In the biological classification of Kingdom Protista, Protozoans represent the heterotrophic group. They are unicellular eukaryotes that lack a cell wall and exhibit animal-like behaviors, specifically predation and parasitism.
Classification based on Locomotory Organelles
Protozoans are categorized into four major groups depending on how they move and capture prey.
Amoeboid Protozoans
- Habitat: Found in freshwater, seawater, or moist soil.
- Mechanism: They move and capture prey by putting out pseudopodia (false feet).
- Marine Forms: Some marine species have silica shells on their surface.
- Parasitic Example: Entamoeba histolytica, which causes amoebic dysentery in humans.
Flagellated Protozoans
- Habitat: These are either free-living or parasitic.
- Mechanism: They possess flagella for movement.
- Pathogenic Impact: Members of this group are responsible for several chronic diseases.
- Key Example: Trypanosoma, the causative agent of African Sleeping Sickness.
Ciliated Protozoans
- Habitat: Always aquatic and highly active.
- Mechanism: They possess thousands of cilia. The coordinated movement of rows of cilia allows them to steer water (laden with food) into the gullet (a cavity that opens to the outside of the cell surface).
- Key Example: Paramecium.
Sporozoans
- Characteristics: This group includes diverse organisms that have an infectious spore-like stage in their life cycle.
- Mechanism: They lack specific locomotory organelles in their adult infectious stage.
- Key Example: Plasmodium (the malarial parasite), which has a staggering impact on human populations.
Protozoan Diseases and Vectors
The following table summarizes the diseases caused by protozoans, which is a frequent area of questioning in General Science.
| Disease | Causative Protozoan | Vector/Transmission | Target Organ/Effect |
| Malaria | Plasmodium sp. | Female Anopheles mosquito | Liver and RBCs; causes recurring chills/fever |
| Kala-azar | Leishmania donovani | Sandfly (Phlebotomus) | Bone marrow, spleen, and liver |
| Sleeping Sickness | Trypanosoma | Tsetse fly | Central Nervous System; disrupts sleep cycles |
| Amoebic Dysentery | Entamoeba histolytica | Contaminated food/water | Large intestine; causes abdominal pain/mucus stool |
| Giardiasis | Giardia lamblia | Fecal-oral (water) | Small intestine; causes “Beaver Fever” |
Comparative Physiology of Protozoa
Nutrition and Digestion
- Type: Heterotrophic (Holozoic or Saprozoic).
- Process: Food is captured and digested within food vacuoles using digestive enzymes. This is intracellular digestion.
Osmoregulation
- Contractile Vacuole: Freshwater protozoans (like Amoeba and Paramecium) possess a contractile vacuole. This organelle is critical for osmoregulation, pumping out excess water to prevent the cell from bursting in a hypotonic environment.
Reproduction
- Asexual: Most common method, occurring via binary fission or multiple fission (e.g., Plasmodium).
- Sexual: Occurs through conjugation (exchange of nuclear material) in ciliates like Paramecium or through syngamy (fusion of gametes).
Quick Facts for UPSC Prelims
- Contractile Vacuoles: Usually absent in marine and parasitic protozoans because their environment is isotonic or hypertonic to their cytoplasm.
- Discovery: Antonie van Leeuwenhoek was the first to observe protozoa, referring to them as “animalcules.”
- Biological Indicator: Some protozoans are used as indicators of water pollution; high populations of certain ciliates can indicate sewage-polluted water.
- The “Brain-Eating” Amoeba: Naegleria fowleri is a rare but deadly amoeboid protist found in warm freshwater that enters the body through the nose.