Write the distinct characters of protozoa.

Protozoa are unicellular, eukaryotic, heterotrophic organisms. They are either free-living or parasites. There are around 65000 species of protozoans categorised in different groups. They lack agid  ricell wall. There are many different cell organelles, that perform various tasks performed by different organs in higher animals, e.g. mouth, anus, intestinal tract, etc.

General characteristics of Protozoa are:

Habitat- Protozoa are found in the aquatic environment. They live in freshwater or oceans. Some are free-living and some are parasitic in plants and animals. Mostly they are aerobic but some are anaerobic and present in the rumen or human intestine. Some of the species are found in extreme environments like hot springs. Some of them form resting cyst to overcome dry environments.

Size and Shape- The size and shape of Protozoa vary greatly, from microbial (1µm) to large enough and can be seen by the naked eye. The shell of unicellular foraminifera can have a diameter of 20 cm.

They lack a rigid cell wall, so they are flexible and found in various shapes. Cells are enclosed in a thin plasma membrane. Some of the species have a hard shell on the outer surface. In some of the protozoans especially in ciliates, the cell is supported by Pellicle, which may be flexible or rigid and give organisms the definite shape and help in locomotion.

Nutrition- Protozoa are heterotrophic and have holozoic nutrition. They ingest their food by phagocytosis. Some of the protozoan groups have a specialised structure called cytostome for phagocytosis. The pseudopodia of amoeboids help in catching the prey. Thousands of cilia present in ciliates drive the food-laden water into the gullet.

Locomotion- Most of the protozoa species have flagella, cilia or pseudopodia. Sporozoa, which don’t have any locomotory structure, have subpellicular microtubules, which help in the slow movement.

Reproduction- Mostly they reproduce by asexual means. They multiply by binary fission, longitudinal fission, transverse fission or budding. In some of the species, sexual reproduction is present. The sexual reproduction is by conjugation, syngamy or by gametocytes formation.