What are Protists?

Protists are simple eukaryotic organisms that are neither animals, plants nor fungi. Protists are unicellular in nature, or they can be found as a colony of cells. Most protists live in water, damp, terrestrial environments, or even as parasites.

Kingdom Protista

Euglena, a eukaryotic protist

‘Protista’ is derived from Greek, which translates to “the very first“.  These organisms are usually unicellular and the cell of these organisms contain a nucleus which is bound to the organelles. Some of them even possess structures that aid locomotion like flagella or cilia.

Scientists speculate that protists form a link between plants, animals, and fungi as these three kingdoms diverged from a common protist-like ancestor, billions of years ago. Though this “protists-like” ancestor is a hypothetical organism, we can trace some genes found in modern animals and plants to these ancient organisms.

Therefore, these organisms are traditionally considered as the first eukaryotic forms of life and a predecessor to plant, animals, and fungi.

Detailed Insight: Eukaryotic Cells

Characteristics of Kingdom Protista

The primary feature of all protists is that they are eukaryotic organisms. This means that they have a membrane-enclosed nucleus. Other characteristic features of Kingdom Protista are as follows:

  • These are usually aquatic, present in the soil or in areas with moisture.
  • Most protist species are unicellular organisms, however, there are a few multicellular protists such as kelp. Some species of kelp grow so large that they exceed over 100 feet in height. (Giant Kelp).

  • Just like any other eukaryotes, the cells of these species have a nucleus which is bound by organelles.

  • They may be autotrophic or heterotrophic in nature. An autotrophic organism can create their own food and survive. A heterotrophic organism, on the other hand, has to acquire nutrition from other organisms such as plants or animals to survive.

  • Symbiosis is observed in the members of this class. For instance, kelp (seaweed) is a multicellular protist that provides otters protection from predators amidst its thick kelp. In turn, the otters eat sea urchins that tend to feed on kelp.

  • Parasitism is also observed in protists. Species such as Trypanosoma protozoa can cause sleeping sickness in humans.

Read more: Kingdom Protista

Classification of Protista

Classification of Protista

Classification of Protista has resulted in 3 subdivisions, namely protozoans, algae and moulds


Protozoans are unicellular organisms. These are also called as animal protists. All protozoans are heterotrophic, meaning: they have to acquire nutrients through other organisms, either by ingesting them or feeding off on their organic waste. There are also parasitic protozoans which live in the cells of larger organisms. Most of the members do not have a predefined shape. For instance, an amoeba can change its shape indefinitely but a paramecium has an almost definite slipper-like shape. The most well-known examples of protozoans are Amoeba, Paramecium, Sporozoa, Euglena etc.


Moulds are saprophytic organisms (they live on the dead and decaying matter). They are basically divided into two types viz. slime moulds and the water moulds. These are tiny organisms that have many nuclei. Usually, moulds are characterized by the presence of spores and are even visible to the naked eye. One of the best-known moulds is Penicillium chrysogenum, which is used to create the antibiotic penicillin.

Read more: Slime moulds


These form another category under kingdom Protista. These are generally single-celled or multicellular organisms. These are photosynthetic organisms that are mostly found in freshwater sources or marine lakes. They are characterized by a stiff cell wall. Usually, the algae are of six types namely green algae, brown algae, red algae, diatoms, pyrrophytes, and euglenoids. Well-known examples of algae are Volvox, Netrium, Spirogyra etc.

Read more: Algae

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Protists Questions and Answers

1. What are Protists?

Protists include a vast collection of single-celled and multicellular organisms that have a nucleus. They also possess highly specialized cellular machinery called cell organelles that aid in performing various life processes. Most protists are free-living autotrophs (such as algae) while others are heterotrophic (Amoeba) or even parasitic (Trypanosoma protozoa).

2. State a few examples of Protists.

Amoeba, paramecium, sporozoans, algae, Plasmodium, etc.

3. How are Protists classified?

Protists are broadly classified into 3 subdivisions based on their general characteristic features. They are classified as:

  • Algae
  • Moulds
  • Protozoans

4. Outline the characteristics of Kingdom Protista.

  • All protists are eukaryotic organisms. This means that they have a membrane-enclosed nucleus.

  • Most protists are aquatic, others are found in moist and damp environments.

  • Most are unicellular, however, there are a few multicellular protists such as the giant kelp.

  • They may be autotrophic or heterotrophic in nature.

  • Parasitism is also observed in some protists.

  • Others exhibit symbiosis.

For more detailed information about Protista, or any other related topics, please explore BYJU’S Biology.

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