The Origin and Evolution of Eukaryotes PDF

The Origin and Evolution of Eukaryotes The term "eukaryote" is derived from the Greek words "eu," meaning true or well, and "karyon," meaning nucleus. Eukaryotic cells are more structurally and functionally complex than prokaryotic cells, which lack a true nucleus and membrane-bound organelles. Ma...

The Origin and Evolution of Eukaryotes The term "eukaryote" is derived from the Greek words "eu," meaning true or well, and "karyon," meaning nucleus. Eukaryotic cells are more structurally and functionally complex than prokaryotic cells, which lack a true nucleus and membrane-bound organelles. Many members of the domain Eukarya are familiar to us Plants, Fungi and Animals are readily recognizable However, a dazzling assortment of Eukaryotes, mostly microscopic single-celled organisms Eukaryotes that are not plants, animals, or fungi have traditionally been “dropped” into the category protists Protists are highly diverse and not closely related to each other. word “protist” does not describe a formal taxonomic group, but is a convenience term—a shorthand way of saying “all the eukaryotes that are not plants, animals, or fungi Derived from Greek Word “Protistos” – the very first. The diversity of protists is reflected in both morphology and phylogeny Some are motile, while others do not move some are photosynthetic, others heterotrophic most are unicellular, but some are multicellular Most are microscopic, but a few are huge Unicellular species of protists as microbial eukaryotes, but you should keep in mind that there are large, multicellular protists as well The phylogeny of the major eukaryote lineages remains a subject of research and debate Some groups of protists are closely related to animals and fungi, whereas others are closely related to the land plants, and still others are only distantly related to any of these familiar eukaryotes Eukaryotes are Monophyletic In other words, there was a single eukaryotic ancestor which diversified into the many different lineages of protists, as well as plants, animals, and fungi. Eukaryotes are Monophyletic Given the nature of evolutionary processes, the many synapomorphies of eukaryotes undoubtedly did not arise simultaneously  The origin of a flexible cell surface The The origin of a cytoskeleton eukaryotic cell The origin of a nuclear envelope, arose in which enclosed a genome organized into chromosomes several steps The appearance of digestive vacuoles The acquisition of certain organelles via endosymbiosis Whether it is cuboid (A) or spheroid (B), as an object grows larger its volume increases more rapidly than its surface area. Cells must maintain a large surface area-to-volume ratio in order to function. This fact explains why large organisms must be composed of many small cells rather than a few huge ones.  The origin of a flexible cell surface The first step toward the eukaryotic condition was the loss of the cell wall by an ancestral prokaryotic cell. Compartmentalization is the key to eukaryotic cell evolution and function The membranous compartments of eukaryotic cells are called organelles. Each type of organelle has a specific role in its particular cell. Some of the organelles have been characterized as factories that make specific products. Others are like power plants that take in energy in one form and convert it into a more useful form. These functional roles are defined by the chemical reactions each organelle can carry out The endomembrane system is a complex network of membrane-bound organelles and structures within eukaryotic cells. Nucleus: The nucleus is the central organelle of eukaryotic cells and is enclosed by a double membrane known as the nuclear envelope. It contains the cell's genetic material (DNA) and is involved in DNA replication and transcription. Endoplasmic Reticulum (ER): Rough ER: Studded with ribosomes, the rough ER is involved in protein synthesis and processing. Smooth ER: Lacks ribosomes and is involved in lipid metabolism, detoxification, and calcium storage. The endomembrane system Golgi Apparatus (Golgi Complex): The Golgi apparatus modifies, sorts, and packages proteins and lipids produced in the ER for transport to their final destinations. Vesicles: Small membrane-bound sacs that transport materials within the cell and to and from other organelles. Lysosomes: Membrane-bound organelles containing digestive enzymes that break down cellular waste materials and cellular components. The endomembrane system Vacuoles: Plant Vacuole: Large membrane-bound organelles in plant cells that store water, ions, and various substances. Contractile Vacuole (in some protists): Involved in osmoregulation, regulating water content. The endomembrane system Endosomes: Membrane-bound vesicles that transport, sort, and process endocytosed materials, including nutrients and signaling molecules. Peroxisomes: Membrane-bound organelles that participate in lipid metabolism and detoxification reactions, including the breakdown of hydrogen peroxide. Secretory Vesicles: Vesicles that transport and release cellular products, such as hormones, enzymes, and neurotransmitters, from the cell.  Internal membranes and the nuclear envelope probably came from the plasma membrane  The origin of a flexible cell surface If the cell’s surface is flexible, it can fold inward and elaborate itself, creating more surface area for gas and nutrient exchange With a surface flexible enough to allow infolding, the cell can exchange materials with its environment rapidly enough to sustain a larger volume and more rapid metabolism The loss of the rigid prokaryotic cell wall allowed the plasma membrane to fold inward and create more surface area.  The formation of ribosome-studded internal Changes In Cell Structure membranes, some of which surrounded the DNA; the appearance of a cytoskeleton; and and function the evolution of digestive vacuoles  A cytoskeleton composed of microfilaments and microtubules would support the cell and allow it to manage changes in shape, to distribute Origin of a Cytoskeleton daughter chromosomes, and to move materials from one part of the now much larger cell to other parts.  Microfilaments (Actin Filaments): Microfilaments are composed of actin protein subunits Microtubules: Microtubules are composed of tubulin protein subunits that polymerize to form long, hollow tubes. Intermediate Filaments: Intermediate filaments are composed of various proteins, depending on the cell type, such as keratins in epithelial cells and vimentin in connective tissue cells  The presence of microtubules in the Emergence of a cytoskeleton could have evolved in some cells to give rise to the characteristic eukaryotic Flagellum flagellum. The origin of the cytoskeleton is becoming clearer, as homologs of the genes that encode many cytoskeletal proteins have been found in modern prokaryotes.  The DNA of a prokaryotic cell is attached to a site on its plasma membrane. If that region of the plasma membrane were to fold into the cell, Emergence of a Nucleus the first step would be taken toward the evolution of a nucleus, a primary feature of the eukaryotic cell (A) The endomembrane system and cell nucleus may have been formed by infolding and then fusion of the plasma membrane. (B) The endosymbiosis theory proposes that some organelles may be descended from prokaryotes that were engulfed by other, larger cells From an intermediate kind of cell, the next step was probably phagocytosis—the ability to engulf and digest other cells. ENDOSYMBIOSIS It was founded on the concept of symbiosis — from the Greek ‘together’ and ‘living’ The theory of endosymbiosis proposes that certain organelles are the descendants of prokaryotes engulfed, but not digested, by ancient eukaryotic cells ENDOSYMBIOSIS It was founded on the concept of symbiosis — from the Greek ‘together’ and ‘living’  cyanobacteria were generating oxygen gas as a product of photosynthesis. Emergence of The increasing O2 levels in the atmosphere had disastrous consequences because most organisms Mitochondria through of the time (archaea and other anaerobic bacteria) were unable to tolerate the newly oxidizing environment. Endosymbiosis But some prokaryotes managed to cope with these changes, and—fortunately for us—so did some of the new phagocytic eukaryotes.  Oxygen, while essential for most aerobic life on Earth today, was toxic to early life forms that evolved in an anaerobic (low-oxygen) environment. This phenomenon is known as the "oxygen catastrophe" or "Great Oxygenation Event," and it occurred approximately 2.4 billion years ago. In summary, oxygen was toxic to early life on Earth because it was a radical shift from the anaerobic conditions in which life initially evolved. The rise of oxygen in the atmosphere during the Great Oxygenation Event posed a significant challenge to anaerobic organisms, which eventually led to the evolution of oxygen-tolerant and oxygen-dependent life forms.  Initially, the new organelle’s primary Emergence of function was probably to detoxify O2 by reducing it to water. Later, this reduction Mitochondria through became coupled with the formation of ATP in cellular respiration. Upon completion of Endosymbiosis this step, the essential modern eukaryotic cell was complete. In the case of mitochondria, evidence points very clearly to an endosymbiont of a-proteobacterial ancestry  Some important eukaryotes are the result of yet Emergence of another endosymbiotic step, the incorporation of a prokaryote related to today’s Chloroplasts (plastids) cyanobacteria, which became the chloroplast.  Eukaryotes in several different groups possess chloroplasts, and groups with chloroplasts appear in several distantly related clades  Plants: Land plants (embryophytes) and green algae are the most well-known photosynthetic eukaryotes. They include various species of trees, grasses, flowering plants, and other terrestrial and aquatic plants. Green Algae: Besides the green algae closely related to plants, there are numerous other groups of green algae. Examples include the Chlorophyta (green algae), Charophyceae (stoneworts), and Ulvophyceae. Red Algae: Red algae, or Rhodophyta, are another group of photosynthetic eukaryotes found in marine environments. Some species are used in the production of agar and carrageenan. Brown Algae: Brown algae, or Phaeophyta, are a group of large multicellular algae found primarily in marine environments. Kelp is a well-known example of brown algae. Diatoms: Diatoms are single-celled photosynthetic organisms that belong to the group Bacillariophyta. They are abundant in both marine and freshwater environments. (Stramenopiles) Euglenoids: Euglenoids are single-celled protists that can be both photosynthetic and heterotrophic, depending on environmental conditions. (Discoba) Dinoflagellates: Dinoflagellates are a diverse group of single-celled eukaryotes found in marine and freshwater environments. Some are photosynthetic, while others are heterotrophic. (Alveolata) Cryptophytes: Cryptophytes are unicellular algae with a complex plastid derived through secondary endosymbiosis. Haptophytes: Haptophytes are a group of single-celled algae with distinctive calcium carbonate scales. They are found in marine environments. Glaucophytes: Glaucophytes are a group of unicellular algae that are considered to be one of the earliest branching lineages of photosynthetic eukaryotes. Golden Algae: Golden algae, or Chrysophytes, are a diverse group of unicellular and colonial photosynthetic protists found in aquatic habitats. (Stramenopiles) Primary endosymbiosis All chloroplasts trace their ancestry back to the engulfment of one cyanobacterium by a larger eukaryotic cell. This event, the step that gave rise to the photosynthetic eukaryotes, is known as primary endosymbiosis. The cyanobacterium, a Gram-negative bacterium, had an inner and outer membrane. Thus, the original chloroplasts had two surrounding membranes—the inner and outer membranes of the cyanobacterium. Primary endosymbiosis Remnants of the peptidoglycan-containing wall of the bacterium are present in the form of a bit of peptidoglycan between the chloroplast membranes of glaucophytes, the first microbial eukaryote group to branch off following primary endosymbiosis of the cyanobacterium. Primary endosymbiosis Primary endosymbiosis gave rise to the chloroplasts of the “green algae” (including chlorophytes and charophytes) and the red algae. Studies of phylogeny indicate that each of these distinct lineages trace back to a single primary endosymbiosis. The photosynthetic land plants arose later from a green algal ancestor. The red algal chloroplast retains certain pigments of the original cyanobacterial endosymbiont that are absent in green algal chloroplasts. Secondary endosymbiosis Almost all remaining photosynthetic eukaryotes are the result of additional rounds of endosymbiosis. For example, the photosynthetic euglenids derived their chloroplasts from secondary endosymbiosis. Their ancestor took up a unicellular chlorophyte, retaining the endosymbiont’s chloroplast and eventually losing the rest of the constituents of the chlorophyte. This history explains why the photosynthetic euglenids have the same hotosynthetic pigments as the chlorophytes and land plants. Secondary endosymbiosis—the uptake and retention of a chloroplast-containing cell by another eukaryotic cell—took place several times, independent of each other.  Secondary endosymbiosis occurs when an organism that has already undergone primary endosymbiosis is engulfed by another eukaryotic host cell. In this case, the endosymbiont becomes an endosymbiont once again but within a eukaryotic host. This process is more complex than primary endosymbiosis, as it involves multiple membranes. For example, a eukaryotic cell that has already incorporated a primary endosymbiont (like a mitochondrion or a plastid) can be engulfed by another eukaryotic cell. The engulfed cell then becomes a secondary endosymbiont. This process can result in eukaryotic cells with multiple organelles of endosymbiotic origin. Secondary endosymbiosis is responsible for the diversity of photosynthetic eukaryotes, including various algae and protists, as it led to the acquisition of chloroplasts through the engulfment of primary endosymbiotic cells that already contained chloroplasts.  Lateral gene transfer accounts for the presence of some prokaryotic genes in eukaryotes An endosymbiotic origin of mitochondria and chloroplasts accounts for the presence of bacterial genes encoding enzymes for energy metabolism (respiration and photosynthesis) in eukaryotes, but it does not explain the presence of some other bacterial genes