Protista or Protoctista, in the five-kingdom system of classification, a kingdom comprising a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms. Protists, which are eukaryotes, have cells that have a membrane-bound nucleus, DNA that is associated with histone proteins, and organelles (e.g., mitochondria and chloroplasts). A recently proposed system of classification designates the eukaryotes as one of three great groups of life (beside bacteria and archaea) and places the protists within it.

It has been hypothesized that the organelles in protists descend evolutionarily from specialized symbiotic bacteria living within the cells of other bacteria, contributing at least in part to the transition from prokaryotic (bacterial) cells (the earliest form of life on the planet, dating back at least 3.5 billion years) to early eukaryotic cells (the cells that define protists, dating back 1.5 billion years) and the more complex life forms of later plants and animals.

The protists comprise a very diverse group of organisms. They include some algae, the protozoans, and multicellular or multinucleate autotrophs, such as the water molds. Many have flagella that enable them to move about. Before the advent of modern biochemistry and the electron microscope, these organisms were fit into the plant and animal kingdoms. It is now thought that, although green plants probably evolved from the green algae and animals from some other early forms, most modern protists have followed independent evolutionary lines. There are approximately 60,000 living species of protists.

Any member of a kingdom (Protista) of diverse eukaryotes, including algae, protozoans, and lower fungi (see fungus). Most are single-celled organisms, though the algae tend to be multicellular. Many can move, mainly by using flagella (see flagellum), cilia (see cilium), or footlike extensions (pseudopodia). The kingdom was developed to accommodate intermediate organisms that, even though they possessed some plant or animal characteristics, did not exhibit the specialized features indicative of those groups. Some protists are considered the ancestors of multicellular plants, animals, and fungi. The term was first suggested in 1866 by Ernst Haeckel. With the development of advanced biochemical, genetic, and imaging techniques, previously established relationships have come under scrutiny, and it is now thought that some groups are less closely related to one another than once believed. As a result, the classification of protists, while convenient, is no longer entirely satisfactory.

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