The Protists Review Flashcards

Evolutionary Timeline and General Characteristics of Protists

  • Timeline of Eukaryotic Evolution

    • Chemical Signatures: Evidence of eukaryotic cells dates back as far as 2.72.7 billion years ago.

    • Fossil Evidence: The earliest known eukaryotic fossils date back approximately 2.12.1 billion years.

    • Protist Dominance: For about 22 billion years, the eukaryotic landscape was dominated by microscopic organisms referred to by the informal name "protists."

    • Biological Significance: The evolution of protists coincides with two major biological events: the origin of the eukaryotic cell and the emergence of multicellularity.

  • Diversity and Taxonomy

    • Scope: There are more than 60,00060,000 known species of protists.

    • Morphology: While the majority are unicellular, some species are colonial and others are truly multicellular.

    • Cellular Complexity: Unicellular protists are considered the most elaborate of all cells at the cellular level. This is because a single cell must independently perform all basic biological functions that, in plants and animals, are divided among specialized collective cells.

    • Taxonomic Status: The former "Kingdom Protista" is now recognized as a paraphyletic group. Because some protists are more closely related to plants, animals, or fungi than to each other, systematists are splitting the group into as many as 2020 separate kingdoms.

  • Habitats and Ecology

    • Moisture Requirements: Protists are found almost anywhere there is water, including oceans, ponds, lakes, damp soil, and leaf litter.

    • Aquatic Life: In water, they may be bottom-dwellers (attached to rocks or creeping through sand/silt) or members of the plankton (organisms that drift passively or swim weakly).

    • Ecological Roles: Protists are divided into three broad ecological categories:

      • Protozoa: Animal-like, ingestive protists.

      • Absorptive Protists: Fungus-like protists.

      • Algae: Plant-like, photosynthetic protists.

  • Nutritional Diversity

    • Aerobic Nature: Most protists are aerobic and utilize mitochondria for cellular respiration.

    • Photoautotrophs: Contain chloroplasts for photosynthesis.

    • Heterotrophs: Ingest larger food particles or absorb organic molecules.

    • Mixotrophs: Organisms that combine photosynthesis and heterotrophic nutrition.

    • Example: Euglena: A single-celled mixotroph that performs photosynthesis when light is available but becomes a heterotroph (absorbing nutrients) in the dark.

  • Motility and Reproduction

    • Movement: Most protists utilize flagella or cilia at some point in their life cycle. Eukaryotic flagella are extensions of the cytoplasm supported by a 9+29 + 2 microtubule system (not homologous to prokaryotic flagella).

    • Reproduction: Highly varied.

      • Asexual: Many reproduce exclusively via mitosis.

      • Sexual: Many employ meiosis and syngamy (union of two gametes) to shuffle genes between individuals.

Evolutionary Theories: Endosymbiosis and Autogenesis

  • Serial Endosymbiosis Theory

    • Definition: Proposes that the eukaryotic cell originated from a symbiotic coalition of multiple prokaryotic ancestors.

    • Mitochondria: Evolved from small, aerobic heterotrophic prokaryotes that were engulfed by and lived within larger cells.

    • Chloroplasts: Evolved from photosynthetic prokaryotes (cyanobacteria) living within larger host cells.

    • Evidence: Each endosymbiotic event adds a membrane derived from the vacuole membrane of the host cell.

  • Secondary Endosymbiosis

    • This occurs when a heterotrophic eukaryote engulfs a photosynthetic eukaryote (like a red or green alga), which then becomes a plastid within the new host (e.g., Alga ABC).

  • Autogenesis (The Endomembrane System)

    • Structures such as the nuclear envelope, endoplasmic reticulum, and Golgi apparatus likely evolved from infoldings of the plasma membrane.

The Four Supergroups of Domain Eukarya

  1. Excavata

  2. SAR Clade (Stramenopiles, Alveolates, and Rhizarians)

  3. Archaeplastida

  4. Unikonta

Supergroup Excavata

  • General Characteristics: Defined by cytoskeleton similarities; some members possess an "excavated" feeding groove. Includes organisms with modified mitochondria and unique flagella.

  • Kingdom Diplomonads

    • Mitochondria: Feature modified mitochondria called mitosomes that lack electron transport chains; energy is derived anaerobically.

    • Structure: Two equal-sized nuclei and multiple flagella.

    • Example: Giardia lamblia, an intestinal parasite in mammals.

  • Kingdom Parabasalids

    • Mitochondria: Reduced mitochondria called hydrogenosomes that generate energy anaerobically.

    • Example: Trichomonas vaginalis, a pathogen in the human reproductive tract. It features flagella and an undulating membrane.

  • Kingdom Euglenozoans

    • Distinguishing Feature: A spiral or crystalline rod of unknown function inside the flagella.

    • Storage Molecule: A unique glucose polymer called paramylon.

    • Subgroup: Euglenids: Have one or two flagella emerging from a pocket; typically autotrophic but can be mixotrophic or heterotrophic.

    • Subgroup: Kinetoplastids: Feature a single large mitochondrion containing an organized mass of DNA called a kinetoplast.

      • Includes Trypanosoma, the cause of African sleeping sickness in humans.

      • Includes the cause of Chagas’ disease.

The "SAR" Clade

  • Stramenopila Sub-Supergroup

    • Characteristic: Presence of numerous fine, hairlike projections on the flagella, usually paired with a smooth flagellum.

    • Diatoms (Bacillariophyta): Have glasslike walls of hydrated silica in an organic matrix; abundant in freshwater and marine plankton.

    • Golden Algae (Chrysophyta): Named for yellow and brown carotene pigments; biflagellated; often colonial and mixotrophic.

    • Brown Algae (Phaeophyta): Large, complex, multicellular marine "seaweeds." Olive color comes from carotenoid pigments. Structures include the Blade (leaf-like), Stipe (stem-like), and Holdfast (root-like).

    • Alternation of Generations: A complex life cycle alternating between multicellular haploid (nn) and diploid (2n2n) forms.

      • Heteromorphic: Generations are structurally different.

      • Isomorphic: Generations look similar.

    • Oomycetes (Water Molds): Once misidentified as fungi due to multinucleate filaments (hyphae). Differ from fungi by having cellulose cell walls (not chitin).

  • Alveolata Sub-Supergroup

    • Characteristic: Small membrane-bound cavities (alveoli) under the cell surface.

    • Dinoflagellates: Components of phytoplankton; reinforced by cellulose plates. Two flagella in perpendicular grooves produce a spinning movement.

      • Karenia brevis causes "red tide" algal blooms that produce dinotoxins.

    • Apicomplexans: Parasites of animals; use an "apex" of organelles to penetrate host cells. They disseminate as sporozoites.

      • Plasmodium: Parasite causing malaria. Requires mosquitoes and humans to complete its life cycle. Sporozoites infect the liver, become merozoites, and then infect red blood cells.

    • Ciliophora (Ciliates): Use cilia for movement and feeding.

      • Nuclei: Have a large macronucleus and several tiny micronuclei for genetic variation.

      • Example: Paramecium: Cilia in the oral groove draw in food for phagocytosis.

  • Rhizarian Sub-Supergroup

    • Characteristic: Many are amoebas with threadlike pseudopodia (extensions for feeding and movement).

    • Foraminiferans (Forams): Have porous, multichambered shells called tests.

    • Cercozoans: Common in marine and soil ecosystems; mostly heterotrophic parasites or predators.

    • Radiolarians: Marine protists with delicate silica tests fused in one piece; pseudopodia radiate from the central body.

Supergroup Archaeplastida

  • Origin: Formed when a heterotrophic protist acquired a cyanobacterial endosymbiont over 11 billion years ago.

  • Red Algae (Rhodophyta): Named for the red pigment phycoerythrin. Most are multicellular seaweeds common in warm tropical coastal waters. Example: Nori.

  • Green Algae (Chlorophyta): Named for grass-green chloroplasts similar to land plants. Over 7,0007,000 species.

    • Types: Ranges from biflagellated unicells to colonial, filamentous, and large multicellular seaweeds.

    • Chlorophytes and Charophyceans: Charophyceans are the group most closely related to land plants.

Supergroup Unikonta

  • Diversity: Includes animals, fungi, and amoebozoans.

  • Amoebozoans: Amoebas with lobe- or tube-shaped pseudopodia (true amoebas). Includes tubulinids, slime molds, and entamoebas.

  • Pathogenic Example: Brain-Eating Amoeba (Naegleria):

    • Infection Path: Water goes up the nose during swimming.

    • Migration: Amoebae penetrate the nasal mucosa and migrate to the brain via olfactory nerves.

    • Condition: Causes primary amebic meningoencephalitis (PAM).

    • Stages: Life cycle includes Cyst, Trophozoite (infective stage), and Flagellated stages.

Questions & Discussion

  • Q: Why are unicellular protists considered the most elaborate of all cells?

  • A: Because a single cell must perform all vital functions—such as digestion, respiration, and reproduction—that are handled by specialized tissues in multicellular organisms like plants and animals.

  • Q: How does Plasmodium cause disease?

  • A: It is an apicomplexan parasite that travels from a mosquito host as sporozoites, infects the human liver to become merozoites, and subsequently destroys red blood cells while evading the immune system.

  • Q: What defines the Rhizarians?

  • A: They are primarily identified as amoebas that utilize threadlike pseudopodia for movement and feeding, as opposed to the lobe-shaped pseudopodia of the Amoebozoans in the Unikonta group.