Introduction
Protozoa are unicellular, eukaryotic, mostly microscopic organisms that are either free-living or parasitic. They are usually heterotrophic and feed on organic matter. In NCERT, they are broadly divided into four major groups mainly on the basis of their locomotory organs and mode of life.
1. Amoeboid Protozoans (Sarcodines)
- These protozoans are found in freshwater, seawater or moist soil. Many are free-living; some are parasitic.
- They move and capture prey with the help of temporary, finger-like projections of the cytoplasm called pseudopodia (false feet).
- Food is engulfed by phagocytosis; nutrition is holozoic (ingestive).
- The body is usually irregular in shape and lacks a rigid cell wall.
- Example: Amoeba proteus (free-living), Entamoeba histolytica (parasitic, causes amoebic dysentery in humans).
2. Flagellated Protozoans (Mastigophora)
- These protozoans possess one or more long, whip-like structures called flagella for locomotion.
- They may be free-living (in water) or parasitic in animals and humans.
- Nutrition may be holozoic, saprobic or parasitic; some forms can be autotrophic or mixotrophic (e.g., Euglena has chlorophyll).
- Parasitic flagellates often live in the blood or body fluids of their hosts and may cause serious diseases.
- Example: Trypanosoma (causes sleeping sickness), Giardia, Trichomonas.
3. Ciliated Protozoans
- These protozoans are mostly aquatic and possess numerous short, hair-like structures called cilia that cover the cell surface.
- Cilia beat in a coordinated manner to help in locomotion and in driving food-laden water into a specialized feeding groove or oral cavity (gullet).
- They usually have a well-developed cytostome (cell mouth) and cytopyge (anal pore).
- Many ciliated protozoans show the presence of two types of nuclei: a large macronucleus and a small micronucleus.
- Example: Paramecium, Vorticella.
4. Sporozoans
- These protozoans are exclusively endoparasitic in animals and humans.
- They lack locomotory organs such as pseudopodia, cilia or flagella in the adult stage.
- They are characterized by the presence of a spore-like stage in their life cycle, which helps in transmission from one host to another.
- Their mode of nutrition is parasitic; they absorb nutrients directly from the host body fluids.
- Example: Plasmodium (malarial parasite), which causes malaria in humans and is transmitted by female Anopheles mosquito.
Short Exam-style Answer
Protozoa are unicellular eukaryotic organisms divided into four major groups. Amoeboid protozoans live in water or moist soil and move and capture prey by pseudopodia (e.g., Amoeba). Flagellated protozoans possess flagella for locomotion and may be free-living or parasitic (e.g., Trypanosoma). Ciliated protozoans are aquatic, have numerous cilia for movement and feeding, and often possess two nuclei (e.g., Paramecium). Sporozoans are endoparasites that lack locomotory organs and have a spore-like infectious stage in their life cycle (e.g., Plasmodium).