Practice questions · Topic 2.4

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

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 tells you what a wrong choice assumed. 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 your answer in full sentences as you would in the test, 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. Substances are added to one chamber, one at a time, each at 10 mmol/L (thousandths of a mol/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 small nonpolar gas B. Potassium ions (K⁺), which are smaller than lactose C. Sucrose, a large sugar with many –OH groups D. Phosphate ions (PO₄³⁻), which are small and charged

Question 2

Protein-free bubbles of bilayer are floated in a solution of three substances, each at the same concentration outside the bubbles and none inside. After ten minutes the amount inside, as a percentage of the amount outside, is: substance 1, 100%; substance 2, 9%; substance 3, 0%.

Which set of identities fits the three results?

A. 1 chloride ions; 2 ethylene, a small nonpolar gas; 3 hydrogen peroxide, a small polar molecule with no charge B. 1 hydrogen peroxide; 2 chloride ions; 3 ethylene C. 1 ethylene; 2 hydrogen peroxide; 3 chloride ions D. 1 ethylene; 2 chloride ions; 3 hydrogen peroxide

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. Put outside protein-free bubbles of bilayer, methane reaches 100% of its outside level inside within a minute, while methanol reaches 20%.

Why does methane cross freely while methanol crosses only a little?

A. Methanol is the larger molecule, so fewer of its molecules fit between the tails B. Water holds methanol's –OH group, which the tails have nothing to hold C. The phospholipid heads repel methanol's –OH group and let methane pass D. Methanol dissolves into the tails as easily as methane but moves more slowly

Question 4

Ethylene, a small nonpolar molecule, crosses a protein-free bilayer freely. The fluoride ion (F⁻), which is smaller than an ethylene molecule, is blocked completely.

What explains both results?

A. Size decides: ethylene is small enough to slip between the tails and F⁻ is too large B. F⁻ is blocked because it is heavier than ethylene and sinks in the water C. The heads pull F⁻ into the water and push ethylene through the middle D. The interior has nothing to hold a charge: water keeps F⁻, while ethylene dissolves into the tails

Question 5

Two proteins in a nerve cell's membrane let through substances that the bare bilayer 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 bare bilayer on its own, so a cell needs no carrier for it D. The data say nothing about 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 them to one another B. Water entered the cells, and their swollen contents pressed out on the walls, which pressed back C. The cold water stiffened the cell walls, which had softened in the warm air D. Water dissolved the cell walls, which then set again in a firmer form

Question 9

Pond algae, whose cells have walls, live in nearly pure water without harm. Amoebas, single cells with no wall, taken from soil and put into the same water swell and burst within an hour.

Why do the algae survive in the water that bursts the amoebas?

A. The algae's walls keep water from entering their cells at all B. The algae's membranes are far stronger than an amoeba's C. Water enters the algae's cells too, but their walls stop the swelling D. The water is hypotonic to an amoeba 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. Under a microscope, the cells' contents have pulled away from their walls, while each wall keeps its shape and size.

What has happened to the cells?

A. Salt entered the cells and dissolved part of each wall B. Water entered the cells, and the walls stretched away from the contents C. The walls shrank around the contents, squeezing liquid out D. Water left the cells toward the salty surface, and the contents shrank from the 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: Ethylene crossed freely: after ten minutes it was at 100%, as concentrated inside as outside. Hydrogen peroxide crossed in small amounts, reaching 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 bare 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 meeting in the middle. The tails carry no charges or partial charges, so there is nothing in the interior that can hold on to a polar or charged particle: it 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, meeting in the middle; the tails carry no charges or partial charges (nothing that can hold 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 Ca²⁺ stayed outside the bubbles although it is smaller than an ethylene molecule. (1 point)

Hint: The stem says Ca²⁺ is the smaller particle, so size is ruled out. What other difference between Ca²⁺ and ethylene is left, and how does your answer to (b) bear on it?

A full-credit answer: Ca²⁺ carries two full charges, so water holds on to it strongly. The hydrocarbon tails in the interior carry no charges or partial charges, so there is nothing there to hold the ion, and it cannot leave the water on its own side. Ethylene offers water nothing to hold, dissolves into the tails and passes through. What decides is charge, not size: the smaller particle is the one that stayed out.

Check the box for each point your answer earns

Ca²⁺ carries a charge, so water holds on to it, and the hydrocarbon interior has no charges or partial charges to hold it, so it cannot leave the water to cross; ethylene offers water nothing to hold and 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. It is the smaller of the two; its charge, held by water, is what keeps it out.

(d) Explain why lactose also stayed outside. (1 point)

Hint: Lactose has no charge, yet it stayed out. What does water do to an –OH group, and how does lactose compare in size with hydrogen peroxide, which got a little through?

A full-credit answer: Lactose is large and polar. Water's partial charges hold on to every one of its –OH groups, and the hydrocarbon interior has nothing to hold them, so lactose stays in the water. Hydrogen peroxide is polar too but small, so a little of it slips through; lactose is both polar and large, and none of it does.

Check the box for each point your answer earns

lactose is a large polar molecule; water's partial charges hold every one of its –OH groups, and the interior has nothing to take their place, so it 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 holds it all over and the tails hold none of it".

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

(e) Predict how a living gut cell takes in Ca²⁺, and justify your prediction. (1 point)

Hint: The bubbles are bilayer and nothing else. How does a living cell's membrane differ from them, and could that difference be the route in?

A full-credit answer: A living gut cell takes Ca²⁺ in through a membrane protein: a channel protein, a water-lined tunnel that lets the ion through, or a carrier protein that binds it and changes shape to move it across. The bubbles show that Ca²⁺ cannot cross the bilayer itself, so a protein embedded in the membrane is the only way in.

Check the box for each point your answer earns

through a membrane protein, a channel (a water-lined tunnel for the ion) or a carrier; because Ca²⁺ cannot cross the bilayer on its own, as the bubbles show, so a protein door is the only way in.

Accept: "through a channel protein" or "through a carrier protein" with the reason that the bilayer blocks ions.

Common slip: Saying the cell 'lets it in' with no route named. The point is earned by naming a protein door and saying why one is needed.

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 lies outside the plasma membrane and is a rigid boundary: it gives the cell its shape and holds back some substances. The plasma membrane inside it does the selecting, deciding 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 wall holds the shape; the membrane selects".

Common slip: Giving the wall the job of deciding what enters. The 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 them by osmosis. The swelling contents press outward on each cell wall and the wall presses back, so every cell is firm, or turgid. That outward push against the wall, 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 wall and the wall presses back, making each cell turgid; this turgor pressure holds the soft stem stiff.

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

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

(c) Predict 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. The membrane and contents shrink away from each wall, which keeps its shape: plasmolysis. With nothing pressing on the walls, turgor is lost, and the stem droops.

Check the box for each point your answer earns

water leaves the cells; the membrane and contents shrink away from the wall (plasmolysis) while the wall keeps its shape; turgor is lost and the stem droops.

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) Justify your prediction, using what the cell wall and the plasma membrane each let through. (1 point)

A full-credit answer: The plasma membrane is the selector: it 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 toward the sugar. The wall is a rigid boundary that keeps its shape whether or not the contents press on it, and it holds back only some large substances, so the water passes on out through it freely. The membrane and contents therefore shrink inward, away from a wall that stays where it was.

Check the box for each point your answer earns

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 wall is a rigid boundary that holds its shape whatever the contents do and holds back only some large substances, so water passes out through it freely while the membrane and contents shrink away from it.

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

Common slip: Giving the wall the job of keeping water in, or having it shrink with the cell. The 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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