Paramecium Cells

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Paramecium cells are large unicellular organisms. Paramecium is a genus, there are four different species; paramecium aurelia, paramecium bursaria, paramecium caudatum and paramecium tetraurelia.[1] They are part of the eukaryotic family, thus meaning that they have membrane-bound organelles.

Paramecium is free living ciliated Protozoa, its cell body is surrounded by cilia.  THere are two main functions of these cilia.  Firstly they allow paramecium to move around its freshwater habitat.  Secondly, they are also used to waft small bacteria and algaes into the gullet (a large invagination in the cell membrane) where they are endocytosed and assymilated into the cell. All waste excess is exctreted via the anal pore.

Paramecium also use trichocysts (spear-like structures that protrude from the cell) as a defence mechanism to help protect themselves from predators.[2] This YouTube video shows the discharge of trichocysts: youtu.be/5eDYfcdE7ns

A Paramecium cell has two nuclei, the germinal nucleus also known as the micro-nucleus is involved in sexual processes. Fundamentally the transfer of genetic information; meiosis is conducted as this nucleus. The somatic nucleus, also known as the macro-nucleus participates in the process of transcription and ensures expression of genetic information.

Paramecium lives in a freshwater environment which in the abscence of contractile vacuoles would burst this is caused by the osmotic uptake of water, by a process known as osmoregulation [3]. Near to the cell surface membrane, contractile vacuoles have canals. These contain vacuole fluid with an osmolarity, controlled by cl- and k+, that is higher than the osmolarity of the cytoplasm. This allows water to enter the canals passively through osmosis[4]. Once the water has entered the vacuole the pore opens and the vacuole contracts expelling the water. Within close proximity of the contractile vacuoles are many mitochondria and this is due to the face that the organelles require ATP as its source of energy.

The Paramecium cell reproduces by a process called conjugation [5] and asexual fission.   Asexual fission creates two genetically identical daughter cells.

Paramecium has action potentials not unlike those that occur in neurons.  However, in Paramecium, calcium ions enter the cell through voltage gated channels and cause the rapid depolarisation of the membrane[6] and generate an action potential, rather than the sodium ions (as in neurones). The repolarising phase is due to the closing of the calcium ion channels and the opening of the potassium ion channels [7].

The length of a typical paramecium varies from 100 μm to 300 μm [8]. They can be found in freshwater areas, like rivers, ponds and lakes [9].

Paramecium cells are capable of regulated exocytosis when triggered by an external stimulus. This exocytosis is similar to the release of neurotransmitters by the presynaptic membrane at a synapse. However instead of using it for signalling and depolarising the postsynaptic membrane, it is used as a defence mechanism against predators [10][11].

According Beale; ‘one hypothesis suggests that Paramecium has been round even before the continents separated and has not moved; only continents have’. That is why the ciliated protozoa is readily found all over the world living in fresh water and feed on microscopic organisms such as bacteria and single-celled algae and move by propelling their cilia, back and forth in prompted quick succession (Beale & Preer., 2008: 16).

As one of the oldest primitive organisms on earth, Paramecia are among the first organisms used to clarify the Universal genetic code. It is still of much historical interest to geneticists, today, known to use a variant genetic code (UAA and UAG = Glu not stop).

Like most other single celled organism, they divide by binary fission. Occasionally, Paramecia exchange genetic material in a kind of primitive sexual reproduction using a parole cone like protuberance which passes gamete nuclei from one conjugate to another.

A peculiar behavioral response is demonstrated by Paramecia; when exposed to any physical or chemical stimuli they propel faster or discharge a spine-like structure from their outer coating called trichocyst at the stimulus as a protective defense measure against being pursued or devoured by predators.

Studying Paramecium cell has produced concepts that are widely accepted to advance knowledge leading to a better understanding of mechanisms like the muscle sliding filament phenomenon and the evolution of the neuronal functions of higher organisms[12][13].

References:

  1. Lynn H. (2008) The ciliated protozoa characterisation, classification and guide to the literatuer, New York: Springer
  2. Beale, Geoffrey (2008) Paramecium Genetics and Epigentics CRC Press, Taylor and Fancis Group, Pg 23
  3. Beale, Goffey and Preer, John R. Jr. (2008) Paramecium Genetics and Epigenetics CRC Press, Taylor and Francis Group.
  4. Stock C., Gronlein H.K., Allen D., Naitoh Y., 2002, Osmoregulation in paramecium; in situ ion gradients permit water to cascade through cytosol to the contractile vacuole, Journal of cell science, vol. 115 pages 2339-2348
  5. L.Prescott, J.Hardley and D.Klein Microbiology 6th Edition New York:McGraw-Hill
  6. Hinrichsen, R. and Schultz, J. (1988) “Paramecium: a model system for the study of excitable cells” Trends in Neurosciences, vol 11, no. 1, pp. 28
  7. Eckert, R. and Brehm, P. (1979) Ionic Mechanisms of Excitation in Paramecium. Annual Review of Biophysics and Bioengineering. 8, 353-383
  8. Brock Biology of Micro-organisms 12th Edition, Madigan Dunlap Clark, Pg 69
  9. Holtzman E, Novikoff A (1984) Cells and Organelles, 3rd edition, USA, CBS College Publishing
  10. Genoscope (2007) Paramecium tetraurelia: Paramecium, a model ciliate
  11. Wincker P (1st November 2006) Nature 444 171-178 Global trends of the whole-genome duplications revealed by the ciliate Paramecium tetraurelia 29th November 2012
  12. Beale, G.H. and Preer, J. R. (2008). Paramecium Genetics and Epigenetics, e-book, accessed 26 November 2012 from http://www.ncl.ac.uk/library/e-books.
  13. Hames, D. and Hooper, N. (2011) BIOS Instant notes: Biochemistry, 4th edition, New York: Garland Science.
 
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