Introduction:

• Protozoa are the unicellular and microscopic (minute) animals which are found in both marine and fresh water surroundings and as well in the damp soil.
• They mostly fit in to the kingdom protista.
• Several of them are free-living (example:  Amoeba prnteus and paramecium).
• Euglena viridis is a protist which exhibits both animal-like and plant-like characteristics.

Protozoa are mainly single-celled, animal-like organisms. However a few of them are colonial or form loose aggregations, nearly all live and function as a separate cellular individual. Most of the protozoa are chemoheterotrophs, that is, organisms employing preformed organic compounds for both carbon and energy. Nearly all don't have green pigments and are not able of employing light as an energy source (though there are exemptions). They live in diversity of habitats comprising fresh and salt water and within multi-cellular organisms comprising humans and other animals. Protozoa are free-living organisms which get nutrients from decomposing organic materials or feed on bacteria and smaller eukaryotic cells. A few are parasites and some are pathogens able of causing disease in animals and as well in humans.

As protozoa live in diverse habitats and function as individuals subject to a massive amount of environmental confronts, it is not shocking that they (as a group) have developed a range of specializations. Some of them are explained below.

1) Locomotor structures: These structures are specializations permitting cells to travel via their environment as a mode of dispersing, finding food sources or escaping potential predators.

a) Cilia - cilia (singular = cilium) are small, hair-like structures found on the surfaces of protozoa termed as ciliates (phylum Ciliophora). Every cilium is surrounded with the cell membrane and is supported through a cytoskeleton of microtubules arranged in the characteristic 9 plus 2 prototypes. They are able of whip-like motion comprising MAPs like dynein and are coordinated by microtubules arranged just within the cell membrane. Cilia can be allocated more or less evenly all over the cell surface, can take place in rows or patches, or be grouped altogether in tufts. Mainly cilia are employed for swimming and let ciliates to move smoothly via their watery habitats, some give a jumping motility, and cilia arranged in tufts termed as cirri let cells to walk or jump all along the solid surfaces.

b) Flagella - flagella (singular = flagellum) are long, whip-like formations found on the surfaces of several eukaryotic cells. Similar to cilia they are surrounded with the cell membrane and have microtubules arranged in the characteristic 9 plus 2 prototype.

c) Pseudopodia - Pseudopodia (singular = pseudopodium) are extensions of the protoplasm related with amoeba-like organisms. Such differ considerably in size and shapes however are like cilia and flagella are surrounded with the cell membrane and often supported through microtubules (not arranged in a 9 plus 2 prototype).

2) Food gathering structures: The structures comprised in food gathering actions are often those employed for locomotion, comprising flagella, cilia and pseudopodia. Moreover to these are structures comprised in ingestion and digestion of foods. Most of the protozoa are holozoic, that is, organisms which take in entire organisms for food. They encompass no means of biting off small parts of their prey as would big, multicellular animals.

a) Cytostome: The Cytostome or cell mouth (meaning cyto = cell and stoma = mouth) is an area on the surface of a cell where endocytosis can take place. Ciliated protozoa like Paramecium can let in food only via their Cytostome as the rest of the cell is covered with tough pellicle.

b) Lysosomes: It includes the digestive enzymes required to break down food materials taken in via endocytosis.

c) Cilia and flagella can sweep food all along the cell surface to the Cytostome and at times line an area of the cell termed as an oral funnel or oral groove.

d) Pseudopodia: Amoeba-like protozoa employ their pseudopodia to confine food by extending them out and around the food and fusing them to make food vacuoles.

3) Osmoregulatory structures: The contractile vacuoles present in numerous kinds of fresh-water protozoa are employed mainly to pump surplus water out of cells. They are joined to the endoplasmic reticulum, so as well encompass circulatory function and might be employed to remove liquid wastes, that is, as well encompass excretory function.

4) Protective Structures: Protozoa live in potentially hazardous atmospheres and encompass evolved a diversity of protective structures which assist them survive.

a) Pellicle: The pellicle is a tough, flexible layer found exterior to the cell membrane on all kinds of ciliated protozoa. It assists in providing the cell with a characteristic shape and secures it against the physical damage.

b) Skeletons: The skeletons of protozoa are generally made of glass (that is, silica dioxide) or calcium carbonate. Radiolaria encompass glass skeletons perforated with many holes permitting pseudopodia to expand out via them. Foraminifera contain skeletons of calcium carbonate which look like the shells of mollusks (example: the chambered nautilus). Skeletons give protection against predation and as well support the protoplasm.

c) Trichocysts: These are dart-like structures which can be shot out from certain cells. They are building up of protein and are often barbed and joined to the cell surface through microscopic threads. Trichocysts are liberated in response to chemical and/or physical stimuli and might be employed for defense or attachment.

5) Life cycle phases: Protozoa mostly live in habitats subject to change due to climate and seasons, whereas a few live parts of their lives within different kinds of hosts. To survive variations in living conditions, protozoa can switch among the two different phases.

a) Trophozoites: Trophozoites (meaning troph = activity), are active protozoa at times termed as vegetative cells. Whereas in the trophozoite form, protozoa are engage in feeding, reproducing and moving actively. Vernal pools, that is, such filled with water throughout the spring, include lots of trophozoites. Since the weather warms up and pools dry out in the summer, the protozoa go into the resting state.

b) Cysts: These are dormant structures generated by numerous kinds of protozoa beneath certain conditions. They are metabolically not active and much more resistant to drying, heat, radiation and chemicals than are trophozoites (active vegetative cells).

Protozoan Reproduction:

Protozoa similar to algae and fungi can reproduce themselves both sexually and asexually. There are numerous variations on such fundamental themes; however some of the most generally encountered forms of reproduction are described below.

1) Asexual reproduction:

It is a reproductive process which let individuals to reproduce devoid of interacting with other cells. In the eukaryotic organisms, asexual reproduction needs mitosis (that is, the separation of chromosome) and cytokinesis (that is, the separation of cytoplasm making new daughter cells). A few particular illustrations comprise:

a) Binary fission: It is a method comprising the separation of the cytoplasm across the long axis of the cell. Mainly protozoa reproduce through binary fission.

b) Schizogony: Schizogony or multiple-fission is a method comprising the division of one cell into numerous daughter cells. Sporozoans in the genus Plasmodium reproduce by means of schizogony whereas within human RBCs.

c) Budding: It comprises an unequal splitting up of the protoplasm and outcomes in the making of a bud at the margin of a cell. When conditions are favorable, the bud will grow up and ultimately separate from the cell, however when conditions are poor, the bud might die with little effect to the cell.

2) Sexual reproduction:

In order to reproduce sexually, protozoa should interact by other, genetically dissimilar cells. At times this comprises karyogamy, plasmogamy and meiosis, however not always and such terms are seldom employed in zoology texts. Two illustrations of sexual reproduction comprise:

a) Syngamy: It includes the fusion of the two haploid cells to form a diploid zygote. Protozoa in the genus Plasmodium experience syngamy whereas within mosquitoes.

b) Conjugation: It needs those two cells with dissimilar genetic content meet up and place themselves side-by-side. Parts of the cell membranes fuse letting the creation of a cytoplasmic bridge and then segments of genetic material (DNA) are exchanged among the two cells.

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