Practice questions · Topic 4.1

Unit 4 · Practice for the Topic 4.1 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: Topic 4.1 summary: how a message travels

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A message passes by touch, to the cells next door, or through the blood; the same molecule reaches many cells; only the cells that carry a receptor for it respond.

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.

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. For the free-response questions, write one short sentence per step of your reasoning, in order, and show any calculation. In the exam, write the steps as a paragraph: a bulleted list is not scored. The first free-response question walks you through the reasoning one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it. Where a graph carries error bars, the caption says what the bars represent.

Question 1

Before a caterpillar sheds its outer layer, cells of a gland in the front of its body release a molecule into its blood. Skin cells all over its body detect the molecule and begin building a new outer layer.

Which of the following gives the signaling cell and the chemical signal?

A. The skin cell; the molecule it detects B. The gland cell; the blood that carries the molecule C. The skin cell; the new outer layer it builds D. The gland cell; the molecule it releases

Question 2

The molting hormone in the caterpillar's blood reaches its muscle cells at the same concentration as its skin cells. The muscle cells show no change.

Why do the muscle cells show no change?

A. Muscle cells receive less of the hormone than skin cells do B. Muscle cells carry no receptor that binds the hormone C. Muscle cells detect the hormone but have no use for a new outer layer D. Muscle cells destroy the hormone before it reaches their surface

Question 3

Amount of the receptor for the molting hormone carried by three cell types of a caterpillar, in units per cell.

The table below gives how much of the receptor for the molting hormone three cell types of the caterpillar carry. In this measurement, cells that lack the receptor read 0.7 units. The hormone reaches all three in the blood at the same concentration.

Which of the following predicts which cell types respond to the hormone, and explains why?

A. Skin cells and gut-lining cells respond, because only they carry the receptor B. Skin cells respond only, because gut-lining cells carry too little receptor C. All three cell types respond, in proportion to the receptor each carries D. All three cell types respond, because the hormone reaches all three

Question 4

Response of two cell types to three amounts of the molting hormone.

A researcher injects caterpillars with the molting hormone at the usual amount, ten times the usual amount and thirty times the usual amount, and measures the response of two cell types. The table below gives the results, in units. Muscle cells carry 0.7 units of the receptor, the level measured in cells that lack the receptor.

Which of the following predicts what the muscle cells do at one hundred times the usual amount, and explains why?

A. The muscle cells respond weakly, in proportion to the extra hormone B. The muscle cells respond as strongly as cells that carry the receptor, because the extra hormone makes up for the missing receptor C. The muscle cells stay as they are, because they carry almost no receptor D. The muscle cells stay as they are, because the injected hormone never reaches them

Question 5

When a sperm cell reaches an egg, within seconds the egg's surface changes so that no second sperm can attach. Sperm held just out of reach of the egg by a mesh that lets molecules through, and fluid that sperm swam in, both leave the egg unchanged.

Which of the following gives the kind of signaling this is, and what shows it?

A. A quorum-sensing signal; the egg changes only once enough sperm have gathered around it B. Direct contact; the egg changes only when a sperm touches it C. A local regulator; the sperm releases a molecule that diffuses through the mesh to the egg D. A local regulator; the fluid the sperm swam in carried the sperm's molecule to the egg

Question 6

Concentration of the molecule at five distances from the plug, one minute and ten minutes after the release.

Cell fragments that plug a tear in a blood vessel release a burst of a molecule. A researcher measures the concentration of the molecule at distances from the plug, one minute and ten minutes after the release. The table below gives the results as a percent of the concentration at the plug at one minute. Muscle cells at every distance carry the same receptor for the molecule; only the muscle cells within 4 mm of the plug contract.

Why does the molecule act only on the muscle cells near the plug?

A. The blood carries the molecule only as far as the nearest muscle cells B. The muscle cells near the plug pull the molecule toward them, so none is left to reach the far cells C. The molecule is too large to diffuse further than 4 mm D. The molecule is broken down within minutes, so it diffuses only a short way before it is gone

Question 7

Two molecules act on the muscle cells in the wall of the bladder. A nerve ending less than 0.1 micrometers from a muscle cell releases molecule 1; the muscle cell contracts within a millisecond, and an enzyme in the gap destroys molecule 1 within five milliseconds. Molecule 2 arrives in the blood from a gland in the brain and stays in the blood for about an hour.

Which of the following classifies the two molecules?

A. Molecule 1 is a hormone; molecule 2 is a local regulator B. Molecule 1 is a contact signal; molecule 2 is a hormone C. Molecule 1 is a neurotransmitter; molecule 2 is a hormone D. Both molecules are neurotransmitters

Question 8

Cells in a hen's ovary release a molecule into the blood. Within an hour, liver cells and cells lining the tube where the eggshell forms, both far from the ovary, have changed what they make.

Which of the following gives the kind of signaling this is, and what shows it?

A. A hormone in the blood; one molecule reached distant organs of other cell types B. A local regulator; the molecule diffused from the ovary to the nearby tube C. Direct contact; the ovary cells touched the cells that responded D. A local regulator; the response took an hour to appear

Question 9

A soil bacterium releases an antibiotic that kills rival bacteria. A sparse culture releases almost none; a dense culture releases a great deal. Each cell releases a small signal molecule and carries a receptor for it. When a researcher adds the purified signal molecule to a sparse culture at the concentration found in a dense culture, the sparse culture releases the antibiotic within an hour.

Which of the following makes the sparse culture given the signal release the antibiotic?

A. Crowding, because the added molecules fill the space between the cells B. The antibiotic carried into the culture with the added molecule C. The cells decide, from the signal, that enough neighbors are present, and choose to release the antibiotic D. The signal molecule, at a concentration high enough to bind the cells' receptors

Question 10

To test whether the antibiotic response comes from the added signal molecule, two sparse cultures are set up: one given the purified signal molecule in fresh medium, and one given fresh medium alone. After three hours the antibiotic each culture has released is measured.

Which of the following statements is the null hypothesis for this comparison?

A. The purified signal molecule makes a sparse culture release more antibiotic B. The purified signal molecule makes no difference to the amount of antibiotic a sparse culture releases C. Fresh medium alone makes a sparse culture release less antibiotic than the signal molecule does D. Crowding makes no difference to the amount of antibiotic a sparse culture releases

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

Roots of many plants form a partnership with a soil fungus. Root cells release a molecule into the soil water, and the fungus's threads grow toward the root. Researchers test how the message travels. In every dish the researchers place a small piece of the fungus 10 mm from the source and measure the growth of its threads toward the source after 24 hours, in six dishes per treatment. Dish A: a living root tip. Dish B: a living root tip behind a mesh that lets molecules through but keeps root cells and fungal threads apart. Dish C: a drop of water that roots grew in for a day, with every root cell removed. Dish D: a drop of fresh water. The table below gives the mean growth toward the source for each dish, with ±2SE.

Mean growth of the fungus's threads toward the source in 24 hours, six dishes per treatment, with ±2SE for each mean.

(a) Identify the signaling cells and the chemical signal in this partnership. (1 point)

Hint: The signaling cell is the one that sends the message, not the one that reacts to it; the signal is the thing that travels.

A full-credit answer: The signaling cells are the root cells, and the chemical signal is the molecule they release into the soil water.

Check the box for each point your answer earns

both: the root cells are the signaling cells, and the molecule they release into the soil water is the chemical signal.

Do not award the point for naming the fungus as the signaling cell or the water as the signal.

Common slip: Naming the fungus as the signaling cell. The fungus responds; the signaling cell is the one that releases the molecule.

(b) Identify the control dish, and explain what it shows. (1 point)

Hint: Which dish is treated like the others but has the tested factor left out, and what would you expect to see in it if the factor were doing nothing?

A full-credit answer: The control is dish D, the drop of fresh water, because it is treated like the others but has the tested factor, anything from roots, left out.
It shows that water on its own gives almost no growth toward the source (0.4 mm), so growth toward the other sources can be credited to something from the roots.

Check the box for each point your answer earns

dish D (fresh water) is the control: it lacks the factor under test (anything from roots) and shows that water on its own gives almost no growth toward the source (0.4 mm), so growth in the other dishes can be credited to something from the roots.

Do not award the point for dish C as the control (it carries the tested factor) or for 'it shows the experiment worked'.

Common slip: Naming dish C as the control. Dish C carries the tested factor, water that roots grew in; the control is the dish that lacks it.

(c) State the null hypothesis for the comparison between dish C and dish D. (1 point)

Hint: A null hypothesis names the factor changed and the quantity measured. What does it say about the effect of the one on the other?

A full-credit answer: The null hypothesis is that the water the roots grew in makes no difference to the growth of the fungus's threads toward the source, compared with fresh water.

Check the box for each point your answer earns

water that roots grew in makes no difference to the growth of the fungus's threads toward the source, compared with fresh water.

Do not award the point for a prediction of a difference in either direction, or for a statement that names neither the root water nor the growth.

Common slip: Writing the researchers' prediction ('root water makes the fungus grow toward it') as the null. The null predicts no difference.

(d) Explain how dish B and dish D together show that the message from the root is a released molecule rather than contact between root and fungus. (1 point)

Hint: What did the mesh stop, and what did it let through? And what does the fresh-water dish rule out?

A full-credit answer: In dish B the mesh kept every root cell and every fungal thread apart.
The fungus still grew 5.8 mm toward the root.
So the message crossed the mesh as a molecule dissolved in the water, and contact is ruled out.
In dish D the fungus was given fresh water and grew only 0.4 mm toward it.
So water on its own gives almost no growth.
Therefore the molecule that crossed the mesh came from the roots.

Check the box for each point your answer earns

in dish B the fungus grew toward the root (5.8 mm) although the mesh kept root cells and fungal threads apart, so the message crossed as a molecule in the water and contact was ruled out; dish D shows that water on its own gives almost no growth (0.4 mm), so the molecule came from the roots.

Do not award the point for using dish C alone (it shows the water works but does not by itself rule out contact in the other dishes) or for an answer with no reference to what the mesh does.

Common slip: Explaining from dish C alone. Dish C shows the root water works; dish B is what rules out contact, and dish D is what rules out the water itself.

(e) A strain of the fungus carries no receptor for the root's molecule. Determine how far its threads grow toward the source in dish C, and justify your decision. (1 point)

Hint: What has to happen at the fungus's surface before anything inside it can change?

A full-credit answer: The strain’s threads grow about 0.4 mm toward the source, the same as toward fresh water.
A cell responds to a signal only through a receptor that binds it.
The molecule reaches this strain in the water.
But nothing in the strain binds the molecule.
So nothing inside it changes, and its threads grow no more toward the source than toward fresh water.

Check the box for each point your answer earns

the decision (about 0.4 mm, the same as dish D: no growth toward the source) AND the reasoning it rests on (the molecule reaches the fungus, but nothing in it binds the molecule, so nothing inside it changes).

Do not award the point for the decision alone, or for 'less growth, because it takes up less of the molecule'.

Common slip: Deciding on slower growth toward the source. A cell without the receptor shows no response at all; the amount of signal changes only how strongly receptor-carrying cells respond.

Free-response score: 0 of 5

Free response 2 · Scientific Investigation · 4 points

A soil bacterium releases an antibiotic that kills rival bacteria. Each cell releases a small signal molecule and carries a receptor for it. Researchers grow four sets of six flasks: a sparse culture (30,000 cells per mL); a dense culture (300,000,000 cells per mL); a sparse culture given the purified signal molecule in fresh medium, at the concentration found in a dense culture; and a sparse culture given fresh medium alone. After three hours they measure the antibiotic released into the fluid, in units per mL. The graph below shows the means, and the error bars represent ±2SE.

Antibiotic released into the fluid in three hours by four cultures of the soil bacterium, six flasks each, in units per mL. Error bars represent ±2SE. Gridlines every 5 units per mL.

(a) Explain why the sparse culture releases almost no antibiotic. (1 point)

A full-credit answer: Every cell releases the signal molecule, but in a sparse culture it diffuses away into the fluid and stays dilute, below the threshold concentration that binds the receptors.
With nothing bound, the cells stay switched off and release almost no antibiotic.

Check the box for each point your answer earns

with few cells, the signal molecule each cell releases diffuses away and stays below the threshold concentration that binds the receptors, so the cells stay switched off.

Do not award the point for 'sparse cells make no signal' or for 'the cells do not need the antibiotic'.

Common slip: Saying sparse cells make no signal. They release it as dense cells do; the concentration stays too low because few cells are releasing it.

(b) Identify the control for the culture given the purified signal, and explain what it shows. (1 point)

A full-credit answer: The control is the sparse culture given fresh medium alone.
It is treated the same way as the signal flasks but lacks the factor under test, the signal molecule.
It shows that adding medium by itself gives almost no antibiotic, 0.6 units per mL, so the release in the signal flasks is credited to the signal.

Check the box for each point your answer earns

the sparse culture given fresh medium alone is the control: it lacks the tested factor (the signal) and shows that adding medium by itself gives almost no antibiotic (0.6 units per mL), so the release in the signal flasks can be credited to the signal molecule.

Do not award the point for the dense culture as the control or for 'it shows the experiment worked'.

Common slip: Naming the dense culture as the control. The dense culture shows what a crowded population does; the control for the added signal is the same sparse culture given medium with the signal left out.

(c) Support the claim that the signal molecule, rather than crowding, switches on the antibiotic, using evidence from the error bars. (1 point)

A full-credit answer: The bars represent ±2SE.
The signal flasks’ bar runs from 34 to 44 units per mL, and the fresh-medium bar from 0.3 to 0.9 units per mL.
The two bars do not overlap, so the difference is unlikely to be chance.
The signal flasks stayed sparse, yet the signal raised their antibiotic.
So the signal molecule, not crowding, switches on the antibiotic.

Check the box for each point your answer earns

the evidence (the signal flasks' bar, 34 to 44 units per mL, does not overlap the fresh-medium flasks' bar, 0.3 to 0.9) AND the reasoning (the signal flasks stayed sparse, so crowding cannot explain the rise; the signal alone raised the antibiotic, and the non-overlap shows the difference is unlikely to be chance).

Accept adding that the signal flasks' bar overlaps the dense culture's (38 to 46), so the signal alone gives a release these data cannot tell from a dense culture's. Accept: readings within half a gridline (2.5 units per mL) of those values. Do not award the point for a comparison of means with no use of the bars, or for the bars with no link to the claim.

Common slip: Quoting the bars and stopping. Supporting the claim needs the link: the flasks stayed sparse, so the rise came from the signal, not from crowding.

(d) The dense culture is diluted a hundred-fold with fresh medium, to 3,000,000 cells per mL. In this bacterium the signal molecule reaches the concentration that binds the receptors only when a culture is denser than about 10,000,000 cells per mL. Predict what happens to the diluted culture's release of antibiotic three hours after the dilution, and justify your prediction. (1 point)

A full-credit answer: Three hours after the dilution, the release of antibiotic has fallen to almost nothing.
The diluted culture holds 3,000,000 cells per mL, below the 10,000,000 per mL threshold.
So the signal’s concentration falls below what binds the receptors, and each cell switches off.
No new antibiotic-making protein is made, and the proteins already built soon stop.
So the release fades to almost nothing.

Check the box for each point your answer earns

release falls to almost nothing, because 3,000,000 cells per mL is below the density (about 10,000,000 cells per mL) at which the signal reaches the concentration that binds the receptors, so the receptors are no longer bound and each cell switches off.

Accept 'falls to almost nothing' with or without the delay while proteins already built keep working. Do not award the point for 'release continues because the cells are already switched on' with no fading, for 'release rises because each cell has more room', or for a prediction that does not compare the new density with the threshold density.

Common slip: Predicting that release continues because the cells 'have already switched on'. Each cell responds only while the signal's concentration stays above the threshold; dilution removes it.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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