Lipid

Lipids are a broad and diverse group of organic compounds.

They include familiar substances like fats, waxes, and oils. They also encompass sterols, fat-soluble vitamins like A, D, E, and K, and phospholipids. Lipids serve several vital biological functions. They act as long-term energy stores for organisms. They also function as signaling molecules to communicate within the body. Furthermore, they serve as the primary structural components of cell membranes. Lipids are broadly defined by their relationship with water. Many are hydrophobic, meaning they do not dissolve in water. Others are amphiphilic, possessing both water-loving and water-fearing parts. This amphiphilic nature allows them to form complex structures like vesicles or membranes in aqueous environments.

Biological lipids are built from two distinct types of biochemical subunits. These building blocks are known as ketoacyl groups and isoprene groups. Using these subunits, scientists divide lipids into eight main categories. The first six categories are derived from ketoacyl subunits. These include fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, and polyketides. The remaining two categories come from isoprene subunits. These are sterol lipids and prenol lipids. Fatty acyls are a fundamental category. They consist of a hydrocarbon chain that ends with a carboxylic acid group. This structure creates a polar, hydrophilic end and a nonpolar, hydrophobic end. The carbon chains are typically between four and 24 carbons long. These chains can be saturated or unsaturated. If a chain has a double bond, it may exhibit cis or trans isomerism. Cis-double bonds cause the chain to bend. This bending is crucial for maintaining membrane fluidity in various environments.

Glycerolipids are another major class of lipids. They are composed of mono-, di-, or tri-substituted glycerols. The most well-known members are triglycerides, also called triacylglycerols. In these molecules, three fatty acids are attached to a glycerol backbone through ester bonds. Triglycerides serve as the primary energy storage in animal tissues. When the body needs energy, it uses hydrolysis to break these ester bonds. This process releases glycerol and fatty acids for metabolism. Some glycerolipids, called glycosylglycerols, include sugar residues attached to the glycerol. Examples include digalactosyldiacylglycerols found in plant membranes.

Glycerophospholipids, or phospholipids, are essential components of the lipid bilayer in cell membranes. They are also involved in metabolism and cell signaling. These molecules can be subdivided based on their polar headgroups. In eukaryotes and bacteria, these headgroups are at the sn-3 position of the glycerol backbone. In archaebacteria, they are at the sn-1 position. Common examples include phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine.

Some glycerophospholipids, such as phosphatidylinositols, act as second messengers in cells. Neural tissue contains high amounts of these lipids. Changes in their composition are linked to various neurological disorders.

Sphingolipids form a complicated family of compounds. They share a common structural feature called a sphingoid base backbone. This backbone is synthesized from the amino acid serine and a long-chain fatty acyl CoA. This process produces ceramides, which can then be converted into other compounds. The primary sphingoid base in mammals is sphingosine. Mammals use sphingomyelins as their major phosphosphingolipids.

Insects primarily contain ceramide phosphoethanolamines. Fungi use different versions, such as phytoceramide phosphoinositols. Glycosphingolipids are a diverse group that includes cerebrosides and gangliosides. These molecules contain one or more sugar residues linked to the sphingoid base.

Sterols are a specialized group of lipids that include cholesterol. They are critical components of cell membranes alongside phospholipids and sphingomyelins. Sterols are defined as steroids that have a hydroxyl group at the third carbon position. They share a fused four-ring core structure with other steroids. In mammals, the liver synthesizes bile acids, which are oxidized derivatives of cholesterol. Plants use different versions called phytosterols, such as β-sitosterol. Fungi rely on ergosterol for their cell membranes.

Steroids also function as important hormones. These include the eighteen-carbon estrogen family and the nineteen-carbon androgens like testosterone. The twenty-one-carbon subclass includes progestogens and glucocorticoids. Vitamin D belongs to a group called secosteroids.

The history of lipid science shows how our understanding has grown. In 1815, Henri Braconnot classified lipids into solid greases (suifs) and fluid oils (huiles). Michel Eugène Chevreul expanded this in 1823 to include waxes and resins. In 1827, William Prout identified fat as a vital nutrient for humans and animals. The first synthetic triglyceride was created by Théophile-Jules Pelouze in 1844. Later, Theodore Gobley discovered phospholipids in 1847, calling them lecithins. The term "lipid" was introduced by Gabriel Bertrand in 1923. This term was officially approved by the international commission of the Société de Chimie Biologique on July 3, 1923. Today, lipids are essential to many industries. They are used in food, cosmetics, and even nanotechnology.