Protozoa : Habitat, Morphology, Structure, Classification & Life Cycle
Introduction
- Protozoans (Protozoa) are Single-celled Eukaryotic microorganisms belonging to the animal kingdom. (Greek protos—first; zoon—animal).
- Within its single cell, the protozoon contains all structures required for performing its various functions. Some free-living protozoa resemble plants in containing green plastids that enable them to perform photosynthesis.
Habitat
Numerous varieties of protozoans have evolved to suit all manner of environmental conditions.
- Free-living protozoa are found in all habitats—in deep ocean or in shallow fresh waters, in hot springs or in ice, under the soil or in snow on mountain tops.
- Parasitic protozoa have, however adapted to different host species, with more restricted physicochemical requirements.
Morphology
- Protozoa exhibit a wide range of size, shape and structure, yet all possess certain essential common features.
Cell Structure
- The typical protozoan cell is bounded by a trilaminar unit membrane, supported by a sheet of contractile fibrils which enable the cell to change its shape and to move.
- The cytoplasm can often be differentiated into an outer rim of relatively homogeneous ectoplasm and a more granular inner endoplasm.
- The ectoplasm serves as the organ of locomotion and for engulfment of food materials by putting forth pseudopodial processes. It also functions in respiration, in discharging waste materials and also as a protective covering for the cell.
- Within the endoplasm is the nucleus within a tough nuclear membrane. The nucleus is usually single, but may be double or multiple, some species having as many as a hundred nuclei in one cell. The nucleus contains one or more nucleoli or a central endosome or karyosome. The chromatin may be distributed along the inner surface of the nuclear membrane (peripheral chromatin) or as condensed masses around the karyosome.
- The endoplasm shows a number of structures—the endoplasmic reticulum, mitochondria and Golgi bodies.
- Contractile vacuoles may be present which serve to regulate the osmotic pressure. Several food vacuoles also may be seen.
Feeding
- The active feeding and growing stage of the protozoa is called the trophozoite (G.trophos-nourishment).
- The cell may obtain nourishment from the environment by diffusion or by active transport across the plasma membrane.
- Larger food particles are taken in by phagocytosis through pseudopodia. Some species ingest food through special mouth-like structures or cytostomes. Minute droplets of food may also enter by pinocytosis.
Life Cycle
Some protozoans have life phases alternating between proliferative stages (e.g., trophozoites) and dormant cysts.
- Several species possess a resting or resistant cystic stage which enables prolonged survival under unfavourable conditions. The cystic stage may also involve reproduction by the nucleus dividing once or more to give rise to daughter trophozoites on excystation. The cyst is usually the infective stage for the vertebrate host.
Reproduction
Reproduction is usually asexual.
- The most common method is binary fission by mitotic division of the nucleus, followed by division of the cytoplasm.
- In amoebae, division occurs along any plane, but in flagellates division is along the longitudinal axis and in ciliates in the transverse plane. Some protozoa, as for instance the malaria parasites exhibit schizogony in which the nucleus undergoes several successive divisions within the schizont to produce a large number of merozoites.
Sexual stages are seen in ciliates and sporozoa. In ciliates the sexual process is conjugation in which two organisms join together and reciprocally exchange nuclear material. In sporozoa, male and female gametocytes are produced, which after fertilisation form the zygote giving rise to numerous sporozoites by sporogony.
Classification
Protozoan parasites of medical importance have been classified into the following groups or Phyla: Sarcomastigophora, Apicomplexa, Microspora and Ciliophora.
Kingdom : ANlMALIA
Subkingdom : PROTOZOA
A. Phylum : Sarcomastigophora (having flagella or pseudopodia)
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B. Phylum : Apicomplexa (possessing apical complex)
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C. Phylum Microspora (contains many minute intracellular protozoan parasites)
D. Phylum Ciliophora (possessing cilia)
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