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Monday, 4 June 2018

Describe the ultra structure of a becterial cell


1.Describe the ultra structure of a becterial cel.
Ans. Ultra structure of a bacterial cell :
The prokaryotic cells very in size from a Mycoplasma (a sphere of about 0.12 diameter) to a blue green bacterium like oscillatoria (a rod of dimensions as much as 40 x 12 micrometre), but the majority have a diameter in the region of 1.0 micrometre. The components of a typical prokaryotic cell are shown in figure. These are as follows :-
Ultra structure of a bacterial cell as seen under electron microscope
Fig. Ultra structure of a bacterial cell as seen under electron microscope
 (1) Cytoplasm or cytosol :
  - The cytoplasm is requestered within the cell membrane. Does cytosol has structure or is it just like a solution of enzymes and metabolites in a test tube. Whwther cytosol in vivo is just a disordered solution or indeed highly organised is a matter of a same debate. Though there is evidence that it has structure but the concept is still poorly understood. in electron microscope it is not organised. It is a concentrated solution which contains a variety of enzymes, coenzymes and metabolites, perhaps in the from of an aqueous fluid or semifluid ground substance or matrix. The matrix is a complex mixture containing in solution a variety of inorganic ions, amino acids, some proteins, lipocomplexes, peptides, nitrogenous bases; sugars, vitamins, enzyme, coenzymes etc. its main function is in intermediary metabolism and in providing an equable chemical environment for cellular activities.
(2) Flagella :
 - most motile bacteria possess long (up to 20 micrometre), thin (20 nm diameter) helical appendages called flagella. Unlike eukaryotic flagellum, the prokaryotic flagellum has no definite membrane. The flagellum is not visible using the light microscope without increasing its effective diameter by coating it with a suitable precipitate. In the electron microscope, negative-staining with phosphotungstic acid shows that flagellum is made up of identical helically along the axis of the flagellum to give a hollow tube. These sub-units can be separated from each other by acidification and consist of protein molecules called flagellin.
(3) Pili, fimbriae and Spinae : 
  - The term pili was introduced by brinton (1959) and Fimbriae by duguid etal (1955). Both are non-flagellar appendages of cell. They are extremely fine, non-flagellar appendages which look superficially similar to flagella. They have been observed mostly in Gram- negative rods. They measure 3-25 nm in diam. and 0.5-20 micrometre in length. They are also made up of individual protein sub- units of pilin, arranged helically to from filament. However, They differ from flagella in the following respects:_
(i) The filament is usually straight and is shorter than a flagellum.
(ii) The diameter is smaller (about 10 micrometre)
(iii) The function is not in motility.
(4) Capsule :  
 - Some prokaryotes have agel layer called the capsule surrounding the cell wall. It can be seen in the light microscope by negative staining with a dye as indian ink. In electron microscope it normally looks as an amorphous shrunken layer. They are synthesised by cell membrane. The gel is usually fromed of a polysaccharide (1-2% dry wet. ) in water and there is a wide variety of different monosaccharide components joined in may different ways. Occasionally, there may be polypeptides also, as the peculiar polymer of the unusual D-glutamic acid found in anthrax-causing bacteria.
(5) Cell wall :
 The cell wall is the danse layer surrounding the cell membrane. The main function is to provide a mechanically strong bounding layer. Some, as Mycoplasma do not have a wall and thus exist only in restricted habitats. The cell wall is not semi-permeable membrane but it can act a molecular sieve preventing large molecules passing through. In gram-negative cells some enzymes and metabolites are trapped between the cell membrane and the outer membrane of cell wall to from the periplasm. The wall components of the cell can be strongly antigenic.
(6) Regular surface layers :
- These are present external to cell wall in all Gram positive and Gram negative bacteria even in Those without peptidoglycans in their cell wall. These main kinds of S-layers are mainly composed of single homogeneous polypeptides with carbohydrates occasionally as minor component. These layers have a predominantly scidic aminoacid composition. These layers may act as  a barrier or molecular sieve controlling the movement of external and internal factors such as toxic macromolecules. These may also protect the peptidoglycan from action of lytic enzymes as lysozyme.

(7) Cell membrane : 
 This is the boundary layer of the protoplast and is about 6-8 nm thick. In thin sections it appears a triple-layered structure consisting of two electrondense (each about 2.5 nm thick) layers surrounding an electron transparent one, a type of structure typical of most selectively permeable membranes in living organisms and called a unit membrane.
(8) intracytoplasmic membranes : 
 The cytoplasmic membrane may not exist as a simple structure underlying the cell wall and following its contours. All such structures may be grouped into two main categories-
(a) Chromatophores : 
 These are pigment-bearing membranous structures of photosynthetic chromatiaceae and cyanophyceae and very in from as vesicles, tubes,bundled tubes, stacks, membranes or thylakoids (as in cyanobacteria).
(b) mesosomes : 
 They were earlier called peripheral body or chondroid and have been seen in ultrathin sections of all Gram-positive and occasionally in Gram-negative becteria.
(9) Ribosomes :
 They are seen in thin sections as relatively dense particles about 20 nm in diameter present as polysomes. They can be easily obtained from ruptured cells by differential centrifugation on a sucrose gradient. They are made up to two sub-units of sedimentation constants 30 S and 50 S, which combine to give the characteristic prokaryotic 70 S ribosome.
(10) Nuclear body : 
 It is of primitive type and also referred to as nucleoid. It is possible to use coloured or fluorescent stains to delineate a central DNA- rich region in bacterial cells. From a lysed protoplast it can be isolated as a structure consisting of loops of super coiled DNA together with about 10% RNA and 10% protein (mostly RNA polymerase). This nuclear body. an amorphous, lobular mass of fibrillar, intensely chromatinic material occupies about 10-20% of the cell volume. In the electron microscope this body shows a diffuse area containing fibrous material with no limiting membrane.

 

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