Mechanisms of Transport

Unit 2 · Topic 2.8 end-of-topic test

Suggested time: about 44 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.

Answer every question. For each multiple-choice question, pick one option. When you have answered every question, press Submit the test; the feedback then gives the reasoning for each. For the free-response questions, write one short sentence for each step of your reasoning, each on its own line, and make every link clear (so, because, therefore). That is what the exam’s ‘paragraph form’ means for you: linked sentences, not bullet points. Then open the scoring guide and mark your own work against it.

Question 1

Over a short time, a single sodium–potassium pump in a liver cell moves 150 Na⁺ out of the cell.

Over the same cycles, how many K⁺ did the pump move into the cell, and how many ATP did it use?

A. 150 K⁺ in, 150 ATP B. 225 K⁺ in, 75 ATP C. 100 K⁺ in, 50 ATP D. 100 K⁺ in, 150 ATP

Question 2

In a nerve cell the cytosol holds 15 mmol/L Na⁺ and 140 mmol/L K⁺; the extracellular fluid holds 145 mmol/L Na⁺ and 5 mmol/L K⁺. The sodium–potassium pump moves Na⁺ out and K⁺ in.

Why does the pump need ATP to do this?

A. Both ions are moved from lower to higher concentration B. Both ions are moved from higher to lower concentration C. Na⁺ goes against its gradient but K⁺ goes down its gradient D. Any crossing through a membrane protein uses ATP

Question 3

A membrane protein in a muscle cell moves Ca²⁺ out of the cell, from 0.0001 mmol/L inside to 1.2 mmol/L outside. When the cell’s ATP supply is blocked, the movement of Ca²⁺ stops.

Which name fits this protein, and why?

A. A calcium channel: Ca²⁺ runs through it down its gradient, so the movement needs a route only B. A calcium carrier for facilitated diffusion: the protein changes shape but the cell uses no energy C. A calcium synthase: the protein makes ATP as Ca²⁺ passes through it D. A Ca²⁺ ATPase: the protein uses ATP to power the moving of Ca²⁺ against its gradient

Question 4

Inside a liver cell the concentration of Na⁺ is low and of K⁺ is high. Outside the cell the concentration of Na⁺ is high and of K⁺ is low.

Which of the following is the work of the sodium–potassium pump?

A. Na⁺ entering and K⁺ leaving the cell, both down their gradients, through open channels B. Na⁺ leaving and K⁺ entering the cell, both against their gradients, while ATP is present C. K⁺ leaving the cell down its gradient, through a channel, whether or not ATP is present D. Glucose entering the cell through a carrier, down its gradient, with no ATP used

Question 5

Four membranes are described.

Which of the following membranes is polarized?

A. A membrane whose inner face carries slightly less positive charge than its outer face B. A membrane with Na⁺ at 145 mmol/L on one side and 15 mmol/L on the other, and equal charge on its two faces C. A membrane that ions cross in both directions at equal rates, with equal charge on its two faces D. A membrane whose pump has stopped, leaving both faces with the same charge

Question 6

A thin artificial membrane separates two salt solutions, A and B, that start out identical, each with equal positive and negative charge. A protein in the membrane then moves positive ions from side A to side B; the negative ions stay where they are.

What is the result?

A. A is now positive relative to B: the membrane is polarized B. Both sides stay neutral: the negative ions follow C. B is now positive relative to A: the membrane is polarized D. No charge difference: only concentrations changed

Question 7

In a resting nerve cell, one sodium–potassium pump completes 1,000 cycles.

Compare the positive charge that left the cell through the pump over the 1,000 cycles with the positive charge that entered.

A. 2,000 more positive charges left than entered B. 1,000 more positive charges entered than left C. No difference: each cycle carries positive ions both ways, so the charges cancel D. 1,000 more positive charges left than entered

Question 8

A newly formed animal cell starts with Na⁺ and K⁺ at the same concentrations inside as outside, and no charge difference across its membrane. Its sodium–potassium pumps then start pumping, with plenty of ATP. Its channels stay shut.

Predict how the cell changes over the next hour.

A. Na⁺ falls and K⁺ rises inside, and the inside becomes slightly negative B. Na⁺ and K⁺ both rise inside, and the inside becomes positive C. The two gradients build, but the charge stays equal, since both ions are positive D. Nothing changes until a channel opens; pumps need an existing gradient to work on

Question 9

A resting animal cell membrane: which ion is high on each side, and the charge on each face. One channel is drawn. The extracellular fluid is shaded darker than the cytosol.

The figure shows a resting animal cell: the concentration of sodium ions (Na⁺) is high outside and low inside, the concentration of potassium ions (K⁺) is high inside and low outside, and the inside is slightly negative. The channel drawn lets only Na⁺ through, and it opens.

Which way do the two pulls on Na⁺ act, and what happens?

A. Concentration pulls Na⁺ in, charge pulls it out; little moves B. Concentration pulls Na⁺ out, charge pulls it in; little moves C. The gradient and the charge both pull Na⁺ in; Na⁺ enters the cell D. The gradient and the charge both pull Na⁺ out; Na⁺ leaves the cell

Question 10

A resting animal cell membrane: which ion is high on each side, and the charge on each face. One channel is drawn. The extracellular fluid is shaded darker than the cytosol.

The resting animal cell in the figure: the concentration of K⁺ is high inside and low outside, and the inside is slightly negative. Now suppose the channel drawn lets only K⁺ through, and it opens.

How do the two pulls on K⁺ compare?

A. Both favor leaving the cell B. Concentration favors leaving; the negative inside favors staying C. Both favor staying in the cell D. Concentration favors staying; the negative inside favors leaving

Question 11

A student says the direction an ion moves through an open channel depends only on its concentration on the two sides of the membrane.

What is missing from the student's account?

A. Ions move through the bilayer itself, not through channels B. Nothing: for an ion only concentration matters C. The ion must also be pumped using ATP D. The charge across the membrane also pulls on an ion

Question 12

In an experiment a scientist makes the inside of a muscle cell slightly positive relative to the outside, while Ca²⁺ stays far more concentrated outside the cell than inside.

How do the two pulls on Ca²⁺ now compare?

A. Concentration favors entering; the positive inside favors staying out B. Both favor entering, as in a resting cell C. Both favor staying out D. Concentration favors staying out; the positive inside favors entering

Question 13

A student reads that a resting nerve cell's membrane is polarized, with the inside slightly negative, and concludes: 'So the cytosol must be full of negative ions and the fluid outside full of positive ions.'

Which statement corrects the student?

A. The student is right: polarizing a membrane means sorting positive ions to one side and negative ions to the other B. The inside is negative because K⁺, the ion piled up inside, carries a negative charge C. Both fluids still hold both kinds of ion; the inside face just carries slightly more negative charge D. The extra charge sits inside the membrane's oily middle, not in either fluid

Question 14

A plant toxin binds the sodium–potassium pumps of a kidney cell and stops them. The cell keeps making ATP, and its membrane is undamaged.

What happens to the cell’s Na⁺ and K⁺ gradients and to its membrane potential over the next hours, and why?

A. The gradients and the potential both hold: the cell’s ATP keeps the ions where they are without the pump B. The gradients and the potential both fade: ATP does nothing without a working pump, so the ions drift downhill C. The gradients and the potential both hold: once the pump has set them up, the gradients keep themselves D. The gradients fade but the membrane potential holds: the charge difference was set once and stays

Question 15

Nerve cells kept at 4 °C for several hours lose much of their Na⁺ and K⁺ gradients, and their insides become less negative, because the cold slows their sodium–potassium pumps almost to a stop. The cells are then warmed back to body temperature, with plenty of ATP.

What happens over the next hours?

A. The pumps restart, but a gradient cannot be rebuilt once it is lost B. The gradients return, but the inside stays less negative C. Na⁺ is moved in and K⁺ out until the two are equal D. The pumps restart: Na⁺ out, K⁺ in, and the inside turns negative again

Question 16

A student explains why the inside of a resting cell is negative: "The sodium–potassium pump carries negative ions into the cell."

Which statement corrects the student?

A. Both ions the pump moves are positive; each cycle carries more positive charge out of the cell than it brings in, which helps make the inside negative B. Both ions the pump moves are positive, so the pump adds as much positive charge to one face as it removes from the other; the inside is negative for some other reason C. The inside is negative because the pump moves Na⁺ out and K⁺ in in equal numbers, and a Na⁺ ion carries more positive charge than a K⁺ ion D. The inside is negative because Na⁺ leaving carries positive charge out of the cell, while K⁺ entering carries no charge in with it

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

Nerve cells keep the concentration of Na⁺ low and of K⁺ high in their cytosol (about 15 mmol/L Na⁺ and 140 mmol/L K⁺ inside, against 145 and 5 mmol/L outside) and keep their inside slightly negative relative to the outside. Ions cross the membrane only through channels, and only when those channels are open; a few channels are open even in a resting cell. A scientist adds a poison that stops the cells from making ATP; the membrane is not damaged.

(a) Describe how the sodium–potassium pump keeps the concentration of Na⁺ low and of K⁺ high inside a resting cell. (1 point)

A full-credit answer: Each cycle of the pump moves Na⁺ out of the cell, three Na⁺ per cycle, and K⁺ into the cell, two K⁺ per cycle; the pump can do this because it uses one ATP each cycle.
Both ions are moved against their concentration gradients.

Check the box for each point your answer earns

the pump uses ATP (one per cycle) to move three Na⁺ out of the cell and two K⁺ in, both against their concentration gradients.

Accept "against their gradients" without the numbers 15/145 and 5/140, but the counts three and two and the direction (Na⁺ out, K⁺ in) must be given. Do not award the point for a pump that moves both ions the same way, or that moves them down their gradients.

Common slip: Moving both ions the same way, or down their gradients. Three Na⁺ out, two K⁺ in, both uphill.

(b) Explain how the pump helps make the inside of the cell slightly negative. (1 point)

A full-credit answer: Na⁺ and K⁺ each carry one positive charge.
Each cycle moves three positive charges out and only two in.
So more positive charge leaves than enters.
So the inside ends up slightly negative relative to the outside.

Check the box for each point your answer earns

each cycle moves three positive charges out and only two in, so more positive charge leaves than enters and the inside ends up slightly negative relative to the outside (a membrane potential; the membrane is polarized).

Do not award the point for an explanation in which the pump moves negative charge, or for "because Na⁺ is outside" with no count of the charges moved.

Also accept, in addition or in place: the pump keeps K⁺ high inside, and K⁺ drifting out through open channels down that gradient carries positive charge out, leaving the inside negative.

Common slip: Saying the inside is negative because Na⁺ is outside, with no count of the charges moved. The three-for-two count is the point.

(c) Make a claim about what happens to the Na⁺ and K⁺ concentrations inside the cells, and to the charge across the membrane, over the hours after the scientist adds the poison. (1 point)

A full-credit answer: Na⁺ inside rises, because Na⁺ drifts in.
K⁺ inside falls, because K⁺ drifts out.
The inside becomes less negative.

Check the box for each point your answer earns

the claim, all three parts: Na⁺ inside rises (Na⁺ drifts in), K⁺ inside falls (K⁺ drifts out), and the inside becomes less negative. No reasoning is required for this point.

Accept 'the gradients fade' for the first two if the direction of each drift is stated, and 'less negative' or 'closer to zero' for the charge. Do not require the charge difference to reach zero. Do not award the point for 'nothing changes because the membrane is intact', or for the inside becoming more negative.

Common slip: Saying nothing changes because the membrane is intact, or that the inside becomes more negative. Open channels let the ions run down their gradients once the pump stops.

(d) Support your claim with the chain of events from ATP to the charge across the membrane. (1 point)

A full-credit answer: Without ATP the pump stops.
So nothing moves the ions against their gradients.
A few channels are open.
So each ion drifts down its concentration gradient: Na⁺ in and K⁺ out.
So the gradients fade.
No pump is sending three positive charges out for every two in.
So the charge difference the pump was maintaining fades too, and the inside becomes less negative.

Check the box for each point your answer earns

the evidence AND the reasoning: without ATP the pump stops, so nothing moves the ions against their gradients; each ion drifts down its concentration gradient whenever a channel for it is open (Na⁺ in, K⁺ out), so the gradients fade; and with no pump sending three positive charges out for every two in, the charge difference it was maintaining fades too (the inside becomes less negative; it need not reach zero).

Accept support that names the pump's need for ATP and the ions drifting down their gradients. Do not award the point for "the poison lets ions through the membrane" (the membrane is not damaged) or for "the pump works in reverse".

Common slip: Saying the poison lets ions through the membrane, or that the pump works in reverse. The membrane is undamaged; the ions use channels, and the pump simply stops.

Free-response score: 0 of 4

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

The model shows one sodium–potassium pump in the plasma membrane of a resting animal cell, with the concentration of Na⁺ and K⁺ on each side. The direction each ion is carried, the number of ions moved in one cycle, the pump’s energy source and the charges on the two faces of the membrane are not drawn.

One sodium–potassium pump in a resting animal cell membrane, with the concentration of each ion on each side. The direction each ion is carried, the number of ions moved per cycle, the pump’s energy source and the charges on the two faces are not drawn. The extracellular fluid is shaded darker than the cytosol.

(a) Describe one cycle of the pump: how many of each ion it moves, in which direction each ion is carried, and what the pump uses as its energy source. (1 point)

A full-credit answer: In one cycle, three Na⁺ move from the cytosol, at 15 mmol/L, to the outside, at 145 mmol/L, and two K⁺ move from the outside, at 5 mmol/L, into the cytosol, at 140 mmol/L; each ion is moved from where it is less concentrated to where it is more concentrated, against its gradient.
The pump uses one ATP for the cycle.

Check the box for each point your answer earns

three Na⁺ are moved from the cytosol to the outside and two K⁺ from the outside into the cytosol, using one ATP; each ion is moved from where it is less concentrated to where it is more concentrated (against its gradient).

Accept directions given as concentrations (Na⁺ from 15 mmol/L to 145 mmol/L; K⁺ from 5 mmol/L to 140 mmol/L). Do not award the point if the direction of either ion is reversed, the 3:2 count is missing, or ATP is not named.

Common slip: Reversing the direction of either ion, leaving out the three-to-two count, or leaving out the ATP. The model gives only the concentrations; the directions, the numbers of ions and the ATP are yours to supply.

(b) State the charge on each face of the membrane that results from many cycles of the pump, and the direction in which the charge difference pulls a Na⁺ ion. (1 point)

A full-credit answer: The inside face is slightly negative and the outside face is positive.
Na⁺ is a positive ion.
Opposite charges attract.
So the charge difference pulls Na⁺ into the cell.

Full-credit drawing: plus signs on the outside face and minus signs on the inside face (inside slightly negative), and the Na⁺ arrow pointing into the cell, toward the negative face.

Check the box for each point your answer earns

the inside face slightly negative and the outside face positive, AND Na⁺ pulled into the cell.

Accept: "inside negative, outside positive; the charge pulls Na⁺ in". Do not award the point for the inside given as positive, or for Na⁺ pulled out of the cell.

Common slip: Calling the inside positive, or sending Na⁺ out. Opposite charges attract: the negative inside pulls a positive ion in.

(c) Explain, using the concentrations in the model, why this pump needs an energy source while a channel uses no energy. (1 point)

A full-credit answer: Both ions are moved toward the side where they are already more concentrated.
So both are moved against their concentration gradients.
Moving a substance against its gradient takes energy.
That energy comes from ATP, so this is active transport.
A channel only lets an ion move down its gradient.
Movement down a gradient needs no energy, so a channel uses no energy.

Check the box for each point your answer earns

both ions are moved toward the side where they are already more concentrated, so the pump moves them against their concentration gradients, which takes energy; that energy comes from ATP (active transport, defined by the energy the cell uses). A channel only lets an ion move down its gradient, which needs no energy.

Accept "uphill" for against the gradient. Do not award the point for "because it is a pump" or "because the ions are charged" with no reference to the direction relative to the gradients.

Common slip: Saying because it is a pump, or because the ions are charged. The reason is the direction relative to the gradients.

(d) Explain how the model relates to the larger idea that a cell must use energy to keep its inside different from its surroundings. (1 point)

A full-credit answer: Ions drift down their gradients whenever channels are open.
So the differences the model shows, a low concentration of Na⁺ and a high concentration of K⁺ inside and a negative inside, would fade on their own.
The cell keeps them only by using ATP in the pump, cycle after cycle.
So the cell holds a steady internal state by using energy.

Check the box for each point your answer earns

ions drift down their gradients whenever channels are open, so the differences the model shows (a low concentration of Na⁺ and a high concentration of K⁺ inside, inside negative) would fade on their own; the cell keeps them only by continually using ATP in the pump, a steady internal state (homeostasis) held by using energy.

Accept "without ATP the gradients and the charge difference fade" as the link. Do not award the point for "the membrane keeps the ions in" with no mention of energy.

Common slip: Saying the membrane keeps the ions in, with no mention of energy. The membrane has channels; the pump, using ATP, is what holds the differences.

Free-response score: 0 of 4

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Multiple choice checked: 0 of 16 correct.

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