A lipid is a hydrophobic (does not interact with water molecules) macromolecule that may provide storage, structure, or nutrients in organisms. Lipids can be categorized into three different types, each with its own important functions.Triglycerides are molecules better known as fats. They are called triglycerides because they consist of three fatty acids connected to a single glycerol molecule.
Fats contain the greatest energy per gram of the three macronutrient molecules (carbohydrates, proteins, and fats) and, therefore, are the most efficient for energy storage. When fats are metabolized, the bonds between the fatty acids and the glycerol are hydrolyzed (broken apart by a reaction with water). The fatty acids that are released move to the liver where they are converted to glucose.
By a reverse process, excess glucose in the body is converted to several two-carbon molecules, which are then joined together to form a fatty acid. These molecules are joined to a glycerol molecule via a condensation reaction, which produces a triglyceride that can be stored in the body's fatty tissue.
A phospholipid is a lipid that usually consists of two fatty acid tails covalently linked to a common phosphate group. The phosphate group makes one end of the molecule polar, while the fatty acid tails make the other part of the molecule nonpolar. The polar end of the molecule is hydrophilic, and the fatty acid tails are hydrophobic. Phospholipids make up the cell membranes of animals, plants, fungi, and some bacteria.A steroid is a lipid that consists of a core with four fused ring structures (three 6- and one 5-membered ring) that serves as a precursor to hormones, cholesterol, or other molecules. Steroids have a very different structure than triglycerides and phospholipids. Some steroids have hydrocarbon tails, but others have only alcohol groups attached to one or more of the rings. All steroids are insoluble in water. Steroids serve a variety of functions, being the precursors of steroid hormones and cholesterol.