The phospholipid bilayer is the structure which surrounds a cell. It consists of a molecule that is composed of a fatty acid chain at one end, and a glycerol-phosphate-nitrogen group at the other end. The phospholipids arrange themselves in two layers, with the fatty acid chains facing each other. This arrangement provides a secure buffer between the interior of the cell and the external environment. The bilayer allows certain types of molecules to pass into the cell and restricts others.

The phospholipid has a polar head and a non-polar tail. The polar head can interact with water molecules while the non-polar tail avoids water molecules. Water itself is polar. This dual nature of the phospholipid causes it to orient itself in such a way that the polar head interacts with water and the non-polar tail avoids water.

The external environment and the internal environment of the cell contains water molecules. Phospholipids orient themselves so the polar heads face the inside of the cell and the outer environment while the non-polar tail is comfortably in between. To accomplish this, two layers of phospholipids develop. One layer of phospholipids lines up with the polar head facing the external environment. The second layer is stacked beneath the first layer, but oriented upside down, with the polar head facing the cell. The non-polar tail of the phospholipids in the first layer face the non-polar tail of the phospholipids in the second layer. The result is the phospholipid bilayer.

The phospholipid bilayer is the major constituent of the cell membrane. It also contains various molecules that can embed in between the phospholipids, such as sphingolipids, cholesterol and membrane proteins. Sphingolipids clump together in raised sections of the phospholipid bilayer. They appear to hover over the rest of the cell membrane. Cholesterol helps stabilize the phospholipid bilayer by reducing fluidity of the phospholipids.

Membrane proteins help transport ions and molecules into and out of the cell. Membrane proteins either reside on one side of the membrane or penetrate completely, forming channels for incoming ions and molecules. Non-polar ions such as sodium, potassium and calcium require membrane proteins for transport. The process requires energy. Molecules that are non-polar can pass through the phospholipid bilayer without assistance. This includes oxygen and carbon dioxide. Some proteins extending from the phospholipid bilayer have a sugar chains attached to it, which help to identify the cell to neighboring cells.