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Chapter_16_Solutions - Chapter 16 The Cytoskeleton THE...

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DEFINITIONS 16–1 Protofilament 16–2 Dynamic instability 16–3 Plus end 16–4 Intermediate filament 16–5 Treadmilling 16–6 Cytoskeleton TRUE/FALSE 16–7 True. Each protofilament in a microtubule is assembled from subunits that all point in the same direction; thus, each protofilament has a -tubulin at one end and b -tubulin at the other. Since the protofilaments in a micro- tubule are aligned in parallel, a -tubulin is always at one end and b -tubulin is always at the other. 16–8 True. When ATP in actin filaments (or GTP in microtubules) is hydrolyzed, much of the free energy released by cleavage of the high-energy bond is stored in the polymer lattice, making the free energy of the ADP-containing polymer higher than that of the ATP-containing polymer. This shifts the equilibrium toward depolymerization so that ADP-containing actin fila- ments disassemble more readily than ATP-containing actin filaments. 16–9 False. In contrast to actin filaments and microtubules, which are present in all eucaryotic organisms, intermediate filaments are found only in some metazoans, including vertebrates, nematodes, and snails. Even in these organisms intermediate filaments are not required in every cell type. The nuclear lamins, which are the ancestors of the intermediate filaments, form a meshwork of protein that lines the nuclear membrane; they are much more widely distributed among eucaryotes. THOUGHT PROBLEMS 16–10 Intermediate filaments provide mechanical stability and resistance to shear stress. Microtubules determine the positions of membranous organelles and direct intracellular transport. Actin filaments determine the shape of the cell’s surface and are necessary for whole-cell locomotion. THE SELF-ASSEMBLY AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTS In This Chapter THE SELF-ASSEMBLY A367 AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTS HOW CELLS REGULATE A378 THEIR CYTOSKELETAL FILAMENTS MOLECULAR MOTORS A384 THE CYTOSKELETON A387 AND CELL BEHAVIOR A367 Chapter 16 16 The Cytoskeleton
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A368 Chapter 16: The Cytoskeleton 16–11 Although the subunits are indeed held together by noncovalent bonds that are individually weak, there are a very large number of them, distributed among a very large number of filaments. As a result, the stress a human being exerts by lifting a heavy object is dispersed over so many subunits that their interaction strength is not exceeded. By analogy, a single thread of silk is not nearly strong enough to hold a human, but a rope woven of such fibers is. 16–12 A. Phase A corresponds to a lag phase (Figure 16–47A), during which actin monomers must assemble to form a nucleus for polymerization (thought to be a trimer of subunits). Formation of a nucleus (nucleation) is followed by rapid growth (phase B), as actin monomers are added to the ends of the growing filaments. At phase C, equilibrium is reached between the rate of addition of actin at the ends and its rate of release. Once equilibrium is reached, the concentration of free actin remains constant.
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Chapter_16_Solutions - Chapter 16 The Cytoskeleton THE...

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