Cholesterol contains the bulky steroid ring structure with a hydroxyl group at one end. Within a lipid bilayer, the polar hydroxyl group hydrogen-bonds to a carbonyl group in the polar head of a phospholipid. The nonpolar steroid section interacts with the fatty acid tails through Van der Waals interactions. The presence of cholesterol acts to regulate membrane fluidity. Cholesterol is present in the plasma membrane but not in organelle membranes.