The plasma membrane is composed of a phospholipid bilayer, which is two layers of phospholipids back-to-back. Phospholipids are lipids with a phosphate group attached to them. The phospholipids have one head and two tails. The head is polar and hydrophilic, or water-loving. The tails are nonpolar and hydrophobic, or water-fearing.Since water is the key component of living organisms, both inside and outside of the cell, this causes the phospholipids to line up in two layers with the heads pointing outward and the tails hidden in the middle. It basically looks like a phospholipid sandwich with the heads forming the bread and the tails forming the meat in the middle.