Locomotion in amoeba, Biology


Continuous formation of new  pseudopodia keeps amoeba in constant locomotion .This is called  amoeboid  movement .It  occurs  in many  other  protozoans , in  amoebocytes of  sponges  and in leucocytes  of vertebrates, It is the simplest   mode of animals locomotion;

As  observed ,microscopically   amoeboid movement  in amoebae  depends  upon three basic factors:

1.       Attachment to substratum :  Since  amoeboid  movement involves   crawling, it needs contact of  the body  with  a solid  substrantum . Adhesion with substratum is facilitated by  glycocalyx  and also  by traces of inorganic ions *(Ca++, Mg++. K+) that  are always  present in  surrounding water .

2.       Continuous  viscosity conversions : Conversion  of plasmagel (ectoplasm)  into sol  at the  trailing  end  of plasmasol  (endoplasm ) into gel at the  advanceing  end  must constantly occur.

3.       There must  be some  force  to cause  forward  flow  of plasmasol  (endoplasm) from behind  into a pseudoodium.

Amoeba  crawls  about 0.02 to 0.03 mm per minute. A pseudopodium  can form  at any point  on body   surface. It first  appears  a small  fingerlike  bulge  of homogeneous  glistening  fluid, called  hyaline cap .As  the  bulge  touches  a sub stratum, there  is a sudden  rush  or granular endoplasm into it ,The bulge therefore elongates  and become    a pseudopodium   .In a roteus several  pseudopodia  usually  being  to form  simultaneously in different  directions, but only  one ultimately  elongates all   other  are withdrawn  back  into  the body .Consequently  there is some  displacement  of body  in the  direction   of the completed pseudopodium ,soon a new  pseudopodium   similary  forms  elsewhere  and the organism, therefore  moves in a   different direction .The  movement  is thus  quite  erratic  and  never  in the  same  direction for long.

How pseudopodia  form and locomotion is affected is still imperfectly  understood. The following  theories  have  been postulated in this connection.

1.    Adhesion Theory : According   to this  earliest view   amoeba   moves like a drop of water  streaming  out in  the path  of greatest  adhesion  upon  rough surfaced  substratum.

2.    Contraction Theory :: In early 19th century , a pseudopodium   was  regarded   a hernia like  protrusion  bulged  out a weak point , probably due to  contraction of  body  mass elsewhere  .In 1835, scientists  believed  that amoeba   contains  contractile  strands  of gel which pull the whole  body mass  in the  direction of a  advancing  pseudopodium  .Later  when Heitzmann advocated that  the body of  amoeba was a 3-dimensioanl . network ( reticular theory of cytoplasmic structure ) of live  contractile fibres embedded  in a non  living  and non contractile  fluid schultze (1861)  postulated that a contraction  at the  rear end of body  pushes or squezes  the sol ahead .

3.    Surface tension  theory  :Berthold (1886) postulated that  constant  internal  tension  forces the cytoplasm to shoot out from  surface  in the form of a pseudopodium  wherever  surface tension suddenly lower due to local  effects. Butschli (1892)  and   rhumbler (1898) supported this  view, surface tension does exits  on a fluid surface . but  its existence upon  the surface of plasmalemma of  amoeba  is doubtful.

4.     Theory  of rolling movement : By  dropping  a particle of lamp  black  on the surfac  of a verrucosa  Jennings (1904)  noted  that amoeba  simply  rolls  like a ball  during locomotion ,Such  a movement  is possible  only  in unipodal forming a single  pseudopodium  at a time   species  like A verrucosa  but not in bi or mulitpodal amoeba . 

5.    Walking  movement theory :: When  we observe  an amoeba under  the  microscope  , we  get  the impression  that it is  creeping  in close  contact  with the  substratum. Dellinger (1906) viewed  amoebae  from a side  and  noted  that, during  locomotion  the main  part of body  remains  lifted from the substratum  and supported on tips   of pseudopodium  thus  amoeba  virtually walk  on pseudopodia  just  as higher  animals  move on the legs .He  explained  that amoeba extends a pseudopodium  free in water  and swings it about .As  the pseudopodium   strikes an  objects  it sticks to it, this  causes contraction of an endoplasmic  of an endoplasmic  fibrous network  of contractile  elements, pulling  the whole body  mass close to  the  pseudopodium .

6.     Pressure or Ectoplasmic  contraction  or sol gel  theory : When  cytoplasm was finally  recognized  as a colloidal system  Hyman (1917) schaudinn  and others suggested  that con version  of ectoplasm into  endoplasm  and vice versa  was the chief  basis  of amoeboid  movement ,actually  worked  out by pantin  (1923)  and mast (1926,1931)  this theory  postulated that pseudopodia  are formed and withdrawn  due  to cycle  viscosity  change in that  colloidal  cytoplasm from  sol to gel  and gel  states. Locomotion  in this way  was explained by  four main processes .a

a.       Contact  of plasmalemma   lof advancing  side  of body  with a substratum and  resultant weakening  of plasmagel ( ectoplasm) layer  here due to  its partal solation .

b.      Sudden forward  streaming of   plasmasol ( endoplasm ) under tension which  breaks  the weakened plasmagel sheet   and converts  into  gel after  striking  against  the hyaline  cap.

c.       Conversion ofplasmagel into  plasmasol  in trailing  part of body

d.      Simultaneous  contraction  in the liquefying  plasmagel at the rear end.  

Posted Date: 10/4/2012 3:03:04 AM | Location : United States

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