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17.3 Lipid Composition of Cell Membranes
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17.5Lipid compositionof cell membrones
AIM: Torecognizethe generol structuresof the following three
phosphoglycerides,
typesof lipid molecules:
sphingomyelins,
ond glycolipids.
Cellular membranes are made
primarily of complex lipids.
It is possible to break cells,empty them of their contents, and isolate the cell
membranes.The cellmembrane is the "snck"that holds the contentsof cells
and actsas a selectiuebarrier for the passageofcertain substancesin and out
of the cell. The interior of cells also contains membrane structures, as
described in A Closer Look Cells. Chemical analysis of the isolated membranes shows that lipids are the major components. These lipids are not
triglycerides, but another group of compounds called complex lipids. Complex lipids contain parts madefrom substancesbesidesfatty acids and glycerol; some contain no glycerol.The complex lipids fall into two categories:
phospholipids and glycolipids.
PhosphoHpids are lipids that are estersof phosphoric acid. There are
two main types of phospholipid molecules in cell membranes: phospho- y
glycerides and sphingomyelins.
Phosphoglycerrde moleculesare builtfrom long-chainfatty acids (14 to
24 carbons), glycerol, and phosphoric acid. Two fatty acids are covalently
bonded to adjacent hydroxyl groups ofglycerol by ester linkages.The phosphoric acid is bonded through phosphate ester linkages to the remaining
hydroxyl function of glycerol. The resulting molecule is called a phosphatidic acid, which is a phosphoglyceride.
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cH2-o-cR
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o
CH-OH
+ 2RC-OH
o
+ HO-P-OH
I
OH
o
+
cH_o_cR
o
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cH2oH
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Glycerol
Fatty acids
Phosphoric
acid
Aphosphatidic
acid
+ 3H2O
550
CHAPTERl7 Lipids
Cells
The two major cell designs are prokaryotic and
eukaryotic. The former is the more ancient of the
two. Microscopic examination of fossilized.remains shows that prokaryotes were present on
Earth at least 3 billion years ago, whereas eukaryotes did not appear until 2 billion yearslater. In the
modern world, the prokaryotic cell design is limited to bacteria and blue-greenalgae.The cells of
other cellular organisms, including green plants
and people, are eukaryotic.
Endoplasmic
reticulum
Mito chondrion_",.----
Golgibody
"'=-.-.-.-/./
I
Nucleus
',
' 1",' ',
Both types of cells are essentially packagesof
chemicalsnecessaryforlife encasedin a cell membrane. Eukaryotic cells are considerably larger and
somewhat more complicated than prokaryotes,
but the chemical processescarried out by both
typesof cellsare very similar,and both are exceedingly efflcient chemicalfactories.The major feature that distinguishesprokaryotes from eukaryotes is the latter's organelles("little organs")-small
membrane-enclosedbodies suspended in the
interior cellular fluid or cltoplasm {seefigure). The
organelles are the sites of many specialized functions in eukaryotes.The most prominent membrane-encasedorganellesand their functions are
asfollows:
Organelle
Function
Nucleus
Mitochondrion
Cellreproduction
Production of most cellular
energy in cells using oxygen
for respiration
Processing of proteins into
glycoproteins
Golgi body
Lysosome
Lysosome
Cell membrane
Theeukaryotic
cell
Digestion of cellular wastes
and substancestaken into
cells
Yet another membrane structure in eukaryotes is the highly folded, netlike endoplasmic reticulum (ER).Among its variousfunctions,the ER
servesas an attachment site for ribosomes-small
organellesthat are not membrane-encased
but
are the sites where proteins are made. Endoplasmic reticulawith and without attachedribosomes
are called, respectively,rough ER and smooth ER
becauseof their appearanceunder a microscope.
Living cells contain little or no free phosphatidic acid. Usually, the
phosphorus of the phosphatidic acid is linked to the hydroxyl group of a
second alcohol. Choline, a common amino alcohol constituent of cell
membrane phospholipids, is an example.In Figure 17.2, the hydroxyl
group of choline is attached to the phosphorus of the phosphatidic acid
through a phosphateesterbond. This phospholipidis a phosphoglyceride
17.3 Lipid Compositionof Cell Membranes
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Aphosphatidyl choline
(or lecithin)
#
Charged head
// \
\ Hydrophobic tail
/
Figure17.2
A phosphatidyl
(phosphoglyceride)
choline
or lecithinmoleculeis constructed
from
the aminoalcoholcholine,
phosphoric
acid,glycerol,
andtwo fattyacidmolecules.
A
simplifiedrepresentation
of the molecule(lowerleft) hasbeenadoptedfor all membranelipids.Thehydrophilic
(charged)
headis shownasa purplesphere,
andthe
hydrophobic
tailsasredwavylines.A space-filling
modeloi a phosphatidyl
choline
moleculeis shownat the lowerrightof the figure.
\
becauseit contains phosphorus and has a backbone of glycerol. It also
m1y be called a phosphatidyl choline becauseit is an ester,of phosphatidic acid and choline . An older name for phosphatidyl choline;J leciitrin.
It is important to recognize that the names phosphoglyceride,phosphatidic acid, and phosphatidyl choline are only general names for classesof
compounds. The lengths of the hydrocarbon chains of the fatty acids may
vary and these chains may be saturated or contain one or more double
bonds.
5t2
CHAPTER
l7 Lipids
-,:-: PRACTICE
EXERCISE
I7.5
'.4-;
:= When the phosphorus of a phosphatidic acid is linked to the hydroxyl
group of the amino alcohol ethanolamine, NH2CH2CH2OH,the com"== pound formed is phosphatidyl ethanolamine. Phosphatidyl ethanolamine, also called cephalin, is found in brain tissue and is important in
=
= blood clotting. Draw the general structure for cephalin.
The second type of phospholipid moleculei encountered in cell rnetnbranes ls sphingomyelins. Sphingomyelins do not contain glycerol.
Instead, they contain sphingosine, a long-chain unsaturated amino alcohol. Only one fatty acid is attached to sphingosine, as shovrrnin Figure
17.3, through an amide linkage. The structure of the nonpolar end of
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Fatty acid
*-
Ho- cH, - H'- :; -
*
""
L.-
: cHcH2cH2cH zcH2cHzcH2clr2cH2
cH2cH zcH2cHzcH
s
Phosphoric
acid
Sphingosine
(an amino alcohol)
?"
CHz-CH-CH-CH:CHCH2CH2CH,CH2CH.CH.CH2CH.CH.CH.CH?CH2CH3
Asphingomyelin
p-cCharged head
/
\ Hydrophobictail
Figure17.5
A sphingomyelin
moleculeconsists
of one moleculeeachof choline,phosphoric
acid,sphingosine,
anda fattyacid.A space-filling
modelof a sphingomyelin
moleculeis shownat the bottomof the figure.
17.3 Lipid Compositionof Cell Membranes
The abnormal metabolism and
accumulation of certain types of
lipid molecules occur in a number
of genetic diseases.For example,
a glycolipid accumulates and
damagesthe brain in Tay-Sachs
disease.
555
sphingomyelins may differ somewhat, depending on the length and t
degree of saturation of the fatty acid attached to the sphingosine amino\
group. Sphingomyelins are the only phospholipids that are not built on ' \
glycerol. Large amounts of sphingomyelins are found in brain and nervous tissue and in the myelin sheath,the protective coat of nerves.
Glycolipids are lipid molecules that contain carbohydrates, usually
simple sugars such as glucose or galactose.Figure 17.4 shows a glycolipid
that consists of sphingosine, a fatty acid, and a sugar. These are called
cerebrosidesbecause
of their abundancein the brain. The cerebrosidesare
not phospholipids, becausethey do not contain phosphorus.
The classificationof lipids is summarizedin Figure 17.5.
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(anaminoalcohol)
Glucose
cHz-cH-
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CH- CH:CHCHzCH2CH2CH2CH2CH.CH.CH2CH2CH'CH2CH,CH3
A cerebroside
#
// \
Polar head
\ Hydrophobic tail
Figure17.4
(glycolipid)
A cerebroside
consists
of a sugar,a sphingosine,
andonefatty
acidmolecule.
Thesugarunitshownis glucose,
but it maybe galactose'
A
space-filling
modelof a cerebroside
is shownat the bottomof the figure.
CHAPTER
17 Lipids
Sphingosine
Triglyceride
Phosphoglyceride
Sphingomyelin
Glycolipid
Figure17.5
Lipidclassification
diagram.
17,4Structareof liposomesond cellmembrones
AIMS: To sketchsecfionsof the liposomolbiloyerin woter,lobeling
Toexplointhe relotionthe polar end of the lipid molecules.
ship betweenthe degreqof unsoturotionin phospholipid
molecules
ond membraleflexibility.To usethe fluid mosoic
in
modelto describethe movementof lipid molecules
membrones.
Complex Hpids form the lipid
bilayers of liposomes and cell
membranes.
Phosphatidyl choline is a typical membrane phospholipid. It contains a
charged head consisting of negatively charged phosphate and positively
charged choline attached through glycerol to two hydrophobic fatty acid
tails. If we vigorously shake a mixture of phosphatidyl choline and water,
the lipid molecules form microscopic spheresrather than dispersing evenly
in water. These lipid spheres,or liposomes, are packages of water surrounded by alipid,bilayet-a two-layer-thick wall of phosphatidyl choline.
Figure 17.6showsa crosssection of a liposome.The lipid moleculesof the
liposomal bilayer are more ordered than the sulfonic acid molecules in
detergentmicelles (Sec.14.3).
All the hydrophobic hydrocarbon tails of the lipids are protected from
water, and all the hydrophilic phospholipid heads interact with water. Lipo-