CHAPTER 12: TRANSPORT ACROSS CELL MEMBRANES

The Nernst equation can be used to calculate the membrane potential based on the ratio of the outer and inner ion concentration. In a resting cell, the membrane potential is calculated taking only K+ ions into account. What is V when Co = 15 mM and Ci =106 mM?

(a) 438.1 mV (b) -52.7 mV (c) 52.7 mV (d) -5.3 mV

When the net charge on either side of the plasma membrane is zero, what else is true?

(a) There an equal number of K+is ions on each side of the plasma membrane. (b) The K+ leak channels are open. (c) The electrochemical potential across the membrane is zero. (d) The resting membrane potential is between -20 mV and -200 mV.

Which of the following gated ion channels are involved in inhibitory synaptic signaling?

(a) voltage-gated Na channels 2+ (b) voltage-gated Ca channels (c) g lycine-gated Cl-channels (d) glutamate-gated cation channels

The flow of ions through a gated channel can be studied using a method called "patch- clamp recording." A. How is a detached patch-clamp experiment set up, and what exactly does it mean to "clamp" an ion channel?

A detached patch-clamp experiment requires the removal of a portion of the cell membrane by sealing the microelectrode to the membrane surface. After lifting the patch of membrane stuck to the microelectrode, it is placed into a solution of controlled medium. The voltage applied to the membrane patch can be fixed (clamped) while other parameters are studied.

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

Doubling the amount of Na+ in the extracellular fluid will increase the height of the peak of the action potential. Again, this is because now the driving force for Na+ to enter the cell is greater than it was before. Thus, when Na channels open, the flux of Na+ ions is now greater. (Remember that flux is the number of ions entering per second.)

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. B. Action potentials are usually mediated by voltage-gated Ca2+ channels.

FALSE. Action potentials are usually mediated by voltage-gated Na channels.

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. D. Voltage-gated K+ channels also open immediately in response to local depolarization, reducing the magnitude of the action potential.

FALSE. Action potentials are usually mediated by voltage-gated Na channels. Because voltage-gated K+channels do not open until the action potential reaches its peak, they do not affect its magnitude. Instead, they help to restore the local membrane potential quickly while the voltage-gated Na+ channels are in the inactivated conformation.

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. The extracellular concentration of Ca2+4 D. The primary mechanism by which Ca2+ acts as a signaling molecule is by increasing the net charge in the cytosol.

FALSE. Ca2+ binds tightly to many proteins in the cell, which in turn changes their activity. This interaction is the primary mechanism by which Ca2+ signaling occurs.

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. The extracellular concentration of Ca2+4 C. Cytosolic Ca2+ concentration is kept low by the use of chelators such as EDTA.

FALSE. Ca2+ concentrations in the cytosol are kept low by the action of ATP- driven calcium pumps in the endoplasmic reticulum membrane and the plasma membrane.

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. B. Aquaporin channels are found in the plasma membrane and allow the rapid passage of water molecules and small ions in and out of cells.

FALSE. Charged molecules (even protons, which are very small) are not able to pass through aquaporins.

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. A. Gap junctions are large pores that connect the cytosol to the extracellular space.

FALSE. Gap junctions are used to connect the cytosol of adjacent cells, allowing the sharing of ions and small metabolites. Because gap junctions are large channels, if they were open while facing the extracellular environment, the ability of the plasma membrane to serve as a permeability barrier would be greatly reduced.

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. D. The net negative charge on the cytosolic side of the membrane enhances the rate of glucose import into the cell by a uniporter.

FALSE. Glucose is an uncharged molecule, and its import is not directly affected by the voltage difference across the membrane if glucose is being transported alone.If the example given were the Na/glucose symporter, we would have to consider the charge difference across the membrane.

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. C. The ion selectivity of a channel depends solely on the charge of the amino acids lining the pore inside the channel.

FALSE. Selectivity depends on three parameters: the diameter, shape, and charge of the ion trying to pass through the pore of the channel.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. B. The differences in permeability between artificial lipid bilayers and cell membranes arise from variations in phospholipid content.

FALSE. The primary difference between cell membranes and artificial membranes is that cell membranes have proteins responsible for creating a selective permeability, which varies with the location and function of the membrane.

18 Indicate whether the statements below are true or false. If a statement is false, explain why it is false. A. Facilitated diffusion can be described as the favorable movement of one solute down its concentration gradient being coupled with the unfavorable movement of a second solute up its concentration gradient.

FALSE. This describes coupled transport, which is one type of active transport. Facilitated diffusion can also be called passive transport, in which a solute always moves down its concentration gradient.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. C. larger, foldedTransporters are similar to channels, except that they are allowing proteins as well as smaller organic molecules to pass through them.

FALSE. Transporters work by changing conformation after specific binding of the solute to be transported. Channels exclude molecules on the basis of size and charge, but do not depend on specific recognition of the molecules moving through.

Active transport requires the input of energy into a system so as to move solutes against their electrochemical and concentration gradients. Which of the following is not one of the common ways to perform active transport?

K+-coupled; because K+ is a positively charged ion and the outside of the plasma membrane is positively charged, K+ has a very small electrochemical gradient across the membrane even though its concentration gradient is large. Because there is a little net movement across the membrane for K+, it would not make a good source of energy to drive the transport of other molecules against their respective gradients.

Which of the following channels would not be expected to generate a change in voltage by movement of its substrate across the membrane where it is found?

AQUAPORIN; where the channels are found in the plasma membrane of some cells, facilitate diffusion of water across the membrane. Because water is an uncharged molecule, its movement would not be expected to alter the voltage across the membrane.

If ATP production is blocked in an animal cell, the cell will swell up. Explain this observation.

ATP is required to power the Na++ pump, which is necessary for maintaining osmotic -K+ balance. The pump requires ATP hydrolysis to drive its pumping cycle. So, in the absence of ATP production, the Na+ concentration inside the cell will increase. This is followed cells to swell by passive diffusion of water across the membrane, causing the ATP to provide energy for the Na++ pumps, no ions will be

You have prepared lipid vesicles (spherical lipid bilayers) that contain Na++ pumps as -K the sole membrane protein. All of the Na++ pumps are oriented in such a way that the -K portion of the molecule that normally faces the cytosol is on the inside of the vesicle and the portion of the molecule that normally faces the extracellular space is on the outside of the vesicle. Assume that each pump transports one Na++ion in one direction and one K ion in the other direction during each pumping cycle (see Figure Q12-24 for how the Na+-K+ pump normally functions in the plasma membrane). Predict what would happen in each of the following conditions: C. The solution outside contains Na++; the solution inside contains Na and ATP.

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 the release of Na+from the vesicle. However, because there is no K+ outside the vesicle, the pump will get stuck at that step and subsequent steps of the cycle will not occur.

Neurotransmitter release is stimulated by the opening of voltage-gated __________________ in the nerve-terminal membrane.

Ca2+ channels

27 You have generated antibodies that recognize the extracellular domain of the Ca2+ -pump. Adding these antibodies to animal cells blocks the active transport of Ca2+ from the cytosol into the extracellular environment. What do you expect to observe with respect to intracellular Ca2+?

Ca2+-pumps in the endoplasmic reticulum membrane keep cytosolic calcium levels low.

Although the extracellular environment has a high sodium ion concentration and the intracellular environment has a high potassium ion concentration, both must be neutralized by negatively charged molecules. In the extracellular case, what is the principal anion?

Cl-

Cells use membranes to help maintain set ranges of ion concentrations inside and outside the cell. Which of the following negatively charged ions is not primarily used to buffer positive charges inside the cell?

Cl-

Negatively charged ions are required to balance the net positive charge from metal ions ++2+ such as K, Na, and Ca. Which of the following negatively charged ions is the most abundant in outside the cell and which ion does most often neutralize (written parentheses)?

Cl-(Na+)

The stimulation of a motor neuron ultimately results in the release of a neurotransmitter at the synapse between the neuron and a muscle cell. How is the chemical signal converted into an electrical signal in the postsynaptic muscle cell?

Most neurotransmitter receptors function as ligand-gated ion channels. These ion channels are similar to voltage-gated channels, except that they do not open in response to a change in voltage across the membrane, but to the binding of a neurotransmitter. In the neuromuscular junction, the neurotransmitter acetylcholine binds to the acetylcholine receptor, which allows Na+ to enter the muscle cell, altering its membrane potential. In this way, a chemical signal (acetylcholine) is converted back into an electrical signal (change in membrane potential).

Cells use membranes to help maintain set ranges of ion concentrations inside and outside the cell. Which of the following ions is the most abundant outside a typical mammalian cell?

Na+

Voltage-gated channels contain charged protein domains, which are sensitive to changes in membrane potential. By responding to a threshold in the membrane potential, these voltage sensors trigger the opening of the channels. Which of the following best describes the behavior of a population of channels exposed to such a threshold?

Some channels remain closed and some open completely.

For each of the following sentences, fill in the blank with the appropriate type of gating for the ion channel described. You can use the same type of gating mechanism more than once. B. _________ ion channels are found in the hair cells of the mammalian cochlea.

Stress-gated

Which of the following statements best reflects the nature of synaptic plasticity?

Synaptic response changes in magnitude depending on frequency of stimulation.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. CO2 and O2 are water-soluble molecules that diffuse freely across cell membranes.

TRUE

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. D. Cells expend energy in the form of ATP hydrolysis so as to maintain ion concentrations that differ from those found outside the cell.

TRUE

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. A. Neurotransmitters are small molecules released into the synaptic cleft after the fusion of synaptic vesicles with the presynaptic membrane.

TRUE

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. B. Transporters undergo transitions between different conformations, depending on whether the substrate-binding pocket is empty or occupied.

TRUE

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. C. The electrochemical gradient for K+ across the plasma membrane is small. Therefore, any movement of K+ solely by its concentration gradient.

TRUE

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. C. Voltage-gated Na+ channels become automatically inactivated shortly after opening, which ensures that the action potential cannot move backward along the axon.

TRUE

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. D. Most ion channels are gated, which allows them to open and close in response to a specific stimulus, rather than allowing the constant, unregulated flow of ions.

TRUE

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. The extracellular concentration of Ca2+4 A. is approximately 10-fold higher than the 2+ the concentration of Ca in the cytosol.

TRUE

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. The extracellular concentration of Ca2+4 B. The low cytosolic Ca2+2+ concentration sensitizes the cell to an influx of Ca, ensuring a rapid response to environmental stimuli.

TRUE

Fill in Table Q12-23. In the "Type of transport" column, designate whether the transporter works by uniport, symport, or antiport mechanisms.

Table A12-23

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?

The action potential in this case will reach a height that is less than that normally achieved. (Choline is added in this case to maintain bulk electrical neutrality. Because Na+ channels are not permeable to choline, choline does not contribute to the electrochemical gradient.) You have now halved the concentration of Na+ and thus decreased the driving force for Na+to enter the cell.

Which of the following statements does not accurately describe the events involved in the propagation of an action potential?

The opening of transmitter-gated K+ channels helps to repolarize the membrane.

When using the Nernst equation to calculate membrane potential, we are making several assumptions about conditions in the cell. ] Which of the following is not a good assumption?

The plasma membrane is primarily permeable to Na+; The cell has K+ leak channels. At rest, the cell is mostly permeable to K because of the presence of these channels.

Which of the following statements about resting membrane potential is not true?

The resting membrane potential for most animal cells is negative because the inside of the cell is more negatively charged than the outside of the cell.

K+ leak channels are found in the plasma membrane. These channels open and close in an unregulated, random fashion. What do they accomplish in a resting cell?

They keep the electrochemical gradient for K+ at zero.

Which of the following statements about GABA receptors is not true?

They promote neuronal uptake of Na+

Describe how synaptic signaling is influenced by the action of tranquilizers (such as Valium®) compared to the effects of antidepressants (such as Prozac®).

Tranquilizers promote inhibitory signaling at the synapse. They do this by making GABA-gated channels open more easily in response to the inhibitory neurotransmitter GABA. This channel, when open, allows Cl- to flow into the neuron, making the cell more difficult to depolarize. Antidepressants are used to enhance the neuronal signaling in neurons that have serotonin receptors. Prozac® specifically blocks the reuptake of serotonin in the synaptic cleft. This results in a net increase in serotonin available for excitatory signaling to the postsynaptic neuron.

__________________ are highly selective in the solutes they transport, binding the solute at a specific site and changing conformation so as to transport the solute across the membrane.

Transporter proteins

For each of the following sentences, fill in the blank with the appropriate type of gating for the ion channel described. You can use the same type of gating mechanism more than once. C. _________ ion channels in the mimosa plant propagate the leaf-closing response.

Voltage-gated

For each of the following sentences, fill in the blank with the appropriate type of gating for the ion channel described. You can use the same type of gating mechanism more than once. D. _________ ion channels respond to changes in membrane potential.

Voltage-gated

In a method called patch-clamping, a glass capillary can be converted into a microelectrode that measures the electrical currents across biological membranes. Which of the following is not true about the patch-clamp method?

a) The glass capillary adheres to a "patch" of membrane through the application of suction. (b) The aperture in the glass capillary used to make a microelectrode is about 1 μm in diameter. (c) If the experimental conditions are held constant, fluctuations in electrical currents across the patch of membrane are still observed.( d) Single-channel patch-clamp recordings have demonstrated that gated membrane channels will only open and close in response to specific stimuli.

Both excitatory and inhibitory neurons from junctions with muscles. By what mechanism do inhibitory neurotransmitters prevent the postsynaptic cell from firing an action potential?

a) by closing Na+ channels (b) by preventing the secretion of excitatory neurotransmitters (c) by opening K+ channels (d) by opening Cl- channels

The stimulation of a motor neuron ultimately results in the release of a neurotransmitter at the synapse between the neuron and a muscle cell. What type of neurotransmitter is used at these neuromuscular junctions?

acetylcholine

Figure Q12-55 illustrates changes in membrane potential during the formation of an action potential. What membrane characteristic or measurement used to study action potentials is indicated by the arrow?

action potential

Pumps are transporters that are able to harness energy provided by other components in the cells to drive the movement of solutes across membranes, against their concentration gradient. This type of transport is called _____________.

active transport.

The action potential travels along the neuron's __________________ to the nerve terminals.

axon

On the other hand, __________________ discriminate between solutes mainly on the basis of size and electrical charge.

ion channels

This wave is triggered by a local change in the membrane potential to a value that is __________________ negative than the resting membrane potential.

less

For each of the following sentences, fill in the blank with the appropriate type of gating for the ion channel described. You can use the same type of gating mechanism more than once. A. The acetylcholine receptor in skeletal muscle cells is a(n) _________ ion channel.

ligand-gated

For each of the following sentences, fill in the blank with the appropriate type of gating for the ion channel described. You can use the same type of gating mechanism more than once. E. Many receptors for neurotransmitters are _________ ion channels.

ligand-gated

On the other hand, for a charged molecule, the __________________ must also be considered.

membrane potential

During an action potential, the membrane potential changes from __________________ to __________________.

negative; positive

A molecule moves down its concentration gradient by __________________ transport, but requires __________________ transport to move up its concentration gradient.

passive; active

Many neurotransmitter receptors are ligand-gated ion channels that open transiently in the __________________ cell membrane in response to neurotransmitters released by the __________________ cell.

postsynaptic; presynaptic

Figure Q12-54 illustrates changes in membrane potential during the formation of an action potential. What membrane characteristic or measurement used to study action potentials is indicated by the arrow?

resting membrane potential

Some cells have aquaporins—channels that facilitate the flow of water molecules through the plasma membrane. For these cells, what regulates the rate and direction of water diffusion across the membrane?

solute concentrations on either side of the membrane

Transporters, in contrast to channels, work by ________________.

specific binding to solutes.

The stimulation of auditory nerves depends on the opening and closing of channels in the auditory hair cells. Which type of gating mechanism do these cells use?

stress-gated

When this transporter moves both ions in the same direction across the membrane, it is considered a(n) __________________; if the ions move in opposite directions, the transporter is considered a(n) __________________. antiport coupled membrane potentialATP hydrolysis electrochemical symportconcentration light-driven uniport

symport; antiport

Neurons communicate with each other through specialized sites called __________________.

synapses

The Na++ ATPase is also known as the Na++ pump. It is responsible for -K-K maintaining the high extracellular sodium ion concentration and the high intracellular potassium ion concentration. What happens immediately after the pump hydrolyzes ATP?

the pump is phosphorylated; the pump causes the conformational change, and it occurs after the binding of Na+.

Approximately, how many distinct synapses are established on the dendrites and cell body of a motor neuron in the spinal cord?

thousands

Figure Q12-53 illustrates changes in membrane potential during the formation of an action potential. What membrane characteristic or measurement used to study action potentials is indicated by the arrow?

threshold potential

The action potential is propagated by the opening of __________________- gated channels.

voltage

Which of the following is required for the secretion of neurotransmitters in response to an action potential?

voltage-gated Ca2+ channels

A hungry yeast cell lands in a vat of grape juice and begins to feast on the sugars there, producing carbon dioxide and ethanol in the process: C6H12O6 + 2ADP + 2P + H+ ◊ 2CO + 2CH3CH2OH + 2ATP + 2H2O Unfortunately, the grape juice is contaminated with proteases that attack some of the transport proteins in the yeast cell membrane, and the yeast cell dies. Which of the following could account for the yeast cell's demise?

inability to import sugar into the cell

Which of the following occur without coupling transport of the solute to the movement of a second solute?

export of Ca2+ from the cytosol

What would you expect to happen if you treat vesicles as in lane F, but before determining the phosphorylation state of the protein, you wash away the outside buffer and replace it with a buffer containing only Zn2+ ?

(d) A small amount of Zn2++ will move into the vesicle; no K will move out of the vesicle; the protein will become unphosphorylated.

Below is a list of molecules with different chemical characteristics. Knowing that all molecules will eventually diffuse across a phospholipid bilayer, select the option below that most accurately predicts the relative rates of diffusion of these molecules (fastest to slowest). alanine estrogen propanol sodium

(d) estrogen > propanol > alanine > sodium Estrogen is a steroid hormone and will diffuse the fastest across the membrane. Propanol is a small, uncharged molecule with a polar group. Alanine is an amino acid, and although it has a small, nonpolar side group, amino acids are charged molecules. Sodium is an ion and will move the slowest across the bilayer.

Circle the molecule in each pair that is more likely to diffuse through the lipid bilayer.

A. benzene (small nonpolar versus larger uncharged) B. ethanol (polar versus charged) C. glycerol (small polar versus very large, highly charged) D. O2 (nonpolar versus polar) E. adenosine (polar versus highly charged)

You have prepared lipid vesicles (spherical lipid bilayers) that contain Na++ pumps as -K the sole membrane protein. All of the Na++ pumps are oriented in such a way that the -K portion of the molecule that normally faces the cytosol is on the inside of the vesicle and the portion of the molecule that normally faces the extracellular space is on the outside of the vesicle. Assume that each pump transports one Na++ion in one direction and one K ion in the other direction during each pumping cycle (see Figure Q12-24 for how the Na+-K+ pump normally functions in the plasma membrane). Predict what would happen in each of the following conditions: A. The solutions inside and outside the vesicles contain both Na++ ions but no and K ATP

A.-K pumped.

It is thought that the glucose transporter switches between two conformational states in a completely random fashion. How is it possible for such a system to move glucose across the membrane efficiently in a single direction?

Although the opening of the glucose transporter on one side of the membrane or the other is random, the binding of glucose into the binding site of the transporter is not a random event. The affinity between the glucose molecule and the transporter governs the binding event: transporter + glucose ↔ transporter-glucose At high glucose concentrations, the complex formation is favored; at low glucose concentrations, dissociation of glucose from the transporter is favored. So, as long as there is a large concentration gradient, efficient transport can occur by the simple rules of binding equilibria.

You have prepared lipid vesicles (spherical lipid bilayers) that contain Na++ pumps as -K the sole membrane protein. All of the Na++ pumps are oriented in such a way that the -K portion of the molecule that normally faces the cytosol is on the inside of the vesicle and the portion of the molecule that normally faces the extracellular space is on the outside of the vesicle. Assume that each pump transports one Na++ion in one direction and one K ion in the other direction during each pumping cycle (see Figure Q12-24 for how the Na+-K+ pump normally functions in the plasma membrane). Predict what would happen in each of the following conditions: B. The solution outside the vesicles contains both Na++ ions; the solution and K inside contains both Na++ ions and ATP and K

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

You are testing the rate of glucose transport into vesicles using the Na+-glucose pump. A. In experiment 1, you employ liposomes that have the pump in the same orientation as that found in the plasma membrane in epithelial cells. These liposomes contain glucose but no Na+ions. You then transfer the liposomes to a series of tubes with solutions containing the same glucose concentration as that inside the vesicle and 0, 1, 2, 3, or 10 mM Na+. You measure the initial rates of glucose transport and plot your results (Figure Q12-33). Why do the initial rates of glucose transport into the liposome reach a plateau as the concentration of Na+ increases?

Because transporters require recognition and binding of the transport substrate before moving it across the membrane, the transporters can reach a maximum capacity (saturation) at which we observe the maximum rate at which transport can occur, regardless of any further increase in the concentration of transport substrate. In this case, it seems that 10 mM Na+is close to saturating the experimental system being used.

The flow of ions through a gated channel can be studied using a method called "patch- clamp recording." B. How is it possible to collect the recordings shown in Figures Q12-37A and Q12- 37B from a single ion channel?

By manipulating ion concentrations in the two chambers or simply reversing the direction of the current in the system, the ion flow through the channel can be reversed, resulting in the recording of negative values for current when the channel opens.

Describe the process by which gut epithelial cells use transporters to take up ingested glucose (against the concentration gradient) and to distribute glucose to other tissues by moving it back out of the cell (down the concentration gradient).

Gut epithelial cells use two different transporters to take glucose up from the gut and distribute it into the bloodstream and to other tissues. These transporters are located at opposite sides of the cell: the apical side of the cell (which es the gut) contains a Na+ fac-glucose symporter. This symporter couples the entry of Na+ down its electrochemical gradient to the active import of glucose against its concentration gradient. The Na+-glucose symporter is restricted to the apical side of the cell by tight-junction complexes in the plasma membrane, which link neighboring epithelial cells together. On the basolateral side of the cell, there is another transporter that facilitates movement of glucose down its concentration gradient, out of the cell. This transporter is a uniporter that only transports glucose in one direction: from the cytosol to the extracellular matrix. The location of this uniporter is also restricted by the presence of the tight junctions, so that the epithelial cell will not transport glucose back into the lumen of the gut.

Ion channels are classified as membrane transport proteins. Channels discriminate by size and charge. In addition to Na+, which one of the following ions would you expect to able to freely diffuse through a Na+ channel? Explain your answer.

H+ If an ion channel is open, it will allow any ion that is under a certain size and that has the correct charge to pass through. H+is the only ion listed that is both smaller and has the same charge of +1.

If Na+ channels are opened in a cell that was previously at rest, how will the resting membrane potential be affected?

It becomes more positive.

Which of the following best describes the behavior of a gated channel?

It opens more frequently in response to a given stimulus.

Cells use membranes to help maintain set ranges of ion concentrations inside and outside the cell. Which of the following ions is the most abundant inside a typical mammalian cell?

K+

The field of neurobiology is seeing rapid advances in our understanding of neural circuitry in the brain. Part of this work involves the physical mapping of all synapses (a project dubbed the connectome); another critical advance is in the area of optogenetics, which allows scientists to dissect neural circuits that determine specific behaviors in a range of organisms from fruit flies to monkeys. B. Optogenetics has great potential for deepening our understanding of behavior, learning, memory, and cognitive development. However, it is not as likely to be used directly for the treatment of problems such as depression or anxiety. Why not? Consider the limitations of the method when answering this question.

Optogenetics currently uses viral vectors to introduce the genes encoding the light-gated channels into the neurons of interest. This approach can be potentially problematic, as insertion of this engineered DNA at off-target sites in the genome could cause mutations that would alter or destroy the activity of critical genes. In addition, the stimulation of neurons by optogenetics requires a light source. In the case of a human patient, this would mean inserting a fiber optic light source into the region of the brain that contains the foreign channels. This would represent a highly experimental treatment, hence it is currently only used in animal models to explore circuitry we do not yet fully understand.

The field of neurobiology is seeing rapid advances in our understanding of neural circuitry in the brain. Part of this work involves the physical mapping of all synapses (a project dubbed the connectome); another critical advance is in the area of optogenetics, which allows scientists to dissect neural circuits that determine specific behaviors in a range of organisms from fruit flies to monkeys. A. Describe the method of optogenetics.

Optogenetics is a method in which genetic engineering techniques are used to introduce light-gated channels into a selected set of target neurons. Light of a specific wavelength is then used to open the channel, which allows the investigator to directly control the activity of these neurons in the living organism. When the channels used are light-gated Na channels, stimulation will allow Na + to enter the neurons, triggering an action potential. Light-gated Cl- channels can similarly, be used to inhibit neural activity.

Describe the two forces that drive an ion across the plasma membrane and explain how the Nernst equation takes into account both of these forces. Use the components of the equation to support your explanation and be sure to specify the assumptions being made when using the Nernst equation to calculate membrane potential.

The forces that drive the movement of an ion across the plasma membrane include a concentration gradient (that is, there is a negative change in free energy associated with an increase in entropy for ions in solution) and an electrical component (the force resulting from the attraction between molecules of opposite charges). The Nernst equation expresses the change in voltage across the membrane as it relates to a change in the ratio of ions on either side of the plasma membrane. As written below, the voltage changes by 62 millivolts with every tenfold change in the ion concentration ratio across the membrane.

You are testing the rate of glucose transport into vesicles using the Na+-glucose pump. B. In experiment 2, there is one new variable: you have included leaky Na+ channels in the liposomal membrane. Figure Q12-33 shows your results. Explain the reason for obtaining such different results in experiment 2 from those in experiment 1.

The leaky Na+ channels allow rapid equilibration of Na ions across the liposomal membranes, destroying the gradient required for glucose transport into the liposome. Thus, there is no net movement of glucose into the liposome at any of the Na+ion concentrations tested.

The movement of glucose into the cell, against its concentration gradient, can be powered by the co-transport of Na+ into the cell. Explain this movement with respect to the net entropy of the system (that is, thermodynamics).

The movement of Na+ ions from an area that has a high Na concentration to a new area of low Na+ concentration is energetically favorable because the net entropy in the system is increasing. As long as the difference in Na+ ion concentration across the membrane is large, the entropic factor will be sufficient to drive the import of glucose into the cell, which represents a decrease in entropy with respect to the population of glucose molecules inside the cell.

Ligand-gated ion channels in nerve cell membranes convert __________________ signals into __________________ ones.

chemical; electrical

For an uncharged molecule, the direction of passive transport across a membrane is determined solely by its __________________ gradient.

concentration

Active transport allows the movement of solutes against this gradient. The transporter proteins called __________________ transporters use the movement of one solute down its gradient to provide the energy to drive the uphill transport of a second solute.

coupled

Neurons chiefly receive signals at their highly branched __________________.

dendrites

The action potential is a wave of __________________ that spreads rapidly along the neuronal plasma membrane.

depolarization

Figure Q12-56 illustrates changes in membrane potential during the formation of an action potential. What membrane characteristic or measurement used to study action potentials is indicated by the arrow?

effect of a depolarizing stimulus

The net driving force for a charged molecule across a membrane therefore has two components and is referred to as the __________________ gradient.

electrochemical

We can test the relative permeability of a phospholipid bilayer by using a synthetic membrane that does not contain any protein components. Some uncharged, polar molecules are found to diffuse freely across these membranes, to varying degrees. Which of the following has the lowest rate of diffusion across an artificial membrane? Why?

glucose

Transporter proteins and ion channels function in membrane transport by providing a __________________ pathway through the membrane for specific polar solutes or inorganic ions.

hydrophilic

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 to which the membrane is impermeable. In this way, you double the internal concentration of K+ while maintaining the bulk electrical balance of the cytoplasmic solution. Will this make the resting potential of the membrane more or less negative?

in the cytoplasm of the squid axon will make the membrane potential more negative. Doubling the amount of K+ increases the driving force for K+ to move out of the cell, leaving the inside of the cell more negative and thus decreasing the membrane potential. (Remember, from the Nernst equation, the driving force for an ion across a membrane is proportional to the ratio of the concentration of the ion on the outside to the concentration of the ion on the inside.)