5t4
l7 Lipids
CHAPTER
,sphingosine
Triglyceride
Phosphoglyceride
Sphingomyelin
Glycolipid
Figure17.5
Lipidclassification
diagram.
17.4Structureo:f liposomesond cellmembranes
AIMS: To sketchsedionsof the liposomalhiloyerin woter,labeling
the polor end of the lipid molecules.
Toexplointhe relationship betweenthe degreeof unsoturotionin phospholipid
moleculesond membraneflexibility.To usethe fluid mosoic
modelto describethe movementof lipid molecules
in
membranes.
Complexlipids 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 moleculesform microscopicspheresrather than dispersingevenly
in water. These lipid spheres,or liposomes, Are packages of water surrounded by alipid,bilayer - 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-
17.4 Structure of Liposomes and Cell Membranes
Outer aqueous
Hydrophilic heads
envlronmenf
Figure17.6
Cross-section
of a liposome.
Thewholeliposome
is spherical.
somes are stable structures.When broken to exposethe lipid hydrocarbons
to water, liposomes spontaneously reseal. Lipid bilayers are tight, so any
leakageof the contents of a liposome to the outside is quite slow.
Figure17.7
The space-filling
modelsshow
that the moleculeof the saturated fatty acid palmiticacid (a) is
straighterthan that of the unsaturatedfatty acid oleic acid (b). The
lighly orderedpackingof three
n\oleculesof palmiticacid (c)
is disruptedby the presenceof a
moleculeof oleic acid between
two moleculesof palmiticacid (d).
Forthis reason,the phospholipids
rich in saturatedfatty acidsmake
more tightly packed,lessflexible
membranes(e) than thoserich in
unsaturatedfatty acids (f).
PRACTICE
EXERCISE
I7.6
Predict the result if the liposome experiment (shaking phosphatidyl
choline in a solvent) is repeated using a nonpolar solvent (such as carbon tetrachloride) instead of water.
Structure
of cell membranes
Cell membranesalso consist of lipid bilayerssimilar to those describedfor
liposomes. Some cell membranes are more flexible than others, due to differing properties of the different fatty acids of the lipids that make the lipid
bilayers. Lipid bilayers made of phospholipids having a high percentage of
unsaturated fatty acids are more flexible than those having a high percentage of saturated fatty acids.As shoum in Figure 17.7,unsaturated fatty acid
chains are bent and fit into bilayers more loosely than saturated fatty acid
chains. The looser the packing in the bilayer, the more flexible is the membrane. A high percentage of glycolipids also tends to increase membrane
flexibility.
afffifr
(a) Palmitic acid
(saturated)
(b) Oleic acid
(unsaturated)
uuuu
uu
(f)
556
CHAPTER
17 Lipids
The fluid mosaic model of membrane structure
The flurd mosaic model of membrane structure proposesthat lipids of tln
bilayer are in constant motion, gliding from one pert of their bilayer to
another at high speed.Althoughlipids move freelywithin their ornn layer of
the bilayer, they cannot easily cross or flip-flop to the other lipid layer (see
Fig. l7.B).
Stuckin the massof movinglipids are protein molecules,some moving
and some apparently anchored in place. Figure 17.g shows two kinds of
membrane proteins: peripheral and integral. Peripheral proteins perch on
either side of the lipid bilayer They can usually be removed from the membrane by high salt concentrations. Integral proteins may be partial\.
embeddedin one side of the bilayer or jut all the way through.
Many integral proteins are glycoproteins-protein molecules with
attached carbohydrates.The carbohydrate portion of the embedded glycoproteins is found at the surface of the bilayer, where its hydroxyl groups can
hydrogen bond with water. The protein portion of membrane glycoproteins
is usually hydrophobic. This ensures a favorable interaction of the protein
with the hydrophobic lipid tails of the bilayer. Many membrane proteins
facilitate the transport of ions and molecules into and out of the cell.
Figure17.8
Lipidmolecules
moveeasilywithin
theirown layerof the bilayerbut
do not readilyflip-flopto the other
layer.
Figure17.9
Fluidmosaicmodel.Mostmembraneproteins
areintegral;
some
membrane
proteins
areperipheral.
Thebeadlikestructures
attachedto
the proteinsrepresent
the carbohydrateportionsof glycoproteins.
Lipid molecules
cannot cross easily
from one layer
to another