Protista

Protista, group of comparatively simple organisms, called protists, that have characteristics of both plants and animals. Most protists are unicellular (consisting of a single cell) and can only be seen with a microscope, although there are some that are composed of more than one cell. There are a wide variety of protists, and they inhabit many different environments—fresh water, seawater, soils, and the intestinal tracts of animals, where they perform crucial digestive processes. Like plants, many species of protists can make their own food by the process of photosynthesis. Like animals, many protists can move around under their own power. Unlike plants and animals, however, protists do not have cells organized into specialized tissues.The protists include such familiar organisms as seaweeds, amoebas, and slime molds.
The kingdom Protista contains many economically important members, including organisms that cause diseases, such as malaria. Biologists theorize that members of the kingdom Protista gave rise to the kingdoms Plantae, Animalia, and Fungi about 600 million years ago.All protists are eukaryotes. This means that their cells contain a nucleus, a membrane-bounded structure that encloses the cell’s genetic material. (Organisms without nuclei—the bacteria and cyanobacteria, or blue-green algae—are called prokaryotes.) Although most protists have a single nucleus, protists are unique in that some contain multiple nuclei—up to ten thousand in a single cell—and others, such as ciliates, have two different-sized nuclei in a single cell. The deoxyribonucleic acid (DNA) of protists (like that of other eukaryotes) is organized in long molecules called chromosomes within the nucleus. When the cell divides, these chromosomes replicate (duplicate themselves) and then divide in a process called mitosis. Among some protists, the nuclear membrane (the thin layer around the nucleus) stays intact during mitosis, whereas in plants, animals, and fungi the nuclear membrane breaks down.Some protists form structures known as spores that are resistant to chemicals and drying and that disperse in the environment. Spores are often the infectious stage of organisms that cause diseases in humans when ingested.