Practice questions · Topic 2.4

Unit 2 · Practice for the Topic 2.4 end-of-topic test

You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.

Watch first: membrane permeability, summed up

From the bubble of pure membrane to its hydrophobic interior and the two protein doors: what crosses, and why.

Watch next: the cell wall and water, summed up

Which cells have a cell wall, turgid and plasmolyzed, why a walled cell does not burst, micrometers and the central vacuole.

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

These practice questions have the shape of the Topic 2.4 test. Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. For the first free-response question, you work through one scenario in small steps, and each step offers a hint if you want one. For the second, write one short sentence for each step of your reasoning, each on its own line, and make every link clear (so, because, therefore). That is what the exam’s ‘paragraph form’ means for you: linked sentences, not bullet points. Then open the scoring guide and mark your own work against it.

Question 1

A sheet of pure phospholipid bilayer, with no proteins in it, separates two chambers of water. One chamber receives each substance in turn, at 10 mmol/L. Nitric oxide (NO), a small nonpolar gas, appears in the other chamber within seconds. Calcium ions (Ca²⁺) and lactose, a large sugar with many –OH groups, have still to appear after an hour.

Which of these substances would also cross the bilayer freely on its own?

A. Ethylene (C₂H₄), a hydrocarbon with two carbon atoms B. Potassium ions (K⁺), which are smaller than lactose C. Sucrose, a sugar with many –OH groups D. Phosphate ions (PO₄³⁻), which are smaller than sucrose

Question 2

Protein-free bubbles of bilayer are floated in a solution of three substances: nitric oxide (NO), a small nonpolar gas; urea, a small polar molecule with no charge; and potassium ions (K⁺). Each substance is at the same concentration outside the bubbles, and none is inside at the start. After ten minutes the amount of each inside, as a percentage of the amount outside, is measured: one substance reached 100%, one reached 9% and one stayed at 0%.

Which set of results fits the three substances?

A. Nitric oxide 0%; urea 9%; potassium ions 100% B. Nitric oxide 9%; urea 100%; potassium ions 0% C. Nitric oxide 100%; urea 9%; potassium ions 0% D. Nitric oxide 100%; urea 0%; potassium ions 9%

Question 3

Methane (CH₄) and methanol (CH₃OH) are molecules of nearly the same size. Methane is nonpolar. Methanol is polar, with an –OH group, but carries no charge. Outside protein-free bubbles of phospholipid bilayer, methane reaches 100% of its outside level inside within a minute, while methanol reaches 20%. A student explains: "Methanol crosses less because methanol is the bigger molecule."

Which statement corrects the student?

A. Nothing needs correcting: the bigger of two molecules always crosses less B. Methanol is no bigger than methane: water attracts methanol's –OH group, and nothing in the tails does C. Methanol crosses less because the phospholipid heads repel its –OH group D. Methanol crosses less because it dissolves into the tails and then moves through them more slowly

Question 4

Three substances are tested, one at a time, outside protein-free bubbles of phospholipid bilayer. Ethylene, a small nonpolar molecule, reaches 100% of its outside level inside within a minute. Propane, a nonpolar molecule larger than ethylene, also reaches 100%. The fluoride ion (F⁻), which is smaller than an ethylene molecule, stays at 0%.

Which conclusion do the three results support?

A. Size decides: the smallest particle crosses first, so the F⁻ reading of 0% must be a mistake in the measurement B. Size decides: F⁻ is too small for the tails to catch and carry across, while both nonpolar molecules are big enough to be carried C. The heads decide: the heads pull F⁻ into the water outside and push the two nonpolar molecules through the middle of the membrane D. Charge decides: water attracts F⁻ and nothing in the tails does, while both nonpolar molecules dissolve into the tails whatever their size

Question 5

Two proteins in a nerve cell's membrane let through substances that the phospholipid bilayer itself blocks. When protein A is open, potassium ions (K⁺) stream through it, thousands every millisecond, with no pause. Protein B binds one amino acid molecule at a time, changes shape, and releases the molecule on the other side.

What kinds of protein are A and B?

A. Both carrier proteins; A binds K⁺ more loosely B. A is a channel protein; B is a carrier protein C. A is a carrier protein; B is a channel protein D. Both channel proteins; B is a narrower one

Question 6

A student predicts that a new drug molecule, which is small and nonpolar, will need a carrier protein to get into cells. The class tests it on protein-free bubbles of bilayer: after five minutes the drug inside the bubbles is at 95% of its level outside.

Do the data support the student's prediction?

A. Yes: the drug reached only 95%, so a carrier would be needed for the last 5% B. Yes: small molecules always need a carrier, and the bubbles are a special case C. No: the drug crossed a phospholipid bilayer on its own, so a cell needs no carrier for it D. The data cannot test the prediction, since the bubbles have no proteins

Question 7

Five organisms are examined: a mushroom, which is a fungus; a frog; an oak tree; a bacterium from yogurt; and an archaean, a single-celled organism with no nucleus, from a hot spring. Every cell of each has a plasma membrane.

Which of these organisms have cells with a cell wall outside the plasma membrane?

A. The oak tree only B. The oak tree and the mushroom C. All five, the frog included D. All but the frog

Question 8

A wilted lettuce leaf is put into a bowl of cold water for twenty minutes and comes out crisp. Only water has entered the leaf.

What made the leaf crisp again?

A. Water filled the spaces between the cells and glued the cells to one another B. Water entered the cells; the swollen contents pressed on the cell walls, which pressed back C. The cold water stiffened the cell walls, which had gone soft in the warm air D. Water dissolved the cell walls, which then set again in a firmer, stiffer form

Question 9

Pond algae, whose cells have cell walls, live in nearly pure water without harm. Red blood cells, which have no cell wall, put into the same water swell and burst within minutes.

Why do the algae survive in the water that bursts the red blood cells?

A. The algae's cell walls keep water from entering their cells at all B. The algae's membranes are far stronger than a red blood cell's C. Water enters the algae's cells too, but their cell walls stop the swelling D. The water is hypotonic to a red blood cell but isotonic to an alga

Question 10

Slices of eggplant are sprinkled with salt. Within half an hour the slices are limp and sitting in liquid.

What has happened to the cells of the slices?

A. Salt entered the cells and dissolved part of each cell wall B. Water entered the cells, and the cell walls stretched away from the contents C. The cell walls shrank around the contents, squeezing liquid out D. Water left the cells toward the salty surface, and the contents shrank from the cell walls

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Scientific Investigation · 5 points

A class floats protein-free bubbles of bilayer in a solution of four substances, each at the same concentration outside the bubbles and none inside: ethylene (C₂H₄), a small nonpolar gas; hydrogen peroxide (H₂O₂), a small polar molecule with no charge; calcium ions (Ca²⁺), which are smaller than an ethylene molecule; and lactose, a large sugar with –OH groups all over it. After ten minutes the amount of each inside the bubbles, as a percentage of the amount outside, is: ethylene 100%; hydrogen peroxide 8%; Ca²⁺ 0%; lactose 0%.

(a) Identify the substance that crossed the bilayer freely and the substance that crossed in small amounts. (1 point)

Hint: What percentage would a substance reach if it crossed until it was as concentrated inside as outside? Which value fits neither that nor zero?

A full-credit answer: The substance that crossed freely is ethylene, at 100%: after ten minutes ethylene was as concentrated inside as outside.
The one that crossed in small amounts is hydrogen peroxide, at 8%: hydrogen peroxide reached 8% of its outside level.

Check the box for each point your answer earns

ethylene crossed freely (100%); hydrogen peroxide crossed in small amounts (8%).

Accept: the two named with their percentages in either order. Do not award the point if 8% is read as no crossing.

Common slip: Reading 8% as none. Small polar molecules with no charge trickle through a phospholipid bilayer; 8% is a small crossing, and 0% is the value that says none.

(b) Describe the interior of the bilayer: what it is made of and what it lacks. (1 point)

Hint: Think about which part of each phospholipid ends up in the middle, and what kind of chemical groups it has and has not got.

A full-credit answer: The interior of the bilayer is made of the hydrocarbon tails of the phospholipids, the tails of both layers touching in the middle, which carry no charges or partial charges.
So nothing in the interior attracts a polar or charged particle.
This band of tails is the hydrophobic interior.

Check the box for each point your answer earns

the interior is the hydrocarbon tails of the phospholipids of both layers, touching in the middle; the tails carry no charges or partial charges (nothing that attracts a polar or charged particle), so the interior is hydrophobic.

Accept: "the oily middle, made of tails with no charges".

Common slip: Describing the heads instead of the tails. The heads face the water on each side; the interior, which decides what crosses, is the tails.

(c) Explain why the calcium percentage is 0% although a calcium ion is smaller than an ethylene molecule. (1 point)

Hint: Compare Ca²⁺ with ethylene property by property: size, polarity, charge. Then use what you said in (b) the interior lacks.

A full-credit answer: Ca²⁺ carries a full +2 charge, so water attracts Ca²⁺ strongly.
The interior has hydrocarbon tails with no charges or partial charges, so nothing in the interior attracts Ca²⁺.
So Ca²⁺ cannot leave the water on its own side.
Ethylene, by contrast, has nothing that attracts water.
So ethylene dissolves into the tails and passes through.
Charge decides, not size: the smaller particle is the one that stayed out.

Check the box for each point your answer earns

Ca²⁺ carries a full +2 charge, so water attracts Ca²⁺, and the hydrocarbon interior has no charges or partial charges that attract Ca²⁺, so Ca²⁺ cannot leave the water to cross; nothing in ethylene attracts water, so ethylene dissolves into the tails. Charge, not size, decides.

Accept: "water keeps the ion; the oily middle gives it nothing" with the contrast to ethylene stated or implied.

Common slip: Saying Ca²⁺ is blocked because it is too big. Ca²⁺ is the smaller of the two; water attracts its charge, and that attraction is what keeps Ca²⁺ out.

(d) Explain why the lactose percentage is also 0%. (1 point)

Hint: Hydrogen peroxide is polar too, and a little of it got through. List the ways lactose differs from hydrogen peroxide, and think about what water does to each polar group.

A full-credit answer: Lactose is large and polar; its –OH groups carry partial charges, so water attracts every one of them.
Nothing in the hydrocarbon interior attracts lactose.
So lactose stays in the water.
Hydrogen peroxide is polar too, but hydrogen peroxide is small, so a little of it slips through.
Lactose is both polar and large.
So none of the lactose gets through.

Check the box for each point your answer earns

lactose is a large polar molecule; water's partial charges attract every one of its –OH groups, and nothing in the interior attracts them, so lactose stays in the water; being large as well as polar, it does not even trickle through as the small polar hydrogen peroxide does.

Accept: "large and polar: water attracts it all over and the tails attract none of it".

Common slip: Saying lactose stayed out only because it is large. Size matters among polar molecules; what keeps lactose in the water is that water attracts its many –OH groups.

(e) Make a claim about how a living gut cell takes in Ca²⁺, and support your claim with the bubble result. (1 point)

Hint: Compare the bubbles with a living gut cell’s plasma membrane. Ask what the cell’s membrane has in it that the bubbles lack, and whether that could give Ca²⁺ a way in.

A full-credit answer: A living gut cell takes in Ca²⁺ through a membrane protein, a channel protein or a carrier protein.
Ca²⁺ reached 0% inside the protein-free bubbles.
So Ca²⁺ cannot cross the bilayer itself.
A channel protein is a water-lined tunnel that lets the ion through.
A carrier protein binds the ion and changes shape to move it across.
Therefore a protein set into the membrane is the only way in.

Check the box for each point your answer earns

the claim that Ca²⁺ enters through a membrane protein, a channel (a water-lined tunnel for the ion) or a carrier, supported by the evidence AND the reasoning: Ca²⁺ reached 0% inside the protein-free bubbles, so Ca²⁺ cannot cross the bilayer on its own, so a protein door is the only way in.

Accept: "through a channel protein" or "through a carrier protein" with the 0% result and the reason that the bilayer blocks ions. Do not award the claim alone.

Common slip: Making the claim with no route named, or naming the route with no evidence from the bubbles. The point needs the protein door, the 0% result and the link.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

A florist keeps cut tulip stems standing in tap water, and the stems stay stiff and upright. A student moves one stem into a strong sugar solution and leaves it there for an hour.

(a) Describe what the cell wall does for a plant cell and what the plasma membrane inside it does. (1 point)

A full-credit answer: The cell wall is a rigid boundary outside the plasma membrane and gives the cell its shape; the cell wall also holds back some large substances.
The plasma membrane inside it does the selecting: the plasma membrane decides which substances enter and leave the cell.

Check the box for each point your answer earns

the cell wall, outside the plasma membrane, is a rigid boundary that gives the cell its shape and holds back some substances; the plasma membrane inside it is what decides which substances enter and leave (does the selecting).

Accept: "the cell wall holds the shape; the membrane selects".

Common slip: Giving the cell wall the job of deciding what enters. The cell wall holds back only some large substances; the membrane inside it is the selector.

(b) Explain why the stem in tap water is stiff. (1 point)

A full-credit answer: Tap water is hypotonic to the stem's cells.
So water enters the cells by osmosis.
The swelling contents press outward on each cell wall.
The cell wall presses back.
So every cell is firm, or turgid.
That outward push against the cell wall is turgor pressure.
Turgor pressure is what holds the soft stem stiff.

Check the box for each point your answer earns

tap water is hypotonic to the stem's cells, so water enters them by osmosis; the swelling contents press outward on each cell wall and the cell wall presses back, making each cell turgid; this turgor pressure holds the soft stem stiff.

Accept: "water enters, the contents push on the cell wall, the cell wall pushes back: turgor".

Common slip: Saying the cell walls are what make the stem stiff. A cell wall with shrunken contents inside it gives a limp stem; it is the pressure of water-filled contents against the cell walls that stiffens it.

(c) Make a claim about what happens to the cells' contents and to the stem during the hour in the strong sugar solution. (1 point)

A full-credit answer: Water leaves the cells.
So the membrane and the contents shrink away from each cell wall.
The cell wall keeps its shape.
This is plasmolysis.
Nothing presses on the cell walls, so turgor is lost.
Therefore the stem droops.

Check the box for each point your answer earns

the claim that water leaves the cells, the membrane and contents shrink away from the cell wall (plasmolysis) while the cell wall keeps its shape, turgor is lost and the stem droops. No reasoning is required for this point.

Accept: water moving out with the loss of turgor and the stem going limp. Do not award the point for sugar entering the cells or for the cells swelling.

Common slip: Having the sugar enter the cells, or the cells swell. Sugar is a large polar molecule the membrane blocks; only water moves, and it moves out toward the sugar.

(d) Support the claim you made in (c), using what the cell wall and the plasma membrane each let through. (1 point)

A full-credit answer: The plasma membrane lets water through and blocks sugar, a large polar molecule.
The strong sugar solution is hypertonic to the cells.
So water crosses the membrane outward by osmosis.
The cell wall is rigid, so it keeps its shape whether or not the contents press on it.
The cell wall lets water pass freely.
Therefore the membrane and the contents shrink inward, away from a cell wall that stays where it was.

Check the box for each point your answer earns

the evidence AND the reasoning: the plasma membrane lets water through but blocks sugar, a large polar molecule, so with more dissolved sugar outside than inside (a hypertonic surrounding) water crosses the membrane outward by osmosis; the cell wall is a rigid boundary that holds its shape and lets water pass freely, so the membrane and contents shrink away from it.

Accept: "the membrane blocks sugar and passes water, so water leaves by osmosis; the cell wall is rigid and lets water through, so it keeps its shape while the contents shrink". Do not award the point for support that gives the cell wall the job of selecting.

Common slip: Giving the cell wall the job of keeping water in, or having it shrink with the cell. The cell wall is rigid and lets water pass; the membrane blocks the sugar and lets the water out.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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