Answer:
Phospholipids
Step-by-step explanation:
The phospholipids are located in the plasma membrane and are main part of the membrane. The hydrophobiticity of membrane is only due to presence of phospholipids. The hydrophobic portion in the membrane allows delivery of drugs because most of the drugs are non-polar and can easily cross the cell membrane.
Thus, phospholipids forms a double layer as the primary structure of cell membranes.