The answer is the cell membrane Because the cell membrane is made up of two layer of phospholipids and the proteins molecules are also attached with them, through elctro-statical charge.
The protein molecules can be found in between the lipids molecules of the same layer or at the peripheri of the membrane.
Phospholipid bilayer is not an any kind of part of a cell, but it is an content of the part of the cell eg: Cell membrane etc....
Hope I helped.