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Protozoa

 

Introduction
Occurance
Nutrition of protozoa

Reproduction

 

Introduction

Protozoa(Profo=first+Zoon=animal) are the most primitive among the living beings that are traditionally classified as animals. Currently however they are included under eukaryotic protists which includes many non animals also.

Portozoans are generally regarded as unicellular organisms. But according to many zoologists, it is not correct to designate protozoans as unicellular as they are complete organisms and not loose cells moving around. The body construction of some protozoans may be more complicated than some of the simple metazoa. Hence protozoans are to be regarded as-non celluar or acelluar animals.

The protozoans may be distinguished from other eukaryotic protists by their lack of cell wall and locomotion that is seen atleast in some stage of the life cycle; if not always. The protozoa consist of some 50,000 (some estimate it to be 65,000)organisms whose cell is bound only by a membrane. Many zoologists regard protozoa as a heterogeneous assemblage of animals as the a cellular organization seems to be the only common character for all of them.

Protozoans were first observed by Leeuwenhoek (1671). The other prominent protozoologists who have contributed towards the understanding of these unique living beings are-Joblott (1718); Pasteur(1870); Butschili(1881); Ronald Ross(1898) etc., The term protozoa was first used by Goldfuss(1817).

As has been mentioned already, protozoa in the present day usage does not indicate a taxonomic group or phylum even though it was so early. Of the estimated 65,000 described species which can be regarded as protozoans, more than 50 percent are fossils. Of the remaining 50% (about 35,000 species) some 22,000 are free living while 10,000 are parasitic. Among the 10,000 parasitic Protozoans only a few are human pathogens but this itself is causing enough havoc in terms of human health. The notorious malarial parasite which was once thought to have been conquered, is raising its head causing alarm as the parasites are becoming resistant to traditional anti malarial drugs (quinine).

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Occurrence:

 Protozoa prefer a moist habitat. They are ubiquitous and are found wherever nature offers a damp surface. They are found in sea water, fresh water and also in moist soil. Some of the free living protozoa have even been recorded from the icy Polar regions. Many protozoa also occur association with other animals. The relationship may be mutualis commensalism or parasitism.

Many of the protozoans are adapted to dry conditions also, which the withstand by undergoing encystment returning to the active state during favourable conditions.

From the standpoint of ecological habitat, protozoans are of two kinds-free living and symbiotic (including parasitism). Free living protozoa are found in a variety of moist habitats. The chief factors which govern the distribution of protozoa are moisture content, temperature, light, availability and type Of nutrients etc.

Many of the free living protozoans which are chlorophyllous (Euglena) require sunlight for their nutrition.

pH of the medium has a bearing on the distribution of protozoa. Although protozoans can survive in the range of 3.2-8, a range of 6.0-8.0 is regarded as optimum.

A medium rich in inorganic and organic nutrients is ideal for the growth of protozoa. Some protozoa thrive very well in water low in organic matter but rich in oxygen content. Others may prefer a reverse situation. Some of the ciliate protozoa like Paramecium prefer a medium rich in bacteria and other tiny protozoans.

Protozoa prefer a temperature range of 16-25 C although they can survive upto 40 C occasionally even warm water springs (30-56) are known to harbour protozoans.

Protozoa as a link in the food chain

Protozoans, particularly the marine inhabiting ones constitute an important level in the trophic organization of the marine ecosystem. In marine waters zooplanktons (free floating animal like individuals) feed on the phytoplanktons (free floating plant like individuals). Most of these zooplanktons are protozoans They in turn are consumed by other larger organisms like larvae, crustaceans, fish etc.

The food chain starting with phyto planktons may be represented as follows.

Solar energy                     Phytoplanktons                         Zooplanktons

       (producers)                                    (Consumers)

    Higher animals. (Secondary and tertiary consumers)

 

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Nutrition of Protozoa

Most of the protozoans are microscopic rarely reaching a size of more than few microns. Some of the fossil members (belonging to Foramniferida) However had reached a size of 15cm. Anaplasma a blood parasite is so small that It occupies 1/10 of the space in a red blood cell. Leishmania donovani, the human pathogen causing kala azar measures 1-4m in length. Amoebaproteus may reach 600 m. Some of the ciliates are very large (comparatively) Paramecium is about 2mm; Spirostamum is about 3mm long.

The body symmetry in protozoa is non radial, spherical or bilateral. The body is either naked or covered by pellicle. Sometimes an exoskeleton may also be present.

The cells of protozoa have an outer boundary cell membrane enclosing cytoplasm and other cell organelles like, nucleus vacuoles, golgi bodies, mitochondria etc. In some Protozoa (Euglena) plastids are also present.

Cell membrane: This is the outer most boundary enveloping the cell. It is thin, elastic and permeable. It has the structure of a unit membrane and is also known asplasmalemma. The cell membrane not only provides rigidity and shape to the cell but being selectively permeable also regulates the movement of substances from and into the cell. The cell membrane also has sites or receptors to receive mechanical and chemical stimuli.

The cell membrane has a tripartite arrangement as revealed by electron microscope. There is a central electron thin layer surrounded on either side by electron dense layers. The membrane is about 75 A thick with each layer ranging between 20-30 A in thickness. Chemically the membrane is lipoproteinaceous and accounts for the permeability of the membrane.

Some unit membranes are also present inside the cell. For e.g. endoplasmic reticulum, nuclear envelop etc.

In some protozoa, in addition to the cell membrane, a compound envelop of a modified structure is present. Called Pellicle, this envelop helps in protection, support and movement which the normal cell membrane can not afford. The pellicle is made up of a continuous layer of filamentous molecules as in some I amoebae. In Euglena, the pellicle is longitudinally striated and ensures flexibility ; while in Paramecium it is quite rigid. In Paramecium the pellicle has three membranes in which the outer one is fashioned into an array of polygons.

Some protozoa have additional protective coverings external to the pellicle which adds to their diversity. Known as thecae, shells, tests or loricae, these envelops offer protection. Small pores present in these envelops help in communication.

Cytoplasm: Internal to the membrane is the cytoplasm. Though it is a homogeneous matrix in some, in the majority of protozoans two zones are distinguishable in the cytoplasm. These are the ectoplasm and the endoplasm, The ectoplasm is more a jelly like substance while the endoplasm is some what thin. Cytoplasmic organelles are found predominantly in endoplasm.

Cytoplasm of protozoa encloses a membrane system as in other eukaryotes This is the endoplasmic reticulum (ER) made up of canals and lacunae. The ER is covered by ribosomes. In addition to ER the cytoplasm has golgi bodies mitochondria, plastids, vacuoles and nucleus.

Golgi bodies or dictyosomes are membranous structures associated with the endoplasmic reticulum and are in the form of vesicles or cisternae. They are believed to be involved in the storage of products of cell synthesis. Dictyosomes are numerous in flagellated protozoans while in ciliates they are poorly developed.

Mitochondria are found distributed in the cytoplasm. They have the envelop of a plasma membrane with the inner layer forming the cristae.

Plastids are found in flagellated protozoans such as Euglena. Each plastid is bound by a double membrane enclosing a number of lamellae. Photosynthetic pigements are concentrated in the lamellae. Pyrenoids are often found in the lamellae.

The vacuoles of protozoans are of two types-contractile vacuoles and food vacuoles. Contractile vacuoles are found in most ciliates, flagellates and amoebae. The number may vary from one to many per cell. These vacuoles are osmoregulatory in function. They collect the waste from cytoplasm, swell in size and burst releasing the waste to the exterior. Contractile vacuoles may be simple membranous enclosures or complex structures having six radiating canals surrounding a central vacuole as in Paramecium.

Food vacuoles are concerned with the ingestion and digestion of food. These are two types of food vacuoles phagocytic and pinocytic. Phagocytic vacuoles consist of large food particles while pinocytic ones have minute food particles in solution form. Food is digested by the activity of the enzymes produced by lysosomes. Lysosomes are formed in the region of the golgibodies.

The cytoplasm of many protozoans also has fibrous structures belonging to two classes. These are microtubules of 20 cm thick and tiny filaments of 4-10 m thick. The filaments have globular protein molecules arranged in one or two rows while microtubules are cylindrical structures having many rows of protein molecules.

Nucleus: All protozoans cells have one, two or many nuclei. The nucleus is eukaryotic. In Amoeba there is a single nucleus, while many ciliates have multiple nuclei. In Paramecium where the cell has two nuclei - one is larger than the other. The larger one is called the macronucleus while the smaller one is called the micronucleus. The macronucleus regulates the metabolic activities and regeneration while the micronucleus is involved in reproduction.

Structurally the nucleus has nuclear membrane, nucleoplasm, nucleolai substance and chromosomes. The number of chromosomes formed during cell division is constant for a given species.

Chemically the nucleus has nucleoproteins. Both DNA and RNA are found. RNA is concentrated into nucleoli which may be one or two in number. Protozoan nuclei divide by mitosis as well as meiosis.

Locomotion: Protozoans are capable of movement. Movement may be brought about by specialized locomotor organelles such as - pseudopodia, flagella or cilia. A few protozoa lack locomotor organelles but exhibit gliding movements.

Pseudopodia (false foot) are characteristic of amoebae, where the cytoplasm forms temporary projections in the direction of movement. The pseudopodia also help in the trapping of food.

Flagella are long fibrous extensions of the cell and are permanent features of the cell. The number of flagellaper cell varies. It is one to many. The flagellum has two parts - a central filament (axoneme) and a surrounding contractile sheath.

Cilia are fine, short threads that extend from the body surface. All cilia in a cell may be of the same length or may vary. They are arranged in longitudinal, oblique or spiral rows. Cilia not only help in locomotion but also in driving the food towards the oral grove (Paramecium).

Nutrition: Protozoans are autotrophic (holophytic) or heterotrophic (holozoic). Some are parasitic also. In holozoic protozoans many feeding organelles develop in the cell. These are varied. In Amoeba, pseudopodia engulf a food particle and convert it into a food vacuole. In ciliates, well developed mouths are present. In Paramecium, an elaborate food ingesting mechanism is Present (see Paramecium for details).

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Reproduction

Protozoa reproduce asexually as well as sexually, eventhough the Principal means of multiplication is by asexual method.

Asexual reproduction takes place by cell division. This is of two types with respect to the number of daughter cells formed. In binary fission two daughter cells are formed, while in multiple fission many daughter cells are formed.

In binary fission, the nucleus divides into two and a constriction appears in the center dividing the cell into two halves. In protozoans like Ameoba which have a soft envelop, the cell constricts along the longitudinal plane dividing it into two. In protozoans with rigid envelop, the cytoplasm first projects o through an opening in the envelop and this portion develops an envelop of its own. Only later the nucleus divides.

Ciliates have transverse fission, while flagellates divide longitudinally In ciliates where two kinds of nuclei are present, the division process is a bit complicated. In Paramecium during binary fission, the diploid micronucleus divides mitotically, while the macronucleus divides amitotically.

After the division, the two daughter cells develop the typical organelles characteristic of the parent even if they do not get them during division.

In multiple fission, the nucleus divides repeatedly into a number of units followed by cytoplasmic cleavage resulting in a number of individuals. Multiple fission is infrequent in ciliates, while quite common in sporozoa (Plasmodium). rhizopoda (Amoeba) radiolaria, foramnifera etc.

Budding: This is another kind of asexual increase seen in some protozoa. It is not similar to the process of budding seen in protozoa like Saccharomyces. In protozoa, budding refers to the formation of motile cells or swarmers from the stationary parent cells. Many ciliates exhibit budding. Buds may be formed externally (exogenous) or internally (endogenous).

Sexual Reproduction

Sexual reproduction is of quite frequent occurrence in protozoa. The methods of sexual increase are varied. Syngamy, conjugation and autogamy are some of the methods of sexual reproduction.

In syngamy, the parent cell(s) forms gametes, which may be similar (isogamy) or dissimilar (anisogamy). When the gametes are dissimilar they are called micro gametes and macrogametes. Microgametes are smaller and motile while macrogametes are larger and sluggish.

In ciliates like Parameuium, sexual reproduction is brought about by means of conjugation. Conjugation is a temporary or partial union of two individuals to exchange nuclear material. After the exchange the two individuals separate. In some ciliates, a permanent conjugation is also seen where the two individuals fuse permanently.

Regeneration: The ability to produce afresh the lost portions of an organ or a cell is called regeneration. Many of the protozoans exhibit this capacity. When the cell of the protozoan is cut into two, the nucleated portion can regenerate the other half of the cell, while the non nucleated portion degnerates. Nucleus is necessary for regeneration.

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