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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.
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Animal-like protist
(amoeba)
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Plant-like protist
(Volvox)
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Fungi-like protist
(slime mold)
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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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the fungus kingdom