Lipid

Lipids are a diverse set of organic molecules that encompass fats, waxes, sterols, fat-soluble vitamins (A, D, E, and K), monoglycerides, diglycerides, phospholipids, and other related compounds. They serve several key roles in living organisms: storing energy, transmitting signals, and forming structural parts of cell membranes. Beyond biology, lipids are used in cosmetics, food manufacturing, and nanotechnology.

Broadly speaking, lipids are small molecules that are either hydrophobic (water-repelling) or amphiphilic (having both water-loving and water-repelling parts). This amphiphilic property allows some lipids to assemble into structures like vesicles, liposomes (both multilamellar and unilamellar), or cell membranes when placed in water. Biologically, lipids are built from two main types of subunits: ketoacyl groups and isoprene groups. Based on this, they fall into eight categories: fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, and polyketides (all derived from ketoacyl subunits), plus sterol lipids and prenol lipids (derived from isoprene subunits).

Though the word "lipid" is sometimes used interchangeably with "fats," fats are actually a specific subgroup called triglycerides. Lipids also include fatty acids and their derivatives—such as tri-, di-, and monoglycerides, and phospholipids—as well as sterol-containing compounds like cholesterol. Humans and other mammals can break down and synthesize many lipids through various metabolic pathways, but certain essential lipids cannot be made internally and must come from the diet.

**History** In 1815, Henri Braconnot divided lipids into two groups: solid greases (suifs) and fluid oils (huiles). Michel Eugène Chevreul refined this in 1823 with categories including oils, greases, tallow, waxes, resins, balsams, and volatile (essential) oils. In 1827, William Prout identified fat as a key nutrient for humans and animals, alongside protein and carbohydrates. The first synthetic triglyceride, tributyrin, was made by Théophile-Jules Pelouze in 1844 by treating butyric acid with glycerin and concentrated sulfuric acid. Later, Marcellin Berthelot synthesized tristearin and tripalmitin using fatty acids, glycerin, and gaseous hydrogen chloride at high heat.

For about a century, chemists considered fats only simple lipids made of fatty acids and glycerol (glycerides), but new forms emerged. In 1847, Theodore Gobley discovered phospholipids—which he called "lecithins"—in mammalian brain and hen eggs. Thudichum later found phospholipids (cephalin), glycolipids (cerebroside), and sphingolipids (sphingomyelin) in human brain. Terms like lipoid, lipin, lipide, and lipid have been used inconsistently. In 1912, Rosenbloom and Gies suggested replacing "lipoid" with "lipin." In 1920, Bloor proposed a classification: simple lipoids (greases and waxes), compound lipoids (phospholipoids and glycolipoids), and derived lipoids (fatty acids, alcohols, sterols). The word "lipide," from Greek *lipos* (fat), was introduced in 1923 by French pharmacologist Gabriel Bertrand, covering both traditional fats and complex "lipoids." It was officially approved by the international commission of the Société de Chimie Biologique on July 3, 1923, and later anglicized to "lipid" due to its pronunciation. In 1947, T. P. Hilditch defined simple lipids as greases and waxes (including true waxes, sterols, and alcohols).

**Categories** The Lipid MAPS consortium classifies lipids into eight categories. Fatty acyls include fatty acids and their derivatives. These are made by elongating an acetyl-CoA primer with malonyl-CoA or methylmalonyl-CoA groups. Each fatty acid has a hydrocarbon chain ending in a carboxylic acid group, giving it a polar, water-attracting head and a nonpolar, water-repelling tail. This structure is a fundamental building block for more complex lipids. The carbon chain, usually 4 to 24 carbons long, can be saturated or unsaturated and may carry oxygen, halogen, nitrogen, or sulfur groups. Double bonds can be *cis* or *trans*, affecting the molecule's shape. *Cis* bonds cause the chain to bend, and more double bonds increase bending. For example, linolenic acid (an 18-carbon fatty acid with three double bonds) is abundant in plant thylakoid membranes, keeping them fluid even in cold temperatures and producing strong signals in 13-C NMR spectra of chloroplasts. This flexibility is crucial for cell membrane structure and function. Most natural fatty acids are *cis*, though *trans* forms occur in some natural and partially hydrogenated fats and oils. Biologically important fatty acids include eicosanoids (like prostaglandins, leukotrienes, and thromboxanes), which are mainly derived from arachidonic acid and eicosapentaenoic acid.