distinct nuclei and other membrane-enclosed structures that is not an animal plant or fungus. The protists do not form a natural group or clade by having a characteristic common origin but like algae or invertebrates they are often grouped together for convenience. In some systems of biological classification such as the popular five-kingdom scheme proposed by Robert Whittaker in the protists make up a kingdom called Protista composed of organisms which are unicellular or unicellular-colonial and which form no tissues. their relatively simple levels of organization protists do not necessarily have much in common. When used the term is now considered to mean similar-appearing but diverse taxa biological groups 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 kingdomsn cladistic systems classifications based on common ancestry 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 such as amoebas protozoa and some algae Archaeplastida such as land plants and some algae Excavata which are a group of unicellular organisms Opisthokonta such as animals and fungi 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
