Eukaryotes

Topic 4: Eukaryotes

Overview of Eukaryotic Cells

  • Morphology of typical eukaryal cells: Eukaryotic cells have distinct structural features that differentiate them from prokaryotic cells (bacteria and archaea).

    • Membrane-bound nucleus: Larger than bacterial or archaeal cells, acts as the control center of the cell.

    • Organelles: Eukaryotic cells contain various membrane-bound organelles that perform specific functions (see complex details below).

    • Cell wall: Present in some eukaryotes (plants, fungi) providing structural support.

    • Complex internal cytoskeleton: A network of protein filaments and tubules providing shape and facilitating movement, intracellular transport, and cell division.

Organelles

Table 3.1: Selected Internal Organelles of Eukarya: This table outlines the main organelles found within eukaryotic cells, their functions, and interesting features.

  • Nucleus

    • plays a role in the storage and expression of information

    • double membrane structure

    • contains linear chromosomes of cell

    • non-membrane bound nucleolus exists within nucleus (ribosome synthesis)

    • spatial separation

      • transcription occurs in nucleus

      • translation occurs in cytoplasm

  • Main function: Contains most of the cell's DNA, serves as the site for transcription.

  • Interesting features: Double membrane with nuclear pores, outer membrane is continuous with the endoplasmic reticulum.

  • Mitochondrion

  • Main function: Energy production.

  • Interesting features: Double membrane, contains DNA, capable of independent replication; not present in amitochondriates.

  • plays a role in cell metabolism - TCA cycle

  • use electron transport chain to produce ATP ( chemiosmosis via proton motive force )

  • Chloroplast

  • Main function: Photosynthesis, converting sunlight into energy.

  • Interesting features: Double membrane containing DNA, specific to photosynthetic organisms.

  • play a role in cell metabolism

  • leverage electron transport chains produce ATP (chemiosmosis via the proton motive force)

  • use ATP they produce to fix carbon into organic compounds

Mitochondria and Choroplast

  • semi-autonomous

  • each has a DNA genome, ribosome and transcription machinery

  • can replicate indepently of the rest of the cell

  • most of their protein originated from the DNA in the nucleus of the cell

  • Rough Endoplasmic Reticulum (ER)

  • Main function: Site of translation and protein folding.

  • Interesting features: Has ribosomes attached, connected to the smooth ER through vesicles.

  • Golgi apparatus

  • proteins are often extensivly modified in these structures proir to reaching their destinations

  • Main function: Modifies, sorts, and transports proteins.

  • Interesting features: Composed of flattened membranous sacs.

  • Vacuole

  • Main function: Storage and structure, contributes to turgor pressure in plant cells.

  • Interesting features: Can serve as sites of digestion (food vacuoles), contractile vacuoles help maintain water balance.

  • Lysosome

  • Main function: Digestion of macromolecules through hydrolysis.

  • Interesting features: Contains digestive enzymes, a highly acidic interior.

  • Peroxisome

  • Main function: Breakdown of fatty acids and detoxification.

  • Interesting features: Contains various oxidative enzymes like catalase.

  • Hydrogenosome

  • Main function: Production of H₂ and ATP.

  • Interesting features: Double membrane, found in some amitochondriates, may be a remnant of a mitochondrion.

Nucleus Structure and Function

  • Role of nucleus: Involvement in storage and expression of genetic information.

    • Structure: Double membrane with linear chromosomes, contains a nucleolus (non-membrane bound) for ribosome synthesis.

    • Spatial separation of processes:

    • Transcription: Occurs in the nucleus.

    • Translation: Occurs in the cytoplasm.

A. Nuclear Structure

  • Components:

    • Nuclear membrane: Composed of inner and outer membranes.

    • Nuclear pore: Complex structures that allow selective transport of molecules.

    • Nucleolus: Site for ribosomal RNA synthesis and ribosome subunit assembly.

B. Nuclear Function

  • Process flow:

    1. DNA replication occurs within the nucleus, providing two identical copies of chromosomes.

    2. Transcription of pre-mRNA for protein coding.

    3. Processing of mRNA before export to the cytoplasm via nuclear pores.

    4. Translation of processed mRNA into proteins at ribosomes in the rough ER.

Membrane Systems

  • Secretory Pathway: Utilizes the endoplasmic reticulum and Golgi apparatus, where proteins undergo extensive modifications before reaching their final destinations.

Plasma Membrane Functionality

  • Structure: Phospholipid bilayer with embedded proteins; allows for molecular transport.

    • Facilitated diffusion: Passive transport requiring no energy.

    • Active transport: Requires energy expenditure by the cell to move molecules against a concentration gradient.

    • Homeostasis: Maintains a stable internal environment.

Comparison of Plasma Membranes

  • Table 3.3: Comparison of plasma membranes in Bacteria, Archaeons, and Eukarya.

    • Eukarya: Phospholipid bilayer, diverse lipid compositions, sterols present for membrane stability.

    • Archaeons: Can have either bilayers or monolayers, ether linkages, branched isoprenoid chains.

    • Bacteria: Mostly phospholipid bilayers, ester linkages, straight fatty acid chains.

Cell Wall

  • Function: Provides support and protection to the cell; composition differs across domains.

    • Eukaryotic examples: Cellulose in plants, chitin in fungi.

Cellulose and Chitin Structures

  • Chemical structure of cellulose: Composed of β-1,4-glycosidic bonds between glucose monomers.

  • Chemical structure of chitin: Composed of N-acetylglucosamine units linked by specific glycosidic bonds.

Cytoskeleton

  • Function of the cytoskeleton: Responsible for cell shape, intracellular transport, cell division, and motility.

    • Components:

    • Microtubules (tubulin): 25 nm in diameter, involved in intracellular transport and separation of chromosomes during cell division.

    • Microfilaments (actin): 7 nm in diameter, facilitate cell movement and maintain shape.

    • Intermediate filaments: 8-11 nm in diameter, provide structural support and anchor organelles.

Cytoskeletal Functions

  • Intracellular trafficking: Facilitates movement of vesicles and organelles within the cell.

  • Cell motion: Essential for movement of cilia and flagella.

  • Cell division: Assists in chromosome separation and the formation of the mitotic spindle.

Eukaryote Diversity

  • Types of Eukaryal Microorganisms: Various eukaryal organisms defined by their functions, structures, and metabolic pathways (refer to Table 3.7 for details).

  • highly conserved genes can be used to enhance our understanding of enkaryal phylogeny (tubulins)

    • Major categories:

    • Fungi: Heterotrophic, typically non-motile with chitin cell walls.

      • eg.Saccharomyces cerevisiae

      • heterotrophic:cell walls of chitin,used to make bread, beer, wine

      • easy/cheap tool to study eukaryotic structure/gene expression

    • Fungal phylogeny

      • Chytridiomycota ;early branching, “watermolds”,Laurel Creek banks

      • Zygomycota:Rhizopus, bread mold,lab contamination

      • Glomeromycota : mycorrhizal fungi- extremly important for plant/trees

      • Ascomycota;’spore shooter”, cup/sac fungi, yeast

      • Basidiomycota:”spore droppers”, “club fungi”, traditional mushroom producing fungi

    • Protozoa: Varied metabolisms (some photosynthetic) and movements (e.g., pseudopods, cilia).

      • some heterotrophic, some photosynthesis

      • variable cell walls

      • different motility strategies

      • different reproduction strategies

    • diverse habitats, ranging from aquatic to terrestrial environments, allowing for adaptation and evolution.

      • GIARDIA LAMBIA

        • genetically “odd”, lacks mitochondria

        • cause human disease

    • Slime molds: Unique lifecycle stages and behaviors, can form multicellular structures.

      • Dityostelium disciodeum

        • model for studying ecology, cell motility, and cell-cell communication

      • Physarum

        • fuses many cells into a continuous multinucleate giant cell

    • Algae: Photosynthetic, generally have cellulose cell walls, can be unicellular or multicellular.

      • many are multicellular

      • all are photosyntheic with cellulose cell walls

      • Chlamydomonas - has two-flagella form good for studying eukarayal flagella biogenesis/funcition

      • durable and easy to grow

Replication - eukaryotic microorganism

  • life cycles are more complicated due to haploid/diploid states

  • possibilities for sexual or asexual reproduction

Mitosis

  • basic cell divison

  • produces two identical cells from one original cell

Meiosis

  • four haploid cells from one original diploaid cell

  • one round of DNA replication followed by two rounds of cell division

  • genetic recombination

    • segregation of maternal/paternal chromosomes

    • ‘crossing over”between chromosomes prior to segregation

    • ensures each haploid cell is genetically distinct

Saccharomyces life cycles

  • can undergo meiosis to form an ascus

  • haploid mating types can fuse to reproduce sexually or be maintained by asexual mitosis

  • Saccharomyces not limited to ascus formation

  • budding off ofsmaller cells can occur or fission of identically sized cells

Chylamdomanas life cycle

  • maintains a motile haploid state

  • haploid cells differentiate and fuse into a diploid form in bad conditions

    • spore formation

Dictyostelium life cycle

  • exists in a haploid unicellular form until conditions worsen

  • multicellular “slug” is formed with a stalk and a fruiting body

  • spores form in the fruiting body, restarting the life cycle as haploid cells

  • haploid cells can fuse into a diploid macro-cyst form

  • macrocyst more haploid cells

Cell Division in Eukaryotes

  • Mitosis: A process that results in two genetically identical diploid cells from a single diploid cell, involving DNA replication and cytokinesis.

  • Meiosis: A process leading to four genetically distinct haploid cells from one diploid cell, involves one round of DNA replication followed by two cell divisions, genetic recombination, and segregation of chromosomes.

Eukaryotic Life Cycles and Reproduction

  • Saccharomyces cerevisiae: Model organism showing both sexual (meiosis, ascus formation) and asexual (budding or fission) reproduction.

  • Chlamydomonas: Inhabits both haploid and diploid forms, transitioning based on environmental conditions.

  • Dictyostelium: Shows complex life cycle stages including unicellular and multicellular forms during stress.

Origins of Eukaryotes

  • Endosymbiotic Theory: Suggests eukaryotes arose from symbiotic relationships between primitive archaeal cells and engulfed bacteria.

    • Evidence supporting this theory includes similarities in DNA and structure between mitochondria/chloroplasts and bacteria.

    • life started 4.5 to4 bya, but enkaryotes appeared around 2.1 to 1.6 bya

    • one primitive microorganism(archaea) egulfred/injested another (bateria), forming a symbiosis

    • at least two endosymbiotic events must have occured

      • mitochondria

      • chloroplasts

    • mitchondria/chloroplasts resemble bacteria in both size and shape

    • double membranes (host and bacterium)

    • “Cell” division with FtsZ

    • eachhas its own DNA rRNA more similar to bacterial sequences than eukaryal ones

    • circular chromosome

EXCEPTION: Amitochndraties lack mitchondria.cells likely evoled out of using them to obtain energy

  • two cells better than one

  • paramecium ingesting algae and using them for photosynthesis

Questions about endosymbiotic theory

  • if we can show it occurs in experiments why has it only been stable twice in history

  • what was the thing that was first engulfred,

  • How did the initial “engulfing” deal with a cell wall strucutre, if there was one

  • are other organelles the result of endosymbiosis? The nucleus has a double membrane as well

Interactions with Other Organisms

  • protozoa can cause significant human disease

  • Faungi are likely to cause dieases, but can so in immuno-compromised indiviuals

  • protozoa and fungi can cause signficant disease in plants

  • potato blight and the grea trish famine,mid-1800s

  • Diseases caused by eukaryal microorganisms: Can lead to significant human and plant diseases (e.g., malaria, fungal infections).

  • Beneficial roles of eukaryal microorganisms: They serve as primary producers, contribute to oxygen production, and are important for recycling nutrients (e.g., cellulose degradation by termites).

    • biodgraders recycle nutrients

    • some eukaryal microbes can degrade cellulose, recycling plant matter better than animals can

Summary of Essential Takeaways

  • Eukaryotes are defined by complex cellular structures and diverse metabolic pathways, encompassing a wide array of organisms, all possessing unique attributes that drive their functioning and interactions within ecosystems.