Bacterial Cell: Structures internal to the cell wall

The structures that present interior to the cell wall include cell membrane, mesosomes, cytoplasm, nuclear material, cytoplasmic inclusions and vacuoles.

i) Cell membrane (or) cytoplasmic membrane:

Immediately beneath the cell wall there is a thin membrane around the cytoplasm. This membrane is called the cytoplasmic membrane or plasma membrane. It contains phospholipids, proteins and polysaccharides. The cell membrane is a vital structure and critical barrier that seperates the inside of the cell from the outer environment. The most widely accepted current model for cell membrane structure is the fluid mosaic proposed by Singer and Nicholson.

The thickness of the cell membrane is 7.5nm. the plasma membrane is semi permeable and regulates the transport of nutrients and waste products into and out of the cell. Cell membrane is composed of 20 – 80% of phospholipids and 60 to 70% of proteins. The phospholipids are amphipatic and from a lipid bilayer with hydrophobic group (fatty acid) towards inside and hydrophilic groups towards outside. Proteins are embedded in the lipid matrix and are called as Integral or intrinsic proteins (70 to 80%). 20 to 30% of protein are loosely connected to the membrane are called as peripheral or extrinsic proteins.

Functions:

1. selective permeability and transport of solutes.

2. Electron transport and oxidative phosphorylation in aerobic species.

3. Excretion of hydrolytic exocoenzymes.

4. Bearing the enzymes and carrier molecules that function in the biosynthesis of DNA, cell wall polymer and membrane lipids.

5. Bearing the receptors and other proteins of the chemo tactic and other sensory transduction system.

ii) Mesosomes:

In gram positive bacteria, the plasma membrane forms in folding. The infolding can give rise to mesosomes within cytoplasm. The mesosome is associated with bacterial nuclear material and its replication. Respiratory enzymes are also associated with thin mesosomes.

Functions:

1. They involve in cell wall formation during cell division.

2. They play a role in replication of chromosome and distribution to daughter cells.

3. They also involve in secretary processes.

iii) Cytoplasm:

The cytoplasm is an aqueous solution of soluble proteins and metabolites. It contains RNA, ribosomes and reserve food materials. Is also contains nuclear body or nucleoid.

iv) Nuclear material:

The prokaryotic cell is strikingly different from eukaryotic cell in the lack of a well defined nucleus. The nuclear body or nucleoid lacks nuclear envelope. It consist of single molecule of DNA which is circular. It embedded directly in the cytoplasm. It consists of a nuclear zone at the centre of the cell. This nuclear zone is also called as nuclear body or chromatin body or nuclear region or nucleoplasm or nucleoid.

In addition to this genetic material (DNA), many bacteria possess other genetic material called as Plasmids. The plasmids is non-chromosomal genetic material. Plasmids are circular, double stranded DNA molecules with independent replicating capacity, they are smaller in size.

v) Ribosomes:

In bacterial cell ribosomes occur freely in cytoplasm and in clusters of 4 to 6 ribosomes. The clusters are called the polysomes. The ribosome are of 70S consisting of 50S and 80S particles.the cytoplasmic area, granular in appearance and rich in the macromolecular RNA protein bodies known as Ribosomes, on which protein are synthesized.

vi) Cytoplasmic inclusions and vacuoles:

Concentrated deposits of certain substances are detectable in the cytoplasm of some bacteria. The reserved food materials are fats, polysaccharides, volutin etc.

Volutin granules are also known as meta-chromatic granules are composed of polyphosphate. Volutin serve as a reserve source of phosphate. Fats are stored in highly retractile globules in the form of polymerized Beta-hydroxy butyric acid, which can serve as a reserve carbon and energy source polysaccharides (glycogen) are stored in granules.

Another type of inclusions is represented by the intracellular globules of elemental sulphur that may accumulate in certain bacteria growing in environments rich in hydrogen sulphite.

Some bacteria that live in aquatic habitats form gas vacuoles that provide buoyancy, these are bright retractile bodies and have a regular shape, hallow, rigid, cylinders with more or less conical ends and having a striated protein boundary. This boundary is impermeable to water.

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