Protists are single-celled eukaryotes which are organisms with a nucleus. The term Protista was first used by Ernst Haeckel in 1866.

It is a rather old-fashioned term which includes microorganisms from several distantly related phyla. Some are autotrophic (which means they make their own food by photosynthesis), and others are heterotrophic (which means they eat organic material).

Most protists are very small. They are made up of one or a few cells at most – they are microscopic and usually invisible to the naked eye. Some algae are protists, if they are single-celled. Many protists are part of the plankton and are very important for the ecosystem. The cells found in protists may be extremely complex, and are often little understood. It is now possible to do DNA sequencing, and a number of protists have been analysed. The results show that the Protista is not a monophyletic group. It is paraphyletic, and not a single clade. The taxonomy of the Protista is therefore rather confused.

Some protists cause diseases. Plasmodium falciparum causes malaria; sleeping sickness is also caused by a protist.

An example of a single celled organism in the protist kingdom is the Paramecium or "slipper animalcule." The Paramecium moves using its small, hair-like fibers called cilia, and eats using the cilia to sweep the food into its food vacuole. Other protists can be amoebas, which move by extending pseudopods and flowing into them, or flowing around food particles and engulfing them.

Nutrition in protists
Nutritional type Source of energy Source of carbon Examples
 Phototrophs   Sunlight   Organic compounds or carbon fixation  Algae, Dinoflagellates or Euglena 
 Organotrophs  Heterotrophs  Organic compounds   Organic compounds   Apicomplexa, Trypanosomes or Amoebae 

Origin and taxonomy

Protists are not a monophyletic clade. The term is a convenient holdall for about 20 different kinds of single-celled eukaryotes. They differ in their cell organelles, specialised units which carry out well-defined functions, like mitochondria and plastids. This proves they have made the transition from prokaryotes in different ways. It is fairly clear now that all or most of these organelles have their origin in once-independent prokaryotes (bacteria or archaea), and that the eukaryote cell is a 'community of micro-organisms' working together in 'a marriage of convenience'. Admittedly, the Protista is a collection of disparate single-celled forms, but while a more sophisticated taxonomy is in flux (changing), Protista is still a useful term.

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Protist for Kids. Kiddle Encyclopedia.