The plasma membrane (cell membrane) is made up of a (n)

The plasma membrane is composed of a phospholipid bilayer, which is two layers of phospholipids back-to-back

The plasma membrane is made of a phospholipid bilayer consisting of two layers of phospholipids with hydrophilic phosphate heads facing the exterior and interior of the cells with a hydrophobic core in the middle. The phospholipid bilayer allows for fluidity of the cell membrane and controls entry of substances into the cell.