Structural organization and function of protozoans, Biology tutorial

Body Structure of Protozoans Animals:

Protozoans could be separated in two according to simplicity or complexity of structure. In more complex ones there is a greater, more extremely specialized portion of protoplasm for performing definite functions efficiently:

1. Simple protozoans like Trypanosoma, Monocystis and plasmodium (parasitic animals) and Amoeba (free - living.)

2. Complex protozoans e.g. Eugelna

Parasitic protozoans contain simple undifferentiated bodies. As they don't have to search for food, they don't develop food - catching organelles; they just absorb food over their body surface food catching and food ingesting organelles like are found in free- living counterpart protozoan are absent. Also, osmotic pressure of fluid in their environment is isotonic to that of protoplasm, therefore osmoregulatory organelles like contractile vacuoles found in free -living ones are absent. Each parasitic protozoans comprises of the mass of protoplasm with the simple nucleus. Both Trypanosoma and Monocystis are lengthened cells pointed at both ends, but plasmodium is spherical.

Trophozoite stage of plasmodium takes place in erythrocyte. Tryapnosoma is flagellate with the long flagellum that arises from darkly staining object, blepharoblast, located in anterior end. It then runs alongside of body pulling out part of body as undulating membrane, before becoming free at other end. Around nucleus that is situated in center are certain granules known as metachronal granules.

i) Amoeba (a free - living protozoan):

This is free - living protozoan with the more complex structure than any of parasitic ones. Being free - living it also searches for its food, therefore pressure of locomotory organelles in form of pseudopodia that provide animal an irregular outline. Unlike parasitic ones it feeds on solid organic food, therefore it has food - catching and food ingesting organelles like pseudopodia and food vacuoles. Living in fresh water where osmotic pressure is hypotonic to that of the protoplasm there is continuous inflow of water by osmosis. thus it has developed the contractile vacuole to eradicate excess water.

ii) Euglena and Paramecium:

In Euglena at the anterior end is gullet leading in reservoir. From reservoir flagelluim arises by 2 roots from basal granules. On one side of gullet is eye - spot or stigma and in vicinity of reservoir is contractile vacuole. Chloroplast may comprise of scattered elements in cytoplasm, or may take form of star. In the posterior part of organism lies nucleus, and scattered throughout cytoplasm are paramylum granules.

Paramecium has definite shape similar to sole of the slippers; therefore it is frequently referred to as slipper - animacule. Rounded end is anterior and pointed and is posterior. Whole body is enclosed with cilia. On ventral side in highly developed food - ingesting organelles in form of oral groove leading in cytopharynx that terminates in small rounded end cytostome, portion where pellicle is lacking.

Nutrition in Protozoan Animals:

Protozoa divided into 2 groups according to mode of nutrition:

i) Heterotrophic organisms like Monocystia, Trypanosima plasmodium, Amoeba and Paramecium. Heterotrotis rely on organic foods from external sources. They are divided usually in two sub-groups:

a) Saprozoic or Osmotrophs like parasitic protozoa that take in organic food solution by absorption. Examples are Monocystis, Trypanosoma and plasmodium.

b) Holozoic or phagotrophs: they ingest solid food particles of animals or plant origin. Therefore they may also feed on bacteria or other small protistans that are ingested whole and digested inside food vacuoles like Amoeba, paramecium. Movement is one of protoplasmic streaming or cyclosis without active proportion by specializes part of it. Most extensively held explanation of protoplasmic streaming relates to property of protoplasm in effecting gel - sol transformation. As outward flow continues, solidification takes place along sides while centre remains fluid. Therefore a pseudopodium can be considered as gelatinous tube given with liquid interior and solid exterior. Local liquefaction or isolation of gelatinous layer possibly results from the chemical reaction that induces gelatin to become solution, a mere change of stage of colloid being involved.

ii) Myxotrophic Animals:

Organisms that are both heterotrophic and autotrophic that is they are also able to produce their own food like Euglena viridis which is both holophytic (autotrophic) and saprophytic (heterotrophic). When chlorophyll fails Euglena turns into heterotrophic and saprophytic absorbing organic food from watery environment. Monocystis (saprozoic) is the parasite in seminal veside of earthworm and feeds on sperm - mother cells known as sperm morulae. Being saprozoic food is digested outside and taken in solution or absorbed throughout the body surface into the endoplasm. There is no requirement to create digestive organelles.

Locomotion in protozoa:

Protozoan locomotor organelle may be flagella, cilia or pseudopodia; these are of substantial value in categorization of protozoans. There are two primitive kinds of movements in protozoa 1). Amoeboid 2). Ciliary flagellate

Amoeboid movement: Amoeba is characteristics Euglenoid Movement Euglenoid and greagarines movements shown by Monocystis and euglena are worm - such as processor of local expansion and contractions which recommend peristalsis invertebrate intestine.

Ciliary and Flagellal Movement: Cilia and flagella essentially have similar structure of two inner rings and peripheral bends or outer ring of spirally arranged longitudinally strengthening structures.

Osmoregulation and Excretion:

Fresh water Protozoa animals will face the problem with excess of wafer which enters in body by Osmosis. Process by which water balance in body is maintained is known as Osmoregulation. In protozoans animal's osmoregulation is performed by contractile vacuoles.

Waste materials in protozoans are eradicated from cell by diffusion by the cell.  They are transported out of cell by food vacuoles which come in contact with surface.  This is known as exocytosis.


Gas exchange takes place by diffusion of oxygen across cell membrane. Few protozoans use this oxygen but are also able to anaerobic respiration. Examples are parasitic protozoans such as Monocystis. Metabolic wastes like carbon dioxide and ammonia diffuse out of organism.

Mechanisms for Response:

Protozoa are sensitive to several types of stimuli such as temperature changes, touch, light, chemicals, etc. it is not clear how they do so but amoebae are continuously changing and protoplasmic mass can receive stimuli and conduct them. Cilia and flagella are extremely sensitive to touch. One special sensory organelle is stigma or eyespot.

Reproduction and life cycles:

Asexual reproduction takes place in all protozoans by fission, budding and cyst formation. Organism reproduces to form its own kind without forming gametes. Parent organism transfers the complete set of diploid chromosome to daughter cell.

Sexual reproduction involves meiotic nuclear divisions which give rise to haploid gametes from diploid nucleus and union of gametes then restores diploid. It may engage gametes from different parents in which case it is known as amphimictic or may involve gametes which may arise from same parent that is automitic.


Two gametes may be morphologically similar (isogametes) or dissimilar, (anisogametes). Gametes also differ in form; they may be flagellated or amoeboid. Zygote generally enters in the quiescent state and later provides rise to new individuals.


Conjugation is characteristics of ciliates. The details differ from species to species. The general features can be observed in paramecium that has one macronucleus and one micronucleus. Two ciliates set for conjugation partly unite. The body surface and pellicle undergo considerable changes. After numerous posts conjugation division's macronucleus and micronucleus are formed restoring normal nuclear complement.


Encystment is characteristics of life cycle of several protozoans. Protozoan secretes the thickened envelope (cyst) around itself and turns inactive with almost complete shutdown of metabolic activity. This protective cyst is opposed to to either desiccation or low temperature and allows the animal to pass by unfavorable environmental situations like food deficiency, desiccation, decreased oxygen concentration, pH, etc. Cysts of some soil inhabiting and freshwater protozoa illustrate amazing durablilty. Soil ciliate Colpoda has been recognized to survive in dried soil for up to 38 years. In contrast not all cysts are as durable. Cysts of Entamoeba histolytica can tolerate acidic medium of stomach but don't survive desiccation, temperature above 50°C or sunlight.

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