S. J Singer and G. L Nicholson proposed fluid mosaic model of cell membranes. According to this model, the plasma membrane is formed of "protein icebergs in the sea of lipids". The membrane is not solid but is quasi-fluid. The lipids of the membrane form a bilayer which is interrupted by the embedded protein molecules.
Related Theory
Depending on the degree of association and the ability of separation, the membrane proteins are classified into two types, which are: internal proteins also known as intrinsic or integral proteins and the external proteins also known as extrinsic or peripheral proteins. The integral or intrinsic proteins account for $70 \%$ of the total membrane proteins and pass into the lipid bilayer to different depths. They cannot be easily separated from the membrane without disrupting it.