, a lipid is a macro biomolecule
that is soluble in nonpolar
solvents. Non-polar solvents
are typically hydrocarbon
s used to dissolve other naturally occurring hydrocarbon lipid molecule
s that do not (or do not easily) dissolve in water, including fatty acid
s, fat-soluble vitamin
s (such as vitamins A, D, E, and K), monoglyceride
s, and phospholipid
The functions of lipids include storing energy, signaling
, and acting as structural components of cell membrane
Lipids have applications in the cosmetic and food industries
as well as in nanotechnology
Scientists sometimes define lipids as hydrophobic
small molecules; the amphiphilic nature of some lipids allows them to form structures such as vesicles
, multilamellar/unilamellar liposome
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
Using this approach, lipids may be divided into eight categories: fatty acid
s, and polyketide
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
s, fats are a subgroup of lipids called triglyceride
s. Lipids also encompass molecules such as fatty acid
s and their derivatives (including tri-
s, and phospholipid
s), as well as other sterol
s such as cholesterol
Although humans and other mammals use various biosynthetic pathway
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.
''Lipid may be regarded as organic substances relatively insoluble in water, soluble in organic solvents(alcohol, ether etc.) actually or potentially related to fatty acid and utilized by the living cells.''
In 1815, Henri Braconnot
classified lipids (''graisses'') in two categories, ''suifs'' (solid greases or tallow) and ''huiles'' (fluid oils). In 1823, Michel Eugène Chevreul
developed a more detailed classification, including oils, greases, tallow, waxes, resins, balsams and volatile oils (or essential oils).
The first successful synthesis of a triglyceride molecule was by Théophile-Jules Pelouze
in 1844, when he produced tributyrin
by reacting butyric acid
in the presence of concentrated sulfuric acid
. Several years later, Marcellin Berthelot
, one of Pelouze's students, synthesized tristearin
by reaction of the analogous fatty acid
s with glycerin in the presence of gaseous hydrogen chloride
at high temperature.
In 1827, William Prout
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
discovered in human brain some phospholipids (cephalin
), glycolipids (cerebroside
) and sphingolipids (sphingomyelin
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, 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.
Even though the word ''lipide'' was unanimously approved by the international commission of the ''Société de Chimie Biologique'' during the plenary session on the 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
divided lipids into "simple lipids", with greases and waxes (true waxes, sterols, alcohols).
Lipids have been classified into eight categories by the Lipid MAPS
(an example of a leukotriene
, an eicosanoid fatty acid)
, or fatty acid residues when they are part of a lipid, are a diverse group of molecules synthesized by chain-elongation of an acetyl-CoA
primer with malonyl-CoA
groups in a process called fatty acid synthesis
They are made of a hydrocarbon chain
that terminates with a carboxylic acid
group; this arrangement confers the molecule with a polar
end, and a nonpolar, hydrophobic
end that is insoluble
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
s containing oxygen
, and sulfur
. If a fatty acid contains a double bond, there is the possibility of either a ''cis'' or ''trans'' geometric isomerism
, which significantly affects the molecule's configuration
. ''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
'', 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
s, derived primarily from arachidonic acid
and eicosapentaenoic acid
, that include prostaglandin
s, and thromboxane
s. Docosahexaenoic acid
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
s, fatty acid thioester coenzyme A
derivatives, fatty acid thioester ACP
derivatives and fatty acid carnitines. The fatty amides include N-acyl ethanolamines
, such as the cannabinoid
300px|Example of an unsaturated fat triglyceride (C55H98O6). Left part: glycerol
; right part, from top to bottom: [[palmitic acid, [[oleic acid, [[alpha-linolenic acid.
Glycerolipids are composed of mono-, di-, and tri-substituted [[glycerols,
the best-known being the fatty acid [[esters|triesters of glycerol, called triglycerides
. 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 animal tissues. The hydrolysis of the ester
bonds of triglycerides and the release of glycerol and fatty acids from adipose tissue
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
. Examples of structures in this category are the digalactosyldiacylglycerols found in plant membranes
and seminolipid from mammalian sperm cells
Glycerophospholipids, usually referred to as phospholipid
s (though sphingomyelin
s are also classified as phospholipids), are ubiquitous in nature and are key components of the lipid bilayer
as well as being involved in metabolism
and cell signaling
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
s and eubacteria, or the ''sn''-1 position in the case of archaebacteria
Examples of glycerophospholipids found in biological membrane
s are phosphatidylcholine
(also known as PC, GPCho or lecithin
(PE or GPEtn) and phosphatidylserine
(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
s and phosphatidic acid
s are either precursors of or, themselves, membrane-derived second messengers
. 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, as well as dialkylether variants in archaebacteria.
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
and a long-chain fatty acyl CoA, then converted into ceramide
s, phosphosphingolipids, glycosphingolipids and other compounds. The major sphingoid base of mammals is commonly referred to as sphingosine
. Ceramides (N-acyl-sphingoid bases) are a major subclass of sphingoid base derivatives with an amide
-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
s (ceramide phosphocholines),
whereas insects contain mainly ceramide phosphoethanolamines
and fungi have phytoceramide phosphoinositols and mannose
The glycosphingolipids are a diverse family of molecules composed of one or more sugar residues linked via a glycosidic bond
to the sphingoid base. Examples of these are the simple and complex glycosphingolipids such as cerebroside
s and ganglioside
s, such as cholesterol
and its derivatives, are an important component of membrane lipids,
along with the glycerophospholipids and sphingomyelins. Other examples of sterols are the bile acid
s and their conjugates,
which in mammals are oxidized derivatives of cholesterol and are synthesized in the liver. The plant equivalents are the phytosterols
, such as β-sitosterol
, and brassicasterol
; the latter compound is also used as a biomarker
The predominant sterol in fungal
cell membranes is ergosterol
Sterols are steroid
s in which one of the hydrogen atoms is substituted with a hydroxyl group
, 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
s and signaling molecules
. The eighteen-carbon (C18) steroids include the estrogen
family whereas the C19 steroids comprise the androgen
s such as testosterone
. The C21 subclass includes the progestogens
as well as the glucocorticoid
s and mineralocorticoids
. The secosteroid
s, comprising various forms of vitamin D
, are characterized by cleavage of the B ring of the core structure.
lipids are synthesized from the five-carbon-unit precursors isopentenyl diphosphate
and dimethylallyl diphosphate
that are produced mainly via the mevalonic acid
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
units. Structures containing greater than 40 carbons are known as polyterpenes. Carotenoid
s are important simple isoprenoids that function as antioxidant
s and as precursors of vitamin A
Another biologically important class of molecules is exemplified by the quinone
s and hydroquinone
s, which contain an isoprenoid tail attached to a quinonoid core of non-isoprenoid origin.
and vitamin K
, as well as the ubiquinone
s, are examples of this class. Prokaryotes synthesize polyprenols (called bactoprenol
s) in which the terminal isoprenoid unit attached to oxygen remains unsaturated, whereas in animal polyprenols (dolichol
s) the terminal isoprenoid is reduced.
s describe compounds in which fatty acids are linked to a nucleotide backbone, forming structures that are compatible with membrane bilayers. In the saccharolipids, a monosaccharide
substitutes for the glycerol backbone present in glycerolipids and glycerophospholipids. The most familiar saccharolipids are the acylated glucosamine
precursors of the Lipid A
component of the lipopolysaccharide
s in Gram-negative bacteria
. Typical lipid A molecules are disaccharides
of glucosamine, which are derivatized with as many as seven fatty-acyl chains. The minimal lipopolysaccharide required for growth in ''E. coli''
-Lipid A, a hexa-acylated disaccharide of glucosamine that is glycosylated with two 3-deoxy-D-manno-octulosonic acid (Kdo) residues.
Polyketides are synthesized by polymerization of acetyl
subunits by classic enzymes as well as iterative and multimodular enzymes that share mechanistic features with the fatty acid synthase
s. They comprise many secondary metabolite
s and natural products
from animal, plant, bacterial, fungal and marine sources, and have great structural diversity.
s are cyclic molecules whose backbones are often further modified by glycosylation
, or other processes. Many commonly used anti-microbial
, and anti-cancer
agents are polyketides or polyketide derivatives, such as erythromycin
s, and antitumor epothilone
cells feature the compartmentalized membrane-bound organelle
s that carry out different biological functions. The glycerophospholipids
are the main structural component of biological membranes
, as the cellular plasma membrane
and the intracellular membranes of organelle
s; in animal cells, the plasma membrane physically separates the intracellular
components from the extracellular
environment. The glycerophospholipids are amphipathic
molecules (containing both hydrophobic
regions) that contain a glycerol core linked to two fatty acid-derived "tails" by ester
linkages and to one "head" group by a phosphate
ester linkage. While glycerophospholipids are the major component of biological membranes, other non-glyceride lipid components such as sphingomyelin
s (mainly cholesterol
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. ]
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 lipid bilayer
. 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
; depending on the concentration
of the lipid, this biophysical interaction may result in the formation of micelle
, or lipid bilayer
s. Other aggregations are also observed and form part of the polymorphism of amphiphile
(lipid) behavior. Phase behavior
is an area of study within biophysics
and is the subject of current academic research.
Micelles and bilayers form in the polar medium by a process known as the hydrophobic effect
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
s to the lipophilic areas of the amphiphile
. So in an aqueous environment, the water molecules form an ordered "clathrate
" cage around the dissolved lipophilic molecule.
The formation of lipids into protocell
membranes represents a key step in models of abiogenesis
, the origin of life.
Triglycerides, stored in adipose tissue, are a major form of energy storage both in animals and plants. They are a major source of energy because carbohydrates are fully reduced structures. In comparison to glycogen which would contribute only half of the energy per its pure mass, triglyceride carbons are all bonded to hydrogens, unlike in carbohydrates. The adipocyte
, 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
The complete oxidation of fatty acids provides high caloric content, about 38 kJ/g (9 kcal/g
), compared with 17 kJ/g (4 kcal/g) for the breakdown of carbohydrate
s and protein
s. Migratory birds that must fly long distances without eating use stored energy of triglycerides to fuel their flights.
Evidence has emerged showing that lipid signaling
is a vital part of the cell signaling
Lipid signaling may occur via activation of G protein-coupled
or nuclear receptor
s, and members of several different lipid categories have been identified as signaling molecules and cellular messengers
These include sphingosine-1-phosphate
, a sphingolipid derived from ceramide that is a potent messenger molecule involved in regulating calcium mobilization,
cell growth, and apoptosis;
(DAG) and the phosphatidylinositol
phosphates (PIPs), involved in calcium-mediated activation of protein kinase C
, which are one type of fatty-acid derived eicosanoid involved in inflammation
the steroid hormones such as estrogen
, which modulate a host of functions such as reproduction, metabolism and blood pressure; and the oxysterol
s such as 25-hydroxy-cholesterol that are liver X receptor agonist
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
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.
The "fat-soluble" vitamins (A
) – 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
. Polyprenols and their phosphorylated derivatives also play important transport roles, in this case the transport of oligosaccharide
s across membranes. Polyprenol phosphate sugars and polyprenol diphosphate sugars function in extra-cytoplasmic glycosylation reactions, in extracellular polysaccharide biosynthesis (for instance, peptidoglycan
polymerization in bacteria), and in eukaryotic protein N-glycosylation
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
Lipids also form the basis of steroid hormones.
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.
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
and the esterification
of fatty acids in the production of triglycerides, a process called lipogenesis
. Fatty acids are made by fatty acid synthase
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
it to an alkene
group and then reduce it again to an alkane
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
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
and secreted from the liver.
The synthesis of unsaturated fatty acid
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
by stearoyl-CoA desaturase-1
produces oleic acid
. The doubly unsaturated fatty acid linoleic acid
as well as the triply unsaturated α-linolenic acid
cannot be synthesized in mammalian tissues, and are therefore essential fatty acid
s and must be obtained from the diet.
[Stryer ''et al.'', p. 643.]
Triglyceride synthesis takes place in the 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.
s and isoprenoids
, including the carotenoid
s, are made by the assembly and modification of isoprene
units donated from the reactive precursors isopentenyl pyrophosphate
and dimethylallyl pyrophosphate
These precursors can be made in different ways. In animals and archaea
, the mevalonate pathway
produces these compounds from acetyl-CoA,
while in plants and bacteria the non-mevalonate pathway
uses pyruvate and glyceraldehyde 3-phosphate
One important reaction that uses these activated isoprene donors is steroid biosynthesis
. Here, the isoprene units are joined together to make squalene
and then folded up and formed into a set of rings to make lanosterol
Lanosterol can then be converted into other steroids such as cholesterol
is the metabolic process by which fatty acids are broken down in the mitochondria
or in peroxisomes
to generate acetyl-CoA
. 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
, and oxidation
to form a beta-keto acid
, which is split by thiolysis
. The acetyl-CoA is then ultimately converted into ATP
, and H2
O using the citric acid cycle
and the electron transport chain
. 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
, and K
) and carotenoids
. Humans and other mammals have a dietary requirement for certain essential fatty acids, such as linoleic acid
(an omega-6 fatty acid
) and alpha-linolenic acid
(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
differing in the number and position of the double bonds. Most vegetable oil
s are rich in linoleic acid (safflower
, and corn
oils). Alpha-linolenic acid is found in the green leaves of plants, and in selected seeds, nuts, and legumes (in particular flax
, and soy
s are particularly rich in the longer-chain omega-3 fatty acids eicosapentaenoic acid
(EPA) and docosahexaenoic acid
[Bhagavan, p. 388.]
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
s, such as those present in partially hydrogenated vegetable oil
s, are a risk factor for cardiovascular disease
. Fats that are good for you can be turned into trans fats by overcooking.
A few studies have suggested that total dietary fat intake is linked to an increased risk of obesity
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 Harvard School of Public Health
, summarizes the current evidence on the impact 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."
* , a class of natural products composed of long aliphatic chains and phenolic rings that occur in plants, fungi and bacteria
IntroductoryList of lipid-related web sitesNature Lipidomics Gateway
– Round-up and summaries of recent lipid researchLipid Library
– General reference on lipid chemistry and biochemistryCyberlipid.org
– Resources and history for lipids.
– Modeling of Lipid MembranesLipids, Membranes and Vesicle Trafficking
– The Virtual Library of Biochemistry, Molecular Biology and Cell Biology
NomenclatureIUPAC nomenclature of lipids
– Comprehensive lipid and lipid-associated gene/protein databases.LipidBank
– Japanese database of lipids and related properties, spectral data and references.
– Provides dyslipidemia and cardiovascular disease prevention and treatment information as well as continuing medical education programsNational 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.
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