Introduction:

Protozoa are basically unicellular, non-photosynthetic eukaryotic organisms. They are differentiated from other eukaryotic protists by their capability to move at some phase of their life-cycle and by their lack of cell walls. A few protozoa are free living whereas some are parasitic. Some protozoa form colonies, in a colony, the individual cells are attached through a cytoplasmic thread or embedded in a common matrix, therefore colonies of protozoa are basically a cluster of independent cells. The study of protozoa is termed as Protozoology.

General features of Protozoa:

1) They are unicellular and non-photosynthetic microorganisms.

2) They are mainly microscopic in size.

3) They arise usually as single cells.

4) They are deficient in cell walls.

5) They have capability to move at some phases of their life cycle. Most of them are motile.

6) The greater part of protozoa is between 5 to 250µm in diameter.

7) They occur in colonies having each colony independent individual cells.

8) Protozoa might be splitted into free-living forms and those living on or in other organisms.

Occurrence and distribution of Protozoa:

Protozoa are found in all the moist habitats. They are widespread in the sea, in soil and in fresh-water.

Free-living protozoa have even been found in the Polar areas and at very high altitudes. Parasitic protozoa might be found in relation with most of the animal groups. Most of the protozoa survive dry conditions by the formation of a resistant cyst or dormant phase.

Ecology of Protozoa:

From the ecological point of view, protozoa might be splitted into free-living forms and those living on or in other organisms. The latter group is termed to as the symbiotic protozoa. A few of the symbiotic ones are parasitic and might cause disease. The others such as those found in the gut of the termite are valuable to the host (that is, live in a mutualistic relationship).

1) Free-Living Protozoa:

Free-living protozoa are found in a diversity of habitats. The factors that influence the distribution and number of free-living protozoa in a habitat are: temperature, moisture, light, available nutrients and other physical-chemical conditions.

2) Symbiotic Protozoa:

This is a kind of co-existence among protozoa and other organisms that differ in numerous ways and comprise:

a) Commensalism: In this the host is neither hurt nor benefitted however the commensal (protozoa) is benefitted, example: the protozoa living in the lumen of alimentary area.

b) Mutualism in which a few flagellates are present in the gut of the termites and aid to digest the woody materials eaten by termite to a form which can be employed by the host cells. If deprived of such flagellates, the termite dies, if the flagellates are eradicated from the termite gut, they as well die.

c) A few protozoa are parasites; they live at the expense of other organisms and an instance is Plasmodium that is a parasite of man and causes malaria in man.

Morphology of Protozoa:

The shape and size of such organisms exhibit considerable variable. Similar to all eukaryotic cells, the protozoan cell as well comprises of cytoplasm, separated from the surrounding medium through a special cell envelope and the nucleus or nuclei.

1) The Cytoplasm: 

The cytoplasm is a less or more homogeneous substance comprising of globular protein molecules loosely linked altogether to form a 3-dimensional molecular framework. Embedded in it are the different structures which give protozoan cells their feature appearance.  

Submicroscopic protein fibrils (that is, fibrillar bundles, myonemes and microtubules) are groups of parallel fibrils in the cytoplasm. Protozoan contractility is almost certainly due to such fibrils.  

In some forms of protozoa, pigments are diffused all through the cytoplasm. These are abundant. They can be green, blue, brown, purple or rose.

Similar to other eukaryotic cells, protozoa contain membrane systems in the cytoplasm. They form a more or less incessant network of canals and lacunae giving mount to the endoplasmic reticulum of the cell. Other structures in the cytoplasm comprise ribosomes, Golgi complexes and dictyosomes (that is, piles of membranous sacs) mitochondria, kinetosomes and blepharoplasts (that is, intracytoplasmic basal bodies of cilia and flagella), food vacuoles, contractile vacuoles and nuclei.

2) Nucleus:

The protozoan cell consists of at least one eukaryotic nucleus. Most of the protozoa, though, have multiple nuclei (example: almost all ciliates all through the greater portion of the life-cycle). The protozoan nuclei are of different forms, sizes and structures. In some species, each individual organism consists of two similar nuclei. In the ciliates, two dissimilar nuclei, one big (that is, macronucleus) and one small (that is, micronucleus) are present. The macronucleus regulates the metabolic actions and regeneration processes; the micronucleus is mainly concerned by the reproductive activity.

3) Cysts:

Most of the protozoa form resistant cysts at certain times of their life-cycle. The cysts are capable to survive in unfavorable environment conditions like desiccation, low nutrient supply, and even anaerobiosis. In parasitic protozoa, the growth phases are often transmitted from host to host in a cyst.

4) Locomotory Organelles:

Protozoa might move by three kinds of specialized organelles: Pseudopodia, flagella and cilia. Moreover, a few protozoa devoid of such organelles can carry out a gliding movement by body flexion.

Feeding Structure:

In protozoa, the food-gathering structures are diverse and range from the pseudopodia of amoebas via the tentacular feeding tubes of suctorians to the well-developed 'mouths' of numerous ciliates. Amoebas collect food by means of pseudopodia engulfment. In ciliates the cytostome is the real opening via which food is ingested.

An oral groove is the indentation in the pellicle of several ciliates. It guides food toward the cytostome and acts as the concentrating device.  The adding up of membranelles to the oral groove makes it a peristome.

Nutrition:

a) Nutrition in the protozoa is heterotrophic.

b) They get cellular energy from organic substances like proteins.

c) Protozoa swallow up and ingest their food sources.

Reproduction of Protozoa:

Protozoa generally multiply by asexual reproduction. Most of the protozoa are capable to carry out both sexual and asexual processes. A few parasitic forms might have an asexual stage in one host and a sexual stage in the other host.

Asexual Reproduction:  

Asexual reproduction takes place by simple cell division, which can be equivalent or unequal - the daughter cells are of equivalent or unequal sizes, correspondingly. When two daughter cells are formed, then the procedure is termed as binary fission. When many daughter cells are formed, it is termed as multiple fission. Budding is the variation of unequal cell division.

1) Binary Fission:

The common form of binary fission is found in the amoebas.  The pseudopodia are inhibited before the nucleus splits. After the nucleus splits, the organism elongates and tightens in the centre in order to form two daughter cells.

2) Multiple Fission:

In the process of multiple fission, a single mother (parental) cell splits to form many daughter (filial) cells. Division is generally preceded by the formation of multiple nuclei in the mother cell, which then cleaves quickly to form a corresponding number of daughter cells.

Multiple fission is not as common as binary fission however it often occurs in addition to the latter procedure.  In ciliates and flagellates, this kind of fission is found in relatively some species.

3) Budding:

In protozoology, it is often employed to explain the varied processes through which sessile protozoa generate motile offspring. That is, the mother cell remains sessile and discharges one or more swarming daughter cells. The swarmer distinct from the parent cell not simply in a lower degree of differentiation however as well in the possession of special locomotor organelles. Several form of budding is found in all sessile ciliates and is employed to disseminate the species whereas the mother cell remains in situ.

Sexual Reproduction:

Different kinds of sexual reproduction have been noticed among protozoa. Sexual fusion of two gametes (that is, syngamy or gametogamy) takes place in different groups of protozoa. They comprise:

a) Conjugation that is usually a temporary union of the two individuals for the aim of exchanging nuclear material, is a sexual method found exclusively in the ciliates. After exchange of nuclei, the conjugant separate and each of them gives mount to its respective progency through fission or budding. If the gametes (that build up from trophozoites) are morphologically similar, they are termed as isogametes. If they are unlike in morphology (and also physiology), they are anisogametes and can be either macrogametes or microgametes.

Importance of Protozoa:

1) Protozoa are significant links in the food chain of communities in the aquatic atmosphere where they act as the primary consumers.

2) They are employed in biological treatment of sewage or industrial waste matters.

3) Some of the protozoa can cause diseases in the mammals comprising man.

4) They are vital research organisms for chemists and biologists.

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