What Is a Lipid?A lipid is a fat-soluble molecule. To put it another way, lipids are insoluble in water but soluble in at least one organic solvent. The other major classes of organic compounds (nucleic acids, proteins, and carbohydrates) are much more soluble in water than in an organic solvent. Lipids do not share a common molecule structure.
Examples of Common LipidsThere are many different types of lipids. Examples of common lipids include butter, vegetable oil, cholesterol and other steroids, waxes, phospholipids, and fat-soluble vitamins. The common characteristic of all of these compounds is that they are essentially insoluble in water yet soluble in one or more organic solvents.
What Are the Functions of Lipids?Lipids are used by organisms for energy storage, as a signalling molecule (e.g., steroid hormones), and as a structural component of cell membranes.
Lipid StructureAlthough there is no single common structure for lipids, the most commonly occurring class of lipids are triglycerides, which are fats and oils. Trigylcerides have a glycerol backbone bonded to three fatty acids. If the three fatty acids are identical then the triglyceride is termed a simple triglyceride. Otherwise, the triglyceride is called a mixed triglyceride.
The second most abundant class of lipids are the phospholipids, which are found in animal and plant cell membranes. Phospholipids also contain glycerol and fatty acids, plus the contain phosphoric acid and a low-molecular-weight alcohol. Common phospholipids include lecithins and cephalins.