Unit 2 · Topic 2.4 end-of-topic test
Sealed bubbles of pure bilayer (liposomes), with no proteins in them, float in a solution. At the start each substance measures 10 units outside the bubbles and 0 inside. After ten minutes: methane (CH₄) 7 outside, 3 inside; sodium ions (Na⁺) 10 outside, 0 inside; chloride ions (Cl⁻) 10 outside, 0 inside; glucose 10 outside, 0 inside.
Which substance crossed the bilayer, and what kind of substance is it?
Oxygen molecules (O₂) and sodium ions (Na⁺) are both present outside a muscle cell, and both are tiny. A sodium ion is in fact smaller than an oxygen molecule.
Which crosses the bare bilayer on its own, and why?
Four substances are tested one at a time against a protein-free bilayer: nitrogen gas (N₂); ammonia (NH₃), a small polar molecule with no charge; potassium ions (K⁺); and sucrose, a large sugar with many –OH groups.
Which one passes through in small amounts only?
Two substances of about the same small size are put outside a protein-free bubble of bilayer in equal amounts. Substance X is nonpolar. Substance Y carries a full charge. After ten minutes, X inside has reached 40% of its outside level; Y inside is below 1%.
Why did X get in while Y stayed out?
A student explains: "The membrane blocks sodium ions because they are too big to fit through the oily middle."
Which statement corrects the student?
Two proteins in a red blood cell's membrane let through substances that the bare bilayer blocks. When protein P is open, chloride ions stream through it, millions every second, without stopping. Protein Q takes glucose one molecule at a time, holding each one for a moment before it appears on the other side.
What kinds of protein are P and Q?
Yeast cells are given equal amounts of glucose and of molecule N, a small nonpolar molecule. Glucose inside the cells rises by 12 units every two minutes. A drug is then added that blocks one glucose-binding protein in the membrane; glucose now rises by only 2 units every two minutes. Molecule N rises by 9 units every two minutes both before and after the drug.
What do the results show?
Before an experiment, a student predicts: "Anything with no charge crosses a bare bilayer freely, whatever its size." Protein-free bubbles of bilayer are placed in four solutions. After ten minutes, the amount inside as a fraction of the amount outside: carbon dioxide (CO₂) 100%; water (H₂O) 12%; glucose (no charge, six –OH groups) 0%; chloride ions (Cl⁻) 0%.
Do the data support the student's prediction?
Using the same protein-free bubbles, a researcher tests a new molecule: it is about the size of glucose, and it is polar, with many –OH groups but no charge.
What should the data show after ten minutes, and would a real cell need a protein to take it in?
Four cells are examined: a bacterium, a yeast cell (a fungus), a cell from a leaf, and a human cheek cell. Each has a plasma membrane.
Which of them also have a cell wall outside the membrane?
To test whether the cell wall decides what enters a root cell, researchers strip the wall from some cells, leaving the plasma membrane intact. Equal amounts of two dissolved substances, P and Q, are supplied to both groups. After ten minutes, cells with a wall hold 72% of the P supplied and 4% of the Q; cells with no wall hold 70% of the P and 3% of the Q.
What do the results show?
A fresh celery stalk snaps when it is bent. The same stalk left in dry air overnight bends without snapping. Nothing has entered or left the stalk but water.
Why was the fresh stalk stiff?
A plant cell is placed in pure water, a hypotonic surrounding. Water enters by osmosis and the contents swell.
What is happening at the cell wall, and what is the cell now called?
Bacteria in a very dilute solution take in water but do not burst. A drug that stops the bacteria from building their cell wall is added to bacteria growing in such a solution.
What happens to the bacteria, and why?
A red blood cell and a plant cell are both placed in pure water. The red blood cell swells and bursts. The plant cell swells a little and becomes firm.
Why does the plant cell stay whole?
Onion skin cells were drawn in two different solutions. In one drawing the wall and the membrane lie together; in the other the wall keeps its shape while the contents have pulled away from it. The wall measures 80 μm across in both drawings.
Which drawing shows a cell in concentrated salt water, and what happened to it?
Onion skin cells in concentrated salt water have their contents shrunk away from their walls, and the piece of skin is limp. The cells are then moved into fresh water.
Predict what happens.
(a) Describe how readily each of the four numbered substances crosses the bilayer itself, on its own, away from proteins X and Y. (1 point)
A full-credit answer: O₂ (1) passes freely through the bilayer; H₂O (2) passes in small amounts; Na⁺ (3) and glucose (4) do not cross the bilayer on their own.
Check the box for each point your answer earns
Accept: "a little", "slowly" or "only some" for water. All four must be placed correctly for the point.
Common slip: Letting Na⁺ through because it is small, or blocking water entirely. Small polar molecules with no charge trickle through; ions and large polar molecules do not.
(b) Explain, using the structure of the interior of the membrane, why O₂ crosses the bilayer on its own while Na⁺ stays in the water. (1 point)
A full-credit answer: The hydrocarbon tails in the middle of the membrane carry no charges or partial charges. Na⁺ carries a full charge that water holds on to, and nothing in the interior can hold it, so it cannot cross on its own. O₂ has no charge or partial charge for water to hold, so it dissolves into the tails and passes out the other side.
Check the box for each point your answer earns
Accept: "the interior is hydrophobic (oily), so the charged ion is kept in the water while the nonpolar O₂ dissolves through" provided the answer mentions the charge on Na⁺ and the uncharged tails. Do not award the point for "Na⁺ is too big" or for "oxygen is smaller".
Common slip: Saying Na⁺ is too big or oxygen is smaller. Na⁺ is smaller than O₂; its charge is what keeps it out.
(c) On paper, draw the path by which glucose enters the cell and the path by which Na⁺ enters, and name the kind of protein each passes through. (1 point)
A full-credit answer: Glucose passes through Y, a carrier protein, which binds glucose and changes shape to move it across. Na⁺ passes through X, a channel protein, an open water-lined tunnel. Neither passes between the phospholipids.
Check the box for each point your answer earns
Accept: "transport protein" for Y. Do not award the point if either substance is drawn crossing the bilayer between the phospholipids, or if the two proteins are named the wrong way around.
Common slip: Drawing either substance slipping between the phospholipids, or naming the two proteins the wrong way around. A tunnel is a channel; a pocket that binds is a carrier.
(d) Explain how what the model shows lets a cell keep the solution inside it different from the solution outside. (1 point)
A full-credit answer: The bilayer itself lets only small nonpolar molecules through, and a little of small polar ones, and holds back ions and large polar molecules, so those cross only through the proteins the cell has put in its membrane. The membrane is selectively permeable, so the cell controls what enters and leaves, and the cytosol can be kept different from the extracellular fluid in its amounts of salt and sugar.
Check the box for each point your answer earns
Accept: "the membrane lets some substances through and holds others back" (the membrane is selectively permeable) together with a link to the cell controlling its inside. Do not award the point for "nothing crosses the membrane" or for "the proteins block everything".
Common slip: Saying nothing crosses, or that the proteins block everything. The membrane lets some substances through and holds others back; that selectivity is what the cell uses.
(a) Describe how the hydrophobic interior of a bilayer decides which substances cross it on their own. (1 point)
A full-credit answer: The hydrocarbon tails in the interior carry no charges or partial charges. A small nonpolar molecule dissolves into them and passes through, while an ion or a polar molecule, which water holds on to, has nothing to hold it in the interior and does not cross on its own; small polar molecules with no charge get through in small amounts. That is what makes the bilayer selectively permeable.
Check the box for each point your answer earns
Accept: "nonpolar things dissolve in the oily middle; charged and polar things are held by the water" provided both halves are given. Do not award the point for "small things cross and big things do not".
Common slip: Saying small things cross and big things do not. Size is not the sorter; charge and polarity are.
(b) Explain why the class made the bubbles with no proteins in them. (1 point)
A full-credit answer: So that anything found inside a bubble must have crossed the bilayer itself and could not have passed through a channel or carrier protein. The experiment tests the bilayer alone.
Check the box for each point your answer earns
Accept: "to remove the protein doors so only the bilayer is tested". Do not award the point for "because proteins are hard to make" or "to keep the bubbles small".
Common slip: Saying proteins are hard to make or would change the bubbles’ size. The reason is about what the experiment tests.
(c) State whether the data support or contradict the student's prediction about Na⁺, and predict the result of a fifth trial with potassium ions (K⁺). (1 point)
A full-credit answer: The data contradict the prediction: Na⁺ was at 0% inside while O₂ reached 100%. K⁺ in a fifth trial would also be at about 0% inside.
Check the box for each point your answer earns
Accept: "K⁺ does not cross" for the prediction. Both the judgment on the prediction and the K⁺ prediction are needed for the point.
Common slip: Giving the judgment on Na⁺ without the K⁺ prediction, or the other way around. Both are needed for the point.
(d) Justify your prediction for K⁺ using the structure of the bilayer. (1 point)
A full-credit answer: K⁺ carries a full charge, which water holds on to. The hydrocarbon tails in the interior carry no charges or partial charges, so nothing there can hold the ion, and it stays in the water outside. Its small size does not help: Na⁺ is also small and did not cross. In a real cell K⁺ would enter only through a channel protein.
Check the box for each point your answer earns
Accept: an answer that adds that in a real cell K⁺ would enter only through a channel protein. Do not award the point for "K⁺ is too big" or for "ions are blocked" with no reference to charge and the uncharged tails.
Common slip: Saying K⁺ is too big, or that ions are blocked, with no reference to the charge and the uncharged tails.