End-of-topic test: Cell Communication

Unit 4 · Topic 4.1 end-of-topic test

Suggested time: about 48 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.

Answer every question. For each multiple-choice question, pick one option. When you have answered every question, press Submit the test; the feedback then gives the reasoning for each. For the two 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. Then 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

When a tadpole is ready to become a frog, cells of its thyroid gland release a molecule into its blood. Cells of the tail detect the molecule and the tail begins to shrink; cells of the growing legs detect it and grow faster.

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

A. The tail cell; the molecule it detects B. The thyroid gland cell; the molecule it releases into the blood C. The thyroid gland cell; the blood that carries the molecule D. The tail cell; the shrinking of the tail

Question 2

When a salmon swims from the sea into a river, cells of a gland near its brain release a hormone into its blood. Gill cells respond by pumping less salt out of the fish. Fin cells, bathed in the same blood at the same concentration, show no change.

Which of the following cells are target cells for the hormone?

A. The gill cells only B. The gland cells and the gill cells C. The gill cells and the fin cells D. Every cell the blood carries the hormone to

Question 3

Amount of the receptor for the gut hormone carried by three cell types, in units per cell, six samples of each. Error bars represent ±2SE. Gridlines every 1 unit.

After a meal, cells of the gut wall release a hormone into the blood. The graph below shows how much of the receptor for this hormone three cell types carry, as the mean of six samples of each; the error bars represent ±2SE. In this measurement, cells that lack the receptor read 0.5 units. A researcher then gives each cell type the same concentration of the hormone.

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

A. Pancreas cells and gallbladder muscle cells respond, because only they carry the receptor B. Pancreas cells respond only, because gallbladder muscle 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

In a developing embryo, R cells lie next to P cells. A protein set in the surface of an R cell binds a receptor on the surface of the P cell touching it, and that P cell becomes a nerve cell. Researchers grow R cells and P cells on opposite sides of a mesh that lets molecules through but keeps the cells apart.

Which of the following predicts what the P cells do, and explains why?

A. The P cells become nerve cells, because the mesh lets R's surface protein through with other molecules B. The P cells become nerve cells, but only once enough R cells have built up on their side of the mesh C. The P cells do not change, because R's surface protein stays in R's membrane and cannot reach P's receptor D. The P cells do not change, because the mesh breaks down the molecules that R releases

Question 5

A pollen grain lands on the stigma at the tip of a flower. Within minutes the stigma cell beneath it softens its surface so the pollen can grow into it. Pollen grains held 0.1 mm above the stigma, and fluid washed off pollen grains and dropped onto the stigma, both leave the stigma cells unchanged.

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

A. Direct contact; the stigma cell changes only when a pollen grain touches it B. A local regulator; the pollen grain releases a molecule that diffuses across the short gap to the stigma cell C. A local regulator; the fluid washed off the pollen carried the pollen's molecule to the stigma cell D. A quorum-sensing signal; the stigma cell changes only once enough pollen grains have landed on it

Question 6

Cells lining the inside of a blood vessel release a small gas molecule. Muscle cells wrapped around the vessel within a few cell-widths of the lining relax within seconds, and the vessel widens there. Muscle cells further along the vessel carry the same receptor and stay contracted. An enzyme destroys the gas within a few seconds of its release. A researcher then adds a drug that blocks this enzyme.

Predict the effect of the drug on which muscle cells relax.

A. Only the nearest muscle cells relax, as before B. The nearest muscle cells stop relaxing C. Muscle cells throughout the body relax D. Muscle cells further along the vessel relax too

Question 7

Measurements of the molecule released at the gut nerve ending.

Researchers study a molecule that a nerve ending releases beside a muscle cell in the wall of the gut. The table below gives what they measured.

Which of the following kinds of signal is the molecule?

A. A hormone B. A neurotransmitter C. A contact signal D. A quorum-sensing signal

Question 8

When extra blood stretches the upper chambers of a person's heart, cells lining those chambers release a molecule into the blood. About twenty minutes later, cells in the kidney, in the lining of blood vessels in the legs and in the adrenal glands have all changed their activity; these organs lie far apart.

Which of the following identifies the kind of signaling, and the evidence for it?

A. A local regulator; the molecule diffused from the heart to the nearby blood vessels B. Direct contact; the heart pressed against each organ that responded C. A hormone in the blood; one molecule reached several distant organs of other cell types D. A local regulator; the response took twenty minutes to appear

Question 9

In a female mammal, a hormone released by the ovary reaches cells lining the uterus and milk-making cells in the breast, and both begin responding at about the same time, well after the release. A neurotransmitter released by a nerve ending acts on its target within a millisecond.

Which of the following explains why the hormone reaches both the uterus cells and the breast cells, and why its response starts more slowly?

A. The hormone diffuses through the tissues from the ovary to the uterus and then on to the breast B. The ovary cells touch the cells of the uterus and of the breast C. The uterus cells and breast cells take a long time to detect the hormone because they carry few receptors D. The blood carries the hormone through the whole body, a slower journey that also dilutes it

Question 10

Wall-digesting enzyme released into the fluid by cultures of the vegetable-rotting bacterium grown to four densities.

A bacterium that rots vegetables releases enzymes that digest plant cell walls. Cultures were grown to four densities, and the enzyme released into the fluid was measured; the table below gives the results. Each cell releases a small signal molecule and carries a receptor for it.

Why do the two sparse cultures release almost no enzyme?

A. A sparse culture has too few cells to make any of the signal molecule B. In a sparse culture the signal molecule diffuses away and stays below the concentration that binds the receptors C. Each cell in a sparse culture releases the enzyme, but the enzyme is too dilute in the fluid to measure D. The signal molecule in a sparse culture is a different molecule from the one in a dense culture

Question 11

A strain of the vegetable-rotting bacterium carries no receptor for the signal molecule but releases the signal normally. A dense culture of this strain releases almost no cell-wall-digesting enzyme. A researcher then adds the purified signal molecule at the concentration found in a dense culture.

Which of the following predicts what the receptor-lacking culture does, and explains why?

A. The culture releases the enzyme, because the added signal reaches every cell B. Almost none of the enzyme appears, because the added signal is broken down before it can bind C. The culture still releases almost no enzyme, because its cells have nothing that binds the signal D. The culture releases the enzyme, because the added signal takes the place of the missing receptor

Question 12

Cells taken from the back of a chick embryo (B cells) make nearby cells (N cells) start producing a cartilage protein. Researchers give N cells and skin cells from the same embryo fluid that B cells grew in, with every B cell removed. The N cells make the protein; the skin cells, in the same fluid, stay as they were.

Which of the following explains how the message reaches the N cells, and why only the N cells answer it?

A. A molecule the B cells release spreads through the fluid; the N cells carry a receptor for it and the skin cells carry none B. A molecule the B cells release spreads through the fluid; the N cells use it up before it reaches the skin cells C. The B cells pass the message by touching the N cells; the skin cells are never touched D. A molecule the B cells release spreads through the fluid; the skin cells detect it but have no use for a cartilage protein

Question 13

Four behaviors of single-celled organisms are described.

Which of the following is an example of quorum sensing?

A. A gut bacterium switches on the genes for digesting a sugar when the concentration of that sugar in the gut rises B. A yeast cell stops dividing once neighboring cells press against its surface on every side C. A soil bacterium forms a tough coat once the cells are dense enough for a molecule they each release to build up around them D. A pond bacterium swims toward the light once the pigment inside each cell has absorbed enough light to change shape

Question 14

When a sea star's arm is bitten, cells damaged at the wound release a molecule. Within an hour, cells up to 0.4 mm from the wound begin laying down new skeleton; cells further along the arm, which carry the same receptor, do nothing. The molecule is gone from the tissue within a few minutes of its release.

By which of the following routes did the message travel from the damaged cells to the cells that responded?

A. By diffusion through the fluid between cells, to cells within a short distance B. By the body fluid that circulates through the sea star, to cells throughout the arm C. By surface proteins, to the cells touching the damaged cells D. By the fluid between cells, to every cell of the arm at once

Question 15

A tadpole’s thyroid gland releases one molecule into its blood. That one molecule makes the tail cells shrink and the leg cells grow.

Which of the following does this show about the molecule?

A. The molecule carries one set of instructions for tail cells and another for leg cells B. The molecule changes into a different molecule on its way to the legs C. The tail cells receive the molecule first and pass a changed form of it to the legs D. The molecule carries no instructions; each cell type responds in its own way once its receptor binds it

Question 16

Lung cells show no change when the gut hormone reaches them. Researchers make a line of lung cells that carries the receptor for the gut hormone in its membrane. When the hormone reaches these cells, they respond.

Which of the following factors do the results show decides whether a cell responds to the hormone?

A. Whether the hormone can reach the cell in the blood B. Whether the cell carries a receptor that binds the hormone C. Whether the cell is the kind of cell the hormone is meant for D. Whether the concentration of hormone around the cell is high enough

Question 17

Four signals are described.

Which of them is a hormone?

A. A molecule released by the testes into the blood that makes muscle cells all over the body grow larger B. A molecule released by a nerve ending into the gap beside a muscle cell, gone within milliseconds C. A molecule released by a cut cell that acts on cells a few cell-widths away and is gone within minutes D. A protein on the surface of one immune cell that binds a protein on the surface of another

Question 18

In a dense culture of the vegetable-rotting bacterium, every cell begins releasing the cell-wall-digesting enzyme within the same few minutes.

Why do the cells all switch on together?

A. The cells signal each other by touch, and the message spreads from cell to cell across the culture B. The first cell to switch on releases the enzyme, and the enzyme switches on the other cells C. The cells wait until they sense that the population is large enough, then agree to switch on D. The signal passes its threshold concentration for every cell at once, and each cell responds automatically

Question 19

Amount of the receptor for hormone Z carried by four cell types of a frog, in units per cell.

The table below gives how much of the receptor for hormone Z four cell types of a frog carry. In this measurement, cells that lack the receptor read 0.5 units. A researcher then adds hormone Z to all four cell types at the same concentration.

Which of the following cell types respond to hormone Z?

A. Skin cells only B. Skin cells and muscle cells C. Skin cells, muscle cells and lens cells D. Skin cells, muscle cells, lens cells and kidney cells

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 · 4 points

Cells of a soil amoeba live as single cells while there are bacteria to eat. When the bacteria are gone, some cells release a small molecule, and nearby cells crawl toward the source. Well-fed cells carry a receptor for the molecule. Researchers set up four dishes, drawn below, each holding well-fed F cells. Dish 1: starved S cells beyond a mesh that lets molecules through but keeps cells apart. Dish 2: fluid S cells had lived in, every S cell removed. Dish 3: fresh fluid. Dish 4: the S-cell fluid, given to F cells from a line with no receptor for the molecule. After 20 minutes the researchers count the percent of F cells crawling toward the source (in dish 1, the mesh) in six fields of view per dish. The graph gives the means for dishes 1 to 3 with ±2SE error bars; dish 4 has still to be counted.

Top: the four dishes. Bottom: percent of F cells crawling toward the source after 20 minutes, mean of six fields of view per dish. Error bars represent ±2SE. Gridlines every 10%. Dish 4 has still to be counted.

(a) Identify the independent variable and the dependent variable in this investigation. (1 point)

A full-credit answer: Independent variable: what the F cells are given (starved cells behind a mesh, fluid the starved cells lived in, or fresh fluid; and, in dish 4, whether the F cells carry the receptor).
Dependent variable: the percent of F cells crawling toward the source after 20 minutes.

Check the box for each point your answer earns

both: independent variable, what is added to the F cells (S cells behind a mesh, fluid the S cells lived in, or fresh fluid; accept 'the source of the fluid' or 'whether starved cells or their fluid are present'); dependent variable, the percent of F cells crawling toward the source after 20 minutes.

Accept 'the treatment condition of each dish (dish 4 varies the cell line as well as the fluid)' as the independent variable. Accept the F cells' receptor status (dish 4) as a further part of the independent variable. Do not award the point if the two variables are reversed, or if the amoeba species or the 20-minute count time is named as a variable.

Common slip: Naming the crawling as the independent variable. The crawling is what is measured; what the researchers change is what they add to each dish.

(b) Explain why dish 3 is included in the investigation. (1 point)

A full-credit answer: Dish 3 is the control.
The F cells in dish 3 get fresh fluid: the same treatment as dish 2, but lacking the factor under test, the fluid the S cells lived in.
Dish 3 shows that fluid on its own makes only 4% of F cells crawl toward the source.
So the rise to 57% in dish 2 can be credited to a molecule the S cells released.

Check the box for each point your answer earns

dish 3 is the control: it shows how many F cells crawl toward fluid that no S cell has lived in (4%), so a rise above that in dish 2 can be credited to a molecule the S cells released into their fluid.

Accept 'it shows that fluid on its own gives no response'. Do not award the point for 'it shows the experiment worked' or for naming a condition kept the same (the 20 minutes, the temperature) as the reason.

Common slip: Saying the control 'shows the experiment worked'. The control gives the response with the tested factor absent; without it, the 57% in dish 2 could not be credited to anything the S cells released.

(c) Support the claim that a molecule released by the S cells, rather than contact with S cells, causes the crawling, using evidence from the error bars. (1 point)

A full-credit answer: Dish 2 held no S cells, so no F cell could touch one.
The dish 2 bar runs from 51% to 63%.
The dish 3 bar runs from 2% to 6%.
The bars do not overlap, so the difference is unlikely to be chance.
So the fluid the S cells lived in carried a released molecule, and that molecule made the F cells crawl.

Check the box for each point your answer earns

the evidence (dish 2 held no S cells, and its bar, 51 to 63%, does not overlap the dish 3 bar, 2 to 6%) AND the reasoning that links it to the claim (no S cell was present to touch, so the fluid the S cells lived in carried a released molecule that made the F cells crawl, and the non-overlap shows the rise is unlikely to be chance).

Accept the dish 1 evidence instead (the mesh kept every S cell away, yet the F cells crawled; the dish 1 bar, 56 to 66%, overlaps the dish 2 bar), with the same link stated. Accept: readings within half a gridline (5 percentage points) of those values, that is 46–56 to 58–68 for dish 2 and 0–7 to 1–11 for dish 3. Do not award the point for evidence with no link to the claim, for reasoning with no data named, or for a comparison of the means alone.

Common slip: Naming the evidence and stopping. Supporting a claim needs the link: say why dish 2 against dish 3 rules out contact and leaves a released molecule as the only signal.

(d) Predict the result for dish 4, and justify your prediction. (1 point)

A full-credit answer: About 4% of the F cells in dish 4 crawl toward the source, the same as in dish 3.
The molecule reaches these cells in the S-cell fluid.
A cell responds to a signal only through a receptor that binds the signal.
This line carries no receptor for the molecule.
So nothing in the cells binds the molecule, and nothing inside them changes.
They crawl no more than cells given fresh fluid.

Check the box for each point your answer earns

about 4% of the F cells crawl toward the source, the same as dish 3 (no rise), because these F cells carry no receptor for the molecule: the molecule reaches them in the fluid, nothing in them binds it, and nothing inside them changes.

Do not award the point for 'fewer crawl, because they take up less of the molecule' (a cell with no receptor shows no response at all) or for a prediction with no reason.

Common slip: Predicting a weaker response. A cell without the receptor does nothing at all; the amount of signal changes only how strongly receptor-carrying cells respond.

Free-response score: 0 of 4

Free response 2 · Conceptual Analysis · 4 points

When the blood carries too little oxygen, cells in the kidney release a hormone into the blood. Over the following days, cells in the bone marrow of every bone make more red blood cells. Liver cells and skin cells, bathed in the same blood, show no change. Researchers measured how much of the receptor for the hormone each cell type carries, in units per cell, in six samples of each; the graph below shows the means, and the error bars represent ±2SE. In this measurement, cells that lack the receptor read 0.5 units.

Amount of the receptor for the kidney hormone carried by three cell types, in units per cell, six samples of each. Error bars represent ±2SE. Gridlines every 1 unit.

(a) Describe how the hormone gets from the kidney cells to the marrow cells of the leg bones. (1 point)

A full-credit answer: The kidney cells release the hormone into the blood, and the blood carries it through the whole body.
It reaches the marrow cells of the leg bones, and of every other bone, in the same blood.

Check the box for each point your answer earns

the kidney cells release the hormone into the blood, and the blood carries it through the whole body, so it reaches the marrow cells wherever they are (including the leg bones).

Accept with or without a note that the response starts more slowly than a local signal's or that the hormone reaches many organs at once.

Do not award the point for 'it diffuses through the tissues to the bones' or for 'the kidney cells touch the marrow'.

Common slip: Saying the hormone diffuses through the tissues from the kidney to the bones. Diffusion carries a signal only a few cell-widths before it is destroyed or diluted away; a hormone rides the blood.

(b) Explain why the liver cells show no change although the hormone reaches them. (1 point)

A full-credit answer: Liver cells carry almost none of the receptor for the hormone: 0.6 units per cell against 7.6 in marrow cells, and 0.5 units is the reading of cells that lack the receptor.
The hormone reaches the liver cells in the blood.
A cell responds to a signal only through a receptor that binds it.
With almost no receptor, nothing in a liver cell binds the hormone.
So nothing inside the liver cell changes.

Check the box for each point your answer earns

liver cells carry almost none of the receptor (0.6 units against 7.6 in marrow cells; 0.5 units is the no-receptor reading), so no protein on or in them binds the hormone and nothing inside them changes; only cells that carry the receptor are target cells.

Do not award the point for 'the liver does not need more red blood cells', 'the liver ignores the hormone', or 'less hormone reaches the liver'.

Common slip: Saying the liver cells 'have no need' for the hormone or 'ignore' it. A cell that does not respond has no protein that binds the signal; nothing is being weighed or chosen.

(c) The researchers then raise the concentration of the hormone in the blood twenty-fold and measure the hormone bound to each cell type, in units per cell, before and after: marrow cells 6.8 before and 7.1 after; liver cells 0.2 and 0.2; skin cells 0.2 and 0.2. In this measurement, cells with no receptor read 0.2 units. Using these data, predict how the liver cells respond at the higher concentration, and justify your prediction. (1 point)

A full-credit answer: The liver cells still show no change.
At the normal concentration the liver cells bind 0.2 units of hormone, the no-receptor reading.
At twenty times the concentration they still bind 0.2 units, the no-receptor reading.
A cell responds only through a receptor that binds the hormone, and the liver cells bind none at either concentration.
So the liver cells show no change at twenty times the concentration.

Check the box for each point your answer earns

the liver cells still show no change, because the bound hormone stays at the no-receptor reading (0.2 units) at twenty times the concentration, so nothing in the liver cells binds the hormone at either concentration.

Do not award the point for 'the liver cells respond a little now' or for a prediction that does not use the bound-hormone data.

Common slip: Predicting a weak response in proportion to the extra hormone. More signal cannot make a receptor-less cell respond; it only strengthens the response of cells that already carry the receptor.

(d) Determine, using the error bars, whether the data show that liver cells and skin cells differ in how much of the receptor they carry. Justify your decision. (1 point)

A full-credit answer: The bars represent ±2SE, the range each true mean is likely to lie in.
The liver bar runs from 0.4 to 0.8 units.
The skin bar runs from 0.3 to 0.7 units.
The bars overlap, so the difference between 0.6 and 0.5 could be chance.
So the data do not show that the two cell types differ.
Both cell types read close to the 0.5 units of cells that lack the receptor.

Check the box for each point your answer earns

the decision (the data do not show a difference) AND the reasoning it rests on (the liver bar, 0.4 to 0.8 units, and the skin bar, 0.3 to 0.7 units, overlap, so the difference between the means could be chance and the null hypothesis of no difference is not rejected).

Do not award the point for the decision alone, for a comparison of the means alone (0.6 against 0.5), or for deciding that liver cells carry more of the receptor than skin cells, or for deciding that the two cell types carry the same amount of receptor (overlap shows no difference, not equality).

Common slip: Deciding from the means alone, 0.6 against 0.5. Determining needs the decision and what it rests on: the two bars overlap.

Free-response score: 0 of 4

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Multiple choice checked: 0 of 19 correct.

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