We often tend to think of fats as substances we should avoid at all costs--the ultimate enemies in our efforts to attain a healthier lifestyle. But despite their bad reputation, fats and other fat-like molecules called lipids play vital roles in the chemistry of life. Some of the most important lipids are three types of molecules called phospholipids, sterols and triglycerides.
Phospholipids, triglycerides and sterols are all members of a family of organic molecules called lipids. They share similarities but have very different functions; their key defining characteristic they all share in common is they are hydrophobic, meaning they mix poorly with water. Phospholipids, unlike the others, are actually amphipathic, meaning that one region of the phospholipid molecule is hydrophilic ("water-loving") and the other is hydrophobic ("water-fearing").
The functions of each molecule are closely related to its structure. Triglycerides, or fats, have three fatty acids linked to a molecule of glycerol; a fatty acid is a hydrocarbon chain with a central carbon "spine" bonded to hydrogens on either side. If all the carbons in the chain have only a single bond to their neighboring carbons, the molecule is a saturated fatty acid; if one or more of the carbons has a double bond with one of its neighbors, however, the fatty acid is unsaturated.
Like triglycerides, phospholipids are composed of fatty acid chains linked to a glycerol molecule. Unlike a triglyceride, however, phospholipids have only two fatty acids; moreover, they also have a phosphate group and a polar head group attached to the glycerol. These last two groups make one end of the phospholipid attracted to water or hydrophilic, while the fatty acid chains are hydrophobic. You can think of a phospholipid as being shaped a little like a small head with two tails.
Sterols are members of a class of molecules called steroids; they have four fused rings of carbon atoms and are hydrophobic like triglycerides.
Triglycerides are a potent store of energy for animals. When broken down, they release twice as much energy as a comparable weight of sugars or carbohydrates--one of the reasons it's so difficult to burn off extra weight. Phospholipids, by contrast, form the membranes in your cells. Since one end of each phospholipid is hydrophilic and the other is hydrophobic, the phospholipids form a bilayer that's two molecules thick; the hydrophilic heads point outwards towards the water and the hydrophobic tails point inwards towards the center of the bilayer. Sterols like cholesterol help keep cell membranes stable and fluid at different temperatures.
The membrane or lipid bilayer is a crucial component of your cells; it keeps the contents of the cell enclosed and regulates traffic by determining which substances can cross. Sterols are also essential in limited amounts. Cholesterol not only ensures the stability and flexibility of human cell membranes, but is also used to synthesize steroid hormones like estrogen and testosterone. Other organisms use different kinds of sterols to perform similar functions for their cell membranes; fungi, for example, use a sterol molecule called ergosterol.
Just because a substance may be good for you in moderation doesn't mean it's healthy in excess--and that's especially true of sterols and triglycerides. Since triglycerides (fats) are high in energy, consuming foods that are high in fat can increase your calorie intake; moreover, a diet rich in saturated fats is thought to contribute to your chances of developing a condition called atherosclerosis, where deposits called plaque form on the inside of blood vessels and increase the likelihood of a heart attack.