What are membranes composed of?

Cell membranes are mainly composed of lipids (phospholipids and cholesterol), carbohydrates (glycoproteins and glycolipids) and proteins (integral and peripheral).

cellular membranes — including plasma membranes and internal membranes — are made of glycerophospholipids, molecules composed of glycerol, a phosphate group, and two fatty acid chains. 

Cell Membranes are composed of: Phospholipids, Cholesterol, Glycolipids, Proteins & Glycoproteins