Essential Cell Biology 5th edition

Questions
607
• In skeletal muscle cells, repeating arrays of overlapping filaments of
actin and myosin-II form highly ordered myofibrils, which contract as
these filaments slide past each other.
• Muscle contraction is initiated by a sudden rise in cytosolic Ca 2+ ,
which delivers a signal to the myofibrils via Ca 2+ -binding proteins
associated with the actin filaments.
KEY TERMS
actin-binding protein
actin filament
cell cortex
centriole
centrosome
cilium
cytoskeleton
dynamic instability
dynein
filopodium
flagellum
intermediate filament
keratin filament
kinesin
lamellipodium
microtubule
microtubule-associated protein
motor protein
myofibril
myosin
myosin-I
myosin-II
myosin filament
nuclear lamina
polarity
Rho protein family
sarcomere
tubulin
QUESTIONS
QUESTION 17–11
Which of the following statements are correct? Explain your
answers.
A. Kinesin moves endoplasmic reticulum (ER) membranes
along microtubules so that the network of ER tubules
becomes stretched throughout the cell.
B. Without actin, cells can form a functional mitotic spindle
and pull their chromosomes apart but cannot divide.
C. Lamellipodia and filopodia are “feelers” that a cell
extends to find anchor points on the substratum that it will
then crawl over.
D. GTP is hydrolyzed by tubulin to cause the bending of
flagella.
E. Cells having an intermediate-filament network that
cannot be depolymerized would die.
F. The plus ends of microtubules grow faster because they
have a larger GTP cap.
G. The transverse tubules in muscle cells are an extension
of the plasma membrane, with which they are continuous;
similarly, the sarcoplasmic reticulum is an extension of the
endoplasmic reticulum.
H. Activation of myosin movement on actin filaments is
triggered by the phosphorylation of troponin in some
situations and by Ca 2+ binding to troponin in others.
QUESTION 17–12
The average time taken for a molecule or an organelle to
diffuse a distance of x cm is given by the formula
t = x 2 /2D
where t is the time in seconds and D is a constant called the
diffusion coefficient for the molecule or particle. Using the
above formula, calculate the time it would take for a small
molecule, a protein, and a membrane vesicle to diffuse from
one side to another of a cell 10 μm across. Typical diffusion
coefficients in units of cm 2 /sec are: small molecule, 5 × 10 –6 ;
protein molecule, 5 × 10 –7 ; vesicle, 5 × 10 –8 . How long
would a membrane vesicle take to reach the end of an axon
10 cm long by free diffusion? How long would it take if it
was transported along microtubules at 1 μm/sec?
QUESTION 17–13
Why do eukaryotic cells, and especially animal cells, have
such large and complex cytoskeletons? List the differences
between animal cells and bacteria that depend on the
eukaryotic cytoskeleton.
QUESTION 17–14
Examine the structure of an intermediate filament shown in
Figure 17−4. Does the filament have a unique polarity—that
is, could you distinguish one end from the other by chemical
or other means? Explain your answer.
QUESTION 17–15
There are no known motor proteins that move on
intermediate filaments. Suggest an explanation for this.