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Chapter 16: 13-19

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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