These are original Marlbridge practice questions, not past exam questions. The mark points are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. Data for every question: the sodium–potassium pump moves 3 Na⁺ out of the cell and 2 K⁺ into the cell for each ATP hydrolysed. A calculator is assumed. Keep full calculator values until the last step. All data sets are fictional.
Question 1 (multiple choice · foundation)
What is the direct role of ATP in one cycle of the sodium–potassium pump?
- (A) ATP transfers a phosphate group to the pump protein, causing a change in shape that releases Na⁺ outside the cell.
- (B) ATP binds three Na⁺ ions and carries them across the membrane.
- (C) ATP opens a channel in the pump so that Na⁺ and K⁺ can diffuse down their concentration gradients.
- (D) ATP is split outside the cell, and the energy released pulls K⁺ ions into the pump.
Answer and explanation
Answer: (A). After three Na⁺ bind on the inside, ATP is hydrolysed and its phosphate attaches to the pump (phosphorylation). The pump changes shape, opens to the outside and releases the Na⁺.
- (B) ATP is not a carrier. It never crosses the membrane; the pump protein carries the ions.
- (C) describes a channel and passive diffusion. The pump moves both ions against their gradients, which needs energy.
- (D) puts ATP in the wrong place: ATP is in the cytoplasm and is split on the inside of the membrane.
Question 2 (multiple choice · core)
The sodium–potassium pumps in a cell hydrolyse 5.0 × 10⁶ ATP molecules per second. How many K⁺ ions do they move into the cell each second?
- (A) 2.5 × 10⁶
- (B) 5.0 × 10⁶
- (C) 1.0 × 10⁷
- (D) 1.5 × 10⁷
Answer and explanation
Answer: (C). Each ATP drives one cycle, and each cycle brings in 2 K⁺: 2 × 5.0 × 10⁶ = 1.0 × 10⁷ K⁺ per second.
- (A) divides by 2 instead of multiplying.
- (B) assumes one K⁺ per ATP.
- (D) is the number of Na⁺ ions moved out (3 × 5.0 × 10⁶), not K⁺ moved in.
Question 3 (multiple choice · core)
In a resting animal cell, the inside of the membrane is negative and Na⁺ is far more concentrated outside than inside. When Na⁺ channels open, Na⁺ enters very rapidly. Which statement best explains why?
- (A) The concentration gradient and the electrical gradient both push Na⁺ into the cell.
- (B) The sodium–potassium pump pushes Na⁺ inward through the open channels.
- (C) Na⁺ is attracted to the high concentration of K⁺ inside the cell.
- (D) The open channels use ATP to move Na⁺ quickly into the cell.
Answer and explanation
Answer: (A). Na⁺ moves down its concentration gradient (high outside, low inside), and as a positive ion it is also drawn toward the negative inside. Both parts of its electrochemical gradient point inward, so the flow is fast.
- (B) reverses the pump’s job: it moves Na⁺ out. The rapid inflow is passive.
- (C) is wrong: K⁺ is also positive, so it would repel Na⁺, not attract it.
- (D) Channels allow passive movement and do not use ATP.
Question 4 (data analysis · core)
A researcher records the membrane potential of a fictional animal cell. At time 0 she adds a drug that stops all sodium–potassium pumps. The cell’s ion channels are not affected.
| Time / min | 0 | 1 | 10 | 20 | 30 | 45 | 60 |
|---|---|---|---|---|---|---|---|
| Membrane potential / mV | −70 | −66 | −62 | −57 | −52 | −46 | −41 |
(a) Describe the two phases of change, using numbers from the table. (b) Explain the small, rapid change in the first minute. (c) Explain the slower change from 1 to 60 minutes. (d) The drug is washed away and the cell has plenty of ATP. Predict what happens to the membrane potential, and explain.
Worked solution and suggested Marlbridge rubric
(a) In the first minute, the potential becomes less negative by 4 mV (−70 to −66 mV). Over the next 59 minutes it changes more slowly and at a fairly steady rate, by a further 25 mV (−66 to −41 mV), about 0.42 mV per minute.
(b) Each pump cycle moves 3 Na⁺ out and 2 K⁺ in, so one net positive charge leaves the cell. While the pumps run, this keeps the inside slightly more negative. When the drug stops them, this direct charge movement stops at once, so the inside becomes a little less negative almost immediately.
(c) Without the pumps, ions keep leaking through channels down their electrochemical gradients: Na⁺ leaks in and K⁺ leaks out. Nothing moves them back, so the gradients slowly run down. The difference in charge across the membrane depends on these gradients, so it shrinks gradually.
(d) The potential should return toward −70 mV. The pumps use ATP to move Na⁺ out and K⁺ in against their gradients, rebuilding them, and the net outward movement of positive charge resumes.
| Point | What earns it |
|---|---|
| 1 | Both phases described with numbers (a fast 4 mV change, then a slow 25 mV change) |
| 1 | Rapid change linked to loss of the pump’s net outward movement of one positive charge per cycle |
| 1 | Slow change linked to Na⁺ leaking in and/or K⁺ leaking out, so the gradients run down |
| 1 | Recovery predicted, with pumps using ATP to rebuild the gradients |
Question 5 (constructed response · core)
Lysosomes contain digestive enzymes that work best in acidic conditions. Their membranes contain proton pumps (H⁺-ATPases) that move H⁺ from the cytoplasm into the lysosome. In a fictional experiment, a drug that blocks these pumps is added at time 0. The cytoplasm stays at pH 7.2 throughout.
| Time / min | 0 | 10 | 20 | 30 | 40 | 50 |
|---|---|---|---|---|---|---|
| pH inside lysosomes | 4.6 | 5.2 | 5.9 | 6.6 | 7.0 | 7.1 |
(a) Each fall of one pH unit means a tenfold rise in H⁺ concentration. Calculate how many times more concentrated H⁺ is inside the lysosome than in the cytoplasm at time 0. (b) Explain why keeping the lysosome at pH 4.6 requires ATP. (c) Explain the change in pH after the drug is added. (d) Predict the effect of the drug on the digestion of material inside lysosomes.
Model answer and suggested Marlbridge rubric
(a) The difference is 7.2 − 4.6 = 2.6 pH units, so H⁺ is 10²·⁶ ≈ 400 times (398) more concentrated inside the lysosome.
(b) The pumps move H⁺ from the cytoplasm, where H⁺ is less concentrated, into the lysosome, where it is about 400 times more concentrated. This is movement against the concentration gradient, so it needs a membrane protein and a direct input of energy from ATP.
(c) Once the pumps stop, H⁺ leaks out of the lysosome down its concentration gradient into the cytoplasm. Nothing replaces it, so the H⁺ concentration inside falls and the pH rises, from 4.6 to 7.1 in 50 minutes. The rise slows as the inside approaches the cytoplasm’s pH, because the gradient becomes small.
(d) Digestion inside lysosomes slows or stops, because the enzymes work best at acidic pH and the inside is now close to neutral.
| Point | What earns it |
|---|---|
| 1 | About 400 times (accept 10²·⁶) |
| 1 | H⁺ moved from low to high concentration, so ATP and a pump protein are needed |
| 1 | H⁺ leaks out down its gradient and is not replaced, so pH rises toward 7.2 |
| 1 | Prediction that digestion slows, linked to enzymes needing acidic conditions |
Question 6 (constructed response · stretch)
In a fictional animal cell at rest, the Ca²⁺ concentration is about 0.0001 mM in the cytoplasm and about 1.2 mM outside. The inside of the cell is negative. Calcium pumps in the plasma membrane use ATP to move Ca²⁺ out of the cell. Many cells use a brief rise in cytoplasmic Ca²⁺ as a signal.
(a) Calculate how many times more concentrated Ca²⁺ is outside than inside. (b) Use both parts of the electrochemical gradient to predict what happens when Ca²⁺ channels in the plasma membrane open briefly. (c) Explain why the calcium pumps must keep working after the channels close. (d) The calcium pumps of a cell are damaged. Predict how cytoplasmic Ca²⁺ changes over time, and explain why the cell would respond less clearly to later signals.
Model answer and suggested Marlbridge rubric
(a) 1.2 ÷ 0.0001 = 12 000 times.
(b) Ca²⁺ is far more concentrated outside, so the concentration push is inward. Ca²⁺ is positive and the inside is negative, so the electrical push is also inward. Both point the same way, so Ca²⁺ rushes into the cytoplasm as soon as the channels open, and cytoplasmic Ca²⁺ rises sharply.
(c) The signal must end, and the cell must be ready for the next one. After the channels close, the pumps move Ca²⁺ back out, against its very steep gradient, using ATP. This returns cytoplasmic Ca²⁺ to its very low resting level. Ca²⁺ also leaks in slowly at rest, so pumping must continue.
(d) Cytoplasmic Ca²⁺ would rise gradually and stay high, because Ca²⁺ keeps leaking in and is not removed. A signal works because Ca²⁺ is normally very low, so a small inflow makes a large relative change. If Ca²⁺ is already high, the gradient is smaller and the same inflow makes a smaller relative change, so the cell cannot tell clearly when a signal arrives (and the signal would not switch off).
| Point | What earns it |
|---|---|
| 1 | 12 000 times |
| 1 | Both concentration and electrical pushes inward, so fast inflow |
| 1 | Pumps restore the low resting level against the gradient using ATP, and must offset continuous leak |
| 1 | Damaged pumps: Ca²⁺ stays high, so later inflows give a weaker or unclear signal |
Question 7 (calculation · stretch)
During a burst of activity, a fictional cell gains 6.0 × 10⁸ extra Na⁺ ions. The cell has 5.0 × 10⁵ sodium–potassium pumps, each completing 80 cycles per second. Assume no more Na⁺ leaks in while the pumps work.
(a) Calculate how long the pumps take to remove the extra Na⁺. (b) Calculate how many ATP molecules this uses and how many K⁺ ions are brought in. (c) A toxin blocks half of the pumps. How long would the same recovery take?
Worked solution
(a) Na⁺ removed per second = 3 × 5.0 × 10⁵ pumps × 80 s⁻¹ = 1.2 × 10⁸ Na⁺ s⁻¹. Time = 6.0 × 10⁸ ÷ 1.2 × 10⁸ = 5.0 s.
(b) ATP = 6.0 × 10⁸ ÷ 3 = 2.0 × 10⁸ ATP. K⁺ in = 2 × 2.0 × 10⁸ = 4.0 × 10⁸ K⁺.
(c) Half as many pumps remove Na⁺ at half the rate, 6.0 × 10⁷ s⁻¹, so recovery takes 10 s, twice as long. The ATP used is unchanged (2.0 × 10⁸), because the same number of Na⁺ must still be moved.
Suggested mark points (3): 1 for 5.0 s using 3 Na⁺ per cycle; 1 for 2.0 × 10⁸ ATP and 4.0 × 10⁸ K⁺; 1 for 10 s with the reason.
Common errors: using 1 Na⁺ per cycle gives 15 s; using 2 Na⁺ per cycle (confusing Na⁺ with K⁺) gives 7.5 s; counting 6.0 × 10⁸ ATP assumes one ATP per ion.
How did you do?
- Q1 wrong: re-read “The sodium–potassium pump” in the study guide and look again at Figure 1.
- Q2 or Q7 calculations wrong: rework Worked example 1 and part (c) of Worked example 2; write the 3 : 2 : 1 ratio before you start.
- Q3 or Q6 wrong: re-read “Electrochemical gradients and membrane potential” and Worked example 2 (b).
- Q4 incomplete: re-read how the pump helps maintain the membrane potential, directly and through the gradients.
- Q5 incomplete: re-read “Three ways across the membrane” and the paragraph on other pumps.
Then tick off the topic checklist.