Bacteria

 ULTRASTRUCTURE OF BACTERIAL CELL

Under electron microscope, a typical bacterial cell shows the following structures –

            1. Capsule:- In some bacterial cell, a gelatinous layer is present surrounding the cell. It is composed of polysaccharides and amino acids. When the constituents of the layer are only polysaccharides, it is known as slime layer, but when amino acids are present along the polysaccharides, it is called capsule. The main function of this layer is to protect the cell from antibodies.

            2. Cell Wall:-  Just below the capsule is the cell wall. It varies in thickness from 50 to 100 A­⁰. It is rigid and provides a definite shape and protection against mechanical injury, pathogen and chemicals. The cell wall is composed of peptidoglycans or mucopeptides. Few lipids, phosphorous, muramic acid and a few inorganic salts are also present.

            3. The Protoplasm:- Beneath the cell wall lies the living protoplasm. Protoplasm can be divided into two parts – the plasma membrane and the cytoplasm.

            (a) Plasma membrane:- It is a semi-permeable membrane which surrounds the protoplasm. It is 75 A⁰ in thickness. Plasma membrane is mainly composed of lipid and protein (50 – 65%). The main function of plasma membrane is to control passage of dissolved substances in and out of the wall. This membrane posses a special type of enzyme known as permease which helps in the transport of specific proteins. It also helps in cell division and in the formation of mesosomes.

            (b) Cytoplasm:- It lies immediately beneath the plasma membrane. Cytoplasm is a homogenous aqueous solution of soluble proteins, large number of enzymes, amino acids and nucleotides. Due to the presence of reserve materials, it appears granular under electron microscope.

            Bacterial cytoplasm also contains ribosomes, mesosomes, chromatophores, etc. Besides, the cytoplasm also contains non living inclusions such as volutin, glycogen, lipid globules or protein crystals. Sulphur and iron are also present in some bacteria.

            (c) Nuclear material:- There is no well organized nucleus in bacteria. Instead of this, a naked DNA serves as the function of the nucleus which has no surrounding membrane. This structure is known as nucleoid or genophore. It appears to be fibrillar and composed of a double or single stranded DNA which is about 10,000 A⁰long. The part of the cytoplasm occupied by DNA is known as nucleoplasm.

          (d) Photosynthetic material:- In photoautotrophic bacteria, photosynthetic materials are found in the form of small membranous vesicle. Bacterial chlorophyll is the example of such material which is found within the cytoplasm.