Exam 2 review

When the hormone insulin is released into the bloodstream, what form of cell-cell signaling is being used? -contact-dependent -paracrine -endocrine -neuronal

-endocrine Part of the pancreas, for example, is an endocrine gland that produces several hormones, including insulin, which regulates glucose uptake in cells all over the body.

Glucose enters the cell by which process? -osmosis -simple diffusion -facilitated diffusion -active transport

-facilitated diffusion Glucose enters the cell by facilitated diffusion. As seen in the illustration, when there is a concentration gradient, glucose passively diffuses into the cell through the glucose transporter. Protein-mediated passive diffusion is called facilitated diffusion. In this case, conformational changes in a transporter mediate the passive transport of glucose. The glucose transporter is shown in three conformational states. In the outward-open state (left), the binding sites for solute are exposed on the outside. In the inward-open state (right), the sites are exposed on the inside of the bilayer. And in the occluded state (center), the sites are not accessible from either side.

In passive transport, the net movement of a charged solute across the membrane is determined by which of the following? -its osmotic gradient alone -its concentration gradient -its electrochemical gradient -the membrane potential

-its electrochemical gradient In passive transport, the net movement of a charged solute across the membrane is determined by its electrochemical gradient, which is a composite of two forces: one due to the concentration gradient and the other due to the membrane potential.

Enzymes that add a phosphate group to a switch protein are called -phosphatases. -GTPases. -G-proteins. -kinases.

-kinases. Enzymes called kinases add a phosphate to a protein; the phosphate could either activate or inactivate the protein, depending on the protein. Phosphatases remove phosphates.

What is the voltage difference across a membrane of a cell called? -membrane potential -gradient establishment -electrical current -potential balance

-membrane potential Although the electrical charges inside and outside the cell are generally kept in balance, tiny excesses of positive or negative charge, concentrated in the neighborhood of the plasma membrane, do occur. Such electrical imbalances generate a voltage difference across the membrane called the membrane potential. In animal cells, for example, the resting membrane potential can be anywhere between -20 and -200 millivolts (mV), depending on the organism and cell type. The value is expressed as a negative number because the interior of the cell is more negatively charged than the exterior.

Many of the extracellular signal molecules that regulate inflammation are released locally at the site of infection. What form of cell-cell signaling is being used? -paracrine -contact-dependent -endocrine -neuronal

-paracrine In paracrine signaling, the extracellular signaling molecule acts as a local mediator on cells located nearby the signaling cell that produced it.

The movement of an ion down its concentration gradient is called what? -osmosis -passive transport -pumping -active transport

-passive transport 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.

One of the two types of GTP-binding proteins, often called G-proteins, are membrane bound. These are the -G-protein coupled receptors. -trimeric GTP-binding proteins. -cyclic GMPs. -monomeric GTP-binding proteins.

-trimeric GTP-binding proteins. Trimeric GTP-binding proteins are often called G-proteins, and are membrane bound. These are distinct from the monomeric GTP-binding proteins, which are soluble proteins.

The depolarization of the nerve-terminal plasma membrane triggers the secretion of neurotransmitters by opening which of the following? -voltage-gated Na+ channels in the plasma membrane -voltage-gated Ca2+ channels in the sarcoplasmic reticulum -transmitter-gated Ca2+ channels in the plasma membrane -voltage-gated Ca2+ channels in the plasma membrane -voltage-gated K+ channels in the plasma membrane

-voltage-gated Ca2+ channels in the plasma membrane The opening of voltage-gated Na+ channels causes depolarization of the nerve-terminal membrane. The depolarization of the nerve-terminal plasma membrane triggers the secretion of neurotransmitters by opening voltage-gated Ca2+ channels in the plasma membrane. The resulting increase in Ca2+ concentration in the cytosol triggers the fusion of vesicles with the plasma membrane, which releases neurotransmitters into the synaptic cleft. There is no sarcoplasmic reticulum in a nerve terminal and the opening of voltage-gated K+ channels allows recovery from membrane depolarization.

For voltage-gated channels, a change in the membrane potential has what effect on the channel? -A. It changes which ions can pass through the channel. -B. It makes the channel more sensitive to the binding of neurotransmitters. -C. It changes the width of the channel opening. -D. It either opens the channel or closes it, depending on the voltage. -E. It alters the probability that the channel will be found in its open conformation.

E. It alters the probability that the channel will be found in its open conformation. Voltage-gated channels respond to changes in the membrane potential: a change in the membrane potential alters the probability that the channel will be found in its open conformation. Note that when channels are open, they are all the way open; they do not open to different degrees. Voltage-gated ion channels play a major role in propagating electrical signals along all nerve cell extensions, such as those that relay signals from our brain to our muscles.

Determine whether the following statement is true or false: A symport protein would function as an antiport protein if its orientation in the membrane were reversed. This statement is...

False A symport protein would not function as an antiport protein if its orientation in the membrane were reversed. A symport protein binds two different solutes on the same side of the membrane; an antiport protein binds two different solutes on opposite sides of the membrane. Thus, if a symport were "upside down" in the membrane, then both solutes likely would be moved in the opposite direction compared to normal. Just inverting a protein does not alter the mechanics of the protein from symport to antiport or vice versa.

Which type of membrane transport protein can perform either passive or active transport? -A. Neither type of membrane transport protein can perform both passive and active transport. -B. both channels and transporters -C. transporters -D. channels

-C. transporters 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.

How does an action potential spread along the cell membrane? -A. A change in membrane potential triggers the opening of nearby voltage-gated sodium channels in a one-way direction. -B. Potassium leak channels quickly reverse the action potential to move the membrane depolarization away from the original site. -C. Voltage-gated Ca2+ channels are activated by the action potential and the calcium diffuses along the membrane. -D. The ions entering the cell upon triggering an action potential travel laterally along the membrane to carry the charge.

-A. A change in membrane potential triggers the opening of nearby voltage-gated sodium channels in a one-way direction. 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.

A sodium-potassium antiport maintains the extracellular concentration of sodium at levels that are about 20-30 times higher than inside the cells. What directly supplies the energy for maintaining this gradient? -A. ATP hydrolysis drives the function of the pump. -B. A proton gradient in the mitochondria drives the antiport. -C. Potassium supplies the energy, as it is moving along its concentration gradient. -D. Sodium supplies the energy, as it is moving along its concentration gradient.

-A. ATP hydrolysis drives the function of the pump. The sodium-potassium pump is an antiport that pumps sodium out and potassium into the cell. This pump moves both ions against their concentration gradient, and ATP supplies the energy for this active transport.

Why would a cell express the aquaporin protein if water can cross the membrane in the absence of aquaporin? -A. Aquaporin facilitates the faster movement of water molecules across the membrane. -B. Aquaporin limits the movement of water molecules so they do not move too quickly across the membrane. -C. Aquaporin moves a positively charged ion along with water across the membrane. -D. Water molecules cannot cross the membrane in the absence of a pore like aquaporin.

-A. Aquaporin facilitates the faster movement of water molecules across the membrane. 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.

How do transporters and channels select which solutes they help move across the membrane? -A. 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. -B. Transporters discriminate between solutes mainly on the basis of size and electric charge; channels bind their solutes with great specificity in the same way an enzyme binds its substrate. -C. Both channels and transporters discriminate between solutes mainly on the basis of size and electric charge. -D. Channels will allow the passage of any solute as long as it has an electrical charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate. -E. Channels allow the passage of solutes that are electrically charged; transporters facilitate the passage of molecules that are uncharged.

-A. 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. 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.

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. A leaky Na+ channel would not affect glucose transport because these two transporters are unrelated. -C. Glucose transport would increase because the Na+ gradient is strengthened by the Na+ channel. -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. 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.

Which of the following statements is true? -A. Inside the cell, the quantity of positively charged ions is almost equal to the quantity of negatively charged ions. -B. Inside the cell, there are no negatively charged ions. -C. Inside the cell, the quantity of positively charged ions is much less than the quantity of negatively charged ions. -D. Inside the cell, there are no positively charged ions. -E. Inside the cell, the quantity of positively charged ions is much greater than the quantity of negatively charged ions.

-A. Inside the cell, the quantity of positively charged ions is almost equal to the quantity of negatively charged ions. Inside the cell, the quantity of positively charged ions is almost equal to the quantity of negatively charged ions. For the cytoplasm of a cell to avoid being significantly disrupted by electrical forces, the quantity of positively charged ions must be balanced by an almost exactly equal quantity of negatively charged ions. The high concentration of K+ inside is balanced by a variety of negatively charged inorganic and organic ions (anions), including nucleic acids, proteins, and many cell metabolites. Although the electrical charges inside and outside the cell are generally kept in balance, tiny excesses of positive or negative charge, concentrated in the neighborhood of the plasma membrane, do occur. Such electrical imbalances generate a voltage difference across the membrane called the membrane potential. The resting membrane potential is integral to many activities that occur across the plasma membrane.

What determines the direction that glucose is transported across the membrane, through a glucose transporter? -membrane potential -a molecule's charge -concentration gradient -a molecule's size

-concentration gradient Because glucose is uncharged, the direction it is transported is determined by its concentration gradient alone. Some membrane channels will select based on size, but glucose does not enter the cell through a size-exclusion channel, and because of the lack of chemical charge on a molecule of glucose, the membrane potential does not govern the diffusion of this solute.

When an individual ion channel is stimulated to open (for example, by the binding of a neurotransmitter), what is the typical activity of the ion channel? -A. It continues to flicker between open and closed states, but spends more time open while the neurotransmitter is bound. -B. It opens and then very quickly closes, triggering the neurotransmitter to detach. -C. It opens to allow the neurotransmitter to enter the cell. -D. It opens partially and will open fully once neighboring channels begin to open. -E. It opens and stays open until the neurotransmitter detaches.

-A. It continues to flicker between open and closed states, but spends more time open while the neurotransmitter is bound. When an individual gated ion channel is stimulated to open (for example, by the binding of a neurotransmitter), the channel continues to flicker between open and closed states, but spends more time open while the neurotransmitter is bound. When neurotransmitters are not present, channels open rarely. This came as a surprise when investigators first observed this phenomenon using the patch-clamp technique, and it suggests that the opening and closing of the channels involves moving parts.

When voltage-gated Na+ channels in a nerve cell open, what happens to the axonal membrane? -A. It depolarizes. -B. No change in the membrane potential occurs. -C. The nerve cell becomes more negatively charged inside than outside. -D. It becomes electrically charged. -E. The membrane potential rises to 0 mV and stays there.

-A. It depolarizes. 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.

Which of the following statements is not true regarding active transport by transmembrane pumps? -A. Some solutes are transported across the membrane in tandem with other molecules, both moving from lower concentration to higher concentration. -B. Some solutes are moved against their concentration gradients, from one side of a membrane to the other, using energy from ATP hydrolysis. -C. Some solutes are moved across a membrane against their concentration gradient using energy from sunlight.

-A. Some solutes are transported across the membrane in tandem with other molecules, both moving from lower concentration to higher concentration. It is true that some solutes are transported across the membrane in tandem with other molecules, but both do not move from lower concentration to higher concentration. Coupled pumps can link the uphill transport of one solute across a membrane to the downhill transport of another. In this manner, the concentration gradient for one solute is used to drive the mechanism of the pump that moves another solute against its concentration gradient. In terms of other energy sources that drive membrane pumps, some solutes are moved across a membrane against their concentration gradient using energy from sunlight; such movement is facilitated by light-driven pumps. Additionally, some solutes are moved against their concentration gradients, from one side of a membrane to the other, using energy from ATP hydrolysis; such movement is facilitated by ATP-driven pumps.

Why do cells lack membrane transport proteins that are specific for the movement of O2? -A. because oxygen dissolves readily in lipid bilayers -B. because transport of oxygen across cell membranes is energetically unfavorable -C. because oxygen is transported in and out of the cell by special oxygen-binding proteins such as hemoglobin -D. because oxygen, dissolved in water, can enter cells via aquaporins -E. because oxygen concentrations must be kept low inside cells to avoid creating reactive superoxide radicals that can damage DNA and proteins

-A. because oxygen dissolves readily in lipid bilayers Cells lack membrane transport proteins that are specific for the movement of O2 because oxygen dissolves readily in lipid bilayers. This small, nonpolar molecule can diffuse across the cell membrane without the need for a membrane transport protein. The channels of aquaporins are lined with amino acids that provide an environment for the formation of transient hydrogen bonds that facilitate the passage of water molecules, which line up in single file. The channels of aquaporins exclude ions and most other molecules, including O2.

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 increasing the amount of serotonin released from the presynaptic neuron, increasing the amount available in the synapses that use it -C. by making serotonin-gated channels easier to open -D. by decreasing the amount of serotonin released from the presynaptic neuron, decreasing 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 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 -B. agonist that leads to opening of the Ca2+ channel on the presynaptic cell -C. competitive inhibitor of neurotransmitter reuptake by the presynaptic cell -D. All three of these mechanisms could potentially cause muscle paralysis.

-A. competitive inhibitor of the ligand-gated ion channel on the postsynaptic cell 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.

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? -A. placing cells in an environment with a lower solute concentration than that in the cells -B. patch-clamp recording to monitor ion channel activity -C. placing cells in an environment with lower temperatures than the cells were previously exposed to -D. placing cells in an environment with a higher solute concentration than that in the cells

-A. placing cells in an environment with a lower solute concentration than that in the cells 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). In their article "Exploiting osmosis for blood cell sorting," the researchers suggest that after exposure to deionized water, different cell populations swell at different rates due to the relative abundance of aquaporins.

Which of the following correctly describes osmosis? -A. the movement of water from an area of low solute concentration to an area of high solute concentration -B. the movement of water from an area of low water concentration to an area of high water concentration -C. the movement of water from an area of high solute concentration to an area of low solute concentration -D. the movement of water from an area of low solvent concentration to an area of high solvent concentration

-A. the movement of water from an area of low solute concentration to an area of high solute concentration 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.

Which of the following is supported by the information in the figure? -A.Nucleotides enter the cell by facilitated diffusion. -B.Glucose enters the cell by simple diffusion. -C.Sodium and potassium are involved in co-transport.

-A.Nucleotides enter the cell by facilitated diffusion. -C.Sodium and potassium are involved in co-transport. As shown in the image, nucleotides enter the cell through a membrane transport protein. The name for this type of protein-mediated transport is facilitated diffusion. When sodium ions are pumped from the cell and potassium ions pumped into the cell, this is a mechanism of co-transport. Glucose does not enter the cell by simple diffusion through membrane lipids, but enters like nucleotides do, by facilitated diffusion. An important example of a transporter that mediates passive transport is the glucose transporter in the plasma membrane of many mammalian cell types. The protein, which consists of a polypeptide chain that crosses the membrane at least 12 times, can adopt several conformations—and it switches reversibly and randomly between them. In one conformation, the transporter exposes binding sites for glucose to the exterior of the cell; in another, it exposes the sites to the cell interior.

Which of the following characteristics of aquaporins ensure that the channel selectively transports only water molecules and not other solutes? -A.Two asparagines in the center of the pore prevent protons from passing through the channel. -B.The channel undergoes conformational changes to push water through the channel. -C.The channel has a narrow pore that is only wide enough for a single water molecule to pass through. -D.A glutamate at the entrance to the channel prevents positive ions from entering the channel.

-A.Two asparagines in the center of the pore prevent protons from passing through the channel. -C.The channel has a narrow pore that is only wide enough for a single water molecule to pass through. 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 diuretic drug that triggers removal of excess Na+ from the body -B.a drug that blocks the calcium channel in heart muscle cells -C.a drug that decreases the activity of the Na+/Ca2+ transporter -D.a drug that inhibits the Na+-K+ pump from establishing a strong Na+ gradient

-A.a diuretic drug that triggers removal of excess Na+ from the body -C.a drug that decreases the activity of the Na+/Ca2+ transporter -D.a drug that inhibits the Na+-K+ pump from establishing a strong Na+ gradient 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.

What is the name of the specialized junction between a neuron and a target cell? -A. nerve terminal -B. dendrite -C. synaptic vesicle -D. synapse -E. axon

-D. synapse The synapse is the junction between a neuron and a target cell. At this junction, the plasma membranes of the two cells are found in close proximity, but not touching each other. The electrical impulse traveling along the neuron is converted to a chemical signal by triggering the release of a secreted molecule, called a neurotransmitter, into the synaptic cleft, or space, between the two cells. Binding of the neurotransmitter on the target cell triggers the response of that cell to the action impulse from the neuron.

When Na+ channels are opened in an animal cell, what happens to the membrane potential? -A. It becomes more negative inside the cell. -B. It becomes less negative inside the cell. -C. It rapidly reaches the resting membrane potential. -D. It disappears, and membrane potential stabilizes at 0 mV. -E. It stays the same.

-B. It becomes less negative inside the cell. 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.

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? -A. It inhibits the transporters that pump Na+ into the muscle cell cytosol during an action potential. -B. It inhibits the opening of acetylcholine-gated Na+ channels in the muscle cell membrane. -C. It inhibits the transporters that pump Ca2+ into the muscle cell cytosol during an action potential. -D. It inhibits the opening of Ca2+ channels in the sarcoplasmic reticulum. -E. It inhibits the opening of voltage-gated K+ channels.

-B. It inhibits the opening of acetylcholine-gated Na+ channels in the muscle cell membrane. 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.

Which statement about positive feedback regulation is accurate? -A. Positive feedback is rare in biological systems. -B. Positive feedback regulation can generate an abrupt, all-or-none response in which the cell moves from ignoring a signal to responding to it very strongly. -C. In positive feedback, a molecular switch enhances the response to a signal by activating a component that lies downstream in the pathway. -D. Positive feedback can generate responses that oscillate on and off as the activities or concentrations of the participating components rise and fall. -E. In positive feedback, a downstream component acts to inhibit an earlier component in the pathway to diminish the response to the initial signal.

-B. Positive feedback regulation can generate an abrupt, all-or-none response in which the cell moves from ignoring a signal to responding to it very strongly. In some cases, such a response can be self-sustaining and will persist even after the signal is no longer present.

In the technique called optogenetics, light-gated Na+ channels are introduced into the brains of living animals. Activation of these channels by light can depolarize the membranes of neurons that contain them, selectively activating these target cells.Since its inception, optogenetics has been expanded to include other types of light-gated channels, such as a channel that is selective for Cl- instead of Na+. If this light-gated Cl- channel were introduced into neurons in a region of the brain that stimulates feeding, what might you expect to see? -A. In response to light activation, the animals would overeat, even when they are full. -B. The animals would avoid eating, even when they are hungry—but only when the channels are activated by light. -C. The animals would avoid eating, but only during the day. -D. The animals would avoid eating, even when they are hungry. -E. The channels would have no effect on behavior because the animal's normal Na+ channels would allow normal depolarization of neurons that regulate feeding.

-B. The animals would avoid eating, even when they are hungry—but only when the channels are activated by light. For this experiment, the animals would avoid eating, even when they are hungry—but only when the channels are activated by light. Because the cells are deep in the animal's brain, normal daylight will not reach them, so the activating light is supplied by a special fiber-optic cable implanted in the animal's brain. The reverse experiment works as well. When light-gated Na+ channels are introduced into these neurons instead of the Cl- channels, activation by light causes the animals to overeat—even when they have recently fed.

When transmitter-gated ion channels in the membrane of a postsynaptic cell open in response to neurotransmitter binding, what happens? -A. The channels remain open until an inhibitory neurotransmitter triggers their closure. -B. The channels alter the ion permeability of the postsynaptic membrane, which in turn may depolarize the postsynaptic membrane. -C. The channels always trigger an immediate and sustained action potential. -D. The channels convert an electrical signal to a chemical signal.

-B. The channels alter the ion permeability of the postsynaptic membrane, which in turn may depolarize the postsynaptic membrane. When transmitter-gated ion channels in the membrane of a postsynaptic cell open in response to neurotransmitter binding, they alter the ion permeability of the postsynaptic membrane, which in turn may depolarize the postsynaptic membrane. The channels open transiently in response to the binding of the neurotransmitter, changing the ion permeability of the postsynaptic membrane. This, in turn, causes a change in the membrane potential, thus encouraging another action potential. An action potential can be triggered only if the membrane potential becomes positive enough to fully depolarize the postsynaptic membrane.

Which of the following requires an input of energy to occur?Choose one: -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 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.

In bacteria, the transport of many nutrients, including sugars and amino acids, is driven by the electrochemical H+ gradient across the plasma membrane. In E. coli, for example, an H+-lactose symporter mediates the active transport of the sugar lactose into the cell. Given what you know about coupled transport, which is likely true of the H+-lactose symporter? -A. To undergo the conformational change that releases lactose into the cell, the transporter hydrolyzes ATP. -B. The transporter oscillates randomly between states in which it is open to either the extracellular space or the cytosol. -C. The transporter goes through an intermediate state in which the lactose-binding site is open to both sides of the membrane. -D. Lactose and H+ ions bind to two different conformations of the transporter. -E. If the H+ gradient were reversed, the transporter could serve as an H+-lactose antiport.

-B. The transporter oscillates randomly between states in which it is open to either the extracellular space or the cytosol. For a transporter to move solutes in the same direction across a membrane, the solutes must bind to the same conformation of the transporter. The H+-lactose symporter oscillates randomly between states in which it is open to either the extracellular space or the cytosol. In one state, the transporter is open to the extracellular space; in the other, it is open to the cytosol. To transition from one state to the other, the transporter must pass through an "occluded" state in which the transporter is either empty or both solutes are bound. Coupled transporters use the flow of one solute down its electrochemical gradient to drive the transport of a second solute against its electrochemical gradient; therefore, no ATP hydrolysis is needed—the energy source is, in this case, the ion gradient.

What is typically true of ion channels? -A. They hydrolyze ATP. -B. They are gated. -C. They operate by active transport. -D. They are open all the time. -E. They are nonselective.

-B. They are gated. Selective ion channels are not open continuously; they open briefly and then close again. This is referred to as a "gated" channel because the flow of ions can happen only when the channel is in the proper conformation. For most of these ion channels, a specific stimulus triggers them to open. When open, these ion channels facilitate passive diffusion, allowing solutes to move down their electrochemical gradient. This process does not require an input of energy.

How is an electrical signal converted to a chemical signal at a nerve terminal? -A. The influx of ions leads to a pH change, chemical transformation, and signaling. -B. Voltage-gated Ca2+ channels are activated and the influx of Ca2+ triggers the release of neurotransmitters. -C. Ligand-gated channels are bound by ions and open to allow the flow of neurotransmitters out of the cell. -D. Mechanically gated channels change conformation due to the electrical signal and create a mechanical signal.

-B. Voltage-gated Ca2+ channels are activated and the influx of Ca2+ triggers the release of neurotransmitters. 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

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. antiport -C. symport -D. channel

-B. antiport 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.

What condition must exist for glucose to be transported into a cell using the glucose-Na+ symport? -A. high Na+ concentration inside the cell -B. high Na+ concentration outside the cell -C. high glucose concentration outside the cell -D. high ATP concentration inside the cell for phosphorylation of the glucose-Na+ symport

-B. high Na+ concentration outside the cell 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 of the following events occur in the hypothalamus of a mouse brain expressing channelrhodopsin when the blue light is switched on? -A.Cl- floods out of the neuron. -B.The neuron membrane is depolarized. -C.Channelrhodopsin is activated and opened. -D.Na+ floods into the neuron.

-B.The neuron membrane is depolarized. -C.Channelrhodopsin is activated and opened. -D.Na+ floods into the neuron. Channelrhodopsin opens in response to blue light. Na+ ions flow into the cell through the open channel, which leads to depolarization of the plasma membrane. In the algae Chlamydomonas reinhardtii, channelrhodopsin regulates flagellar beating in response to light. Expressing this channel in neurons of other organisms can make those neurons responsive to light. This is an important experimental tool allowing neuroscientists to study the function of specific neurons and circuits.

Which of the following describes negative feedback regulation? -A. A component generates an all-or-none, switch-like mechanism. -B. A component acts to further activate the signaling pathway and enhance the cell's response. -C. A component late in the pathway inhibits an enzyme early in the pathway. -D. A component amplifies the signal for a more robust response.

-C. A component late in the pathway inhibits an enzyme early in the pathway. Negative feedback is when a product of a signaling pathway inhibits steps earlier in the pathway. This leads to a dampening of the response in response to strong activation. Hence, negative feedback can help a pathway switch rapidly between on and off states.

What does a target cell require to respond to an extracellular signal molecule? -appropriate intracellular signaling pathways -access to the signal molecule -a receptor that recognizes the signal molecule -effector molecules that alter cell behavior in response to the signal molecule -the appropriate machinery to produce and secrete the signal molecule

-appropriate intracellular signaling pathways -access to the signal molecule -a receptor that recognizes the signal molecule -effector molecules that alter cell behavior in response to the signal molecule

Which of the following statements is true about the concentration of calcium ions in cells? -A. Calcium levels are kept the same in the cytosol compared to outside the cell. -B. Calcium levels are kept high in the cytosol compared to outside the cell. -C. Calcium levels are kept low in the cytosol compared to outside the cell.

-C. Calcium levels are kept low in the cytosol compared to outside the cell. Calcium levels are kept low in the cytosol compared to outside the cell. Eukaryotic cells in general maintain a very low concentration of free Ca2+ in their cytosol (about 10-4 mM) compared to the much higher concentration of Ca2+ outside of the cell (typically 1-2 mM). This huge concentration difference is achieved mainly by means of ATP-driven Ca2+ pumps in both the plasma membrane and the endoplasmic reticulum membrane, which actively remove Ca2+ from the cytosol. The Na+ and Ca2+ pumps have similar amino acid sequences and structures, indicating that they share a common evolutionary origin, and both pump their ions out of the cytoplasm.

What is not true about a nerve impulse? -A. It is another term for an action potential. -B. It can travel at speeds of 100 meters per second. -C. It depends entirely on the action of ligand-gated ion channels. -D. It is electrical excitation of the plasma membrane that is propagated rapidly along the membrane and sustained by automatic renewal along the way. -E. It can travel long distances without weakening

-C. It depends entirely on the action of ligand-gated ion channels. The nerve impulse is another term for an action potential, and it does not depend entirely on the action of ligand-gated ion channels. Rather, action potentials are propagated by the action of voltage-gated ion channels. By the automatic renewal of the impulse by these voltage-gated ion channels, the action potential can travel long distances without weakening and can even travel at speeds of 100 meters per second.

Which of the following accurately describes the role of the Na+-K+ pump? -A. It maintains a lower Na+ concentration outside the cell. -B. It equilibrates the concentrations of Na+ and K+ across the plasma membrane. -C. It maintains a higher Na+ concentration outside the cell. -D. It maintains a higher K+ concentration outside the cell.

-C. It maintains a higher Na+ concentration outside the cell. The Na+-K+ pump uses energy from the hydrolysis of ATP to establish a strong electrochemical gradient of Na+ and K+ ions, with Na+ ions pumped to the extracellular space and K+ ions pumped to the cytosol. Establishing these electrochemical gradients is such a fundamental process in cells that about one-third of the ATP consumed by the cell is used by this pump. In addition to ATP hydrolysis, the function of the pump requires a carefully orchestrated set of conformational changes to operate and ensures that only Na+ and K+ ions are moved across the membrane by the pump.

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 remains open during the action potential and then rapidly returns to the closed state after the action potential passes. -B. The Na+ channel closes during the action potential and then rapidly returns to the open state after the action potential passes. -C. The Na+ channel becomes inactivated and refractory to reopening for a short time after the action potential passes. -D. The Na+ channel becomes permanently inactivated after the action potential passes.

-C. The Na+ channel becomes inactivated and refractory to reopening for a short time after the action potential passes. 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.

The epithelial cells that line the gut have glucose-Na+ symport proteins that actively take up glucose from the lumen of the gut after a meal, creating a high glucose concentration in the cytosol. How do these cells release that glucose for use by other tissues in the body? -A. Glucose diffuses down its concentration gradient through the lipid bilayer of the plasma membrane. -B. The cells have glucose channels in their plasma membrane. -C. The cells have glucose uniports in their plasma membrane. -D. The cells run the glucose-Na+ symport proteins in reverse. -E. The cells have a glucose pump that expels the glucose needed by other tissues.

-C. The cells have glucose uniports in their plasma membrane. Two types of glucose transporters enable gut epithelial cells to transfer glucose across the epithelial lining of the gut. Epithelial cells that have absorbed intestinal glucose release that glucose for use by other tissues in the body through glucose uniports in their plasma membrane. These passive glucose uniports allow glucose to move down its concentration gradient, out of the cell. The image shows this movement of glucose into the connective tissue. The glucose uniport is only found on the basal and lateral regions of the plasma membrane, ensuring it doesn't flow back into the gut lumen.

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 Na+ channels remain in the inactive, refractory state. -B. The Na+ channel does not open wide enough to allow enough Na+ through the channel. -C. The membrane depolarization is not amplified along the axon. -D. The axon membranes become over-depolarized.

-C. The membrane depolarization is not amplified along the axon. 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.

The first step in a signaling pathway that responds to a molecule that stays in the extracellular space is -diffusion through the plasma membrane into the cell. -binding of the signal molecule to a receptor. -activation of gene expression. -phosphorylation and activation of the receptor protein.

-binding of the signal molecule to a receptor. Signaling pathways that are activated in response to an extracellular signal must begin with the signal molecule binding to the receptor, often a membrane protein. Then the signal can be relayed and amplified inside the cell to produce cellular responses like activation of growth, gene expression, etc.

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? -A. The pump ran out of both Na+ and K+ because an equal number of both ions is pumped in each cycle. -B. The liposomes ran out of pumps to pump ions. -C. The pump ran out of Na+ to pump because it pumps 3 Na+ out for every 2 K+ pumped in. -D. The pump ran out of K+ to pump because it pumps 3 K+ out for every 2 Na+ pumped in.

-C. The pump ran out of Na+ to pump because it pumps 3 Na+ out for every 2 K+ pumped in. 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.

What is the role of K+-gated ion channels in an action potential? -A. They lead to the action potential reaching its highest state of cell depolarization. -B. They provide the energy for the sodium-potassium pump to reestablish resting potential. -C. They help reverse the action potential by repolarizing the cell. -D. They do not have a role in action potentials.

-C. They help reverse the action potential by repolarizing the cell. K+-gated ion channels open after membrane depolarization to allow K+ movement across the membrane to reestablish resting potential.

An electrochemical gradient has a chemical component and an electrical component. Which of the following will have the largest electrochemical gradient? -A. a positively charged ion, such as K+, at high concentrations inside the cell -B. a negatively charged ion, such as Cl-, at high concentrations outside the cell -C. a positively charged ion, such as Na+, at high concentrations outside the cell

-C. a positively charged ion, such as Na+, at high concentrations outside the cell Because an electrochemical gradient has a chemical component and an electrical component, a positively charged ion such as Na+ at high concentrations outside the cell will have the largest electrochemical gradient. In this situation, the concentration gradient works in the same direction as the membrane potential, creating a steep electrochemical gradient. Because a cell is more negatively charged inside, the membrane potential would oppose the movement of (1) positively charged ions out of the cell and (2) negatively charged ions into the cell.

Which organelle is important for controlling the concentration of calcium ions in the cytosol? -A. nucleus -B. Golgi apparatus -C. endoplasmic reticulum -D. lysosome

-C. endoplasmic reticulum The endoplasmic reticulum is important for controlling the concentration of calcium ions in the cytosol. Ca2+ pumps in the endoplasmic reticulum membrane, as well as the plasma membrane, keep cytosolic Ca2+ concentrations low. Eukaryotic cells in general maintain a very low concentration of free Ca2+ in their cytosol (about 10-4 mM) compared to the much higher concentration of Ca2+ outside of the cell (typically 1-2 mM). This huge concentration difference is achieved mainly by means of ATP-driven Ca2+ pumps in both the plasma membrane and the endoplasmic reticulum membrane, which actively remove Ca2+ from the cytosol.

What is the conformation of the voltage-gated Na+ channel that keeps the action potential from traveling backward along the axonal membrane? -A. open -B. closed -C. inactivated -D. triggered

-C. inactivated During an action potential, Na+ channels change conformations from closed to open to inactivated, then back to closed. Channels are in the inactivated state directly after opening, which keeps them from being reactivated immediately, and leads to the movement of the action potential away from the site of activation.

Which of the following activities helps restore the ion gradients across the plasma membrane of an axon after an action potential has occurred? -A. the activity of K+ leak channels -B. the opening of voltage-gated Na+ channels -C. the action of Na+ pumps -D. the closing of voltage-gated K+ channels

-C. the action of Na+ pumps The opening of voltage-gated Na+ channels allows Na+ to rush into the cell during an action potential. The opposite is needed for the restoration of the resting membrane potential. That is, the action of Na+ pumps helps to restore the ion gradients across the plasma membrane of an axon after an action potential has occurred. About 20% of the energy generated by the metabolism of food is used to operate the Na+ pumps that restore the balance of ions following action potentials.

During an action potential, which of the following actions does not help return the membrane to its resting potential? -A. the inactivation of voltage-gated Na+ channels -B. the flow of K+ through K+ leak channels -C. the opening of voltage-gated Na+ channels -D. the opening of voltage-gated K+ channels

-C. the opening of voltage-gated Na+ channels 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 mechanisms prevents osmotic swelling in plant cells? -A. the expulsion of water from contractile vacuoles -B. turgor pressure -C. tough cell walls -D. the collection of water in contractile vacuoles -E. the activity of Na+ pumps

-C. tough cell walls Cells use different tactics to avoid osmotic swelling. Plant cells are prevented from swelling by their cell walls, allowing them to tolerate a large osmotic difference across their plasma membrane. This results in turgor pressure when the pressure from swelling presses against the cell wall and is utilized by plants to help them achieve structural stability.

Many drugs act by binding to what? -extracellular signal molecules -cell-surface receptors -plasma membrane -cyclic AMP -nuclear receptors

-cell-surface receptors These receptors therefore present attractive targets for the development of drugs that can alter our physiology in a desirable way, for example, providing relief from pain or anxiety.

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.Na+ is transported against its electrochemical gradient. -B.The transporter directly uses ATP as an energy source for transporting ions. -C.The transporter uses the Na+ electrochemical gradient as an energy source for transporting ions. -D.Ca2+ is transported against its electrochemical gradient.

-C.The transporter uses the Na+ electrochemical gradient as an energy source for transporting ions. -D.Ca2+ is transported against its electrochemical gradient. 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 ions has a low cytosolic concentration so that a flood of this ion into the cell can be used as a signal for cell processes like fertilization? -H+ -K+ -phosphate -Ca2+

-Ca2+ Calcium ions are a common intracellular signaling molecule. The concentrations in the cell are kept extremely low so that Ca2+ channels can rapidly change the concentration and activate cellular processes.

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? -Ca2+ -Na+ -Cl- -K+

-Ca2+ 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.

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. -A. KCl, because Cl- is needed for glucose entry. -B. KCl, because K+ is needed for glucose entry. -C. HCl, because H+ is needed for glucose entry. -D. NaCl, because Na+ is needed for glucose entry.

-D. NaCl, because Na+ is needed for glucose entry. 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.

Why does optogenetics hold the potential to help scientists better understand neurobiology? -A. Optogenetics can be used to visualize how neurons contact other cells. -B. Optogenetics can be used to study how Na+ channels transport Na+. -C. Optogenetics can help us understand how light stimulates neurons in the brain. -D. Optogenetics can be used to analyze neural circuits and complex behavior.

-D. Optogenetics can be used to analyze neural circuits and complex behavior. Optogenetics is an experimental method in which neuroscientists can express channelrhodopsin from Chlamydomonas reinhardtii in the cells of other organisms. When expressed in mice brains, scientists can use light to activate specific neurons and measure the response. This technique holds the promise of helping us to understand neurons and circuits that control complex behavior.

Which of the following describes the resting membrane potential of a neuron? -A. a voltage difference across the plasma membrane when the neuron has been stimulated -B. a voltage difference that is chiefly a reflection of the electrochemical Na+ gradient across the plasma membrane -C. a voltage difference across the plasma membrane, with more positive membrane potential inside -D. a state in which the flow of positive and negative ions across the plasma membrane is precisely balanced -E. a voltage difference of 0 millivolts (mV) across the membrane

-D. a state in which the flow of positive and negative ions across the plasma membrane is precisely balanced 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.

Which of the following form tiny hydrophilic pores in the membrane through which solutes can pass by diffusion? -A. pumps -B. transporters -C. anions -D. channels -E. liposomes

-D. channels 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.

Which of the following types of cell signaling is long range and uses hormones as signals? -A. paracrine -B. neuronal -C. contact-dependent -D. endocrine

-D. endocrine Endocrine signaling is a long-range type of cell-cell signaling that uses hormones secreted in the blood by specialized cells. This is distinguished from paracrine signaling, which also uses chemical signals that are secreted but over a shorter range.

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? -A. glucose, sodium ions, oxygen -B. sodium ions, oxygen, glucose -C. oxygen, sodium ions, glucose -D. oxygen, glucose, sodium ions -E. glucose, oxygen, sodium ions

-D. oxygen, glucose, sodium ions As summarized in the figure, various molecules pass directly through lipid bilayers at dramatically different rates. 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.

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? -A. It inhibits the opening of voltage-gated Na+ channels. -B. It accelerates the opening of voltage-gated K+ channels. -C. It slows the inactivation of voltage-gated K+ channels. -D. It prolongs the inactivation of voltage-gated Na+ channels. -E. It slows the inactivation of voltage-gated Na+ channels.

-E. It slows the inactivation of voltage-gated Na+ channels. 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.

In general, which of the following will diffuse across a lipid bilayer most rapidly? -A. a small hydrophilic molecule -B. a large hydrophobic molecule -C. water -D. a large hydrophilic molecule -E. a small hydrophobic molecule

-E. a small hydrophobic molecule 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 inhibits inorganic ions, such as Na+ and Cl-, from passing through a lipid bilayer? -A. the ions' large size -B. the carbohydrate layer on the surface of the lipid bilayer -C. the hydrophilic exterior of the lipid bilayer -D. the watery environment on either side of the lipid bilayer -E. the hydrophobic interior of the lipid bilayer

-E. the hydrophobic interior of the lipid bilayer 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.

Which statement about cell signaling is correct? -Each type of extracellular signal molecule induces the same response in all target cells. -All extracellular signal molecules act by binding to receptors on the cell surface. -Each receptor triggers one particular type of cell behavior, for example, activating gene expression. -Each receptor is generally activated by only one type of signal molecule. -All cell types are able to respond to the same set of signal molecules.

-Each receptor is generally activated by only one type of signal molecule. Small molecules like acetylcholine, for example, usually bind to a pocket formed by amino acids from several transmembrane segments of the receptor. These amino acids form noncovalent interactions with the signal molecule, allowing the receptor to recognize and bind specifically to its signal.

The active form of a monomeric GTP-binding protein is the -GTP-bound form. -guanine nucleotide exchange factor (GEF). -GDP-bound form. -GTPase-activating protein (GAP).

-GTP-bound form. Monomeric GTP-binding proteins (also known as monomeric GTPases) are in their active form when bound to GTP. An enzyme called GEF helps activate the GTP-binding protein by promoting its binding to GTP.

In fungi, plants, and bacteria, which pump helps to drive the import of solutes? -H+ pumps -Ca2+ pumps -Na+ pumps -K+ pumps -ATP pumps

-H+ pumps In fungi, plants, and bacteria, the H+ pump, which functions quite similar to the Ca2+ pump, helps to drive the import of solutes. Plant cells, bacteria, and fungi rely mainly on an electrochemical gradient of H+ to import solutes. When H+ are pumped from the cell, a proton (H+) motive force is built and this is a strong force because when these ions diffuse down their gradient, the electrical and chemical (electrochemical) gradient can be used. Plant cells, many bacteria, and fungi do not have K+ or Na+ pumps in their plasma membranes. Instead, many cells—fungi, plants, animals, and bacteria—rely on the proton motive force.

What is true of the inside of a cell? -It is slightly more negative than the outside of a cell. -It has the same charge as the outside of the cell. -It is slightly more positive than the outside of a cell.

-It is slightly more negative than the outside of a cell. The inside of a cell is slightly more negative than the outside of a cell. This uneven charge distribution tends to pull positively charged solutes into the cell and drive negatively charged ones out. In animal cells, for example, the resting membrane potential can be anywhere between -20 and -200 millivolts (mV), depending on the organism and cell type. The value is expressed as a negative number because the interior of the cell is more negatively charged than the exterior.

In most animal cells, which ion can move through "leak" channels? -H+ -Na+ -K+ -Ca2+ -Cl-

-K+ In most animal cells, K+ ions can move through "leak" channels. When leak channels are open, they allow K+ to move freely out of the cell. In a resting cell, these are the main ion channels open in the plasma membrane, rendering the membrane much more permeable to K+ than to other ions. When K+ flows out of the cell—down the concentration gradient generated by the ceaseless operation of the Na+ pump—the loss of positive charge inside the cell creates a voltage difference, or membrane potential.

Lipid bilayers are highly impermeable to which molecule(s)? -oxygen -steroid hormones -Na+ and Cl- -water -carbon dioxide

-Na+ and Cl- 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+ 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 both excluded from cells. -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. 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.

Signaling at a synapse occurs in postsynaptic cells...

-Neurotransmitter binding begins a new action potential. -Ligand-gated ion channels open in response to neurotransmitter. 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.

Signaling at a synapse occurs in presynaptic cells...

-Neurotransmitter is released into the synaptic cleft. -Voltage-gated Ca2+ channels open. -Reuptake of the neurotransmitter occurs. 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.

Which is true of the GTP-binding proteins that participate in intracellular signaling? -Only trimeric GTP-binding proteins relay messages from G-protein-coupled receptors. -G-protein-coupled receptors interact with all types of GTP-binding proteins. -Only trimeric GTP-binding proteins participate in intracellular cell signaling. -Only trimeric GTP-binding proteins interact with guanine nucleotide exchange factors (GEFs). -Only monomeric GTP-binding proteins relay messages from G-protein-coupled receptors.

-Only trimeric GTP-binding proteins relay messages from G-protein-coupled receptors. Two main types of GTP-binding proteins participate in intracellular signaling. The first type—the large, trimeric GTP-binding proteins (also called G proteins)—relay messages from G-protein-coupled receptors. Other cell-surface receptors, such as RTKs, rely on a second type of GTP-binding protein—the small, monomeric GTPases—to help relay their signals. These switch proteins are generally aided by two sets of regulatory proteins that help them bind and hydrolyze GTP: guanine nucleotide exchange factors (GEFs) activate the switches by promoting the exchange of GDP for GTP, and GTPase-activating proteins (GAPs) turn them off by promoting GTP hydrolysis.Trimeric G proteins undergo a conformational change when they interact with an activated GPCR. This change in conformation causes the α subunit of the G protein to exchange its GDP for GTP.

Which of the following statements is false? -Intracellular signaling pathways can include both tyrosine kinases and serine/threonine kinases. -Serine/threonine kinases are opposed by the action of tyrosine kinases. -Many protein kinases are themselves activated by phosphorylation. -Serine/threonine kinases phosphorylate intracellular proteins on serines or threonines. -Tyrosine kinases phosphorylate intracellular proteins on tyrosines.

-Serine/threonine kinases are opposed by the action of tyrosine kinases. Two main types of protein kinases operate in intracellular signaling pathways. The most common are serine/threonine kinases, which (as the name implies) phosphorylate proteins on serines or threonines; others are tyrosine kinases, which phosphorylate proteins on tyrosines.However, for every activation step in a signaling pathway, there exists an inactivation mechanism. For molecules whose activity is regulated by phosphorylation, the switch is thrown in one direction by a protein kinase, which covalently attaches a phosphate group onto the switch protein, and in the opposite direction by a protein phosphatase, which takes the phosphate off again. The activity of any protein that is regulated by phosphorylation therefore depends, moment by moment, on the balance between the activities of the protein kinases that phosphorylate it and the protein phosphatases that dephosphorylate it.

When cells respond to an extracellular signal, they most often convert the information carried by this molecule from one form to another. What is this process called? -signal integration -signal transduction -signal amplification -signal production -signal detection

-Signal transduction Signal transduction begins when the receptor on a target cell receives an incoming extracellular signal and then produces intracellular signaling molecules that alter cell behavior.

Which statement is true about cell-surface receptors? -All extracellular signal molecules bind to a single type of receptor. -Each cell-surface receptor can bind to an endless variety of extracellular signal molecules. -All cell-surface receptors bind to a single, unique extracellular signal molecule. -Some cell-surface receptors can bind to multiple signal molecules. -All drugs block the activation of ion-channel-coupled receptors.

-Some cell-surface receptors can bind to multiple signal molecules. Many receptors can even bind foreign substances that can alter their activity. For example, nicotine interacts with ion-channel-coupled receptors that normally recognize acetylcholine.

What is true of the GTP-binding proteins that act as molecular switches inside cells? -They are active when GTP is bound. -They turn themselves on by hydrolyzing GTP to form GDP. -They turn themselves on by phosphorylating GDP to form GTP. -They are active only in their trimeric forms. -They are active when GDP is bound.

-They are active when GTP is bound. Two main types of GTP-binding proteins participate in intracellular signaling. The first type—the large, trimeric GTP-binding proteins (also called G proteins)—relay messages from G-protein-coupled receptors. Other cell-surface receptors rely on a second type of GTP-binding protein—the small, monomeric GTPases—to help relay their signals.GTP-binding proteins toggle between an active and an inactive state depending on whether they have GTP or GDP bound to them, respectively. Once activated by GTP binding, many of these proteins have intrinsic GTP-hydrolyzing (GTPase) activity, and they shut themselves off by hydrolyzing their bound GTP to GDP. Others are aided by two sets of regulatory proteins that help them bind and hydrolyze GTP: guanine nucleotide exchange factors (GEFs) activate the switches by promoting the exchange of GDP for GTP, and GTPase-activating proteins (GAPs) turn them off by promoting GTP hydrolysis.

Most extracellular signal molecules act on cell-surface receptors rather than intracellular receptors. Which statements are true about these extracellular molecules? -They are too small to pass directly across the plasma membrane. -They are too hydrophilic to pass directly across the plasma membrane. -They are too hydrophobic to pass directly across the plasma membrane. -They are too large to pass directly across the plasma membrane.

-They are too hydrophilic to pass directly across the plasma membrane. -They are too large to pass directly across the plasma membrane.

Which is true of intracellular signaling molecules? -They act as local, paracrine mediators. -They are carried through the bloodstream and represent a form of endocrine signaling. -They can be proteins. -They are always hydrophobic. They can act through cell-surface receptors or intracellular receptors.

-They can be proteins Target cells possess receptors that recognize and bind to extracellular signal molecules and then generate intracellular signaling molecules in response. These molecules can be proteins, small messenger molecules, or even ions. Each of these molecules further relays the signal, passing the message "downstream" from one intracellular signaling molecule to another, each activating or generating the next signaling molecule in the pathway, until they activate an effector molecule that changes the behavior of the cell. These effects can be metabolic enzymes that are kicked into action, cytoskeletal elements that are tweaked into a new configuration, or genes that are switched on or off. This final outcome is called the response of the cell.

What is true of ion-channel-coupled receptors? -They transduce signals in a simple and direct manner. -They integrate multiple signals at the same time. -They are found only in nerve cells. -They are voltage-gated. -They transduce signals via elaborate intracellular signaling pathways.

-They transduce signals in a simple and direct manner. Of all the types of cell-surface receptors, ion-channel-coupled receptors function in the simplest and most direct way. These receptors transduce a chemical signal, in the form of a pulse of secreted neurotransmitter molecules delivered to a target cell, directly into an electrical signal, in the form of a change in voltage across the target cell's plasma membrane. They are therefore considered "transmitter-gated" receptors.Although ion-channel-coupled receptors are especially important in nerve cells, they are also present in other electrically excitable cells such as muscle cells.

Following an action potential, a nerve cell goes through a brief refractory period during which it cannot be stimulated. What is true during this refractory period? -Voltage-gated K+ channels in the nerve cell membrane are inactivated. -Voltage-gated Ca2+ channels are open. -Voltage-gated Na+ channels in the nerve cell membrane are open. -The membrane potential remains unchanged. -Voltage-gated Na+ channels in the nerve cell membrane are inactivated.

-Voltage-gated Na+ channels in the nerve cell membrane are inactivated. Following an action potential, a nerve cell goes through a brief refractory period during which it cannot be stimulated. In this refractory period, voltage-gated Na+ channels in the nerve cell membrane are inactivated. This temporary inactivation prevents the advancing front of membrane depolarization from spreading backward along the nerve cell axon. During the refractory period, membrane potential begins to return to the negative resting membrane potential and voltage-gated K+ channels stay open as long as the membrane is depolarized. The figure below provides a graphical representation of how the refractory period ensures unidirectional movement of an action potential.

The movement of an ion against its concentration gradient is called what? -osmosis -passive transport -facilitated diffusion -active transport

-active transport 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.

Each of these mechanisms of transport, as illustrated above (gradient-driven pump, ATP-driven pump, light-driven pump) , is best categorized by which term? -A. facilitated diffusion -B. active transport -C. osmosis -D. simple diffusion

-active transport In each of these examples, there is a solute that is moving up its concentration gradient. This means that there is an energetic investment that is being used to pump a solute—the definition of active transport. First, gradient-driven pumps link the uphill transport of one solute across a membrane to the downhill transport of another. Second, ATP-driven pumps use the energy released by the hydrolysis of ATP to drive uphill transport. And third, light-driven pumps, which are found mainly in bacterial cells, use energy derived from sunlight to drive uphill transport.

Inhibitory neurotransmitters such as glycine and GABA make a postsynaptic cell harder to depolarize by allowing what? -an influx of K+ -the escape of Na+ -an influx of Cl- -an influx of Na+

-an influx of Cl- 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.

Determine whether the following statement is true or false: Communication between neurons involves an interconversion of electrical and chemical signals. This statement is...

True Communication between neurons involves an interconversion of electrical and chemical signals. When an action potential reaches a nerve terminal, this electrical signal is converted to a chemical signal that crosses the synapse. Transmitter-gated ion channels in the membrane of the target cell convert this chemical signal back into an electrical one.

Determine whether the following statement is true or false: Water passes through the cell membrane only through specialized channels called aquaporins.This statement is

false The ability to regulate water balance across the plasma membrane is critically important for the normal function of cells. Water molecules are uncharged and relatively small, meaning they can move across the membrane on their own, albeit at a somewhat slow rate. The presence of aquaporins in the membrane allows for even more efficient transit of water molecules through the membrane, giving the cell the ability to quickly and precisely regulate its water balance.

Determine whether the following statement is true or false: An electrical signal can jump across the synaptic cleft between the presynaptic and postsynaptic cells. This statement is...

false To cross a synapse, electrical signals must be converted into chemical signals. This is accomplished when the electrical signal triggers the release of a small signal molecule called a neurotransmitter into the synaptic cleft. Neurotransmitters are stored in the nerve terminals within membrane-enclosed synaptic vesicles. When an action potential reaches the nerve terminal, some of the synaptic vesicles fuse with the plasma membrane, releasing their neurotransmitter into the synaptic cleft and that chemical can then impact a target cell.