Practice questions · Topic 2.8

Unit 2 · Practice for the Topic 2.8 end-of-topic test

You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.

Watch first: mechanisms of transport, summed up

From the gill cell’s chloride pump to the sodium–potassium pump, the charge it leaves across the membrane, and the two pulls on an ion.

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. 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. The first free-response question walks you through the reasoning one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it.

Question 1

In one second, a single sodium–potassium pump in a kidney cell completes about 100 cycles.

Which statement describes what the pump has done in that second?

A. Moved Na⁺ into the cell and K⁺ out of it, using 100 ATP in all B. Moved Na⁺ out of the cell and K⁺ into it, using one ATP for all 100 cycles C. Moved Na⁺ out of the cell and K⁺ into it, using 100 ATP in all D. Moved Na⁺ and K⁺ both out of the cell, using 100 ATP in all

Question 2

A squid's nerve fiber holds K⁺ at 400 mmol/L inside and 20 mmol/L outside, and Na⁺ at 50 mmol/L inside and 440 mmol/L outside. Its sodium–potassium pumps move Na⁺ out and K⁺ in. A student says the pump needs ATP only for moving the Na⁺, because K⁺ enters the cell anyway.

Which statement corrects the student?

A. K⁺ is moved from 20 mmol/L to 400 mmol/L, against its gradient, so it needs ATP too B. Na⁺ is moved down its gradient, so it needs no ATP; only the K⁺ needs ATP C. Neither ion needs ATP; the pump is a channel that ATP holds open D. The student is right: an ion entering a cell is always moving down its gradient

Question 3

The inside face of a mouse muscle cell’s plasma membrane carries slightly more negative charge than the outside face. A scientist then gives the cell a drug, and two hours later the two faces carry equal charge.

How should the change be described?

A. The cell had a concentration gradient across its membrane and has now lost that gradient B. The membrane was polarized and has now lost its membrane potential C. The cell was in dynamic equilibrium and the drug has now pushed the cell out of it D. The membrane was carrying out active transport and the drug has now stopped that transport

Question 4

A pump in the cells lining the stomach moves H⁺ out of the cell and K⁺ into it, one H⁺ out for every one K⁺ in. H⁺ and K⁺ each carry one positive charge.

Considered on its own, does this pump’s cycling leave the inside of the cell negative, and why?

A. Yes: H⁺ piling up outside the cell makes the outside face positive B. Yes: any pump that uses energy polarizes the membrane C. No: this pump moves both ions down their gradients D. No: one positive charge out for every one in leaves the charge on the two faces unchanged

Question 5

Red blood cells for transfusion are stored in a refrigerator at 4 °C. The cold slows their sodium–potassium pumps to almost nothing. Over two weeks in the bag, the fluid around the cells gains K⁺ and the cells gain Na⁺; the membranes stay intact.

Explain the change.

A. The cold dissolves part of each membrane, so ions leak through the gaps B. The pumps work in reverse in the cold, pushing K⁺ out and pulling Na⁺ in C. With the pumps nearly stopped, Na⁺ drifts in and K⁺ drifts out down their gradients D. The cells use their ATP to push K⁺ out, since a cold cell has no use for it

Question 6

In an experiment a scientist makes the inside of a nerve cell slightly positive relative to the outside, while K⁺ stays at 140 mmol/L inside and 5 mmol/L outside. A potassium channel then opens.

How do the two pulls on K⁺ compare?

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

Question 7

Two cells with the same Na⁺ concentrations. Cell 1 has a charge difference across its membrane (inside slightly negative); cell 2 has none. Each cell has one open sodium channel.

The figure shows two cells holding Na⁺ at 145 mmol/L outside and 15 mmol/L inside. Cell 1 has its inside slightly negative relative to the outside. Cell 2 has no charge difference across its membrane. A sodium channel opens in each cell at the same moment.

Compare the net movement of Na⁺ into the two cells at that moment, and explain the difference.

A. Equal in both: only the concentration difference moves an ion B. Faster into cell 2: a charge difference slows every ion down C. Into cell 1 only: with no charge difference, Na⁺ has no reason to move D. Faster into cell 1: its negative inside pulls the positive ion in as well

Question 8

A newly formed animal cell builds its Na⁺ and K⁺ gradients and its membrane potential from scratch. Four events are listed out of order: (1) the concentration of Na⁺ is low and of K⁺ high inside, and the inside is slightly negative; (2) the cell makes ATP; (3) the pump moves Na⁺ out of the cell and K⁺ in; (4) ATP powers the pump.

Which order runs from the first cause to the final result?

A. 2, 4, 1, 3 B. 2, 4, 3, 1 C. 4, 2, 3, 1 D. 1, 3, 4, 2

Question 9

A membrane protein found in the cells lining the stomach moves K⁺ into the cell against its concentration gradient. It is named an ATPase.

What does the name tell you about how the protein works?

A. The protein uses ATP to power the moving of K⁺ B. The protein makes ATP as K⁺ passes through it C. The protein carries ATP across the membrane along with K⁺ D. The protein is a channel for K⁺ that stays open while ATP is present

Question 10

A student measures Na⁺ at 145 mmol/L outside a cell and 15 mmol/L inside, and writes: 'This difference is the cell's membrane potential.'

Which statement corrects the student?

A. The student is right: any difference across a membrane is a membrane potential B. That is a membrane potential only once the sodium channels open and Na⁺ flows in C. That is a concentration gradient; a membrane potential is a difference in charge across the membrane D. That is a membrane potential only for K⁺, the ion the cell holds high inside

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 · 5 points

Scientists build artificial cells: small membrane sacs with sodium–potassium pumps set into their membranes. At the start, the fluid inside and outside every sac holds Na⁺ at 100 mmol/L and K⁺ at 100 mmol/L, and there is no charge difference across the membrane. A few ion channels in each membrane are open throughout. The scientists then add ATP to the inside of the sacs and follow the sacs for an hour, until the ATP has all been used.

(a) Identify what one cycle of the pump moves, and the energy source it uses. (1 point)

Hint: How many Na⁺ and how many K⁺ cross in one cycle, and which way does each go? What does the pump use up each cycle?

A full-credit answer: One cycle moves three Na⁺ out of the sac and two K⁺ into it, using one ATP.

Check the box for each point your answer earns

three Na⁺ out of the sac and two K⁺ into it, using one ATP.

The counts (three and two), the directions (Na⁺ out, K⁺ in) and ATP are all needed for the point.

Common slip: Swapping the counts, or sending both ions the same way. Three Na⁺ out, two K⁺ in, one ATP.

(b) Predict how the Na⁺ and K⁺ concentrations inside the sacs change over the hour. (1 point)

Hint: Both ions start at 100 mmol/L on each side. Which way does the pump send each one?

A full-credit answer: Inside the sacs, Na⁺ falls below 100 mmol/L and K⁺ rises above 100 mmol/L, because the pump sends Na⁺ out and brings K⁺ in, cycle after cycle.
So the pump builds the two gradients from nothing.

Check the box for each point your answer earns

Na⁺ inside falls below 100 mmol/L and K⁺ inside rises above 100 mmol/L, because the pump keeps sending Na⁺ out and bringing K⁺ in, cycle after cycle, faster than the open channels let them drift back.

Accept: 'Na⁺ ends up at a low concentration inside and K⁺ at a high concentration inside'. Do not award the point for both rising or both falling, or for 'nothing changes because the concentrations start equal'.

Common slip: Saying nothing changes because the pump needs a gradient to work on. The pump makes the gradients; it does not need them.

(c) Explain how the running pump produces a charge difference across the membrane, and state which face ends up negative. (1 point)

Hint: Think about the charge each ion carries. Then count the charges that cross in one cycle, out and in.

A full-credit answer: Each cycle moves three positive charges out and two positive charges in, so more positive charge leaves the sac than enters it; the inside face ends up negative.
Each Na⁺ and each K⁺ carries one positive charge, so three Na⁺ out is three positive charges out and two K⁺ in is two positive charges in.
So the membrane has a membrane potential, and the membrane is polarized.

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 face ends up slightly negative relative to the outside (a membrane potential; the membrane is polarized).

Do not award the point for 'because Na⁺ is outside' with no count of the charges, or for the outside face named as negative.

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

(d) Make a claim about what happens to the two gradients and to the charge difference after the ATP has all been used. (1 point)

Hint: What happens to the pump without ATP? A few channels are open: what can the ions do through them once nothing opposes it?

A full-credit answer: With the ATP gone, the pump stops.
Na⁺ drifts back in through the open channels, so Na⁺ inside rises.
K⁺ drifts back out, so K⁺ inside falls.
No pump is sending three positive charges out for every two in.
So the charge difference fades, and the inside becomes less negative.

Check the box for each point your answer earns

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

Accept: 'the gradients fade and the membrane potential fades'. 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. An intact membrane still has open channels, and the ions run down their gradients through them.

(e) Support your claim in (d) using the chain of events that runs from ATP to the membrane potential. (1 point)

Hint: Write the chain out, from ATP to the negative inside. Then find the link that the end of the hour takes away.

A full-credit answer: The chain is ATP → ATP powers the pump → three Na⁺ out and two K⁺ in → Na⁺ concentration low and K⁺ concentration high inside → inside slightly negative.
Every step depends on the one before it.
With no ATP the pump stops, so nothing moves the ions against their gradients.
So they drift back down their gradients through the open channels.
No pump is sending three positive charges out for every two in, so the charge difference fades as well.

Check the box for each point your answer earns

the evidence AND the reasoning: the chain is ATP → ATP powers the pump → three Na⁺ out and two K⁺ in → Na⁺ concentration low and K⁺ concentration high inside → inside slightly negative; every step depends on the one before it, so with no ATP the pump stops, nothing moves the ions against their gradients, they drift back down them through the open channels, and with no pump sending three positive charges out for two in, the charge difference fades too.

Accept: support that names the pump's need for ATP and the ions drifting down their gradients through open channels. Do not award the point for the chain alone with no link to the claim, or for 'the pump works in reverse'.

Common slip: Saying the pump works in reverse. It simply stops; the ions do the rest by drifting down their gradients.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

A drug used to treat some heart conditions binds to the sodium–potassium pump and slows it. Scientists give the drug to heart muscle cells in a dish and measure the Na⁺ and K⁺ concentrations inside the cells over four hours (table). Before the drug, the cells held Na⁺ at 10 mmol/L and K⁺ at 140 mmol/L inside, against 145 mmol/L Na⁺ and 5 mmol/L K⁺ outside, and their inside was slightly negative relative to the outside. The membranes stay intact, and the cells keep making ATP.

Concentrations of Na⁺ and K⁺ inside heart muscle cells after a pump-slowing drug is given. Outside the cells: Na⁺ 145 mmol/L, K⁺ 5 mmol/L throughout.

(a) Describe what the sodium–potassium pump does in an untreated heart muscle cell. (1 point)

A full-credit answer: In an untreated cell, each cycle of the pump moves three Na⁺ out of the cell and two K⁺ into it, both against their concentration gradients, using one ATP.
That is what keeps Na⁺ at 10 mmol/L and K⁺ at 140 mmol/L inside.

Check the box for each point your answer earns

each cycle uses one ATP to move three Na⁺ out of the cell and two K⁺ in, both against their concentration gradients, keeping Na⁺ low and K⁺ high inside.

The counts, the directions and 'against the gradient' (or 'uphill') are needed; the numbers 10/145 and 140/5 are not required.

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

(b) Explain how the changes in the table show that the drug has slowed the pumps. (1 point)

A full-credit answer: Na⁺ inside rises from 10 mmol/L to 30 mmol/L.
K⁺ inside falls from 140 mmol/L to 124 mmol/L.
Each ion is drifting down its gradient through open channels.
A working pump undoes that drift as fast as it happens.
Here the drift shows through.
So the pumps are moving fewer ions back than before.
So the drug has slowed them.

Check the box for each point your answer earns

Na⁺ inside rises (10 to 30 mmol/L) and K⁺ inside falls (140 to 124 mmol/L), which is each ion drifting down its gradient through open channels; a working pump undoes that drift, so the drift showing through means the pump is moving fewer ions back than before.

Accept: 'the gradients are fading, and only a slowed pump lets that happen'. Do not award the point for reading the numbers with no link to the pump, or for 'the drug lets ions through the membrane' (the membrane is intact).

Common slip: Reading off the numbers with no link to the pump, or blaming the drug for 'letting ions through'. The membrane is intact; the ions use channels, and the pump is what no longer keeps up.

(c) Predict how the charge across the membrane changes over the four hours. (1 point)

A full-credit answer: The inside becomes less negative relative to the outside over the four hours.
The membrane potential fades toward zero.

Check the box for each point your answer earns

the inside becomes less negative relative to the outside (the membrane potential fades toward zero).

Do not award the point for the inside becoming more negative, or for the charge reversing to positive; 'less negative' or 'closer to zero' earns the point.

Common slip: Predicting the inside becomes more negative because Na⁺ is entering. The charge difference depends on the pump's three-out, two-in count, and the pump is doing less of it.

(d) Justify your prediction, using the pump's three-out, two-in count and the changes in the table. (1 point)

A full-credit answer: The pump keeps the inside negative by sending three positive charges out for every two it brings in.
The drug slows the pump, so the pump exports less positive charge each second.
Meanwhile Na⁺ keeps drifting in and K⁺ keeps drifting out through the open channels.
The table shows Na⁺ inside rose by 20 mmol/L while K⁺ inside fell by only 16 mmol/L, so more positive charge entered than left.
So the charge difference shrinks, and the inside becomes less negative.

Check the box for each point your answer earns

the pump keeps the inside negative by sending three positive charges out for every two it brings in; slowed, it exports less positive charge each second, while Na⁺ drifting in and K⁺ drifting out through open channels carry charge that evens the two faces, so the charge difference the pump was maintaining shrinks.

Accept a count from the table: Na⁺ inside rose by 20 mmol/L while K⁺ inside fell by 16 mmol/L, so more positive charge entered than left and the inside became less negative. Accept: 'less pumping, so less of the three-for-two excess that made the inside negative'. Do not award the point for 'the drug makes the membrane leaky'.

Common slip: Saying the drug makes the membrane leaky. The membrane is intact; the change is that the pump no longer keeps up with the drift through the channels.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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