Question

4.During the course of the development of our understanding of the structure of the plasma membrane, it was at one point hypothesized that the proteins in the membrane coated the lipid bilayer on both sides (the sandwich model). What two pieces of evidence suggested that the sandwich model was incorrect?​

Answer 1

According to Sandwitch model that was proposed by Davson & Danial, all the proteins of the plasma membrane are integral in nature. But the plasma membrane proteins are 1. Integral; 2. External and 3. Transmembrane in nature

The nonpolar protein portions would separate the polar portions of the phospholipids from water, causing the bilayer to dissolve. Meaning, the Davson-Danielli model is not only incorrect, but it is also impossible. Membranes are not identical. The differ in thickness and the ratio of proteins:lipids. Membranes have distinct inside and outside layers (defined by the membrane proteins which are present on the surface of the membrane). Other than predicted by the model, the membrane proteins do not have a very good solubility in water - in fact they are amphiphatic, meaning they have hydrophilic and hydrophobic regions. The hydrophobic side is anchored inside the membrane. When the membrane proteins would cover the lipid bilayer, their hydrophobic regions would be in contact with water, which destabilizes this construct. Even if they would be oriented towards the membrane, they would face towards the hydrophilic heads of the phospholipids causing the same effect. Additionally the proteins would also seperate the hydrophilic phospholipid heads from the water. So there is no real stable solution in embedding the membrane with proteins.