Protozoans are the types of microorganisms which are single-celled and yet live independently.
The examples of protozoa are many and they are of different shape, size, structure, habitat, etc.
Many of them are free-living while others live as parasites.
Being single-celled feature resembles the bacteria.
But unlike bacteria, they are eukaryotes and do not possess the cell wall.
Further, they are grouped into animal kingdom due to marked similarity to animals.
Most of them are harmless and live in fresh water or sea water.
Some of them make their food by photosynthesis while others are dependent on other organic debris for food. To the extreme, few of them also live as parasites.
Their physical structure is a simple cell. They have a distinct nucleus and other essential organelles and also possess an ability to move.
They multiply by sexual and asexual reproduction types.
Examples of Protozoa
Amoeba is a protozoan that has no specific shape. It is the most common protozoa found in fresh water. They live individually and can move in search of food. Most freshwater bacteria are microscopic while marine ones are visible to naked eye.
The structure is so simple with no specialized appendages or sexual parts. They move by pseudopodia which are the extensions of cell membrane and cytoplasm. The cell has a contractile vacuole which helps to remove excess water from the cell. Since the cell cytoplasm is hypertonic than the surrounding fresh water, the water tends to accumulate inside the cell. This water is expelled out by the vacuole.
They do not have the mouth but just engulf solid particles by phagocytosis from any point of the cytoplasmic membrane. They also drink dissolved form of liquid nutrients by pinocytosis. Their food includes live microbes like bacteria and also dead organic matter.
They reproduce by binary fission asexually. But recent studies show them to even have sexual means of reproduction. Some types of amoeba also cause severe diseases to humans.
Euglena is an eye-shaped freshwater protozoan. They impart the green color to the water ponds when grown extensively. Unlike amoeba, they have well developed with cell organelles like eyespot (Stigma), flagella, etc.
Unlike amoeba, they can act as autotrophs, as they make their own food. The eyespot contains chlorophyll like pigment by which they perform photosynthesis. They can also live as heterotrophs and eat external substances. Hence they have both plant and animal characters.
3. Paramecium: This is a slipper-shaped protozoan. It has cilia all over its body surface. These cilia help in its locomotion.
They are found in freshwater and more evolved than the amoeba. They have a mouth, cilia and also go for sexual reproduction.
4. Entamoeba histolytica: This is an amoeba which infects large intestine and other tissues. It spreads through contaminated water and food. It is anaerobic and resides in the low oxygen atmosphere.
The infection causes severe pain in the abdomen and loose stools with mucus. Unlike typical amoeba, it has a single pseudopod. It is a mono-genetic parasite, i.e., completes its life cycle in man (single host). In the intestine, it resides in the mucous and sub-mucus layers. It secretes an enzyme namely “Histolysin” which dissolves the mucus tissue.
5. Trichomonas vaginalis; This is a protozoan which causes vaginal infection. It is a type of sexually transmitted diseases. The protozoa are flagellate and are anaerobic.
6. Leishmania: Leishmania is of two kinds as Leishmania donovani and Leishmania tropica. Donovani infects the liver, spleen, white blood cells, etc. It causes kala-azar (dum dum fever) and visceral leishmaniasis. While the L.tropica affects the endothelial cells of skin capillaries. The infection is spread by sandflies.
7. Plasmodium: Commonly known as a malarial parasite. It is spread from human to human by the bite of a female Anopheles mosquito. This protozoan is of 4 types like plasmodium vivox, Plasmodium falciparum, Plasmodium malaria, Plasmodium ovale. Of them Plasmodium falciparum is deadly.
The protozoa in the humans reside in the red blood cells. It eats up hemoglobin and converts it into a poisonous substance Haemozoin. When the RBC breaks down, the hemozoin is released into the bloodstream causing chills, headache and fever. They have a cycle of 24 hours so, they break down the infected RBC and attack new one every 24 hours. So the infected person experiences feverish symptoms in intervals of 24 hours.
8. Trypanosoma gambiense is protozoa which cause sleeping sickness. They are spread by the bite of a tsetse fly.
The parasite is oval shaped and has flagella. It is a digenetic parasite. The first host is man or animal and the second host is an insect called tsetse fly.
9. Giardia intesinalis: It resides in the small intestine and causes dysentery.
10. Balantidium coli: It is a ciliated protozoan living in the large intestine.
It causes ulcers and chronic dysentery. The transmission occurs from man to pig, fecal-oral route and contaminated water.
Vorticella: These are bell-shaped ciliates living in mud and fresh water. They eat bacteria and sometimes live on surfaces of mosquitoes, prawns, etc.
While moving, they make a vortex in the water and hence the name.
Stentors: These are trumpet-shaped protozoa. They live in freshwater and some are found in marine waters. They live in symbiotic relationship with few algae.
Read more on single celled parasite.