ch. 12

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

- Mg2+ - H+! - K+ - Cl-

The Na+-K+ pump in the plasma membrane uses the energy from ATP hydrolysis to pump the two ions against their electrochemical gradients. In what directions are the ions pumped across the membrane?

- Na+ in and K+ out - Both Na+ and K+ in - Both Na+ and K+ out - Na+ out and K+ in!

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

- Na+ is bound. - ADP is bound. - The pump is phosphorylated.! - The pump changes conformation.

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

- Na+-coupled - ATP-driven - Light-driven - K+-coupled!

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

- Na+-coupled - K+-coupled! - ATP-driven - light-driven

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

- Some channels remain closed and some open completely.! - All channels open completely. - All channels open partly, to the same degree. - All channels open partly, each to a different degree.

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

- The membrane potential is not affected by Na+. - It becomes more negative. - It becomes more positive.! - It is permanently reset.

Which organelle is most important for controlling the concentration of calcium ions in the cytosol using the Ca2+ ATPase pump?

- The nucleus - The lysosome - The endoplasmic reticulum! - The Golgi apparatus

During an action potential, which of the following helps return the membrane potential to its resting potential?

- The opening of voltage-gated Ca2+ channels - The closing of K+ leak channels - The opening of voltage-gated K+ channels! - The opening of voltage-gated Na+ channels

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

- The opening of voltage-gated K+ channels - The activity of K+ leak channels - Pumping by the Na+-K+ ATPase - The opening of voltage-gated Ca2+ channels!

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

- The temperature is 37°C. - The plasma membrane is primarily permeable to Na+.! - At rest, the interior of the cell is more negatively charged than the exterior. - K+ is the principal positive ion in the cell.

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

- They set the K+ concentration gradient to zero. - They set the membrane potential to zero. - They disrupt the resting membrane potential. - They keep the electrochemical gradient for K+at zero.!

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

- acetylcholine! - glutamate - GABA - glycine

Studies on the squid giant axon were instrumental in our current understanding of how action potentials are generated. You decide to do an experiment on the squid giant axon yourself. You add NaCl to the extracellular fluid and effectively double the amount of extracellular Na+cation. How does this affect the action potential?

- action potential will be smaller - action potential will be larger!

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

- active transport! - free diffusion - facilitated diffusion - passive transport

Which molecule is more likely to diffuse through the lipid bilayer?

- amino acids - benzene!

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

- by closing Na+ channels - by preventing the secretion of excitatory neurotransmitters - by opening K+ channels - by opening Cl- channels!

The figure below illustrates changes in membrane potential during the formation of an action potential. What membrane characteristic or measurement used to study action potentials is indicated by the arrow? (arrow at 0 mV)

- effect of a depolarizing stimulus - resting membrane potential - threshold potential - action potential!

The figure below illustrates changes in membrane potential during the formation of an action potential. What membrane characteristic or measurement used to study action potentials is indicated by the arrow? (arrow at -40 mV)

- effect of a depolarizing stimulus - resting membrane potential - threshold potential! - action potential

The figure below illustrates changes in membrane potential during the formation of an action potential. What membrane characteristic or measurement used to study action potentials is indicated by the arrow? (arrow at -60 mV)

- effect of a depolarizing stimulus - resting membrane potential! - threshold potential - action potential

The figure below illustrates changes in membrane potential during the formation of an action potential. What membrane characteristic or measurement used to study action potentials is indicated by the arrow? (arrow at first curve)

- effect of a depolarizing stimulus! - resting membrane potential - threshold potential - action potential

Transporters, in contrast to channels, work by _____________________.

- filtering solutes by charge - a gating mechanism - filtering solutes by size - specific binding to solutes!

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

- glucose! - water - glycerol - ethanol

Which molecule is more likely to diffuse through the lipid bilayer?

- glycerol! - RNA

Ca2+ pumps in the plasma membrane and endoplasmic reticulum are important for _____________.

- maintaining osmotic balance - preventing Ca2+ from altering the activity of molecules in the cytosol! - providing enzymes in the endoplasmic reticulum with Ca2+ions that are necessary for their catalytic activity - maintaining a negative membrane potential

Studies on the squid giant axon were instrumental in our current understanding of how action potentials are generated. You decide to do an experiment on the squid giant axon yourself. You remove the cytoplasm in an axon and replace it with an artificial cytoplasm that contains twice the normal concentration of K+ by adding KOAc, where OAc- is an anion that is impermeable to the membrane. In this way you double the internal concentration of K+ while maintaining the bulk electrical balance of the cytoplasmic solution. Will this make the resting potential of the membrane more or less negative?

- more negative! - less negative

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

- neurotransmitter receptors - Na+-K+ pumps - voltage-gated K+ channels - voltage-gated Ca2+ channels!

Transporters, in contrast to channels, work by ________________.

- specific binding to solutes! - a gating mechanism - filtering solutes by charge - filtering solutes by size

Studies on the squid giant axon were instrumental in our current understanding of how action potentials are generated. You decide to do an experiment on the squid giant axon yourself. You replace half of the NaCl in the extracellular fluid with choline chloride. (Choline is a monovalent cation much larger than Na+. Note that the presence of choline will not impede the flow of Na+ through its channels.) How will this affect the action potential?

- the action potential will be smaller! - the action potential will be larger

A hungry yeast cell lands in a vat of grape juice and begins to feast on the sugars there, producing carbon dioxide and ethanol in the process:C6H12O6 + 2ADP + 2Pi + H+ → 2CO2 + 2CH3CH2OH + 2ATP + 2H2O. Unfortunately, the grape juice is contaminated with proteases that attack some of the transport proteins in the yeast cell membrane, and the yeast cell dies. Which of the following could account for the yeast cell's demise?

- toxic buildup of carbon dioxide inside the cell - toxic buildup of ethanol inside the cell - diffusion of ATP out of the cell - inability to import sugar into the cell!

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

- voltage-gated - extracellular ligand-gated - intracellular ligand-gated - stress-gated!

Action potentials are usually mediated by voltage-gated Ca2+ channels.

False

Aquaporin channels are found in the plasma membrane, allowing the rapid passage of water molecules and small ions in and out of cells.

False

Facilitated diffusion can be described as the favorable movement of one solute down its concentration gradient being coupled with the unfavorable movement of a second solute up its concentration gradient.

False

Gap junctions are large pores that connect the cytosol to the extracellular space.

False

The differences in permeability between artificial lipid bilayers and cell membranes arise from variations in phospholipid content.

False

The ion selectivity of a channel completely depends solely on the charge of the amino acids lining the pore inside the channel.

False

The net negative charge on the cytosolic side of the membrane enhances the rate of glucose import into the cell by a uniporter.

False

Transporters are similar to channels, except that they are larger, allowing folded proteins as well as smaller organic molecules to pass through them.

False

Voltage-gated K+ channels also open immediately in response to local depolarization, reducing the magnitude of the action potential.

False

For each missing section of the paragraph, select the best word or phrase from the pull-down menus below. A molecule moves down its concentration gradient by ___A___ transport, but requires ___B___ transport to move up its concentration gradient. Transporter proteins and ion channels function in membrane transport by providing a ___C___ pathway through the membrane for specific polar solutes or inorganic ions. ___D___ are highly selective in the solutes they transport, binding the solute at a specific site and changing its conformation so as to transport the solute across the membrane. On the other hand, ___E___ discriminate between solutes mainly on the basis of size and electrical charge.

A: passive B: active C: hydrophilic D: transporter proteins E: ion channels

For each missing section of the paragraph, select the best word or phrase from the pull-down menus below. Neurons communicate with each other through specialized sites called ___A___. Many neurotransmitter receptors are ligand-gated ion channels that open transiently in the ___B___ cell membrane in response to neurotransmitters released by the ___C___ cell. Ligand-gated ion channels in nerve cell membranes convert ___D___ signals into ___E___ ones. Neurotransmitter release is stimulated by the opening of voltage-gated ___F___ in the nerve terminal membrane.

A: synapses B: postsynaptic C: presynaptic D: chemical E: electrical F: Ca2+ channels

CO2 and O2 are water-soluble molecules that diffuse freely across cell membranes.

True

Cells expend energy in the form of ATP hydrolysis so as to maintain ion concentrations that differ from those found outside the cell.

True

Most ion channels are gated, which allow them to open and close in response to a specific stimulus rather than allowing the constant, unregulated flow of ions.

True

Neurotransmitters are small molecules released into the synaptic cleft after the fusion of synaptic vesicles with the presynaptic membrane.

True

The electrochemical gradient for K+ across the plasma membrane is small. Therefore, any movement of K+ from the inside to the outside of the cell is driven solely by its concentration gradient.

True

Transporters undergo transitions between different conformations, depending on whether the substrate-binding pocket is empty or occupied.

True

Voltage-gated Na+ channels become automatically inactivated shortly after opening, which ensures that the action potential cannot move backward along the axon.

True

Many receptors for neurotransmitters are _______ ion channels.

- Ligand gated! - Stress gated - Voltage gated

The acetylcholine receptor in skeletal muscle cells is a(n) _______ ion channel.

- Ligand gated! - Stress gated - Voltage gated

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

- It stays open continuously when stimulated. - It opens more frequently in response to a stimulus.! - It remains closed if unstimulated. - It opens more widely as the stimulus becomes stronger.

_______ ion channels respond to changes in membrane potential.

- Ligand gated - Stress gated - Voltage gated!

_______ ion channels are found in the hair cells of the mammalian cochlea.

- Ligand gated - Stress gated! - Voltage gated

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

- 438.1 mV - -52.7 mV! - 52.7 mV - -5.3 mV

Which one of the following statements about passive and/or active transport is FALSE?

- ATP driven pumps move solutes down their concentration gradients.! - The transport of glucose across the membrane can be active or passive, depending on the nature of the transporter. - The movement of an ion against it electrochemical gradient is a form of active transport. - Coupled transporters use electrochemical gradients to drive active transport of other solutes.

Which of the following statements is true?

- Amoebae have transporter proteins that actively pump water molecules from the cytoplasm to the cell exterior. - Bacteria and animal cells rely on the Na+-K+pump in the plasma membrane to prevent lysis resulting from osmotic imbalances. - The Na+-K+ pump allows animal cells to thrive under conditions of very low ionic strength. - The Na+-K+ pump helps to keep both Na+ and Cl- ions out of the cell.!

Both excitatory and inhibitory neurons can form synapses with their target cells. How do inhibitory neurotransmitters prevent the post-synaptic cell from firing another action potential?

- By opening Cl- channels! - By blocking secretion of excitatory neurotransmitters - By closing voltage-gated Na+ channels By opening K+ channels

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

- Ca2+ pumps in vesicles membranes keep cystosolic calcium levels low. - Ca2+ pumps in the ER membrane keep cystosolic calcium levels low.! - Ca2+ pumps in the Golgi apparatus keep cystosolic calcium levels low. - Ca2+ concentrations in the cytosol increase at a steady rate.

Which molecule is more likely to diffuse through the lipid bilayer?

- Cl- - ethanol!

Which molecule is more likely to diffuse through the lipid bilayer?

- H2O - O2!

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

- HCO3- - Cl- - PO43-! - OH-

When voltage-gated Na+ channels are stimulated in a nerve cell, what happens to the membrane potential?

- It is permanently reset. - It becomes less negative.! - It becomes more negative. - It stays the same.

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

- It stays open continuously when stimulated. - It opens more frequently in response to a given stimulus.! - It opens more widely as the stimulus becomes stronger. - It remains closed if unstimulated.

For each missing section of the paragraph, select the best word or phrase from the pull-down menus below. For an uncharged molecule, the direction of passive transport across a membrane is determined solely by its ___A___ gradient. On the other hand, for a charged molecule, the ___B___ must also be considered. The net driving force for a charged molecule across a membrane therefore has two components and is referred to as the ___C___ gradient. Active transport allows the movement of solutes against this gradient. The transporter proteins called ___D___ transporters use the movement of one solute down its gradient to provide the energy to drive the uphill transport of a second gradient. When this transporter moves both ions in the same direction across the membrane, it is considered a(n) ___E___; if the ions move in opposite directions, the transporter is considered a(n) ___F___.

A: concentration B: membrane potential C: electrochemical D: coupled E: symport F: antiport

For each missing section of the paragraph, select the best word or phrase from the pull-down menus below. The action potential is a wave of ___A___ that spreads rapidly along the neuronal plasma membrane. This wave is triggered by a local change in the membrane potential to a value that is ___B___ negative than the resting membrane potential. The action potential is propagated by the opening of ___C___-gated channels. During an action potential, the membrane potential changes from ___D___ to___E___. The action potential travels along the neuron's ___F___ to the nerve terminals. Neurons chiefly receive signals at their highly branched ___G___.

A: depolarization B: less C: voltage D: negative E: positive F: axon G: dendrites