Lipids (Greek letter: lipos means fat) are substances of biological origin that are soluble in organic solvents such as chloroform and methanol but are only sparingly soluble, if at all, in water. Fatty Acids Waxes Triacylglycerols Phosphoglycerides Sphingolipids Sterols And Terpenes

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ASSIGNMENT
ON
LIPIDS: STRUCTURE AND
FUNCTION
SUBJECT: BIOMOLECULES
SUBMITTED TO: SUBMITTED BY:
PROFESSOR M.Sc. BIOCHEMISTRY(SEMESTER I)
DEPT. OF BIOCHEMISTRY DEPT. OF BIOCHEMISTRY
FACULTY OF LIFE SCIENCES FACULTY OF LIFE SCIENCES
JAMIA HAMDARD UNIVERSITY JAMIA HAMDARD UNIVERSITY

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Introduction
Lipids (Greek letter: lipos means fat) are substances of biological origin that are soluble in
organic solvents such as chloroform and methanol but are only sparingly soluble, if at all, in
water. Hence, they are easily separated from other biological materials by extraction into
organic solvents and may be further fractionated by such techniques as adsorption
chromatography, thin layer chromatography, and reverse-phase chromatography. Fats, oils,
certain vitamins and hormones, and most non-protein membrane components are lipids.
Fats and oils are typical lipids in terms of their solubility, but that fact does not really define
their chemical nature. In terms of chemistry, lipids are a mixed bag of compounds that share
some properties based on structural similarities, mainly a preponderance of non-polar groups.
Classified according to their chemical nature, lipids fall into two main groups. One group,
which consists of open-chain compounds with polar head groups and long nonpolar tails,
includes fatty acids (waxes), triacylglycerols, sphingolipids, phosphoacylglycerols, and
glycolipids. The second major group consists of fused ring compounds, the steroids; an
important representative of this group is cholesterol.
A. Fatty Acids
Fatty acids are carboxylic acids with long-chain hydrocarbon side groups. They are rarely
free in nature but, rather, occur in esterified form as the major components of the various
lipids. In higher plants and animals, the predominant fatty acid residues are those of the C16
and C18 species palmitic, oleic, linoleic, and stearic acids. Fatty acids with <14 or >20
carbon atoms are uncommon. Most fatty acids have an even number of carbon atoms because
they are usually biosynthesized by the concatenation of C2 units. Over half of the fatty acid
residues of plant and animal lipids are unsaturated (contain double bonds) and are often
polyunsaturated (contain two or more double bonds). Bacterial fatty acids are rarely
polyunsaturated but are commonly branched, hydroxylated, or contain cyclopropane rings.
Unusual fatty acids also occur as components of the oils and waxes (esters of fatty acids and
long-chain alcohols) produced by certain plants.
a. The Physical Properties of Fatty Acids Vary with Their Degree of Unsaturation
The first double bond of an unsaturated fatty acid commonly occurs between its C9 and C10
atoms counting from the carboxyl C atom (a Δ9- or 9-double bond). In polyunsaturated fatty
acids, the double bonds tend to occur at every third carbon atom toward the methyl terminus
of the molecule (such as ¬CH=CH¬CH2¬CH=CH¬). Double bonds in polyunsaturated fatty
acids are almost never conjugated (as in ¬CH=CH¬CH=CH¬). Triple bonds rarely occur in
fatty acids or any other compound of biological origin. Two important classes of
polyunsaturated fatty acids are denoted n – 3 (or ω – 3) and n – 6 (or ω – 6) fatty acids. This
nomenclature identifies the last double-bonded carbon atom as counted from the methyl
terminal (ω) end of the chain.
Saturated fatty acids are highly flexible molecules that can assume a wide range of
conformations because there is relatively free rotation about each of their C¬C bonds.

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Nevertheless, their fully extended conformation is that of minimum energy because this
conformation has the least amount of steric interference between neighboring methylene
groups. The melting points (mp) of saturated fatty acids, like those of most substances,
increase with molecular mass.
Fatty acid double bonds almost always have the cis configuration. This puts a rigid 30° bend
in the hydrocarbon chain of unsaturated fatty acids that interferes with their efficient packing
to fill space. The consequent reduced van der Waals interactions cause fatty acid melting
points to decrease with their degree of unsaturation. Lipid fluidity likewise increases with the
degree of unsaturation of their component fatty acid residues. This phenomenon has
important consequences for membrane properties.
b. Waxes Serve as Energy Stores and Water Repellents
Biological waxes are esters of long-chain (C14 to C36) saturated and unsaturated fatty acids
with long-chain (C16 to C30) alcohols. Their melting points (60 to 100 ̊ C) are generally
higher than those of triacylglycerols. In plankton, the free-floating microorganisms at the
bottom of the food chain for marine animals, waxes are the chief storage form of metabolic
fuel.
Triacontanoylpalmitate, the major component of beeswax, is an ester of palmitic acid
with the alcohol triacontanol.
Waxes also serve a diversity of other functions related to their water-repellent properties and
their firm consistency. Certain skin glands of vertebrates secrete waxes to protect hair and
skin and keep it pliable, lubricated, and waterproof. Birds, particularly waterfowl, secrete
waxes from their preen glands to keep their feathers water-repellent. The shiny leaves of
holly, rhododendrons, poison ivy, and many tropical plants are coated with a thick layer of
waxes, which prevents excessive evaporation of water and protects against parasites.
Biological waxes find a variety of applications in the pharmaceutical, cosmetic, and other
industries. Lanolin (from lamb’s wool), beeswax, carnauba wax (from a Brazilian palm tree),
and wax extracted from spermaceti oil (from whales) are widely used in the manufacture of
lotions, ointments, and polishes.

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B. Triacylglycerols
The fats and oils that occur in plants and animals consist largely of mixtures of
triacylglycerols (also referred to as triglycerides or neutral fats). These nonpolar, water-
insoluble substances are fatty acid triesters of glycerol:
Triacylglycerols function as energy reservoirs in animals and are therefore their most
abundant class of lipids even though they are not components of biological membranes.
Triacylglycerols differ according to the identity and placement of their three fatty acid
residues. The so-called simple triacylglycerols contain one type of fatty acid residue and are
named accordingly. For example, tristearoylglycerol or tristearin contains three stearic acid
residues, whereas trioleoylglycerol or triolein has three oleic acid residues. The more
common mixed triacylglycerols contain two or three different types of fatty acid residues and
are named according to their placement on the glycerol moiety.
Fats and oils (which differ only in that fats are solid and oils are liquid at room temperature)
are complex mixtures of simple and mixed triacylglycerols whose fatty acid compositions
vary with the organism that produced them. Plant oils are usually richer in unsaturated fatty
acid residues than are animal fats, as the lower melting points of oils imply.

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a. Triacylglycerols Are Efficient Energy Reserves and Insulation
Fats are a highly efficient form in which to store metabolic energy. This is because fats are
less oxidized than are carbohydrates or proteins and hence yield significantly more energy on
oxidation. Furthermore, fats, being nonpolar substances, are stored in anhydrous form,
whereas glycogen, for example, binds about twice its weight of water under physiological
conditions. Fats therefore provide about six times the metabolic energy of an equal weight of
hydrated glycogen. Complete oxidation of fats yields about 9 kcal g⁻¹, in contrast with 4 kcal
g⁻¹ for carbohydrates and proteins.
In animals, adipocytes (fat cells) are specialized for the synthesis and storage of
triacylglycerols. Whereas other types of cells have only a few small droplets of fat dispersed
in their cytosol, adipocytes may be almost entirely filled with fat globules. Adipose tissue is
most abundant in a subcutaneous layer and in the abdominal cavity. The fat content of normal
humans (21% for men, 26% for women) enables them to survive starvation for 2 to 3 months.
In contrast, the body’s glycogen supply, which functions as a short-term energy store, can
provide for the body’s metabolic needs for less than a day. The subcutaneous fat layer also
provides thermal insulation, which is particularly important for warm-blooded aquatic
animals, such as whales, seals, geese, and penguins, which are routinely exposed to low
temperatures.
b. Hydrolysis of triacylglycerols
When an organism uses fatty acids, the ester linkages of triacylglycerols are hydrolyzed by
enzymes called lipases. The same hydrolysis reaction can take place outside organisms, with

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acids or bases as catalysts. When a base such as sodium hydroxide or potassium hydroxide is
used, the products of the reaction, which is called saponification , are glycerol and the sodium
or potassium salts of the fatty acids. These salts are soaps. When soaps are used with hard
water, the calcium and magnesium ions in the water react with the fatty acids to form a
precipitate—the characteristic scum left on the insides of sinks and bathtubs. The other
product of saponification, glycerol, is used in creams and lotions as well as in the
manufacture of nitroglycerin.
C. Phosphoglycerides
Glycerophospholipids (or phosphoglycerides) are the major lipid components of biological
membranes. They consist of sn-glycerol-3-phosphate esterified at its C1 and C2 positions to
fatty acids and at its phosphoryl group to a group, X, to form the class of substances.
Glycerophospholipids are therefore amphiphilic molecules with nonpolar aliphatic “tails”
and polar phosphoryl-X “heads.” The simplest glycerophospholipids, in which X = H, are
phosphatidic acids; they are present only in small amounts in biological membranes. In the
glycerophospholipids that commonly occur in biological membranes, the head groups are
derived from polar alcohols. Saturated C16 and C18 fatty acids usually occur at the C1
position of glycerophospholipids, and the C2 position is often occupied by an unsaturated
C16 to C20 fatty acid. Glycerophospholipids are, of course, also named according to the
identities of these fatty acid residues. Some glycerophospholipids have common names. For
example, phosphatidylcholines are known as lecithins; diphosphatidylglycerols, the “double”
glycerol phospholipids, are known as cardiolipins (because they were first isolated from heart
muscle).
Plasmalogens are glycerophospholipids in which the C1 substituent to the glycerol moiety is
bonded to it via an α,β-unsaturated ether linkage in the cis configuration rather than through
an ester linkage. Ethanolamine, choline, and serine form the most common plasmalogen head
groups.

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The common glycerophospholipids
a. Some Glycerophospholipids Have Ether-Linked Fatty Acids
Some animal tissues and some unicellular organisms are rich in ether lipids, in which one of
the two acyl chains is attached to glycerol in ether, rather than ester, linkage. The ether-linked
chain may be saturated, as in the alkyl ether lipids, or may contain a double bond between C-
1 and C-2, as in plasmalogens. Vertebrate heart tissue is uniquely enriched in ether lipids;
about half of the heart phospholipids are plasmalogens. The membranes of halophilic
bacteria, ciliated protists, and certain invertebrates also contain high proportions of ether
lipids. The functional significance of ether lipids in these membranes is unknown; perhaps
their resistance to the phospholipases that cleave ester-linked fatty acids from membrane
lipids is important in some roles.

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At least one ether lipid, platelet-activating factor, is a potent molecular signal. It is released
from leukocytes called basophils and stimulates platelet aggregation and the release of
serotonin (a vasoconstrictor) from platelets. It also exerts a variety of effects on liver, smooth
muscle, heart, uterine, and lung tissues and plays an important role in inflammation and the
allergic response.
b. Chloroplasts Contain Galactolipids and Sulfolipids
The second group of membrane lipids are those that predominate in plant cells: the
galactolipids, in which one or two galactose residues are connected by a glycosidic linkage to
C-3 of a 1,2-diacylglycerol. Galactolipids are localized in the thylakoid membranes (internal
membranes) of chloroplasts; they make up 70% to 80% of the total membrane lipids of a
vascular plant, and are therefore probably the most abundant membrane lipids in the
biosphere. Phosphate is often the limiting plant nutrient in soil, and perhaps the evolutionary
pressure to conserve phosphate for more critical roles favored plants that made phosphate-
free lipids. Plant membranes also contain sulfolipids, in which a sulfonated glucose residue is
joined to a diacylglycerol in glycosidic linkage. The sulfonate group bears a negative charge
like that of the phosphate group in phospholipids.
Two galactolipids of chloroplast thylakoid membranes.

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c. Archaea Contain Unique Membrane Lipids
Some archaea that live in ecological niches with extreme conditions—high temperatures
(boiling water), low pH, high ionic strength, for example—have membrane lipids containing
long-chain (32 carbons) branched hydrocarbons linked at each end to glycerol. These
linkages are through ether bonds, which are much more stable to hydrolysis at low pH and
high temperature than are the ester bonds found in the lipids of bacteria and eukaryotes. In
their fully extended form, these archaeal lipids are twice the length of phospholipids and
sphingolipids, and can span the full width of the plasma membrane. At each end of the
extended molecule is a polar head consisting of glycerol linked to either phosphate or sugar
residues. The general name for these compounds, glycerol dialkyl glycerol tetraethers
(GDGTs), reflects their unique structure. The glycerol moiety of the archaeal lipids is not the
same stereoisomer as that in the lipids of bacteria and eukaryotes; the central carbon is in the
R configuration in archaea, in the S configuration in bacteria and eukaryotes.
An unusual membrane lipid found only in some archaea.
D. Sphingolipids
Sphingolipids, which are also major membrane components, are derivatives of the C18 amino
alcohols sphingosine, dihydrosphingosine, and their C16, C17, C19, and C20 homologs.
Their N-acyl fatty acid derivatives, ceramides, occur only in small amounts in plant and
animal tissues but form the parent compounds of more abundant sphingolipids:
1. Sphingomyelins, the most common sphingolipids, are ceramides bearing either a
phosphocholine or a phosphoethanolamine moiety, so that they can also be classified as
sphingophospholipids. Although sphingomyelins differ chemically from phosphatidylcholine
and phosphatidylethanolamine, their conformations and charge distributions are quite similar.

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The membranous myelin sheath that surrounds and electrically insulates many nerve cell
axons is particularly rich in sphingomyelin.
2. Cerebrosides, the simplest sphingoglycolipids (alternatively glycosphingolipids), are
ceramides with head groups that consist of a single sugar residue. Galactocerebrosides, which
are most prevalent in the neuronal cell membranes of the brain, have a β-D-galactose head
group.
Glucocerebrosides, which instead have a β-D-glucose residue, occur in the membranes of
other tissues. Cerebrosides, in contrast to phospholipids, lack phosphate groups and hence are
most frequently nonionic compounds. The galactose residues of some galactocerebrosides,
however, are sulfated at their C3 positions to form ionic compounds known as sulfatides.
More complex sphingoglycolipids have unbranched oligosaccharide head groups of up to
four sugar residues.
3. Gangliosides form the most complex group of sphingoglycolipids. They are ceramide
oligosaccharides that include among their sugar groups at least one sialic acid residue (N-
acetylneuraminic acid and its derivatives). The structures of gangliosides GM1, GM2, and
GM3, three of the hundreds that are known.

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Gangliosides are primarily components of cellsurface membranes and constitute a significant
fraction (6%) of brain lipids. Other tissues also contain gangliosides but in lesser amounts.
Gangliosides have considerable physiological and medical significance.Their complex
carbohydrate head groups, which extend beyond the surfaces of cell membranes, act as
specific receptors for certain pituitary glycoprotein hormones that regulate a number of
important physiological functions. Gangliosides are also receptors for bacterial protein toxins
such as cholera toxin. There is considerable evidence that gangliosides are specific
determinants of cell–cell recognition, so they probably have an important role in the growth
and differentiation of tissues as well as in carcinogenesis (cancer generation). Disorders of
ganglioside breakdown are responsible for several hereditary sphingolipid storage diseases,
such as Tay-Sachs disease, which are characterized by an invariably fatal neurological
deterioration
E. Sterols And Terpenes
Steroids, which are mostly of eukaryotic origin, are derivatives of cyclopentanoperhydro-
phenanthrene. The much maligned cholesterol, the most abundant steroid in animals, is
further classified as a sterol because of its C3-OH group and its branched aliphatic side chain
of 8 to 10 carbon atoms at C17.
Cholesterol is a major component of animal plasma membranes, where it is typically present
at 30 to 40 mol %, and occurs in lesser amounts in the membranes of their subcellular
organelles. Its polar OH group gives it a weak amphiphilic character, whereas its fused ring
system provides it with greater rigidity than other membrane lipids. Cholesterol is therefore
an important determinant of membrane properties. It is also abundant in blood plasma
lipoproteins, where ~ 70% of it is esterified to long-chain fatty acids to form cholesteryl
esters.

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Cholesterol is the metabolic precursor of steroid hormones, substances that regulate a great
variety of physiological functions including sexual development and carbohydrate
metabolism, the much-debated role of cholesterol in heart disease, Cholesterol metabolism
and the biosynthesis of steroid hormones.
Plants contain little cholesterol. Rather, the most common sterol components of their
membranes are stigmasterol and β-sitosterol which differ from cholesterol only in their
aliphatic side chains. Yeast and fungi have yet other membrane sterols such as ergosterol,
which has a C7 to C8 double bond. Prokaryotes, with the exception of mycoplasmas, contain
little, if any, sterol.

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REFERENCES
 NELSON D., COX M.: Principles of Biochemistry-LEHNINGER (2013), SIXTH
EDITION, W. H. Freeman and Company, New York.
Chapter 10: Lipids; Page No. 357-370
ISBN-13: 978-1-4641-0962-1
 CAMPBELL M., FARRELL S.: Biochemistry (2012), SEVENTH EDITION,
Brooks/Cole, Cengage Learning, CA, USA.
Chapter 8: Lipids and Proteins Are Associated in Biological Membranes; Page No.
193-199
ISBN-13: 978-0-8400-6858-3
 VOET D., VOET J.: Biochemistry(2011), FOURTH EDITION, JOHN WILEY &
SONS , INC., USA.
Chapter 12: Lipids and Membranes; Page No. 386-393
ISBN 13 978-0470-57095-1
 Eoin Fahy, Dawn Cotter, Manish Sud, and Shankar Subramaniam: Lipid
classification, structures and tools, Published online 2011 Jun 16.
PMCID: PMC3995129
Direct Link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3995129/