Paramecium

Paramecium
Paramecium aurelia
Scientific classification
Domain:	Eukarya
Superphylum:	Alveolata
Phylum:	Ciliophora
Class:	Ciliatea
Order:	Peniculida
Family:	Parameciidae
Genus:	Paramecium Müller, 1773
Species
Paramecium aurelia Paramecium bursaria Paramecium caudatum Paramecium tetraurelia

Paramecium is a genus of unicellular ciliate protozoa, commonly studied as a representative of the ciliate group. The cell ranges from about 50 to 350 µm in length and is covered with simple cilia, allowing the cell to move at speeds of approximately 12 body lengths per second. There is a deep oral groove containing inconspicuous tongue-like compound oral cilia (as found in other peniculids) used to draw food inside. In general, they feed on bacteria and other small cells, making them heterotrophs. Osmoregulation is carried out by a pair of contractile vacuoles, which actively expel water from the cell absorbed by osmosis from its surroundings. They are relatively large protists and can easily be seen with a medium-power microscope.

Paramecia are widespread in freshwater environments, and are especially common in scums. Recently, some new species of Paramecium have been discovered in the oceans.

Certain single-cell eukaryotes, such as Paramecium, are examples for exceptions to the universality of the genetic code: in their translation systems a few codons differ from the standard ones.

Physiology

The cell approximates a prolate spheroid, rounded at the front and pointed at the back. The pellicle is a stiff but elastic membrane that gives the Paramecium its definite shape. Covering the outer edge are hairlike structures, called cilia. On the side, beginning near the front end continuing down half way, is the oral groove, which collects food until it is swept into the cell mouth. There is an opening near the back end called the anal pore. The contractile vacuole and its radiating canals, referred to previously for osmoregulation of the organism, are also found on the outside of a Paramecium. Paramecium is commonly confused with Blepharisma.

The Paramecium contains cytoplasm, trichocysts, the gullet, food vacuoles, the macronucleus, and the micronucleus. It also has a contractile vacuole that removes water so that the cell does not burst due to the excessive water that can freely enter the membrane due to its semi-permeable nature.

Movement

Cilia are the locomotive structures of the Paramecium. In order for the Paramecium to move forward, its cilia beat at an 120-degree angle, backwards in unison (i.e., the cilium wiggles from tip-to-base). This means that the Paramecium moves by spiraling through the water on an invisible axis. The Paramecium can also move backwards when the cilia beat forward at an angle in unison.

If the Paramecium hits an obstacle it moves back, turns slightly and goes forward again. If it runs into the solid object again, it will repeat this process until it can get past the object.

Gathering food

Paramecia feed on micro-organisms like bacteria, algae, and yeasts. To gather its food, the Paramecium uses its cilia to sweep up food along with some water into the cell mouth after it falls into the oral groove. The food goes through the cell mouth into the gullet. When enough food has accumulated at the gullet base, it forms a food vacuole in the cytoplasm, and travels through the cell, through the back end first. As it moves along, enzymes from the cytoplasm enter the vacuole to digest the contents, digested nutrients then go into the cytoplasm, and the vacuole shrinks. When the vacuole reaches the anal pore, it ruptures, expelling its waste contents to the exterior.

Symbiosis

One of the most interesting known symbiotic relationships is that of Paramecium aurelia and its bacterial endosymbionts. See also the Chlorella symbiosis with Paramecium bursaria.

Genome

The Paramecium genome has been sequenced (species: Paramecium tetraurelia), providing evidence for three whole-genome duplications.

In some ciliates, like Stylonychia and Paramecium, only UGA is decoded as a stop codon, while UAG and UAA are reassigned as sense codons.

Learning

The question of whether paramecia exhibit learning has been the object of a great deal of experimentation, yielding equivocal results. In one of the most recent experiments published, the authors, by using a voltage as a reinforcement, concluded that Paramecium may indeed learn to discriminate between different brightness levels.

External links

• Paramecium/ Paramecium tetraurelia brief facts, modes of reproduction

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