Una turbia época de entreguerras

12-30 Studies on the squid giant axon were instrumental in our current understanding of how action potentials are generated. You decide to do some experiments on the squid giant axon yourself.

A. You remove the cytoplasm in an axon and replace it with an artificial cytoplasm that contains twice the normal concentration of K⁺ by adding KOAc, where OAc^– is an anion that is impermeable to the membrane. In this way you double the internal concentration of K⁺ while maintaining bulk electrical balance of the cytoplasmic solution. Will this make the resting potential of the membrane more or less negative?

B. You add NaCl to the extracellular fluid and effectively double the amount of extracellular Na⁺ cation. How does this affect the action potential?

C. You replace half of the NaCl in the extracellular fluid with choline chloride.

(Choline is a monovalent cation much larger than Na⁺. Note that the presence of choline will not impede the flow of Na⁺ through its channels.) How will this affect the action potential?

Answers

12-1 (d)

12-2 The two basic properties governing the likelihood of whether a molecule will diffuse through a lipid bilayer are the size of the molecule and the charge of the molecule. A smaller molecule will be more likely to diffuse through the lipid bilayer than a larger molecule. A nonpolar (hydrophobic) molecule will be more likely to diffuse through the lipid bilayer than a polar molecule, which is more likely to diffuse through the lipid bilayer than a charged molecule.

A. benzene (small nonpolar vs. larger uncharged) B. ethanol (polar vs. charged)

C. glucose (large polar vs. very large highly charged) D. O2 (nonpolar vs. polar)

E. adenosine (polar vs. highly charged)

12-3 Choice (d) is the correct answer. Because the lipid bilayer is permeable to carbon dioxide and ethanol, destroying membrane proteins is unlikely to affect their exit (choices (a) and (b)). The lipid bilayer is also permeable to water (choice (e)). On the other hand glucose requires a membrane transport protein to be imported into the cell. ATP, which is a highly charged molecule, also requires a transport protein to cross a membrane and thus could not be lost from the cell by simple diffusion (choice (c)).

12-4 A molecule moves down its concentration gradient by passive transport, but requires active transport to move up its concentration gradient. Carrier proteins and ion channels function in membrane transport by providing a hydrophilic pathway through the

membrane for specific polar solutes or inorganic ions. Carrier proteins are highly selective in the solutes they transport, binding the solute at a specific site and changing its conformation in order to transport the solute across the membrane. On the other hand, ion channels discriminate among solutes mainly on the basis of size and electrical charge.

12-5 A. Molecule B is more likely to utilize a carrier protein. From the graph, the yeast cell’s uptake of molecule B levels off when the concentration of carbon source is high; this indicates a saturation point for the uptake of molecule B. Carrier proteins work by specifically binding their solutes and transferring the solute molecules across the membrane one at a time, down the concentration gradient of the solute. Thus, the rate at which a solute can be transported by a carrier protein is limited by the number of carrier proteins in the membrane and will reach a maximum when the solute concentration is high enough for solute molecules to saturate the carrier proteins. Since the graph indicates that the uptake of molecule B reaches a saturation point at high concentrations, molecule B is more likely to utilize a carrier protein.

B. Molecule A is more likely to be ethanol. Uptake of molecule A is proportional to its concentration, while the uptake of molecule B reaches a saturation point at high concentration. The linear dependence of molecule A upon its concentration suggests that molecule A can diffuse into the cell. Since ethanol (a small polar molecule) is more likely to diffuse into the cell compared to acetate (a charged molecule), ethanol is likely to be molecule A. Since acetate is charged, it is more likely to require a carrier protein in order to be transported across the membrane.

12-6 ATP-driven transport, coupled transport, light-driven transport 12-7 (a), (b), and (e)

12-8 (d) If the pump is mechanistically similar to the Na⁺–K⁺ pump, then the transport of ions is driven by ATP hydrolysis, the pump is transiently phosphorylated;

phosphorylation is stimulated by one ion and dephosphorylation is stimulated by the other ion. Since all of the protein is in the phosphorylated form in the absence of Zn²⁺ (lane F), Zn²⁺ is probably required for dephosphorylation. K⁺, then, probably binds to the dephosphorylated form and stimulates the

ATPase/autophosphorylation. So, if Zn²⁺ is added to the phosphorylated pump, Zn²⁺ will stimulate dephosphorylation, trigger a conformational change, and be injected into the vesicle. K⁺ will stimulate the kinase activity of the pump, but since there is no ATP to be hydrolyzed in the interior of the vesicle, no

phosphorylation and hence no movement of K⁺ will occur.

12-9 Na⁺ is commonly used to drive coupled transport in animal cells because a steep

concentration gradient of Na⁺ (high outside and low inside) is maintained by the Na⁺–K⁺ pump. Na⁺ readily flows back into the cell down this gradient because of the negative membrane potential. The energy provided by the flow of Na⁺ down this steep

electrochemical gradient can be harnessed by coupled transporters.

12-10 (d) Ouabain inhibits the Na⁺–K⁺ pump and, therefore, leads to an increase in the intracellular concentration of Na⁺, which is continually leaking into the cell.

Uptake of glucose into epithelial cells occurs via a Na⁺-glucose symport, which uses the Na⁺ gradient to drive movement of glucose into the cell.

12-11 Choice (e) is the correct answer. The Na⁺–K⁺ pump keeps Na⁺ out directly by pumping it out and keeps Cl^– out indirectly by helping to maintain the negative membrane potential.

Cells do not have pumps for moving water molecules across the membrane (choice (a)), since the lipid bilayer is permeable to water. Bacteria do not have Na⁺–K⁺ pumps in their plasma membranes (choice (b)). The Na⁺–K⁺ pump cannot directly remove water

molecules from the cell; it helps maintain osmotic balance by pumping out the Na⁺ that leaks in, which would not help if the cell is in a solution of very low ionic strength (choice (c)). The plant cell wall is permeable to water and therefore cannot prevent osmosis (choice (d)).

12-12 (a)

12-13 Choice (b) is the correct answer. The major purpose of the Ca²⁺ pumps is to keep the cytosolic concentration of Ca²⁺ low. When Ca²⁺ does move into the cytosol, it alters the behavior of many proteins; hence Ca²⁺ is a powerful signaling molecule. It is not

involved in the catalytic activity of ER enzymes (choice (c)). Since the levels of Ca²⁺ are very low relative to the levels of K⁺ and Na⁺, the Ca²⁺ gradient does not have a

significant effect on the osmotic balance of the cell (choice (a)) or the membrane potential (choice (d)). They are not involved in K⁺ import (choice (e)).

12-14 Disagree. A symporter functions by transporting two solutes in the same direction while an antiporter transports two different solutes to opposite sides of the membrane.

Therefore, flipping the symporter around would not change its characteristics into that of an antiporter, as it should still transport both solutes in the same direction.

12-15 A. Without any ATP to provide energy for the Na⁺–K⁺ pumps, no ions will be pumped.

B. The pumps will utilize the energy from ATP hydrolysis to transport Na⁺ out of the vesicles and K⁺ into the vesicles. (The pumps will stop working when either the amount of ATP inside the vesicle is depleted or, when the K⁺ outside of the vesicles is depleted.)

C. The pump will bind a molecule of Na⁺, causing the ATPase activity to hydrolyze ATP and transfer the phosphate group onto the pump. A conformational change will occur, leading to release of Na⁺ from the vesicle. However, since there is no K⁺ outside of the vesicle, the pump will get stuck at that step and subsequent steps of the cycle will not occur.

12-16 For an uncharged molecule, the direction of passive transport across a membrane is determined solely by its concentration gradient. On the other hand, for a charged molecule, an additional force called the membrane potential must also be considered.

The net driving force for a charged molecule across a membrane therefore has two components and is referred to as the electrochemical gradient. Active transport allows the movement of solutes against this gradient. The carrier proteins called coupled

transporters utilize the movement of one solute down its gradient to provide the energy to drive the uphill transport of a second gradient. When this transporter moves both ions in the same direction across the membrane, it is considered a(n) symport; if the ions move in opposite directions, the transporter is considered a(n) antiport.

12-17 Choice (e) is the correct answer. Ions can pass either way through a channel; the direction in which flow takes place depends on the concentration gradient and the membrane potential. Some ion channels require a specific stimulus to open them; others, such as K⁺ leak channels, do not (choice (a)). Ion channels are passive transporters and therefore require no energy source in order to function (choice (b)). Ion channels are very fast relative to carrier proteins but are limited by the rate at which ions can move through the channel (choice (c)). An ion channel allows specific positive or negative ions to pass, but not both (choice (d)).

12-18 (b)

12-19 1—c; 2—d; 3—b; 4—a

12-20 A. The acetylcholine receptor in skeletal muscle cells is a ligand-gated ion channel.

B. Stress-activated ion channels are found in the hair cells of the mammalian cochlea.

C. Voltage-gated ion channels in the mimosa plant propagate the leaf-closing response.

D. Voltage-gated ion channels respond to changes in membrane potential.

E. Many receptors for neurotransmitters are ligand-gated ion channels.

12-21 (b), (e) The Na⁺–K⁺ pump continually transports K⁺ into the cell. The negative membrane potential also helps to retain K⁺ in the cell. The K⁺ leak channels allow K⁺ to move both into and out of the cell and so do not contribute to the accumulation of K⁺ in the cell.

12-22 Agree. The membrane potential arises from a thin layer of ions that are close to the membrane. Only a small number of ions (compared to the number of ions present) must move across the membrane to set up the membrane potential.

12-23 A—4; B—1; C—3; D—2

12-24 The action potential is a wave of depolarization that rapidly spreads along the neuronal plasma membrane. This wave is triggered by a local change in the membrane potential to a value that is less negative than the resting membrane potential. The action potential is propagated by the opening of voltage-gated channels. During an action potential, the membrane potential changes from negative to positive. The action potential travels from the neuron’s cell body along the axon to the nerve terminals. Neurons chiefly receive signals at their highly branched dendrites.

12-25 See figure A12-25.

Figure A12-25

12-26 The ability of the voltage-gated Na⁺ channel to adopt an inactivated conformation allows for the action potential to move along the membrane in a wave-like fashion. During the firing of an action potential, the voltage-gated Na⁺ channel will open and then, after a delay, adopt the inactive form. When the voltage-gated Na⁺ channel adopts an inactive form, the membrane potential begins to return to its resting potential and cannot support the production of another action potential (i.e., further membrane depolarization) until the channel changes from an inactive to a closed state.

12-27 Neurons communicate with each other through specialized sites called synapses. Many neurotransmitter receptors are ligand-gated ion channels that open transiently in the postsynaptic cell membrane in response to neurotransmitters released by the

presynaptic cell. Ligand-gated ion channels in nerve cell membranes convert chemical signals into electrical ones. Neurotransmitter release is stimulated by the opening of voltage-gated Ca²⁺ channels in the nerve terminal membrane.