Term
| 12-8 The diagram in figure 12-6 shows a passive transporter that mediates the transfer of a solute down its concentration gradient across the membrane. how would you need to change the diagram to convert the transporter into a pump that moves the solute up its electrochemical gradient by hydrolyzing ATP? explain the need for each of the steps in your new illustration. |
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Definition
| By analogy to the Na+-K+ pump shown in figure 12-11(page 395), ATP might be hydrolyzed and donate a phosphate group to the transporter when-and only when-it has the solute bound on the "inside" face of the membrane (step 1 to 2). the attachment of the phosphate would trigger an immediate conformational change (step 2 to 3), thereby capturing the solute and exposing it to the "outside." the phosphate would be removed from the protein when - and only when- the solute had dissociatd, and the now empty, nonphosphorylated transporter would switch back to the starting position (step 3 to 4) (figure A12-8) page A:33(back of book) |
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Term
| 12-9 A. The plasma membrane is highly impermeable to all charged molecules. |
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Definition
| False. the plasma membrane contains proteins that confer selective permeability to many charged molecules. in contrast, a pure lipid bilayer lacking proteins is highly impermeable to all charged molecules. |
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Term
| 12-10 List the following compounds in order of increasing lipid bilayer permeability: RNA, Ca2+, glucose, ethanol, N2, Water |
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Definition
| The permeabilities are N2 (small and nonpolar) > ethanol (small and slightly polar) > water (small and polar) > glucose (large and polar) > Ca2+ (small and charged) > RNA (Very large and charged). |
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Term
| 12-12 Discuss the following statement: "the differences between a channel and a transporter are like the differences between a bridge and a ferry." |
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Definition
| a bridge allows vehicles to pass over a river in a steady stream; the entrance can be designed to exclude, for example, oversized trucks, and it can be intermittently closed to traffic by a gate. by analogy, channels allow ions to flow in a gated stream across the membrane, imposing size and charge restrictions.a ferry, in contrast, loads vehicles on one riverbank and then, after movement of the ferry itself, unloads on the other side of the river. this process is slower. during loading, particular vehicles could be selected from the waiting line because they fit particularly well on the car deck. by analogy, transporters bind solutes on one side of the membrane and then, after a conformational movement, release them on the other side. specific binding leads to the selection of the molecules to be transported. as in the case of a coupled transport, sometimes you have to wait until the ferry is full before you can go. |
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Term
| 12-13 The neurotransmitter acetylcholine is made in the cytosol and then transported into synaptic vesicles, where its concentration is more than 100-fold higher than in the cytosol. when synaptic vesicles are isolated from neurons, they can take up additional acetylcholine added to the solution in which they are suspended, but only when ATP is present. Na+ ions are not required for acetylcholine uptake, but, curiously, raising the pH of the solution in which the synaptic vesicles are suspended increases the rate of acetylcholine uptake. furthermore, transport is inhibited when drugs are added that make the membrane permeable to H+ ions. suggest a mechanism that is consistent with all of these observations. |
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Definition
| acetylcholine is being transported into the vesicles by an H+-acetylcholine antiport in the vesicle membrane. the H+ gradient that drives the uptake is generated by an ATP-driven H+ pump in the vesicle membrane, which pumps H+ into the vesicle (hence the dependence of the reaction on ATP). raising the pH of the solution surrounding the vesicles increases the H+ ions in the solution outside the vesicles while the number inside remains the same. this explains the observed enhanced rate of uptake |
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Term
| 12-15 Phospholipid bilayers form sealed spherical vesicles in water. assume you have constructed lipid vesicles that contain Na+-K+ pumps as the sole membrane protein, and assume for the sake of simplicity that each pump transports one Na+ one way and one K+ the other way in each pumping cycle. all the Na+-K+ pumps have the portion of the molecule that normally faces the cytosol oriented toward the outside of the vesicles. With the help of figure 12-11, determine what would happen if: A. Your vesicles were suspended in a solution containing both Na+ and K+ ions and had a solution with the same ionic composition inside them. |
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Definition
| Nothing. You require ATP to drive the Na+-K+ pump. |
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Term
| 12-18 Amino acids are taken up by animal cells using a symport in the plasma membrane. what is the most likely ion whose electrochemical gardient drives the import? Is ATP consumed in the process? if so, how? |
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Definition
| Animal cells drive most transport processes across the plasma membrane with the electrochemical gradient of Na+. ATP is needed to fuel the Na+-K+ pump to maintain the Na+ gradient. |
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Term
| 15-9 A. Ribosomes are cytoplasmic structures that, during protein synthesis, become linked by an mRNA molecule to form polyribosomes |
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Definition
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Term
| 15-10 Some proteins shuttle back and forth between the nucleus and the cytosol. they need a nuclear export signal to get out of the nucleus. How do you suppose they get into the nucleus |
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Definition
| they must contain a nuclear localization signal as well. proteins with nuclear export signals shuttle between the nucleus and the cytosol. an example is A1 protein, which binds to mRNAs in the nucleus and guides them through the nuclear pores. once in the cytosol, a nuclear localization signal ensures that the a1 protein is reimported so that it can participate in the export of further mRNAs |
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Term
| 15-16 compare and contrast protein import into the ER and into the nucleus. list at least two major differences in the mechanisms, and speculate why the ER mechanism might not work for nuclear import and vise versa |
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Definition
| (1) proteins are imported into the nucleus after they have been synthesized, folded, and, if appropriate, assembled into complexes. in contrast, unfolded polypeptide chains are translocated into the ER as they are being made by the ribosomes. ribosomes are assembled in the nucleus yet function in the cytosol, and the enzyme complexes that catalyze RNA transcription and splicing are assembled in the cytosol yet function in the nucleus. thus, both ribosomes and these enzyme complexes need to be transported through the nuclear pores intact. (2) nuclear pores are gates, which are always open to small molecules; in contrast, translocation channels in the ER membrane are normally closed (indicated by the "plug" in figure 15-15), and open only after the ribosome has attached to the membrane and the translocating polypeptide chain has sealed the channel from the cytosol. it is important that the ER membrane remain impermeable to small molecules during the translocation process, as the ER is a major store for Ca2+ in the cell, and Ca2+ release into the cytosol must be tightly controlled. (3) nuclear localization signals are not cleaved off after protein import into the nucleus; in contrast, ER signal peptides are usually cleaved off. nuclear localization signals are needed to repeatedly reimport nuclear proteins after they have been released into the cytosol during mitosis, when the nuclear envelop breaks down. |
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Term
| 15-17 during mitosis, the nuclear envelope breaks down and intranuclear proteins completely intermix with cytosolic proteins. is this consistent with the evolutionary scheme proposed in figure 15-3 Page 499 |
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Definition
| the transient intermixing of nuclear and cytosolic contents during mitosis supports the idea that the nuclear interior and the cytosol are indeed evolutionarily related. In fact, one can consider the nucleus as a subcompartment of the cytosol that has become surrounded by the nuclear envelope, with access only through the nuclear pores. |
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Term
| 15-19 Dr. Outonalimb's claim to fame is her discovery of forgettin, a protein predominantly made by the pineal gland in human teenagers. the protein causes selective short term unresponsiveness and memory loss when the auditory system receives statements like "please take out the garbage!" her hypothesis is that forgettin has a hydrophobic ER signal sequence at its C-terminus that is recognized by an SRP and causes it to be translocated across the ER membrane by the mechanism shown in Figure 15-14. she predicts that the protein is secreted from pineal cells into the bloodstream, from where it exerts its devastating systemic effects. you are a member of the committee deciding whether she should receive a grant for further work on her hypothesis. critique her proposal, and remember that grant reviews should be polite and constructive. |
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Definition
| Critique: "Dr. Outonalimb proposes to study the biosynthesis of forgettin, a protein of significant interest. the main hypothesis on which this proposal is based, however, requires further support. In particular, it is questionable whether forgettin is indeed a secreted protein, as proposed. ER signal sequences are normally found at the N-Terminus. C-terminal hydrophobic sequences will be exposed outside the ribosome only after protein synthesis has already terminated and can therefore not be recognized by an SRP during translation. it is therefore unlikely that forgettin will be translocated by an SRP-dependent mechanism; it is more likely that it will remain in the cytosol. Dr. Outonalimb should take these considerations into account when submitting a revised application." |
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Term
| 12-9 B. Channels must first bind to solute molecules before they can select those that they allow to pass |
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Definition
| False. channels do not bind the solute that passes through them. selectivity of a channel is achieved by the size of the internal pore and by charged regions at the entrance of the pore that attract or repel ions of the appropriate charge. |
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Term
| 12-9 C. Transporters allow solutes to cross a membrane at much faster rates than do channels. |
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Definition
| false. transporters are slower. they have enzymelike properties, i.e., they bind solutes and need to undergo conformational changes during their functional cycle. this limits the maximal rate of transport to about 1000 solute molecules per second, whereas channels can pass up to 1000000 solute molecules per second. |
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Term
| 12-9 D. Certain H+ pumps are fueled by light energy |
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Definition
| True. the bacteriorhodopsin of some photosynthetic bacteria moves H+, using energy captured from visible light |
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Term
| 12-9 E. The plasma membrane of many animal cells contains open K+ channels, yet the K+ concentration in the cytosol is much higher than outside the cell. |
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Definition
| True. most animal cells contain K+ leak channels in their plasma membrane that are predominantly open. the K+ concentration inside the cell still remains higher than outside, because the membrane potential is negative and therefore inhibits the positively charged K+ from leaking out. K+ is also continually pumped into the cell by the Na+-K+ pump. |
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Term
| 12-9 F. A symport would function as an antiport if its orientation in the membrane were reversed (i.e., if the portion of the molecule normally exposed to the cytosol faced the outside of the cell instead). |
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Definition
| False. a symport binds two different solutes on the same side of the membrane. turning it around would not change it into an antiport, which must also bind to different solutes, but on opposing sides of the membrane. |
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Term
| 12-9 G. The membrane potential of an axon temporarily becomes more negative when an action potential excites it. |
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Definition
| False. the peak of an action potential corresponds to a transient shift of the membrane potential from a negative to a positive value. the influx of Na+ causes the membrane potential first to move toward zero and then to reverse, rendering the cell positively charged on its inside. eventually, the resting potential is restored by an efflux of K+ through voltage gated K+ channels and K+ leak channels. |
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Term
| 12-15 Phospholipid bilayers form sealed spherical vesicles in water. assume you have constructed lipid vesicles that contain Na+-K+ pumps as the sole membrane protein, and assume for the sake of simplicity that each pump transports one Na+ one way and one K+ the other way in each pumping cycle. all the Na+-K+ pumps have the portion of the molecule that normally faces the cytosol oriented toward the outside of the vesicles. With the help of figure 12-11, determine what would happen if: B. You add ATP to the suspension described in (A. your vesicles were suspended in a solution containing both Na+ and K+ ions and had a solution with the same ionic composition inside them. |
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Definition
| The ATP becomes hydrolyzed, and Na+ is pumped into the vesicles, generating a concentration gradient of Na+ across the membrane. at the same time, K+ is pumped out of the vesicles, generating a concentration gradient K+ of opposite polarity. when all the K+ had been pumped out of the vesicle or the ATP ran out, the pump would stop. |
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Term
| 12-15 Phospholipid bilayers form sealed spherical vesicles in water. assume you have constructed lipid vesicles that contain Na+-K+ pumps as the sole membrane protein, and assume for the sake of simplicity that each pump transports one Na+ one way and one K+ the other way in each pumping cycle. all the Na+-K+ pumps have the portion of the molecule that normally faces the cytosol oriented toward the outside of the vesicles. With the help of figure 12-11, determine what would happen if: C. You add ATP, but the solution- outside as well as inside the vesicles contains only Na+ ions and no K+ ions |
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Definition
| The Na+-K+ pump would go through states 1, 2, 3 in figure 12-11. because all reaction steps must occur strictly sequentially, however, dephosphorylation and the conformation switch cannot occur in the absence of K+. the Na+-K+ pump will therefore become stuck in the phosphorylated state, waiting indefinitely for a potassium ion. the number of sodium ions transported would be minuscule, because each pump molecule would have functioned only a single time. Similar experiments, leaving out individual ions and analyzing the consequences, were used to determine the sequence of steps by which the Na+-K+ pump works. |
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Term
| 12-15 Phospholipid bilayers form sealed spherical vesicles in water. assume you have constructed lipid vesicles that contain Na+-K+ pumps as the sole membrane protein, and assume for the sake of simplicity that each pump transports one Na+ one way and one K+ the other way in each pumping cycle. all the Na+-K+ pumps have the portion of the molecule that normally faces the cytosol oriented toward the outside of the vesicles. With the help of figure 12-11, determine what would happen if: D. Half of the pump molecules embedded in the membrane of each vesicle were oriented the other way around so that the normally cytosolic portions of these molecules faced the inside of the vesicles. You then add ATP to the suspension |
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Definition
| ATP would become hydrolyzed and Na+ and K+ would be pumped across the membrane as described in scenario(B). however, the pump molecules that sit in the membrane in the reverse orientation would be completely inactive (ex: they would not-as one might have erroneously assumed- pump ions in the opposite direction), because ATP would not have access to the site on these molecules where phosphorylation occurs. this site is normally exposed to the cytosol. ATP is highly charged and cannot cross membranes without the help of specific transporters. |
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Term
| 12-15 Phospholipid bilayers form sealed spherical vesicles in water. assume you have constructed lipid vesicles that contain Na+-K+ pumps as the sole membrane protein, and assume for the sake of simplicity that each pump transports one Na+ one way and one K+ the other way in each pumping cycle. all the Na+-K+ pumps have the portion of the molecule that normally faces the cytosol oriented toward the outside of the vesicles. With the help of figure 12-11, determine what would happen if: E. You add ATP to the suspension described in (A. Your vesicles were suspended in a solution containing both Na+ and K+ ions and had a solution with the same ionic composition inside them), but in addition to Na+-K+ pumps, the membrane of your vesicles also contains K+ leak channels |
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Definition
| ATP becomes hydrolyzed and Na+ and K+ are pumped across the membrane, as described in scenario (B). K+, however, immediately flows back into the vesicles through the K+ leak channels. K+ moves down the K+ concentration gradient formed by the action of the Na+-K+ pump. with each K+ that moves into the vesicle through the leak channel, a positve charge is moved across the membrane, building a membrane potential that is positve on the inside of the vesicles. eventually, K+ will stop flowing through the leak channels when the membrane potential balances the concentration gradient. the scenario described here is a slight oversimplification: the Na+-K+ pump in mammalian cells actually moves three sodium ions out of cells for each two potassium ions that it pumps into the cell, thereby driving an electric current across the membrane and making a small additional contribution to the resting membrane potential (which therefore corresponds only approximately to a state of equilibrium for K+ moving via K+ leak channels). |
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Term
| 15-9 B. the amino acid sequence Leu-His-Arg-Leu-Asp-Ala-Gln-Ser-Lys-Leu-Ser-Ser is a signal sequence that directs proteins to the ER |
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Definition
| False. The signal sequences that direct proteins to the ER contain a core of eight or more hydrophobic amino acids. the sequence shown here contains many hydrophilic amino acid side chains, including the charged amino acids His, Arg, Asp, and Lys, and the uncharged hydrophilic amino acids Gln and Ser. |
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Term
| 15-9 C. All transport vesicles in the cell must have a v-SNARE protein in their membrane |
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Definition
| True. Otherwise they could not dock at the correct target membrane or recruit a fusion complex to a docking site |
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Term
| 15-9 D. Transport vesicles deliver proteins and lipids to the cell surface |
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Definition
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Term
| 15-9 E. if the delivery of prospective lysosomal proteins from the trans golgi network to the late endosomes were blocked, lysosomal proteins would be secreted by the constitutive secretion pathways shown in figure 15-27 |
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Definition
| True. Lysosomal proteins are selected in the trans Golgi network and packaged into transport vesicles that deliver them to the late endosome. if not selected, they would enter by default into transport vesicles that move constitutively to the cell surface. |
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Term
| 15-9 F. lysosomes digest only substances that have been taken up by cells by endocytosis |
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Definition
| False. Lysosomes are digest internal organelles by autophagy. |
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Term
| 15-9 G. N-linked sugar chains are found on glycoproteins that face the cell surface, as well as on glycoproteins that face the lumen of the ER, trans Golgi network and mitochondria |
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Definition
| False. Mitochondria do not participate in vesicular transport, and therefore N-linked glycoproteins, which are exclusively assembled in the ER, cannot be transported to mitochondria. |
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