According to widely accepted "fluid mosaic model" cell membranes...

According to this model, plasma membrane is formed of "protein icebergs in the sea of lipids." The phospholipid form a bilayer which is interrupted by the embedded protein molecules. The amount of proteins varies in different membranes. The phospholipid bilayer is a twodimensional fluid which allows the movement of proteins and phospholipid. The phospholipid molecules show two types of movements in this layer. (i) Transition Movement: Change in the position of molecules within the same layer. (ii) Flip-flop Movement : The molecules of the two layers of the bilayer can be interchanged. 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. Some of them run throughout the lipid bilayer called tunnel proteins or transmembrane proteins and form channels for the passage of water-and water-soluble substances individually or in a group.