Overview of Protists

Estimated Reading Time: 2 Minutes

Welcome to our continuing AP Biology/ General Biology series here on STEM Talks! The purpose of this post is to give you an overview of Protists.

All protists were once classified into a single kingdom, but scientists found protists to be polyphyletic: some protists are more closely related to plants, fungi, or animals than they are to other protists. This is why scientists have abandoned the kingdom system of protists.

Most Eukaryotes are single-celled organisms

Protists are eukaryotes- they have cells with a nucleus and membrane-bound organelles that provide specific locations in which particular functions are accomplished.

Cellular Structures

Unicellular protists carry out essential functions using subcellular organelles- not multicellular. They use organelles such as the nucleus, ER, Golgi apparatus, and lysosomes. Some protists have contractile vacuoles that pump excess water out from the protistan cell.


Protists may be aerobic and anaerobic. In addition, they can be photoautotrophs, containing chloroplasts; heterotrophs that absorb organic molecules; and mixotrophs that combine photosynthesis and heterotrophic nutrition.


Reproduction is also highly varied among protists. Some protists reproduce asexually and others reproduce sexually.

Endosymbiosis: Theory for Eukaryotic Evolution

Endosymbiosis is a process in which certain unicellular organisms engulf each other cells, which become endosymbionts and ultimately organelles in the host cell. In this theory, it is stated that the first eukaryotes acquired mitochondria by engulfing aerobic prokaryote. The early origin of mitochondria is supported by the fact that all eukaryotes either have mitochondria or show signs that their ancestors had them. Later in eukaryote history, one lineage of heterotrophic eukaryotes acquired an additional endosymbiont- a photosynthetic cyanobacterium- evolved into plastids gave rise to green and red algae. Then red and green algae went through secondary endosymbiosis; they were ingested in the food vacuole of heterotrophic eukaryotes and became endosymbionts. Eventually, these endosymbionts evolved to become protists such as euglenids and stramenopiles.

When seeing the characteristics above, you can see why the kingdom Protista has been abandoned. Currently, biologists have separated the protists into five supergroups: Excavata, Chromalveolata, Rhizaria, Unikonta, and Archaeplastida. Some of these supergroups are well supported by morphological and DNA data, others are more controversial.

In the next few posts, I will discuss each of these five supergroups in detail. Feel free to leave any question down below in the comments!

– Tahmid Islam

Co-Founder and Editor of StemTalksNC


In all my posts, I follow multiple sources for AP Biology. However, I mainly go off of the Campbell Biology 8th and 11th edition.



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