cisternae migrate toward the cell membrane.
Some vesicles bud off the cisternae and move in a retrograde (backward) fashion toward the ER.
Some vesicles bud off to form lysosomes or storage vesicles.
Secretory vesicles fuse with the cell membrane to release contents outside the cell (exocytosis).

Eukaryotic Cell Transport: Endocytosis and Exocytosis

Endocytosis: An active transport process (requires energy) that moves large molecules or whole cells into the cell by folding the plasma membrane around them to form endocytic vesicles.
- Phagocytosis: "Cell-eating." The cell engulfs a target (cell debris or microorganisms) using pseudopodia to form a phagosome.
- Pinocytosis: "Cell-drinking." The cell takes up smaller molecules and extracellular fluid.
- Receptor-mediated Endocytosis: Relies on specific receptors in a coated pit to attract and bind certain chemicals (metabolites or hormones) before forming a coated vesicle.
Many viruses rely on endocytosis to invade host cells, which means blocking endocytosis is one possible approach to treating a variety of viral infections, including human immunodeficiency virus (HIV)
Normally, HIV invades cells by binding to specific proteins on the plasma membrane of T cells, a type of human immune system cell. When HIV binds to the T cell’s surface proteins, it triggers endocytosis. allowin the virus to enter the cell.
An HIV fusion inhibitor drFkindug called enfuvirtide limits HIV endocytosis by blocking HIV‘s binding to T cells
doesnt eradicate infection, not a cure of HIV.
Phagocytosis Process:
1. Target is engulfed to form a phagosome.
2. Phagosome fuses with a lysosome containing hydrolytic enzymes, forming a phagolysosome.
3. Hydrolytic enzymes destroy the target (e.g., bacteria like Staphylococcus aureus).
4. Waste products are expelled from the cell.
Exocytosis: The process of removing particles from the cell.
- Vesicles (often budding from the Golgi) deliver contents to the plasma membrane.
- Vesicle membranes fuse with the plasma membrane.
- Contents are either integrated into the membrane or secreted (e.g., waste, toxins, neurotransmitters).
- Endocytosis: cell is receiving something from outside.
  - 3 types: pinocytosis (cell drinking), phagocytosis (cell eating), receptor mediated.
  - cell membrane changes its formation,
- Exocytosis: making a vesiclethat transports substances out of the cell

Clinical Case: The Case of the Infectious Hike

This case study explored how understanding eukaryotic cell structures and functions can explain medical mysteries associated with infections encountered during outdoor activities. Details involve NCLEX, HESI, and TEAS preparation materials available in the Mastering Microbiology Study Area.