Unit 2 · Topic 2.8 end-of-topic test
A muscle cell's membrane carries many sodium–potassium pumps. Inside the cell Na⁺ is low and K⁺ is high; outside, Na⁺ is high and K⁺ is low. One pump completes one cycle, using one ATP.
Which ions did that cycle move, and which way?
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?
The sodium–potassium pump is also called the Na⁺/K⁺ ATPase.
What does the second name tell you about the pump?
A sodium–potassium pump in a cell membrane completes 10 full cycles.
In total, what has it moved and used?
A fine probe is placed inside a cell and another outside. Together they show that the inside face of the membrane carries slightly more negative charge than the outside face.
What is this condition called?
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?
Over a day, a resting nerve cell's pumps run continuously. Each cycle moves three Na⁺ out and two K⁺ in.
How does this keep the inside of the cell slightly negative relative to the outside?
A poison stops a nerve cell from making ATP. Its membrane stays intact.
Over the next few hours, what happens to the cell's Na⁺ and K⁺ gradients and to the charge across its membrane?
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 begin to run, with plenty of ATP. Its channels stay shut.
Predict how the cell changes over the next hour.
The figure shows a resting animal cell: Na⁺ is high outside and low inside, 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?
Same cell as in the figure. Now suppose the channel drawn lets only K⁺ through, and it opens.
How do the two pulls on K⁺ compare?
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?
In an experiment the inside of a cell is made slightly positive relative to the outside, while Na⁺ stays high outside and low inside.
How do the two pulls on Na⁺ now compare?
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 cell keeps its inside slightly negative and holds Na⁺ low and K⁺ high. A student claims that giving the cell extra ATP while blocking its sodium–potassium pumps would keep these conditions as they are.
Is the claim right?
Nerve cells whose ATP supply was blocked for several hours had lost much of their Na⁺ and K⁺ gradients, and their insides had become less negative. The block is then removed and the cells make ATP again.
What happens next?
An artificial membrane separates two identical salt solutions. No ions have crossed it.
Is there a membrane potential?
(a) Describe how the sodium–potassium pump keeps Na⁺ low and K⁺ high inside a resting cell. (1 point)
A full-credit answer: The pump uses one ATP per cycle to move three Na⁺ out of the cell and two K⁺ in, both against their concentration gradients.
Check the box for each point your answer earns
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: 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, with the membrane polarized.
Check the box for each point your answer earns
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.
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) Predict what happens to the Na⁺ and K⁺ concentrations inside the cells, and to the charge across the membrane, over the hours after the poison is added. (1 point)
A full-credit answer: Na⁺ inside rises as Na⁺ drifts in; K⁺ inside falls as K⁺ drifts out; and the inside becomes less negative.
Check the box for each point your answer earns
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) Justify your prediction. (1 point)
A full-credit answer: 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 and K⁺ out, so the gradients fade. With no pump sending three positive charges out for every two in, the charge difference it was maintaining fades too, and the inside becomes less negative.
Check the box for each point your answer earns
Accept a justification 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 runs backward".
Common slip: Saying the poison lets ions through the membrane, or that the pump runs backward. The membrane is undamaged; the ions use channels, and the pump simply stops.
(a) Describe what the model shows one cycle of the pump doing. (1 point)
A full-credit answer: One cycle moves three Na⁺ from the cytosol (15 mmol/L) to the outside (145 mmol/L) and two K⁺ from the outside (5 mmol/L) into the cytosol (140 mmol/L), using one ATP. Each ion is moved from where it is less concentrated to where it is more concentrated, against its gradient.
Check the box for each point your answer earns
Accept a description that reads the arrows and the concentrations (Na⁺ from 15 to 145 mmol/L; K⁺ from 5 to 140 mmol/L). Do not award the point if the direction of either ion is reversed or the 3:2 count is missing.
Common slip: Reversing the direction of either ion, or leaving out the three-to-two count. Read the arrows and the concentrations.
(b) Explain, using the concentrations in the model, why this pump needs ATP while a channel runs for free. (1 point)
A full-credit answer: Both ions are moved toward the side where they are already more concentrated, against their concentration gradients, which takes energy; that energy comes from ATP, so this is active transport. A channel only lets an ion move down its gradient, which needs no energy.
Check the box for each point your answer earns
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.
(c) On a copy of the model, mark the charge on each face of the membrane that results from many cycles of the pump, and draw an arrow showing the direction in which the charge difference pulls a Na⁺ ion. (If you are typing, describe exactly what you would draw.) (1 point)
A full-credit answer: Minus signs on the inside face and plus signs on the outside face: inside slightly negative, outside positive. The arrow for Na⁺ points into the cell, because a positive ion is pulled toward the negative inside.
Check the box for each point your answer earns
Accept the written statement "inside negative, outside positive; the charge pulls Na⁺ in" in place of drawn marks. Do not award the point for the inside marked positive, or for a Na⁺ arrow pointing out of the cell.
Common slip: Marking the inside positive, or pointing the Na⁺ arrow out. Opposite charges attract: the negative inside pulls a positive ion in.
(d) Explain how the model relates to the larger idea that a cell must spend 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, Na⁺ low and K⁺ high inside and the inside negative, would fade on their own. The cell keeps them only by continually spending ATP in the pump: a steady internal state held at a cost in energy.
Check the box for each point your answer earns
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, spending ATP, is what holds the differences.