Ch 12. Cells Exam 2

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. A. Gap junctions are large pores that connect the cytosol to the extracellular space. B. Aquaporin channels are found in the plasma membrane, and allow the rapid passage of water molecules and small ions in and out of cells. C. The ion selectivity of a channel depends solely on the charge of the amino acids lining the pore inside the channel. D. Most ion channels are gated, which allows them to open and close in response to a specific stimulus, rather than allowing the constant, unregulated flow of ions.

A. False. Gap junctions are used to connect the cytosol of adjacent cells, allowing the sharing of ions and small metabolites. Because gap junctions are large channels, if they were open while facing the extracellular environment, the ability of the plasma membrane to serve as a permeability barrier would be greatly reduced. B. False. Charged molecules (even protons, which are very small) are not able to pass through aquaporins. C. False. Selectivity depends on three parameters: the diameter, shape, and charge of the ion trying to pass through the pore of the channel. D. True.

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. A. Facilitated diffusion can be described as the favorable movement of one solute down its concentration gradient being coupled with the unfavorable movement of a second solute up its concentration gradient. B. Transporters undergo transitions between different conformations, depending on whether the substrate-binding pocket is empty or occupied. C. 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. D. The net negative charge on the cytosolic side of the membrane enhances the rate of glucose import into the cell by a uniporter.

A. False. This describes coupled transport, which is one type of active transport. Facilitated diffusion can also be called passive transport, in which a solute always moves down its concentration gradient. B. True. C True. D. False. Glucose is an uncharged molecule, and its import is not directly affected by the voltage difference across the membrane if glucose is being transported alone. If the example given were the Na/glucose symporter, we would have to consider the charge difference across the membrane.

Indicate whether the statements below are true or false. If a statement is false, explain why it is false. A. Neurotransmitters are small molecules released into the synaptic cleft after the fusion of synaptic vesicles with the presynaptic membrane. B. Action potentials are usually mediated by voltage-gated Ca2+ channels. C. Voltage-gated Na+ channels become automatically inactivated shortly after opening, which ensures that the action potential cannot move backward along the axon. D. Voltage-gated K+ channels also open immediately in response to local depolarization, reducing the magnitude of the action potential.

A. True. B. False. Action potentials are usually mediated by voltage-gated Na+ channels. C. True. D. False. Voltage-gated K+ channels respond more slowly than the voltage-gated Na+ channels. Because voltage-gated K+ channels do not open until the action potential reaches its peak, they do not affect its magnitude. Instead, they help to restore the local membrane potential quickly while the voltage-gated Na+ channels are in the inactivated conformation

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. CO2 and O2 are water-soluble molecules that diffuse freely across cell membranes. B. The differences in permeability between artificial lipid bilayers and cell membranes arise from variations in phospholipid content. C. Transporters are similar to channels, except that they are larger, allowing folded proteins as well as smaller organic molecules to pass through them. D. Cells expend energy in the form of ATP hydrolysis so as to maintain ion concentrations that differ from those found outside the cell.

A. True. B. False. The primary difference between cell membranes and artificial membranes is that cell membranes have proteins responsible for creating a selective permeability, which varies with the location and function of the membrane. C. False. Transporters work by changing conformation after specific binding of the solute to be transported. Channels exclude molecules on the basis of size and charge, but do not depend on specific recognition of the molecules moving through. D. True.

Circle the molecule in each pair that is more likely to diffuse through the lipid bilayer. A. amino acids or benzene B. Cl- or ethanol C. glycerol or RNA D. H2O or O2 E. adenosine or ATP

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

Large molecules, such as glucose, and ions can diffuse easily through plasma membrane

False

Cells use membranes to help maintain set ranges of ion concentrations inside and outside the cell. Which of the following ions is the most abundant outside a typical mammalian cell? (a) Na+ (b) K+ (c) Ca2+ (d) Cl-

a

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 _____________. (a) active transport. (b) free diffusion. (c) facilitated diffusion. (d) passive transport.

a

Transporters, in contrast to channels, work by ________________. (a) specific binding to solutes. (b) a gating mechanism. (c) filtering solutes by charge. (d) filtering solutes by size.

a

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? (a) Some channels remain closed and some open completely. (b) All channels open completely. (c) All channels open partly, to the same degree. (d) All channels open partly, each to a different degree.

a

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

a

Which of the following channels would not be expected to generate a change in voltage by movement of its substrate across the membrane where it is found? (a) an aquaporin (b) a sodium channel (c) a calcium channel (d) a proton channel

a

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? (a) Na+-coupled (b) K+-coupled (c) ATP-driven (d) light-driven

b

Cells use membranes to help maintain set ranges of ion concentrations inside and outside the cell. Which of the following ions is the most abundant inside a typical mammalian cell? (a) Na+ (b) K+ (c) Ca2+ (d) Cl-

b

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? (a) Mg2+ (b) H+ (c) K+ (d) Cl-

b

Which of the following best describes the behavior of a gated channel? (a) It stays open continuously when stimulated. (b) It opens more frequently in response to a given stimulus. (c) It opens more widely as the stimulus becomes stronger. (d) It remains closed if unstimulated.

b

If Na+ channels are opened in a cell that was previously at rest, how will the resting membrane potential be affected? (a) The membrane potential is not affected by Na+. (b) It becomes more negative. (c) It becomes more positive. (d) It is permanently reset.

c

Some cells have aquaporins—channels that facilitate the flow of water molecules through the plasma membrane. For these cells, what regulates the rate and direction of water diffusion across the membrane? (a) aquaporin conformation (b) resting membrane potential (c) solute concentrations on either side of the membrane (d) availability of ATP

c

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? (a) Na+ is bound (b) ADP is bound (c) the pump is phosphorylated (d) the pump changes conformation

c

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

c

Which of the following statements about resting membrane potential is not true? (a) The resting membrane potential for most animal cells is 0 mV, because the positive and negative ions are in balance. (b) The resting membrane potential for most animal cells is positive, because Na+ ions are so plentiful inside cells. (c) The resting membrane potential for most animal cells is negative, because the inside of the cell is more negatively charged than the outside of the cell. (d) At the resting membrane potential, no ions enter or exit the cell.

c

Which of the following is required for the secretion of neurotransmitters in response to an action potential? (a) neurotransmitter receptors (b) Na+-K+ pumps (c) voltage-gated K+ channels (d) voltage-gated Ca2+ channels

d

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once. For an uncharged molecule, the direction of passive transport across a membrane is determined solely by its __________________ gradient. On the other hand, for a charged molecule, the __________________ 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 __________________ gradient. Active transport allows the movement of solutes against this gradient. The transporter proteins called __________________ transporters use the movement of one solute down its gradient to provide the energy to drive the uphill transport of a second solute. When this transporter moves both ions in the same direction across the membrane, it is considered a(n) __________________; if the ions move in opposite directions, the transporter is considered a(n) __________________. antiport coupled membrane potential ATP hydrolysis electrochemical symport concentration light-driven uniport

concentration; membrane potential; electrochemical; coupled; symport; antiport

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? (a) toxic buildup of carbon dioxide inside the cell (b) toxic buildup of ethanol inside the cell (c) diffusion of ATP out of the cell (d) inability to import sugar into the cell

d

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

d

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? (a) They set the K+ concentration gradient to zero. (b) They set the membrane potential to zero. (c) They disrupt the resting membrane potential. (d) They keep the electrochemical gradient for K+ at zero.

d

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? (a) voltage-gated (b) extracellular ligand-gated (c) intracellular ligand-gated (d) stress-gated

d

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once. A molecule moves down its concentration gradient by __________________ transport, but requires __________________ transport to move up its concentration gradient. Transporter proteins and ion channels function in membrane transport by providing a __________________ pathway through the membrane for specific polar solutes or inorganic ions. __________________ are highly selective in the solutes they transport, binding the solute at a specific site and changing conformation so as to transport the solute across the membrane. On the other hand, __________________ discriminate between solutes mainly on the basis of size and electrical charge. active hydrophilic noncovalent amino acid hydrophobic passive amphipathic ion channels transporter proteins

passive; active; hydrophilic; transporter proteins; ion channels

The rate at which a molecule crosses by simple diffusion depends on its

size and solubility

Which molecules can move more quickly through lipid bilayer?

small non polar molecules

Inorganic ions and small polar organic molecules can only cross a cell membrane by using

transporters and channels

Transporters transport only molecules or ions that fit into specific binding sited on a protein

true

channels transport any ions or molecules that are small enough and carries the appropriate charge through plasma membrane

true