Structure and Physiological
Significance of Lipids
脂类的结构和功能
Deqiao Sheng PhD
Biochemistry Department
Summary


Definition
Classes（分类）
Fats :
Lipids 脂肪
脂类
Lipoids
类脂

Function（功能）
triacylglycerols(TG) 甘油三酯
cholesterol, Ch 胆固醇
cholesteryl ester, CE 胆固醇
phospholipids, PL 磷脂
glucolipids, GL 糖脂
Lipids 脂类, protein蛋白质and
carbohydrates碳水化合物



Like proteins and carbohydrates, lipids are
essential components（组分） of all living
organisms.
Unlike proteins and carbohydrates, however,
lipids have widely varied structures.
They are often defined as water-insoluble（水溶性）
organic compounds found in biological systems.
Lipids have high solubility in nonpolar organic
solvents（非极性有机溶剂）.
Definition


Lipids are non-polar (hydrophobic 疏水的)
compounds which can be soluble in organic
solvents.
Lipids have a variety of biological roles:
they serve as fuel molecules, highly
concentrated energy stores, signal molecules,
and components of membranes.
The biological functions of lipids
are diverse
1.
2.
Certain lipids (e.g., triacylglycerols) serve as
efficient reserves for the storage of energy.
--Storage lipids储存脂
Lipids (including mainly
glycerophospholipids, sphingolipids, and
sterols) are the major structural elements of
the biomembranes.
--Membrane lipids膜脂
3.
4.
The water-insoluble vitamins(维生素) like
vitamin A, D, E, K and some hormones
(like steroids（类脂）, prostaglandins（前
列腺素）) are lipids.
Lipids also serve as enzyme cofactors,
light-absorbing pigments, intracellular
messengers.
Lipids have diverse biological functions
as well as diverse structures
Biological membranes contain a variety of
amphipathic lipids, including glycerophospholipids and sphingolipids.
 In some organisms, triacylglycerols (fats
and oils) function as intracellular storage
molecules for metabolic energy.

Fats also provide animals with thermal
insulation and padding.
 Waxes in cell walls, exoskeletons, and skins
protect the surfaces of some organisms.
Some lipids have highly specialized
functions.

Lipids Functions
1.
2.
3.
4.
5.
6.
7.
Excellent energy reserves
Structure of cell membranes
Organ padding
Body thermal insulation
Essential fatty acids (EFA)
Hormone synthesis
Fat soluble vitamin absorption
Lipids Disorder
Lipids deficiency (Shortage in
Lipids intake)
 Lipids exceeding (Overtaking in
Lipids intake)

Lipids Deficiency
Fat should comprise of 3% of total
calories to prevent fatty acid deficiency
 Fatty acid deficiency syndromes

– Dry scaly skin, dermatitis (皮炎，Linoleic
acid deficiency)
– Hand tremors (Prostaglandin deficiency)
– Inability to control blood pressure
Lipids Exceeding


Fat should comprise not more than 30% of
total calories to prevent lipids exceeding.
To prevent overtaking, we should consume fat
breakdown (% total calories)
– <8% from saturated fat
– 10% from polyunsaturated fat
– 10-15% from monounsaturated fat
Health Problems
Energy Intake > Energy needed =
Lipids overtaking
 Develop medical problem

–
–
–
–
–
–
Cancer
Heart disease
Diabetes
Obesity
High blood pressure
High blood cholesterol
Lipids are classified as simple or
complex
1. Simple lipids: Esters of fatty acids with
various alcohols.
a. Fats: Esters of fatty acids with glycerol. Oils
are fats in the liquid state.
b. Waxes: Esters of fatty acids with higher
molecular weight monohydric alcohols.
2. Complex lipids: Esters of fatty acids
containing groups in addition to an alcohol
and a fatty acid.
a. Phospholipids: Lipids containing, in addition
to fatty acids and an alcohol, a phosphoric
acid residue. They frequently have nitrogen
containing bases and other substituents.
b. Glycolipids (glycosphingolipids): Lipids
containing a fatty acid, sphingosine, and
carbohydrate.
c. Other complex lipids: Lipids such as
sulfolipids and aminolipids. Lipoproteins may
also be placed in this category.
3. Precursor and derived lipids: These
include fatty acids, glycerol, steroids, other
alcohols, fatty aldehydes, and ketone
bodies, hydrocarbons, lipid-soluble
vitamins, and hormones.
Fatty acids Are Key
Constituents of Lipids
Fatty acids consist of a hydrocarbon chain
with a carboxylic acid at one end.
 Fatty acids occur mainly as esters in natural
fats and oils but do occur in the unesterified
form as free fatty acids（FFA）, a
transport form found in the plasma.

The chain may be saturated (containing no
double bonds) or unsaturated (containing
one or more double bonds).
 Fatty acids also play important roles in
signal-transduction pathways

16:0 palmitic acid
软脂酸；十六碳酸
A fatty acid is composed of a
long hydrocarbon chain
(“tail”) and a terminal
carboxyl group (or “head”).
Nomenclature
 Fatty acids (even number of carbon atoms）
 Saturated fatty acids
 palmitic
acid 16C
 stearic acid 18C

Unsaturated fatty acids
 Linolenic
acid 18C three unsaturated bonds
 Linoleate 18C two unsaturated bonds
 Arachidonic acid 20C four unsaturated bonds

Essential fatty acids
Required for the growth of mammals and they must
be obtained from food. Including linoleate（亚油酸）、
linolenate（亚麻酸）, arachidonic acid（花生四烯酸）
amount unsaturated in plant
The packing of
fatty acids into
stable aggregates
The Naming of Fatty Acids
Fatty acids can be referred to by either
International Union of Pure and Applied
Chemistry (IUPAC) names or common
names.
 Fatty acids are hydrocarbon chains of
various lengths and degrees of unsaturation
that terminate with carboxylic acid groups.


The number of carbon atoms in the most
abundant fatty acids ranges from 14 to 24 and is
almost always even since fatty acids are
synthesized by the sequential addition of twocarbon units.


In IUPAC nomenclature, the carboxyl carbon is labeled
C-1 and the remaining carbon atoms are numbered
sequentially.
In common nomenclature, Greek letters are used to
identify the carbon atoms. The carbon adjacent to the
carboxyl carbon (C-2 in IUPAC nomenclature) is
designated a, and the other carbons are lettered b, g, d,
e and so on .
g
4
b
a
3
2
O
C
1
O
fatty acid with a cis-9
double bond


The configuration of the double bonds in
unsaturated fatty acids is generally cis. In
IUPAC nomenclature, the positions of double
bonds are indicated by the symbol where the
superscript Δn indicates the lower-numbered
carbon atom of each double-bonded pair.
The notation 18:0 denotes a fatty acid with no
double 18 bonds, whereas 18:2 signifies that
there are two double bonds.


A shorthand notation for identifying fatty acids
uses two numbers separated by a colon; the first
refers to the number of carbon atoms in the fatty
acid, and the second refers to the number of
carbon–carbon double bonds, with their positions
indicated as superscripts following a Greek
symbol, Δ .
In this notation, palmitate is written as 16:0, oleate
as 18:1 Δ9 and arachidonate as 20:4 Δ5,8,11,14 .
g
4
b
a
3
2
O
C
1
O
fatty acid with a cis-9
double bond
There is free rotation about C-C bonds in the fatty acid
hydrocarbon, except where there is a double bond.
Each cis double bond causes a kink in the chain.
Rotation about other C-C bonds would permit a more
linear structure than shown, but there would be a kink.
Some fatty acids and their common names:
14:0 myristic acid; 16:0 palmitic acid; 18:0 stearic acid;
18:1 cis9 oleic acid(油酸)
18:2 cis9,12 linoleic acid(亚油酸)
18:3 cis9,12,15 a-linonenic acid （亚麻酸）
20:4 cis5,8,11,14 arachidonic acid(花生四烯酸)
20:5 cis5,8,11,14,17 eicosapentaenoic acid (二十碳五烯酸,
an omega-3)
Various conventions Δ use for indicating the number and
position of the double bonds ; eg Δ9 indicates a double bond
between carbons 9 and 10 of the fatty acid;
Some naturally occurring fatty acids
in animals
ω9 indicates a double bond on the ninth
carbon counting from the ω- carbon. In
animals additional double bonds are
introduced only between the existing double
bond (eg, ω9, ω6, or ω3) and the carboxyl
carbon, leading to three series of fatty acids
known as the ω9, ω6, and ω3 families,
respectively.
Essential Fatty Acids
Omega-3 (ω-3) and omega-6 (ω-6) fatty acids are
unsaturated “Essential Fatty Acids” . The two EFAs
are linolenic (ω-3) and linoleic (ω-6). The “3” and “6”
indicate where the first double bond occurs in the fatty
acid molecule.
Example:
DHA (docosahexenoic acid, C22:6,廿二碳六烯酸 )
and AA (arachidonic acid, C20:4, 花生四烯酸) are
both crucial to the optimal development of the
brain and eyes.
Fatty Acids Vary in Chain Length and
Degree of Unsaturation


Fatty acids in biological systems usually contain
an even number of carbon atoms, typically
between 14 and 24 . The 16- and 18-carbon fatty
acids are most common.
The properties of fatty acids and of lipids derived
from them are markedly dependent on chain
length and degree of saturation. Unsaturated fatty
acids have lower melting points than saturated
fatty acids of the same length.



Fatty acids that do not contain any carbon–carbon
double bonds are classified as saturated, whereas
those with at least one carbon–carbon double
bond are classified as unsaturated.
Unsaturated fatty acids with only one carbon–
carbon double bond are called monounsaturated,
and those with two or more are called
polyunsaturated.
The configuration of the double bonds in
unsaturated fatty acids is generally cis.
Chemical structures of three C18 fatty acids. (a)
Stearate (octadecanoate), a saturated fatty acid. (b)
Oleate (cis- Δ9 –octadecenoate), a
monounsaturated fatty acid. (c) Linolenate (all-cisΔ9,12,15 –octadecatrienoate), a polyunsaturated fatty
acid.
Triacylglycerols Are the Main
Storage Forms of Fatty acids
Fatty acids are generally stored as neutral
lipids called triacylglycerols.
 The triacylglycerols are esters of the
trihydric alcohol glycerol and fatty acids.
 Mono- and di-acylglycerols wherein one or
two fatty acids are esterified with glycerol
are also found in the tissues. These are of
particular significance in the synthesis and
hydrolysis of triacylglycerols.

Triacylglycerol (TG)
Triacylglycerols are composed of three
fatty acids each in ester linkage with
a single glycerol.
Most lipids in the average human diet are
triacylglycerols. These lipids are broken
down in the small intestine by the action of
lipases (pancreas).
 Pancreatic lipase catalyzes hydrolysis of the
primary esters (at C-1 and C-3) of
triacylglycerols, releasing fatty acids and
generating monoacyl-glycerols.


In mammals, most fat is stored in adipose
tissue, which is composed of specialized cells
known as adipocytes. Each adipocyte
contains a large fat droplet that accounts for
nearly the entire volume of the cell .
Adipose tissue
Large adipocytes (brown) are filled with fat droplets.
They are embedded in collagen matrix. Most cells
are close to capillaies (red).
Triacylglycerols Provide Stored Energy
and Insulation
In most eukaryotic cells, triacylglycerols
form a separate phase of microscopic, oily
droplets in the aqueous cytosol, serving as
depots of metabolic fuel.
 In vertebrates, specialized cells called
adipocytes, or fat cells, store large amounts of
triacylglycerols as fat droplets that nearly fill
the cell .

Humans have fat tissue (composed
primarily of adipocytes) under the skin, in
the abdominal cavity, and in the mammary
glands.
 Moderately obese people with 15 to 20 kg of
triacylglycerols deposited in their
adipocytes could meet their energy needs
for months by drawing on their fat stores.


In some animals, triacylglycerols stored
under the skin serve not only as energy
stores but as insulation against low
temperatures. Seals, walruses, penguins,
and other warm-blooded polar animals are
amply padded with triacylglycerols. In
hibernating animals (bears, for example),
the huge fat reserves accumulated before
hibernation serve the dual purposes of
insulation and energy storage .
Many Foods Contain Triacylglycerols

Most natural fats, such as those in vegetable
oils, dairy products, and animal fat, are
complex mixtures of simple and mixed
triacylglycerols.
Phospholipids Are the Major
Class of Membrane Lipids
There Are Three Common Types of Membrane Lipids
Sphingosine
Glycerophospholipids

Phospholipids may be regarded as
derivatives of phosphatidic acid, in which
the phosphate is esterified with the -OH of
a suitable alcohol.

Two fatty acids are attached in ester linkage to the
first and second carbons of glycerol, and a highly
polar or charged group is attached through a
phosphodiester linkage to the third carbon.
乙醇胺
胆碱
Structure of Phospholipid
Phosphatidylcholines (Lecithins, 卵磷脂)
Occur in Cell Membranes


Phosphoacylglycerols containing choline are
the most abundant phospholipids of the cell
membrane and represent a large proportion
of the body’s store of choline.
Choline is important in nervous
transmission, as acetylcholine(乙酰胆碱), and
as a store of labile methyl groups.
Sphingolipids
After glycerophospholipids, the most
abundant lipids in plant and animal
membranes are sphingolipids(神经鞘脂类).
 Sphingolipids are derivatives of sphingosine.
 In mammals, sphingolipids are particularly
abundant in tissues of the central nervous
system.

Sphingolipids
Glycolipids Are Important In
Nerve Tissues & In The Cell
Memberane



Glycolipids, as their name implies, are
sugar-containing lipids.
Glycolipids are widely distributed in every
tissue of the body, particularly in nervous
tissue such as brain.
They occur particularly in the outer leaflet
of the plasma membrane, where they
contribute to cell surface carbohydrates.
Sphingolipids

The major glycolipids found in animal
tissues are glycosphingolipids. They contain
ceramide and one or more sugars.
Structure of galactosylceramide (galactocerebroside, R
= H), and sulfogalactosylceramide (a sulfatide, R =
SO42- )
In sphingomyelins, phosphocholine is
attached to the C-1 hydroxyl group of a
Ceramide.
 Cerebrosides are glycosphingolipids that
contain one monosaccharide residue
attached by a linkage to C-1 of a ceramide.
 Gangliosides are more complex
glycosphingolipids in which oligosaccharide
chains containing N-acetylneuraminic acid
(NeuNAc) are attached to a ceramide.

Sphingolipids
Steroids Play Many
Physiologically Important Roles


Cholesterol (胆固醇) is a lipid with a
structure quite different from that of
phospholipids. It is a steroid (类固醇), built
from four linked hydrocarbon rings.
Cholesterol, the major sterol in animal
tissues, is amphipathic, with a polar head
group (the hydroxyl group at C-3) and a
nonpolar hydrocarbon body.
Steroids contain four fused
rings: three six-carbon rings
designated A, B, and C and a
five-carbon D ring.
The steroid nucleus
Cholesterol

Plant and animal food contain sterols (固醇)
but only animal food contain cholesterol(胆
固醇).
 Cholesterol is needed to make bile, sex
hormones, steroids and vitamin D.
 Sources – egg yolks, liver, shellfish, organ
foods
Cholesterol Is a Significant
Constituent of Many Tissues


Cholesterol is widely distributed in all cells
of the body but particularly in nervous
tissue.
It is a major constituent of the plasma
membrane and of plasma lipoproteins.


Cholesterol often accumulates in lipid
deposits (plaques) on the walls of blood
vessels. These plaques have been implicated
in cardiovascular disease, which can
precipitate heart attacks or strokes.
Many people limit their intake of cholesterol!


Despite its implication in cardiovascular
disease, cholesterol plays an essential role in
mammalian biochemistry.
Cholesterol is synthesized by mammalian
cells. It is not only a component of certain
membranes but also an essential precursor
of steroid hormones and bile salts.
Vitamin D3 production and metabolism.
(a) Cholecalciferol (vitamin D3) is produced in the skin by
UV irradiation of 7-dehydrocholesterol, which breaks the
bond shaded pink. In the liver, a hydroxyl group is added
at C-25 (pink); in the kidney, a second hydroxylation at C1 (pink) produces the active hormone, 1,25dihydroxycholecalciferol. This hormone regulates the
metabolism of calcium (Ca2+) in kidney, intestine, and bone.

(b) Dietary vitamin D prevents rickets(软骨病), a disease
once common in cold climates where heavy clothing
blocks the UV component of sunlight necessary for the
production of vitamin D3 in skin. On the left is a 2 1/2year-old boy with severe rickets; on the right, the same
boy at age 5, after 14 months of vitamin D therapy.
Biological Membranes Are Composed
of Lipid Bilayers and Proteins
Biological membranes define the external
boundaries of cells and separate
compartments within cells. They are
essential components of all living cells.
 A typical membrane consists of two layers
of lipid molecules and many embedded
proteins.

Membrane lipid and bilayer.
(a) An amphipathic membrane lipid.
(b) Cross-section of a lipid bilayer. The hydrophilic head groups
(blue) of each leaflet face the aqueous medium, and the
hydrophobic tails (yellow) pack together in the interior of the
bilayer.
Structure of a typical eukaryotic plasma membrane.
A lipid bilayer forms the basic matrix of biological membranes, and
proteins (some of which are glycoproteins) are associated with it in
various ways. The oligosaccharides of glycoproteins and glycolipids
are on the exterior surface of the membrane.
Biological membranes have a wide
variety of complex functions



Some proteins contained in membranes serve as
selective pumps that strictly control the transport
of ions and small molecules into and out of the cell.
Membranes are also responsible for generating and
maintaining the proton concentration gradients
essential for the production of ATP.
Receptors in membranes recognize extracellular
signals and communicate them to the cell interior.
SUMMARY
1. Lipids have the common property of being
relatively insoluble in water (hydrophobic) but
soluble in nonpolar solvents. Amphipathic lipids
also contain one or more polar groups, making
them suitable as constituents of membranes at
lipid:water interfaces.
2. The lipids of major physiologic significance are
fatty acids and their esters, together with
cholesterol and other steroids.
3. Long-chain fatty acids may be saturated,
monounsaturated, or polyunsaturated,
according to the number of double bonds
present. Their fluidity decreases with
chain length and increases according to
degree of unsaturation.
4. The esters of glycerol are quantitatively
the most significant lipids, represented by
triacylglycerol (“fat”), a major constituent
of lipoproteins and the storage form of
lipid in adipose tissue.
5. Glycolipids are also important
constituents of nervous tissue such as
brain and the outer leaflet of the cell
membrane, where they contribute to the
carbohydrates on the cell surface.
6. Cholesterol, an amphipathic lipid, is an
important component of membranes. It is
the parent molecule from which all other
steroids in the body, including major
hormones such as the adrenocortical and
sex hormones, D Vitamins, and bile acids,
are synthesized.