Chapter 12 - Transport Across Cell Membranes

Question 1

Why would a cell express the aquaporin protein if water can cross the membrane in the absence of aquaporin?

• Aquaporin facilitates the faster movement of water molecules across the membrane.
• Aquaporin moves a positively charged ion along with water across the membrane.
• Water molecules cannot cross the membrane in the absence of a pore like aquaporin.
• Aquaporin limits the movement of water molecules so they do not move too quickly across the membrane.

Answer 1

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

(The movement of water molecules across a membrane is slow in the absence of aquaporin. Aquaporin speeds up the flow of water across the membrane.)

Question 2

Which of the following characteristics of aquaporins ensure that the channel selectively transports only water molecules and not other solutes?
Choose one or more:

• The channel has a narrow pore that is only wide enough for a single water molecule to pass through.
• Two asparagines in the center of the pore prevent protons from passing through the channel.
• The channel undergoes conformational changes to push water through the channel.
• A glutamate at the entrance to the channel prevents positive ions from entering the channel.

Answer 2

• The channel has a narrow pore that is only wide enough for a single water molecule to pass through.
• Two asparagines in the center of the pore prevent protons from passing through the channel.

(Channels like aquaporin are specific for the substrate that can pass through the channel. Aquaporin is a narrow channel with two asparagines that block other ions from entering and only allow water to pass through.)

Question 3

Lipid bilayers are highly impermeable to which molecule(s)?

• steroid hormones
• carbon dioxide
• water
• oxygen
• Na+ and Cl–

Answer 3

Na+ and Cl-

(Lipid bilayers are highly impermeable to charged ions, and Na+ and Cl– are common examples of ions that are excluded from the hydrophobic interior of a lipid bilayer.)

Question 4

What is the voltage difference across a membrane of a cell called?

• gradient establishment
• potential balance
• electrical current
• membrane potential

Answer 4

Membrane potential

(The voltage difference across a membrane is called the 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.)

Question 5

The movement of an ion against its concentration gradient is called what?

• facilitated diffusion
• passive transport
• osmosis
• active transport

Answer 5

Active transport

Active transport requires an input of energy and it is carried out by a special class of transporters called pumps.

Question 6

Which of the following mechanisms prevents osmotic swelling in plant cells?

• the activity of Na+ pumps
• the expulsion of water from contractile vacuoles
• turgor pressure
• tough cell walls
• the collection of water in contractile vacuoles

Answer 6

Tough cell walls

(Plant cells are prevented from swelling by their rigid cell walls, so they can tolerate a large osmotic difference across their plasma membrane.)

Question 7

What determines the direction that glucose is transported across the membrane, through a glucose transporter?

• membrane potential
• a molecule’s size
• concentration gradient
• a molecule’s charge

Answer 7

Concentration gradient

Because glucose is uncharged, the direction it is transported is determined by its concentration gradient alone.

Question 8

Glucose enters the cell by ________.

Answer 8

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.)

Question 9

Nucleotides enter the cell through a _______.

Answer 9

membrane transport protein.

The name for this protein-mediated transport is facilitated diffusion.

Question 10

When sodium ions are pumped from the cell and potassium ions pumped into the cell, this is a mechanism of _______.

Answer 10

co-transport

Question 11

Nucleotides enter the cell by ________.

Answer 11

facilitated diffusion

Question 12

Which organelle is important for controlling the concentration of calcium ions in the cytosol?

• Golgi apparatus
• lysosome
• nucleus
• endoplasmic reticulum

Answer 12

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.)

Question 13

Gradient-driven pumps can act as ________ or ________.

Answer 13

symports or antiports

(They transfer solutes either in the same direction (symports, left panel) or in opposite directions (antiports, center panel).)

Question 14

_______ only facilitate the movement of a solute down its concentration gradient.

Answer 14

Uniports

(Because such movement does not require an additional energy source, uniports are not pumps and do not exhibit co-transport.)

Question 15

Which of the following characteristics of K+ channels are important for the selectivity for K+ rather than other ions?
Choose one or more:

• Four rigid protein loops line the narrowest part of the pore.
• Acidic side chains line the wall of the pore.
• Carbonyl groups line the wall of the pore.
• Basic side chains line the wall of the pore.

Answer 15

• Four rigid protein loops line the narrowest part of the pore.
• Carbonyl groups line the wall of the pore.

(The selectivity filter of the K+ channel does contain four rigid protein loops that have carbonyl groups at just the right spacing to interact with the K+ ions.)

Question 16

Ions in solution are found in a hydration shell of water. This shell must be removed for an ion to pass through the channel. How does the K+ channel accomplish removal of the water from the shell around the ion?

• Carbonyl groups lining the wall of the pore can interact with the unsolvated K+ ion, balancing the energy needed to remove the hydration shell.
• The K+ channel has four subunits; one subunit removes the hydration shell as the ion passes through the pore formed by the three other subunits.
• Rigid protein loops strip the hydration shell from the potassium so that the ion is the right diameter to pass through the pore.
• The K+ channel uses the energy in ATP hydrolysis to remove the hydration shell from the K+ ion

Answer 16

Carbonyl groups lining the wall of the pore can interact with the unsolvated K+ ion, balancing the energy needed to remove the hydration shell.

(The carbonyl groups found on the four rigid protein loops of the four subunits are spaced precisely to bind the unsolvated K+ ion. Their binding balances the energy needed to remove the hydration shell.)

Question 17

What is typically true of ion channels?

• They hydrolyze ATP.
• They operate by active transport.
• They are open all the time.
• They are gated.
• They are nonselective.

Answer 17

They are gated.

This means that for most of these ion channels, a specific stimulus triggers them to open.

Question 18

For voltage-gated channels, a change in the membrane potential has what effect on the channel?

• It changes the width of the channel opening.
• It makes the channel more sensitive to the binding of neurotransmitters.
• It alters the probability that the channel will be found in its open conformation.
• It either opens the channel or closes it, depending on the voltage.
• It changes which ions can pass through the channel.

Answer 18

It alters the probability that the channel will be found in its open conformation.

(For voltage-gated channels, a change in the membrane potential alters the probability that the channel will be found in its open conformation.)

Question 19

In most animal cells, which ion can move through “leak” channels?

• K+
• Na+
• Ca2+
• H+
• Cl–

Answer 19

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.)

Question 20

To pass through the pore of an ion channel, what must be true of an ion?

• It must be positively charged.
• It must surround itself with water molecules.
• It must avoid contact with the channel wall.
• It must interact with polar groups in the narrowest part of the channel.
• It must interact with nonpolar groups in the selectivity filter.

Answer 20

It must interact with polar groups in the narrowest part of the channel.

(To pass through the pore of an ion channel, an ion must interact with polar groups in the narrowest part of the channel. These polar groups can include the regularly spaced oxygen atoms that are part of the polypeptide backbone.)

Question 21

During the activation of a neuron, the action potential propagates in only one direction. How is this achieved in the neuron?

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

Answer 21

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

(The Na+ channel opens in response to the action potential and then becomes inactivated and refractory to reopening for a few milliseconds before returning to the closed state, ready for the next action potential.)

Question 22

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?

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

Answer 22

The membrane depolarization is not amplified along the axon.

(Na+ channel opening is required to further depolarize the membrane and amplify the signal along the axon. If the Na+ channels do not open, the signal will not be propagated to the muscle at the end of the axon.)

Question 23

When a neuron has been stimulated by a signal, the change in membrane potential first spreads locally to adjoining regions of the plasma membrane by what means?

• action potential
• active transport
• passive spread
• opening of ligand-gated ion channels
• opening of voltage-gated ion channels

Answer 23

Passive spread

(When a neuron has been stimulated by a signal, the change in membrane potential first spreads locally to adjoining regions of the plasma membrane by passive spread. Such a passively spread signal becomes weaker with increasing distance from the initial point of stimulation.)

Question 24

What is the name of the specialized junction between a neuron and a target cell?

• nerve terminal
• axon
• synapse
• synaptic vesicle
• dendrite

Answer 24

Synapse

The plasma membranes at the synapse are separated by a short gap, called a synaptic cleft.

Question 25

Inhibitory neurotransmitters such as glycine and GABA make a postsynaptic cell harder to depolarize by allowing what?

• an influx of Cl–
• the escape of Na+
• an influx of Na+
• an influx of K+

Answer 25

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.)

Question 26

Which is not true about the acetylcholine receptor on vertebrate muscle cells?

• It is a voltage-gated cation channel.
• It depolarizes the muscle cell membrane when bound to acetylcholine.
• It does not discriminate between Na+, K+, and Ca2+.
• Its pore includes negatively charged amino acid side chains at both ends.
• Even with acetylcholine bound, it flickers randomly between open and closed states.

Answer 26

It is a voltage-gated cation channel.

(You have identified that acetylcholine receptor on vertebrate muscle cells is not a voltage-gated cation channel. Rather, it is a transmitter-gated cation channel.)

Question 27

Sodium ions, oxygen (O2), and glucose pass directly through lipid bilayers at dramatically different rates. Which of the following choices presents the correct order, from fastest to slowest?

• oxygen, glucose, sodium ions
• sodium ions, oxygen, glucose
• glucose, sodium ions, oxygen
• glucose, oxygen, sodium ions
• oxygen, sodium ions, glucose

Answer 27

Oxygen, glucose, sodium ions

Sodium ions, oxygen (O2), and glucose pass directly through lipid bilayers at dramatically different rates.

Question 28

How do transporters and channels select which solutes they help move across the membrane?

• Both channels and transporters discriminate between solutes mainly on the basis of size and electric charge.
• 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.
• 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 allow the passage of solutes that are electrically charged; transporters facilitate the passage of molecules that are uncharged.
• 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.

Answer 28

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.

(Each type of cell membrane has its own characteristic set of transport proteins, which determines exactly which solutes can pass into and out of that cell or organelle.)

Question 29

In one experiment, investigators create a liposome—a vesicle made of phospholipids—that contains a solution of 1 mM glucose and 1 mM sodium chloride. If this vesicle were placed in a beaker of distilled water, what would happen the fastest?

• Na+ would diffuse out.
• Cl– would diffuse out.
• H2O would diffuse in.
• NaCl would diffuse out.
• Glucose would diffuse out.

Answer 29

H2O would diffuse in.

(Water molecules can diffuse (slowly) across a lipid bilayer. In this instance, water would move down its concentration gradient, toward the area of high solute concentration—a process called osmosis.)

Question 30

A group of researchers wanted to sort different white blood cell types (monocytes, lymphocytes, and granulocytes) apart from each other based on size differences and to remove unwanted contaminating red blood cells. After a particular manipulation, the red blood cells lysed. The remaining white blood cells increased in size and, more importantly, the size differences among cells increased, allowing for size-based sorting (which requires minimum size differences among cells). What manipulation did the researchers use to increase cell size?

• placing cells in an environment with a lower solute concentration than that in the cells
• placing cells in an environment with a higher solute concentration than that in the cells
• placing cells in an environment with lower temperatures than the cells were previously exposed to
• patch-clamp recording to monitor ion channel activity

Answer 30

Placing cells in an environment with a lower solute concentration than that in the cells.

(Since the solute concentration was higher inside the cell, water entered the cells via osmosis, leading to cell swelling.)

Question 31

Your friend is attempting to study the function of the Na+-K+ pump and has created spherical liposomes that contain only the Na+-K+ pump. She has inserted the pumps so that the extracellular side of the pump is also outside the liposome. She has added different ions and energy sources to the beaker with the liposomes, but no pumping of ions occurs. You explain that components must be in the proper location inside or outside the liposome for the pump to work. Help her by adding the proper components to the inside or outside of the liposome so that proper pumping occurs. Place unneeded components in the unneeded box. The figure shows a spherical liposome formed from phospholipids.

What goes inside the liposome?
What goes outside the liposome?
What is unneeded?

• ATP
• GTP
• Cl-
• K+
• Na+

Answer 31

Inside the liposome

• ATP
• Na+

Outside the liposome
-K+

Unneeded

• GTP
• Cl-

(Na+ and ATP are inside the liposome and K+ outside the liposome for the pump to function.)

Question 32

Your friend now has the pumps successfully pumping ions. She added an equal concentration of both ions to the correct sides of the liposomes along with an excess of the energy source. She is surprised when the pumps stop working after a short time. Which of the following could explain why the transporter stopped pumping ions?

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

Answer 32

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

(The pump transports three Na+ ions out of the cell for every two K+ pumped in. 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.)

Question 33

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?

• The transporter goes through an intermediate state in which the lactose-binding site is open to both sides of the membrane.
• The transporter oscillates randomly between states in which it is open to either the extracellular space or the cytosol.
• If the H+ gradient were reversed, the transporter could serve as an H+–lactose antiport.
• Lactose and H+ ions bind to two different conformations of the transporter.
• To undergo the conformational change that releases lactose into the cell, the transporter hydrolyzes ATP.

Answer 33

The transporter 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.)

Question 34

The P-type ATPase Ca2+ pump transports Ca2+ ions into the sarcoplasmic reticulum in muscle cells in a series of steps. What is the correct order of steps?

Answer 34

1. Two Ca2+ ions from the cytosol side bind to a pocket in the Ca2+ pump.
2. ATP is hydrolyzed and a conserved aspartate group is phosphorylated.
3. ADP is exchanged for a new ATP.
4. The pump opens to the lumen side of the sarcoplasmic reticulum, releasing Ca2+.
5. Two protons bind to the pump and are exported from the sarcoplasmic reticulum.

(Ca2+ is pumped across the membrane by first binding the pump. ATP is then hydrolyzed and ADP is exchanged for new ATP, enabling the pump to release Ca2+ inside the ER. Finally, two protons bind the pump and are transported out.)

Question 35

You join a laboratory to study muscle function. You decide to inhibit the pumping of Ca2+ into the sarcoplasmic reticulum to determine how excess cytosolic Ca2+ will affect muscle function. Which of the following strategies would be effective in blocking Ca2+ pumping?

• block the hydrolysis of ATP to AMP by the Ca2+ pump
• block binding of ATP to the pump in the lumen of the sarcoplasmic reticulum
• block the phosphorylation of the conserved aspartate in the Ca2+ pump
• enhance the binding of ATP to the Ca2+ pump

Answer 35

Block the phosphorylation of the conserved aspartate in the Ca2+ pump

(Blocking phosphorylation of the pump would block Ca2+ transport by inhibiting the conformational changes required for pump function.)

Question 36

Most sports drinks contain both carbohydrates and salts. The carbohydrates replace glucose burned during exercise and the salts replace salts lost in sweat. The salt also helps the small intestine absorb glucose. Pick the answer that accurately describes which salt is most beneficial for glucose absorption.

• NaCl, because Na+ is needed for glucose entry.
• KCl, because K+ is needed for glucose entry.
• KCl, because Cl– is needed for glucose entry.
• HCl, because H+ is needed for glucose entry.

Answer 36

NaCl, because Na+ is needed for glucose entry.

The apical domain of intestinal epithelial cells contains a glucose–Na+ symporter.

Question 37

Which of the following describes the resting membrane potential of a neuron?

• a voltage difference across the plasma membrane, with more positive membrane potential inside
• a voltage difference of 0 millivolts (mV) across the membrane
• a state in which the flow of positive and negative ions across the plasma membrane is precisely balanced
• a voltage difference that is chiefly a reflection of the electrochemical Na+ gradient across the plasma membrane
• a voltage difference across the plasma membrane when the neuron has been stimulated

Answer 37

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

(The best description of the neuron resting membrane potential is a state in which the flow of positive and negative ions across the plasma membrane is precisely balanced. In this steady-state condition, no further differences in charge will accumulate.)

Question 38

When a neuron is activated by a stimulus, its plasma membrane will change until it reaches a membrane potential of about +40 mV. What is special about this value?

• It is the threshold potential at which voltage-gated Na+ channels close.
• It is the opposite of the resting membrane potential.
• It is approximately the membrane potential at which the electrochemical gradient for K+ is zero.
• It is approximately the membrane potential at which the electrochemical gradient for Na+ is zero.
• It is the threshold potential that opens voltage-gated Na+ channels.

Answer 38

It is approximately the membrane potential at which the electrochemical gradient for Na+ is zero.

(When a neuron is activated by a stimulus, its plasma membrane will change until it reaches a membrane potential of about +40 mV. This value is special because it is approximately the membrane potential at which the electrochemical gradient for Na+ is zero. Around +40 mV, Na+ ions have no further tendency to enter or leave the cell. In other words, they are near their theoretical equilibrium potential, as determined by the Nernst equation.)

Question 39

In the patch-clamp technique (shown here), a small glass electrode forms a tight seal with a portion of the cell membrane (A), allowing detection of ion flow through channels (B).

This technique has clinical uses. For example, some individuals with high blood pressure (hypertension) have a genetic defect in an epithelial Na+ channel (ENaC) that makes the channel hyperactive (open more than normal). Such individuals can be treated with amiloride, which inhibits sodium ion movement through ENaC. Researchers predicted that amiloride would not effectively treat hypertension in individuals with normal ENaC activity and tested this prediction by analyzing ENaC activity in patch-clamped cells from individuals with high blood pressure, some of whom showed hyperactive ENaC and some of whom showed normal ENaC activity. Which of the following results align with researcher expectations?
Choose one or more:

• Patch-clamp data showed normal ENaC activity prior to amiloride treatment; patient did not benefit from amiloride.
• Patch-clamp data showed hyperactive ENaC prior to amiloride treatment; patient did not benefit from amiloride.
• Patch-clamp data showed normal ENaC activity prior to amiloride treatment; patient benefited from amiloride.
• Patch-clamp data showed hyperactive ENaC prior to amiloride treatment; patient benefited from amiloride.

Answer 39

• Patch-clamp data showed normal ENaC activity prior to amiloride treatment; patient did not benefit from amiloride.
• Patch-clamp data showed hyperactive ENaC prior to amiloride treatment; patient benefited from amiloride.

(In patch-clamp analysis, only some hypertensive patients had high ENaC function compared to normotensive controls and only the hypertensive patients with high ENaC function had lowered blood pressure after amiloride treatment.)

Question 40

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?

• The animals would avoid eating, but only during the day.
• The animals would avoid eating, even when they are hungry.
• The animals would avoid eating, even when they are hungry—but only when the channels are activated by light.
• In response to light activation, the animals would overeat, even when they are full.
• The channels would have no effect on behavior because the animal’s normal Na+ channels would allow normal depolarization of neurons that regulate feeding.

Answer 40

The animals would avoid eating, even when they are hungry—but only when the channels are activated by light.

(The animals would avoid eating, even when they are hungry—but only when the channels are activated by light. 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.)

Question 41

A toxin present in scorpion venom prolongs the duration of action potentials in nerve cells. Which of these actions would best explain how this toxin exerts its effect?

• It prolongs the inactivation of voltage-gated Na+ channels.
• It slows the inactivation of voltage-gated Na+ channels.
• It inhibits the opening of voltage-gated Na+ channels.
• It accelerates the opening of voltage-gated K+ channels.
• It slows the inactivation of voltage-gated K+ channels.

Answer 41

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

(This toxin exerts its effect by slowing the inactivation of voltage-gated Na+ channels, causing them to get stuck in the open conformation, and prolonging the action potential.)

Question 42

The drug scopolamine is used to treat dizziness, motion sickness, and smooth muscle spasms. When isolated muscle cells are incubated with scopolamine, addition of acetylcholine no longer depolarizes the muscle cell membrane or stimulates muscle cell contraction. Which would best explain how scopolamine exerts its muscle-relaxing effects?

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

Answer 42

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. Acetylcholine triggers muscle contraction by opening a ligand-gated Na+ channel, which leads to membrane depolarization and contraction of the muscle cell.)

Question 43

Optogenetics is a powerful tool that uses light to control the activity of specific neurons. These neurons contain artificially introduced light-gated ion channels. A number of different light-gated channels with different ion specificities have been either found in nature (such as the sodium-specific channelrhodopsin, originally found in green algae) or produced via genetic engineering (the production of a chloride ion-specific form of channelrhodopsin). There are also light-gated ion channels specific for potassium or calcium. Sort each light-activated channel type based on whether activation of this channel will tend to depolarize cells or not.

• Sodium channel
• Calcium channel
• Chloride channel
• Potassium channel

Answer 43

Promote

• Sodium channel
• Calcium channel

Inhibit

• Chloride channel
• Potassium channel

(Movement of sodium or calcium cations into cells will depolarize cells, while movement of potassium cations out of cells inhibits depolarization. Movement of a chloride anion into cells down its concentration gradient also inhibits depolarization.)