Bacteria are unicellular organisms. The structural details of the organism can be properly seen under electron microscope.
Under light microscope, cell wall and protoplast containing nuclear mass can be made out. External to the cell wall may be present a thin layered envelope called slime. The bacterial shape also can be detected under light microscope.
The detailed structure of the bacteria as seen under electron microscope is given in the following.
1) Slime
ADVERTISEMENTS:
Viscous or gelatinous substances are sometimes secreted by the protoplast of the bacterial cell. It is called slime and when this forms a thick conspicuous layer around the cell wall, it is called sheath or capsule. It is usually composed of polysaccharides or of polypeptides of certain amino acids.
The sheath protects the enclosed organism against desiccation and antibodies. The encapsulated pathogenic bacteria can withstand phagocytosis.
2) The Cell wall
The cell wallencloses the bacteria as a thin, sharply defined layer around the protoplast. It ranges in thickness may be around 0.02fi. The cell wall is composed mainly of peptidoglycans like acctylglucosamine and acctylmuramic acid. The other components of cell wall are polysaccharides, amino acids and lipids. Cellulose and chitin are absent as constituents of the cell wall.
ADVERTISEMENTS:
The cell wall provides rigidity and shape to the bacterial cell.
3) The Protoplast
This constitutes the living components of the bacterial cell. The protoplast contains viscous fluid. It is differentiated into plasma membrane, cytoplasm and chromatin network.
(a) Plasma membrane
ADVERTISEMENTS:
The delicate, envelope which encloses the protoplast and lies close to the cell wall is called plasma membrane or plasma lemma or cytoplasm membrane.
Chemically, it is composed of phospholipids, with proteins and some polysaccharides. It is differentially permeable membrane, which regulates the passage of dissolved substances. The enfolding of the plasma membrane form complex structures within the cytoplasm. These are called embosoms.
(b) The cytoplasm
The bacterial cells are fairly homogeneous with numerous minute, spherical, bodies called ribosome. No other fine structure is seen in the bacterial cytoplasm. The ribosome’s are again of 70S type.
ADVERTISEMENTS:
The cytoplasm’s show no streaming movements and contain no obvious vacuoles. Sulphur and iron are found in some bacteria. The nonliving inclusions are storage granules. Of volition, glycogen, lipid and protein crystals. Organic matters occur in the colloidal state. The cytoplasm of bacteria is a complex mixture of carbohydrates, lipids, minerals, nucleic acids and water.
(c) Chromatin Network
Here, the chromatin or genetic materials are concentrated. The bacterial cell lacks nuclear membrane and nucleolus. The nuclear material lacks any definite shape
since it has no membrane.
Genetic material occurs in the form of a single long circular molecule of double stranded DNA. Unlike the chromosomes of eukaryotes, the bacterial chromosomes do not have histoncs. The chromosomes occur in the form of dispersed fibrous areas called nucleotides or chromatin network.
ADVERTISEMENTS:
In addition to bacterial chromosomes, bacteria often contain extra chromosomal double stranded circular DNA molecules called plasmids.
These extra chromosomal strands of DXA do not carry genetic material that is essential the organism. They are independent and self replicating. Another type of plasmid called episome may be present. Episomes can integrate into the bacterial chromosomes in contrast to the plasmids.
Plasmids have variable properties F-Plasmids, R-Plasmids and Ti Plasmids respectively enhance fertility factors, resistance to antibiotics and tumor inducing properties. Similarly, virulent plasmids cause disease symptoms. These characters have great importance in genetic engineering and crop improvement.
The bacterial cell differs from eukaryotic cell in the following aspects.
ADVERTISEMENTS:
(1) Nucleoli and nuclear membrane are absent.
(2) The cell organelles like mitochondria and endoplasmic reticulum do not occur.
(3) The nuclear mass does not divide by mitosis.
(4) Photosynthetic apparatus when present are not organized into chloroplasts.
(5) Extra nuclear chromosomal materials called plasmids occur in some bacteria.
(6) The protoplasm shows no streaming movement.