KINGDOM PROTISTA

Protista

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.

Euglena, a eukaryotic protist

The term ‘Protista’ is derived from the Greek word “protistos”, meaning “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.

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:

1. These are usually aquatic, present in the soil or in areas with moisture.
2. 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).
3. Just like any other eukaryotes, the cells of these species have a nucleus which is bound by organelles.
4. 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.
5. 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.
6. Parasitism is also observed in protists. Species such as Trypanosoma protozoa can cause sleeping sickness in humans.
7. Protists exhibit locomotion through cilia and flagella. A few organisms belonging to kingdom Protista have pseudopodia that help them to move.
8. Protista reproduces by asexual means. The sexual method of reproduction is extremely rare and occurs only during times of stress.

Classification of Protista

Kingdom Protista is classified into the following:

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

Protozoa

Protozoans are unicellular organisms. These are also called animal protists. All protozoans are heterotrophic, i.e., they feed on other organisms to obtain nutrition.

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 a definite slipper-like shape. The most well-known examples of protozoans are an amoeba, paramecium, Sporozoa, euglena etc.

The protozoans can be divided into four major groups:

1. Amoeboid protozoans – Mostly found in water bodies, either fresh or saline. They have pseudopodia (false feet) which help to change their shape and in capturing and engulfing food. Eg., Amoeba
2. Flagellated protozoans – As the name suggests, the members of this group have flagella. They can be free-living as well as parasitic. Eg., Euglena
3. Ciliated protozoans – They have cilia all over their body which help in locomotion as well as nutrition. They are always aquatic. Wg., Paramecium
4. Sporozoans – These organisms are so-called because their life cycle has a spore-like stage. For example, the malarial parasite, Plasmodium.

Moulds

Moulds are saprophytic organisms (they feed 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.

Algae

These form another category under kingdom Protista. These are generally single-celled or multicellular organisms. These are photosynthetic, found mostly in freshwater sources or marine lakes. They are characterized by a stiff cell wall.

The algae are divided into six types, namely, green algae, brown algae, red algae, diatoms, pyrrophytes, and euglenoids. Well-known examples of algae are Volvox, Netrium, Spirogyra etc.

Frequently Asked Questions

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).

State a few examples of Protists.

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

How are Protists classified?

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

• Algae
• Moulds
• Protozoans

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.

Are all Protists unicellular?

No, not all Protists are unicellular. Protists such as moulds and algae are multicellular, i.e., they are made up of more than one cell. Amoeba, paramaecium and euglena are unicellular organisms belonging to kingdom Protista.