Facilitated Diffusion

Unit 2 · Topic 2.6 end-of-topic test

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback tells you what a wrong choice assumed. For the two free-response questions, write your answer in full sentences, then open the scoring guide and mark your own work against it.

Question 1

Glucose is at 8 mmol/L (thousandths of a mol/L) in the fluid around a muscle cell and 2 mmol/L inside. Glucose enters the cell through a carrier protein.

How should this crossing be classified?

A. Simple diffusion: glucose is small enough to pass between the tails B. Facilitated diffusion: down its gradient, through a carrier, at no cost C. Active transport: using a carrier protein always costs the cell some energy D. Active transport: glucose is moving against its gradient

Question 2

A nerve cell holds K⁺ at 140 mmol/L inside and 5 mmol/L outside. One of its potassium channels opens. The cell spends no energy while K⁺ moves.

Which way does K⁺ move, and what is the crossing called?

A. In, through the channel; facilitated diffusion B. Out, through the channel; simple diffusion C. Out, through the bilayer; facilitated diffusion D. Out, through the channel; facilitated diffusion

Question 3

Two artificial membranes are tested. Model 1 is a bare bilayer. Model 2 is the same bilayer with glucose carrier proteins in it. Both oxygen and glucose start at higher concentrations outside each model than inside, and no energy source is present. Result: oxygen crosses both membranes toward the inside; glucose crosses only Model 2, also toward the inside.

Which explanation fits both results?

A. Both need carriers; the oxygen result for Model 1 must be an error in the measurement B. Both dissolve through the bilayer; the carriers in Model 2 make no difference C. O₂ passes the bilayer itself; glucose needs the carrier, still downhill and free D. The carriers push glucose against its gradient using energy from the membrane

Question 4

Cells lining the gut take in glucose even when it is 2 mmol/L in the gut and 8 mmol/L inside the cells, and the uptake almost stops when the cells' energy supply is cut off. A membrane protein is involved.

Is this crossing facilitated diffusion?

A. Yes: a membrane protein is involved B. Yes: glucose is a large polar molecule C. No: it runs against the gradient and needs energy D. No: here glucose crosses the bilayer directly

Question 5

A nerve cell has Na⁺ at 145 mmol/L outside and 15 mmol/L inside. During interval 1 every sodium channel is shut; during interval 2 they open. The cell's energy supply is blocked throughout, and no other protein moves Na⁺.

What is the net movement of Na⁺ in each interval?

A. Inward in both: through the bilayer, then through the channels B. Almost none in 1; inward through the channels in 2 C. None in either: charged ions cannot cross a membrane D. Almost none in 1; outward through the channels in 2

Question 6

Two kinds of animal cell are placed in a dilute solution, in which water enters cells. Cells of type 1 have many aquaporins in their membranes; cells of type 2 have very few. In both kinds, the water crosses for free.

Which cells gain water faster, and why?

A. Type 1: more aquaporins give water more routes, so it enters faster B. Type 2: with fewer channels, water is forced through the bilayer faster C. Both equally: water crosses the bilayer itself, so channels make no difference D. Type 1: aquaporins pump water in, and more pumps move more water

Question 7

Two batches of red blood cells start with Cl⁻ at 120 mmol/L outside and 20 mmol/L inside. In batch 1 the chloride channels are open; after five minutes the concentrations are 110 outside and 30 inside. In batch 2 the chloride channels are blocked; after five minutes the concentrations are unchanged.

What do the two results show?

A. Cl⁻ left the cells through the open channels B. Cl⁻ crossed the bilayer itself; ions are small C. Cl⁻ entered through the blocked channels D. Cl⁻ entered through the open channels, down its gradient

Question 8

A nerve cell holds Na⁺ at 15 mmol/L inside against 145 outside, and K⁺ at 140 mmol/L inside against 5 outside. A signal opens only the potassium channels.

Which ions move, and which way?

A. K⁺ leaves; Na⁺ stays where it is B. K⁺ leaves and Na⁺ enters C. Na⁺ enters; K⁺ stays where it is D. Neither: the two gradients cancel

Question 9

Kidney cells are placed in a dilute solution, in which water enters them, for five minutes, and their gain in mass is measured. Cells with working aquaporins and a normal energy supply gain 12%. Cells with their aquaporins blocked gain 3%. Cells with working aquaporins but an energy supply cut to under 5% of normal gain 11%.

What do the three results show about aquaporins?

A. They pump water in, using the cell's energy B. They are the only route water can take C. They speed water's crossing and use no energy from the cell D. Blocking them made water leave the cells

Question 10

A membrane protein lets water molecules stream through it, several hundred million every second, but lets no ions and no glucose through.

What is the protein?

A. A carrier that binds each water molecule and changes shape B. An aquaporin: a channel protein for water C. A protein that pushes water in using the cell's energy D. A glycoprotein on the outer face of the membrane

Question 11

Water can cross a bare bilayer in small amounts. Yet kidney cells, which must move large volumes of water across their membranes each day, are packed with aquaporins.

Why do these cells need aquaporins?

A. Because water cannot cross a bilayer at all B. To move water against its gradient C. To carry dissolved salts along with the water D. To move a lot of water fast; the bilayer alone is too slow

Question 12

Two airway cells hold Cl⁻ at 30 mmol/L inside, with 120 mmol/L in the fluid outside. Cell P has 100 open chloride channels; cell Q has 200. In both cells, the Cl⁻ crosses for free.

Compare the net movement of Cl⁻ in the two cells at the start.

A. Out of both cells; faster in Q B. Into P, out of Q C. Into both cells; faster in Q D. Into both cells; equally fast

Question 13

A red blood cell with glucose carriers sits in a solution. At 0 minutes glucose is 8 mmol/L outside and 4 mmol/L inside. At 10 minutes both are 6 mmol/L. The solution is then swapped, and at 20 minutes glucose is 3 mmol/L outside and 5 mmol/L inside.

What is the net movement of glucose at 20 minutes?

A. Into the cell, through the carriers B. Out of the cell, through the carriers, at no cost C. Out of the cell, but only if the cell spends energy D. None: net movement ended at 10 minutes and cannot restart

Question 14

Red blood cells with open chloride channels sit in a solution with Cl⁻ at 120 mmol/L; inside the cells it is 30 mmol/L, and Cl⁻ is entering. The solution is then replaced by one with Cl⁻ at 10 mmol/L. The channels stay open and the cells spend no energy on Cl⁻.

What happens to the movement of Cl⁻ after the change?

A. Cl⁻ now leaves the cells through the same channels, down the new gradient B. Cl⁻ keeps entering: a channel lets an ion through in one direction only C. Cl⁻ stops moving: the channels close when the outside falls below the inside D. Cl⁻ leaves only if the cells spend energy to push it out

Question 15

Two groups of muscle cells start with glucose at 2 mmol/L inside; the solution around them is held at 10 mmol/L. Group L has 1,000 glucose carriers per cell; group H has 10,000. Once inside, the glucose stays as it is.

Compare the two groups.

A. H fills faster and ends above 10 mmol/L B. Both fill equally fast; both end near 10 mmol/L C. H fills faster; L never changes D. H fills faster; both end near 10 mmol/L inside

Question 16

A nerve cell membrane with a potassium channel, blocked by a toxin (X), and a glucose carrier.

The figure shows a nerve cell's membrane with a potassium channel and a glucose carrier. K⁺ is at 140 mmol/L inside and 5 mmol/L outside; glucose is at 8 mmol/L outside and 2 mmol/L inside. A toxin blocks the potassium channel, marked with an X.

What happens to the two substances while the channel is blocked?

A. K⁺ stops crossing; glucose keeps entering through its carrier B. Both stop: one blocked door shuts the membrane C. K⁺ crosses the bilayer instead; glucose is unchanged D. K⁺ leaves through the glucose carrier instead

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Conceptual Analysis · 4 points

A nerve cell holds K⁺ at 140 mmol/L inside and 5 mmol/L outside, and Na⁺ at 15 mmol/L inside and 145 mmol/L outside. Its membrane has potassium channels and sodium channels, most of them shut while the cell is at rest. When a potassium channel opens, K⁺ leaves the cell, and the cell spends no energy on this. A researcher then applies a drug that doubles the number of potassium channels that open.

(a) Describe how K⁺ crosses the membrane when a potassium channel opens, and name the kind of transport. (1 point)

A full-credit answer: K⁺ passes through the open channel protein, a water-lined tunnel, moving down its concentration gradient from 140 toward 5 mmol/L with no energy spent by the cell. This is facilitated diffusion, a form of passive transport.

Check the box for each point your answer earns

K⁺ passes through the open channel protein, a water-lined tunnel, moving down its concentration gradient from 140 toward 5 mmol/L with no energy spent by the cell; this is facilitated diffusion (a form of passive transport).

Accept: "through the channel, from high to low, for free: facilitated diffusion". Both the route and the name are needed for the point.

Common slip: Giving the route without the name, or the name without the route. Both are needed.

(b) Explain what keeps Na⁺ out of the cell while its channels are shut, even though its concentration is much higher outside. (1 point)

A full-credit answer: Na⁺ is a charged ion, and the hydrocarbon tails in the middle of the membrane carry no charges or partial charges, so Na⁺, which water holds on to, has nothing to hold it in the hydrophobic interior and cannot cross on its own. It can cross only through a channel, so with its channels shut it has no route.

Check the box for each point your answer earns

Na⁺ is a charged ion, and the hydrocarbon tails in the middle of the membrane carry no charges or partial charges, so Na⁺, which water holds on to, has nothing to hold it in the hydrophobic interior and cannot cross on its own; it can cross only through a channel, so with its channels shut it has no route.

Accept: "ions cross only through channels, and the sodium channels are closed", provided the reason (the charge on the ion and the uncharged interior) is given.

Common slip: Saying the sodium channels are closed, with no reason why Na⁺ needs a channel at all. The charge on the ion and the uncharged interior are the reason.

(c) Predict how the drug changes the movement of K⁺ when the potassium channels open. (1 point)

A full-credit answer: K⁺ still leaves the cell, in the same direction as before, but faster, because twice as many channels are open.

Check the box for each point your answer earns

K⁺ still leaves the cell (the direction is unchanged) but leaves faster, because twice as many channels are open.

Accept: "more K⁺ leaves per second" or "K⁺ leaves about twice as fast". Do not award the point for a change of direction or for "the cell now needs energy".

Common slip: Changing the direction, or saying the cell now needs energy. More doors speed a crossing; the gradient sets its direction.

(d) The cell is then bathed in a solution holding K⁺ at 140 mmol/L, equal to the inside, with the drug still present and the channels open. Considering only the concentrations (set aside any charge across the membrane), predict the net movement of K⁺ and justify your prediction. (1 point)

A full-credit answer: No net movement of K⁺. With equal concentrations on both sides, K⁺ ions cross the channels in both directions in equal numbers, a dynamic equilibrium. Channels provide only the route, and with no gradient there is no net direction, however many channels are open.

Check the box for each point your answer earns

no net movement of K⁺, because with equal concentrations on both sides K⁺ ions cross the channels in both directions in equal numbers (a dynamic equilibrium); channels provide only the route, and with no gradient there is no net direction, however many channels are open.

Accept: "K⁺ still crosses both ways but the two flows cancel". Do not award the point for "K⁺ stops moving" or for "K⁺ still leaves because the drug opened more channels".

Accept also: an answer that gives 'no net movement by concentration' and adds that a negative inside would pull a little K⁺ inward; the concentration argument earns the point and the charge remark is not penalized.

Common slip: Saying K⁺ stops moving, or that it still leaves because the drug opened more channels. Ions still cross both ways; the two flows cancel.

Free-response score: 0 of 4

Free response 2 · Analyze Model or Visual Representation · 4 points

The model shows a cross-section of a plasma membrane with the extracellular fluid above and the cytosol below, and three routes across it. Route 1 is the bilayer itself. Route 2 is a channel protein: a water-lined tunnel through the membrane. Route 3 is a carrier protein, which binds one kind of molecule and changes shape. Concentrations are marked: O₂ at 40 units outside and 10 inside; K⁺ at 5 mmol/L outside and 140 mmol/L inside; glucose at 8 mmol/L outside and 2 mmol/L inside. The cell spends no energy on any of these three crossings.

Three routes across a plasma membrane, with the concentration of each substance marked outside (above) and inside (below).

(a) For each of O₂, K⁺ and glucose, state which route it takes across this membrane. (1 point)

A full-credit answer: O₂ takes route 1, straight through the bilayer. K⁺ takes route 2, the channel. Glucose takes route 3, the carrier.

Check the box for each point your answer earns

O₂ takes route 1, straight through the bilayer; K⁺ takes route 2, a channel; glucose takes route 3, a carrier.

All three are needed for the point.

Common slip: Giving glucose the channel or K⁺ the carrier. A channel is a tunnel for one kind of ion; a carrier binds a molecule and changes shape.

(b) Explain what prevents K⁺ and glucose from using route 1, and why their crossings through routes 2 and 3 still cost the cell nothing. (1 point)

A full-credit answer: The hydrocarbon tails in the middle of the membrane carry no charges or partial charges, so an ion (K⁺) or a polar molecule (glucose), which water holds on to, has nothing to hold it in the hydrophobic interior and cannot cross on its own. Through a channel or carrier each moves down its own concentration gradient, and movement down a gradient happens by itself, because more particles leave the more concentrated side than arrive from the less concentrated one, so no energy is needed: facilitated diffusion.

Check the box for each point your answer earns

the hydrocarbon tails in the middle of the membrane carry no charges or partial charges, so an ion (K⁺) or a polar molecule (glucose), which water holds on to, has nothing to hold it in the hydrophobic interior and cannot cross on its own; through a channel or carrier each moves down its own concentration gradient, and movement down a gradient happens by itself, because more particles leave the more concentrated side than arrive from the less concentrated one, so no energy is needed (facilitated diffusion).

Accept: "charged or polar things cannot enter the oily middle; the proteins give a route, and downhill movement is free". Both halves are needed for the point.

Common slip: Explaining why they need a protein but not why the crossing is free, or the other way around. Both halves are needed.

(c) On a copy of the model, or on paper, draw an arrow through route 2 showing the net direction of K⁺ and an arrow through route 3 showing the net direction of glucose, and label each arrow with the name of the ion or molecule. (1 point)

A full-credit answer: The K⁺ arrow through route 2 points from the cytosol to the extracellular fluid, out of the cell, from 140 toward 5 mmol/L. The glucose arrow through route 3 points from the extracellular fluid into the cytosol, from 8 toward 2 mmol/L. Each arrow is labeled with its ion or molecule.

Check the box for each point your answer earns

the K⁺ arrow through route 2 points from the cytosol to the extracellular fluid (out), and the glucose arrow through route 3 points from the extracellular fluid to the cytosol (in), each labeled.

Both arrows must be correct for the point; an arrow for K⁺ pointing inward earns nothing.

Common slip: Pointing the K⁺ arrow inward because most things seem to come in. Read the concentrations: K⁺ is higher inside, so its net movement is out.

(d) Kidney cells have many aquaporins in their membranes. Describe what an aquaporin is in terms of this model, and what it does for the cell. (1 point)

A full-credit answer: An aquaporin is a route-2 protein: a channel for water. Water crosses the bilayer (route 1) only slowly on its own; aquaporins let large quantities of water cross the membrane quickly, down water’s own gradient and at no cost to the cell.

Check the box for each point your answer earns

an aquaporin is a route-2 protein, a channel, for water; water crosses the bilayer (route 1) only slowly on its own, and aquaporins let large quantities of water cross the membrane quickly, down water's own gradient and at no cost to the cell.

Accept: "a water channel; it speeds water's crossing greatly". Do not award the point for "it pumps water" or for calling it a carrier.

Common slip: Saying an aquaporin pumps water, or calling it a carrier. It is an open channel; water moves through it only the way it would move anyway.

Free-response score: 0 of 4

Multiple choice checked: 0 of 16 correct.

Next: Pumping uphill