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Wednesday 4 June 2003


Definition: Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others.

The whole of lipids is the lipidome.

Lipids are broadly defined as any fat-soluble (lipophilic), naturally-occurring molecule. Lipids are usually defined as hydrophobic or amphipathic biological molecules that will dissolve in organic solvents such as benzene or chloroform.

The emulsion test is a crude method for determining the presence or absence of lipids in a given sample.

Although the term lipid is sometimes used as a synonym for fats, fats are a subgroup of lipids called triglycerides.

Lipids encompass molecules such as fatty acids and their derivatives (including tri-, di-, and monoglycerides and phospholipids), as well as sterol-containing metabolites such as cholesterol.

Although humans and other mammals use various biosynthetic pathways to both break down and synthesize lipids, some essential lipids cannot be made this way and must be obtained from the diet.


Lipids are a diverse group of compounds that have many key biological functions, such as acting as structural components of cell membranes, serving as energy storage sources and participating in signaling pathways.

The main biological functions of lipids include energy storage, as structural components of cell membranes, and as important signaling molecules.

Their main structural uses are as part of biological membranes such as the cell membrane, or as a source of energy.


Lipids may be broadly defined as hydrophobic or amphiphilic small molecules that originate entirely or in part from two distinct types of biochemical subunits or "building blocks": ketoacyl and isoprene groups.

Most lipids consist of a polar or hydrophilic head and one to three nonpolar or hydrophobic fatty acid tails, and therefore they are amphiphilic.


Lipids are the most diverse group of biochemicals. Lipids encompass molecules such as fatty acids and their derivatives (including tri-, di-, and monoglycerides and phospholipids), as well as other sterol-containing metabolites such as cholesterol.

- The fats (or glycerids) are a large group of compounds that contain fatty acids and glycerol; a glycerol molecule attached to three fatty acid esters is a triacylglyceride.

- Several variations on this basic structure exist, including alternate backbones such as sphingosine in the sphingolipids, and hydrophilic groups such as phosphate in phospholipids.

- Steroids such as cholesterol are another major class of lipids that are made in cells.

Using this approach, lipids may be divided into eight categories :

- fatty acyls
- glycerolipids
- glycerophospholipids
- sphingolipids
- saccharolipids
- polyketides (derived from condensation of ketoacyl subunits)
- sterol lipids and prenol lipids (derived from condensation of isoprene subunits).

Fatty acids

Lipids are chiefly fatty acid esters, and are the basic building blocks of biological membranes. Another biological role is energy storage (e.g., triglycerides).

Fatty acids consist of unbranched chains of carbon atoms that are connected by single bonds alone (saturated fatty acids) or by both single and double bonds (unsaturated fatty acids). The chains are usually 14-24 carbon groups long.

Biological membranes

For lipids present in biological membranes, the hydrophilic head is from one of three classes:

- glycolipids, whose heads contain an oligosaccharide with 1-15 saccharide residues.
- phospholipids, whose heads contain a positively charged group that is linked to the tail by a negatively charged phosphate group.
- sterols, whose heads contain a planar steroid ring, for example, cholesterol.

By cellular localization

- intranuclear lipids

By types

- triglycerides (fatty acids )
- steroids
- glyoclipids
- phospholipids
- polyisoprenoids
- ether lipids
- inositol lipids

See also

- lipid intakes

See also

- lipid homeostasis
- lipid biosynthesis
- lipid catabolism