Paramecium- Classification, Locomotion, Nutrition, Osmoregulation

Paramecium

Classification / Systematic Position of Paramecium

Following is the classification of Paramecium -:

Phylum – Protozoa 🡪 Microscopic and acellular

Sub phylum – Ciliophora 🡪 Locomotory organs cilia

Class – Ciliata 🡪 Locomotory organs cilia which persists throughout life

Sub class – Euciliata 🡪 Cilia not uniformly distributed and of unequal size

Order – Holotricha 🡪 Cilia uniformly distributed all over the body

• Paramecium is commonly known as slipper animalcule.
• Body slipper shaped measuring about 0.3 mm in length.
• Covering of the body is by pellicle.
• On the oral side is a groove, the oral groove or peristomial groove. This groove ends in the mouth or cytosome which leads into a narrow cytopharynx ending in the protoplasm.
• Locomotion by lashing of cilia.
• Nutrition usually holozoic. Food consists of bacteria, diatoms and other small organisms.
• Reproduction both asexual and sexual.
  Asexual – Transverse binary fission
  Sexual – Conjugation, endomixis, autogamy and hemixis

 

Habit and Habitat

• Paramecium is found in freshwater ponds, pools, ditches, streams, lakes, reservoirs and rivers.
• It is especially found in abundance in stagnant ponds rich in decaying matter, in organic infusions and in the sewage water.

Locomotion

• Paramecium has a streamlined body which enables it to swim about in water with a minimum amount of friction.
• The rapid swimming is facilitated by the beating of fine hair-like cellular organelles, called cilia, that covers the animal’s entire cell-body.
• Paramecium moves with a speed of 1500 micron per second.

1. Ciliary Beats

• During movement, a cilium oscillates like a pendulum.
• Each oscillation comprises a fast effective stroke and a slow recovery stroke.
• During the effective stroke or the strong backward lash, the cilium becomes slightly curved and rigid and strikes the water like an oar, so that the body is propelled forward in the opposite direction of stroke.
• The recovery stroke which follows immediately brings the cilium again into position for the next effective stroke.

 

* All the cilia of the body do not move simultaneously and independently but progressively in a characteristic wave-like manner, called metachronal rhythm.

2. Mode of swimming

• The animal does not follow a straight tract but rotates spirally like a rifle bullet along a left handed helix (i.e., in an anticlockwise spiral path).

The reason for this twofold (i.e., having two parts) is – the body cilia which rotates the cell body towards the left and forward ; the cilia present on the oral groove which helps it to move in circles.

 

Nutrition

1. Food

• Paramecium feeds in the holozoic manner, like Amoeba.
• The food consists chiefly of bacteria which float in water in which it lives.
• It has been estimated that 2 to 5.5 million individuals of Bacillus coli are devoured in 24 hours by a single Paramecium.
• In a sense, Paramecia are also beneficial to bacteria, or else the bacteria might reproduce too rapidly so as to endanger their own existence by overcrowding.
• It also feeds upon small Protozoa, unicellular plants (algae, diatoms. Yeasts, e.t.c.) and small bits of animals and vegetables.
• It will reject most of non-digestible material and lay waste certain kinds of food.

* One species, Paramecium bursaria, is interesting, being green in colour due to the presence of numerous unicellular alga, the symbiotic Zoochlorella in its endoplasm. It can thus live holophytically for long periods on food substances manufactured by Zoochlorella. During scarcity of food, it can digest even its own Zoochlorella and can live apparently indefinitely without them.

2. Feeding Mechanism

• Food is ingested by a definite cell mouth or cytosome lying at the bottom of buccal cavity.
• The constant lashing movements of cilia of oral groove drive a current of water with food particles towards the vestibule.
• Ciliary tracts of vestibule direct the food particles into buccal cavity.
• Paramecium is a selective feeder,i.e., many kinds of particles may be carried with water current into vestibule, but only the selected ones are passed on the side of buccal cavity, Rest of the particles are rejected,i.e., discharged to outside.
• Passage along which ciliary action drives selected food particles is termed as the selection path whereas passage along which the unwanted food particles are driven outside vestibule is termed as rejection path.
• Beating of cilia of membranelles of buccal cavity drives the selected food particles through cytosome into cell gullet or cytopharynx.
• The food now gradually collects at the bottom of cytopharyns into a membranous vesicle which is later nipped (removed by pinching/ clipping) off as food vacuole.

3. Digestion

• Each food vacuole consists of food particles surrounded by a thin film of water.
• Rapid and irregular movement of endoplasm does not occur in Paramecium, but the food vacuole is circulated around the body along a more or less definite path by a slow streaming movement of endoplasm known as cyclosis.
• Several vacuoles may be seen thus circulating in a definite direction in the endoplasm of well-fed Paramecium.
• Digestion and assimilation of food take place during this journey. Digestive enzymes (proteases, carbohydrases, lipases) are secreted by the lysosomes into the food vacuoles.
• In Amoeba, the contents of a vacuole first become increasingly acidic, but later become alkaline. The alkaline phase results from the secretion of enzymes within an alkaline medium into the vacuole.
• Products of digestion are diffused into the surrounding cytoplasm and either stored or used for vital activity and growth.

 

4. Egestion

• The vacuole gradually becomes smaller as digestion and absorption proceed.
• Finally, the undigested residual matter is eliminated from body, through a definite anal spot or cytopyge on ventral surface, posterior to cytosome.

Osmoregulation

The function of contractile vacuoles in Paramecium is osmoregulation.

• An excess of water accumulates in the body because of continuous endosmosis, the concentration of body cytoplasm being higher than that of the external medium.
  Small quantities of water are also taken in along the ingested food. This excess of water is got rid off by means of contractile vacuoles which contract (systole) and expand (diastole) at regular intervals, assisted by the contractility of myofibrils.

• Water from the cytoplasm is secreted into some of the tubules of endoplasmic reticulum from where it flows down through the nephridial tubules into the feeder canals to accumulate in radial canals. The ampullae (radial canals) converge and discharge into the contractile vacuole.