Unit 2 · Topic 2.6 end-of-topic test
Suggested time: about 44 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.
Glucose is at 7 mmol/L in the blood around a liver cell and 3 mmol/L inside the cell. Glucose enters the cell through a carrier protein.
Which of the following describes the movement of glucose into the cell?
A guard cell, one of the pair of cells that open and close a pore in a leaf, holds K⁺ at 300 mmol/L inside; the fluid in the cell wall around it holds K⁺ at 10 mmol/L. As the pore closes, one of the cell’s potassium channels opens.
Which way does K⁺ move, and what is the crossing called?
Four substances, W, X, Y and Z, are tested on protein-free phospholipid bilayers and on living cells. W crosses the protein-free bilayer and enters the cells; blocking the cells’ ATP supply leaves its entry unchanged. X is held back by the bilayer but enters the cells; blocking ATP leaves its entry unchanged. Y is held back by the bilayer but enters the cells; blocking ATP stops its entry. Z is held back by the bilayer and stays outside the cells.
Which substance enters the cells by facilitated diffusion?
Cells lining a kidney tubule take in glucose even when it is 0.5 mmol/L in the tubule fluid and 6 mmol/L inside the cells, and the uptake almost stops when the cells' energy supply is cut off. A membrane protein is involved.
Is this crossing facilitated diffusion?
A squid nerve cell has Na⁺ at 440 mmol/L outside and 50 mmol/L inside. At first every sodium channel is shut. Then the sodium channels open. The cell's energy supply is blocked throughout, and no other protein moves Na⁺.
What is the net movement of Na⁺ while the channels are shut, and then once they open?
Two kinds of animal cell sit in a dilute solution, in which water enters cells. Cells of type 1 have many aquaporins in their membranes; cells of type 2 have very few.
Which cells gain water faster, and why?
Cells lining a sweat duct hold Cl⁻ at 20 mmol/L; the fluid outside holds 100 mmol/L. At 0 minutes the cells’ chloride channels are open. At 2 minutes a blocker shuts every chloride channel. Cl⁻ inside the cells is measured at 0, 2 and 4 minutes.
Which of the following gives the Cl⁻ concentration inside the cells, in mmol/L, at 0, 2 and 4 minutes?
A muscle cell in the stomach wall of a frog holds Na⁺ at 15 mmol/L inside against 110 mmol/L outside, and K⁺ at 120 mmol/L inside against 3 mmol/L outside. A drug shuts every sodium channel in its membrane; its potassium channels stay open. A student predicts: “With the sodium channels shut, K⁺ will stop moving too, because the membrane is now closed to ions.”
Which of the following statements about the prediction is correct?
Cells from a salivary gland sit in a dilute solution, in which water enters them, for five minutes. Cells with working aquaporins and a normal energy supply gain 18%. Cells with their aquaporins blocked gain 5%. Cells with working aquaporins but an energy supply cut to under 5% of normal gain 17%.
What do the three results show about aquaporins?
A membrane protein lets water molecules stream through it, several hundred million every second, but lets no ions and no glucose through.
What is the protein?
A potassium channel in a kidney-tubule cell lets K⁺ stream through it. Na⁺ is also present on both sides of the membrane, and a Na⁺ ion is smaller than a K⁺ ion, yet almost no Na⁺ passes through this channel.
What does this show about the channel?
Two cells lining a kidney tubule hold Cl⁻ at 25 mmol/L inside, with 115 mmol/L in the tubule fluid outside. Cell 1 has 100 open chloride channels; cell 2 has 200.
Which of the following describes the net movement of Cl⁻ in the two cells at the start?
A red blood cell with glucose carriers sits in a solution; at 10 minutes it is moved into a second solution. The table gives the glucose concentration outside and inside the cell.
What is the net movement of glucose at 20 minutes?
Cells lining a duct of the pancreas, with open chloride channels, sit in a solution with Cl⁻ at 100 mmol/L; inside the cells it is 40 mmol/L, and Cl⁻ is entering. The solution is then replaced by one with Cl⁻ at 15 mmol/L. The channels stay open.
What happens to the movement of Cl⁻ after the change?
Fat cells sit in a solution held at 6 mmol/L glucose; inside the cells glucose is 1 mmol/L. The hormone insulin makes a fat cell move extra glucose carriers into its membrane: about 500 carriers per cell without insulin, about 5,000 with it. One dish of cells gets insulin and one does not. Inside the cells, glucose stays as glucose.
Which of the following describes the two dishes?
The figure shows part of a fat cell’s plasma membrane, with two membrane proteins and the concentration of K⁺ and of glucose on each side. A toxin binds the protein marked X.
What happens to K⁺ and to glucose while the toxin is bound?
(a) Describe how K⁺ crosses the membrane when a potassium channel opens, and name the kind of transport. (1 point)
A full-credit answer: When a potassium channel opens, K⁺ passes through the open channel protein, a water-lined tunnel.
K⁺ moves down its concentration gradient, from 180 mmol/L inside to 12 mmol/L outside, so K⁺ leaves the cell.
Movement down a gradient happens by itself.
So the cell uses no energy on this crossing.
This kind of transport is facilitated diffusion, a form of passive transport.
Check the box for each point your answer earns
Accept: "through the channel, from high to low, out of the cell, with no energy used: facilitated diffusion". The route, the direction and the name are all needed for the point.
Common slip: Giving the route without the name, or the name without the route, or sending K⁺ inward. The route, the direction from the two concentrations and the name are all needed.
(b) Explain what keeps Na⁺ out of the cell while its channels are shut, even though its concentration is much higher outside. (1 point)
A full-credit answer: Na⁺ is an ion, so it carries a full charge.
The hydrocarbon tails in the middle of the membrane carry no charges or partial charges.
So nothing in the tails attracts a charged ion.
Water is attracted to Na⁺.
So Na⁺ stays in the water and cannot cross the hydrophobic interior on its own.
Na⁺ can cross only through a channel.
With its channels shut, Na⁺ has no route.
Check the box for each point your answer earns
Accept: "ions cross only through channels, and the sodium channels are closed", provided the reason (the charge on the ion and the uncharged interior) is given.
Common slip: Saying the sodium channels are closed, with no reason why Na⁺ needs a channel at all. The charge on the ion and the uncharged interior are the reason.
(c) Make a claim about how the drug changes the movement of K⁺ when the potassium channels open, and support your claim. (1 point)
A full-credit answer: K⁺ still leaves the cell, in the same direction as before.
The gradient sets the direction, and the drug does not change the gradient.
Each open channel is one route.
The drug doubles the number of open channels.
Therefore K⁺ leaves faster.
Check the box for each point your answer earns
Accept: "more K⁺ leaves per second" or "K⁺ leaves about twice as fast" with the reason. Do not award the claim alone, a change of direction, or "the cell now needs energy".
Common slip: Changing the direction, or saying the cell now needs energy. More doors speed a crossing; the gradient sets its direction.
(d) The cell is then bathed in a solution holding K⁺ at 180 mmol/L, equal to the inside, with the drug still present and the channels open. Considering only the concentrations (set aside any charge across the membrane), predict the net movement of K⁺ and justify your prediction. (1 point)
A full-credit answer: There is no net movement of K⁺.
K⁺ is 180 mmol/L on both sides.
So K⁺ ions cross the channels in both directions in equal numbers.
That is a dynamic equilibrium.
A channel provides only the route.
With no gradient there is no net direction, however many channels are open.
Check the box for each point your answer earns
Accept: "K⁺ still crosses both ways but the two flows cancel". Do not award the point for "K⁺ stops moving" or for "K⁺ still leaves because the drug opened more channels".
Accept also: an answer that gives 'no net movement by concentration' and adds that a negative inside would pull a little K⁺ inward; the concentration argument earns the point and the charge remark is not penalized.
Common slip: Saying K⁺ stops moving, or that it still leaves because the drug opened more channels. Ions still cross both ways; the two flows cancel.
(a) For each of O₂, K⁺ and glucose, state which route it takes across this membrane. (1 point)
A full-credit answer: O₂ takes route 1, straight through the bilayer, because O₂ is a small nonpolar molecule; K⁺ takes route 2, the channel, because K⁺ is an ion; glucose takes route 3, the carrier, because glucose is a large polar molecule that a carrier binds.
Check the box for each point your answer earns
All three are needed for the point. No reason is required for the point, but a reason given must not contradict the route.
Common slip: Giving glucose the channel or K⁺ the carrier. A channel is a tunnel for one kind of ion; a carrier binds a molecule and changes shape.
(b) State the net direction of K⁺ and of glucose across this membrane, using the concentrations in the table. (1 point)
A full-credit answer: K⁺ is 150 mmol/L in the cytosol and 4 mmol/L in the extracellular fluid.
So the net direction of K⁺ is out of the cell.
Glucose is 6 mmol/L in the extracellular fluid and 1 mmol/L in the cytosol.
So the net direction of glucose is into the cell.
Check the box for each point your answer earns
Both directions must be correct for the point; K⁺ given as moving inward earns nothing.
Common slip: Sending K⁺ inward because most things seem to come in. Read the table: K⁺ is higher in the cytosol, so its net movement is out.
(c) Explain what decides whether a substance can use route 1, and why a crossing through route 2 or route 3 still needs no energy from the cell. (1 point)
A full-credit answer: The hydrocarbon tails in the middle of the membrane carry no charges or partial charges.
K⁺ is an ion and glucose is a polar molecule; water is attracted to both, and nothing in the tails attracts either.
So neither can cross the hydrophobic interior on its own, and neither can use route 1.
Through a channel or a carrier, each moves down its own concentration gradient.
Movement down a gradient happens by itself, so the cell uses no energy: facilitated diffusion.
Check the box for each point your answer earns
Accept: "charged or polar things cannot enter the oily middle; the proteins give a route, and movement down a gradient needs no energy". Both halves are needed for the point.
Common slip: Explaining why they need a protein but not why the crossing needs no energy, or the other way around. Both halves are needed.
(d) Kidney cells have many aquaporins in their membranes. Describe what an aquaporin is in terms of this model, and what it does for the cell. (1 point)
A full-credit answer: An aquaporin is a route-2 protein: a channel for water.
Water crosses the bilayer, route 1, only slowly on its own.
Through aquaporins, large quantities of water cross the membrane quickly.
Water moves down its own gradient, with no energy used by the cell.
Check the box for each point your answer earns
Accept: "a water channel; it speeds water's crossing greatly". Do not award the point for "it pumps water" or for calling it a carrier.
Common slip: Saying an aquaporin pumps water, or calling it a carrier. It is an open channel; water moves through it only the way it would move anyway.