Lipids are a category of molecules united by their inability to mix well with water. They are composed of carbon, hydrogen, oxygen, and phosphorus (in phospholipids). Lipids tend to be hydrophobic, nonpolar, and made up mostly of hydrocarbon chains, though there are some variations on this, which we’ll explore below. The different varieties of lipids have different structures, and correspondingly diverse roles in organisms. For instance, lipids store energy, provide insulation, make up cell membranes, form water-repellent layers on leaves, and provide building blocks for hormones like testosterone. Types of lipids include fats, oils, saturated fats, unsaturated fats, trans fats, waxes, phospholipids, and steroids.