Butter, olive oil, cholesterol, and the membranes around your cells. These are lipids, a diverse group of organic compounds that share one property: they do not dissolve in water. They dissolve in nonpolar solvents like ether and chloroform. The word comes from the Greek lipos, meaning fat. Lipids include fats, oils, waxes, phospholipids, and steroids.

Fats and oils are triglycerides, made of glycerol and three fatty acid chains. Saturated fats have no double bonds and are solid at room temperature, like butter. Unsaturated fats have double bonds and are liquid, like olive oil. Trans fats are unsaturated fats that have been partially hydrogenated, which straightens the chain and makes them solid. They are linked to heart disease. Phospholipids have a hydrophilic head and hydrophobic tails, which makes them ideal for forming cell membranes. Steroids have a four-ring structure and include cholesterol, testosterone, and estrogen.

Lipids serve many functions. They store energy, providing more than twice the calories per gram of carbohydrates or proteins. They insulate the body and protect organs. They form cell membranes. They carry fat-soluble vitamins. They serve as signaling molecules. They are essential for life, but the type and amount matter.