Why was the unit membrane model rejected?

The Unit Membrane model, proposed by Hugh Davson and James Danielli in 1935 describes the position of proteins within the cell membrane bilayer. According to this model, the cell membrane is made up of 3 layers – a central phospholipid bilayer flanked by two layers of proteins.

Dawson and Danielli arrived at this conclusion because of the characteristic trilaminar appearance of the membrane when viewed under a transmission electron microscope. Based on this appearance, they proposed the Unit Membrane model wherein the two dark outer layers were composed of protein and the lighter inner region was composed of lipids.

The Unit Membrane model was later rejected because it wrongly assumed that:

• All membranes were of uniform thickness
• All membranes would have symmetrical external and internal surfaces
• All membranes would have a constant protein-lipid ratio

Moreover, the Unit Membrane model did not take into consideration the permeability of some substances and the consequent need for hydrophilic pores.

Lastly, the temperatures at which membranes solidified and the temperatures expected under the proposed model did not correlate.

In light of these limitations, scientists today use the Fluid-Mosaic model. Proposed by Seymour Singer and Garth Nicolson in 1972, this model states that proteins are embedded within the lipid bilayer rather than existing as separate layers as proposed by the Membrane Unit model.