The prokaryotic cells are generally smaller and vary in size in different members. In mycoplasma it is about 0.12) in while in Oscillator, a filamentous blue green the size is 40 x 51 (1. A great majority of them however are about in size.
Among the bacteria the smallest are to be found among cocci (0.1 (i) while the largest are the spirilla (60 x 6m). The various components of the prokaryotic cell are described below:
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1. Cell Wall:
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Cell wall is absent in some prokaryotic (mycoplasma, L forms of bacteria) members while it forms the outermost boundary in a large majority of them. While the cell wall is non living and does not have the property of selective permeability it can act as a sieve as it is porous.
In some prokaryotes the space between the cell membrane and cell wall is filled with some enzymes and metabolites constituting the periplasm.
Chemically the cell wall is made up of strong fibres of hetropolymers called mucopeptides ofpeptidogycans. They are also referred to as glycopeptide, muropeptide, murein etc.
The peptidoglycans consist of alternating units made up of N acetyl glucosamine and N acetyl muramic acid with 1 -4 linkages. This type of a linkage is common to all Prokaryotes.
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Attached to the muramic acid is a short peptide chain consisting of alanine, glutamic acid and either lysine or diaminopimelic acid (DAP) and muramic acid. DAP and some of D-amino acids are found only in prokaryotic cell walls.
Cross linkage of these polymers assures structural rigidity with the type and extent of linkage being species specific.
For example in some bacteria a pentaglycine links the tetrapeptide side chains extending from the muramic acid units while in others the terminal D- alanine of one tetrapeptide may be covalently linked to the DAP of an adjacent tetrapeptide.
The peptidoglycans of gram negative bacteria have more cross linkage than those of gram positive bacteria.
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In addition to peptiodglycans the wall has many other chemicals. The wall of gram negative bacteria shows as many as five layers under the electron microscope.
These are-the periplasm space, Peptidoglycan layer, an intermediate zone, the outer membrane and lipopolysaccharides.
2. Cell Membrane:
This is the bounding layer of cytoplasm and is about 6-8nm thick. Unlike in the case of eukaryotic cells it carries out multiple functions. Some of these are –
(i) Presence of enzymes of biosynthetic pathways leading to the synthesis of wall components.
(ii) Occurrence of permease enzymes responsible for the transport of materials across the membrane.
(iii) Membranes of aerobic bacteria have the electron transport chain and oxidative phosohorylation.
(iv) The purple bacteria have their photosynthetic apparatus in the membrane.
(v) The membrane has attachment sites for chromosomal DNA (also plasmid) during DNA partitioning.
(vi) Regulatory mechanism for chemotaxis is located in the membrane.
Structurally the membrane is triple layered consisting of two electron dense layers surrounding an electron translucent layer. In other words it is similar to a unit membrane. The architecture and chemical composition is similar to the fluid mosaic model proposed by Singer and Nicolson (1972).
3. Cytoplasm:
Cytoplasm does not appear to have an organized structure when observed under electron microscope. It may be regarded as a concentrated solution containing a host of metabolites, enzymes, amino acids, inorganic ions, sugars, vitamins etc. Cytoplasm plays an important role in intermediary metabolism. There is no cytoskelcton.
4. Surface Layers:
These are present in all gram negative and gram positive bacteria. Three main types of S layers have been reported in prokaryotic cells (bacteria) The S layers are composed of single homogenous peptides with occasional carbohydrates as minor components.
These layers are believed to act as macromolecular sieves preventing the entry of toxic molecules and also to protect the peptidoglycans of the wall from the action of lytic enzymes
5. Nucleoid:
This represents the genetic material of prokaryotic cells. It is similar in all prokaryotes. (For details see – bacteria) The molecular structure of DNA is same as in eukaryotic cells.
6. Inclusion Bodies:
These are storage granules and other particles distributed in the cytoplasm. There are two kinds of inclusion bodies-membrane enclosed and membraneless (Shiviey 1974).
7. Membrane Enclosed Bodies:
These are enclosed by a non unit type of membrane 2-4nm thick and made up of proteins. The inclusion bodies encountered in the prokaryotic cells are chlorosomes, carboxysomes, magnetosomes, gas vacuoles, polyhydroxybutyrate (PHB) granules, sulphur granules and glycogen granules.
PHB granules occur in cyanobacteria, sulphur granules are found in Thiorhodaccae and glycogen granules in Clostridium spp.
Chlorosomes have been demonstrated in photosynthetic bacteria belonging to chlorlobiaceae. (Cruden and Stanier, 1970) these are different from chromatophores in being non-membranous. Carboxysomes are seen in cyanobacteria (Murphy et al 1974).
Magnetosomes are particles which help the bacterium in orienting itself in relation to the magnetic behaviour of the surroundings. Reported to occur in Aquaspirillum magnetotacticum (Balkwill et al 1980), the magnetosomes allow the bacteria to punise the most efficient aerotactic behaviour.
8. Non membranous Inclusion Bodies:
Phycobilisomes, cyanophycin granules (found in cyanobacteria), polyglucoside granules and several crystalline substances belong to this group
In addition to the above there is R – bodies (retractile structures) demonstrated in bacteria by Lalucat and Mayer (1978) these are supposed to be similar to kappa particles of Paramecium.
Ribosomes (see bacteria)
Mesosomes (see bacteria)
9. Chromatophores:
These are present in the photosynthetic bacteria belonging to the families Rhodospirillaceae, Chromatiaceae and Cyanophyceae. Chromatophores vary in form and have vesicles, tubes, bundles, stacks or thylakoids etc.