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.
Chemical components of bio membranes are Proteins and 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;
- Providing the basic structure.(provides structural integrity) Very strong and stable even when the environment changes.
- Acts as a barrier for water and water soluble molecules.
Membrane lipids are heterogeneous and have a wide diversity.
Lipid molecules on the cell membrane are Amphipathic. They have;
- Water loving/hydrophilic/polar region (head)
- Water hating/hydrophobic/non-polar region (tail)
There are three main membrane lipid groups;
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.
Contains molecules called Sphingosine, an amino alcohol with a long hydrocarbon chain.
Sphingolipids are amphipathic and have a structure similar to Phosphatidylcholine.
Other sphingolipids contain sphingosine, fatty acid chain and sugar(glycolipids)
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 )