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Lipids are insoluble in water and can be divided into three categories: triglycerides, phospholipids, and steroids.

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.


A triglyceride consists of three fatty acids and one glycerol. The fatty acids may be saturated (no double bonds) or unsaturated (at least one double bond). This triglyceride molecule will be metabolized into a molecule of glycerol and three fatty acids. Two of the fatty acids are saturated and one is not.
A fat may contain three of the same triglyceride tails, or the tails may differ. Saturated fatty acids are those that have no double bonds between any of the carbon atoms. Unsaturated fatty acids are those that have at least one double bond between carbon atoms. Unsaturated versions have less efficient packing (more disorder) and weaker London dispersion forces. Saturated fatty acids are more linear and uniform and, therefore, have stronger London dispersion forces and higher melting points. Many fatty acids in animal products are saturated, which makes them solid at room temperature. Fatty acids in nuts and olives, on the other hand, are largely unsaturated, making them less uniform, with weaker attraction for one another, giving them a much lower melting point.

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.

Cholesterol in the Cell Membrane

Cholesterol is often thought of negatively in association with heart disease, but it is a necessary nutrient for cell function and overall health. The cell membrane is composed of a bilayer of phospholipids with their hydrophobic tails facing one another. Cholesterol's nonpolar end is located next to the lipid tails while its polar end is near the polar phosphate. This arrangement lends strength and fluidity to the cell membrane.
Cholesterol is an important component in the cell membrane, where it helps to stabilize the structure and control the flow of water in and out of the cell. Cholesterol is also the precursor to steroid hormones, including most of the sex hormones, as well as vitamin D and bile salts. Many doctors think that excess cholesterol circulating in the bloodstream can lay the foundation for plaque deposits in blood vessels, leading to cardiovascular disease.