What are the major components of the cell membrane?

The major components of the cell membrane are phospholipids, proteins, carbohydrates, and other lipids such as cholesterol. Together these components make up the fluid mosaic model, which is used to describe the structure of the cell membrane.

Phospholipids

Phospholipids are the most important component of the cell membrane. A phospholipid is a specific type of lipid that’s composed of glycerol, a phosphate-linked head and two fatty acid tails. 

The heads, which form the hydrophilic portion of a phospholipid, face outward, contacting the aqueous fluid both inside and outside the cell. The fatty acid tails, which form the hydrophobic portion of a phospholipid, can interact easily with other nonpolar molecules but interact weakly with water. 

Phospholipids are amphipathic in nature and are arranged in a bilayer that makes up the basic fabric of the cell membrane. 

Proteins

Proteins make up the second major component of cell membranes. They may span the membrane entirely, extend partially into the cell membrane, or be loosely attached to its external or internal surface. 

There are two main categories of membrane proteins: integral and peripheral. Integrated proteins are firmly embedded into the cell membrane. Integrated proteins that extend all the way across the membrane are called transmembrane proteins. Peripheral proteins are loosely attached to the outside or inside surfaces of cell membranes. 

Carbohydrates

Carbohydrates are found only on the outer surface of cells. They may be attached to proteins, forming glycoproteins or to lipids, forming glycolipids. Carbohydrates present on cells form a unique cellular marker that acts as a molecular ID, facilitating intercellular recognition. 

Carbohydrates are a crucial component in the immune system. By acting as unique cellular markers, carbohydrates help immune system cells differentiate between self-cells and foreign cells so they only attack foreign cells without damaging the body’s own cells. 

Cholesterol

Cholesterol molecules are interspersed among the phospholipids in the membrane. Cholesterol helps regulate the fluidity of the membrane, making it more stable and less permeable to small water-soluble molecules.