lipid
/ LIP-id /
Olive oil, butter, the fat on a steak, the waxy coating on an apple, even some of your hormones — all of these are lipids. The family is defined not by a shared shape but by a shared habit: lipids do not mix with water. If a biological molecule is greasy and water-fearing, it is probably a lipid.
Lipids are mostly built from long chains of carbon and hydrogen, and because those chains are nonpolar, the whole molecule is hydrophobic. Unlike the other three macromolecule families, lipids are not true polymers — they are not long chains of one repeating monomer — which is why they are the odd member of the four. The family includes fats and oils (triglycerides) for energy storage, phospholipids that build cell membranes, and steroids such as cholesterol and many hormones, which look quite different but share that water-fearing nature.
Lipids do jobs no other family does as well. They store energy at more than twice the density of carbohydrate, which is why your body banks long-term reserves as fat. Their refusal to mix with water is exactly what lets them form the thin oily barrier — the membrane — that separates the inside of every cell from the outside world. Without lipids, there would be no cells to speak of.
Salad dressing separates into an oil layer and a vinegar layer because the lipids in the oil are hydrophobic and refuse to stay mixed with the watery vinegar.
Oil and vinegar separating is the defining trait of lipids made visible.
Lipid is not a synonym for fat: fat is just one kind of lipid. Cholesterol and steroid hormones are lipids too, even though they contain no fatty tails at all.