Systematic Position
Kingdom : Monera
Division : Eubacteria
Order : Eubacteriales
Type : Bacteria
Cell Structure of Bacteria
Bacteria are small unicellular, microscopic organisms. Under electron microscope, bacteria show a small tiny mass of protoplasm surrounded by a cell wall.
- Cell wall :The cell are extremely covered by rigid cell wall. Main component of cell wall is peptidoglycan or mucopeptide or murein. The peptidoglycan is composed of amino sugars and amino acids. Consists of lipids and polysaccharides, cellulose and chitin are absent. Permeable to water, ions and other small molecules, Not to proteins. Functions: Provides definite shape and protection to the protoplast against mechanical injury, pathogens, and chemicals.
- Capsule and Slime Layers :In many bacteria a layer of slime, a viscous or gelatinous substance of polysaccharides or of polypeptides secreted by the protoplasm, surrounds the cell wall. If the mucilage sheath is thick (due to presence of amino acids), it is called capsule. Capsule is developed in non-flagellate stage. Functions: Mucilage protects the cell against chemicals, desiccation (drying), and host action. It holds the cell together after division so that they form filaments or irregular masses.
- Cytoplasmic membrane: It is present inner to the cell wall and for covering of the cytoplasm. It is about 30A° in thickness. It is composed phospholipids, proteins and polysaccharides. It acts as a semipermeable membrane. Cytoplasmic membrane is folded at places, which are called mesosomes. It encloses several respiratory and other enzymes.
- Cytoplasm: It is a granular viscous substance, present between the plasma membrane and nuclear body. It includes gas vacuoles, ribosomes, glycogen, lipids, proteins, etc. As the bacterial cells are prokaryotic, the cytoplasm contains no endoplasmic reticulum, Golgi bodies, mitochondria, and chloroplast. In certain photosynthetic bacteria photosynthetic pigments are lodged in chromatophores. The important cytoplasmic structures are as follows: Mesosomes: These are small circular or semicircular bodies formed by the enfoldings of the cytoplasm membrane. They are usually found in Gram- positive bacteria and a few species of Gram-negative bacteria. Mesosomes increase the surface area of the cell membrane for the enzymes, actively involved in respiration. They initiate DNA replication. They help in cell wall formation.
Chromatophores: They are single membrane-bounded vesicles occupying much of the cytoplasm. It contain photosynthetic pigments like bacteriochlorophyll and carotenoids. They are also known as "vesicles' and act as photosynthetic apparatus and hence only found in photosynthetic bacteria.
Ribosomes: The bacterial ribosomes are 70s type. They take part in protein synthesis.
Gas vacuoles: These are single membrane bounded gas vesicles. They help in floatation of aquatic bacteria.
Reserved food material: The reserved food materials are glycogen or starch. poly f-hydroxybutyric acid, lipid globules and protein crystal.
Volutin granules (metachromatic granules): They are composed of polymerized metaphosphates. In certain sulphur bacteria, the globules of pure elemental sulphur occur in the cell. - Nuclear Body: The bacterial cell lacks an organised nucleus. It differs from the nucleus of higher plants in the absence of nuclear membrane and nucleoli. This type of nucleus is called incipient nucleus or nucleoid. These bodies are composed of naked, double stranded, thread-like molecules of DNA. The DNA molecules are circular and tightly packed, present at the centre of the cell and lack histone protein. It contains all the genetic information and is usually called bacterial chromosome.
Extra DNA (Plasmids): These are extra circular DNA present in most bacteria. They are self replicating circular strands of DNA. The number of plasmid varies from1 to 100 in a cell.
Flagella: These are long, fine, protoplasmic threads found in many forms of bacteria. Each flagellum arises from a basal granule. The basal granule remains embedded in the cytoplasm just below the surface of the cell. Flagella are locomotary organs that help in locomotion of bacteria in the liquid media.
Sex pilli or Fimbriae: These are fine straight hair-like protoplasmic projections from the wall of some gram negative bacteria made up of a pure protein pilin. These help in attachment of bacteria with other bacteria at the time of conjugation.
