Sterols are steroid alcohols and include cholesterol and vitamin D sterols.They are complex alcohols based on a carbon four-ring structure with a hydroxyl group at the polar end. The rest of the molecule is hydrophobic.Cholesterol is manufactured in the liver and intestines and is also taken in the diet.Cholesterol is a component of cell membranes. It regulates the fluidity of the cell membrane, keeping membranes fluid at low temperatures and maintaining the stability of the membrane.The non-polar part of the molecule interacts with the fatty acid tails so is oriented so that the hydroxyl group is positioned at the surface of the bilayer and the rest of the molecule is in the hydrophobic layer.Cholesterol is used in the biosynthesis of vitamin D and steroid hormones such as oestrogen and progesterone.