What is the plasma membrane mostly made of?

Plasma membranes are mainly composed of phospholipids, which consist of fatty acids and alcohol.

Each phospholipid molecule has a hydrophilic, or water-loving, head and two hydrophobic, or water-fearing, tails. The phospholipids in the plasma membrane are arranged in two layers forming a phospholipid bilayer, with the hydrophilic heads forming the exterior of the membrane and the hydrophobic tails on the interior of the membrane. 

In addition to phospholipids, other molecules that make up the plasma membrane include:

• Transmembrane proteins
• Protein channels
• Peripheral proteins
• Glycoproteins
• Glycolipids