Tuesday, January 18, 2011

Biological Membranes

Since a cell is a limited space it needs an outer border. Biological membrane or bio membrane that surrounds the boundary of a cell is called cell membrane or plasma membrane. It creates a very ordered environment within the enclosed area to carry out functions.

Membrane Structure


When observed under Electron Microscope(EM) stained with Osmiun Tetroxide (OsO4) , a lipid stain, it appears as a rail road track, which is called the "Tri laminar appearance"
Since all cell membranes give this structure under EM , it gives evidence for universality of the structure of cell membranes.Any kind of bio membrane can be fused together.

General Structure 


Chemical components of bio membranes are Proteins and Lipids.

Lipids

Lipid molecules are arrayed as a continuous double layer which is maintained by hydrophobic and Van der Waals interactions.Functional properties of lipid bilayer are;

  1. Providing the basic structure.(provides structural integrity) Very strong and stable even when the environment changes.
  2. Acts as a barrier for water and water soluble molecules.
Lipids are organic molecules that contain C H and O. They are poorly soluble (virtually insoluble) in water but soluble in  non-polar organic solvents. They are water repelling/hydrophobic.Lipids are a group that is difficult to classify. The lipids in bio membranes are called membrane lipids.

Membrane lipids
Membrane lipids are heterogeneous and have a wide diversity. 

Lipid molecules on the cell membrane are Amphipathic. They have;

  1. Water loving/hydrophilic/polar region (head)
  2. Water hating/hydrophobic/non-polar region (tail) 
in the same molecule. This is due to the chemical composition/structure of lipid molecules in bio membranes. The head has a phosphate (PO4 3 -  group) which makes it polar and the tail has hydrocarbons which makes it non polar.

There are three main membrane lipid groups;

  1. Phospholipids/Phosphoglycerides
  2. Sphingolipids
  3. Cholesterol

The amount and type of lipids present in a membrane varies from membrane to membrane among cells and within a cell.

The most abundant lipid type is phospholipid.
The most abundant phospholipid is Phosphatidylcholine .

Phosphatidylcholine consists of 2 long fatty acid chains and two C=O groups linked to the two OH groups of the glycerol backbone and a phosphate group linked to the 3rd OH group. To the Phosphate group a choline molecule is linked.

Sphingolipids
Contains molecules called Sphingosine, an amino alcohol with a long hydrocarbon chain.
eg: Sphingomyelin
Sphingolipids are amphipathic and have a structure similar to Phosphatidylcholine.
Other  sphingolipids contain sphingosine, fatty acid chain and sugar(glycolipids)

Cholesterol
Lipid bilayer of many cell membranes contain cholesterol and other steroids.
Of steroids cholesterol is major molecule (in all eukaryotes ;not prokaryotes)
Cholesterol molecules are arranged in between phospholipid molecules.

Together all these lipids creates a hydrophobic region in the cell membrane which acts as a barrier to water.
The presence of different types of  lipids in different amounts in the bio membranes give them unique structural properties.

Glycolipids are when there are sugar group/groups linked to the lipid.When more than one sugar molecule is linked they are called oligosaccharides. These sugar molecules link to sphingosine most of the time. Therefore   Glycolipids are Sphingolipids most of the time. Since sugars are hydrophilic, have OH groups they are never found inside the lipid bilayer because it is very unstable that way.

The best examples for carbohydrates linked to membrane lipids are blood group antegens (A , B ,AB and O )

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