Unit 2 · Topic 2.4 end-of-topic test
Suggested time: about 44 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.
Sealed bubbles of phospholipid bilayer with no proteins in them float in a solution of four substances. At the start each substance is at 10 mmol/L outside the bubbles and 0 mmol/L inside. After ten minutes the concentration inside the bubbles is: propane (C₃H₈), a hydrocarbon, 7 mmol/L; calcium ions (Ca²⁺) 0 mmol/L; nitrate ions (NO₃⁻) 0 mmol/L; fructose, a sugar, 0 mmol/L.
Which substance crossed the bilayer?
Carbon dioxide molecules (CO₂) and potassium ions (K⁺) are both present in the blood outside a liver cell, and both are tiny. A bare potassium ion is smaller than a carbon dioxide molecule.
Which of the two crosses the phospholipid 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?
Four substances are dissolved in the fluid around a liver cell.
Which of the following can cross the cell's plasma membrane only through a membrane protein?
A student explains: "The membrane blocks glucose because glucose carries a charge, and water attracts charged particles."
Which statement corrects the student?
Two proteins in a muscle cell's membrane let through substances that the phospholipid bilayer itself blocks. When protein 1 is open, calcium ions (Ca²⁺) stream through it, millions every second, without stopping. Protein 2 takes lactate, a substance a hard-working muscle makes, one molecule at a time, holding each molecule for a moment before that molecule appears on the other side.
What kinds of protein are protein 1 and protein 2?
Liver cells sit in a solution holding equal amounts of fructose, a sugar, and of molecule Z, a small nonpolar molecule. Fructose inside the cells rises by 3.0 mmol/L every five minutes. Then a drug that blocks one membrane protein reaches the cells; fructose now rises by only 0.5 mmol/L every five minutes. Molecule Z rises by 2.0 mmol/L every five minutes both before and after the drug.
What do the results show?
Protein-free bubbles of bilayer are placed in a solution of a new molecule. The molecule 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 the molecule in?
Before an experiment, a student predicts: "Anything with no charge crosses a phospholipid 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?
Four cells are examined: a cell of a mold (a fungus) growing on bread, a bacterium from soil, a cell from a pine needle, and a cell lining a dog's gut. Each cell has a plasma membrane.
Which of them also have a cell wall outside the membrane?
Cell walls are separated from ground-up leaf tissue, with no membrane attached to them, and placed in a solution of two dissolved substances, glucose and a blue dye. Both substances pass through the isolated cell walls freely. Intact cells of the same leaf, in the same solution, take in the glucose and hold the dye out.
Which boundary decides what enters the intact cells?
A fresh asparagus spear snaps when it is bent. The same spear left in dry air overnight bends without snapping. Nothing has entered or left the spear but water.
Why was the fresh spear stiff?
A root hair cell of a radish seedling is placed in pure water, a hypotonic surrounding.
What happens at the cell wall, and what is the cell then called?
Leaf cells have their cell walls digested away by enzymes, leaving the plasma membrane intact, in a solution that matches the cells' contents. The cells, now with no cell wall, are then moved into pure water.
What happens to these cells with no cell wall, and why?
Onion skin cells were drawn in two different solutions, one cell in each drawing. The cell wall measures 80 μm across in both drawings.
Which drawing shows a cell in concentrated salt water, and what happened to it?
Red cabbage leaf cells in concentrated salt water have their contents shrunk away from their cell walls, and the piece of leaf is limp. The cells are then moved into fresh water.
Predict what happens.
(a) Describe how readily each of the four substances shown, oxygen, water, sodium ions and glucose, crosses the bilayer itself, on its own, away from proteins X and Y. (1 point)
A full-credit answer: O₂ passes freely through the bilayer.
H₂O passes in small amounts.
Na⁺ and glucose 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) Identify which of the four substances passes through protein X and which passes through protein Y, and name the kind of protein each is. (1 point)
A full-credit answer: Glucose passes through Y.
Y is a carrier protein: Y binds glucose and changes shape to move glucose across.
Na⁺ passes through X.
X is a channel protein: an open water-lined tunnel.
Neither substance 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 said to cross the bilayer between the phospholipids, or if the two proteins are named the wrong way around.
Common slip: Sending either substance between the phospholipids, or naming the two proteins the wrong way around. A tunnel is a channel; a pocket that binds is a carrier.
(c) Explain, using the structure of the interior of the membrane, what decides whether a dissolved substance crosses the bilayer on its own. (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, so water attracts Na⁺.
Nothing in the interior attracts Na⁺.
So Na⁺ cannot cross on its own.
O₂ has no charge or partial charge, so nothing in O₂ attracts water.
So O₂ 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 charged and polar particles are kept in the water while nonpolar molecules dissolve through" provided the answer names the uncharged tails and what water attracts. Do not award the point for "small things cross and big things do not".
Common slip: Saying size decides. Na⁺ is smaller than O₂ and still stays out; its charge is what keeps it in the water.
(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 freely, and a little of small polar molecules.
The bilayer holds back ions and large polar molecules.
So ions and large polar molecules cross only through the proteins the cell has put in its membrane.
So the membrane is selectively permeable.
So the cell controls what enters and leaves.
So the cytosol can be kept different from the extracellular fluid, for example 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) State what the protein-free bubble result shows about the route the nonpolar drug takes into liver cells, and explain why the result from the living cells alone leaves that route open. (1 point)
A full-credit answer: The bubble result shows that the nonpolar drug crosses the bilayer itself, with no protein: the bubbles are pure bilayer, and the nonpolar drug reached 95% inside them, the same as in the cells.
A living cell’s membrane has proteins in it as well as the bilayer.
So a molecule found inside a living cell could have come through the bilayer or through a channel or carrier.
The bubble result rules the proteins out.
Check the box for each point your answer earns
Accept: "the bubbles are a control for the proteins". The route and the reason the control was needed are both required.
Common slip: Saying the bubble result shows only that the nonpolar drug "can cross". The point is what it rules out: any protein route.
(b) Explain, using the structure of the hydrophobic interior of the bilayer, why the charged drug stays out of the liver cells while the nonpolar drug gets in. (1 point)
A full-credit answer: The hydrocarbon tails in the interior carry no charges or partial charges.
The nonpolar drug is a small nonpolar molecule, so the nonpolar drug dissolves into the tails and passes through to the other side.
The charged drug carries a full charge, which water attracts, so water keeps the charged drug in the water outside the cell.
Nothing in the tails attracts a charged group.
So the charged drug stays out.
Check the box for each point your answer earns
Accept: "the interior is hydrophobic, so the charged drug cannot enter it", provided the answer says what water does to the charged group. Both drugs must be explained for the point.
Common slip: Saying the charged drug is blocked because it is bigger. The added group makes it only slightly larger; the charge is what water attracts.
(c) The company then makes the charged drug half its original size, keeping the charged group. Make a claim about whether the smaller charged drug enters the liver cells, and support your claim. (1 point)
A full-credit answer: The smaller charged drug still does not enter.
The charged drug still carries a full charge.
Water still attracts that charge.
The hydrocarbon tails still carry no charges or partial charges, so nothing in the tails attracts the molecule.
Size is not what kept the charged drug out.
Therefore shrinking it changes nothing.
Check the box for each point your answer earns
Accept a claim of "a very small amount" if the support says the charge is what decides. Do not award the claim alone, or "it enters because it is now small enough".
Common slip: Predicting that a smaller molecule slips through. A bare Na⁺ ion is smaller than an O₂ molecule and still does not cross; the charge decides.
(d) The company then finds that the charged drug does enter kidney cells, reaching 60% in ten minutes. Make a claim about what the kidney cells’ membrane has that the liver cells’ membrane lacks, and describe one test of your claim. (1 point)
A full-credit answer: The kidney cells’ membrane has a protein that the charged drug can pass through, a channel or a carrier that fits the charged molecule; the liver cells’ membrane has none.
Test: make protein-free bubbles from the kidney cells’ lipids and supply the charged drug.
If the charged drug stays out of the bubbles but enters the kidney cells, a protein is the route.
Or block the proposed protein with a drug: if uptake falls, that protein was the route.
Check the box for each point your answer earns
Accept any test that isolates the protein from the bilayer (bubbles with and without kidney-cell membrane proteins; a drug that blocks one protein). A claim without a test, or a test without a claim, does not earn the point.
Common slip: Claiming that kidney cells have a "weaker" or "thinner" bilayer. A bilayer is a bilayer; only a protein can give a charged molecule a way through.