If a eukaryote is not a fungi, plant, or animal then it is placed in Kingdom Protista.
Most are unicellular (e.g. paramecium, amoebas, etc.).
Some are multicellular (e.g. brown algae like giant kelp!).
Protists are more diverse in structure and function than any other group of organisms.
The only thing that unifies all protists is the fact that they are eukaryotes!
Evidence from the evolutionary history of all living organisms, also known as “phylogeny,” shows that kingdom protista is an artificial group.
Evolutionary History of Protists
Protists are not a monophyletic group: there is not one common ancestor that is unique to only the organisms in Kingdom Protista.
Contrast this with animals which all have one common ancestor—the “first animal,” or fungi which all have one common ancestor—the “first fungi,” or plants which all have one common ancestor—the “first plant.” These three kingdoms are therefore “natural” (monophyletic) groups.
Conclusion: Kingdom Protista is an artificial grouping of organisms.
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Protist Nutrition
Photoautotrophs
Some protists have chloroplasts and “make” their own energy.
Examples:
Unicellular: diatoms—common in fresh water, found everywhere in the marine environment. One of the major photosynthetic producers in coastal waters.
Multicellular: brown algae—one of the most well known in California is the giant kelp which can grow up to 60 meters in length!
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Chemoheterotrophs
Many protists eat other organisms.
Examples:
Didinium captures and swallows a paramecium (another protist).
Amoeba uses its pseudopodia (false feet) to capture its food.
Stentor uses its beating cilia to draw food into its gullet.
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Mixotrophs
Some protists are both autotrophic and heterotrophic.
Example:
Euglena: This protist has chloroplasts and therefore photosynthesizes in the presence of light. When in the dark it switches to a heterotrophic form of nutrition in which it absorbs organic molecules from the environment.
