Several other lipids are also found in your body. Some are vital components of your brain. An important group includes phospholipids, which resemble triglycerides in structure – the main difference is that one of the fatty acid groups is replaced by one that contains phosphorus. This alters the properties of the molecule because half of its structure is soluble in fat whereas the other dissolves in water. These dual solubility properties allow phospholipids to act as detergents and, together with special proteins, phospholipids are able to interact with cholesterol and triglycerides to form packages of lipids that are stable in water.
Cell membranes have phospholipids and glycolipids, with a hydrophilic (water-loving) head group and hydrophobic (water-hating) fatty acid tails that are 14 to 24 carbon atoms long. Cholesterol is also a very common lipid in the body, and has 27 carbon atoms linked together in rings, rather than having long chain fatty acids. Except for a hydrophilic alcohol group on cholesterol, the entire molecule is hydrophobic, and most of the cholesterol molecule is in the center of the membrane. Cholesterol is modified in various tissues to form hormones and fat-soluble vitamins.