The description of a cell membrane is called the fluid mosaic model. As you can see from the figure below, there are three main parts of a cell membrane:
Phospholipid bilayer - You’ll remember from the section on phospholipids that phospholid molecules have a hydrophilic head and two hydrophobic tails. In cell membranes, the phospholipids form two layers called a bilayer with the hydrophilic heads pointing toward the water found inside and outside of the cell.
Cholesterol - The cholesterol molecules stabilize the phosopholipid bilayer so it is not too much of liquid at body temperatures.
Movement of substances through a cell membrane - Substances that are fat-soluble pass through the cell membrane easily but water and water-soluble substances of ions either cannot pass through the cell membrane or pass through it slowly. The fact fat-soluble substance pass through easily and water-soluble substances pass through slowly or not at all will be an important fact to know when discussing membrane transport in the next chapter.
Lipid rafts (video) are groupings of membrane lipids that are often rich in cholesterol. These rafts perform a variety of functions.