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Paramecium Cells

2016-10-21T14:06:02Z

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[[Eukaryotic cells|''Paramecium'' cells]] are large unicellular organisms. Paramecium is a genus, there are four different species; paramecium aurelia, paramecium bursaria, paramecium caudatum and paramecium tetraurelia.<ref>Lynn H. (2008) The ciliated protozoa characterisation, classification and guide to the literatuer, New York: Springer</ref> They are part of the [[Eukaryotes|eukaryotic]] family, thus meaning that they have membrane-bound [[Organelles|organelles]].

Paramecium is free living ciliated [[Protozoa|Protozoa]], its cell body is surrounded by [[Cilia|cilia]]. There are two main functions of these [[Cilia|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 [[Endocytosis|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.<ref name="null">Beale, Geoffrey (2008) Paramecium Genetics and Epigentics CRC Press, Taylor and Fancis Group, Pg 23</ref> This YouTube video shows the discharge of trichocysts: [http://youtu.be/5eDYfcdE7ns youtu.be/5eDYfcdE7ns] 

A Paramecium cell has two [[Nuclei|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 vauoles|contractile vacuoles]] would burst this is caused by the [[Osmosis|osmotic]] uptake of water, by a process known as [[Osmoregulation|osmoregulation]] <ref name="(Beale G. & Preer J,2008).">Beale, Goffey and Preer, John R. Jr. (2008) Paramecium Genetics and Epigenetics CRC Press, Taylor and Francis Group.</ref>. Near to the cell surface membrane, [[Contractile vauoles|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<ref>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</ref>. Once the water has entered the vacuole the pore opens and the vacuole contracts expelling the water. Within close proximity of the [[Contractile vauoles|contractile vacuoles]] are many [[Mitochondria|mitochondria]] and this is due to the face that the [[Organelles|organelles]] require [[ATP|ATP]] as its source of energy. 

The Paramecium cell reproduces by a process called [[Reproduction by Conjugation|conjugation]] <ref>L.Prescott, J.Hardley and D.Klein Microbiology 6th Edition New York:McGraw-Hill</ref> and [[Asexual fission|asexual fission]].   Asexual fission creates two genetically identical daughter cells. 

Paramecium has [[Action potentials|action potentials not]] unlike those that occur in [[Neuron|neurons]].  However, in Paramecium, calcium ions enter the cell through voltage gated channels and cause the rapid depolarisation of the membrane<ref>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</ref> 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 <ref>Eckert, R. and Brehm, P. (1979) Ionic Mechanisms of Excitation in Paramecium. Annual Review of Biophysics and Bioengineering. 8, 353-383</ref>. 

The length of a typical [[Cell|''paramecium'']] varies from 100 μm to 300 μm <ref>Brock Biology of Micro-organisms 12th Edition, Madigan Dunlap Clark, Pg 69</ref>. They can be found in freshwater areas, like rivers, ponds and lakes <ref>Holtzman E, Novikoff A (1984) Cells and Organelles, 3rd edition, USA, CBS College Publishing</ref>. 

Paramecium cells are capable of regulated [[Exocytosis|exocytosis]] when triggered by an external stimulus. This exocytosis is similar to the release of [[Neurotransmitter|neurotransmitters]] by the [[Presynaptic membrane|presynaptic membrane at]] a [[Synapse|synapse]]. However instead of using it for signalling and depolarising the [[Postsynaptic membrane|postsynaptic membrane]], it is used as a defence mechanism against predators <ref>Genoscope (2007) Paramecium tetraurelia: Paramecium, a model ciliate</ref><ref>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</ref>. 

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<ref>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.</ref><ref>Hames, D. and Hooper, N. (2011) BIOS Instant notes: Biochemistry, 4th edition, New York: Garland Science.</ref>. 

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Lactose

2015-12-03T14:56:54Z

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Lactose is a disaccharide present in milk. It is made up of the [[Monosaccharides|monosaccharides]] [[Glucose|glucose]] and [[Galactose|galactose]].  The two monosaccarides join via a condensation reaction forming a [[Glycosidic bond|β1-4 glycosidic bond]]. Commonly associated with the [[Lac operon|Lac Operon]] which is a model complex for understanding selective gene [[Transcription|transcription]] control in ''[[Escherichia coli|E. coli]]''.

In humans the disaccharide is is hydrolysed in humans by the enzyme lactase and by[https://bms.ncl.ac.uk/wiki/index.php/%CE%92-galactosidase β-galactosidase] in bacteria.

Many adults, more commonly outside of Europe, are unable to drink milk as they are unable to hydrolyse the disaccharide into the two monosaccharides. This condition is called lactose intolerance, or hypolactasia. A deficiency of the enzyme lactase is usually the cause of hypolactasia. 

 

 

 

= References =

<references />Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York: WH Freeman.

Saltatory conduction

2015-12-03T14:51:58Z

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Due to the [[Myelin Sheath|myelination of]] [[Neuron|neurones]] within [[Mammals|mammalian]] nervous systems, [[Action potential|action potentials]] may only occur at the [[Nodes of Ranvier|Nodes of Ranvier]]. Myelin is made up of insulating cells which means [[Depolarisation|depolarisation]] cannot occur in myelinated regions. Between these cells however, there are gaps known as the [[Nodes of Ranvier|Nodes of Ranvier]] which are unmyelinated. As depolarisation cannot occur at the cells making up the myelin sheath, the wave of depolarisation can only occur at the [[Nodes of Ranvier|Nodes of Ranvier]]. Thus, [[Action potentials|action potentials appear]] to jump from node to node when travelling down an [[Axon|axon]]. 

This phenomenon is known as saltatory conduction, and serves as a means of increasing the rate of propagation of an [[Action potential|action potential]] <ref>Alberts, B (2008). Molecular Biology of the Cell. New York: Garland Science. 680</ref> (200m/s as opposed to 2m/s)<ref>OMICS international. 2014. saltatory conduction. [ONLINE] Available at: http://research.omicsgroup.org/index.php/Saltatory_conduction#cite_ref-AP_1-0. [Accessed 17 November 15].</ref>

Not only does saltatory conduction increase the speed of impulse transmission by causing the depolarization process to jump from one node to the next, it also conserves energy for the axon as depolarization only occurs at the nodes and not along the whole length of the nerve fibre, as in unmyelinated fibres. This leads to up to 100 times less movement of ions than would otherwise be necessary, therefore conserving the energy required to re-establish the Na+ and K+ concentration differences across the membranes following a series of action potentials being propogated along the fibre. <ref>Linden, R., Ward, J. (2013) Physiology at a Glance, 3rd edition, Oxford: Wiley-Blackwell</ref> 

=== Ion Channels and Action Potentials ===

The influx of Na+ ions and efflux of K+ ions produce the action potential. In the resting state, voltage sensitive Na+ and K+ ion channels are closed. The simultaneous activation of many sodium channels in the membrane of an axon causes an influx of Na+ ions. This influx of positive charges causes the membrane potential (of a neurone) to become more positive, producing a gradual depolarization of the membrane. Once the threshold value of the membrane potential has been reached (-45mV) a series of events is triggered leading to the initiation and generation of an action potential. At threshold level of the membrane potential, more voltage sensitive sodium channels are activated, resulting in a greater influx of Na+ ions. The influx of positive charges depolarises the membrane further. 

At the peak of the [[Action potential]] the membrane is much more permeable to Na+ than K+; consequently the value of the membrane potential is closer to the Na+ equilibrium potential than to the K+ equilibrium potential. After the peak has been reached the inactivation channels close, Na+ influx is blocked, and the membrane potential begins to repolarise. As sodium channels become inavactivated, the potassium channels begin to become activated. This increase in potassium conductance causes the membrane potential to become more negative and contributes to the repolarisation phase of the action potential. Finally, the prolonged opening of K+ channels causes a continued efflux of K+ ions. This removal of positive charges from the cell in turn causes the membrane potential to remain  briefly before returning to the resting level<ref>Human Physiology by Rhoades and Pflanzer - 3rd edition 1996</ref>.

Non myelinated neurones

=== References ===

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