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Protoctista(prō'tŏktĭs`tə), 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 eukaryoteseukaryote
, a cell or organism composed of cells that have a membrane-bound nucleus and organelles (mitochondria, chloroplasts; see cell, in biology) and genetic material organized in chromosomes in which the DNA is combined with histone proteins.
..... Click the link for more information. , have cells that have a membrane-bound nucleus, DNA that is associated with histonehistone
, any of a class of protein molecules found in the chromosomes of eukaryotic cells. They complex with the DNA (see nucleic acid) and pack the DNA into tight masses of chromatin, which have the structure of coiled coils, much like a tangled telephone cord.
..... Click the link for more information. 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 algaealgae
[plural of Lat. alga=seaweed], a large and diverse group of primarily aquatic plantlike organisms. These organisms were previously classified as a primitive subkingdom of the plant kingdom, the thallophytes (plants that lack true roots, stems, leaves, and flowers).
..... Click the link for more information. , the protozoansprotozoan
, informal term for the unicellular heterotrophs of the kingdom Protista. Protozoans comprise a large, diverse assortment of microscopic or near-microscopic organisms that live as single cells or in simple colonies and that show no differentiation into tissues.
..... Click the link for more information. , and multicellular or multinucleate autotrophs, such as the water moldswater mold,
common name for a group of multinucleated organisms that superficially resemble fungi but are now recognized as having an independent evolutionary lineage and are placed in the kingdom Protista.
..... Click the link for more information. . 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.
The kingdom comprising all single-celled forms of living organisms in both the five-kingdom and six-kingdom systems of classification. Kingdom Protista encompasses both Protozoa and Protophyta, allowing considerable integration in the classification of both these animallike and plantlike organisms, all of whose living functions as individuals are carried out within a single cell membrane. Among the kingdoms of cellular organisms, this definition can be used to distinguish the Protista from the Metazoa (sometimes named Animalia) for many-celled animals, or from the Fungi and from the Metaphyta (or Plantae) for many-celled green plants. See Metazoa
The most significant biological distinction is that which separates the bacteria and certain other simply organized organisms, including blue-green algae (collectively, often designated Kingdom Monera), from both Protista and all many-celled organisms. The bacteria are described as prokaryotic; both the Protista and the cells of higher plants and animals are eukaryotic. Structurally, a distinguishing feature is the presence of a membrane, closely similar to the bounding cell membrane, surrounding the nuclear material in eukaryotic cells, but not in prokaryotic ones. See Eukaryotae, Protozoa
The definition that can separate the Protista from many-celled animals is that the protistan body never has any specialized parts of the cytoplasm under the sole control of a nucleus. In some protozoa, there can be two, a few, or even many nuclei, rather than one, but no single nucleus ever has separate control over any part of the protistan cytoplasm which is specialized for a particular function. In contrast, in metazoans there are always many cases of nuclei, each in control of cells of specialized function.
Most authorities would agree that the higher plants, the Metazoa, and the Parazoa (or sponges) almost certainly evolved (each independently) from certain flagellate stocks of protistans.
the totality of unicellular animals and plants. The term “Protista” was introduced by the German biologist E. Haeckel in 1866. He grouped unicellular organisms into a special, third kingdom on a par with the two kingdoms of multicellular organisms—Plantae and Animalia.