Phylum Protozoa (Ultrastructure, Osmoregulation, Locomotion, Nutrition, Reproduction) For Msc. 1st semester  

Ultrastructure of paramecium

1. Shape and size

Paramecium caudatum is a microscopic organism and it appears light gray or white measuring commonly looks like the sole of a slipper or shoe, hence, the animal is commonly known as Slipper animalcule. The forward moving anterior part is slender with a blunt or rounded end, while the posterior end is somewhat pointed or cone shaped. The widest part of the organism is just below the middle. The body of animal is asymmetrical. 

2. Pellicle 

The body is covered by a thin, double layered, elastic and firm pellicle made of gelatin. The pellicle holds the shape of the animal but is elastic enough to permit contractions. The pellicle has double membrane, the outer membrane is continuous with the cilia and the inner membrane with the ectoplasm. Under the higher magnification of microscope, pellicle shows surface of Paramecium. 

3. Cilia

The entire body is covered with numerous, small hair like projections called cilia. Cilia occur in longitudinal rows all over the body, this condition is known as holotrichous in which the body cilia are equal. 

The cilia are organelles of locomotion and food collection, they also act as sensory receptors and detect the stimuli of the external environment. 

4. Cytoplasm

The cytoplasm is differentiated into a narrow, external or cortical zone called the ectoplasm and a larger, internal called the endoplasm.

(a) Ectoplasm

The ectoplasm is a permanent part of the body. Ectoplasm forms a firm, clear, thin and dense outer layer. It contains the trichocysts, cilia and fibrillar structures.

(b) Endoplasm

The endoplasm is the more fluid and voluminous part of the cytoplasm which contains many cytoplasmic granules as well as outer inclusions. 

5. Contractile vacuoles 

There are two large, liquid - filled contractile vacuoles, each situated near one end of the body close to the dorsal surface.

       

 Osmoregulation and Contractile vacuole in Protozoa

Shape, Size and Occurrence : -  The contractile vacuoles are usually large, colourless, pulsatile flute field organelles found in majority of protozoans. These vacuoles are nearly always found in freshwater flagellata, Sarcodina and Ciliata. The contractile vacuoles are also found in marine ciliates but these are not at all found in parasitic protozoans.

The contractile vacuoles are found in their simplest form in Sarcodina like Amoeba. These vacuoles are found surrounded by a circlet of mitochondria which provide energy for their pulsating activity. In flagellata like Euglena, the contractile vacuole is some what complicated and surrounding by a large number of accessory contractile vacuoles. In Ciliata like Paramecium, the contractile vacuoles are much complicated and found surrounded by 5 to 12 radiating canals or feeding canals which collect water from the various parts of the body.

Situation and number : -  The position of contractile vacuole is not definite in Sarcodina and therefore, can be found anywhere in the endoplasm. In Flagellata, e.g. Euglena the contractile vacuole is found situated near the interior and at the side of reservoir. In Ciliata like Paramecium, the contractile vacuoles are usually two in number, situated one at each end of the body. The number of contractile vacuoles various in the different groups of Protozoa but its number remains constant in the same species. 

Structure : - Contractile vacuole is an empty space filled with fluid. Its limiting membrane is lipoprotein in nature like that of the plasma membrane. 

Mode of working : -The mode of working of contractile vacuole include to steps the diastole and the systole. The diastole is the phase of enlargement of contractile vacuole to its maximum size and systole is the phase of its contraction to explain its contents. A contractile vacuole is usually formed by the fusion of a large number of very small droplets in the area where contractile vacuole is to be formed or around the contractile vacuole. However, the exact mechanism of working of contractile vacuole is not yet understood, even then the following theories have been put forth to explain its working:

• Osmotic theory :  This theory explains that the water from the surrounding cytoplasm enters into the contractile vacuole by osmosis.
• Filtration theory :  this theory explains that the water from the cytoplasm is forced into the contractile vacuole through its membrane due to internal hydrostatic pressure.
• Secretion theory : This theory states that the water is actively secreted into the vacuole during diastole throw the vacuole wall. 

Function and significance :- 

Contractile vacuole performs the function of osmoregulation by removing excess of water content from the body. In addition to its water regulatory function, the contract is also believed to be excretory in function.  

The contractile vacuole helps in removing the excess water content of the protoplasm, i.e., it helps in maintaining the internal hydrostatic pressure. Actually, the body fluid is hypertonic to the surrounding medium is nearly isotonic and, therefore,  no excess water enters in the body. Hence, contractile vacuole is usually absent in these forms.

       REPRODUCTION IN PROTOZOA

The process of reproduction is it to continue one's kind in nature. The mode of reproduction in Protozoa is highly variable among different groups although it is primarily a cell division. Protozoa reproduce both a Asexually and sexually.

Asexual reproduction :- 

In a sexual mode of reproduction no special sex sales are involved but their is always some form of prison present. And essential part of the process is the partition of some parent chromatin substance among daughter individuals. Hence, the fission of the cell body is preceded by the division of the nucleus. Asexual reproduction occurs by the following methods. 

(A) Equal or binary fission :-  Equal or binary fission place for reproducing and also for gamete formation. Usually there is a centriole within the nucleus, but unlike Metazoa no asters are formed,Moreover the nuclear membrane persist intact during division in most Protozoa. The nucleus elongates and divides amitotically into two parts which travel apart, then the cell constricts in the middle form two daughter cells. Macronuclei of Ciliates divide amitotically. Binary fission is simple in Sarcodina like Amoeba where the plane of division is not definite and it is usually transverse Ciliates like Paramecium but in most flagellates like Euglena it is longitudinal in which the nucleus Elongates transversely, but the cell divides lengthwise while binary fission is oblique in certain case like Ceratium. In binary fission a single flagellum usually by one daughter cell, and the basal granule divide into two the new basal granule forms a Flagellum in the other daughter cell.

Fig. Binary fission in protozoa. A------Amoeba (irregular) ; B---- Euglena (longitudinal); C---- Paramecium (transverse) ; D------ Ceratium (oblique)