Protists
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Protists: Single-Celled Eukaryotes
Protists are eukaryotic forms, single-celled organisms made of cells that closely resemble those of multi-celled life forms. In the period between 1980 and 2005, the classification of these organisms changed in many ways.
The Tradition Classification of Protista
This table below shows the variety found in many habitats. What criteria does it use to distinguish different groups from each other?
Protozoa: "animal-like" heterotrophs |
Amoeba |
Change shape, surround prey to absorb it. |
Foraminiferans |
Make "tests", shells made of calcium (responsible for deposits like White Cliffs of Dover, England). |
Actinopods |
Have long filaments which they use to trap algae; often live in sybiosis with algae. |
Flagellates |
Have one or more long, whiplike appendages that allow them to move; often symbiotic or parasitic; one type causes sleeping sickness. |
Ciliates |
Have many short "hairs" which help them move. Includes Paramecium |
Sporozoans |
Form spores to live through dry spells or phases between hosts. One type causes malaria. |
|
Algae: "plant-like" autotrophs |
Dinoflagellates |
Marine plankton, photosynthetic. Some are endosymbiotic — they provide energy from photosynthesis to their host. Often part of coral reefs. |
Diatoms |
Primarily unicellular (some form colonies). Have two shells with either radial symmetry or bilateral symmetry; the variety of shapes and sizes and colors is stunning. Both marine and freshwater forms are common; deposits form diatomaceous earth, a natural filter. |
Euglenoids |
Often photosynthetic, shape changers like amoebas. Have complex internal structures and are light-sensitive. |
Green algae |
Very similar to plants in their cell pigments, storage products (sugars and starches) and cell walls. Multicellular forms do not have differentiated tissues, however. |
Red algae |
Primarily multicellular, with interwoven filaments. Similar to cyanobacteria, and again, no differentiation into separate kinds of tissue that marks true plants. |
Brown algae |
Primarily multicellular. Common seaweed; can be up to 200 feet long. |
|
Slime molds |
Plasmodial |
Similar to fungi. In dry conditions, will change form and produce spores which can survive until water becomes available again. |
Cellular |
Multicellular, more like amoeba that like plasmodial molds. During one stage, the organism is a many single cells with individual nuclei; during dry spells, the cells come together to form a fruiting body and produce spores. |
|
The Modern Classification of Protista
This classification system has emerged in recent decades as a different way of grouping single-celled eukaryotic organims. What characteristics are now emphasized in distinguishing between these groups?
Group I |
Diplomonads |
Heterotrophic |
Two nuclei, no mitchondria [Giardia intestinalis] |
Parabasalids |
Heterotrophic |
Modified mitochondria, anaerobic [Trichomonas vaginalis] |
Euglenozoans |
Heterotrophic, photosynthetic autotrophs, parasites |
Unique internal structure (crystalline rod, function unknown) [Trypanosoma] |
|
Group IIA Alveolates (sacs beneath membranes) |
Dinoflagellates |
Photosynthetic autotrophs, heterotrophs |
[Red tide] |
Apicomplexans |
Parasites |
Disease vectors [malaria] |
Cilliates |
Use many cilia for motion |
[Paramecium] |
|
Group IIB Stramenopiles (hairy and smooth flagellum |
Diatoms |
Autotrophic |
Unicellular, silica cell walls |
Brown Algae |
Autotrophic |
Brown chloroplasts [Seaweed, kelp] |
Water molds |
Decompose fungi |
Freshwater habitats [Late Blight] |
|
Group III: Threadlike pseuedopodia |
Forams |
Heterotrophs |
Porous calcium carbonite shells |
Radiolarians |
Heterotrophs |
Internal silica skeleton |
|
Group IV |
Red algae |
Autotrophs |
Red chlorophyll [Agar] |
(Green Algae) Chlorophytes |
Autotrophs |
Green chlorophyll, alternation of generations |
(Green Algae) Charophytes |
Autotrophs |
Green chlorophyll [plant ancestors] |
Land Plants |
Autotrophs |
Multicellular |
|
Group V |
(Amoebozoans) Amoebas |
Heterotrophs |
Lobe-shaped pseudopodia |
(Amoebozonas) Slime Molds |
Heterotrophs |
Multinucleate |
Fungi |
Heterotrophs; decomposers |
Multicellular |
Choanoflagellates |
Heterotrophs |
|
Animals |
Heterotrophs |
Multicellular |
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