Questions and Answers of Molecular Cell Biology

In order for proliferating cells to maintain a relatively constant size, the length of the cell cycle must match the time it takes for the cell to double in size.Which statements are true? Explain
Since there are about 1013 cells in an adult human, and about 1010 cells die and are replaced each day, we become new people every three years.Which statements are true? Explain why or why not.
How is the unidirectional motion of a lamellipodium maintained?
Why is it that intermediate filaments have identical ends and lack polarity, whereas actin filaments and microtubules have two distinct ends with a defined polarity?
Cofilin preferentially binds to older actin filaments and promotes their disassembly. How does cofilin distinguish old filaments from new ones?
A mitochondrion 1 μm long can travel the 1 meter length of the axon from the spinal cord to the big toe in a day. The Olympic men’s freestyle swimming record for 200 meters is 1.75 minutes. In
The movements of single motor-protein molecules can be analyzed directly. Using polarized laser light, it is possible to create interference patterns that exert a centrally directed force, ranging
How does a centrosome “know” when it has found the center of the cell?
A solution of pure αβ-tubulin dimers is thought to nucleate microtubules by forming a linear protofilament about seven dimers in length. At that point, the probabilities that the next αβ-dimer
At 1.4 mg/mL pure tubulin, microtubules grow at a rate of about 2 μm/min. At this growth rate, how many αβ-tubulin dimers (8 nm in length) are added to the ends of a microtubule each second?
Detailed measurements of sarcomere length and tension during isometric contraction in striated muscle provided crucial early support for the sliding-filament model of muscle contraction. Based on
The concentration of actin in cells is 50–100 times greater than the critical concentration observed for pure actin in a test tube. How is this possible? What prevents the actin subunits in cells
In most animal cells, minus-end directed microtubule motors deliver their cargo to the periphery of the cell, whereas plus-end directed microtubule motors deliver their cargo to the interior of the
Motor neurons trigger action potentials in muscle cell membranes that open voltage-sensitive Ca2+ channels in T tubules, allowing extracellular Ca2+ to enter the cytosol, bind to troponin C, and
The role of ATP hydrolysis in actin polymerization is similar to the role of GTP hydrolysis in tubulin polymerization: both serve to weaken the bonds in the polymer and thereby promote
The Wnt planar polarity signaling pathway normally ensures that each wing cell in Drosophila has a single hair. Overexpression of the Frizzled gene from a heatshock promoter (hs-Fz) causes multiple
Phosphorylase kinase integrates signals from the cyclic-AMP-dependent and Ca2+-dependent signaling pathways that control glycogen breakdown in liver and muscle cells (Figure Q15–4). Phosphorylase
Propose specific types of mutations in the gene for the regulatory subunit of cyclic-AMP-dependent protein kinase (PKA) that could lead to either a permanently active PKA or a permanently inactive
Activation (“maturation”) of frog oocytes is signaled through a MAP kinase signaling module. An increase in the hormone progesterone triggers the module by stimulating the translation of Mos
Describe three ways in which a gradual increase in an extracellular signal can be sharpened by the target cell to produce an abrupt or nearly all-or none response.
Consider a signaling pathway that proceeds through three protein kinases that are sequentially activated by phosphorylation. In one case, the kinases are held in a signaling complex by a scaffolding
Why do you suppose that phosphorylation/ dephosphorylation, as opposed to allosteric binding of small molecules, for example, has evolved to play such a prominent role in switching proteins on and
How is it that different cells can respond in different ways to exactly the same signaling molecule even when they have identical receptors?
Why do signaling responses that involve changes in proteins already present in the cell occur in milliseconds to seconds, whereas responses that require changes in gene expression require minutes to
Cells communicate in ways that resemble human communication. Decide which of the following forms of human communication are analogous to autocrine, paracrine, endocrine, and synaptic signaling by
Suppose that the circulating concentration of hormone is 10–10 M and the Kd for binding to its receptor is 10–8 M. What fraction of the receptors will have hormone bound? If a meaningful
Even though plants and animals independently evolved multicellularity, they use virtually all the same signaling proteins and second messengers for cell–cell communication.Is the statement true?
Protein tyrosine phosphatases display exquisite specificity for their substrates, unlike most serine/threonine protein phosphatases, which have rather broad specificity.Is the statement true? Explain
Binding of extracellular ligands to receptor tyrosine kinases (RTKs) activates the intracellular catalytic domain by propagating a conformational change across the lipid bilayer through a single
Most intracellular signaling pathways provide numerous opportunities for amplifying the responses toextracellular signals.Is the statement true? Explain why or why not.
In the regulation of molecular switches, protein kinases and guanine nucleotide exchange factors (GEFs) always turn proteins on, whereas protein phosphatases and GTPase-activating proteins (GAPs)
All second messengers are water-soluble and diffuse freely through the cytosol.Is the statement true? Explain why or why not.
Examine the variegated leaf shown in Figure Q14–3. Yellow patches surrounded by green are common, but there are no green patches surrounded by yellow. Propose an explanation for this
In chloroplasts, protons are pumped out of the stroma across the thylakoid membrane, whereas in mitochondria, they are pumped out of the matrix across the crista membrane. Explain how this
How much energy is available in visible light? How much energy does sunlight deliver to Earth? How efficient are plants at converting light energy into chemical energy? The answers to these questions
ATP synthase is the world’s smallest rotary motor. Passage of H+ ions through the membrane embedded portion of ATP synthase (the Fo component) causes rotation of the single, central, axle-like γ
In actively respiring liver mitochondria, the pH in the matrix is about half a pH unit higher than it is in the cytosol. Assuming that the cytosol is at pH 7 and the matrix is a sphere with a
Normally, the flow of electrons to O2 is tightly linked to the production of ATP via the electrochemical gradient. If ATP synthase is inhibited, for example, electrons do not flow down the
If isolated mitochondria are incubated with a source of electrons such as succinate, but without oxygen, electrons enter the respiratory chain, reducing each of the electron carriers almost
Both H+ and Ca2+ are ions that move through the cytosol. Why is the movement of H+ ions so much faster than that of Ca2+ ions? How do you suppose the speed of these two ions would be affected by
Heart muscle gets most of the ATP needed to power its continual contractions through oxidative phosphorylation. When oxidizing glucose to CO2, heart muscle consumes O2 at a rate of 10 μmol/min per g
In the 1860s, Louis Pasteur noticed that when he added O2 to a culture of yeast growing anaerobically on glucose, the rate of glucose consumption declined dramatically. Explain the basis for this
Mutations that are inherited according to Mendelian rules affect nuclear genes; mutations whose inheritance violates Mendelian rules are likely to affect organelle genes.Is the statement true?
The number of c subunits in the rotor ring of ATP synthase defines how many protons need to pass through the turbine to make each molecule of ATP.Is the statement true? Explain why or why not.
The three respiratory enzyme complexes in the mitochondrial inner membrane tend to associate with each other in ways that facilitate the correct transfer of electrons between appropriate complexes.Is
Melanosomes are specialized lysosomes that store pigments for eventual release by exocytosis. Various cells such as skin and hair cells then take up the pigment, which accounts for their
How does the low pH of lysosomes protect the rest of the cell from lysosomal enzymes in case the lysosome breaks?
The KDEL receptor must shuttle back and forth between the ER and the Golgi apparatus to accomplish its task of ensuring that soluble ER proteins are retained in the ER lumen. In which compartment
If you were to remove the ER retrieval signal from protein disulfide isomerase (PDI), which is normally a soluble resident of the ER lumen, where would you expect the modified PDI to be located?
SNAREs exist as complementary partners that carry out membrane fusions between appropriate vesicles and their target membranes. In this way, a vesicle with a particular variety of v-SNARE will fuse
For fusion of a vesicle with its target membrane to occur, the membranes have to be brought to within 1.5 nm so that the two bilayers can join (Figure Q13–1). Assuming that the relevant portions of
Enveloped viruses, which have a membrane coat, gain access to the cytosol by fusing with a cell membrane. Why do you suppose that these viruses encode their own special fusion protein, rather than
In a nondividing cell such as a liver cell, why must the flow of membrane between compartments be balanced, so that the retrieval pathways match the outward flow? Would you expect the same balanced
All the glycoproteins and glycolipids in intracellular membranes have oligosaccharide chains facing the lumenal side, and all those in the plasma membrane have oligosaccharide chains facing the
There is one strict requirement for the exit of a protein from the ER: it must be correctly folded.Is the statement true? Explain why or why not.
In all events involving fusion of a vesicle to a target membrane, the cytosolic leaflets of the vesicle and target bilayers always fuse together, as do the leaflets that are not in contact with the
All new phospholipids are added to the cytosolic leaflet of the ER membrane, yet the ER membrane has a symmetrical distribution of different phospholipids in its two leaflets. By contrast, the plasma
Examine the multipass transmembrane protein shown in Figure Q12–3. What would you predict would be the effect of converting the first hydrophobic transmembrane segment to a hydrophilic segment?
Why do mitochondria need a special translocator to import proteins across the outer membrane, when the membrane already has large pores formed by porins?
If the enzyme dihydrofolate reductase (DHFR), which is normally located in the cytosol, is engineered to carry a mitochondrial targeting sequence at its N-terminus, it is efficiently imported into
Components of the TIM complexes, the multisubunit protein translocators in the mitochondrial inner membrane, are much less abundant than those of the TOM complex. They were initially identified using
The nuclear pore complex (NPC) creates a barrier to the free exchange of molecules between the nucleus and cytosol, but in a way that remains mysterious. In yeast, for example, the central pore of
Assuming that 32 million histone octamers are required to package the human genome, how many histone molecules must be transported per second per nuclear pore complex in cells whose nuclei contain
Before nuclear pore complexes were well understood, it was unclear whether nuclear proteins diffused passively into the nucleus and accumulated there by binding to residents of the nucleus such as
The rough ER is the site of synthesis of many classes of membrane proteins. Some of these proteins remain in the ER, whereas others are sorted to compartments such as the Golgi apparatus, lysosomes,
What is the fate of a protein with no sorting signal?
Peroxisomes are found in only a few specialized types of eukaryotic cell.Is the statement true? Explain why or why not.
To avoid the inevitable collisions that would occur if two-way traffic through a single pore were allowed, nuclear pore complexes are specialized so that some mediate import while others mediate
ER-bound and free ribosomes, which are structurally and functionally identical, differ only in the proteins they happen to be making at a particular time.Is the statement true? Explain why or why not.
Like the lumen of the ER, the interior of the nucleus is topologically equivalent to the outside of the cell.Is the statement true? Explain why or why not.
Acetylcholine-gated cation channels at the neuromuscular junction open in response to acetylcholine released by the nerve terminal and allow Na+ ions to enter the muscle cell, which causes membrane
The giant axon of the squid (Figure Q11–3) occupies a unique position in the history of our understanding of cell membrane potentials and nerve action. When an electrode is stuck into an intact
In a subset of voltage-gated K+ channels, the N-terminus of each subunit acts like a tethered ball that occludes the cytoplasmic end of the pore soon after it opens, thereby inactivating the channel.
According to Newton’s laws of motion, an ion exposed to an electric field in a vacuum would experience a constant acceleration from the electric driving force, just as a falling body in a vacuum
Microvilli increase the surface area of intestinal cells, providing more efficient absorption of nutrients. Microvilli are shown in profile and cross section in Figure Q11–1. From the dimensions
Ion transporters are “linked” together—not physically, but as a consequence of their actions. For example, cells can raise their intracellular pH, when it becomes too acidic, by exchanging
How is it possible for some molecules to be at equilibrium across a biological membrane and yet not be at the same concentration on both sides?
Order Ca2+, CO2, ethanol, glucose, RNA, and H2O according to their ability to diffuse through a lipid bilayer, beginning with the one that crosses the bilayer most readily. Explain your order.
The membrane potential arises from movements of charge that leave ion concentrations practically unaffected, causing only a very slight discrepancy in the number of positive and negative ions on the
Transporters saturate at high concentrations of the transported molecule when all their binding sites are occupied; channels, on the other hand, do not bind the ions they transport and thus the flux
Transport by transporters can be either active or passive, whereas transport by channels is always passive.Is the statement true? Explain why or why not.
Three mechanisms by which membrane-binding proteins bend a membrane are illustrated in Figure Q10–2A, B, and C. As shown, each of these cytosolic membrane- bending proteins would induce an
Glycophorin, a protein in the plasma membrane of the red blood cell, normally exists as a homodimer that is held together entirely by interactions between its transmembrane domains. Since
You are studying the binding of proteins to the cytoplasmic face of cultured neuroblastoma cells and have found a method that gives a good yield of inside-out vesicles from the plasma membrane.
Monomeric single-pass transmembrane proteins span a membrane with a single α helix that has characteristic chemical properties in the region of the bilayer. Which of the three 20-amino-acid
If a lipid raft is typically 70 nm in diameter and each lipid molecule has a diameter of 0.5 nm, about how many lipid molecules would there be in a lipid raft composed entirely of lipid? At a ratio
Margarine is made from vegetable oil by a chemical process. Do you suppose this process converts saturated fatty acids to unsaturated ones, or vice versa? Explain your answer.
When a lipid bilayer is torn, why does it not seal itself by forming a “hemi micelle” cap at the edges, as shown in Figure Q10–1?Figure Q10-1 tear in bilayer seal with hemi-micelle cap
Although membrane domains with different protein compositions are well known, there are at present no examples of membrane domains that differ in lipid composition.Is the statement true? Explain why
Whereas all the carbohydrate in the plasma membrane faces outward on the external surface of the cell, all the carbohydrate on internal membranes faces toward the cytosol.Is the statement true?
Although lipid molecules are free to diffuse in the plane of the bilayer, they cannot flip-flop across the bilayer unless enzyme catalysts called phospholipid translocators are present in the
Consider a fluorescent detector designed to report the cellular location of active protein tyrosine kinases. A blue (cyan) fluorescent protein (CFP) and a yellow fluorescent protein (YFP) were fused
Figure Q9–3 shows a series of modified fluorescent proteins that emit light in a range of colors. How do you suppose the exact same chromophore can fluoresce at so many different wavelengths?Figure
Antibodies that bind to specific proteins are important tools for defining the locations of molecules in cells. The sensitivity of the primary antibody—the antibody that reacts with the target
Explain the difference between resolution and magnification.
Why do humans see so poorly under water? And why do goggles help?
Figure Q9–2 shows a diagram of the human eye. The refractive indices of the components in the light path are: cornea 1.38, aqueous humor 1.33, crystalline lens 1.41, and vitreous humor 1.38. Where
The diagrams in Figure Q9–1 show the paths of light rays passing through a specimen with a dry lens and with an oil-immersion lens. Offer an explanation for why oil immersion lenses should give
A fluorescent molecule, having absorbed a single photon of light at one wavelength, always emits it at a longer wavelength.Is the statement true? Explain why or why not.
Because the DNA double helix is only 2 nm wide well below the limit of resolution of the light microscope— it is impossible to see chromosomes in living cells without special stains.Is the

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