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9 TH CLASS BIOLOGY Protista--ప్రొటిస్టా

                         9TH CLASS BIOLOGY PROTISTA              
A protist is any eukaryotic organism that is not an animal, plant or fungus. The protists do not form a natural group, or clade, but are often grouped together for convenience, like algae or invertebrates. In some systems of biological classification, such as the popular 5-kingdom scheme proposed by Robert Whittaker in 1969, the protists make up a kingdom called Protista, composed of "organisms which are unicellular or unicellular-colonial and which form no tissues."

Besides their relatively simple levels of organization, protists do not necessarily have much in common.When used, the term “protists” is now considered to mean similar-appearing but diverse taxa that are not related through an exclusive common ancestor, and have different life cycles, trophic levels, modes of locomotion, and cellular structures.In the classification system of Lynn Margulis, the term protist is reserved for microscopic organisms, while the more inclusive term Protoctista is applied to a biological kingdom which includes certain large multicellular eukaryotes, such as kelp, red algae and slime molds.Others use the term protist more broadly, to encompass both microbial eukaryotes and macroscopic organisms that do not fit into the other traditional kingdoms.
In cladistic systems, there are no equivalents to the taxa Protista or Protoctista, both terms referring to a paraphyletic group which spans the entire eukaryotic tree of life. In cladistic classification, the contents of Protista are distributed among various supergroups (SAR, Archaeplastida, Excavata, Opisthokonta, etc.) and "Protista", ''Protoctista'' and "Protozoa" are considered obsolete. However, the term "protist" continues to be used informally as a catch-all term for eukaryotic microorganisms. For example, the phrase "protist pathogen" may be used to denote any disease-causing microbe which is not bacteria, virus, viroid or metazoa.


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