Chapter 23
Protists
By Boundless
![Thumbnail](../../../../../figures.boundless-cdn.com/19110/square/figure-b23-00-01abc.jpeg)
Protists are eukaryotes that first appeared approximately 2 billion years ago with the rise of atmospheric oxygen levels.
![Thumbnail](../../../../../figures.boundless-cdn.com/20603/raw/celltypes.jpg)
Eukaryotes, having probably evolved from prokaryotes, have more complex traits in both cell and DNA organization.
![Thumbnail](../../../../../figures.boundless-cdn.com/20609/raw/endosymbiosis.jpg)
Eukaryotes may have been a product of one cell engulfing another and evolving over time until the separate cells became a single organism.
![Thumbnail](../../../../../figures.boundless-cdn.com/19116/square/figure-23-01-01.jpeg)
Mitochondria are energy-producing organelles that are thought to have once been a type of free-living alpha-proteobacterium.
![Thumbnail](../../../../../figures.boundless-cdn.com/19122/square/figure-23-01-02.jpeg)
Plastids may derive from cyanobacteria engulfed via endosymbiosis by early eukaryotes, giving cells the ability to conduct photosynthesis.
![Thumbnail](../../../../../figures.boundless-cdn.com/19126/square/figure-b23-02-01.jpeg)
Protists are an incredibly diverse set of eukaryotes of various sizes, cell structures, metabolisms, and methods of motility.
Protists live in a wide variety of habitats, including most bodies of water, as parasites in both plants and animals, and on dead organisms.
![Thumbnail](../../../../../figures.boundless-cdn.com/19128/square/figure-23-03-02.jpeg)
Excavata, defined by a feeding groove that is "excavated" from one side, includes Diplomonads, Parabasalids and Euglenozoans.
![Thumbnail](../../../../../figures.boundless-cdn.com/19133/square/figure-23-03-06ab.jpeg)
Alveolates are defined by the presence of an alveolus beneath the cell membrane and include dinoflagellates, apicomplexans and ciliates.
![Thumbnail](../../../../../figures.boundless-cdn.com/19146/square/figure-23-03-10.jpg)
Stramenophiles include photosynthetic marine algae and heterotrophic protists such as diatoms, brown and golden algae, and oomycetes.
![Thumbnail](../../../../../figures.boundless-cdn.com/19148/square/figure-23-03-12.jpeg)
Rhizaria are a supergroup of protists, typically amoebas, that are characterized by the presence of needle-like pseudopodia.
![Thumbnail](../../../../../figures.boundless-cdn.com/19151/square/figure-23-03-15.jpeg)
Archaeplastida are a supergroup of protists that comprise red and green algae, which include unicellular, multicellular, and colonial forms.
![Thumbnail](../../../../../figures.boundless-cdn.com/19153/square/figure-23-03-17.jpeg)
Amoebozoa are a type of protist that is characterized by the presence of pseudopodia which they use for locomotion and feeding.
![Thumbnail](../../../../../figures.boundless-cdn.com/19156/square/figure-23-04-01.jpeg)
Protists function as sources of food for organisms on land and sea.
![Thumbnail](../../../../../figures.boundless-cdn.com/19158/square/figure-23-04-03.jpeg)
Many protists exist as parasites that infect and cause diseases in their hosts.
![Thumbnail](../../../../../figures.boundless-cdn.com/30631/square/figure-23-04-05.jpeg)
Many protists act as parasites that prey on plants or as decomposers that feed on dead organisms.