The lipid that's most important in biological membranes is cholesterol. Cholesterol both builds and protects the membranes of animal cells. The hydroxyl group and hydrocarbon chain that are found in a molecule of cholesterol protects and regulates the fluidity of the cell.
Cholesterol is also necessary in building the myelin sheaths around nerve cells. Without these myelin sheaths, the nerve cells can't function properly, because nerve impulses can't travel efficiently. Diseases such as multiple sclerosis result when the myelin sheaths around nerve cells break down.
Cholesterol is derived from a person's diet, but the body also produces it. Because people get cholesterol from animal products, strict vegetarians might be expected to have low levels of cholesterol. However, the body compensates for the cholesterol that's lacking in the diet by making more of its own.
Besides supporting cell membranes, cholesterol is important for the manufacture of hormones, such as estrogen. It also helps to make bile, which is used to digest other lipids. About half of the cholesterol found in human bile is recycled by the liver, while the rest is excreted. Cholesterol is also important in the production of vitamin D, which is one of the few vitamins that the human body manufactures.