Structure of biological membrane and transport across membrane

STRUCTURE OF BIOLOGICAL MEMBRANE AND TRANSPORT ACROSS MEMBRANE 

  • Biological membranes are thin sheet like structure composed of lipids and proteins. The ratio of proteins to lipids varies between different types of membranes. Generally 40-50% proteins are present. Myelin sheath has 80% lipids and 20% proteins. The inner membrane of mitochondria has 80% proteins and 20% lipids.
  • Some membrane contains carbohydrates up to 10%. It is found on the external surface of the plasma membrane. They are attached to either proteins as glycoproteins and lipids as glycolipids.
  • Three main groups of lipids are found in the membrane viz 

Functions of membranes

  • It is the boundary between the cell interior and exterior and maintains the shape of the cell.
  • Transport of substances in and out of the cell. The lipid bilayer prevents the passage of hydrophilic ions and molecules.
  • Cell cell interaction.
  • Signal transduction.

Arrangements of proteins and lipids

  • Membrane is in the form of phospholipid bilayer, which is composed of two rows of phophoglycerides that have hydrophobic fatty acyl groups oriented towards inward and the hydrophilic phosphate head group outward to the extracellular and cytoplasmic surface.Each lipid bilayer has different phospholipid composition.
  • The arrangements of lipids and proteins in membranes are described by fluid mosaic model. According to this model the membrane is a dynamic structure (they are in constant motion) in which both proteins and lipids rapidly diffuse.
  • Membrane proteins can be divided into two types. 1. peripheral and 2. integral.
  • Peripheral proteins are bound loosely with the surface of either side of the membrane. They can be removed by mild treatment with solutions of high ionic strength.
  • Integral proteins are tightly bound. The proteins may cross the membrane (called transmembrane proteins) or the proteins may be firmly embedded in the bilayer. These proteins can be removed by the use of powerful detergent. These membranes contains high portions of hydrophobic amino acids, which can interact with hydrophobic regions of lipid bilayer.
  • Some proteins may be covalently attached to lipid prosthetic group.
  • Membranes are asymmetric due to the irregular distribution of proteins

Functions of membrane proteins

  • Localization of enzyme activity
  • Energy transduction 
  • Facilitated transport 
  • Receiving of extracellular signal

Fluidity of membrane

  • The degree of motion of the hydrocarbon chain within the lipid bilayer is called fluidity. Several factors influence the fluidity in membranes.
  • Long chain saturated fatty acids decrease fluidity and permeability of membrane. They have straight tail, so that they are packed tightly.
  • Unsaturated fatty acids present in the membrane phospholipids increase the fluidity and permeability of membrane. Greater the number of double bonds greater will be the fluidity and permeability. Unsaturated fatty acids exist in the cis form in the membrane, which makes kinked (twisted) tails. Membrane made of more unsaturated fatty acids has more kinks; hence, they are not highly packed and showing fluid nature.
  • Presence of cholesterol prevents the movements of fatty acids and reduces fluidity. It prevents close packing of the hydrocarbon chains and thereby lowers the melting point.
  • Fluidity increase with increase in temperature.
  • Movements of proteins and lipids within the membrane are lateral. Neither protein nor lipids flip flop from one side to other ( passage of lipids from one leaflet of the bilayer to the other).

 

Last modified: Sunday, 18 December 2011, 4:16 PM