Lipids are a diverse group of hydrophobic or amphipathic molecules that serve as energy stores, structural components of membranes, and signaling molecules. They are broadly defined by their solubility in organic solvents rather than by a common structural feature.
Fatty Acids
Fatty acids are the simplest lipids and the building blocks of many complex lipids. They consist of a carboxylic acid head group attached to a long hydrocarbon chain. Fatty acids are classified by chain length, degree of unsaturation, and the position and configuration of double bonds. Their metabolism is covered in detail in fatty acid oxidation and synthesis.
Saturated fatty acids have no double bonds and pack tightly together, giving them higher melting points. Palmitic acid (16:0) and stearic acid (18:0) are common saturated fatty acids. Unsaturated fatty acids contain one or more double bonds, usually in the cis configuration, which introduces kinks that prevent tight packing and lower the melting point. Oleic acid (18:1, omega-9) is a monounsaturated fatty acid, while linoleic acid (18:2, omega-6) and alpha-linolenic acid (18:3, omega-3) are essential polyunsaturated fatty acids that must be obtained from the diet.
Triacylglycerols
Triacylglycerols, also called triglycerides, consist of three fatty acids esterified to a glycerol backbone. They are the primary form of energy storage in animals, stored in adipose tissue. Triacylglycerols are highly reduced and yield more than twice the energy per gram compared to carbohydrates or proteins.
Phospholipids
Phospholipids are amphipathic lipids containing a glycerol backbone esterified to two fatty acids and a phosphate group linked to a polar head group. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol are the major glycerophospholipids. Their amphipathic nature allows them to form bilayers, making them the fundamental components of biological membranes.
Sphingolipids
Sphingolipids are built on a sphingosine backbone instead of glycerol. Ceramide consists of sphingosine linked to a fatty acid via an amide bond. Sphingomyelins have a phosphocholine or phosphoethanolamine head group and are important components of myelin sheaths. Glycosphingolipids such as cerebrosides and gangliosides contain sugar head groups and are abundant in the nervous system, where they function in cell recognition and signaling.
Sterols
Sterols are lipids with a characteristic four-ring steroid nucleus. Cholesterol is the major sterol in animals, where it modulates membrane fluidity and serves as a precursor for bile acids, steroid hormones, and vitamin D. Ergosterol serves an analogous role in fungi, and phytosterols such as sitosterol and stigmasterol are found in plants.
Waxes
Waxes are esters of long-chain fatty acids with long-chain alcohols. They form waterproof coatings on plant leaves, fruits, and animal fur or feathers. Beeswax, lanolin, and the earwax of mammals are familiar biological waxes.
Eicosanoids
Eicosanoids are signaling molecules derived from arachidonic acid, a 20-carbon polyunsaturated fatty acid. They include prostaglandins, thromboxanes, leukotrienes, and lipoxins, and mediate inflammation, fever, pain, blood clotting, and smooth muscle contraction.
