Life Science: Session 2
Classifying Living Things: The Protist Kingdom
What features distinguish protists from other life forms?
The most diverse group of organisms in this classification system is found among the Protists. This group contains plant-like, animal-like, and fungus-like organisms. This grouping is not based on evolutionary relationships but is more a grouping of convenience — the protists are the eukaryotes that aren’t plants, animals, or fungi. This may seem to violate the precision that seems important in a classification system, but this is what has been used historically and is commonly taught in K-6 classrooms and beyond. There will likely be changes in this grouping as the science of classification progresses.
The majority of organisms classified as protists are unicellular though there are a few multicellular organisms. For example, kelp (“seaweed”) is technically a protist even though it is multicellular. Kelp is not grouped with plants, however, because it lacks the cellular complexity present in plant cells. There are also “colonial protists,” which are aggregates of individual cells of the same species that function together as a group.
Plant-like protists make their own food, and many have cell walls made of cellulose. Animal-like protists cannot make their own food and ingest it. Some animal-like protests have “shells,” called tests, which are made of silica or calcium carbonate. Other animal-like protists lack a cell wall or protective test. The fungi-like protists are actually not evolutionarily related to fungi, although they do take in food by absorption. Like plants, however, their cell walls are made of cellulose.
How are protists important to people?
Animal-like protists include some well-known species. Paramecia and amoeba are frequently used examples of single-celled organisms. Malaria, a world-wide disease occurring in tropical climates, is caused by an animal-like protist, the Plasmodium. In the ocean, many plant-like protists live at the surface where they perform photosynthesis. These organisms, such as diatoms, dinoflagellates, and foraminiferans, serve as a food source to many planktonic, or floating, animals. However, these plant-like protists are not always beneficial. Red tide is caused by a population explosion in poisonous dinoflagellates and contaminates the ecosystem, killing fish and making shellfish unsafe for human consumption. The fungus-like protists are commonly known as slime molds, and include the brightly colored organisms found growing on decomposing trees and the pesky mildew growing in a bathroom.
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