Eukaryotic Organisms and Endosymbiotic Theory Study Guide

The Endosymbiotic Theory and Eukaryotic Evolution

• The Endosymbiotic Theory: This theory explains the evolutionary transition of complex eukaryotic cells from simpler prokaryotic cells.

• Origin of Organelles: The theory proposes that specific organelles within modern eukaryotic cells—most notably mitochondria and chloroplasts—originated as free-living bacteria.

• The Engulfment Process: These free-living bacteria were engulfed by a larger host cell, referred to as a protoeukaryote. Instead of undergoing digestion, the bacteria and the host formed a symbiotic relationship where both entities benefited.

• Specific Origins:

  • Mitochondria: Evolved from aerobic bacteria and are responsible for energy production.

  • Chloroplasts: Evolved from photosynthetic bacteria and are responsible for photosynthesis.

Characteristics and Differences of Eukaryotic and Prokaryotic Cells

• Size and Complexity: Eukaryotes are generally larger and more structurally complex than prokaryotes.

• Internal Structures:

  • Eukaryotes: Possess a defined nucleus and membrane-bound organelles.

  • Prokaryotes: Lack a nucleus and membrane-bound organelles; their genetic material is located in a nucleoid, an open region of the cell. They may contain membranous inclusions.

• Organism Examples:

  • Eukaryotes: Include both unicellular and multicellular organisms such as animals, plants, and most fungi.

• Reproduction:

  • Eukaryotes: Reproduce both asexually through mitosis and sexually through meiosis.

  • Prokaryotes: Reproduce only asexually.

• Cell Walls:

  • Eukaryotes: A cell wall is not universal; it is present only in plants, fungi, and some protists.

  • Prokaryotes: Most possess a cell wall, with exceptions such as mycoplasma and L-forms.

• Ribosomes: Eukaryotic cells contain $80S$ ribosomes.

• Genetic Material:

  • Both cell types utilize DNA as their genetic material.

  • Eukaryotes: Possess multiple linear chromosomes.

  • Prokaryotes: Possess a single circular chromosome.

Cell Division: Mitosis and Meiosis

• Shared Requirements: Both processes are mechanisms for cell division that require a parent cell and the replication of DNA prior to the division.

• Mitosis:

  • A form of asexual division.

  • Results in offspring cells that are genetically identical to the parent.

  • Produces diploid cells.

• Meiosis:

  • A form of sexual cell division.

  • Produces offspring cells with half the number of chromosomes as the parent cell.

  • Produces haploid cells.

  • Results in 4 offspring cells.

Transport Mechanisms in Eukaryotic Cells

• Endocytosis: The process of bringing substances into the cell.

• Exocytosis: The process of exporting substances out of the cell.

• Phagocytosis (Cell-eating):

  • An importation process starting at the plasma membrane.

  • Involves engulfing non-dissolved substances and breaking them down.

• Pinocytosis (Cell-drinking):

  • An importation process starting at the plasma membrane.

  • Involves engulfing substances that are already dissolved.

The Four Kingdoms of Eukaryotes

• Fungi: Includes both unicellular and multicellular organisms.

• Protista: Includes both unicellular and multicellular organisms.

• Animalia: Consists only of multicellular eukaryotes.

• Plantae: Consists only of multicellular eukaryotes.

• Photosynthetic Kingdoms: Includes Plantae and some members of Protista.

Parasitic Helminths

• Definition: Parasitic worms.

• Main Groups:

  1. Flatworms (tape worms).

  2. Round.

Fungal Morphology and Growth: Hyphae

• Hyphae: Tubular extensions that represent the primary mode of growth for most fungi.

• Types of Hyphae:

  1. Septate hyphae: Feature divisions between each cell in the filament, appearing like a string of individual cells. The dividing structure is called the septum.

  2. Aseptate hyphae: Lack divisions and appear as a long, continuous stick containing many nuclei.

Fungal Spores and Classification

• Sexual Spores (formed by meiosis):

  1. Zygospores: Haploid gametes located on the tips of the hyphae.

  2. Ascospores: Haploid gametes that form within a sac known as an ascus.

  3. Basidiospores: Spores that bud off of a pedestal structure called a basidium.

• Asexual Spores (made by mitosis):

  1. Conidiospores: Spores that form in chains and are not enclosed in a sac.

  2. Sporangiospores: Spores that are formed within a sac known as a sporangium.

Mycosis: Fungal Diseases in Humans

• Definition: Mycosis refers to diseases specifically caused by fungi.

• Human Examples:

  1. Pneumocystis pneumonia in AIDS patients.

  2. Vulvovaginal candidiasis (vaginal yest infection).

  3. Histoplasmosis.

  4. Coccidioidomycosis.

Kingdom Protista: The Catchall Category

• Diversity: Protista is a highly diverse group of eukaryotes.

• Catchall Taxonomic Category: It is described this way because its members possess features reminiscent of all other eukaryotic kingdoms (animal-like, plant-like, or fungi-like).

• Scope: It includes organisms that do not fit into the other strictly defined kingdoms.

Protozoans: Classification and Motility

• Definition: Animal-like protists.

• Classification: Primarily classified based on their method of motility in their mature form.

• Grouping by Motility:

  1. Amoeboid: Utilize pseudopods.

  2. Flagellated: Utilize flagella.

  3. Ciliated: Utilize cilia.

  4. Spore forming.

The Eukaryotic Plasma Membrane and Sterols

• Basic Structure: All cells possess a plasma membrane, which is a flexible phospholipid bilayer consisting of two layers of phospholipids.

• Functions: Protects the cell, controls the movement of substances, and assists in cell communication.

• Membrane Stabilizers: Contain sterols, which are a type of lipid or fat-like molecule.

• Sterol Variation by Kingdom:

  • Animal cells: Use cholesterol.

  • Fungal cells: Use ergosterol.

  • Plant cells: Use phytosterols.

The Eukaryotic Cell Wall across Kingdoms

• Location and Function: Located outside the plasma membrane; maintains cell shape and protects against mechanical and osmotic stress.

• Composition by Kingdom:

  • Plants: Use cellulose cell walls.

  • Fungi: Use Chitin cell walls.

  • Protists (many algae): Composition varies by species; contains cellulose, calcium, carbonate, Xylan, silica, or other protein- and carbohydrate-based compounds.

  • Animals: Do not have a cell wall.

The Glycocalyx and Extracellular Protection

• Definition: An extracellular layer enriched with carbohydrates, glycoproteins, and glycolipids.

• Roles:

  • Cell protection.

  • Cell communication.

  • Cell adhesion.

  • In multicellular organisms, it aids in proper tissue development.

Locomotion: Flagella and Cilia

• Eukaryotic Flagella:

  • Built from the protein tubulin.

  • Consists of microtubules arranged in a $9+2$ pattern.

  • Covered by the plasma membrane.

  • Anchored by a basal body; microtubules sprout from a centriole.

  • Movement uses dynein proteins and $ATP$.

  • Displays a wavelike motion (whips back and forth).

• Prokaryotic Flagella:

  • Built from the protein flagellin.

  • Generally not membrane-enclosed (except for periplasmic flagella).

  • Features a hook-and-filament structure anchored by rings.

  • Moves in a rotary (propeller) motion.

• Cilia:

  • Feature a $9+2$ structure made of tubulin.

  • Surrounded by a membrane.

  • Beat in an oar-like motion consisting of a Power stroke and a Recovery stroke (left to right motion).

  • Comparison: Much shorter and more numerous than flagella.

Eukaryotic Ribosomes

• Structure: $80S$ structures.

• Location:

  • Can be free in the cytoplasm.

  • Can be bound to the surface of the rough endoplasmic reticulum (ER).

• Functional Differences:

  • Bound Ribosomes: Produce proteins destined for secretion from the cell.

  • Free Ribosomes: Produce cytosolic proteins.

• Adaptability: Ribosomes can change from free to bound status depending on the protein production needs of the cell.

The Cytoskeleton and Centrosomes

• Cytoskeleton: A protein fiber network responsible for maintaining cell shape, aiding movement, protecting against external forces, directing transport, and coordinating cell division.

• Fibers: Composed of three main types: Microtubules, Intermediate filaments, and Microfilaments.

• Centrosome:

  • The organelle that organizes and builds microtubules in most eukaryotic cells.

  • Contains two centrioles.

  • Acts as the microtubule-organizing center.

  • Microtubules grow outward from the centrosome to form the mitotic spindle, cilia, and flagella.

The Nucleus: Genetic Control Center

• Visibility: Large organelle visible with a light microscope.

• DNA Storage: Houses DNA, which is loosely organized as chromatin floating in the nucleoplasm.

• Nuclear Envelope: A double-membrane structure with pores that regulate traffic (the movement of materials in and out). It gives rise to the endoplasmic reticulum.

• Nucleolus: A dense area within the nucleus with a high concentration of RNA; it is the site where ribosomal subunits begin development.

The Endomembrane System: Endoplasmic Reticulum and Golgi Apparatus

• Endoplasmic Reticulum (ER):

  • A series of interconnected membranes originating from the nuclear envelope.

  • Central to the folding, modifying, and packaging of proteins.

  • Plays a role in building lipids.

• Golgi Apparatus:

  • Responsible for building lipids.

  • Coordinates with the ER to sort and package cellular lipids and proteins to ensure they reach their correct destination.

  • Receives vesicles that bud off the ER.

Vesicles and Vacuoles: Transport and Storage

• Vesicles: Small lipid bilayer sacs.

  • Transport vesicles: Move substances throughout the cell.

  • Secretory vesicles: A type of transport vesicle that delivers substances to the cell surface for expulsion (discharge) from the cell.

  • Lysosomes: Packed with hydrolytic enzymes used to break down substances engulfed by the cell (important for phagocytosis).

  • Peroxisomes: Contain enzymes that oxidize and break down fats and amino acids.

• Vacuoles: Large membranous sacs formed by the merger of many vesicles.

  • Vary in size and shape; common in plants and fungi.

  • Contain mainly water and various organic or inorganic substances (nutrients, toxins).

  • Contractile vacuoles: Found in many freshwater protists; they collect and remove excess water that enters by osmosis to maintain osmotic balance and prevent the cell from bursting.

Mitochondria and Chloroplasts: Energy and Endosymbiosis

• Mitochondria Structure:

  • Surrounded by a double membrane.

  • Outer membrane is smooth.

  • Inner membrane is folded into Cristae to increase surface area.

  • Matrix (the inside) contains enzymes, DNA, and ribosomes.

• Mitochondrian Functions:

  • Produce energy ($ATP$) through cellular respiration; known as the powerhouse of the cell because they fuel cell work.

  • Manufacture amino acids and vitamins.

  • Regulate cell division.

  • Carry out programmed cell death (apoptosis).

• Shared Features with Bacteria (Endosymbiotic Evidence):

  • Both mitochondria and chloroplasts are double-membrane structures.

  • Contain $70S$ ribosomes.

  • Possess a circular genome with genetic features reminiscent of bacteria.

  • Divide through a process resembling binary fission.

• Distribution:

  • Chloroplasts are found only in photosynthetic cells.

  • Mitochondria are found in most eukaryotic cells, including plant cells.