Lipids. Lipids are a structurally heterogeneous group of compounds that are soluble in nonpolar organic solvents and insoluble in water. Three major types of lipids are (1) nonpolar fatty-acid-containing lipids, (2) polar fatty-acid-containing lipids, and (3) non-fatty-acid-containing lipids.

Fatty acids. Fatty acids are long-chain unbranched monocarboxylic acids. The carbon chain may be saturated, monounsaturated, or polyunsaturated. Length of carbon chain, degree of unsaturation, and location of unsaturation influence the properties of fatty acids.

Triacylglycerols. Triacylglycerols are triesters formed by esterification of three fatty acid molecules to glycerol. Fats are triacylglycerols that are solids at room temperature, and oils are triacylglycerols that are liquids at room temperature. Chemical reactions of triacylglycerols include hydrolysis, saponification, hydrogenation, and oxidation. Triacylglycerols represent stored forms of energy for living systems.

Phosphoacylglycerols. Phosphoacylglycerols are triesters of glycerol in which two -OH groups are esterified with fatty acids and one -OH group is esterified with phosphoric acid, which in turn is esterified to an alcohol. Phosphoacylglycerols have a "head and two tails" structure and are major components of cell membranes. Lecithins and cephalins are types of phosphoacylglycerols.

Sphingolipids. Sphingolipids have structures based on the long-chain amino dialcohol sphingosine rather than on glycerol. A fatty acid is bonded to the sphingosine via an amide linkage, and an additional group is bonded to the terminal hydroxyl group of sphingosine through an ester linkage. Sphingolipids have a "head and two tails" structure and are components of cell membranes. Sphingolmyelins and cerebrosides are types of sphingolipids.

Steroids. Steroids are non-fatty-acid-containing lipids that have structures involving a fused-ring system containing three 6?membered rings and one 5-membered ring. Cholesterol is the most abundant steroid in the human body. Bile salts and steroid hormones are cholesterol derivatives.

Lipid bilayer. A lipid bilayer is the fundamental structural feature of a cell membrane. It is a two-layer-thick structure of lipid molecules (phosphoacylglycerols and sphingolipids) aligned so that the nonpolar tails of the lipids form the interior of the structure and the polar heads form the outside surfaces. Cholesterol, proteins, and carbohydrates can also be present in cell membranes.