BACTERIAL PLASMA MEMBRANE

 PLASMA MEMBRANE

—  Plasma membrane is also called as Cytoplasmic Membrane or Cell Membrane.

—  It act as a boundary of the bacterial cell.

—  Plasma membrane is found in the bacterial cells that separates the interior of the cell from the outside environment.

—  Plasma membrane measures about 5 to 10 nm thick.

—  In Prokaryotes, the Plasma membrane is made up of Phospholipids (20 % to 30 %) and Proteins (60 % to 70 %). In Eukaryotes, the Plasma membrane is made up of Carbohydrates and Streols (Cholesterol).

—  Generally, Plasma membrane is a Phospholipid bilayer and Semipermeable.  

—  The Phospholipid structure is Amphipathic (a molecule, especially Protein, having both Hydrophilic and Hydrophobic parts) in nature and measures approximately 8 nm thick.

—  The Phospholipid structure has Hydrophilic (water loving) heads and Hydrophobic (water hating) tails.

 Structure of Phospholipid

FLUID-MOSAIC MODEL STRUCTURE OF PLASMA MEMBRANE

—  The dynamic arrangement of Proteins and Phospholipids in Plasma membrane is called as “Fluid-Mosaic Model Structure”.

—  Discovered by Jonathan Singer and Garth Nicholson in 1974.

 

—  The Fluid mosaic model of plasma membrane is the most accepted hypothesis.

—  The “Fluid-mosaic model” name is derived from the fact that Phospholipids in the membrane are in a fluid state and that Proteins are dispersed among the lipid molecules in the membrane, forming a Mosaic pattern.

—  Membrane proteins (Peripheral Proteins and Integral Proteins), Glycoproteins and Glycolipids are present in the Fluid – Mosaic model.

 Fluid-Mosaic Model Structure of Bacterial Plasma membrane (Source: Microbiology – Principles and Explorations, 8^th Edition by Jacquelyn G. Black)

Membrane Proteins

(i) Peripheral Proteins

·       Connected to the membrane and easily removed by mild treatments.

·       It lies at the inner and outer surface of the membrane.

·       They are soluble in aqueous solution and make upto 20 to 30 % of total Membrane proteins.

·       They may function as enzymes that catalyze chemical reactions, as a "scaffold" for support, and as mediators of changes in membrane shape during movement.

(ii) Integral Proteins

—  Integral proteins can be removed from the membrane only after disrupting the lipid bilayer (by using detergents).

—  Not soluble in aqueous solutions and forms above 70 – 80 % of Membrane proteins.

—  Most integral proteins penetrate the membrane completely and are called Transmembrane proteins which have Channel proteins and Pores for the transport of nutrients.

Glycoproteins and Glycolipids

—  Many of the proteins and some of the lipids on the outer surface of the plasma membrane have carbohydrates attached to them.

—  Proteins attached to carbohydrates are called Glycoproteins.

—  Lipids attached to carbohydrates are called Glycolipids.

—  Both Glycoproteins and Glycolipids helps to protect the cell and lubricate the cells which are involved in cell-to-cell interactions.

Cholesterol

—  A few bacteria such as Mycoplasma sp. have cholesterol molecules in their cell membranes, as do most eukaryotes.

—  Mycoplasma lack cell walls so Cholesterol molecules add rigidity to their Cell membranes.

FUNCTIONS OF PLASMA MEMBRANE

a)     Mechanical boundary of cell and protects the bacterial cell from external sources.

b)     Site of ATP Synthesis by Electron Transport Chain (ETC) in Prokaryotes.

c)     Communication (Cell to cell interaction).

d)     Selective permeable barrier.

e)     Transport of substances (particularly nutrients).

f)      Nutrient and waste transport (entry and exit)

g)     Pigments and enzymes responsible for photosynthesis are present in Cell membrane.

h)    Exocytosis and Endocytosis (Phagocytosis – Cell eating and Pinocytosis – Cell drinking)