Practice questions · Topic 2.8

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

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 tells you what a wrong choice assumed. For the free-response questions, write your answer in full sentences. 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.

In that second, what has the pump moved and used?

A. 200 Na⁺ out, 300 K⁺ in, 100 ATP B. 300 Na⁺ in, 200 K⁺ out, 100 ATP C. 300 Na⁺ out, 200 K⁺ in, 100 ATP D. 300 Na⁺ out, 200 K⁺ in, 1 ATP

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 to 400 mmol/L, against its gradient, so it costs ATP too B. Na⁺ is moved down its gradient, so it costs nothing; only the K⁺ costs ATP C. Neither ion costs 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

Two fine probes, one inside a mouse muscle cell and one in the fluid outside, show that the inside face of the membrane carries slightly more negative charge than the outside face. The cell is then treated with a drug, and two hours later the probes show the two faces carrying equal charge.

How should the change be described?

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

Question 4

Suppose a cell's only pump moved two Na⁺ out and two K⁺ in for each ATP it used, instead of three and two. In this model, only the pump moves ions across the membrane.

Would a cell running only this pump end up with its inside negative?

A. Yes: Na⁺ piling up outside the cell makes the outside face positive B. Yes: any pump that uses ATP polarizes the membrane C. No: this pump would move both ions down their gradients D. No: two positive charges out for every two in leaves the charge 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 run backward 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 spend their ATP pushing K⁺ out, since a cold cell has no use for it

Question 6

In an experiment the inside of a nerve cell is made 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: concentration, and a positive inside repelling a positive ion 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 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.

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) Na⁺ is low and K⁺ high inside, and the inside is slightly negative; (2) ATP is made; (3) three Na⁺ leave and two K⁺ enter per cycle; (4) the pump runs.

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. Researchers name it 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

Researchers 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, so ions can slowly drift down any gradient that forms. The researchers then add ATP to the inside of the sacs and follow them 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, building 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 low inside and K⁺ high 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: What charge does each ion carry, and how many ions leave and how many enter per cycle?

A full-credit answer: Each cycle moves three positive charges out and only two in, so a little more positive charge leaves than enters every cycle. The inside face ends up slightly negative relative to the outside: the membrane has a membrane potential and 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) Predict 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, and what do the open channels in the stimulus allow the ions to do?

A full-credit answer: With the ATP gone, the pump stops. Na⁺ inside rises as Na⁺ drifts back in, K⁺ inside falls as K⁺ drifts back out, and the charge difference fades: the inside becomes less negative, drifting toward zero.

Check the box for each point your answer earns

all three: Na⁺ inside rises (drifts back in), K⁺ inside falls (drifts back out), and the charge difference fades (the inside becomes less negative, toward zero).

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

Hint: Each link in the chain depends on the link before it. Which link has been removed?

A full-credit answer: The chain runs ATP → the pump runs → three Na⁺ out and two K⁺ in → Na⁺ low and K⁺ 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; they drift back down them through the open channels, and with no pump sending three positive charges out for every two in, the charge difference it was maintaining fades as well.

Check the box for each point your answer earns

the chain runs ATP → the pump runs → three Na⁺ out and two K⁺ in → Na⁺ low and K⁺ 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 it maintained fades too.

Accept: a justification 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 pump runs backward'.

Common slip: Saying the pump runs backward. It simply stops; the ions do the rest by running downhill.

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. Researchers 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 and two K⁺ in, 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 to 30 mmol/L and K⁺ inside falls from 140 to 124 mmol/L. Each ion is drifting down its gradient through open channels, which a working pump normally undoes as fast as it happens. The drift showing through means the pumps are moving fewer ions back than before: 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. Slowed by the drug, it exports less positive charge each second, while the ions keep drifting down their gradients through the open channels, Na⁺ in and K⁺ out. The table shows the same thing: Na⁺ inside rose by 20 mmol/L while K⁺ inside fell by only 16 mmol/L, so more positive charge entered than left. The charge difference the pump was maintaining therefore 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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