- In 1902 it was thought that the membranes had only lipids (Overton).
- In 1926 Gorter and Grendell proposed that lipids are capable of forming a double layer.
- In 1935 Danielli and Davson proposed the lipid bilayer model that includes proteins adhering to both lipid-aqueous interfaces.
- Artificial model systems such as the liposomes supported the idea of Danielli and Devson.
- A droplet of lipid made soluble in an organic solvent can be spread over a small hole on a septum that divides two chambers containing water.
- This set up is useful to study biophysical properties of a bilayer such as permeability and electrical resistance.
- Channels for ions can be formed by adding certain proteins or polypeptides.
- Liposomes act as excellent carriers for different molecules such as chemotherapeutic compounds, insulin and antibodies.
Fluid mosaic model
- Fluid mosaic model proposed by S.J. Singer and G.L. Nicolson (1972) was finally acceptable to most biologists.
- This model recognizes that lipids and proteins are in a mosaic arrangement.
- It also recognizes that there is translational movement of lipids and proteins within the lipid bilayer.
- Non covalent interactions ensure a fluid like state for the membranes.
- Integral proteins are intercalated into the continuous lipid bilayer.
- Polar/hydrophilic regions of proteins protrude from the surface while the nonpolar/hydrophobic regions are embedded inside.
Unit membrane model
- Robertson in 1959 postulated the unit membrane model.
- This model stated that the central layer of plasma membranes is made up of hydrocarbon chains of lipids and the proteins constitute the dense surrounding layers on both sides when viewed through an electronmicroscope.
- Unit membrane model turned out to be an over simplification model as it can’t account for the number of protein molecules present across the membranes.
