Essential Cell Biology 5th edition

Answers A:29
slightly water-soluble, so that detergent molecules leave
and reenter micelles frequently in aqueous solution. Because
of this, some monomeric detergent molecules are always
present in aqueous solution and therefore can enter the
lipid bilayer of a cell membrane to solubilize the proteins
(see Figure 11–27).
ANSWER 11–12
A. There are about 4000 lipid molecules, each 0.5 nm wide,
between one end of the bacterial cell and the other.
So if a lipid molecule at one end moved directly in a
straight line it would require only 4 × 10 –4 sec (= 4000
× 10 –7 ) to reach the other end. In reality, however, the
lipid molecule would move in a random path, so that it
would take considerably longer. We can calculate the
approximate time required from the equation: t = x 2 /2D,
where x is the average distance moved, t is the time
taken, and D is a constant called the diffusion coefficient.
Inserting step values x = 0.5 nm and t = 10 –7 sec, we
obtain D = 1.25 × 10 –8 cm 2 /sec. Using this value in the
same equation but with distance x = 2 × 10 –4 cm (= μm)
gives the time taken t = 0.16 seconds.
B. Similarly, if a 4-cm-diameter ping-pong ball exchanged
partners every 10 –7 seconds and moved in a linear
fashion, it would reach the opposite wall in
1.5 × 10 –5 sec, traveling at 1,440,000 km/hr. [But a
random walk would take longer. Using the equation
above, we calculate the constant D in this case to be
8 × 10 7 cm 2 /sec and the time required to travel
6 m about 2 msec (= 600 2 /1.6 × 10 8 ).]
ANSWER 11–13 Transmembrane proteins anchor the plasma
membrane to the underlying cell cortex, strengthening the
membrane so that it can withstand the forces on it when
the red blood cell is pumped through small blood vessels.
Transmembrane proteins also transport nutrients and ions
across the plasma membrane.
ANSWER 11–14 The hydrophilic faces of the five membranespanning
α helices, each contributed by a different subunit,
are thought to come together to form a pore across the
lipid bilayer that is lined with the hydrophilic amino acid
side chains (Figure A11–14). Ions can pass through this
hydrophilic pore without coming into contact with the lipid
tails of the bilayer. The hydrophobic side chains interact with
the hydrophobic lipid tails.
Figure A11–14
HYDROPHILIC PORE
hydrophilic face
hydrophobic face
lipid bilayer
ANSWER 11–15 There are about 100 lipid molecules (i.e.,
phospholipid ECB5 + cholesterol) EA11.14/A11.14 for every protein molecule
in the membrane [(2/50,000)/(1/800 + 1/386)]. A similar
protein/lipid ratio is seen in many cell membranes.
ANSWER 11–16 Membrane fusion does not alter the
orientation of the membrane proteins with their attached
color tags: the portion of each transmembrane protein that
is exposed to the cytosol always remains exposed to the
cytosol, and the portion exposed to the outside always
remains exposed to the outside (Figure A11–16). At 0°C,
the fluidity of the membrane is reduced, and the mixing of
the membrane proteins is significantly slowed.
Figure A11–16
ANSWER 11–17 The exposure of hydrophobic amino acid
side chains to water is energetically unfavorable. There are
two ways that ECB5 such EA11.16/A11.16
side chains can be sequestered away
from water to achieve an energetically more favorable
state. First, they can form transmembrane segments that
span a lipid bilayer. This requires about 20 of them to be
located sequentially in a polypeptide chain. Second, the
hydrophobic amino acid side chains can be sequestered
in the interior of the folded polypeptide chain. This is one
of the major forces that lock the polypeptide chain into
a unique three-dimensional structure. In either case, the
hydrophobic forces in the lipid bilayer or in the interior of a
protein are based on the same principles.
ANSWER 11–18 (A) Antarctic fish live at subzero
temperatures and are cold-blooded. To keep their
membranes fluid at these temperatures, they have a high
percentage of unsaturated phospholipids.
ANSWER 11–19 Sequence B is most likely to form
a transmembrane helix. It is composed primarily of
hydrophobic amino acids, and therefore can be stably
integrated into a lipid bilayer. In contrast, sequence A
contains many polar amino acids (S, T, N, Q), and sequence
C contains many charged amino acids (K, R, H, E, D), which
would be energetically disfavored in the hydrophobic
interior of the lipid bilayer.
ANSWER 11–20 Triacylglycerol is an entirely hydrophobic
molecule. Without a hydrophilic portion, it is unable to form
favorable interactions with water. Thus, triacylglycerol would
be unlikely to become part of a lipid bilayer. Instead, such
purely hydrophobic molecules cluster together to limit their
contact with surrounding water molecules (see Figure 11–9).
In this way, triacylglycerols—which are major components of
animal fats and plant oils—coalesce into fat droplets in an
aqueous environment, including those in fat cells and plant
seeds.
Chapter 12
ANSWER 12–1
A. The movement of a solute mediated by a transporter
can be described by a strictly analogous equation:
equation 1: T + S ↔ TS → T + S*
where S is the solute, S* is the solute on the other