Essential Cell Biology 5th edition

Answers A:45
becomes localized by binding to the evenly spaced ends
of tropomyosin molecules.
B. Calcium ions influence force generation in the actin–
myosin system only if both troponin (to bind the calcium
ions) and tropomyosin (to transmit the information to
the actin filament that troponin has bound calcium)
are present. (i) Troponin cannot bind to actin without
tropomyosin. The actin filament would be permanently
exposed to the myosin, and the system would be
continuously active, independently of whether calcium
ions were present or not (a muscle cell would therefore
be continuously contracted with no possibility of
regulation). (ii) Tropomyosin would bind to actin and
block binding of myosin completely; the system would
be permanently inactive, no matter whether calcium
ions were present, because tropomyosin is not affected
by calcium. (iii) The system will contract in response to
calcium ions.
ANSWER 17–11
A. True. A continual outward movement of ER is required;
in the absence of microtubules, the ER collapses toward
the center of the cell.
B. True. Actin is needed to make the contractile ring that
causes the physical cleavage between the two daughter
cells, whereas the mitotic spindle that partitions the
chromosomes is composed of microtubules.
C. True. Both extensions are associated with
transmembrane proteins that protrude from the plasma
membrane and enable the cell to form new anchor
points on the substratum.
D. False. To cause bending, ATP is hydrolyzed by the
dynein motor proteins that are attached to the outer
microtubules in the flagellum.
E. False. Cells could not divide without rearranging
their intermediate filaments, but many terminally
differentiated and long-lived cells, such as nerve cells,
have stable intermediate filaments that are not known to
depolymerize.
F. False. The rate of growth is independent of the size of
the GTP cap. The plus and minus ends have different
growth rates because they have physically distinct
binding sites for the incoming tubulin subunits; the rate
of addition of tubulin subunits differs at the two ends.
G. True. Both are nice examples of how the same
membrane can have regions that are highly specialized
for a particular function.
H. False. Myosin movement is activated by the
phosphorylation of myosin, or by calcium binding to
troponin.
ANSWER 17–12 The average time taken for a small
molecule (such as ATP) to diffuse a distance of 10 μm is
given by the calculation
(10 –3 ) 2 / (2 × 5 × 10 –6 ) = 0.1 seconds
Similarly, a protein takes 1 second and a vesicle 10 seconds
on average to travel 10 μm. A vesicle would require on
average 10 9 seconds, or more than 30 years, to diffuse to
the end of a 10 cm axon. Motorized transport at 1 μm/sec
would require 10 5 seconds, or 28 hours. These calculations
make it clear why kinesin and other motor proteins evolved
to carry molecules and organelles along microtubules.
ANSWER 17–13 (1) Animal cells are much larger and
more diversely shaped than bacteria, and they do not have
a cell wall. Cytoskeletal elements are required to provide
mechanical strength and shape in the absence of a cell wall.
(2) Animal cells, and all other eukaryotic cells, have a nucleus
that is shaped and held in place in the cell by intermediate
filaments; the nuclear lamins attached to the inner nuclear
membrane support and shape the nuclear membrane, and a
meshwork of intermediate filaments surrounds the nucleus
and spans the cytosol. (3) Animal cells can move by a
process that requires a change in cell shape. Actin filaments
and myosin motor proteins are required for these activities.
(4) Animal cells have a much larger genome than bacteria;
this genome is fragmented into many chromosomes.
For cell division, chromosomes need to be accurately
distributed to the daughter cells, requiring the function of
the microtubules that form the mitotic spindle. (5) Animal
cells have internal organelles. Their localization in the cell
is dependent on motor proteins that move them along
microtubules. A remarkable example is the long-distance
travel of membrane-enclosed vesicles (organelles) along
microtubules in an axon that can be up to 1 m long in the
case of the nerve cells that extend from your spinal cord to
your feet.
ANSWER 17–14 The ends of an intermediate filament are
indistinguishable from each other, because the filaments are
built by the assembly of symmetrical tetramers made from
two coiled-coil dimers. In contrast to microtubules and actin
filaments, intermediate filaments therefore have no polarity.
ANSWER 17–15 Intermediate filaments have no polarity;
their ends are chemically indistinguishable. It would
therefore be difficult to envision how a hypothetical motor
protein that bound to the middle of the filament could
sense a defined direction. Such a motor protein would be
equally likely to attach to the filament facing one end or the
other.
ANSWER 17–16 Katanin breaks microtubules along
their length, and at positions remote from their GTP caps.
The fragments that form therefore contain GDP-tubulin
at their exposed ends and rapidly depolymerize. Katanin
thus provides a very quick means of destroying existing
microtubules.
ANSWER 17–17 Cell division depends on the ability of
microtubules both to polymerize and to depolymerize. This
is most obvious when one considers that the formation of
the mitotic spindle requires the prior depolymerization of
other microtubules to free up the tubulin required to build
the spindle. This rearrangement is not possible in Taxoltreated
cells, whereas in colchicine-treated cells, division is
blocked because a spindle cannot be assembled. On a less
obvious but no less important level, both drugs block the
dynamic instability of microtubules and would therefore
interfere with the workings of the mitotic spindle, even if
one could be properly assembled.
ANSWER 17–18 Motor proteins are unidirectional in
their action; kinesin always moves toward the plus end
of a microtubule and dynein toward the minus end. Thus
if kinesin molecules are attached to glass, only those
individual motors that have the correct orientation in
relation to the microtubule that settles on them can attach
to the microtubule and exert force on it to propel it forward.
Since kinesin moves toward the plus end of the microtubule,