What type of molecule makes up the bi-layer of a cell membrane?

This membrane is called the fluid mosaic model as it is a mixture of phospholipids, cholesterol, proteins and carbohydrates. Most of the membrane is composed of phospholipid molecules. These allow the membrane to be rather fluid.
Embedded in this membrane are proteins which give some structure to the membrane. The 3rd components are proteins or glycolipids. Cholesterol is also found in the membrane. It prevents lower temperatures from inhibiting the fluidity of the membrane and prevents higher temperatures from increasing fluidity.