Detailed and point-wise notes on bacterial cell constituents with clear diagrams, which are also applicable for general prokaryotes. Notes useful for under graduate level. Good reference for PG too.
BACTERIAL CELL STRUCTURE
PLASMA MEMBRANE:
• Bacterial plasma membrane is 7.5nm thick; has 20-30% phospholipids, 60-70% proteins.
• Singer and Nicolson gave the Fluid Mosaic Model of plasma membrane. According to this
model, the membrane is bi-layer of phospholipids.
Each phospholipid is asymmetric with polar and non-polar ends. The non-polar ends are
hydrophobic and don’t interact with water, while the polar end is hydrophilic and interacts with
water.
• There are two types of proteins present in the membrane; Peripheral and Integral proteins.
Peripheral proteins are present on the surface of plasma membrane while the integral proteins are
embedded in the membrane.
Integral proteins can be removed only by destruction of the membrane and treatment with
detergents; Peripheral proteins can be removed by mild treatments like osmotic shock.
• The integral proteins are amphipathic i.e, their hydrophobic regions are buried in lipids while
the hydrophilic regions project out from the membrane surface.
• Many bacterial membranes have pentacyclic sterol-like molecule called Hopanoids.
Hopanoids are thought to provide stability to bacterial membrane, like steroids in eukaryotic
cells.
DIFFERENCES WITH EUKARYOTIC PLASMA MEMBRANE
• Sterols absent, except in mycoplasma which lack cell wall. Sterols stabilize the phospholipids
in eukaryotic membrane and make it more rigid.
• Proportion of protein to phospholipid is high.
Prokaryote – 2:1 , Eukaryote – 1:1 or less.
FUNCTIONS
• Selectively regulates flow of materials in and out of the cell.
• Contains enzymes useful in the synthesis of membrane lipids and other macro molecules that
compose the bacterial cell wall.
• Site of ETS (Electron Transport System) carriers which generate ATP.
• Has specific attachment sites for chromosome and plasmid.
Plays active role in replication of chromosome and plasmid during cell division.
Proteins that need to be transported out of the cell are different from other proteins ; they carry a
signal peptide made of sequence of 20 amino acids. These sequences are added to the proteins at the
time of their synthesis. The membrane recognizes the signal peptide and transports the protein out
of the cell.
, CELL WALL:
• Cell wall is rigid structure that gives shape to cell.
• Cell wall of bacteria is made of Peptidoglycan or Murein.
• Main function of cell wall is to prevent cell from expanding and eventually bursting due to
uptake of water, since most bacteria live in hypotonic environments.
• Based on cell wall composition bacteria are two types; Gram positive and Gram negative.
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