Plasma membrane is composed of proteins, lipids, water and carbohydrates. The ratio of lipids and proteins varies considerably in different types cells.

The major lipids are phospholipids that are arranged in a bilayer. In addition to phospholipids membrane also contains cholesterol

Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic (water-repellent) tails towards the inner part. This ensures that the non-polar tail of saturated hydrocarbons is protected from the aqueous environment.