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Lipids are a broad group of naturally-occurring molecules which includes
fat In nutrition Nutrition is the biochemistry, biochemical and physiology, physiological process by which an organism uses food to support its life. It provides organisms with Nutrient, nutrients, which can be Metabolism, metabolized to ...
s, waxes,
sterol Sterol is an organic compound with formula , whose molecule is derived from that of gonane by replacement of a hydrogen atom in position 3 by a hydroxyl group. It is therefore an alcohol (chemistry), alcohol of gonane. More generally, any compounds ...
s, fat-soluble vitamins (such as vitamins A, D, E and K),
monoglyceride Monoglycerides (also: acylglycerols or monoacylglycerols) are a class of glycerides which are composed of a molecule of glycerol linked to a fatty acid via an ester bond. As glycerol contains both primary and secondary alcohol groups two differen ...
s,
diglyceride A diglyceride, or diacylglycerol (DAG), is a glyceride consisting of two fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Org ...
s,
phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophile, hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an Alcohol (chemistry), alcohol residue (usually a glycerol mol ...
s, and others. The functions of lipids include storing energy,
signaling In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
, and acting as structural components of
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the cytoplasm, interior of all Cell (biology), cells from th ...
s. Lipids have applications in the
cosmetic Cosmetic may refer to: *Cosmetics, or make-up, substances to enhance the beauty of the human body, apart from simple cleaning *Cosmetic, an adjective describing beauty, aesthetics, or appearance, especially concerning the human body *Cosmetic, a t ...
and food industries, and in
nanotechnology Nanotechnology, also shortened to nanotech, is the use of matter on an atomic, molecular, and Supramolecular complex, supramolecular scale for industrial purposes. The earliest, widespread description of nanotechnology referred to the particul ...
. Lipids may be broadly defined as
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly intermolecular force, repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend t ...
or
amphiphilic An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...
small molecules; the amphiphilic nature of some lipids allows them to form structures such as vesicles, multilamellar/
unilamellar liposome A unilamellar liposome is a spherical liposome, a Vesicle (biology and chemistry), vesicle, bounded by a single bilayer of an amphiphilic lipid or a mixture of such lipids, containing aqueous solution inside the chamber. Unilamellar liposomes are u ...
s, or membranes in an aqueous environment. Biological lipids originate entirely or in part from two distinct types of biochemical subunits or "building-blocks": ketoacyl and
isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. Isoprene is an unsaturated hydrocarbon. It is produced by many plants and animals ...
groups. Using this approach, lipids may be divided into eight categories: fatty acyls,
glycerolipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E, E and Vitamin K, K), monoglycerides, diglycerides, phospholipids, and o ...
s,
glycerophospholipid Glycerophospholipids or phosphoglycerides are glycerol-based phospholipids. They are the main component of biological membranes. Two major classes are known: those for bacteria and eukaryotes and a separate family for archaea. Structures The t ...
s,
sphingolipid Sphingolipids are a class of lipids containing a backbone of sphingoid bases, a set of aliphatic amino Alcohol (chemistry), alcohols that includes sphingosine. They were discovered in brain extracts in the 1870s and were named after the mytholog ...
s,
saccharolipid Saccharolipids are chemical compound, chemical compounds containing fatty acids linked directly to a sugar backbone, forming structures that are compatible with membrane bilayers. In the saccharolipids, a monosaccharide substitutes for the glycerol ...
s, and
polyketide Polyketides are a class of natural products derived from a precursor molecule consisting of a chain of alternating ketone (or Carbonyl reduction, reduced forms of a ketone) and methylene groups: (-CO-CH2-). First studied in the early 20th century, ...
s (derived from condensation of ketoacyl subunits); and sterol lipids and prenol lipids (derived from condensation of isoprene subunits). Although the term "lipid" is sometimes used as a synonym for
fat In nutrition Nutrition is the biochemistry, biochemical and physiology, physiological process by which an organism uses food to support its life. It provides organisms with Nutrient, nutrients, which can be Metabolism, metabolized to ...
s, fats are a subgroup of lipids called
triglyceride A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids (from ''wikt:tri-#Prefix, tri-'' and ''glyceride''). Triglycerides are the main constituents of body fat in humans and other ...
s. Lipids also encompass molecules such as
fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an B ...
s and their derivatives (including
tri- Numeral or number prefixes are prefixes derived from Numeral (linguistics), numerals or occasionally other numbers. In English and many other languages, they are used to coin numerous series of words. For example: * unicycle, bicycle, tricycle (1 ...
, di-,
monoglyceride Monoglycerides (also: acylglycerols or monoacylglycerols) are a class of glycerides which are composed of a molecule of glycerol linked to a fatty acid via an ester bond. As glycerol contains both primary and secondary alcohol groups two differen ...
s, and
phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophile, hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an Alcohol (chemistry), alcohol residue (usually a glycerol mol ...
s), as well as other
sterol Sterol is an organic compound with formula , whose molecule is derived from that of gonane by replacement of a hydrogen atom in position 3 by a hydroxyl group. It is therefore an alcohol (chemistry), alcohol of gonane. More generally, any compounds ...
-containing
metabolite In biochemistry, a metabolite is an intermediate or end product of metabolism. The term is usually used for small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, c ...
s such as
cholesterol Cholesterol is any of a class of certain organic compound, organic molecules called lipids. It is a sterol (or chemical modification, modified steroid), a type of lipid. Cholesterol is biosynthesis, biosynthesized by all animal Cell (biology)# ...
. Although humans and other mammals use various
biosynthetic pathway Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
s both to break down and to synthesize lipids, some essential lipids can't be made this way and must be obtained from the diet.


History

In 1815,
Henri Braconnot Henri Braconnot (29 May 1780 – 13 January 1855) was a French people, French chemist and pharmacist. He was born in Commercy, his father being a counsel at the local parliament. At the death of his father, in 1787, Henri began his instruction ...
classified lipids (''graisses'') in two categories, ''suifs'' (solid greases or tallow) and ''huiles'' (fluid oils). In 1823,
Michel Eugène Chevreul Michel Eugène Chevreul (31 August 1786 – 9 April 1889) was a French chemist and centenarian A centenarian is a person who has reached the age of 100 years. Because life expectancies worldwide are below 100 years, the term is invariably asso ...
developed a more detailed classification, including oils, greases, tallow, waxes, resins, balsams and volatile oils (or essential oils). The first synthetic triglyceride was reported by
Théophile-Jules Pelouze Théophile-Jules Pelouze (also known as Jules Pelouze, Théophile Pelouze, Theo Pelouze, or T. J. Pelouze, ; 26 February 180731 May 1867) was a French chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval ...
in 1844, when he produced tributyrin by treating
butyric acid Butyric acid (; from grc, βούτῡρον, meaning "butter"), also known under the systematic name butanoic acid, is a straight-chain alkyl carboxylic acid with the chemical formula CH3CH2CH2CO2H. It is an oily, colorless liquid with an unple ...
with
glycerin Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
in the presence of concentrated
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen ...
. Several years later,
Marcellin Berthelot Pierre Eugène Marcellin Berthelot (; 25 October 1827 – 18 March 1907) was a French chemist and Republican Union (France), Republican politician noted for the Thomsen-Berthelot principle, ThomsenBerthelot principle of thermochemistry. He synt ...
, one of Pelouze's students, synthesized
tristearin Stearin , or tristearin, or glyceryl tristearate is an odourless, white powder. It is a triglyceride derived from three units of stearic acid. Most triglycerides are derived from at least two and more commonly three different fatty acids. Like o ...
and
tripalmitin Tripalmitin is a triglyceride derived from the fatty acid palmitic acid. References

Triglycerides Palmitate esters {{ester-stub ...
by reaction of the analogous
fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an B ...
s with glycerin in the presence of gaseous
hydrogen chloride The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chlorid ...
at high temperature. In 1827,
William Prout William Prout FRS (; 15 January 1785 – 9 April 1850) was an English chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemis ...
recognized fat ("oily" alimentary matters), along with protein ("albuminous") and carbohydrate ("saccharine"), as an important nutrient for humans and animals. For a century, chemists regarded "fats" as only simple lipids made of fatty acids and glycerol (glycerides), but new forms were described later. Theodore Gobley (1847) discovered phospholipids in mammalian brain and hen egg, called by him as "
lecithin Lecithin (, from the Greek ''lekithos'' "yolk") is a generic term to designate any group of yellow-brownish lipid, fatty substances occurring in animal and plant tissues which are amphiphilic – they attract both water and fatty substances (and ...
s". Thudichum discovered in human brain some phospholipids ( cephalin), glycolipids (
cerebroside Cerebrosides is the common name for a group of glycosphingolipid Glycosphingolipids are a subtype of glycolipids Glycolipids are lipids with a carbohydrate attached by a glycosidic linkage, glycosidic (covalent) bond. Their role is to main ...
) and sphingolipids (
sphingomyelin Sphingomyelin (SPH, ˌsfɪŋɡoˈmaɪəlɪn) is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It usually consists of phosphocholine and ceramide, or a ethano ...
). The terms lipoid, lipin, lipide and lipid have been used with varied meanings from author to author. In 1912, Rosenbloom and Gies proposed the substitution of "lipoid" by "lipin". In 1920, Bloor introduced a new classification for "lipoids": simple lipoids (greases and waxes), compound lipoids (phospholipoids and glycolipoids), and the derived lipoids (fatty acids,
alcohols In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, st ...
, sterols). The word ''lipide'', which stems etymologically from Greek λίπος, ''lipos'' 'fat', was introduced in 1923 by the French pharmacologist Gabriel Bertrand. Bertrand included in the concept not only the traditional fats (glycerides), but also the "lipoids", with a complex constitution. The word ''lipide'' was unanimously approved by the international commission of the ''Société de Chimie Biologique'' during the plenary session on July 3, 1923. The word ''lipide'' was later anglicized as ''lipid'' because of its pronunciation ('lɪpɪd). In French, the suffix ''-ide'', from Ancient Greek -ίδης (meaning 'son of' or 'descendant of'), is always pronounced (ɪd). In 1947, T. P. Hilditch defined "simple lipids" as greases and waxes (true waxes, sterols, alcohols).


Categories

Lipids have been classified into eight categories by the
Lipid MAPS LIPID MAPS (Lipid Metabolites and Pathways Strategy) is a web portal designed to be a gateway to Lipidomics Lipidomics is the large-scale study of pathways and networks of cellular lipid Lipids are a broad group of naturally-occurring molec ...
consortium as follows:


Fatty acyls

Fatty acyls, a generic term for describing
fatty acids In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an B ...
, their conjugates and derivatives, are a diverse group of molecules synthesized by chain-elongation of an
acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining che ...
primer with
malonyl-CoA Malonyl-CoA is a coenzyme A Coenzyme A (CoA, SHCoA, CoASH) is a coenzyme, notable for its role in the Fatty acid metabolism#Synthesis, synthesis and Fatty acid metabolism#.CE.B2-Oxidation, oxidation of fatty acids, and the oxidation of pyruvic ac ...
or methylmalonyl-CoA groups in a process called
fatty acid synthesis In biochemistry, fatty acid synthesis is the creation of fatty acids from acetyl-CoA and NADPH through the action of enzymes called fatty acid synthases. This process takes place in the cytoplasm of the Cell (biology), cell. Most of the acetyl-Co ...
. They are made of a hydrocarbon chain that terminates with a
carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with substituent, R referring to the alkyl, alkenyl, aryl, or other group. ...
group; this arrangement confers the molecule with a polar,
hydrophilic A hydrophile is a molecule or other molecular entity that is intermolecular force, attracted to water molecules and tends to be dissolution (chemistry), dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clar ...
end, and a nonpolar,
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly intermolecular force, repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend t ...
end that is
insoluble In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, st ...
in water. The fatty acid structure is one of the most fundamental categories of biological lipids and is commonly used as a building-block of more structurally complex lipids. The carbon chain, typically between four and 24 carbons long, may be saturated or unsaturated, and may be attached to
functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions re ...
s containing
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...
,
halogen The halogens () are a group in the periodic table The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and ...
s,
nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the pnictogens. It is a common element in the ...
, and
sulfur Sulfur (or sulphur in British English British English (BrE, en-GB, or BE) is, according to Oxford Dictionaries, " English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to the ...
. If a fatty acid contains a double bond, there is the possibility of either a ''cis'' or ''trans''
geometric isomerism Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is ca ...
, which significantly affects the molecule's
configuration Configuration or configurations may refer to: Computing * Computer configuration or system configuration * Configuration file, a software file used to configure the initial settings for a computer program * Configurator, also known as choice board ...
. ''Cis''-double bonds cause the fatty acid chain to bend, an effect that is compounded with more double bonds in the chain. Three double bonds in 18-carbon ''
linolenic acid Linolenic acid is a type of naturally-occurring fatty acid. It can refer to either of two octadecatrienoic acids (i.e. with an 18-carbon chain and three double bonds, which are found in the ''Cis-trans isomerism, cis'' configuration), or a mixture ...
'', the most abundant fatty-acyl chains of plant ''thylakoid membranes'', render these membranes highly ''fluid'' despite environmental low-temperatures, and also makes linolenic acid give dominating sharp peaks in high resolution 13-C NMR spectra of chloroplasts. This in turn plays an important role in the structure and function of cell membranes. Most naturally occurring fatty acids are of the ''cis'' configuration, although the ''trans'' form does exist in some natural and partially hydrogenated fats and oils. Examples of biologically important fatty acids include the
eicosanoid Eicosanoids are lipid signaling, signaling molecules made by the enzymatic or non-enzymatic oxidation of arachidonic acid or other polyunsaturated fatty acids (PUFAs) that are, similar to arachidonic acid, around 20 carbon units in length. Eicosa ...
s, derived primarily from
arachidonic acid Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega-6 fatty acid 20:4(ω-6), or 20:4(5,8,11,14). It is structurally related to the saturated arachidic acid found in Cupuaçu, cupuaçu butter. Its name derives from the New Latin word ...
and
eicosapentaenoic acid Eicosapentaenoic acid (EPA; also icosapentaenoic acid) is an omega-3 fatty acid Omega−3 fatty acids, also called Omega-3 oils, ω−3 fatty acids or ''n''−3 fatty acids, are polyunsaturated fatty acids (PUFAs) characterized by the presenc ...
, that include
prostaglandin The prostaglandins (PG) are a group of physiologically active lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E ...
s,
leukotriene Leukotrienes are a family of eicosanoid inflammatory mediators produced in leukocytes by the oxidation of arachidonic acid (AA) and the essential fatty acid eicosapentaenoic acid (EPA) by the enzyme Enzymes () are proteins that ...
s, and
thromboxane Thromboxane is a member of the family of lipids known as eicosanoids. The two major thromboxanes are thromboxane A2 and thromboxane B2. The distinguishing feature of thromboxanes is a 6-membered ether-containing ring. Thromboxane is named for its ...
s.
Docosahexaenoic acid Docosahexaenoic acid (DHA) is an omega-3 fatty acid Omega−3 fatty acids, also called Omega-3 oils, ω−3 fatty acids or ''n''−3 fatty acids, are polyunsaturated fatty acids (PUFAs) characterized by the presence of a double bond, three ...
is also important in biological systems, particularly with respect to sight. Other major lipid classes in the fatty acid category are the fatty esters and fatty amides. Fatty esters include important biochemical intermediates such as
wax ester A wax ester (WE) is an ester of a fatty acid and a fatty alcohol. Wax esters comprise the main components of three commercially important waxes: carnauba wax, candelilla wax, and beeswax.. Wax esters are formed by combining one fatty acid with on ...
s, fatty acid thioester
coenzyme A Coenzyme A (CoA, SHCoA, CoASH) is a coenzyme, notable for its role in the Fatty acid metabolism#Synthesis, synthesis and Fatty acid metabolism#.CE.B2-Oxidation, oxidation of fatty acids, and the oxidation of pyruvic acid, pyruvate in the citric ac ...
derivatives, fatty acid thioester ACP derivatives and fatty acid carnitines. The fatty amides include N-acyl ethanolamines, such as the
cannabinoid Cannabinoids () are several structural classes of compounds found in the cannabis plant primarily and most animal organisms (although insects lack such receptors) or as synthetic compounds. The most notable cannabinoid is the phytocannabinoid te ...
neurotransmitter
anandamide Anandamide (ANA), also known as ''N''-arachidonoylethanolamine (AEA), is a fatty acid neurotransmitter. Anandamide was the first endocannabinoid to be discovered: it participates in the body's endocannabinoid system by binding to cannabinoid rece ...
.


Glycerolipids

Glycerolipids are composed of mono-, di-, and tri-substituted
glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
s, the best-known being the fatty acid triesters of glycerol, called
triglycerides A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids (from ''wikt:tri-#Prefix, tri-'' and ''glyceride''). Triglycerides are the main constituents of body fat in humans and other ...
. The word "triacylglycerol" is sometimes used synonymously with "triglyceride". In these compounds, the three hydroxyl groups of glycerol are each esterified, typically by different fatty acids. Because they function as an energy store, these lipids comprise the bulk of storage
fat In nutrition Nutrition is the biochemistry, biochemical and physiology, physiological process by which an organism uses food to support its life. It provides organisms with Nutrient, nutrients, which can be Metabolism, metabolized to ...
in animal tissues. The hydrolysis of the
ester In chemistry, an ester is a chemical compound, compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an Alcohol ...
bonds of triglycerides and the release of glycerol and fatty acids from
adipose tissue Adipose tissue, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular en ...
are the initial steps in metabolizing fat. Additional subclasses of glycerolipids are represented by glycosylglycerols, which are characterized by the presence of one or more sugar residues attached to glycerol via a
glycosidic linkage A glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate. A glycosidic bond is formed between the hemiacetal or hemiketal group ...
. Examples of structures in this category are the digalactosyldiacylglycerols found in plant membranes and seminolipid from mammalian
sperm cells A spermatozoon (; also spelled spermatozoön; ; ) is a motile sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and ...
.


Glycerophospholipids

Glycerophospholipids, usually referred to as
phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophile, hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an Alcohol (chemistry), alcohol residue (usually a glycerol mol ...
s (though
sphingomyelin Sphingomyelin (SPH, ˌsfɪŋɡoˈmaɪəlɪn) is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It usually consists of phosphocholine and ceramide, or a ethano ...
s are also classified as phospholipids), are ubiquitous in nature and are key components of the
lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cell (biology), cells. The cell membranes of almost all organis ...
of cells, as well as being involved in
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
and
cell signaling In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a Cell (biology), cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property ...
. Neural tissue (including the brain) contains relatively high amounts of glycerophospholipids, and alterations in their composition has been implicated in various neurological disorders. Glycerophospholipids may be subdivided into distinct classes, based on the nature of the polar headgroup at the ''sn''-3 position of the glycerol backbone in
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
s and eubacteria, or the ''sn''-1 position in the case of archaebacteria. Examples of glycerophospholipids found in
biological membrane A biological membrane, biomembrane or cell membrane is a Semipermeable membrane, selectively permeable membrane that separates the interior of a Cell (biology), cell from the extracellular, external environment or creates intracellular compartmen ...
s are
phosphatidylcholine Phosphatidylcholines (PC) are a class of phospholipids that incorporate choline as a headgroup. They are a major component of biological membranes and can be easily obtained from a variety of readily available sources, such as egg yolk or soybean ...
(also known as PC, GPCho or
lecithin Lecithin (, from the Greek ''lekithos'' "yolk") is a generic term to designate any group of yellow-brownish lipid, fatty substances occurring in animal and plant tissues which are amphiphilic – they attract both water and fatty substances (and ...
),
phosphatidylethanolamine Phosphatidylethanolamine (PE) is a class of phospholipids found in biological membranes. They are synthesized by the addition of cytidine diphosphate-ethanolamine to diglycerides, releasing cytidine monophosphate. S-Adenosyl methionine, ''S''-Ade ...
(PE or GPEtn) and
phosphatidylserine Phosphatidylserine (abbreviated Ptd-L-Ser or PS) is a phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophile, hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, join ...
(PS or GPSer). In addition to serving as a primary component of cellular membranes and binding sites for intra- and intercellular proteins, some glycerophospholipids in eukaryotic cells, such as
phosphatidylinositol Phosphatidylinositol (or Inositol Phospholipid) consists of a family of lipids as illustrated on the right, where red is x, blue is y, and black is z, in the context of independent variation, a class of the Glycerophospholipid, phosphatidylglyceri ...
s and
phosphatidic acid Phosphatidic acids are anionic phospholipids important to cell signaling and direct activation of lipid-gated ion channels. Hydrolysis of phosphatidic acid gives rise to one molecule each of glycerol and phosphoric acid and two molecules of fatty ac ...
s are either precursors of or, themselves, membrane-derived
second messengers Second messengers are intracellular signaling molecules released by the cell in response to exposure to extracellular signaling molecules—the first messengers. (Intercellular signals, a non-local form or cell signaling, encompassing both first me ...
. Typically, one or both of these hydroxyl groups are acylated with long-chain fatty acids, but there are also alkyl-linked and 1Z-alkenyl-linked (
plasmalogen Glycerophospholipids of biochemical relevance are divided into three subclasses based on the substitution present at the sn-1 position of the glycerol backbone: acyl, alkyl and alkenyl. Of these, the alkyl and alkenyl moiety in each case form an ...
) glycerophospholipids, as well as dialkylether variants in archaebacteria.


Sphingolipids

Sphingolipid Sphingolipids are a class of lipids containing a backbone of sphingoid bases, a set of aliphatic amino Alcohol (chemistry), alcohols that includes sphingosine. They were discovered in brain extracts in the 1870s and were named after the mytholog ...
s are a complicated family of compounds that share a common structural feature, a sphingoid base backbone that is synthesized ''de novo'' from the amino acid
serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
and a long-chain fatty acyl CoA, then converted into
ceramide Ceramides are a family of waxy lipid molecules. A ceramide is composed of N-acetyl sphingosine and a fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated ...
s, phosphosphingolipids, glycosphingolipids and other compounds. The major sphingoid base of mammals is commonly referred to as
sphingosine Sphingosine (2-amino-4-trans-octadecene-1,3-diol) is an 18-carbon amino alcohol with an unsaturated hydrocarbon chain, which forms a primary part of sphingolipid Sphingolipids are a class of lipids containing a backbone of sphingoid bases, a s ...
. Ceramides (N-acyl-sphingoid bases) are a major subclass of sphingoid base derivatives with an
amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent organic compound, organic functional group, groups or hydrogen atoms ...
-linked fatty acid. The fatty acids are typically saturated or mono-unsaturated with chain lengths from 16 to 26 carbon atoms. The major phosphosphingolipids of mammals are
sphingomyelin Sphingomyelin (SPH, ˌsfɪŋɡoˈmaɪəlɪn) is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It usually consists of phosphocholine and ceramide, or a ethano ...
s (ceramide phosphocholines), whereas insects contain mainly ceramide phosphoethanolamines and fungi have phytoceramide phosphoinositols and
mannose Mannose is a sugar monomer of the hexose, aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several Congenital disorder of glycosylati ...
-containing headgroups. The glycosphingolipids are a diverse family of molecules composed of one or more sugar residues linked via a
glycosidic bond A glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate. A glycosidic bond is formed between the hemiacetal or hemiketal group ...
to the sphingoid base. Examples of these are the simple and complex glycosphingolipids such as
cerebroside Cerebrosides is the common name for a group of glycosphingolipid Glycosphingolipids are a subtype of glycolipids Glycolipids are lipids with a carbohydrate attached by a glycosidic linkage, glycosidic (covalent) bond. Their role is to main ...
s and
ganglioside A ganglioside is a molecule composed of a glycosphingolipid (ceramide and oligosaccharide) with one or more sialic acids (e.g. N-acetylneuraminic acid, ''N''-acetylneuraminic acid, NANA) linked on the sugar chain. NeuNAc, an acetylated derivative ...
s.


Sterols

Sterol Sterol is an organic compound with formula , whose molecule is derived from that of gonane by replacement of a hydrogen atom in position 3 by a hydroxyl group. It is therefore an alcohol (chemistry), alcohol of gonane. More generally, any compounds ...
s, such as
cholesterol Cholesterol is any of a class of certain organic compound, organic molecules called lipids. It is a sterol (or chemical modification, modified steroid), a type of lipid. Cholesterol is biosynthesis, biosynthesized by all animal Cell (biology)# ...
and its derivatives, are an important component of membrane lipids, along with the glycerophospholipids and sphingomyelins. Other examples of sterols are the
bile acid Bile acids are steroid acids found predominantly in the bile of mammals and other vertebrates. Diverse bile acids are synthesized in the liver. Bile acids are conjugated with taurine or glycine residues to give anions called bile salts. Primary b ...
s and their conjugates, which in mammals are oxidized derivatives of cholesterol and are synthesized in the liver. The plant equivalents are the
phytosterols Phytosterols are phytosteroids, similar to cholesterol, that serve as structural components of biological membranes of plants. They encompass plant sterols and stanol ester, stanols. More than 250 sterols and related compounds have been identified ...
, such as
β-sitosterol β-sitosterol (beta-sitosterol) is one of several phytosterol Phytosterols are phytosteroids, similar to cholesterol, that serve as structural components of biological membranes of plants. They encompass plant sterols and stanol ester, stanols. ...
,
stigmasterol Stigmasterol – a plant sterol (''phytosterol'') – is among the most abundant of plant sterols, having a major function to maintain the structure and plant physiology, physiology of cell membranes. In the European Union, it is a food additive ...
, and
brassicasterol Brassicasterol (24-methyl cholest-5,22-dien-3β-ol) is a 28-carbon sterol synthesised by several unicellular algae (phytoplankton) and some terrestrial plants, like oilseed rape, rape. This compound has frequently been used as a biomarker for the ...
; the latter compound is also used as a
biomarker In biomedical contexts, a biomarker, or biological marker, is a measurable wikt:indicator, indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biologic ...
for
algal Algae (; singular alga ) is an informal term for a large and diverse group of photosynthesis, photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from u ...
growth. The predominant sterol in
fungal A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
cell membranes is
ergosterol Ergosterol (ergosta-5,7,22-trien-3β-ol) is a sterol found in cell membranes of fungus, fungi and protozoa, serving many of the same functions that cholesterol serves in animal cell (biology), cells. Because many fungi and protozoa cannot survive ...
. Sterols are
steroid A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes that alter membrane fluidity; and a ...
s in which one of the hydrogen atoms is substituted with a
hydroxyl group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom Chemical bond, covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or ...
, at position 3 in the carbon chain. They have in common with steroids the same fused four-ring core structure. Steroids have different biological roles as
hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and beh ...
s and
signaling molecules In biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology), cel ...
. The eighteen-carbon (C18) steroids include the
estrogen Estrogen or oestrogen is a category of sex steroid, sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogeny (biology), endogenous estrogens th ...
family whereas the C19 steroids comprise the
androgen An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This i ...
s such as
testosterone Testosterone is the primary sex hormone and anabolic steroid in males. In humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate The prostate is both an Male accessory gland ...
and
androsterone Androsterone, or 3α-hydroxy-5α-androstan-17-one, is an endogenous steroid hormone, neurosteroid, and putative pheromone. It is a weak androgen with a potency that is approximately 1/7 that of testosterone. Androsterone is a metabolite ...
. The C21 subclass includes the
progestogens Progestogens, also sometimes written progestagens or gestagens, are a class of natural or synthetic steroid hormone A steroid hormone is a steroid that acts as a hormone. Steroid hormones can be grouped into two classes: corticosteroids (typi ...
as well as the
glucocorticoid Glucocorticoids (or, less commonly, glucocorticosteroids) are a class of corticosteroids, which are a class of steroid hormones. Glucocorticoids are corticosteroids that bind to the glucocorticoid receptor that is present in almost every vertebr ...
s and
mineralocorticoids Mineralocorticoids are a class of corticosteroid Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex of vertebrates, as well as the synthetic analogues of these hormones. Two main classes of corticosteroids, ...
. The
secosteroid A secosteroid () is a type of steroid with a "broken" ring. The word ''secosteroid ''derives from the Latin verb ''secare'' meaning "to cut", and 'steroid'. Secosteroids are alternatively described as a subclass of steroids; ; or derived from st ...
s, comprising various forms of
vitamin D Vitamin D is a group of Lipophilicity, fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group ar ...
, are characterized by cleavage of the B ring of the core structure.


Prenols

Prenol Prenol, or 3-methyl-2-buten-1-ol, is a natural alcohol Alcohol most commonly refers to: * Alcohol (chemistry) In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a Saturat ...
lipids are synthesized from the five-carbon-unit precursors isopentenyl diphosphate and
dimethylallyl diphosphate Dimethylallyl pyrophosphate (DMAPP; or alternatively, dimethylallyl diphosphate (DMADP); also isoprenyl pyrophosphate) is an isoprenoid precursor. It is a product of both the mevalonate pathway and the methylerythritol phosphate, MEP pathway of i ...
, which are produced mainly via the
mevalonic acid Mevalonic acid (MVA) is a key organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chain ...
(MVA) pathway. The simple isoprenoids (linear alcohols, diphosphates, etc.) are formed by the successive addition of C5 units, and are classified according to number of these
terpene Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n > 1. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly Pinophyta, conifers. ...
units. Structures containing greater than 40 carbons are known as polyterpenes.
Carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, and Fungus, fungi. Carotenoids give the characteristic color to pumpki ...
s are important simple isoprenoids that function as
antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to anothe ...
s and as precursors of
vitamin A Vitamin A is a fat-soluble vitamin and an essential nutrient for humans. It is a group of organic compounds that includes retinol, retinal (also known as retinaldehyde), retinoic acid, and several provitamin A carotenoids (most notably beta-c ...
. Another biologically important class of molecules is exemplified by the
quinone The quinones are a class of organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains ...
s and
hydroquinone Hydroquinone, also known as benzene-1,4-diol or quinol, is an aromatic In chemistry, aromaticity is a chemical property of cyclic compound, cyclic (ring (chemistry), ring-shaped), ''typically'' plane (geometry), planar (flat) molecular stru ...
s, which contain an isoprenoid tail attached to a quinonoid core of non-isoprenoid origin.
Vitamin E Vitamin E is a group of eight fat soluble compounds that include four tocopherols and four tocotrienols. Vitamin E deficiency, which is rare and usually due to an underlying problem with digesting dietary fat rather than from a diet low in vitami ...
and
vitamin K Vitamin K refers to structurally similar, fat-soluble vitamers found in foods and marketed as dietary supplements. The human body requires vitamin K for post-translational modification, post-synthesis modification of certain proteins that are ...
, as well as the
ubiquinone Coenzyme Q, also known as ubiquinone and marketed as CoQ10, is a coenzyme A cofactor is a non-protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), resid ...
s, are examples of this class. Prokaryotes synthesize polyprenols (called bactoprenols) in which the terminal isoprenoid unit attached to oxygen remains unsaturated, whereas in animal polyprenols (
dolichol Dolichol refers to any of a group of long-chain mostly unsaturated compound, unsaturated organic compounds that are made up of varying numbers of isoprene units terminating in an α-saturated isoprenoid group, containing an Alcohol (chemistry), al ...
s) the terminal isoprenoid is reduced.


Saccharolipids

Saccharolipid Saccharolipids are chemical compound, chemical compounds containing fatty acids linked directly to a sugar backbone, forming structures that are compatible with membrane bilayers. In the saccharolipids, a monosaccharide substitutes for the glycerol ...
s describe compounds in which fatty acids are linked to a sugar backbone, forming structures that are compatible with membrane bilayers. In the saccharolipids, a
monosaccharide Monosaccharides (from Greek language, Greek ''wikt:μόνος, monos'': single, ''wikt:σάκχαρ, sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates a ...
substitutes for the glycerol backbone present in glycerolipids and glycerophospholipids. The most familiar saccharolipids are the acylated
glucosamine Glucosamine (C6H13NO5) is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylation, glycosylated proteins and lipids. Glucosamine is part of the structure of two polysaccharides, chitosan and chitin. Glucosamine is o ...
precursors of the Lipid A component of the
lipopolysaccharide Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the Bacterial ...
s in
Gram-negative bacteria Gram-negative bacteria are bacteria that do not retain the Crystal violet, crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidogly ...
. Typical lipid A molecules are
disaccharides A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars solubility, soluble in water. Three common examples are s ...
of glucosamine, which are derivatized with as many as seven fatty-acyl chains. The minimal lipopolysaccharide required for growth in ''E. coli'' is Kdo2-Lipid A, a hexa-acylated disaccharide of glucosamine that is glycosylated with two 3-deoxy-D-manno-octulosonic acid (Kdo) residues.


Polyketides

Polyketides are synthesized by polymerization of
acetyl In organic chemistry, acetyl is a functional group with the chemical formula and the Chemical structure, structure . It is Skeletal formula#Pseudoelement symbols, sometimes represented by the symbol Ac (not to be confused with the element ac ...
and propionyl subunits by classic enzymes as well as iterative and multimodular enzymes that share mechanistic features with the
fatty acid synthase Fatty acid synthase (FAS) is an enzyme that in humans is encoded by the ''FASN'' gene. Fatty acid synthase is a multi-enzyme protein that catalyzes fatty acid synthesis. It is not a single enzyme but a whole enzymatic system composed of two iden ...
s. They comprise many
secondary metabolite Secondary metabolites, also called specialised metabolites, toxins, secondary products, or natural products, are organic compounds produced by any lifeform, e.g. bacteria, fungi, animals, or plants, which are not directly involved in the norma ...
s and
natural products A natural product is a natural Chemical compound, compound or chemical substance, substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural product ...
from animal, plant, bacterial, fungal and marine sources, and have great structural diversity. Many
polyketide Polyketides are a class of natural products derived from a precursor molecule consisting of a chain of alternating ketone (or Carbonyl reduction, reduced forms of a ketone) and methylene groups: (-CO-CH2-). First studied in the early 20th century, ...
s are cyclic molecules whose backbones are often further modified by
glycosylation Glycosylation is the reaction in which a carbohydrate (or 'glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not alw ...
,
methylation In the chemical sciences, methylation denotes the addition of a methyl, methyl group on a Substrate (chemistry), substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group repla ...
,
hydroxylation In chemistry, hydroxylation can refer to: *(i) most commonly, hydroxylation describes a chemistry, chemical process that introduces a hydroxyl group () into an organic compound. *(ii) the ''degree of hydroxylation'' refers to the number of OH gr ...
,
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
, or other processes. Many commonly used
anti-microbial An antimicrobial is an agent that kills microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is somet ...
, anti-parasitic, and
anti-cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumor A benign tumor is a mass of Cell (biology), cells (tumor) that does not Ca ...
agents are polyketides or polyketide derivatives, such as
erythromycin Erythromycin is an antibiotic used for the treatment of a number of bacterial infections. This includes respiratory tract infections, skin infections, chlamydia infections, pelvic inflammatory disease, and syphilis. It may also be used ...
s,
tetracyclines Tetracyclines are a group of broad-spectrum antibiotic compounds that have a common basic structure and are either isolated directly from several species of ''Streptomyces'' bacteria or produced semi-synthetically from those isolated compounds. T ...
,
avermectin The avermectins are a series of drugs and pesticides used to treat parasitic worms and insect Pest (organism), pests. They are a group of 16-membered macrocyclic lactone derivatives with potent anthelmintic and Insecticide, insecticidal properties ...
s, and antitumor
epothilone Epothilones are a class of potential cancer drugs. Like taxanes, they prevent cancer cells from dividing by interfering with tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one o ...
s.


Biological functions


Component of biological membranes

Eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
cells feature the compartmentalized membrane-bound
organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to the ...
s that carry out different biological functions. The glycerophospholipids are the main structural component of
biological membranes A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates the interior of a cell from the external environment or creates intracellular compartments by serving as a boundary between one part of th ...
, as the cellular
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the cytoplasm, interior of all Cell (biology), cells from th ...
and the intracellular membranes of
organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to the ...
s; in animal cells, the plasma membrane physically separates the
intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
components from the
extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
environment. The glycerophospholipids are
amphipathic An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...
molecules (containing both
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly intermolecular force, repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend t ...
and
hydrophilic A hydrophile is a molecule or other molecular entity that is intermolecular force, attracted to water molecules and tends to be dissolution (chemistry), dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clar ...
regions) that contain a glycerol core linked to two fatty acid-derived "tails" by
ester In chemistry, an ester is a chemical compound, compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an Alcohol ...
linkages and to one "head" group by a
phosphate In chemistry, a phosphate is an anion, salt (chemistry), salt, functional group or ester derived from a phosphoric acids and phosphates, phosphoric acid. It most commonly means orthophosphate, a derivative of phosphoric acid, orthophosphoric a ...
ester linkage. While glycerophospholipids are the major component of biological membranes, other non-glyceride lipid components such as
sphingomyelin Sphingomyelin (SPH, ˌsfɪŋɡoˈmaɪəlɪn) is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It usually consists of phosphocholine and ceramide, or a ethano ...
and
sterol Sterol is an organic compound with formula , whose molecule is derived from that of gonane by replacement of a hydrogen atom in position 3 by a hydroxyl group. It is therefore an alcohol (chemistry), alcohol of gonane. More generally, any compounds ...
s (mainly
cholesterol Cholesterol is any of a class of certain organic compound, organic molecules called lipids. It is a sterol (or chemical modification, modified steroid), a type of lipid. Cholesterol is biosynthesis, biosynthesized by all animal Cell (biology)# ...
in animal cell membranes) are also found in biological membranes. In plants and algae, the galactosyldiacylglycerols,Heinz E. (1996). "Plant glycolipids: structure, isolation and analysis", pp. 211–332 in ''Advances in Lipid Methodology'', Vol. 3. W.W. Christie (ed.). Oily Press, Dundee. and sulfoquinovosyldiacylglycerol, which lack a phosphate group, are important components of membranes of chloroplasts and related organelles and are the most abundant lipids in photosynthetic tissues, including those of higher plants, algae and certain bacteria. Plant thylakoid membranes have the largest lipid component of a non-bilayer forming monogalactosyl diglyceride (MGDG), and little phospholipids; despite this unique lipid composition, chloroplast thylakoid membranes have been shown to contain a dynamic lipid-bilayer matrix as revealed by magnetic resonance and electron microscope studies. A biological membrane is a form of
lamellar phase Lamellar phase refers generally to packing of polar-headed long chain nonpolar-tail molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may no ...
lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cell (biology), cells. The cell membranes of almost all organis ...
. The formation of lipid bilayers is an energetically preferred process when the glycerophospholipids described above are in an aqueous environment. This is known as the hydrophobic effect. In an aqueous system, the polar heads of lipids align towards the polar, aqueous environment, while the hydrophobic tails minimize their contact with water and tend to cluster together, forming a
vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features o ...
; depending on the
concentration In chemistry, concentration is the Abundance (chemistry), abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: ''mass concentration (chemistry), mass concentration'', ...
of the lipid, this biophysical interaction may result in the formation of
micelle A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloid, colloidal suspension (also known as associat ...
s,
liposomes A liposome is a small artificial vesicle, spherical in shape, having at least one lipid bilayer. Due to their hydrophobicity and/or hydrophilicity, biocompatibility, particle size and many other properties, liposomes can be used as drug deli ...
, or
lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cell (biology), cells. The cell membranes of almost all organis ...
s. Other aggregations are also observed and form part of the polymorphism of
amphiphile An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...
(lipid) behavior. Phase behavior is an area of study within
biophysics Biophysics is an interdisciplinary science that applies approaches and methods traditionally used in physics to study Biology, biological phenomena. Biophysics covers all scales of biological organization, from Molecule, molecular to organismic ...
and is the subject of current academic research. Micelles and bilayers form in the polar medium by a process known as the
hydrophobic effect The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in an aqueous solution and exclude water#Properties, water molecules. The word hydrophobic literally means "water-fearing", and it describes the Segregation in m ...
. When dissolving a lipophilic or amphiphilic substance in a polar environment, the polar molecules (i.e., water in an aqueous solution) become more ordered around the dissolved lipophilic substance, since the polar molecules cannot form
hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily Electrostatics, electrostatic force of attraction between a hydrogen (H) atom which is Covalent bond, covalently bound to a more electronegativity, electronegative "donor" atom or group ( ...
s to the lipophilic areas of the
amphiphile An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...
. So in an aqueous environment, the water molecules form an ordered "
clathrate A clathrate is a chemical substance consisting of a lattice (group), lattice that traps or contains molecules. The word ''clathrate'' is derived from the Latin language, Latin (), meaning ‘with bars, Crystal structure, latticed’. Most clathr ...
" cage around the dissolved lipophilic molecule. The formation of lipids into
protocell A protocell (or protobiont) is a self-organized, endogenously ordered, spherical collection of lipids Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vi ...
membranes represents a key step in models of
abiogenesis In biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology ...
, the origin of life.


Energy storage

Triglycerides, stored in adipose tissue, are a major form of energy storage both in animals and plants. They are a major source of energy in aerobic respiration. The complete oxidation of fatty acids releases about 38 kJ/g (9  kcal/g), compared with only 17 kJ/g (4 kcal/g) for the oxidative breakdown of
carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or may ...
s and
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
s. The
adipocyte Adipocytes, also known as lipocytes and fat cells, are the cell (biology), cells that primarily compose adipose tissue, specialized in storing energy as fat. Adipocytes are derived from mesenchymal stem cells which give rise to adipocytes through ...
, or fat cell, is designed for continuous synthesis and breakdown of triglycerides in animals, with breakdown controlled mainly by the activation of hormone-sensitive enzyme
lipase Lipase ( ) is a family of enzymes that catalyzes the hydrolysis of fats. Some lipases display broad substrate scope including esters of cholesterol, phospholipids, and of lipid-soluble vitamins and sphingomyelinases; however, these are usually tr ...
. Migratory birds that must fly long distances without eating use triglycerides to fuel their flights.


Signaling

Evidence has emerged showing that
lipid signaling Lipid signaling, broadly defined, refers to any biological cell signaling, signaling event involving a lipid messenger that binds a protein target, such as a receptor (biochemistry), receptor, kinase or phosphatase, which in turn mediate the effec ...
is a vital part of the
cell signaling In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a Cell (biology), cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property ...
. Lipid signaling may occur via activation of
G protein-coupled G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of protein family, evolution ...
or
nuclear receptor In the field of molecular biology, nuclear receptors are a class of proteins responsible for sensing steroid hormone, steroids, thyroid hormone, thyroid hormones, vitamins, and certain other molecules. These receptors work with other proteins to ...
s, and members of several different lipid categories have been identified as signaling molecules and cellular messengers. These include
sphingosine-1-phosphate Sphingosine-1-phosphate (S1P) is a signaling sphingolipid Sphingolipids are a class of lipids containing a backbone of sphingoid bases, a set of aliphatic amino Alcohol (chemistry), alcohols that includes sphingosine. They were discovered i ...
, a sphingolipid derived from ceramide that is a potent messenger molecule involved in regulating calcium mobilization, cell growth, and apoptosis;
diacylglycerol A diglyceride, or diacylglycerol (DAG), is a glyceride consisting of two fatty acid chains covalent bond, covalently bonded to a glycerol molecule through ester linkages. Two possible forms exist, 1,2-diacylglycerols and 1,3-diacylglycerols. DAGs ...
(DAG) and the
phosphatidylinositol Phosphatidylinositol (or Inositol Phospholipid) consists of a family of lipids as illustrated on the right, where red is x, blue is y, and black is z, in the context of independent variation, a class of the Glycerophospholipid, phosphatidylglyceri ...
phosphates (PIPs), involved in calcium-mediated activation of
protein kinase C In cell biology, Protein kinase C, commonly abbreviated to PKC (EC 2.7.11.13), is a family of protein kinase enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and th ...
; the
prostaglandins The prostaglandins (PG) are a group of physiologically active lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E ...
, which are one type of fatty-acid derived eicosanoid involved in
inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and mol ...
and
immunity Immunity may refer to: Medicine * Immunity (medical) In biology, immunity is the capability of multicellular organisms to resist harmful microorganisms. Immunity involves both specific and nonspecific components. The nonspecific components act ...
; the steroid hormones such as
estrogen Estrogen or oestrogen is a category of sex steroid, sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogeny (biology), endogenous estrogens th ...
,
testosterone Testosterone is the primary sex hormone and anabolic steroid in males. In humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate The prostate is both an Male accessory gland ...
and
cortisol Cortisol is a steroid hormone, in the glucocorticoid class of hormones. When used as a medication, it is known as hydrocortisone. It is produced in many animals, mainly by the '' zona fasciculata'' of the adrenal cortex in the adrenal ...
, which modulate a host of functions such as reproduction, metabolism and blood pressure; and the oxysterols such as 25-hydroxy-cholesterol that are
liver X receptor The liver X receptor (LXR) is a member of the nuclear receptor family of transcription factors and is closely related to nuclear receptors such as the PPARs, Farnesoid X receptor, FXR and Retinoid X receptor, RXR. Liver X receptors (LXRs) are i ...
agonist An agonist is a chemical that activates a Receptor (biochemistry), receptor to produce a biological response. Receptors are Cell (biology), cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an ...
s. Phosphatidylserine lipids are known to be involved in signaling for the phagocytosis of apoptotic cells or pieces of cells. They accomplish this by being exposed to the extracellular face of the cell membrane after the inactivation of
flippase Flippases (rarely spelled flipases) are transmembrane lipid transporter protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast a ...
s which place them exclusively on the cytosolic side and the activation of scramblases, which scramble the orientation of the phospholipids. After this occurs, other cells recognize the phosphatidylserines and phagocytosize the cells or cell fragments exposing them.


Other functions

The "fat-soluble" vitamins ( A, D, E and K) – which are isoprene-based lipids – are essential nutrients stored in the liver and fatty tissues, with a diverse range of functions. Acyl-carnitines are involved in the transport and metabolism of fatty acids in and out of mitochondria, where they undergo
beta oxidation In biochemistry and metabolism, beta-oxidation is the catabolism, catabolic process by which fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA, which enters the citric ...
. Polyprenols and their phosphorylated derivatives also play important transport roles, in this case the transport of
oligosaccharide An oligosaccharide (/ˌɑlɪgoʊˈsækəˌɹaɪd/; from the Greek wikt:ὀλίγος#Ancient Greek, ὀλίγος ''olígos'', "a few", and σάκχαρ ''sácchar'', "sugar") is a carbohydrate, saccharide polymer containing a small number (typical ...
s across membranes. Polyprenol phosphate sugars and polyprenol diphosphate sugars function in extra-cytoplasmic glycosylation reactions, in extracellular polysaccharide biosynthesis (for instance,
peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units ...
polymerization in bacteria), and in eukaryotic protein N-
glycosylation Glycosylation is the reaction in which a carbohydrate (or 'glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not alw ...
.
Cardiolipin Cardiolipin (IUPAC name 1,3-bis(''sn''-3’-phosphatidyl)-''sn''-glycerol) is an important component of the inner mitochondrial membrane, where it constitutes about 20% of the total lipid composition. It can also be found in the membranes of most ...
s are a subclass of glycerophospholipids containing four acyl chains and three glycerol groups that are particularly abundant in the inner mitochondrial membrane. They are believed to activate enzymes involved with
oxidative phosphorylation Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the metabolic pathway in which Cell (biology), cells use enzymes to Redox, oxidize nutrients, thereby releasing chemical energy in order t ...
. Lipids also form the basis of steroid hormones.


Metabolism

The major dietary lipids for humans and other animals are animal and plant triglycerides, sterols, and membrane phospholipids. The process of lipid metabolism synthesizes and degrades the lipid stores and produces the structural and functional lipids characteristic of individual tissues.


Biosynthesis

In animals, when there is an oversupply of dietary carbohydrate, the excess carbohydrate is converted to triglycerides. This involves the synthesis of fatty acids from
acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining che ...
and the
esterification In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, stru ...
of fatty acids in the production of triglycerides, a process called
lipogenesis In biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural bio ...
. Fatty acids are made by
fatty acid synthase Fatty acid synthase (FAS) is an enzyme that in humans is encoded by the ''FASN'' gene. Fatty acid synthase is a multi-enzyme protein that catalyzes fatty acid synthesis. It is not a single enzyme but a whole enzymatic system composed of two iden ...
s that polymerize and then reduce acetyl-CoA units. The acyl chains in the fatty acids are extended by a cycle of reactions that add the acetyl group, reduce it to an alcohol,
dehydrate In physiology, dehydration is a lack of total body water In physiology, body water is the water content of an animal body that is contained in the tissues, the blood, the bones and elsewhere. The percentages of body water contained in various ...
it to an
alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—its atom making four electro ...
group and then reduce it again to an
alkane In organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms tha ...
group. The enzymes of fatty acid biosynthesis are divided into two groups, in animals and fungi all these fatty acid synthase reactions are carried out by a single multifunctional protein, while in plant
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingd ...
s and bacteria separate enzymes perform each step in the pathway. The fatty acids may be subsequently converted to triglycerides that are packaged in
lipoproteins A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, sur ...
and secreted from the liver. The synthesis of
unsaturated fatty acid In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, struc ...
s involves a desaturation reaction, whereby a double bond is introduced into the fatty acyl chain. For example, in humans, the desaturation of
stearic acid Stearic acid ( , ) is a saturated fatty acid with an 18-carbon chain. The IUPAC name is octadecanoic acid. It is a waxy solid and its chemical formula is C17H35CO2H. Its name comes from the Greek language, Greek word στέαρ "''stéar''", whi ...
by
stearoyl-CoA desaturase-1 Stearoyl-CoA desaturase (Δ-9-desaturase) is an endoplasmic reticulum enzyme that catalyzes the rate-limiting step in the formation of Monounsaturated fat, monounsaturated fatty acids (MUFAs), specifically Oleic acid, oleate and Palmitoleic acid ...
produces
oleic acid Oleic acid is a fatty acid that occurs naturally in various animal fat, animal and vegetable oil, vegetable fats and oils. It is an odorless, colorless oil, although commercial samples may be yellowish. In chemical terms, oleic acid is classified ...
. The doubly unsaturated fatty acid
linoleic acid Linoleic acid (LA) is an organic compound with the formula COOH(CH2)7CH=CHCH2CH=CH(CH2)4CH3. Both alkene groups are cis-trans isomerism, ''cis''. It is a fatty acid sometimes denoted 18:2 (n-6) or 18:2 ''cis''-9,12. A linoleate is a salt (chem ...
as well as the triply unsaturated
α-linolenic acid ''alpha''-Linolenic acid (ALA), also known as α-Linolenic acid (from Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-Eu ...
cannot be synthesized in mammalian tissues, and are therefore
essential fatty acid Essential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health but cannot Biosynthesis, synthesize them. Only two fatty acids are known to be essential for humans: alpha ...
s and must be obtained from the diet. Triglyceride synthesis takes place in the
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ...
by metabolic pathways in which acyl groups in fatty acyl-CoAs are transferred to the hydroxyl groups of glycerol-3-phosphate and diacylglycerol.
Terpene Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n > 1. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly Pinophyta, conifers. ...
s and
isoprenoids The terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carb ...
, including the
carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, and Fungus, fungi. Carotenoids give the characteristic color to pumpki ...
s, are made by the assembly and modification of
isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. Isoprene is an unsaturated hydrocarbon. It is produced by many plants and animals ...
units donated from the reactive precursors
isopentenyl pyrophosphate Isopentenyl pyrophosphate (IPP, isopentenyl diphosphate, or IDP) is an isoprenoid precursor. IPP is an intermediate in the classical, HMG-CoA reductase pathway (commonly called the mevalonate pathway) and in the ''non-mevalonate'' MEP pathway of i ...
and
dimethylallyl pyrophosphate Dimethylallyl pyrophosphate (DMAPP; or alternatively, dimethylallyl diphosphate (DMADP); also isoprenyl pyrophosphate) is an isoprenoid precursor. It is a product of both the mevalonate pathway The mevalonate pathway, also known as the isopreno ...
. These precursors can be made in different ways. In animals and
archaea Archaea ( ; singular archaeon ) is a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacter ...
, the
mevalonate pathway The mevalonate pathway, also known as the isoprenoid pathway or HMG-CoA reductase HMG-CoA reductase (3-hydroxy-3-methyl-glutaryl-coenzyme A reductase, official symbol HMGCR) is the rate-controlling enzyme (NADH-dependent, ; NADPH-dependent, ...
produces these compounds from acetyl-CoA, while in plants and bacteria the
non-mevalonate pathway The non-mevalonate pathway—also appearing as the mevalonate-independent pathway and the 2-''C''-methyl-D-erythritol 4-phosphate/1-deoxy-D-xylulose 5-phosphate (MEP/DOXP) pathway—is an alternative metabolic pathway for the biosynthesis of the is ...
uses pyruvate and
glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is a metabolite In biochemistry, a metabolite is an intermediate or end product of metabolism. The t ...
as substrates. One important reaction that uses these activated isoprene donors is steroid biosynthesis. Here, the isoprene units are joined together to make
squalene Squalene is an organic compound. It is a triterpenoid with the formula C30H50. It is a colourless oil, although impure samples appear yellow. It was originally obtained from shark liver oil (hence its name, as ''Squalus'' is a genus of sharks). A ...
and then folded up and formed into a set of rings to make
lanosterol Lanosterol is a tetracyclic triterpenoid and is the compound from which all animal and fungal steroids are derived. By contrast plant steroids are produced via cycloartenol. Role in biosynthesis of other steroids Elaboration of lanosterol under en ...
. Lanosterol can then be converted into other steroids such as
cholesterol Cholesterol is any of a class of certain organic compound, organic molecules called lipids. It is a sterol (or chemical modification, modified steroid), a type of lipid. Cholesterol is biosynthesis, biosynthesized by all animal Cell (biology)# ...
and
ergosterol Ergosterol (ergosta-5,7,22-trien-3β-ol) is a sterol found in cell membranes of fungus, fungi and protozoa, serving many of the same functions that cholesterol serves in animal cell (biology), cells. Because many fungi and protozoa cannot survive ...
.


Degradation

Beta oxidation In biochemistry and metabolism, beta-oxidation is the catabolism, catabolic process by which fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA, which enters the citric ...
is the metabolic process by which fatty acids are broken down in the
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which i ...
or in
peroxisomes A peroxisome () is a membrane-bound organelle, a type of microbody, found in the cytoplasm of virtually all eukaryotic cells. Peroxisomes are oxidative organelles. Frequently, molecular oxygen serves as a co-substrate, from which hydrogen pero ...
to generate
acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining che ...
. For the most part, fatty acids are oxidized by a mechanism that is similar to, but not identical with, a reversal of the process of fatty acid synthesis. That is, two-carbon fragments are removed sequentially from the carboxyl end of the acid after steps of
dehydrogenation In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, struc ...
, hydration, and
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
to form a beta-keto acid, which is split by thiolysis. The acetyl-CoA is then ultimately converted into ATP, CO2, and H2O using the
citric acid cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the Redox, oxidation of acetyl-CoA derived from carbohydrates, fats, and p ...
and the
electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules that electron transfer, transfer electrons from electron donors to electron acceptors via redox reactions (both Redox#Definitions, reduction and oxidation occu ...
. Hence the citric acid cycle can start at acetyl-CoA when fat is being broken down for energy if there is little or no glucose available. The energy yield of the complete oxidation of the fatty acid palmitate is 106 ATP. Unsaturated and odd-chain fatty acids require additional enzymatic steps for degradation.


Nutrition and health

Most of the fat found in food is in the form of triglycerides, cholesterol, and phospholipids. Some dietary fat is necessary to facilitate absorption of fat-soluble vitamins ( A, D, E, and K) and
carotenoids Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, and Fungus, fungi. Carotenoids give the characteristic color to pumpki ...
. Humans and other mammals have a dietary requirement for certain essential fatty acids, such as
linoleic acid Linoleic acid (LA) is an organic compound with the formula COOH(CH2)7CH=CHCH2CH=CH(CH2)4CH3. Both alkene groups are cis-trans isomerism, ''cis''. It is a fatty acid sometimes denoted 18:2 (n-6) or 18:2 ''cis''-9,12. A linoleate is a salt (chem ...
(an
omega-6 fatty acid Omega-6 fatty acids (also referred to as ω-6 fatty acids or ''n''-6 fatty acids) are a family of polyunsaturated fatty acids Polyunsaturated fatty acids (PUFAs) are fatty acids In chemistry, particularly in biochemistry, a fatty acid is a ...
) and
alpha-linolenic acid ''alpha''-Linolenic acid (ALA), also known as α-Linolenic acid (from Greek ''alpha'' meaning "first" and ''linon'' meaning flax Flax, also known as common flax or linseed, is a flowering plant, ''Linum usitatissimum'', in the family Lina ...
(an omega-3 fatty acid) because they cannot be synthesized from simple precursors in the diet. Both of these fatty acids are 18-carbon
polyunsaturated fatty acids Polyunsaturated fatty acids (PUFAs) are fatty acids In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsa ...
differing in the number and position of the double bonds. Most
vegetable oil Vegetable oils, or vegetable fats, are oils extracted from seeds or from other parts of fruits. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed oils, or fats ...
s are rich in linoleic acid (
safflower Safflower (''Carthamus tinctorius'') is a highly branched, herbaceous Herbaceous plants are vascular plants that have no persistent wood, woody stems above ground. This broad category of plants includes many perennial plant, perennials, and ...
,
sunflower The common sunflower (''Helianthus annuus'') is a large annual plant, annual forb of the genus ''Helianthus'' grown as a crop for its Sunflower seed, edible oily seeds. Apart from sunflower oil, cooking oil production, it is also used as livestoc ...
, and
corn Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn ( North American and Australian English Australian English (AusE, AusEng, AuE, AuEng, en-AU) is the set of variety (linguistics), varieties ...
oils). Alpha-linolenic acid is found in the green leaves of plants and in some seeds, nuts, and legumes (in particular
flax Flax, also known as common flax or linseed, is a flowering plant, ''Linum usitatissimum'', in the family Linaceae. It is cultivated as a food and fiber crop in regions of the world with temperate climates. Textiles made from flax are known in W ...
,
rapeseed Rapeseed (''Brassica napus ''subsp.'' napus''), also known as rape, or oilseed rape, is a bright-yellow flowering member of the family Brassicaceae (mustard or cabbage family), cultivated mainly for its oil-rich seed, which naturally contains a ...
,
walnut A walnut is the edible seed of a drupe of any tree of the genus ''Juglans'' (family Juglandaceae), particularly the Persian or English walnut, ''Juglans regia''. Although culinarily considered a "nut" and used as such, it is not a true Nut ...
, and
soy The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean, which has numerous uses. Traditional unfermented food uses of soybeans include soy milk, from which tofu and ...
).
Fish oil Fish oil is oil derived from the biological tissue, tissues of oily fish. Fish oils contain the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), precursors of certain eicosanoids that are known to reduce inflamma ...
s are particularly rich in the longer-chain omega-3 fatty acids
eicosapentaenoic acid Eicosapentaenoic acid (EPA; also icosapentaenoic acid) is an omega-3 fatty acid Omega−3 fatty acids, also called Omega-3 oils, ω−3 fatty acids or ''n''−3 fatty acids, are polyunsaturated fatty acids (PUFAs) characterized by the presenc ...
(EPA) and
docosahexaenoic acid Docosahexaenoic acid (DHA) is an omega-3 fatty acid Omega−3 fatty acids, also called Omega-3 oils, ω−3 fatty acids or ''n''−3 fatty acids, are polyunsaturated fatty acids (PUFAs) characterized by the presence of a double bond, three ...
(DHA). Many studies have shown positive health benefits associated with consumption of omega-3 fatty acids on infant development, cancer, cardiovascular diseases, and various mental illnesses (such as depression, attention-deficit hyperactivity disorder, and dementia). In contrast, it is now well-established that consumption of
trans fat Trans fat, also called trans-unsaturated fatty acids, or trans fatty acids, is a type of unsaturated fat that naturally occurs in small amounts in meat and milk fat. It became widely produced as an unintentional byproduct in the industrial pro ...
s, such as those present in partially hydrogenated vegetable oils, are a risk factor for
cardiovascular disease Cardiovascular disease (CVD) is a class of diseases that involve the heart or blood vessels. CVD includes coronary artery diseases (CAD) such as angina pectoris, angina and myocardial infarction (commonly known as a heart attack). Other CVDs in ...
. Fats that are good for one may be turned into trans fats by improper cooking methods that result in overcooking the lipids. A few studies have suggested that total dietary fat intake is linked to an increased risk of obesity and diabetes; however, a number of very large studies, including the Women's Health Initiative Dietary Modification Trial, an eight-year study of 49,000 women, the Nurses' Health Study, and the Health Professionals Follow-up Study, revealed no such links. None of these studies suggested any connection between percentage of calories from fat and risk of cancer, heart disease, or weight gain. The Nutrition Source, a website maintained by the department of nutrition at the T. H. Chan School of Public Health at
Harvard University Harvard University is a Private university, private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the History of the Puritans in North America, Puritan cler ...
, summarizes the current evidence on the effect of dietary fat: "Detailed research—much of it done at Harvard—shows that the total amount of fat in the diet isn't really linked with weight or disease."


See also

* * * * * * * * * * , a class of natural products composed of long aliphatic chains and phenolic rings that occur in plants, fungi and bacteria


References


Bibliography

* * * *


External links

Introductory
List of lipid-related web sitesNature Lipidomics Gateway
– Round-up and summaries of recent lipid research
Lipid Library
– General reference on lipid chemistry and biochemistry
Cyberlipid.org
– Resources and history for lipids.

– Modeling of Lipid Membranes
Lipids, Membranes and Vesicle Trafficking
– The Virtual Library of Biochemistry, Molecular Biology and Cell Biology Nomenclature
IUPAC nomenclature of lipids
Databases

– Comprehensive lipid and lipid-associated gene/protein databases.
LipidBank
– Japanese database of lipids and related properties, spectral data and references. General
ApolloLipids
– Provides dyslipidemia and cardiovascular disease prevention and treatment information as well as continuing medical education programs
National Lipid Association
– Professional medical education organization for health care professionals who seek to prevent morbidity and mortality stemming from dyslipidemias and other cholesterol-related disorders. {{Portal bar, Food, Biology * Underwater diving physiology