Updated by: Arabinda Naik
Unicellular or acellular, eukaryotic, and com¬monly motile heterotrophic organisms are called Protozoa. Term protozoa (from Greek protos meaning first, zoon meaning animals) was given by Goldfass. There are about 50,000 known species of Phylum Protozoa.
General Characteristic Features:
Habitat:
1. Protozoa are found in diverse habitats, including freshwater, marine environments, soil, and even within other organisms as parasites.
2. Parasitic Forms: Some protozoa are parasitic, causing diseases in humans and animals. Examples include Plasmodium (causing malaria) and Giardia (causing giardiasis).
3. Some are commensals in habit.
4. Most species occur as single, but many are colonial.
Cell Structure:
1. Protozoa are unicellular organisms, and their size typically ranges from a few micrometers to 0.5 millimeters, making them invisible to the naked eye.
2. Protozoans are minute, usually microscopic, and unicellular (without tissues and organs), eukaryotic organisms.
3. Protozoa are eukaryotes, which means they have well-defined nuclei and other membrane-bound organelles within their cells.
4. Body shape is variable. It may be spherical, oval, elongated or flattened.
5. Body is bounded by a cell membrane or plasmalemma.
6. Body may be naked or is covered by a pellicle or a test, made of silica or calcium carbonate.
7. They are the simplest and primitive of all animals with very simple body organisation. Protoplasmic grade of organisation.
8. They exhibit all types of symmetry. They are asymmetrical but Giardia is bilaterally symmetrical.
9. Usually, uninucleate but may be more than single nucleus in some forms.
10. Nucleus is commonly spherical, oval, or biconvex.
11. Dimorphic nuclei are found in cilia, one larger macronucleus and other small micronucleus.
12. Many protozoa have mitochondria for energy production, although some anaerobic forms lack them.
13. Protozoa have a well-developed cytoskeleton that supports their shape and contributes to their motility.
14. The protoplasm is differentiated into outer ectoplasm and inner endoplasm.
Locomotory organelles:
1. Locomotor organelles may be flagella (e.g., Euglena), cilia (e.g., Paramecium), pseudopodium (e.g., Amoeba) or ab-sent in parasitic forms (contractile myonemes are present in the body).
2. Most protozoa are motile and possess various structures for movement, such as cilia, flagella, or pseudopodia.
3. Cilia: Hair-like structures that beat in coordinated waves for locomotion, found in organisms like Paramecium.
4. Flagella: Whip-like appendages that rotate to propel the cell forward, seen in organisms like Euglena.
5. Pseudopodia: Temporary, cytoplasmic extensions that allow amoeboid movement in organisms like Amoeba.
6. Amoeboid Movement: Amoeboid protozoa, like Amoeba, move by changing their cell shape through the extension and retraction of pseudopodia.
Nutrition:
1. Nutrition may be holozoic, e.g., Amoeba (animal-like), holophytic (e.g., Euglena), saprophytic, mixotrophic or parasitic.
2. In Euglena the mode of nutrition is mixotrophic both holozoic and holophytic.
3. Intracellular type of digestion occurs within the food vacuoles.
4. Heterotrophic Nutrition: Protozoa are primarily heterotrophic, meaning they obtain their nutrients by ingesting other organisms or organic matter in their environment.
5. They have specialized structures, such as food vacuoles, where digestion of ingested particles occurs.
6. In some forms, egestion occurs through a temporary opening in the ectoplasm or through permanent opening called cytopyge.
7. Phagocytosis: Ingestion of solid particles by forming a food vacuole.
8. Pinocytosis: Uptake of liquid or dissolved substances through invagination of the cell membrane.
9. Osmotrophy: Absorption of dissolved organic molecules directly from the environment.
10. Diverse Feeding Strategies: Protozoa employ various strategies for obtaining food, including filter-feeding, predation, and parasitism.
11. Flagellar Apparatus: Flagellated protozoa have distinct flagellar apparatuses, including basal bodies and microtubules.
12. Nutrient Storage: Some protozoa store excess nutrients as reserve granules, which can be used during periods of scarcity.
Respiration:
1. Respiration performs generally through the outer surface of the body by diffusion.
2. Protozoa primarily respire aerobically, using oxygen for energy production.
Excretion:
1. Excretion performs generally through the body surface, and water regula¬tion of the body is accomplished by contractile vacuole. They are ammoniotelic.
2. Contractile Vacuole: Many freshwater protozoa have contractile vacuoles to regulate water balance and prevent cell bursting due to osmotic pressure.
3. Protozoa excrete waste products through specialized structures like contractile vacuoles or simple diffusion.
Reproduction:
1. Asexual reproduction occurs by binary fis¬sion, multiply fission (mitosis), plasmotomy or bud¬ding, sporulation.
2. In certain forms sexual repro¬duction may occur either by conjuga¬tion or fusion by gametes (syngamy).
3. Binary fission may be simple or transverse or longitudinal or oblique.
4. Cyst Formation: In adverse conditions, many protozoa can encyst, forming a protective, dormant stage to survive harsh environments.
5. Cyst Resistance: Cysts are resistant to unfavourable conditions and can survive for extended periods until conditions improve.
Life cycle:
1. Life cycle often exhibits alternation of generation, it includes asexual and sexual phases.
Plasticity:
1. Protozoa can adapt to various environmental conditions, including changes in temperature, salinity, and nutrient availability.
Symbiosis:
1. Some protozoa engage in symbiotic relationships with other organisms, either as mutualists or commensals.
Protozoan Diseases:
1. Some protozoa cause diseases in humans, such as amoebiasis, Chagas disease, and sleeping sickness.
Environmental Sensing:
1. Protozoa can sense changes in their environment and respond to stimuli such as light and temperature.