The plasma membrane is the boundary between the fluids within and outside of a cell. It is composed primarily of phospholipids and proteins. The phospholipids are arranged in 2 layers with their hydrophobicfatty acid tails to the interior of the membrane and their hydrophilic polar heads on the inner and outer surfaces. Some of these molecules have carbohydrate chains attached to them forming glycolipids and glycoproteins. Cholesterol is found in animal plasma membranes. The membrane is a fluid and its components are slowly drifting past one another.