What are cellular membranes made of?

The principal components that make up cellular membranes are phospholipids, proteins, cholesterol, and carbohydrate groups. The proportions of each component vary among different types of cells. 

Phospholipids are a type of lipid composed of glycerol, 2 fatty acid tails and a phosphate-linked head group. They make up the basic fabric of cellular membranes. Biological membranes are typically made up of a phospholipid bilayer, in which phospholipids are arranged in two layers with their tails pointing inward. 

Proteins are the second major component of cellular membranes. There are two main types of membrane proteins. Integral proteins are embedded in the phospholipid bilayer and may traverse the membrane completely or extend only partially into the membrane. Peripheral proteins are not embedded in the phospholipid bilayer. Instead they may be loosely attached to the membrane’s inner or outer surface. 

Cholesterol is another type of lipid that’s found alongside phospholipids in the membrane core. It is composed of 4 fused carbon rings. 

Carbohydrate groups occur only on the outer surface of the plasma membrane where they may be bound to lipids, forming glycolipids or they may be bound to proteins, forming glycoproteins. These carbohydrate chains can be either straight or branched and may consist of about 2-60 monosaccharide units.