Chapter 12 Cellular
Given the type of transporter as determined in Part 1, choose all of the correct statements below that relate to the function of the Na+/Ca2+ transporter. A.Ca2+ is transported against its electrochemical gradient. B.Na+ is transported against its electrochemical gradient. C.The transporter directly uses ATP as an energy source for transporting ions. D.The transporter uses the Na+ electrochemical gradient as an energy source for transporting ions.
-Ca2+ is transported against its electrochemical gradient. -The transporter uses the Na+ electrochemical gradient as an energy source for transporting ions. E: An antiport transports one substrate down its concentration gradient while a second substrate is transported against its gradient. The energy from the movement of the first substrate is used to drive the transport of the second substrate against its gradient. An antiport moves the two substrates in opposite directions across the membrane. The Na+/Ca2+ antiport uses the energy in the Na+ gradient to transport Ca2+ out of the cell against its gradient as Na+ comes into the cell down its gradient.
Which of the following characteristics of aquaporins ensure that the channel selectively transports only water molecules and not other solutes?
-Two asparagines in the center of the pore prevent protons from passing through the channel. -The channel has a narrow pore that is only wide enough for a single water molecule to pass through. Explanation: Aquaporin, like other channels, is specific for one substrate: water. The aquaporin channel is narrow, so only a single water molecule can pass through at one time. The channel is also lined with two asparagine side chains that function as selectivity filters to block protons from moving through the channel. Together, the size of the channel and the two asparagines function to make the channel specific for water. Water will move through the channel down the concentration gradient
Intracellular Ca2+ levels are important in cardiac muscle. Increasing intracellular Ca2+ levels in heart muscle cells leads to an increase in muscle contraction. Lowering the intracellular Ca2+ levels decreases the strength of cardiac muscle contraction. Congestive heart failure can occur when the heart's pumping of blood is weaker than normal, which leads to fluid collecting around organs, including the heart. One treatment method is to give the patient drugs that increase the strength of the heart muscle contraction. Which of the following might function as an effective treatment of congestive heart failure by increasing the strength of heart muscle contraction? A.a drug that inhibits the Na+-K+ pump from establishing a strong Na+ gradient B.a drug that blocks the calcium channel in heart muscle cells C.a diuretic drug that triggers removal of excess Na+ from the body D.a drug that decreases the activity of the Na+/Ca2+ transporter
-a drug that inhibits the Na+-K+ pump from establishing a strong Na+ gradient -a diuretic drug that triggers removal of excess Na+ from the body -a drug that decreases the activity of the Na+/Ca2+ transporter E: Treating congestive heart failure is complex and many lifestyle changes and drugs are often used to control symptoms and lessen worsening of symptoms. One drug, digoxin, inhibits the Na+-K+ pump on heart cells, which leads to a smaller Na+ gradient across the membrane. A smaller Na+ gradient will drive less Ca2+ out of the cell through the Na+/Ca2+ antiport. More Ca2+ will remain in the cytosol, leading to stronger heart contraction and a reduction in symptoms. While used for many years as a drug, digoxin has now been shown to increase risk of death in some patients and so is used more rarely. Other treatments are now more common. Diuretics are still often prescribed for congestive heart failure. Diuretics remove both Na+ and fluid from the body. This helps lessen the congestion symptoms. Additionally, removing Na+ reduces the Na+ gradient as described above, leading to less Ca2+ export and increased cytosolic Ca2+ levels and a stronger heart contraction. Drugs that decrease the activity of the Na+/Ca2+ transporter would increase the Ca2+ concentration inside the cell and increase the strength of heart muscle contraction. These drugs exist and have been prescribed for congestive heart failure. However, they are no longer commonly used for treating congestive heart failure as the risk of side effects and drug interactions is high. Some drugs in this class are still used for other heart problems, though. Calcium channel blockers are contraindicated for the treatment of congestive heart failure. These drugs block the calcium channel used to bring Ca2+ back into the cell down its concentration gradient. These drugs would weaken the heart muscle contraction and worsen congestive heart failure.
The movement of an ion down its concentration gradient is called what? -osmosis -active transport -pumping -passive transport
-passive transport E: The movement of an ion down its concentration gradient is called passive transport. For example, if a solute is present at a higher concentration outside the cell than inside, and an appropriate channel or transporter is present in the plasma membrane, the solute will move into the cell by passive transport, without expenditure of energy by the membrane transport protein. The opposite of passive transport is active transport, where the movement of an ion is against its concentration gradient. Ion pumping is a term that describes the active transport of an ion or molecule against its concentration gradient.
Our hearing relies upon several cellular structures that go through a set of steps to signal the receipt of sound to the nerves. Put the following cellular steps involved in response to sound in the proper order.
1) The basilar membrane vibrates 2) The basilar membrane pushes stereocilia against the tectorial membrane 3) The stereocilia tilt, triggering an electrical response in the hair cell 4) The activated hair cell triggers the auditory nerve to fire E: When sound vibrations enter the ear, the basilar membrane vibrates in response. As the extracellular matrix membrane vibrates, it pushes the stereocilia on a hair cell up against a second extracellular membrane, the tectorial membrane. The stereocilia are tilted by the pressure against the tectorial membrane. Stretch-activated ion channels on the stereocilia open in response to the tilt, triggering an electrical response in the hair cell. The activated hair cell releases neurotransmitters that activate the auditory nerve cell, which passes the signal to the brain that a sound was heard. When exposed to extremely loud sounds, the stereocilia of the hair cells can be pushed against the tectorial membrane so hard that damage can occur. Hair cells do not regenerate, and so the damage from loud sounds leads to permanent hearing loss.
In general, which of the following will diffuse across a lipid bilayer most rapidly? A. water B. a small hydrophobic molecule C. a small hydrophilic molecule D. a large hydrophobic molecule E. a large hydrophilic molecule
A small hydrophobic molecule E: The hydrophobic core of the lipid bilayer, which is formed from the hydrocarbon tails of phospholipids, creates a selective barrier that prevents many molecules from easily diffusing across the plasma membrane. However, small, nonpolar molecules, such as molecular oxygen (O2, molecular mass 32 daltons) and carbon dioxide (CO2, 44 daltons), can diffuse rapidly across the nonpolar, hydrophobic core of the membrane. On the other hand, large, hydrophilic molecules and charged ions do not interact favorably with the hydrophobic region of the lipid bilayer and will not diffuse rapidly across the membrane.
Which of the following describes the resting membrane potential of a neuron? -A voltage difference across the plasma membrane when the neuron has been stimulated -State in which the flow of positive and negative ions across the plasma membrane is precisely balanced -A voltage difference of 0 millivolts (mV) across the membrane -A voltage difference that is chiefly a reflection of the electrochemical Na+ gradient across the plasma membrane -A voltage difference across the plasma membrane, with more positive membrane potential inside
A state in which the flow of positive and negative ions across the plasma membrane is precisely balanced Explanation: The resting membrane potential is the voltage difference across a membrane of an unstimulated cell, and for most animal cells, the resting membrane potential is negative, between -20 and -200 mV. The resting membrane potential is a state in which the movement of positive and negative ions across the membrane is precisely balanced; in this state, no further differences in charge will accumulate and there are not necessarily an equal number of positive and negative charges on both sides of the membrane. Rather, the resting membrane potential in animal cells is chiefly a reflection of the electrochemical K+ gradient across the plasma membrane.
Ions in solution are found in a hydration shell of water. This shell must be removed for an ion to pass through the channel. How does the K+ channel accomplish removal of the water from the shell around the ion? A. Carbonyl groups lining the wall of the pore can interact with the unsolvated K+ ion, balancing the energy needed to remove the hydration shell. B. Rigid protein loops strip the hydration shell from the potassium so that the ion is the right diameter to pass through the pore. C. The K+ channel uses the energy in ATP hydrolysis to remove the hydration shell from the K+ ion. D. The K+ channel has four subunits; one subunit removes the hydration shell as the ion passes through the pore formed by the three other subunits.
A. Carbonyl groups lining the wall of the pore can interact with the unsolvated K+ ion, balancing the energy needed to remove the hydration shell. E: The K+ channel is formed from four identical subunits that together form a pore. The four rigid protein loops each contain amino acids with carbonyl side chains. These carbonyl groups are spaced at precisely the right distance to interact only with an unsolvated K+ ion stripped of its water molecules. The binding of a K+ ion to the carbonyl groups balances the energy needed to remove the hydration shell of water, allowing passage of the K+ ion through the channel.
The glucose-Na+ symport transports glucose into the epithelial cells lining the gut. How would import of glucose into the cells be affected by addition of a leaky Na+ channel to their plasma membrane? A. Glucose transport would slow because the Na+ gradient is dissipated by the Na+ channel. B. Glucose transport would increase because the Na+ gradient is strengthened by the Na+ channel. C. A leaky Na+ channel would not affect glucose transport because these two transporters are unrelated. D. Na+ transport would slow, but glucose transport would remain high because glucose could still be transported by the glucose-Na+ symport.
A. Glucose transport would slow because the Na+ gradient is dissipated by the Na+ channel. E: The glucose-Na+ symport transports Na+ down its concentration gradient while transporting glucose into the cell against its gradient. The Na+ gradient supplies the energy to transport glucose. The Na+ concentration must be higher outside the cell than inside to transport glucose. A leaky Na+ channel would dissipate the Na+ gradient. With a smaller Na+ gradient, the transport of glucose across the membrane would slow.
Tetrodotoxin is a potent toxin found in a variety of organisms including the pufferfish. The toxin binds to the extracellular side of the Na+ channel and prevents channel opening. This leads to paralysis of muscles, including the diaphragm. Death from respiratory failure can occur after ingestion of as little as 1 mg of the toxin. Why does this toxin cause paralysis? A. The membrane depolarization is not amplified along the axon. B. The Na+ channels remain in the inactive, refractory state. C. The axon membranes become over-depolarized. D. The Na+ channel does not open wide enough to allow enough Na+ through the channel.
A. The membrane depolarization is not amplified along the axon. E: The tetrodotoxin binds to the extracellular side of the Na+ channel and prevents the channel from opening. This prevents Na+ from entering the cytosol of the cell and the subsequent depolarization of the membrane. If the membrane does not depolarize, the signal is not propagated along the axon. The muscle at the axon terminus does not receive the proper signal and remains in a relaxed state. When this occurs in the muscles required for breathing, such as the diaphragm, the victim is unable to breathe and can die from respiratory failure.
What is the role of K+-gated ion channels in an action potential? A. They help reverse the action potential by repolarizing the cell. B. They provide the energy for the sodium-potassium pump to reestablish resting potential. C. They do not have a role in action potentials. D. They lead to the action potential reaching its highest state of cell depolarization.
A. They help reverse the action potential by repolarizing the cell. E: K+-gated ion channels open after membrane depolarization to allow K+ movement across the membrane to reestablish resting potential.
How does an action potential spread along the cell membrane? A. Voltage-gated Ca2+ channels are activated by the action potential and the calcium diffuses along the membrane. B. A change in membrane potential triggers the opening of nearby voltage-gated sodium channels in a one-way direction. C. Potassium leak channels quickly reverse the action potential to move the membrane depolarization away from the original site. D. The ions entering the cell upon triggering an action potential travel laterally along the membrane to carry the charge.
A. Voltage-gated Ca2+ channels are activated by the action potential and the calcium diffuses along the membrane. E: An action potential spreads along the cell membrane because the change in membrane potential triggers the opening of nearby Na+ channels, which then change the membrane potential in their local vicinity. This continues down the cell membrane in the direction away from the location of the originally stimulated channels.
Prozac, a common antidepressant medication, functions by altering neurotransmitter levels in the brain. How does Prozac work? A. by blocking the reuptake of serotonin after it has been released, increasing the amount available in the synapses that use it B. by decreasing the amount of serotonin released from the presynaptic neuron, decreasing the amount available in the synapses that use it C. by making serotonin-gated channels easier to open D. by increasing the amount of serotonin released from the presynaptic neuron, increasing the amount available in the synapses that use it
A. by blocking the reuptake of serotonin after it has been released, increasing the amount available in the synapses that use it E: Prozac works by blocking the reuptake of serotonin after it has been released, increasing the amount available in the synapses that use it. It is a member of a class of drugs known as selective serotonin reuptake inhibitors, or SSRI. Prozac blocks the Na+-driven symport that returns serotonin to the cell for reuse. When Prozac is taken by the patient, serotonin remains in the synapse between the presynaptic neuron and the postsynaptic neuron for a longer period of time. This encourages greater serotonin signaling.
Curare is a chemical purified from the bark of a South American vine, Chondrodendron tomentosum. South American native hunters place the curare on arrow tips. Animals shot with these arrows die from respiratory failure. Derivatives of curare have also been developed for medical use as a muscle relaxant that causes paralysis of muscles during surgery or other procedures. In this case, the muscle cell is the postsynaptic cell. Which of the following potential drug mechanisms would fit with the main outcome of muscle paralysis?
A. competitive inhibitor of the ligand-gated ion channel on the postsynaptic cell E: The mechanism of curare is to act as a competitive inhibitor of the ligand-gated ion channel on the postsynaptic cell. Curare blocks the ability of the postsynaptic cell to perceive the neurotransmitter signal and convert the chemical signal into an action potential in the postsynaptic cell. Both increased opening of the Ca2+ channel and inhibition of neurotransmitter reuptake would lead to increased levels of neurotransmitters in the synapse and would not cause the described paralysis.
What condition must exist for glucose to be transported into a cell using the glucose-Na+ symport? A. high Na+ concentration outside the cell B. high Na+ concentration inside the cell C. high ATP concentration inside the cell for phosphorylation of the glucose-Na+ symport D. high glucose concentration outside the cell
A. high Na+ concentration outside the cell E: The glucose-Na+ symport transports both Na+ and glucose into the cell. The Na+ is transported down its concentration gradient, releasing energy that is used to transport glucose into the cell against its concentration gradient. The Na+ concentration must be higher outside the cell than inside to transport glucose. The glucose will be transported with Na+ whether the glucose concentration is high or low inside the cell. ATP is not used by the glucose-Na+ symport, although it is used by the Na+-K+ pump, which establishes the Na+ gradient across the membrane.
Which type of membrane transport protein can perform either passive or active transport? A. transporters B. both channels and transporters C. Neither type of membrane transport protein can perform both passive and active transport. D. channels
A. transporters E: Only transporters can move a solute against its concentration gradient. Some transporters also allow passive transport of molecules down their concentration gradient. Passive transport, which is the movement of solutes across a membrane by following their concentration gradient, and active transport, which uses the input of energy to move solutes across a membrane and can move them against a concentration gradient, can both be accomplished by transporter proteins. A key aspect of active transport is the specificity of the molecule to be transported for its transporter protein. Transporters that accomplish active transport are called pumps.
Which type of ion channel plays the major role in propagating electrical signals in nerve cells? A. voltage-gated B. ligand-gated C. mechanically-gated
A. voltage-gated E: Voltage-gated ion channels play a major role in propagating electrical signals in nerve cells. The opening of voltage-gated channels changes the membrane potential, which opens additional voltage-gated channels, thus amplifying and propagating the electrical signal. Voltage-gated ion channels have domains called voltage sensors that are extremely sensitive to changes in the membrane potential: changes above a certain threshold value exert sufficient electrical force on these domains to encourage the channel to switch from its closed to its open conformation.
Which of the following is supported by the information in the figure? A.Sodium and potassium are involved in co-transport. B.Glucose enters the cell by simple diffusion. C.Nucleotides enter the cell by facilitated diffusion.
A.Sodium and potassium are involved in co-transport. C.Nucleotides enter the cell by facilitated diffusion.
The movement of an ion against its concentration gradient is called what? active transport osmosis facilitated diffusion passive transport
Active Transport E: Active transport is the movement of an ion against its concentration gradient and it is carried out by special types of transporters called pumps. To allow for the accumulation of an excess of an ion on one side of a membrane, energy must be used, as the ions will naturally attempt to reach equilibrium. In contrast to active transport, passive transport is where the ion moves along its concentration gradient and does not need energy input, as it stops once equilibrium is reached.
Why would a cell express the aquaporin protein if water can cross the membrane in the absence of aquaporin?
Aquaporin facilitates the faster movement of water molecules across the membrane. Explanation: Only small numbers of water molecules can diffuse across a membrane in a given amount of time. Adding aquaporins to a membrane facilitates quicker movement of water across a membrane. Aquaporins contribute to cellular function in certain tissues, such as the epithelial cells of the kidney where the flow of water into cells is particularly important.
Which of the following characteristics of K+ channels are important for the selectivity for K+ rather than other ions? A.Acidic side chains line the wall of the pore. B.Carbonyl groups line the wall of the pore. C.Four rigid protein loops line the narrowest part of the pore. D.Basic side chains line the wall of the pore.
B. Carbonyl groups line the wall of the pore. C.Four rigid protein loops line the narrowest part of the pore. E: The K+ channel, like other channels, is selective for one solute only—in this case, K+ ions. The pore contains four rigid protein loops at the narrowest part of the pore that form the selectivity filter for K+ ions. The protein loops lead to a channel the correct size for K+ ions to pass through. The protein loops also contain carbonyl groups on the side chains of amino acids that are spaced to selectively interact with K+ ions and not other positive ions.
Which of the following requires an input of energy to occur? A. the movement of a solute from a region of higher concentration on one side of a membrane to a region of lower concentration on the other side B. The movement of a solute from a region of lower concentration on one side of a membrane to a region of higher concentration on the other side C. Both of these options require energy investment because diffusion is a change in a system, and any change requires energy.
B. The movement of a solute from a region of lower concentration on one side of a membrane to a region of higher concentration on the other side E: Active transport occurs when a solute from a region of lower concentration on one side of a membrane moves to a region of higher concentration on the other side of the membrane. This action requires an input of energy to occur. Passive transport describes the movement of a solute from a region of higher concentration on one side of a membrane to a region of lower concentration on the other side, and does not require an energy investment.
Which factors determine the force driving the passive transport of charged solutes across the membrane? A. membrane potential only B. concentration gradient only C. electrochemical gradient D. ATP gradient
B. electrochemical gradient E: Passive transport of charged solutes depends not only on concentration gradient, but also on the charge distribution across the membrane. The combined contributions of concentration gradient and membrane potential are referred to as "electrochemical gradient."
Which of the following correctly describes osmosis? A. the movement of water from an area of low solvent concentration to an area of high solvent concentration B. the movement of water from an area of high solute concentration to an area of low solute concentration C. the movement of water from an area of low solute concentration to an area of high solute concentration D. the movement of water from an area of low water concentration to an area of high water concentration
B. he movement of water from an area of low solute concentration to an area of high solute concentration E: Osmosis is the movement of water from an area of low solute concentration to an area of high solute concentration. When the solute concentration is high, the water concentration is low; water will then naturally move toward an area of high solute concentration, diluting the solute. Another way to consider this is that like any molecule, water tends to move down its concentration gradient, from an area where its concentration is high to one in which its concentration is lower.
When glucose moves across a phospholipid bilayer by passive transport, which factor determines the direction of its transport? A. the amount of energy available to fuel the transport process B. the concentrations of glucose on either side of the membrane C. whether the cell is metabolically active or not D. the charge difference across the membrane
B. the concentrations of glucose on either side of the membrane E: When glucose moves across a phospholipid bilayer by passive transport, the concentrations of glucose on either side of the membrane determines the direction of its transport. Unlike ions, which move across membranes according to their concentration and membrane potential, glucose is uncharged, so the direction it moves is determined by its concentration gradient alone.
During an action potential, which of the following actions does not help return the membrane to its resting potential? A. the opening of voltage-gated K+ channels B. the opening of voltage-gated Na+ channels C. the inactivation of voltage-gated Na+ channels D. the flow of K+ through K+ leak channels
B. the opening of voltage-gated Na+ channels E: During an action potential, the opening of voltage-gated Na+ channels does not help return the membrane to its resting potential. The opening of voltage-gated Na+ channels promotes depolarization, not repolarization. Repolarization is encouraged by the opening of voltage-gated K+ channels, the inactivation of voltage-gated Na+ channels, and by the flow of K+ through K+ leak channels. K+ leak channels continue to randomly flicker open and closed during an action potential. K+ can flow out of the cell through these channels as well as through voltage-gated K+ channels to encourage repolarization.
Which of the following are consistent with the Na+-K+ pump and the illustration? A.K+ ions are pumped against their electrical gradient. B.Na+ ions are pumped against their chemical gradient. C.K+ ions are pumped against their chemical gradient. D.Na+ ions are pumped against their electrical gradient.
B.Na+ ions are pumped against their chemical gradient. C.K+ ions are pumped against their chemical gradient. D.Na+ ions are pumped against their electrical gradient. E: The ATP-driven Na+ pump plays such a central part in the energy economy of animal cells that it typically accounts for 30% or more of their total ATP consumption. This pump uses the energy derived from ATP hydrolysis to transport Na+ out of the cell as it carries K+ in. The pump is therefore sometimes called the Na+-K+ ATPase, or the Na+-K+ pump. In the activity of the Na+-K+ pump, Na+ ions are pumped against their chemical gradient and against their electrical gradient, meaning that sodium is pumped against its electrochemical gradient. K+ ions, on the other hand, are only pumped against their chemical gradient and not against their electrical gradient. The transport of Na+ ions out, and K+ ions in, takes place in a cycle in which each step depends on the one before. This steep concentration gradient of Na+ across the plasma membrane acts together with the membrane potential to create a large Na+ electrochemical gradient.
When voltage-gated Na+ channels in a nerve cell open, what happens to the axonal membrane? A. No change in the membrane potential occurs. B. It becomes electrically charged. C. It depolarizes. D. The membrane potential rises to 0 mV and stays there. E. The nerve cell becomes more negatively charged inside than outside.
C. It depolarizes. E: Even when the cell is resting, its membrane is "electrically charged" in that there is a voltage difference across the membrane—this is referred to as the resting membrane potential. The entry of Na+ ions makes the nerve cell less negatively charged inside and ultimately positively charged. This is called depolarization and it takes place along the axonal membrane when voltage-gated Na+ channels in the nerve cell open. The wave of depolarization is also called an action potential and it travels very quickly, allowing the nervous system to respond as quickly as it does.
What specific event triggers activation of the stereocilia before they activate the auditory neuron? A. The tectorial membrane vibrates and stretch-activated ion channels open on the stereocilia, polarizing the membrane. B. Stretch-activated ion channels on the tectorial membrane open, both depolarizing and activating the stereocilia. C. The stereocilia tilt when pushed against the tectorial membrane and stretch-activated ion channels open, releasing positive ions into the hair cell. D. The basilar membrane vibrates and stretch-activated ion channels on stereocilia close, depolarizing the membrane.
C. The stereocilia tilt when pushed against the tectorial membrane and stretch-activated ion channels open, releasing positive ions into the hair cell. E: The stereocilia found on hair cells contain stretch-activated ion channels on the plasma membrane. The channels are generally closed when the stereocilia are not pushed against the tectorial membrane and tilted. When the stereocilia are tilted, a linking filament on the channel on the first stereocilium pulls on a channel on neighboring stereocilia and opens the ion channel. Positive ions flow into the cell, depolarizing the membrane. This activates the hair cell, which then goes on to activate the auditory nerve.
Auditory hair cells in the ear depend on what type of ion channel to detect sound vibrations? A. ligand-gated B. voltage-gated C. mechanically-gated
C. mechanically-gated E: As shown in the figure, the auditory hair cells in the ear depend on mechanically-gated ion channels to detect sound vibrations. Sound vibrations pull the mechanically-gated channels open, allowing ions to flow into the hair cells. This ion flow sets up an electrical signal that is transmitted from the hair cell to the auditory nerve, which then conveys the signal to the brain.
All other factors (concentration, solute size, etc.) being equal, which type of solute does a cell tend to pull inside? A. negatively charged solutes B. uncharged solutes C. positively charged solutes
C. positively charged solutes E: All other factors (concentration, solute size, etc.) being equal, a cell tends to pull inside positively charged solutes. The excess of negative charge on the cytosolic side of the plasma membrane tends to pull positively charged solutes into the cell. Specifically, a variety of negatively charged inorganic and organic ions (anions), including nucleic acids, proteins, and many cell metabolites, maintain this relatively constant "positive pull" at the surface of the cell.
When an action potential reaches a nerve terminal, what type of voltage-gated channels are opened and result in the fusion of synaptic vesicles with the cell membrane? Na+ K+ Cl- Ca2+
Ca2+ E: When an action potential reaches a nerve terminal, Ca2+ voltage-gated channels are opened and result in the fusion of synaptic vesicles with the cell membrane. The opening of Ca2+ channels permits an influx of Ca2+, which triggers membrane fusion, as shown below.
How do transporters and channels select which solutes they help move across the membrane?
Channels discriminate between solutes mainly on the basis of size and electric charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate. Explanation: Channels discriminate mainly on the basis of size and electric charge: when the channel is open, only ions of an appropriate size and charge can pass through. A transporter transfers only those molecules or ions that fit into specific binding sites on the protein. Transporters bind their solutes with great specificity, in the same way an enzyme binds its substrate, and it is this requirement for specific binding that gives transporters their selectivity.
When Na+ channels are opened in an animal cell, what happens to the membrane potential? A. It stays the same. B. It rapidly reaches the resting membrane potential. C. It becomes more negative inside the cell .D. It becomes less negative inside the cell. E. It disappears, and membrane potential stabilizes at 0 mV.
D. It becomes less negative inside the cell. E: Na+ channels of cells are usually opened in response to stimulation; the resting membrane potential is associated with an unstimulated cell. When stimulated, Na+ channels open in an animal cell and the membrane potential changes; it becomes less negative inside the cell compared to the resting membrane potential. This is because when Na+ channels are opened, Na+ rushes into the cell. This rapid entry of positive ions makes the membrane potential less negative inside. If this depolarization is sufficiently large, it will cause voltage-gated Na+ channels in the membrane to open transiently at the site.
During the activation of a neuron, the action potential propagates in only one direction. How is this achieved in the neuron? A. The Na+ channel becomes permanently inactivated after the action potential passes. B. The Na+ channel remains open during the action potential and then rapidly returns to the closed state after the action potential passes. C. The Na+ channel closes during the action potential and then rapidly returns to the open state after the action potential passes. D. The Na+ channel becomes inactivated and refractory to reopening for a short time after the action potential passes.
D. The Na+ channel becomes inactivated and refractory to reopening for a short time after the action potential passes. E: During an action potential, the Na+ channels along the axon open in response to membrane depolarization. As Na+ rushes into the cell, the membrane is further depolarized, causing the Na+ channel to close and become inactive. In the inactive state, the Na+ channel is refractory to opening and therefore remains closed. The channels return to the closed state after a few milliseconds. They are then able to respond to another action potential. The closed, inactive state of the Na+ channel prevents the action potential from moving backward toward the cell body of the neuron.
How is an electrical signal converted to a chemical signal at a nerve terminal? A. Mechanically gated channels change conformation due to the electrical signal and create a mechanical signal. B. The influx of ions leads to a pH change, chemical transformation, and signaling. C. Ligand-gated channels are bound by ions and open to allow the flow of neurotransmitters out of the cell. D. Voltage-gated Ca2+ channels are activated and the influx of Ca2+ triggers the release of neurotransmitters.
D. Voltage-gated Ca2+ channels are activated and the influx of Ca2+ triggers the release of neurotransmitters. E: An electrical signal is converted to a chemical signal at a nerve terminal by the action of voltage-gated Ca2+ channels, which allow the influx of Ca2+. Ca2+ triggers the fusion of neurotransmitter-containing vesicles with the plasma membrane to release the neurotransmitter into the synaptic cleft.
Inhibitory neurotransmitters such as glycine and GABA make a postsynaptic cell harder to depolarize by allowing what? A. an influx of Na+ B. an influx of K+ C. the escape of Na+ D. an influx of Cl-
D. an influx of Cl- E: Inhibitory neurotransmitters such as glycine and GABA make a postsynaptic cell harder to depolarize by allowing an influx of Cl-. The main receptors for inhibitory neurotransmitters are ligand-gated Cl- channels. If a neurotransmitter such as glycine and GABA were to be inhibitory, then it would invoke a reaction that would discourage depolarization of the membrane potential; allowing an influx of Cl- accomplishes just that. Cl- influx would draw the membrane potential more negative and reduce the chances of a depolarization and an action potential.
Cardiac muscle cells contain a Na+/Ca2+ transporter responsible for maintaining a low cytosolic Ca2+ concentration, which helps regulate cardiac muscle contraction. Ca2+ is transported out of the cell as Na+ is brought into the cell. What type of transporter is this protein? A. uniport B. symport C. channel D. antiport
D. antiport E: Antiports transport two substrates in opposite directions across the membrane. The Na+/Ca2+ antiport transports three Na+ molecules into the cell for every Ca2+ transported out of the cell. The Na+ is moving into the cell down its electrochemical gradient. The energy from Na+ moving down its gradient is used to drive the movement of Ca2+ against its electrochemical gradient as Ca2+ is pumped out of the cell. This keeps the concentration of Ca2+ very low inside the cytosol of the cell. The strength of cardiac muscle contraction is regulated in part by the concentration of Ca2+ in the cytosol.
Which of the following inhibits inorganic ions, such as Na+ and Cl-, from passing through a lipid bilayer? A. the watery environment on either side of the lipid bilayer B. the carbohydrate layer on the surface of the lipid bilayer C. the ions' large size D. the hydrophilic exterior of the lipid bilayer E. the hydrophobic interior of the lipid bilayer
E. the hydrophobic interior of the lipid bilayer Explanation: The hydrophobic interior of a lipid bilayer inhibits the passage of all ions (including Na+ and Cl-) because ions are charged and the interior of the membrane is very nonpolar. The watery environment on either side of the lipid bilayer is ideal for the solubility of ions because ions will dissolve well in water due to its polar nature. In contrast, ions are repelled by the nonpolar, hydrophobic hydrocarbon tails of the phospholipids that compose the interior of the plasma membrane.
Determine whether the following statement is true or false: The glucose-Na+ symport protein uses the electrochemical Na+ gradient to drive the active transport of glucose into the cell. Once this transporter has bound both Na+ and glucose, it preferentially opens toward the cytosol, where it releases both solutes.
False E: A glucose-Na+ symporter uses the electrochemical Na+ gradient to drive the active import of glucose. The pump oscillates randomly between alternate states. Because conformational transitions are reversible, once the transporter has bound both solutes, two things can happen: (1) it can flip into the inward-open state, allowing both solutes to enter the cytosol, or (2) it can also remain in the outward-open state. In this case, the solutes would dissociate into the extracellular space and nothing would be gained. Even though Na+ and glucose can each bind to the pump in either of these "open" states, the pump can transition between them only through an "occluded" state in which both glucose and Na+ are bound ("occluded-occupied") or neither is bound ("occluded-empty"). An overview of this mechanism is shown in the figure.
Name different kinds of ports.
Gradient-driven pumps can act as symports or antiports, both of which are a type of co-transport. They transfer solutes either in the same direction, in which case they are called symports, or in opposite directions, which are antiports. Uniports, by contrast, only facilitate the movement of a solute down its concentration gradient. Because such movement does not require an additional energy source, uniports are not pumps and do not exhibit co-transport. The glucose transporter is an example of a uniport and the glucose-Na+ transporter is a great example of a symport.
In one experiment, investigators create a liposome—a vesicle made of phospholipids—that contains a solution of 1 mM glucose and 1 mM sodium chloride. If this vesicle were placed in a beaker of distilled water, what would happen the fastest?
H2O would diffuse in. Explanation: In this experiment, the possible molecules that can move across the membrane are water, glucose, and ionized sodium chloride. Glucose requires a transporter to move across a lipid membrane because of its relatively large size and the experimental design does not include these transporter proteins in the liposome. Therefore, glucose will not leave the liposome. Sodium chloride will ionize into Na+ and Cl- when dissolved into solution. While small in size, the charge of these molecules likewise means that they cannot diffuse across the nonpolar liposome membrane without a channel protein. On the other hand, water is small enough, as well as noncharged, meaning that it can cross the membrane. Distilled water outside of the cell lacks dissolved solutes (i.e., high water concentration), whereas the interior of the liposome has a relatively high concentration of two different solutes (i.e., low water concentration). Water will follow its concentration gradient and move into the liposome.
A difference in solute concentrations on either side of a membrane leads to osmosis, the passive movement of water across a membrane from a region of low solute concentration (where the water concentration is high) to a region of high solute concentration (where the water concentration is low).
Inside the Liposome: ATP and Na+ Outside the: K+ Unneeded: GTP and Cl- Explanation: The Na+-K+ pump binds to Na+ ions on the inside of the liposome and is then phosphorylated by a phosphate from the ATP, which is also inside the liposome. The pump changes conformation in response to phosphorylation, releasing the Na+ on the outside. K+ from the outside of the liposome can then bind to the pump and trigger dephosphorylation. Dephosphorylation of the pump triggers switching of conformation back to the original conformation, leading to a release of K+ inside the liposome. The pump is then ready to repeat the cycle of pumping Na+ out and K+ into the liposome. The Na+ and ATP must be inside the liposome and K+ outside the liposome for pumping of ions to occur. Cl- and GTP are not used or pumped by this transporter.
The drug scopolamine is used to treat dizziness, motion sickness, and smooth muscle spasms. When isolated muscle cells are incubated with scopolamine, addition of acetylcholine no longer depolarizes the muscle cell membrane or stimulates muscle cell contraction. Which would best explain how scopolamine exerts its muscle-relaxing effects?
It inhibits the opening of acetylcholine-gated Na+ channels in the muscle cell membrane. Explanation: Scopolamine exerts its muscle-relaxing effects by inhibiting the opening of acetylcholine-gated Na+ channels in the muscle cell membrane. Normally, acetylcholine triggers muscle contraction by opening a ligand-gated Na+ channel, which leads to membrane depolarization and contraction of the muscle cell because during an action potential, Na+ enters the cytosol through voltage-gated Na+ channels. However, in scopolamine-treated cells, the ligand-gated Na+ channel will not open and the action potential will not generate.
When a neuron is activated by a stimulus, its plasma membrane will change until it reaches a membrane potential of about +40 mV. What is special about this value?
It is approximately the membrane potential at which the electrochemical gradient for Na+ is zero. Explanation: When a neuron is activated by a stimulus, its plasma membrane will change until it reaches a membrane potential of about +40 mV. This value is special because it is approximately the membrane potential at which the electrochemical gradient for Na+ is zero. Around +40 mV, Na+ ions have no further tendency to enter or leave the cell. In other words, they are near their theoretical equilibrium potential, as determined by the Nernst equation. According to this equation, the electrochemical gradient for K+ is near zero when a cell is at its resting potential, around -20 to -200 mV.
A toxin present in scorpion venom prolongs the duration of action potentials in nerve cells. Which of these actions would best explain how this toxin exerts its effect?
It slows the inactivation of voltage-gated Na+ channels. Explanation: The toxin in scorpion venom exerts its effect by slowing the inactivation of voltage-gated Na+ channels, causing them to be stuck in the open conformation and prolonging the action potential. In contrast, prolonged inactivation of voltage-gated Na+ channels would delay subsequent action potentials and inhibition of voltage-gated Na+ channels would prevent depolarization of the membrane.
Define Ligand-Gated
Molecules that bind to channels to alter their conformation to open more frequently are called ligands; a channel that is regulated in this way is a ligand-gated channel. The opening of other channels can be triggered by a membrane potential (voltage-gated) or by mechanical forces (mechanically gated).
Lipid bilayers are highly impermeable to which molecule(s)?
Na+ and Cl- Explanation: Lipid bilayers are highly impermeable to many charged ions, and Na+ and Cl- are common examples of ions that are excluded from the hydrophobic interior of a lipid bilayer. For ion transport, cell membranes contain channel proteins that permit passage of ions. In contrast, small, nonpolar molecules, such as CO2 and O2, and hydrophobic steroid hormones easily pass through the lipid bilayer. Water, even though it is polar, is small enough that it does enter the membrane at a measurable rate. However, water moves much more quickly though through membrane proteins called aquaporins.
Which of the following statements is true? K+ and Na+ are both excluded from cells. K+ is the most plentiful positively charged ion outside the cell, while Na+ is the most plentiful inside. K+ and Na+ are both maintained at high concentrations inside the cell compared to out. K+ and Na+ are present in the same concentration on both sides of the plasma membrane. Na+ is the most plentiful positively charged ion outside the cell, while K+ is the most plentiful inside.
Na+ is the most plentiful positively charged ion outside the cell, while K+ is the most plentiful inside. E: Because lipid bilayers are impermeable to inorganic ions, living cells are able to maintain internal ion concentrations that are very different from the concentrations of ions in the medium that surrounds them. Na+ is the most plentiful positively charged ion outside the cell, while K+ is the most plentiful inside. The intracellular concentration of sodium ions is approximately 5-15 mM, whereas the extracellular concentration of sodium ions is approximately 145 mM. Additionally, the intracellular concentration of potassium ions is approximately 140 mM, whereas the extracellular concentration of potassium ions is approximately 5 mM. Cells expend a great deal of energy to maintain this chemical imbalance, and such electrical imbalances generate a voltage difference across the membrane called the membrane potential.
The Na+ pump in the plasma membrane of animal cells uses energy from ATP hydrolysis to pump sodium and potassium ions against their electrochemical gradients. In which direction are the ions pumped across the membrane? Na+ out and K+ in Na+ and K+ both in Na+ and K+ both out Na+ in and K+ out
Na+ out and K+ in E: The Na+ pump in the plasma membrane of animal cells uses energy from ATP hydrolysis to pump sodium and potassium ions against their electrochemical gradients; specifically, Na+ is pumped out of the cell and K+ is pumped into the cell. Thus, Na+ is at a higher concentration outside the cell and K+ is at a higher concentration inside the cell. Because Na+ is maintained at a much higher concentration outside the cell, it can enter cells passively if given an opportunity. Furthermore, this Na+ gradient can be used to drive the import of other substances into the cell by membrane pumps that exhibit a coupled transport mechanism.
Most sports drinks contain both carbohydrates and salts. The carbohydrates replace glucose burned during exercise and the salts replace salts lost in sweat. The salt also helps the small intestine absorb glucose. Pick the answer that accurately describes which salt is most beneficial for glucose absorption.
NaCl, because Na+ is needed for glucose entry. Explanation: Many different transmembrane proteins contribute to glucose absorption across the intestinal epithelium. The Na+-K+ ATPase, located in the basal membrane, keeps intracellular sodium low. Glucose is brought into the epithelial cells from the gut lumen, against its concentration gradient, by the action of a glucose-Na+ symport protein.
Sodium ions, oxygen (O2), and glucose pass directly through lipid bilayers at dramatically different rates. Which of the following choices presents the correct order, from fastest to slowest?
Oxygen, Glucose, Sodium ions Explanation: The correct order, from fastest to slowest, is oxygen, glucose, and lastly, sodium ions. Oxygen is small and nonpolar; glucose is large and polar, but it has no charge; sodium ions, with their positive charge, diffuse across a lipid bilayer least readily. Charged ions, whether cations or anions, will not interact favorably with the hydrophobic interior (the nonpolar lipid hydrocarbons) of the lipid bilayer. Transporter proteins dramatically alter this picture by increasing the rate of movement across a membrane for molecules that would otherwise not readily diffuse across directly.
Signaling at a synapse occurs in presynaptic and postsynaptic cells. Sort each of the following events into the proper location. Presynaptic Cell: Postsynaptic Cell:
Presynaptic Cell: -Voltage gated Ca2+ channels open. -Neurotransmitter is released into the synaptic cleft. -Reuptake of the neurotransmitter occurs. Postsynaptic Cell: -Ligand-gated ion channels open in response to neurotransmitter. -Neurotransmitter binding begins a new action potential. E: As the action potential reaches the nerve terminal of the presynaptic neuron, the voltage-gated Ca2+ channels open and Ca2+ enters the cytoplasm. The increased level of Ca2+ stimulates the fusion of synaptic vesicles containing neurotransmitters with the plasma membrane of the presynaptic neuron, causing release of the neurotransmitter into the synaptic cleft. The neurotransmitter then binds to ligand-gated ion channels on the postsynaptic cells. Opening of the ligand-gated ion channels causes depolarization of the postsynaptic cells and transmission of a new action potential in the postsynaptic cell. The neurotransmitter is either degraded or taken back up by the presynaptic cell.
Optogenetics is a powerful tool that uses light to control the activity of specific neurons. These neurons contain artificially introduced light-gated ion channels. A number of different light-gated channels with different ion specificities have been either found in nature (such as the sodium-specific channelrhodopsin, originally found in green algae) or produced via genetic engineering (the production of a chloride ion-specific form of channelrhodopsin). There are also light-gated ion channels specific for potassium or calcium. Sort each light-activated channel type based on whether activation of this channel will tend to depolarize cells or not.
Promote: Sodium Channel and Calcium Channel Inhibit: Chloride Channel and Potassium Channel Explanation: Calcium and sodium are both cations with higher extracellular concentrations than intracellular concentrations, so light activation of these channels will cause positive ions to enter the cell, leading to depolarization. While chloride ions also have a higher extracellular concentration relative to inside the cell, they are negatively charged anions, leading to membrane hyperpolarization (more negative than usual). Finally, while cells are normally leaky to potassium, opening of additional potassium channels can also hyperpolarize cells, preventing depolarization
Determine whether the following statement is true or false: Transmitter-gated ion channels are insensitive to membrane potential; in the absence of neurotransmitters, they cannot generate an action potential.
T
Your friend now has the pumps successfully pumping ions. She added an equal concentration of both ions to the correct sides of the liposomes along with an excess of the energy source. She is surprised when the pumps stop working after a short time. Which of the following could explain why the transporter stopped pumping ions?
The pump ran out of Na+ to pump because it pumps 3 Na+ out for every 2 K+ pumped in. Explanation: Three Na+ bind to the pump on the inside of the liposome. This triggers pump phosphorylation and a change in conformation so that three Na+ are released outside the liposome. Two K+ bind the pump from outside the liposome. The pump is dephosphorylated and returns to the beginning conformation and releases the K+ inside the liposome. Since the experiment started with an equal concentration of Na+ inside the liposome to the K+ outside the liposome, the Na+ inside the liposome will run out first. In this case, an excess of ATP was added, so that is not the limiting factor.
Determine whether the following statement is true or false: Given enough time, virtually any molecule will diffuse across a lipid bilayer.
True E: Lipid bilayers are selectively permeable, meaning that they more easily allow some molecules to cross than others. For example, small, nonpolar molecules like gases can easily pass while ions, because of their charge, are largely prevented from crossing the membrane. However, with enough time, essentially all molecules will eventually diffuse across the lipid bilayer, even if the diffusion is inefficient. Transmembrane transporter proteins increase the efficiency of movement of molecules that otherwise wouldn't easily cross the membrane, while still providing selectivity.
You join a laboratory to study muscle function. You decide to inhibit the pumping of Ca2+ into the sarcoplasmic reticulum to determine how excess cytosolic Ca2+ will affect muscle function. Which of the following strategies would be effective in blocking Ca2+ pumping?
block the phosphorylation of the conserved aspartate in the Ca2+ pump Explanation: ATP is required for Ca2+ transport by the pump. In the ATP bound state, the pump is open to the cytosol and Ca2+ ions bind from the cytosol. Ca2+ binding triggers closure of the pump and ATP hydrolysis. The phosphate from ATP is transferred to a conserved aspartate on the pump. ADP is released and a new ATP binds the pump, opening it to the lumen side of the sarcoplasmic reticulum where Ca2+ is released. Blocking phosphorylation of the pump will prevent Ca2+ transport.
Which of the following form tiny hydrophilic pores in the membrane through which solutes can pass by diffusion?
channels Explanation: Membrane channels form tiny hydrophilic pores in the membrane through which solutes can pass by diffusion. Solutes that are small enough to pass through the channel will diffuse through, while those that are too large will not. Most channels only permit passage of ions and are therefore also referred to as ion channels. Because ions are electrically charged, their movements can create a powerful electric force—or voltage—across the membrane.
Glucose enters the cell by which process? osmosis facilitated diffusion simple diffusion active transport
facilitated diffusion
A group of researchers wanted to sort different white blood cell types (monocytes, lymphocytes, and granulocytes) apart from each other based on size differences and to remove unwanted contaminating red blood cells. After a particular manipulation, the red blood cells lysed. The remaining white blood cells increased in size and, more importantly, the size differences among cells increased, allowing for size-based sorting (which requires minimum size differences among cells). What manipulation did the researchers use to increase cell size?
placing cells in an environment with a lower solute concentration than that in the cells Explanation: A difference in solute concentrations on either side of a membrane leads to osmosis, the passive movement of water across a membrane from a region of low solute concentration (where the water concentration is high) to a region of high solute concentration (where the water concentration is low).
Which term describes a coupled transporter that moves both solutes in the same direction across a membrane?
symport E: By paying attention to the prefixes of these terms, you can deduce the nature of the movement of molecules through them. The prefix "sym" means same while "anti" means opposed/opposite. Therefore, a symport will move two different molecules across a membrane in the same direction, while an antiport will move the different molecules in opposite directions across the membrane. The prefix "uni" means one and hence a uniport will only move one type of molecule across a membrane. As you can see, paying attention to the parts of terms can give you insight into their function.
