Chapter 23: Protists and Protozoans

Overview of Protists

  • Domain: Eukarya
  • Kingdom: Protista
    • Comprises diverse organisms, including:
    • Land plants
    • Animals
    • Fungi
    • Green algae
    • Amoebas
    • Cellular slime molds
    • Forams
    • Red algae
    • Dinoflagellates

Protozoans

  • Existence: Protozoans have existed for at least 2.7 billion years.
  • Structure: Most are unicellular, with some exceptions.
  • Discovery: First observed by Leeuwenhoek in 1674.

Characteristics of Protozoans

  • Organelles: Similar organelles to other eukaryotes:
    • Nuclei
    • Mitochondria
    • Linear chromosomes
    • Mitosis
    • Sexual reproduction
  • Additional Structures:
    • Complex cytoskeleton
    • Contractile vacuole: maintains osmotic balance by collecting water and discharging it via a pore.
    • Example: Paramecium with contractile vacuoles.
    • Extrusomes: Organelles that eject toxins, mucus, adhesives, anchors, or spikes.

Movement in Protozoans

  • Modes of locomotion:
    • Cilia: Small, hair-like structures for movement.
    • Flagella: Tail-like structures for propulsion.
    • Pseudopodia: Temporary projections used for movement and food collection.

Reproduction in Protozoans

  • Predominantly sexual:
    • Conjugation: Direct exchange of genetic material.
    • Syngamy: Fusion of gametes.
  • Asexual methods include:
    • Fission: Division of a single organism into two.
    • Budding: Growth of a new individual from a parent.

Nutrition in Protozoans

  • Nutritional strategies:
    • Photoautotrophs: Use chloroplasts for photosynthesis.
    • Chemoheterotrophs: Obtain nutrients from organic compounds.
    • Mixotrophs: Combine autotrophic and heterotrophic methods.

Endosymbiosis Theory

  • Definition: A unicellular organism engulfs another cell, which becomes an endosymbiont.
  • The theory explains the origin of organelles such as mitochondria and chloroplasts, believed to be reduced endosymbionts.

Supergroups of Protists

  1. Excavata:

    • Sub-groups: Diplomonads, Euglenozoans, Parabasalia.
    • Examples: Giardia, Leishmania, Peranema.

  2. Chromalveolata:

    • Two main branches: Alveolates and Stramenopiles.
    • Alveolates include Dinoflagellates, Apicomplexans, and Ciliates.
    • Stramenopiles include Diatoms, Brown Algae, Golden Algae, and Oomycetes.

  3. Rhizaria:

    • Characterized by slender pseudopodia and shells.

  4. Archaeplastida:

    • Includes Red algae and Green algae, chloroplasts similar to land plants.

  5. Amoebozoa:

    • Contains lobe-shaped pseudopodia, includes amoebas and slime molds.

  6. Opisthokonta:

    • Includes Choanoflagellates, closely related to the Kingdom Animalia and Fungi.

Taxonomy Notes

  • The Kingdom Protozoa is paraphyletic and not a valid monophyletic kingdom.
  • Vocabulary to understand:
    • Contractile vacuole, Extrusome, Conjugation, Syngamy, Endosymbiosis, Cilium, Flagellum, Pseudopodium.

Review Questions

  • What Domain and Kingdom do protists belong to? Is this Kingdom monophyletic?
  • What characteristics define protozoans?
  • Describe the unique structures found in protozoans and their functions.
  • How do protozoans reproduce and how do they obtain nutrition?
  • Explain the endosymbiosis theory and its historical significance.
  • What are the main supergroups of protists, and what organisms do they include?
  • Discuss the relations between protists and the other kingdoms like Animalia, Plantae, and Fungi.