Protista

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.

Protista(Protoctista) A kingdom in the domainEucaryota that comprises single-celled, eukaryotic organisms (see eukaryote) that may resemble animals or plants. Naked and shelled amoebas, foraminiferans, zooflagellates, ciliates, dinoflagellates, diatoms, and algae are protists. In an earlier five-kingdom classification, Protista was ranked as a kingdom; later some multicellular organisms with protist affinities but previously classed as fungi or plants were transferred into the Protista and the name was changed to Protoctista. Protoctista is now a synonym for Protista.

Protista (domain Eukarya) A paraphyletic kingdom comprising single-celled and multicelled eukaryotes that are not classified as animals, green plants, or true fungi. There are about 60 types and about 200000 species of protists. These include the Oomycota, parasitic and free-living protozoa, and various single-celled and multicelled algae (see ALGA). The multicellular organisms arose from the protists by various independent routes, possibly as many as 17. They were already a diverse group 1000 Ma ago. Protoctista is sometimes used as an alternative name. In earlier classifications the Protista formed a kingdom (or superphylum or phylum) of relatively simple, undifferentiated organisms.