
Lipids are a broad group of organic compounds which include
fats,
waxes,
sterol
A sterol is any organic compound with a Skeletal formula, skeleton closely related to Cholestanol, cholestan-3-ol. The simplest sterol is gonan-3-ol, which has a formula of , and is derived from that of gonane by replacement of a hydrogen atom on ...
s,
fat-soluble vitamins (such as vitamins
A,
D,
E and
K),
monoglycerides,
diglycerides,
phospholipids, and others. The functions of lipids include storing energy,
signaling, and acting as structural components of
cell membranes.
Lipids have applications in the
cosmetic and
food industries, and in
nanotechnology.
Lipids are broadly defined as
hydrophobic or
amphiphilic small molecules; the amphiphilic nature of some lipids allows them to form structures such as
vesicles, multilamellar/
unilamellar liposomes, 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 groups.
Using this approach, lipids may be divided into eight categories:
fatty acyls,
glycerolipid
Lipids are a broad group of organic compounds which include 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 others. The fu ...
s,
glycerophospholipids,
sphingolipids,
saccharolipids, and
polyketides (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 fats, fats are a subgroup of lipids called
triglycerides. Lipids also encompass molecules such as
fatty acid
In chemistry, in particular 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 ...
s and their derivatives (including tri-, di-, monoglycerides, and phospholipids), as well as other
sterol
A sterol is any organic compound with a Skeletal formula, skeleton closely related to Cholestanol, cholestan-3-ol. The simplest sterol is gonan-3-ol, which has a formula of , and is derived from that of gonane by replacement of a hydrogen atom on ...
-containing
metabolites such as
cholesterol
Cholesterol is the principal sterol of all higher animals, distributed in body Tissue (biology), tissues, especially the brain and spinal cord, and in Animal fat, animal fats and oils.
Cholesterol is biosynthesis, biosynthesized by all anima ...
.
Although humans and other mammals use various
biosynthetic pathways both to break down and to synthesize lipids, some essential lipids cannot be made this way and must be obtained from the diet.
History
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 synthetic triglyceride was reported by
Théophile-Jules Pelouze in 1844, when he produced
tributyrin by treating
butyric acid
Butyric acid (; from , meaning "butter"), also known under the systematic name butanoic acid, is a straight-chain alkyl carboxylic acid with the chemical formula . It is an oily, colorless liquid with an unpleasant odor. Isobutyric acid (2-met ...
with
glycerin in the presence of concentrated
sulfuric acid. Several years later,
Marcellin Berthelot, one of Pelouze's students, synthesized
tristearin and
tripalmitin by reaction of the analogous
fatty acid
In chemistry, in particular 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 ...
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 "
lecithins".
Thudichum 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.
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 consortium
as follows:
Fatty acyls

Fatty acyls, a generic term for describing fatty acids, their conjugates and derivatives, are a diverse group of molecules synthesized by chain-elongation of an
acetyl-CoA primer with
malonyl-CoA or
methylmalonyl-CoA groups in a process called
fatty acid synthesis.
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 Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...
group; this arrangement confers the molecule with a
polar,
hydrophilic 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
In organic chemistry, a functional group is any 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 r ...
s containing
oxygen,
halogens,
nitrogen
Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
, 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
eicosanoids, derived primarily from
arachidonic acid and
eicosapentaenoic acid, that include
prostaglandins,
leukotrienes, and
thromboxanes.
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 esters, 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 neurotransmitter
anandamide.
Glycerolipids
Glycerolipids are composed of mono-, di-, and tri-substituted
glycerol
Glycerol () is a simple triol compound. It is a colorless, odorless, sweet-tasting, viscous liquid. The glycerol backbone is found in lipids known as glycerides. It is also widely used as a sweetener in the food industry and as a humectant in pha ...
s,
the best-known being the fatty acid
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

Glycerophospholipids, usually referred to as
phospholipids (though
sphingomyelins are also classified as phospholipids), are ubiquitous in nature and are key components of the
lipid bilayer of cells,
as well as being involved in
metabolism
Metabolism (, from ''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 cellular processes; the co ...
and
cell signaling
In biology, cell signaling (cell signalling in British English) is the Biological process, process by which a Cell (biology), cell interacts with itself, other cells, and the environment. Cell signaling is a fundamental property of all Cell (biol ...
.
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
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s and eubacteria, or the ''sn''-1 position in the case of
archaebacteria.
Examples of glycerophospholipids found in
biological membranes are
phosphatidylcholine (also known as PC, GPCho or
lecithin),
phosphatidylethanolamine (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
phosphatidylinositols and
phosphatidic acids 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.
Sphingolipids

Sphingolipids 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
Ceramides are a family of waxy lipid molecules. A ceramide is composed of sphingosine and a fatty acid joined by an amide bond. Ceramides are found in high concentrations within the cell membrane of Eukaryote, eukaryotic cells, since they are co ...
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
sphingomyelins (ceramide phosphocholines),
whereas insects contain mainly ceramide phosphoethanolamines
and fungi have phytoceramide phosphoinositols and
mannose-containing headgroups.
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
cerebrosides and
gangliosides.
Sterols

Sterols, such as
cholesterol
Cholesterol is the principal sterol of all higher animals, distributed in body Tissue (biology), tissues, especially the brain and spinal cord, and in Animal fat, animal fats and oils.
Cholesterol is biosynthesis, biosynthesized by all anima ...
and its derivatives, are an important component of membrane lipids,
along with the glycerophospholipids and sphingomyelins. Other examples of sterols are the
bile acids 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,
stigmasterol, and
brassicasterol; the latter compound is also used as a
biomarker for
algal growth.
The predominant sterol in
fungal cell membranes is
ergosterol.
Sterols are
steroid
A steroid is an organic compound with four fused compound, fused rings (designated A, B, C, and D) arranged in a specific molecular configuration.
Steroids have two principal biological functions: as important components of cell membranes t ...
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
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 or tissues by complex biological processes to regulate physio ...
s and
signaling molecules. The eighteen-carbon (C18) steroids include the
estrogen family whereas the C19 steroids comprise the
androgen
An androgen (from Greek ''andr-'', the stem of the word meaning ) is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This includes ...
s such as
testosterone and
androsterone. The C21 subclass includes the
progestogens as well as the
glucocorticoids and
mineralocorticoids.
The
secosteroids, comprising various forms of
vitamin D, are characterized by cleavage of the B ring of the core structure.
Prenols
Prenol lipids are synthesized from the five-carbon-unit precursors
isopentenyl diphosphate and
dimethylallyl diphosphate, which are produced mainly via the
mevalonic acid (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 ≥ 2. Terpenes are major biosynthetic building blocks. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predomi ...
units. Structures containing greater than 40 carbons are known as polyterpenes.
Carotenoids are important simple isoprenoids that function as
antioxidants and as precursors of
vitamin A.
Another biologically important class of molecules is exemplified by the
quinones and
hydroquinones, which contain an isoprenoid tail attached to a quinonoid core of non-isoprenoid origin.
Vitamin E and
vitamin K
Vitamin K is a family of 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 ...
, as well as the
ubiquinones, 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 (
dolichols) the terminal isoprenoid is reduced.
Saccharolipids
Saccharolipids 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 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
lipopolysaccharides in
Gram-negative bacteria
Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the Crystal violet, crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelo ...
. 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'' is Kdo
2-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 and
propionyl subunits by classic enzymes as well as iterative and multimodular enzymes that share mechanistic features with the
fatty acid synthases. They comprise many
secondary metabolites and
natural products from animal, plant, bacterial, fungal and marine sources, and have great structural diversity.
Many
polyketides are cyclic molecules whose backbones are often further modified by
glycosylation,
methylation,
hydroxylation,
oxidation
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
, or other processes. Many commonly used
antimicrobial,
antiparasitic, and
anticancer agents are polyketides or polyketide derivatives, such as
erythromycins,
tetracyclines,
avermectins, and antitumor
epothilones.
Biological functions
Component of biological membranes
Eukaryotic
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
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 th ...
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 organelles; in animal cells, the plasma membrane physically separates the
intracellular 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 molecules (containing both hydrophobic and hydrophilic regions) that contain a glycerol core linked to two fatty acid-derived "tails" by ester linkages and to one "head" group by a
phosphate
Phosphates are the naturally occurring form of the element phosphorus.
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...
ester linkage. While glycerophospholipids are the major component of biological membranes, other non-glyceride lipid components such as
sphingomyelin and
sterol
A sterol is any organic compound with a Skeletal formula, skeleton closely related to Cholestanol, cholestan-3-ol. The simplest sterol is gonan-3-ol, which has a formula of , and is derived from that of gonane by replacement of a hydrogen atom on ...
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. ] and sulfoquinovosyldiacylglycerol,
which lack a phosphate group, are important components of membranes of chloroplasts and related organelles and are among 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
micelles,
liposomes, or
lipid bilayers. Other aggregations are also observed and form part of the polymorphism of
amphiphile (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 biological phenomena. Biophysics covers all scales of biological organization, from molecular to organismic and populations ...
.
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 bonds 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.
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
carbohydrates 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 metab ...
s. 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.
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 event involving a lipid messenger that binds a protein target, such as a receptor (biochemistry), receptor, kinase or phosphatase, which in turn mediate the effects of these ...
is a vital part of the
cell signaling
In biology, cell signaling (cell signalling in British English) is the Biological process, process by which a Cell (biology), cell interacts with itself, other cells, and the environment. Cell signaling is a fundamental property of all Cell (biol ...
.
Lipid signaling may occur via activation of
G protein-coupled or
nuclear receptors, 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;
diacylglycerol and the
phosphatidylinositol phosphates (PIPs), involved in calcium-mediated activation of
protein kinase C;
the
prostaglandins, which are one type of fatty-acid derived eicosanoid involved in
inflammation and
immunity;
the steroid hormones such as
estrogen,
testosterone and
cortisol, 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 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 R ...
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
flippases 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. Polyprenols and their phosphorylated derivatives also play important transport roles, in this case the transport of
oligosaccharides 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.
Cardiolipins 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.
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 and the
esterification of fatty acids in the production of triglycerides, a process called
lipogenesis.
Fatty acids are made by
fatty acid synthases 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
plastids 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 acids 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 acids and must be obtained from the diet.
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.
Terpene
Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n ≥ 2. Terpenes are major biosynthetic building blocks. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predomi ...
s and
isoprenoids, including the
carotenoids, 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
Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
, the
mevalonate pathway produces these compounds from acetyl-CoA,
while in plants and bacteria the
non-mevalonate pathway uses pyruvate and
glyceraldehyde 3-phosphate as substrates.
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 and ergosterol.
Degradation
Beta oxidation 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,
hydration, and
oxidation
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
to form a
beta-keto acid, which is split by
thiolysis. The acetyl-CoA is then ultimately converted into
adenosine triphosphate (ATP), CO
2, and H
2O 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,
D,
E, 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 oils are rich in linoleic acid (
safflower,
sunflower, and
corn 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. In 2022, France produced 75% of t ...
,
rapeseed,
walnut, and
soy).
Fish oils are particularly rich in the longer-chain omega-3 fatty acids
eicosapentaenoic acid and
docosahexaenoic acid.
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 fats, such as those present in
partially hydrogenated vegetable oil
Vegetable oils, or vegetable fats, are oils extracted from seeds or from other parts of edible plants. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed oils, o ...
s, are a risk factor for
cardiovascular disease. 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;
Others, 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, United States. Founded in 1636 and named for its first benefactor, the History of the Puritans in North America, Puritan clergyma ...
, 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
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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