Everything about Protists totally explained
Protists,
Greek protiston -a meaning the (most) first of all ones, are a diverse group of
organisms, comprising those
eukaryotes that can't be classified in any of the other eukaryotic
kingdoms as
fungi,
animals, or
plants. They are usually treated as the
kingdom Protista or
Protoctista.
Protoctists (or protists) are a
paraphyletic grade, rather than a natural, (
monophyletic) group, and so don't have much in common besides a relatively simple organization -- either they're
unicellular, or they're
multicellular without highly specialized tissues. The term
protista was coined by
Ernst Haeckel in
1866.
Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms: the
one-celled animal-like
protozoa, the plant-like
protophyta (mostly
one-celled algae), and the fungus-like
slime molds and
water molds. Because these groups often overlap, they've been replaced by
phylogenetic-based
classifications. However, they're still useful as informal names for describing the
morphology and
ecology of protists.
At one time, the non-
nucleated bacteria were also considered protists under the three-kingdom system of Animalia (comprising the
many-celled animals or
metazoans), Plantae (which then included fungi as well as
green land plants), and Protista (which included everything else, except
viruses). However, most current textbooks treat bacteria (and the newly-discovered
archaea) as either a separate kingdom or
domain.
Obtaining nutrients
Protists obtain nutrients and digest nutrients in a complex acquirement and assimilation system. Most protists also feed on
bacteria. Protists acquire their food material through internal digestion. They extend their
cell wall and
cell membrane around the food material to form a
food vacuole via
endocytosis (usually
phagocytosis; sometimes
pinocytosis).
Nutrition in some different types of protists is variable. In flagellates, for example,
filter feeding may sometimes occur where the flagella find the prey. In other multicellular protists, elements like
nitrogen and
oxygen are acquired by constant beating of the
flagella. Protists often occur in hydrophilic conditions and thus have large amounts of oxygen within them, which is necessary for them to conduct
respiration and
photosynthesis to desirable levels.
Organization
Protozoa, the animal-like protists
Protozoa are mostly single-celled,
motile protists that feed by
phagocytosis, though there are numerous exceptions. They are usually only 0.01–0.5 mm in size, generally too small to be seen without
magnification. Protozoa are grouped by method of
locomotion into:
Algae, the plant-like protists
They include many single-celled organisms that are also considered protozoa, such as
Euglena, which many believe have acquired
chloroplasts through secondary
endosymbiosis. Others are non-motile, and some (called
seaweeds) are truly multicellular, including members of the following groups:
The green and red algae, along with a small group called the
glaucophytes, appear to be close relatives of other plants, and so some authors treat them as Plantae despite their simple organization. Most other types of algae, however, developed separately. They include the
haptophytes,
cryptomonads,
dinoflagellates,
euglenids, and
chlorarachniophytes, all of which have also been considered protozoans.
Note some protozoa host endosymbiotic algae, as in
Paramecium bursaria or
radiolarians, that provide them with energy but are not integrated into the cell.
Fungus-like protists
Various organisms with a protist-level organization were originally treated as fungi, because they produce
sporangia. These include
chytrids,
slime molds,
water molds, and
Labyrinthulomycetes. Of these, the chytrids are now known to be related to other fungi and are usually classified with them. The others are now placed among the
heterokonts (which have
cellulose rather than
chitin walls) and the
Amoebozoa (which don't have cell walls).
The term Protoctista
During the latter 20th century, the terms
Protista,
protist and
protistan were increasingly used by biological scientists and laymen alike. Groups devoted to
protistology emerged, while
protozoology seemed to fade as an intellectual construct. In more recent years, however, the terms
Protoctista,
protoctist and
protoctistan have been championed by some scholars in
microbiology and
micropaleontology. For example, the 50-volume
Treatise on Invertebrate Paleontology -- eager to fill in the gaps left by
vertebrate paleontology -- has moved from its 1953 (and onwards) use of
Protista to the 21st-century use of
Protoctista. So a
Protist-Protoctist debate would seem to be inevitable.
The taxonomic category
Protoctista was first coined by an English biologist,
John Hogg, in an article entitled
On the distinctions between a plant and an animal, and on a fourth kingdom of nature (1860). In this article, Hogg argued that the term
Protoctist should be used to include "both the Protophyta ... and Protozoa". Therefore, he said, there should be a "fourth kingdom of nature" in addition to the then-traditional kingdoms of plants, animals and minerals. For nearly a century, however, his ideas were eclipsed by those of Haeckel, the reputed founder of protistology.
Herbert F. Copeland resurrected Hogg's label almost a century later in his article,
Progress report on basic classification (1947). Arguing that "Protoctista" literally meant "first established beings", Copeland complained that Haeckel's term
protista included anucleated microbes such as
bacteria. Copeland's use of the term
protoctista did not.
In contrast, Copeland's term included
nucleated eukaryotes such as
brown and
red algae -- but not the
green algae, which he placed with the other
green plants. Copeland further elaborated on his taxonomic proposal in his 1956 book,
Classification of Lower Organisms (Palo Alto, California: Pacific Books). For a more recent delineation of the protoctists, see the
Handbook of Protoctista (Boston: Jones & Bartlett) by
Lynn Margulis, Heather I. McKhann, and Lorraine Olendzenski (1990).
Phylogenetic classifications
The taxonomy of protists is still changing. Newer classifications attempt to present
monophyletic groups based on
ultrastructure,
biochemistry, and
genetics. Because the protists as a whole are paraphyletic, such systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes. The recent scheme by Adl
et al. (2005) is an example that doesn't bother with ranks (phylum, class, etc.).
Some of the main groups of protists, which may be treated as phyla, are listed in the taxobox at right. Many are thought to be monophyletic, though there's still uncertainty. For instance, the
excavates are probably not monophyletic and the
chromalveolates are probably only monophyletic if the
haptophytes and
cryptomonads are excluded.
Further Information
Get more info on 'Protists'.
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