Essential Cell Biology 5th edition

A:28 Answers
Internal hydrogen bonds between the peptide bonds
stabilize the α helix and β barrel.
ANSWER 11–5 The sulfate group in SDS is charged and
therefore hydrophilic. The OH group and the C–O–C groups
in Triton X-100 are polar; they can also form hydrogen
bonds with water molecules and are therefore hydrophilic.
In contrast, the red portions of the detergents are either
hydrocarbon chains or aromatic rings, neither of which has
polar groups that could form hydrogen bonds with water
molecules; they are therefore hydrophobic. (One example of
a tripeptide with hydrophobic side chains is shown in Figure
A11–5.)
Figure A11–5
O H water
H
CH 3 CH 3
H H
molecules
hydrogen
O
CH
bond
CH
O
3
H
H
H
C N C C OH O
H H H
O H 2 N C C N C
H
H
H
CH
O
O
CH 3 CH 2
H
O H
CH H
3
O
H
H
O
H
valine isoleucine alanine
ANSWER 11–6 Some of the transmembrane proteins are
anchored to the spectrin filaments of the cell cortex. These
molecules are not free to ECB5 rotate EA11.05/A11.05
or diffuse within the plane
of the membrane. There is an excess of transmembrane
proteins over the available attachment sites in the cortex,
however, so some of the transmembrane protein molecules
are not anchored. These proteins are free to rotate
and diffuse within the plane of the membrane. Indeed,
measurements of protein mobility show that there are two
populations of each transmembrane protein, corresponding
to those proteins that are anchored and those that are not.
ANSWER 11–7 The different ways in which membrane
proteins can be restricted to different regions of the
membrane are summarized in Figure 11–31. The mobility
of the membrane proteins is drastically reduced if they are
bound to other proteins such as those of the cell cortex
or the extracellular matrix. Some membrane proteins are
confined to membrane domains by barriers, such as tight
junctions. The fluidity of the lipid bilayer is not significantly
affected by the anchoring of membrane proteins; the sea of
lipid molecules flows around anchored membrane proteins
like water around the posts of a pier.
ANSWER 11–8 All of the statements are correct.
A. The lipid bilayer is fluid because its lipid can undergo
these motions.
B. The lipid bilayer is fluid because its lipid can undergo
these motions.
C. Such exchanges require the action of a transporter.
D. Hydrogen bonds are formed and broken by thermal
motion.
E. Glycolipids are mostly restricted to the monolayer
of membranes that faces away from the cytosol.
Some special glycolipids, such as phosphatidylinositol
(discussed in Chapter 16), are found specifically in the
cytosolic monolayer.
F. The reduction of double bonds (by hydrogenation)
allows the resulting saturated lipid molecules to pack
more tightly against one another and therefore increases
viscosity—that is, it turns oil into margarine.
G. Examples include the many membrane enzymes involved
in signaling (discussed in Chapter 16).
H. Polysaccharides are the main constituents of mucus and
slime; the carbohydrate coat of a cell, which is made
up of polysaccharides and oligosaccharides, is a very
important lubricant, particularly for cells that line blood
vessels or circulate in the bloodstream.
ANSWER 11–9 In a two-dimensional fluid, the molecules
are free to move only in one plane; the molecules in a
normal fluid, in contrast, can move in three dimensions.
ANSWER 11–10
A. You would have a detergent. The diameter of the lipid
head would be much larger than that of the hydrocarbon
tail, so that the shape of the molecule would be a
cone rather than a cylinder and the molecules would
aggregate to form micelles rather than bilayers.
B. The lipid bilayers formed would be much more fluid.
The bilayers would also be less stable, as the shorter
hydrocarbon tails would be less hydrophobic, so the
forces that drive the formation of the bilayer would be
reduced.
C. The lipid bilayers formed would be much less fluid.
Whereas a normal lipid bilayer has the viscosity of olive
oil, a bilayer made of the same lipids but with saturated
hydrocarbon tails would have the consistency of bacon
fat.
D. The lipid bilayers formed would be much more fluid.
Also, because the lipids would pack together less well,
there would be more gaps and the bilayer would be
more permeable to small, water-soluble molecules.
E. If we assume that the lipid molecules are completely
intermixed, the fluidity of the membrane would be
unchanged. In such bilayers, however, the saturated lipid
molecules would tend to aggregate with one another
because they can pack so much more tightly and would
therefore form patches of much-reduced fluidity. The
bilayer would not, therefore, have uniform properties
over its surface. Because in membrane lipid molecules,
one saturated and one unsaturated hydrocarbon tail
are typically linked to the same hydrophilic head, such
segregation does not occur in cell membranes.
F. The lipid bilayers formed would have virtually unchanged
properties. Each lipid molecule would now span the
entire membrane, with one of its two head groups
exposed at each surface. Such lipid molecules are found
in the membranes of thermophilic bacteria, which can
live at temperatures approaching boiling water. Their
bilayers do not come apart at elevated temperatures, as
usual bilayers do, because the original two monolayers
are now covalently linked into a single membrane.
ANSWER 11–11 Phospholipid molecules are approximately
cylindrical in shape. Detergent molecules, by contrast,
are conical or wedge-shaped. A phospholipid molecule
with only one hydrocarbon tail, for example, would be
a detergent. To make a phospholipid molecule into a
detergent, you would have to make its hydrophilic head
larger or remove one of its tails so that it could form a
micelle. Detergent molecules also usually have shorter
hydrocarbon tails than lipid molecules. This makes them