Paramecium
Overview
Paramecium is a group of unicellular ciliate
Ciliate
The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...

 protozoa
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

, which are commonly studied as a representative of the ciliate group, and range from about 0.05 to 0.35 mm in length. Simple cilia cover the body, which allow the cell to move with a synchronous motion (like a caterpillar
Caterpillar
Caterpillars are the larval form of members of the order Lepidoptera . They are mostly herbivorous in food habit, although some species are insectivorous. Caterpillars are voracious feeders and many of them are considered to be pests in agriculture...

) at speeds of approximately 12 body lengths per second. There is also a deep oral groove containing inconspicuous tongue-like compound oral cilia (as found in other peniculid
Peniculid
The peniculids are an order of ciliate protozoa, including the well-known Paramecium and its close relatives. Most are relatively large, freshwater forms that feed on smaller organisms swept into the mouth. They have simple life cycles, and in many cases do not even form resting cysts.Typically...

s) used to draw food inside.
Encyclopedia
Paramecium is a group of unicellular ciliate
Ciliate
The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...

 protozoa
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

, which are commonly studied as a representative of the ciliate group, and range from about 0.05 to 0.35 mm in length. Simple cilia cover the body, which allow the cell to move with a synchronous motion (like a caterpillar
Caterpillar
Caterpillars are the larval form of members of the order Lepidoptera . They are mostly herbivorous in food habit, although some species are insectivorous. Caterpillars are voracious feeders and many of them are considered to be pests in agriculture...

) at speeds of approximately 12 body lengths per second. There is also a deep oral groove containing inconspicuous tongue-like compound oral cilia (as found in other peniculid
Peniculid
The peniculids are an order of ciliate protozoa, including the well-known Paramecium and its close relatives. Most are relatively large, freshwater forms that feed on smaller organisms swept into the mouth. They have simple life cycles, and in many cases do not even form resting cysts.Typically...

s) used to draw food inside. They generally feed on bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and other small cells, making them heterotroph
Heterotroph
A heterotroph is an organism that cannot fix carbon and uses organic carbon for growth. This contrasts with autotrophs, such as plants and algae, which can use energy from sunlight or inorganic compounds to produce organic compounds such as carbohydrates, fats, and proteins from inorganic carbon...

s. Osmoregulation
Osmoregulation
Osmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is it keeps the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a measure of the tendency of water to move...

 is carried out by a pair of contractile vacuole
Contractile vacuole
A contractile vacuole is a sub-cellular structure involved in osmoregulation. It is found predominantly in protists and in unicellular algae. It was previously known as pulsatile or pulsating vacuole....

s, which actively expel water from the cell absorbed by osmosis
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...

 from their surroundings. They are relatively large protists and can easily be seen with a medium-power microscope.

Paramecia are widespread in freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

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

Certain single-celled eukaryotes, such as Paramecium, are examples for exceptions to the universality of the genetic code
Genetic code
The genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells....

: in their translation systems a few codons differ from the standard ones.

Physiology

The Paramecium approximates a prolate spheroid
Prolate spheroid
A prolate spheroid is a spheroid in which the polar axis is greater than the equatorial diameter. Prolate spheroids stand in contrast to oblate spheroids...

, rounded at the front and pointed at the back. The pellicle
Pellicle (biology)
The pellicle is a thin layer supporting the cell membrane in various protozoa, protecting them and allowing them to retain their shape, especially during locomotion, allowing the organism to be more hydrodynamic. They vary from flexible and elastic to rigid. Although stiff, the pellicle is...

 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. The Paramecium is very commonly mistaken for a blepharisma.

The Paramecium contains cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

, trichocyst
Trichocyst
http://media-2.web.britannica.com/eb-media/68/8968-004-A94DB3DF.jpg A trichocyst is a structure in the cortex of certain ciliate and flagellate protozoans consisting of a cavity and long, thin threads that can be ejected in response to certain stimuli...

s, the gullet, food vacuoles, the macronucleus
Macronucleus
A macronucleus is the larger type of nucleus in ciliates. Macronuclei are polyploid and undergo direct division without mitosis. It controls the non-reproductive cell functions, the everyday tasks, such as metabolism...

, 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 microorganisms 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
Paramecium aurelia
Paramecium aurelia are unicellular organisms belonging to the genus Paramecium of the phylum Ciliophora. They are covered in cilia which help in movement and feeding.Paramecium can reproduce sexually, asexually, or by the process of endomixis...

and its bacterial endosymbionts. See also the Chlorella
Chlorella
Chlorella is a genus of single-celled green algae, belonging to the phylum Chlorophyta. It is spherical in shape, about 2 to 10 μm in diameter, and is without flagella. Chlorella contains the green photosynthetic pigments chlorophyll-a and -b in its chloroplast...

symbiosis with Paramecium bursaria
Paramecium bursaria
Paramecium bursaria is a species of ciliate protozoan that has a mutualistic symbiotic relationship with green algae called Zoochlorella. The algae live inside the Paramecium in its cytoplasm and provide it with food, while the Paramecium provides the algae with movement and protection. P...

.

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

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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