Describe the cells structure of a typical bacterial cells
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A bacterial cell shows a typical prokaryotic structure. The cytoplasm is enclosed by three layers, the outermost slime or capsule, the middle cell wall and inner cell membrane. The major cytoplasmic contents are nucleoid, plasmid, ribosome, mesosome etc., and the cell is devoid of endoplasmic reticulum, mitochondria, centrosome and golgi bodies.
A. Slime layer, Capsule and Glycocalyx:
An amorphous viscid secretion of bacterial cell is present as a loose undemarcated region outside the cell, called slime layer (e.g., Leuconostoc). But, when it originates as a sharply defined structure outside the cell wall, it is called capsule (e.g., Pneumococcus). The capsule is about 0.2 μm in width.
The capsules those are much narrower than true capsule, called microcapsule (e.g., Haemophilus influenzae). Both the layers usually consist of polysaccharide and occasionally polypeptide. The capsule contains 2% solid and 98% water. The solid portion is comprised of complex polysaccharide (e.g., Pneumococcus, Enterobacter) or polype) ide (e.g., anthrax bacillus) or hyaluronic acid (e.g., Streptococcus).
Both the layers serve as a protective covering and protect the cells from antibacterial substances like bacteriophage, enzymes etc. They enhance the virulence of bacteria.
The glycocalyx is an extracellular, hygroscopic network of mostly polysaccharides and occasionally of polypeptide or of both. The chemical components of glycocalx are synthesized by the cell and transported through the ceil membrane and cell wall, and finally deposited outside the cell, to form extracellular covering.
The polysaccharides of glycocalyx contain many different sugars such as glucose, galactose, rhamnose and sugar acids like glucuronic acid etc.
B. Cell Wall:
The bacterial cell wall is tough and rigid due to the presence of strong fibres composed of heteropolymers called mucopeptides, peptido- glycans, mucocomplex, murein etc. The peptidoglycan is composed of alternate units of N-acetyl muramic acid and N-acetyl glucosamine residues, cross-linked with tetra-peptide subunits
The structural constituents of wall vary in Gram- negative and Gram-positive bacteria
C. Cytoplasmic Membrane of Bacteria:
The cytoplasmic layer is the boundary layer of the protoplast, situated beneath the cell wall . It is thin (5-10 nm), elastic and semipermeable layer . In section, it appears as a triple-layered structure consisting of a bilayer region of phospholipid molecules, with polar heads on the surface and fatty-acyl chains towards the inner side. The proteins are found embedded in the lipid bilayer.
D. Cytoplasm of Bacteria:
The cytoplasm is a colloidal system containing both organic and inorganic substances. It lacks mitochondria, endoplasmic reticulum, centrosome and golgi bodies. It contains many ribosomes, few mesosomes, soma inclusions and vacuoles .
Ribosome:
Mesosomes (Chondroids):
Chromatophores:.
Cytoplasmic inclusion:
E. Genetic Material of Bacteria:
The genetic material is present both in nucleoid and plasmid .
Nuclear material:
Under light microscope nuclear body cannot be differentiated in the cytoplasm, but it is differentiated only under electron microscope as a central area of lower electron dense region than rest of the cytoplasm.
Plasmid:
Bacterial cytoplasm may contain some genetic material excepting the genophore, called plasmid or episomes. Lederberg (1952) termed as plasmid those extragenophoral genetic materials.
F. Flagella of Bacteria:
Most of motile bacteria (e.g., Spirochaetes) possess long (5-20 µm), thin (12-30 nm), helical appendages, called flagella. Electron microscopy shows that the flagellum consists of three distinct regions — filament, hook and basal body . Filament is attached at one end through the cell wall to the cell membrane by the hook, which, in turn, is attached to the basal body. The rings of basal body remain attached to the cell membrane and cell wall. The filament lies external to the cell.
G. Fimbriae or Pili:
These are extremely thin and short, filamentous, non-flagellar appendages projecting peritrichously from cell surface. Their number is 100-500 per cell and measure 0.5-20 µm in length and 3-25 nm in diameter. They are made up of subunits of protein, the pilin, arranged helically and form hollow filament .
H. Spinae:
Spinae are tubular, pericellular rigid appendages. They are made up of single protein moiety, the spinin. They possibly help to acclimatize the cells in different environmental conditions such as salinity, temperature etc. Spinae have been reported in some Gram-positive bacteria.
A. Slime layer, Capsule and Glycocalyx:
An amorphous viscid secretion of bacterial cell is present as a loose undemarcated region outside the cell, called slime layer (e.g., Leuconostoc). But, when it originates as a sharply defined structure outside the cell wall, it is called capsule (e.g., Pneumococcus). The capsule is about 0.2 μm in width.
The capsules those are much narrower than true capsule, called microcapsule (e.g., Haemophilus influenzae). Both the layers usually consist of polysaccharide and occasionally polypeptide. The capsule contains 2% solid and 98% water. The solid portion is comprised of complex polysaccharide (e.g., Pneumococcus, Enterobacter) or polype) ide (e.g., anthrax bacillus) or hyaluronic acid (e.g., Streptococcus).
Both the layers serve as a protective covering and protect the cells from antibacterial substances like bacteriophage, enzymes etc. They enhance the virulence of bacteria.
The glycocalyx is an extracellular, hygroscopic network of mostly polysaccharides and occasionally of polypeptide or of both. The chemical components of glycocalx are synthesized by the cell and transported through the ceil membrane and cell wall, and finally deposited outside the cell, to form extracellular covering.
The polysaccharides of glycocalyx contain many different sugars such as glucose, galactose, rhamnose and sugar acids like glucuronic acid etc.
B. Cell Wall:
The bacterial cell wall is tough and rigid due to the presence of strong fibres composed of heteropolymers called mucopeptides, peptido- glycans, mucocomplex, murein etc. The peptidoglycan is composed of alternate units of N-acetyl muramic acid and N-acetyl glucosamine residues, cross-linked with tetra-peptide subunits
The structural constituents of wall vary in Gram- negative and Gram-positive bacteria
C. Cytoplasmic Membrane of Bacteria:
The cytoplasmic layer is the boundary layer of the protoplast, situated beneath the cell wall . It is thin (5-10 nm), elastic and semipermeable layer . In section, it appears as a triple-layered structure consisting of a bilayer region of phospholipid molecules, with polar heads on the surface and fatty-acyl chains towards the inner side. The proteins are found embedded in the lipid bilayer.
D. Cytoplasm of Bacteria:
The cytoplasm is a colloidal system containing both organic and inorganic substances. It lacks mitochondria, endoplasmic reticulum, centrosome and golgi bodies. It contains many ribosomes, few mesosomes, soma inclusions and vacuoles .
Ribosome:
Mesosomes (Chondroids):
Chromatophores:.
Cytoplasmic inclusion:
E. Genetic Material of Bacteria:
The genetic material is present both in nucleoid and plasmid .
Nuclear material:
Under light microscope nuclear body cannot be differentiated in the cytoplasm, but it is differentiated only under electron microscope as a central area of lower electron dense region than rest of the cytoplasm.
Plasmid:
Bacterial cytoplasm may contain some genetic material excepting the genophore, called plasmid or episomes. Lederberg (1952) termed as plasmid those extragenophoral genetic materials.
F. Flagella of Bacteria:
Most of motile bacteria (e.g., Spirochaetes) possess long (5-20 µm), thin (12-30 nm), helical appendages, called flagella. Electron microscopy shows that the flagellum consists of three distinct regions — filament, hook and basal body . Filament is attached at one end through the cell wall to the cell membrane by the hook, which, in turn, is attached to the basal body. The rings of basal body remain attached to the cell membrane and cell wall. The filament lies external to the cell.
G. Fimbriae or Pili:
These are extremely thin and short, filamentous, non-flagellar appendages projecting peritrichously from cell surface. Their number is 100-500 per cell and measure 0.5-20 µm in length and 3-25 nm in diameter. They are made up of subunits of protein, the pilin, arranged helically and form hollow filament .
H. Spinae:
Spinae are tubular, pericellular rigid appendages. They are made up of single protein moiety, the spinin. They possibly help to acclimatize the cells in different environmental conditions such as salinity, temperature etc. Spinae have been reported in some Gram-positive bacteria.
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A typical bacterial cell strcture for the conclusion
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