Note that a low temperatures, the bilayer is in a gel state and tightly packed. This also allows movement of other components of the membrane. The figure below is from Wolfe S.L., Molecular and Cellular Biology, Wadsworth Publishing Company, 1993. Another type of lipid in the membrane is cholesterol . The amount of cholesterol may vary with the type of membrane.
Plasma membranes have nearly one cholesterol per phospholipid molecule.
The following figure shows the steroid structure of cholesterol. The cholesterol molecule inserts itself in the membrane with the same orientation as the phospholipid molecules. The figures show phospholipid molecules with a cholesterol molecule inbetween. Note that the polar head of the cholesterol is aligned with the polar head of the phospholipids. Figure modified from Alberts et al.