18.05 Kingdom Protoctista
Definition
- A diverse kingdom of eukaryotic organisms that do not fit into the kingdoms Fungi, Plantae, or Animalia.
- Includes:
- Protozoans: Animal-like protoctists (e.g., amoeba, Stentor roseli).
- Algae: Plant-like protoctists (e.g., seaweeds).
Key Features of Protoctists
- Eukaryotic:
- Cells have a nucleus and membrane-bound organelles.
- Cellular Structure:
- Mostly single-celled (unicellular) or exist as colonies of similar cells.
- Some exhibit specialized regions within their unicellular body (e.g., Stentor roseli).
- Cell Wall:
- Protozoa: Animal-like, no cell wall.
- Algae: Plant-like, cell walls with cellulose; some have chloroplasts for photosynthesis.
- Modes of Nutrition:
- Heterotrophic: Protozoa ingest and digest food (like animals).
- Autotrophic: Algae perform photosynthesis (like plants).
- Diversity:
- Some protoctists share closer relationships with organisms in other kingdoms than with other protoctists (e.g., arguments for classifying algae as plants).
Examples
- Protozoans:
- Stentor roseli: Unicellular protozoan with cilia for movement and feeding; exhibits specialization within its single cell.
- Algae:
- Seaweeds: Multicellular, plant-like protoctists found in aquatic environments.
Key Terms
- Protoctista: Kingdom of eukaryotic organisms, single-celled or simple multicellular.
- Protoctist: Member of the Protoctista kingdom.
FIGURE
- Figure A:
Stentor roseli: A unicellular protoctist with specialized regions, covered in cilia for movement and feeding (magnified ×240).
Summary
- The kingdom Protoctista encompasses highly varied eukaryotic organisms that do not belong to other eukaryotic kingdoms. Their features include:
- Plant-like algae (autotrophic, cellulose walls, chloroplasts).
This kingdom reflects the complexity and transitional nature of eukaryotic life forms.
Eukaryotic cells.
- Unicellular or simple multicellular forms.
- Diversity in structure and nutrition:
- Animal-like protozoa (heterotrophic, no cell wall).