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Chapter 10 Lipids
1. Lipids encompass a large and diverse
group of organic compounds
1.1 Lipids are broadly defined as biological
molecules that are soluble in organic solvents.
1.1.1 Lipids are usually extracted from
biological materials by nonpolar solvents like
ether, chloroform（氯仿）, benzene（苯）.
1.2 The biological functions of lipids are
diverse.
1.2.1 Certain lipids (e.g., triacylglycerols
（三脂酰甘油）, commonly called fats) serve
as efficient reserves for the storage of energy.
1.2.2 Lipids (including mainly
glycerophospholipids （甘油磷脂）, sphingolipids,
and sterols) are the major structural elements of
the biomembranes.
1.2.3 The water-insoluble vitamins like
vitamin A, D, E, K and some hormones (like
steroids（类固醇）, prostaglandins（前列腺素）)
are lipids.
1.2.4 The bile acids help to solubilize (emulsify)
other classes of lipids for better digestion.
1.2.5 Lipids also serve as enzyme cofactors,
light-absorbing pigments, intracellular messengers.
Fat cells of guinea pig
A cotyledon cell from a seed of the plant
arabidopsis
Fatty acid composition of three food fats
Melting point
as affected by the proportion of saturated fat
Beewax: an ester of palmitic acid（软脂酸）
with the alcohol triacontanol
2. Fatty acids are a class of compounds
containing a long hydrocarbon chain and a
terminal carboxylate group
2.1 Fatty acids can be saturated or unsaturated.
2.1.1 Saturated fatty acids do not contain
carbon-carbon double bonds.
2.1.2 The two most common saturated fatty
acids are palmitic and stearic acids（软脂酸和硬
脂酸）, containing 16 and 18 carbons,
respectively (abbreviated as 16:0 and 18:0).
2.1.3 Unsaturated fatty acids contain one or
more carbon-carbon double bonds, usually of cis
configuration in the aliphatic（脂肪族的）
chains.
2.1.4 Positions of double bonds are
specified by superscript numbers following a 
symbol, e.g., linoleic acid（亚油酸） is
abbreviated as 18:2(9,12).
2.1.5 The number of carbons in a fatty acid
is commonly even and the positions of carboncarbon double bonds are regular (usually at
positions 9, 12, 15 (+3), separated by a methylene
group and never conjugate, that is, alternating
single and double bonds). (cis).
2.1.6 Oleic acid（油酸）, 18:1(9), is
the most common monounsaturated fatty
acid in mammals.
2.1.7 Linoleic acid, 18:2(9,12), and
linolenic acid（亚麻酸）, 18:3(9,12,15), can
not be synthesized by mammals and are
called essential fatty acids (like vitamins,
they have to be taken from food).
2.2 The physical properties of fatty acids are
mainly determined by the length and degree of
unsaturation on the hydrocarbon chain.
2.2.1 The shorter the hydrocarbon chain,
the lower its melting point (saturated fatty acids
of 10 or less carbon atoms are liquid, of 12 or
more carbons are waxy or solid at room
temperature).
2.2.2 Existence of carbon-carbon double
bonds lowers the melting point of fatty acids due
to the kink introduced by double bonds
(therefore preventing the tight packing).
2.3 Fatty acids are usually found as
components of complex lipids.
2.3.1 Fatty acids are major
components of the triacylglycerols, the
common storage lipids.
2.3.2 Fatty acids are also components
of the membrane phospolipids and
glycolipids.
2.3.3 A small amount of free fatty acids
are carried by serum albumin（血清白蛋白）
in the blood of vertebrate animals.
2.4 Triacylglycerols are fatty acid esters of
glycerol（甘油） and provide stored energy
and insulation.
2.4.1 Triacylglycerols are nonpolar,
hydrophobic, and essentially insoluble in
water.
2.4.2 Triacylglycerols serve as stored
energy in animal adipocytes (fat cells) and
plant seeds.
2.4.3 Oxidation of triacylglycerols
yields more than twice the amount of energy
than carbonhydrates or proteins, gram for
gram (due to the higher level of reduction of
the acyl（酰基） groups).
2.4.4 Triacylglycerols（三脂酰甘油） are
unhydrated and thus much lighter than
saccharides, which are highly hydrated (e.g.,
one gram of dry glycogen binds two grams of
water!).
2.4.5 Warm-blooded polar animals are
amply padded with triacylglycerols as
insulation against very low temperatures.
Stearic acid
Oleic acid or Oileate
3. Glycerophospholipids（甘油磷脂）,
sphingolipids（鞘脂）, and sterols（胆固醇）
are the three major types of lipids present in
biomembranes.
3.1 Glycerophospholipids are derivatives of
phosphatidic acid （磷脂酸）(i.e., 1,2diacylglycerol-3-phosphate)
3.1.1 Different polar alcohols are joined
to the phosphate（磷酸盐） group to make the
various glycerophospholipids.
3.1.2 Glycerophospholipids are usually
named for their head alcohol groups with the
prefix “phosphatidyl”: e.g.,
phosphatidylethanolamine, phosphatidylcholine,
phosphatidylserine, phosphatidyl glycerol,
phosphatidylinositol.
3.1.3 In cardiolipin（二磷脂酰甘油）, two
phosphatidic acids share a single glycerol (fig.).
3.1.4 In plasmalogens（缩醛磷脂） and
platelet-activating factor（PAF，血小板活化因
子）, one hydrocarbon chain form a ether
linkage to the glycerol backbone.
3.1.5 In general, a glycerophospholipid
（甘油磷脂） contains a saturated fatty
acid at C-1, and an unsaturated fatty acid at
C-2 (commonly 16 or 18 carbon long).
3.2 Sphingolipids are derivatives of
sphingosine（(神经) 鞘氨醇）.
3.2.1 Sphingosine is a long-chain amino
alcohol (with amino group at C-2, hydroxyl
groups at C-1 and C-3) having a carboncarbon double bond between C-4 and C-5.
3.2.2 The two -OH groups at C-1, C-3, and
the -NH2 amino group at C-2 are structurally
homologous with the three -OH groups of
glycerol in glycerophospholipids（甘油磷脂）.
3.2.3 The fundamental structural unit
common to all sphingolipids（神经鞘脂） is
ceramide（神经酰胺） having a fatty acid
attached to the 2-NH2 group in amide linkage.
3.2.4 Sphingolipids can be categorized as
three subclasses, differing in their polar head
groups: sphingomyelins（(神经)鞘磷脂）
(containing phosphoholine or
phosphoethanolamine), neutral glycolipids
(containing one or a few neutral sugars like glucose,
galactose), gangliosides (containing complex
oligosaccharides with at least one negatively
charged N-acetylneuraminic acid or sialic acid).
3.2.5 The glycolipids having only one sugar
residue (either galactose or glucose) at the head
are called cerebroside （脑苷脂）(being the
simplest glycolipids). (function?)
3.2.6 Both glycerophospholipids and
sphingomyelins are phospholipids with similar
molecular structures. (overlap in classification).
3.2.7 Sphingolipids are involved in
biological recognition: e.g., glycosphingolipids
(of various sugar groups at the end of the head
oligosaccharides) are part of the determinants of
the human blood groups A, B, and O. (cell
surface antigens?).
3.3 Sterols contain a characteristic steroid
nucleus of four fused hydrocarbon rings.
3.3.1 Three of the fused rings are sixmembered, one is five-membered.
3.3.2 The fusion of the rings make the C-C
bonds not rotatable, which in turn makes the
steroid nucleus almost planar and rigid.
3.3.3 Cholesterol（胆固醇） is the major
sterol in membranes of animal cells.
3.3.4 The carbon atoms of the cholesterol
are numbered.
3.3.5 The hydroxyl group at C-3 is the
polar head and the rest nonpolar part.
3.3.6 Sterols similar to cholesterol
are found in plants and fungi.
3.3.7 Bacteria often lack sterols.
3.3.8 Sterols serve as precursors
for a variety of products including bile
acids (with a polar side chain at C-17,
acting as emulsifying agents in animal
intestines) and many steroid hormones
(fig.).
3.3.9 Many Nobel Prizes have been
awarded to works related to cholesterol.
3.4 Specific enzymes degrade membrane lipids.
3.4.1 Phospholipases（磷脂酶） A1 and A2
hydrolyze the ester bonds of intact
glycerophospholipids at C-1 and C-2 of glycerol,
respectively.
3.4.2 Phospholipases C and D each
hydrolyze one of the two phosphodiester bonds in
the head group. (fig of the bonds broken).
3.4.3 Some inherited human diseases are
results of abnormal accumulations of membrane
lipids (e.g., Niemann-Pick disease, the Tay-Sachs
disease).
5. Some lipids have specific biological activities.
5.1 Cholesterol is the precursor of five major
classes of steroid hormones
5.1.1 Progestagens (such as progesterone)
prepare the uterus for implantation of an egg
and prevent ovulation during pregnancy.
5.1.2 Androgens, or male sex hormones
(such as testosterone) are responsible for the
development of male secondary sex
characteristics.
5.1.3 Estrogen, or female sex hormones
(such as estradiol) are reponsible for the
development of female secondary sex
characteristics.
5.1.4 Glucocorticoids（糖(肾上腺)皮质激素）
(such as cortisol) promotes gluconeogenesis.
5.1.5 Mineralocorticoids（盐皮质激素）
(such as aldosterone) act on the renal tubules to
increase the reabsorption of Na+ and the
excretion of K+ and H+, leading to an increase in
blood volume and blood pressure.
5.1.6 These steroid hormones are synthesized
from cholesterol mainly in the gonads and adrenal
cortex and carried in the bloodstream to target
tissues.
5.1.7 The steroid hormones bind to highly
specific receptors and move into cell nucleus to
regulate gene transcription.
5.2 Receptor-triggered hydrolysis of
phosphatidylinositol bisphosphate on plasma
membranes generates two secondary messengers.
5.2.1 Binding of certain hormones (e.g.,
vasopressin) to specific receptors on cell surfaces
leads to activation of the membrane bound
phospholipase C.
5.2.2 Phospholipase C catalyzes the
hydrolysis of phosphatidyl inositol 4,5bisphophate (PIP2) to form inositol 1,4,5triphosphate (IP3) and diacylglycerol (DAG).
5.2.3 IP3 further triggers release
of intracellular Ca2+, which leads to
activation of many Ca2+-dependent
enzymes.
5.2.4 DAG activates protein
kinase C, which further activates many
enzymes by phosphorylation.
5.3 Eicosanoids are derivatives of arachidonic
acid, 20:4(5,8,11,14), and have a variety of extremely
potent hormone-like actions on vertebrate tissues.
5.3.1 Arachidonic acid is released from
membrane phospholipids by the catalysis of
phospholipase A2 in response to certain hormone
signals.
5.3.2 Three major classes of eicosanoids
have been identified.
5.3.3 Prostaglandins （前列腺素）(PG)
contain a characteristic five-carbon ring. (fig.).
5.3.4 Prostaglandins seem to have multiple
functions including stimulation of contraction of
smooth muscles, raising body temperature
(producing fever), causing inflammation,
regulating blood flow to particular organs,
generating wake-sleep cycle, resulting in pain.
5.3.5 Thromboxanes（血栓素，凝血噁烷）
(TX) contain a characteristic six-membered ether
ring.
5.3.6 Thromoboxanes are synthesized by
platelets and act in blood clotting.
5.3.7 Leukotrienes（白三烯,白细胞三烯，白
细胞血管收缩素） contain three characteristic
conjugated double bonds.
5.3.8 Overproduction of leukatrienes causes
asthmatic and anaphylactic shocks.
5.3.9 Sue Bergstrom, Bengt Ingemar
Samuelsson, and John Robert Vane won the 1982
Nobel Prize in medicine or physiology for studying
prostaglandins and related bioactive compounds.
5.4 Vitamins A, D, E, K are all isoprenoid（类异
戊二烯(的)） compounds synthesized by the
condensation of isoprene units (also precursors
of sterols).
5.4.1 Vitamin A (retinol) is usually derived
from b-carotenoids, light-absorbing pigments
existing in many yellow-colored plants (like
carrots, sweet potatoes).
5.4.2 Vitamin A (11-trans-retinol) is the
precursor of 11-cis-retinal, an essential pigment
for vision (the cofactor of rhodopsin（视紫质, 视
网膜紫质）).
5.4.3 Vitamin D is derived from 7dehydrocholesterol（脱氢胆甾醇）in the skin
by irradiation.
5.4.4 Vitamin D is the precursor of 1,25dihydroxycholecalciferol, a hormone regulating
the uptake of calcium and phosphate (in what
forms from where, kidney, intestine, bone).
5.4.5 Vitamin D deficiency in children
produces rickets (resulting from inadequate
calcification of cartilage and bone).
5.4.6 Vitamin E (also called tocopherols)
contain a substituted aromatic ring and a long
hydrocarbon side chain.
5.4.7 Vitamin E was first recognized as an
compound that prevented sterility in rats.
5.4.8 The function of vitamin E is still not
clear, but is believed to act as an antioxidant to
prevent peroxidation of polyunsaturated fatty
acids.
5.4.9 Vitamin K is a quinone（醌，苯醌）
needed for blood clotting.
5.4.10 Vitamin K acts as a cofactor in
converting Glu residues into gcarboxylglutamate in prothrombin（凝血素）,
modifications making prothrombin a much
stronger Ca2+ chelator（螯合剂）.
5.4.11 Binding of Ca2+ helps the proteolytic
（ (分)解蛋白的,蛋白水解的） activation of
prothrombin into thrombin（凝血酶）, which
converts fibrinogen（纤维蛋白原） into fibrin
（ (血)纤维蛋白, (血)纤维） which forms blood
blots.
5.4.12 George Wald share the 1967 Nobel
Prize in medicine or physiology for his studies on
vitamin A.
5.4.13 Adolf Otto Reinhold Windaus won the
1928 Nobel Prize in Chemistry for discovering the
cholesterol derived from vitamin D3.
5.4.14 Henrik Dam and Edward A. Doisy won
the 1943 Nobel Prize in medicine or physiology for
their studies on vitamin K.
Vitamin D
Skin to liver
to kidney
4. Membrane lipids are amphipathic（两性分子
的 ） and form ordered structures
spontaneously in water
4.1 All membrane lipids contain a polar
(hydrophilic) head and a nonpolar (hydrophobic)
tail.
4.1.1 Membrane lipids are usually
represented by a circle head and one or two
attached wavy or straight lines as the tails.
4.2 The amphipathic membrane lipids form
ordered structures in water.
4.2.1 Amphipathic lipids form oriented
monolayers at air-water interfaces.
(experiment?).
4.2.2 Fatty acids, lysophospholipids
(glycerophospholipids lacking one fatty acyl
group) forms the globular micelles in water.
4.2.3 Phospholipids and glycolipids in
aqueous media favorably form bimolecular
sheets (lipid bilayers) rather than micelles. This
is because the two fatty acyl tails are too bulky
to fit into the interior of a micelle. (cylindricalshaped versus wedge-shaped).
4.2.4 The formation of lipid bilayers from
phospholipids and glycolipids is rapid and
spontaneous, stabilized by the full array of
weak interactions.
4.2.5 Lipid bilayers have an inherent
tendency to be extensive (due to diffusion?
function?).
4.2.6 Lipid bilayers tend to close
themselves (to limit the amount exposed
hydrocarbon chains), generating artificial
structures called liposomes（脂质体）.
4.2.7 Lipid bilayers are self-sealing（自动封
口） because a hole in a bilayer is energetically
unfavorable (driven by hydrophobic interaction
and diffusion).
4.3 Liposomes can be used to carry membrane
impermeable substances into cells.
4.3.1 Water-soluble substances (e.g., proteins,
nucleic acids, drugs) can be encapsulated into
liposomes.
4.3.2 Liposomes can fuse with cell plasma
membranes (a lipid bilayer), releasing substances
into cells (can be used as drug delivery tools).
4.3.3 Liposomes are used as model systems to
study membrane permeability (or membrane
protein reconstitution).