Draw a neat labelled diagram of a typical bacterial cell to show the detailed structure and describe the function of important organelles.
Q. Draw a neat labelled diagram of a typical bacterial cell to show the detailed structure and describe the function of important organelles.
Or, Describe ultrastructure of bacterial cell.
Ans. Structure of Bacteria : Bacterial cell include (a) cell wall and the (b) cytoplasm. The cytoplasm is surrounded by a cytoplasmic membrane just inside the cell wall contains various granules and other cell inclusion and finally the nuclear material of vital importance. The bacterium also may be enclosed in an envelope or sheath of slime, viscous material forming a more less definite capsule. At the surface of bacterium there may be flagella (organs of locomotion) and some bacilli have pili.
(A) Cell wall and cell membrane:
A bacterial cell usually remains surrounded by three membranes : viz (a) cytoplasmic membrane- the inner most (b) cell wall- the middle one and (c) slime layer- the outer most.
Slime layer: It is the modified outer cell wall of bacteria. Sometimes this slime layer is thick and fixed round the cell wall and then termed as capsule. Slime layer gummy and mucilaginous envelopes. Chemically these are found to be polysaccharides along with some nitrogenous substances. These bacterial capsules act as a protective material in addition to the cell wall.
Cell wall: It is rigid, elastic and mainly protective in function the cell wall also acts as selectively permeable membrane, Chemical composition of cell wall is not fully known. The chief chemical constituents of bacterial cell are polysaccharides, oligosaccharides, glucominopeptides, protein liquids, teichoic acids, lipoprotein, etc.
Gram stain: It is one of man differential stains for bacteria which serve to divide bacteria into Gram positive and Gram negative. Recent studies indicate that amino acid make up in the cell walls of gram positive and gram negative bacteria differ consequently, but it is not well understood whether this difference is responsible for the differences in the staining reactions of the two groups.
Cytoplasmic membrane: It is the protein of cytoplasm lying surrounded by cell wall. It is somewhat thicker and denser than cytoplasm. It consists of huge quantities of lipids and lipoproteins. Its functions is to act as selectively permeable membrane. Because of its complex nature the chemical structure of the cytoplasmic membrane is not defined.
(B) Cytoplasm:
The cytoplasm occupies the volume of the bacterial cell surrounded by a cell wall and membrane. This bacterial cytoplasm in young stage remains packed with tiny, dense particle which have a strong affinity for staining. These particles are the ribosome as revealed by electron microscope. Ribosomes contain ribonucleic acid (RNA) and proteins. They distributed uniformly in cytoplasm.
In order bacterial cultures, the homogeneity of the cytoplasm is usually lost. The formation and separation of some granules or droplets of various substances are known as cell inclusions
, Some important cell inclusions are: ?
(i) Metachromatic granules: These granules may be observed in bacterial cell by treating them with basic dyes such as methylene blue. They may be found either throughout the cell or massed the end of cells. These are actually reserve of nitrogenous food. Sometimes these granules are also referred as past nuclei or as reproductive bodies. These granules are also referred as volutin granules.
(ii) Fats (lipid) droplets: These are usually conscious in these nitrogen fixing soil bacteria (Azotobacter or Rhizobium) in large free living Spirilla, in various acid test Bacilli, etc., The facts granules functions.
(iii) Mesosomes: This is a membranous structure of cytoplasm and arise a invagination of plasma membrane. The function of these is a matter of conjective. These are active in cell wall synthesis and secretion of extra cellular substance.
(C) Nuclear Structure:
A large debated question among bacteriologist was that whether true nuclear material exists in bacterial cells or not and if so, whether it is organized into separate structure distinct from cytoplasm. At present evidences concluded that bacterial cell have a nucleus although it is of primitive type.
Recently, with electron microscopic and improved staining techniques, some bacteriologists have observed the presence of some granules [known as “chromatin granules”] and other internal changes in bacteria which indicates the presence and activities of nuclear structures. Chromatin bodies are in reality nucleus have been found in regular numbers during stage of active growth and have been shown to divide just before the whole bacterial cell divides. Chemically the nuclei contain deoxyribonucleic acid (DNA), probably found in a basic protein. The bacterial nuclei is arranged into bundles of fibres and do not remain surrounded by a nuclear membrane. The bacterial nuclei do not undergo mitosis and their mode of replication during division is not well known.
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