paramecium, unicellular organism of the genus Paramecium, of the ciliate phylum Ciliophora found in freshwater throughout the world. Like other protozoans, paramecia, previously considered one-celled animals, are now customarily placed in kingdom Protista. The paramecium has a stiff outer covering that gives it a permanent slipper shape. It swims rapidly by coordinated wavelike beats of its many cilia—short, hairlike projections of the cell. A paramecium normally moves forward in a corkscrew fashion but is capable of reversing direction when it encounters adverse conditions. This trial-and-error behavior (backing up and then continuing forward in a slightly different direction until an optimum path is found) is conspicuous when the animal is observed through a microscope.

Paramecia and other ciliates are the most complex of all single-celled organisms. The paramecium has an external oral groove lined with cilia and leading to a mouth pore and gullet; food (typically smaller organisms, such as bacteria) is digested in food vacuoles. There are also an anal pore, two contractile vacuoles that regulate the water content of the cell, and two nuclei. The larger nucleus, or macronucleus, is thought to regulate most cell functions, while the smaller nucleus, or micronucleus, is involved in reproduction. Paramecia usually reproduce asexually by cell division but can also exchange genetic information via a process called conjugation, in which two individuals unite at the oral grooves and exchange micronuclei that serve as little packages of DNA, after which the cells divide, yielding daughter cells with DNA from each of the parents.

Paramecia are a group of unicellular ciliate protozoa formerly known as “slipper animalcules” from their slipper shape, are commonly studied as a representative of the ciliate group, and range from about 50 to 350 μm in length, depending on species. Simple cilia cover the body, which allow the cell to move with a synchronous motion (like a caterpillar). There is also a deep oral groove containing inconspicuous compound oral cilia (as found in other peniculids) used to draw food inside. They generally feed on bacteria and other small cells. Osmoregulation is carried out by a pair of contractile vacuoles, which actively expel water absorbed by osmosis from their surroundings.

Paramecia are widespread in freshwater environments, and are especially common in scum. Paramecia are attracted by acidic conditions. Certain single-celled eukaryotes, such as Paramecium, are examples for exceptions to the universality of the genetic code (translation systems where a few codons differ from the standard ones).

Physiology

The paramecium 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 pellicle are many tiny hairs, 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.

The paramecium contains cytoplasm, trichocysts (“thread capsules”), the gullet, food vacuoles, the macronucleus, and the micronucleus.

Locomotion

For the paramecium to move forward, its cilia beat on an angle, backward. This means that the paramecium moves by spiralling through the water on an invisible axis. For the paramecium to move backward, the cilia simply beat forward on an angle. If the paramecium should run into a solid object, the cilia change direction and beat forward, causing the paramecium to go backward. The paramecium 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 microorganisms like bacteria, algae, and yeasts. To gather its food, the paramecium uses its cilia to sweep the 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, which is like the stomach. When enough food has accumulated at the gullet base, it invaginates there to form 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 going 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. The bacteria infect the protozoa, and they produce toxic particles that kill sensitive strains, but not killer strains. See also the Chlorella symbiosis with Paramecium bursaria.

Giant amoebas, for instance, have 2 types of endosymbiotes, which seem to function as mitochondria in these amoebas. Another example involves protozoa bacteria that produce cellulases to assist the host protozoan with cellulose digestion (similar to those found in some in termites). This is a cell that appears at quiet ponds.

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

In some ciliates, like Stylonychia and Paramecium, only UGA 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 experiment and partaying, 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.

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