Protozoans a single celled organisms which live independently.
Being single celled, they appear similar to bacteria.
But unlike bacteria, they are eukaryotes and do not possess cell wall.
Further they have characteristics similar to animals.
Most of them are harmless and live in fresh water around.
Some of them rely on photosynthesis or other organic debris for food while others live as parasites.
Their physical structure is simple. They have a distinct nucleus and are able to move.
They multiply by asexual and sexual reproduction types.
Examples of Protozoa
Amoeba: This is a shapeless protozoa. It is the most common type of protozoa and studied as first one in the class. They live individually and have ability to move and find food.
The structure is so simple with no special appendages or sexual parts. They move by pseudopodia which are extensions of cell membrane and cytoplasm. They do not have mouth but just engulf solid particles by phagocytosis and drink liquids by pinocytosis. They reproduce by binary fission asexually. But recent studies show them to even have sexual means of reproduction. Some types of amoeba also causes severe diseases to humans.
Euglena: An eye shaped fresh water protozoa. It imparts green color to the water ponds. Unlike amoeba seems to be well developed with some special organelles like eye, flagella etc.
Unlike amoeba, they can act as autotrophs i.e. make their own food. They contain chlorophyll like pigment by which they performs photosynthesis. They can also act as heterotrophs and eat external substances.
Paramecium: This is a slipper shaped protozoa. It has cilia all over its body surface. These cilia helps in its locomotion.
They are found in fresh water and more evolved than ameoba. They have a mouth, cilia and also go for sexual reproduction.
Disease causing protozoa
Entamoeba histolytica: This is amoeba which infects intestine and other tissues. It spreads through contaminated water and food. It is anaerobic and reside in low oxygen atmosphere.
The infection causes severe pain in the abdomen and mucous loose stools.
Trichomonas vaginalis: A protozoa which causes vaginal infection. It is a type of sexually transmitted diseases. The protozoa is flagellate and is anaerobic.
Leishmania: Infectious protozoa spread by insect bites.
Plasmodium: Commonly known as malarial parasite. Spread by mosquitoes from human to human.
Trypansoma is protozoa which causes sleeping sickness.