Chemically the plasma membrane consists of proteins (20-70%), lipids (20-79%), carbohydrates (1-5%) and water (20%). Nucleic acids (DNA and RNA) are absent in the plasma membrane. The lipids present in the plasma membrane are phospholipids, glycolipids (sugar lipids) and sterols.
Each lipid molecule consists of a three carbon glycerol poles (head) which is hydrophilic (water-loving) in nature and two long tails of fatty acids which are hydrophobic (water-fearing) in nature.
The hydrophilic glycerol poles of lipid are located towards the outside of the lipid bilayer whereas, the hydrophobic fatty acid tails are repelled by water and face towards the inner side of the membrane.
The hydrophilic and hydrophobic forces in lipid molecules cause the membrane to become a bilayer.