Overview
This lecture covers the biology of protists, including their classification, evolution via endosymbiosis, diversity, nutritional strategies, and the major supergroups as outlined in Chapter 28 of Campbell Biology (10th Ed.).
Introduction to Protists
- Protists are eukaryotes not classified as plants, animals, or fungi.
- Most protists are single-celled but exhibit high structural and functional diversity.
- Protists are found in diverse aquatic environments, e.g., Stentor in freshwater.
Eukaryotic Cell Origins & Endosymbiosis
- Eukaryotic cells are distinguished by internal organelles (e.g., mitochondria, plastids).
- Endosymbiosis is the process where one organism lives inside another; it likely led to eukaryotic cells.
- Mitochondria and plastids (e.g., chloroplasts) originated as free-living prokaryotes engulfed by ancestral eukaryotes.
- Mitochondria derive from proteobacteria; plastids from cyanobacteria.
- Evidence: mitochondria/plastids have their own DNA and prokaryote-like ribosomes/membranes.
Diversity and Classification of Protists
- Protists show more diversity in structure and function than animals, plants, or fungi.
- Nutrition varies: protists can be photoautotrophic, chemoheterotrophic, or mixotrophic.
- Reproduction can be asexual (like mitosis or binary fission) or sexual (involving meiosis and fertilization).
- Four main supergroups of eukaryotes: Excavata, SAR (Stramenopiles, Alveolates, Rhizarians), Archaeplastida, Unikonta.
Endosymbiosis and Protist Diversity
- Primary endosymbiosis: eukaryote engulfs prokaryote (e.g., formation of chloroplasts in green/red algae).
- Secondary endosymbiosis: eukaryote engulfs another eukaryote (e.g., red/green algae engulfment).
Supergroup: Excavata
- Excavata includes protists with unique cytoskeletons and some with feeding grooves.
- Main groups: Diplomonads (two nuclei, reduced mitochondria called mitosomes, e.g., Giardia), Parabasalids (reduced mitochondria called hydrogenosomes, e.g., Trichomonas vaginalis), Euglenozoans.
- Euglenozoans: diverse nutrition; characterized by a crystalline rod in their flagella.
- Kinetoplastids (e.g., Trypanosoma—causes sleeping sickness); Euglenids (some mixotrophic and can switch nutrition modes).
Supergroup: SAR (focus on Stramenopiles)
- Stramenopiles often possess one hairy and one smooth flagellum.
- Diatoms: single-celled, glasslike cell walls, major phytoplankton group.
- Golden algae: mostly single-celled or colonial, have yellow/brown pigments, some are mixotrophs.
- Brown algae: multicellular, marine (seaweeds like kelp), body parts include holdfast, stipe, and blades. Analogous, not homologous, to plant structure.
Key Terms & Definitions
- Protist — any eukaryote not in the kingdoms Plantae, Animalia, or Fungi.
- Endosymbiosis — a relationship where one organism lives inside another, leading to organelle formation.
- Mitochondria — energy-producing organelle derived from proteobacteria.
- Plastid (Chloroplast) — photosynthetic organelle derived from cyanobacteria.
- Primary Endosymbiosis — eukaryote engulfs a prokaryote symbiont.
- Secondary Endosymbiosis — eukaryote engulfs a eukaryotic alga.
- Mixotroph — organism capable of both photosynthesis and heterotrophy.
- Excavata — protist supergroup with distinctive cytoskeleton/feeding groove.
- Stramenopiles — subgroup of SAR, often with two distinct flagella.
Action Items / Next Steps
- Read Chapter 28 in Campbell Biology (10th Ed.).
- Observe living protists (e.g., Stentor, diatoms, Euglena, Amoeba) in lab.
- Avoid mixing living protists during lab to prevent predation.
- Review diagrams on endosymbiosis and protist supergroups in your textbook.