All the living organisms on the planet are basically divided into five kingdoms, of which, Kingdom Protista is one among them. Protists are a group of eukaryotic organisms (having a true nucleus) which are mostly aquatic in nature but are also found in moist places. These organisms are usually unicellular and the cell of these organisms contain nucleus which is bound to organelles. Some of them even possess structures that aid locomotion like flagella or cilia. They form a link between the plants, animals, and fungi by having an ancient ancestor that shared the same features with members of the other kingdom millions of years ago (such as having a “true” nucleus).

‘Protista’ is a Greek word. These are the organisms that were traditionally considered as the first eukaryotic forms of living, main predecessors to the organisms in the plant, animal, and the fungus kingdoms.

Characteristics of Protista

The usual characteristics of Protista are given below-

  • They are unicellular organisms, however, there are a few multicellular protists such as kelp. Some species of kelp grow so large that they exceed over a 100 feet in height. (Giant Kelp).
  • 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 it’s thick kelp. In turn, the otters eat sea urchins that tend to feed on kelp.
  • These are usually aquatic, present in soil or in areas with moisture.

Classification of Protista

Classification of Protista

The kingdom Protista is divided into three types naming Protozoa, 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 by either ingesting them or feeding off on their organic waste. These 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.


Algae are another category of the 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. Few examples of algae are Volvox, Netrium, Spirogyra 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.

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Practise This Question

The cover of air around the earth is known as_____.