Paramecium

Paramecia are a group of unicellular ciliate protozoa formerly known as slipper animalcules from their slipper shape. They are commonly studied as a representative of the ciliate group. Paramecia 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. There is also a deep oral groove containing inconspicuous compound oral cilia (as found in other peniculids) that is used to draw food inside. They generally feed upon 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 scums. 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).

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Physiology

The paramecium is 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 and 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 the radiating canals are also found on the outside of a paramecium.

The paramecium contains cytoplasm, trichocysts, 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

Paramecium feed on micro-organisms 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 there is enough food in it so that it has reached a certain size it breaks away and forms a food vacuole. The food vacuole travels through the cell, through the back end first. As it moves along enzymes from the cytoplasm enter the vacuole and digest it. The digested food then goes into the cytoplasm and the vacuole gets smaller and smaller. When the vacuole reaches the anal pore the remaining undigested waste is removed.

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.

Giant amoeba, for instance, have 2 types of endosymbiotes, which seem to function as mitochondria in these amoeba. Another example involves protozoa bacteria that produce cellulases that assist the protozoa in cellulose digestion. It is a cell that appears at quiet ponds.

Genome

The paramecium genome has been sequenced (species: Paramecium tetraurelia), providing evidence for three whole genome duplication.^[1]

In some ciliates, like Stylonychia and Paramecium, only UGA decoded as a stop codon, while UAG and UAA are reassigned as sense codons.^[2]

References

1. ^ Aury, J. M., O. Jaillon, et al. (2006). "Global trends of whole-genome duplications revealed by the ciliate Paramecium tetraurelia." Nature 444(7116): 171-8. [1]
2. ^ Lekomtsev, S, Kolosov, P., et al. (2007) "Different modes of stop codon restriction by the Stylonychia and Paramecium eRF1 translation termination factors", PNAS, 104(26):10824-9 [2]