All cells are surrounded by a lipid membrane but not all membranes are made up of just lipid molecules. The bilayer is interrupted by the inclusion of membrane proteins. These proteins have hydrophobic surfaces which allow them to associate in the hydrophobic core of the membrane, also excluding them from the aqueous phases(cytoplasm). The cytoplasmic face of the membrane is meshed with cytoskeletal proteins which form chains which give the cell its shape, and acting as a motor to move cell contents through the cytoplasm.
These proteins have many functions including;