Which includes both unicellular and multicellular species

This simple cellular-level organization distinguishes protists from other eukaryotes, such as fungi, animals, and plants. There are thought to be between 60, and , protist species, and many have yet to be identified. Protists live in almost any environment that contains liquid water. Many protists, such as the algae , are photosynthetic and are vital primary producers in ecosystems.

Other protists are responsible for a range of serious human diseases, such as malaria and sleeping sickness. The term protista was first used by Ernst Haeckel in Protists were traditionally placed into one of several groups based on similarities to a plant, animal, or fungus: the animal-like protozoa , the plant-like protophyta mostly algae , and the fungus-like slime molds and water molds.

These traditional subdivisions, which were largely based on non-scientific characteristics, have been replaced by classifications based on phylogenetics evolutionary relatedness among organisms. This service is more advanced with JavaScript available. Encyclopedia of Astrobiology Edition. Editors: Muriel Gargaud, William M. Contents Search. Authors Authors and affiliations Linda Amaral-Zettler. If no button appears, you cannot download or save the media. Text on this page is printable and can be used according to our Terms of Service.

Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. A cell is one of the building blocks of life. Cells are membrane-bound groups of organelles that work together to allow it to function. Some of the major organelles include the nucleus, mitochondria, lysosomes, the endoplasmic reticulum, and the Golgi apparatus.

Plant cells also include chloroplasts, which are responsible for photosynthesis. Use these classroom resources to examine how cells function with your students. Even the most basic parts of a cell can enable complex cellular processes, and multifunctional organelles expand these capabilities to make advanced activities possible for higher life-forms.

Organelles are specialized structures that perform various tasks inside cells. Join our community of educators and receive the latest information on National Geographic's resources for you and your students.

Skip to content. Image frontonia protist There are many types of unicellular organisms in the world, including protists like this one, which feed mainly on diatoms, amoebas, bacteria, and algae. Three domains Life on earth is classified into three domains: Bacteria, Archaea and Eukarya.

Bacteria and arachaea are unicellular and lack a nucleus. Life without a nucleus Bacteria and Archaea seem to have a lot in common at first. More complex life The third domain, Eukarya, differs greatly from the other two.