End-of-topic test: Cell Communication

Unit 4 · Topic 4.1 end-of-topic 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 two free-response questions, write your answer in full sentences, 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 is the signaling cell, and which is 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 the blood carries too little oxygen, kidney cells release a hormone that reaches every organ. Bone marrow cells respond by making more red blood cells. Skin cells, bathed in the same blood at the same concentration, show no change.

Why do the skin cells show no change?

A. Skin cells receive less of the hormone than the marrow cells do B. Skin cells detect the hormone but have no use for more red blood cells C. Skin cells break the hormone down before it can reach their surface D. Skin cells carry no protein that binds the hormone, so nothing inside them changes

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. Researchers measured how much of the receptor for this hormone three cell types carry, in six samples of each; the graph below shows the means, and the error bars represent ±2SE. Each cell type is then given the same concentration of the hormone.

Predict which cell types respond to the hormone.

A. Pancreas cells and gallbladder muscle cells respond; lung cells do nothing B. Pancreas cells respond; gallbladder muscle cells carry too little receptor to respond 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

Lung cells carry 0.3 units of the receptor for the gut hormone, the level measured in cells that lack the receptor. Researchers give lung cells fifty times the concentration of the hormone that makes pancreas cells respond.

Predict what the lung cells do.

A. The lung cells respond weakly, in proportion to the extra hormone B. The lung cells respond as strongly as the pancreas cells do C. The lung cells stay as they are, because they carry almost no receptor D. The lung cells respond after a delay, once the hormone has built up around them

Question 5

A pollen grain resting on a stigma cell.

A pollen grain lands on the stigma at the tip of a flower. The drawing below shows a protein on the pollen grain's surface sitting in the pocket of a receptor on the surface of a stigma cell. Within minutes the stigma cell 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, both leave the stigma cells unchanged.

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

A. Direct contact; the surface proteins bind only when the two cells touch B. Direct contact; the pollen's surface protein diffuses across the gap to the stigma cell C. A local regulator; the pollen releases a molecule that diffuses to the stigma cell D. A local regulator; the fluid washed off the pollen carried the molecule to the stigma

Question 6

Cells lining the inside of a blood vessel release a small gas molecule. The muscle cells wrapped around that stretch of vessel relax within seconds, and the vessel widens. Muscle cells a centimeter along the vessel carry the same receptor and stay contracted. The gas is destroyed within a few seconds of its release.

Why does the gas act only on the muscle cells right beside the cells that released it?

A. The gas is swept away by the blood before it can reach cells further along B. Only the nearest muscle cells carry the receptor for the gas C. A gas molecule is too large to diffuse more than a few cell-widths D. The gas is destroyed within seconds, so it diffuses only a short way before it is gone

Question 7

A nerve ending sits a fraction of a micrometer from a heart muscle cell. When the nerve fires, it releases a molecule into that gap; the heart cell responds within a millisecond, and the molecule is broken down in the gap within a few milliseconds.

Which kind of signal is the molecule?

A. A hormone: a molecule the blood carries from the nerve ending to the heart cell B. A neurotransmitter: a local regulator released by a nerve cell into the gap beside its target C. A contact signal: a protein on the nerve ending's surface that touches the heart cell D. A quorum-sensing signal: a molecule that builds up as more nerve endings release it

Question 8

Cells in the wall of a hungry person's stomach release a molecule into the blood. About twenty minutes later, cells in the brain, in fat tissue under the skin and in the pancreas have all changed their activity; these organs lie far apart.

Which kind of signaling is this, and what in the description shows it?

A. A local regulator; the molecule diffused from the stomach to the nearby pancreas B. Direct contact; the stomach wall 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

The kidney hormone reaches the bone marrow in the arm bones and in the leg bones, and both begin responding at about the same time, hours after the release. A neurotransmitter released by a nerve ending acts on its target within a millisecond.

Why does the hormone reach the marrow of both arms and legs, and why is its response slower to start?

A. The hormone diffuses through the tissues from the kidney to every bone in turn B. The kidney cells touch the marrow cells of the arm bones and the leg bones C. The marrow cells take hours 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 wall-digesting enzyme. Researchers then add the purified signal molecule at the concentration found in a dense culture.

Predict what the receptor-lacking culture does.

A. The culture releases the enzyme, because the added signal reaches every cell B. The culture releases the enzyme only once its own density rises further 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. Dish 1: B cells and N cells separated by a mesh that lets molecules through but keeps the cells apart; the N cells make the protein. Dish 2: N cells given fluid that B cells grew in, with every B cell removed; the N cells make the protein. Dish 3: N cells given fresh fluid; the N cells show no change.

Which dish is the control, and what does it show?

A. Dish 3; fluid on its own gives no response B. Dish 2; a molecule on its own gives the response C. Dish 1; the mesh gives the response D. Dish 3; N cells respond to any fluid they are given

Question 13

In the chick-embryo experiment, the researchers compare dish 2 (N cells given fluid that B cells grew in) with dish 3 (N cells given fresh fluid), measuring the amount of cartilage protein the N cells make.

Which statement is the null hypothesis for this comparison?

A. The fluid that B cells grew in makes the N cells produce more cartilage protein B. Fresh fluid makes the N cells produce less cartilage protein than the B cells' fluid does C. The fluid that B cells grew in makes no difference to the amount of cartilage protein the N cells make D. Contact with B cells makes no difference to the amount of cartilage protein the N cells make

Question 14

When a zebrafish's fin is cut, the damaged cells at the edge release a molecule. Within an hour, cells up to half a millimeter from the cut begin dividing; cells further away, which carry the same receptor, do nothing. The molecule is gone from the tissue within minutes of its release.

By which route 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 blood, to cells throughout the fish C. By surface proteins, to the cells touching the damaged cells D. By the fluid between cells, to every cell of the fin at once

Question 15

The same thyroid molecule makes a tadpole's tail cells shrink and its leg cells grow.

What 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 carry the receptor for the gut hormone in their membranes. Given the hormone, these cells now respond.

Which factor 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 a neck gland into the blood that sets how fast cells all over the body use fuel 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 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 K carried by four cell types of a frog, in units per cell.

The table below gives how much of the receptor for hormone K four cell types of a frog carry. Hormone K is then added to all four cell types at the same concentration.

Which cell types respond to hormone K?

A. Skin cells only B. Skin cells, muscle cells and lens cells C. Skin cells and muscle cells D. All four cell types

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 food runs out, some cells release a small molecule into the water around them, and cells nearby crawl toward the source within minutes. Well-fed cells carry a receptor for the molecule. Researchers set up the four dishes drawn below, each holding well-fed cells (F cells). Dish 1: starved cells (S cells) on the far side of a mesh that lets molecules through but keeps cells apart. Dish 2: fluid that S cells had lived in for an hour, with every S cell removed, added to the F cells. Dish 3: fresh fluid added to the F cells. Dish 4: the S-cell fluid added to F cells from a line that carries no receptor for the molecule. After 20 minutes the researchers count the percent of F cells crawling toward the source of the fluid (in dish 1, toward the mesh) in six fields of view per dish. The graph gives the mean for dishes 1 to 3, and the error bars represent ±2SE. 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). 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.

Do not award the point if the two variables are reversed, or if the receptor or the amoeba species 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 it get fresh fluid, the same treatment as dish 2 but lacking the factor under test, the fluid the S cells lived in. It shows that fluid on its own makes only 4% of F cells crawl toward the source, so the rise 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) Using the error bars, justify the claim that a molecule released by the S cells, rather than contact with S cells, makes the F cells crawl toward the source. (1 point)

A full-credit answer: Dish 2 held no S cells, only the fluid they had lived in, yet its bar runs from 51 to 63%, and the dish 3 bar runs from 2 to 6%. The bars do not overlap, so the rise is unlikely to be chance: something in the S cells' fluid, a molecule they released, made the F cells crawl, with no contact possible. Dish 1 agrees: the F cells crawled toward the mesh that kept the S cells away, and its bar (56 to 66%) overlaps dish 2's, so the mesh made no difference these data can show.

Check the box for each point your answer earns

dish 2 held no S cells at all, yet its bar (51 to 63%) does not overlap the dish 3 bar (2 to 6%), so the fluid the S cells lived in carried a molecule that made the F cells crawl, and the difference is unlikely to be chance; contact was ruled out because no S cell was present.

Accept the dish 1 argument instead: the F cells crawled toward the mesh although it kept every S cell away, and the dish 1 bar (56 to 66%) overlaps the dish 2 bar, so keeping the cells apart made no difference these data can show. Do not award the point for a comparison of the means alone with no use of the bars, or for 'the bars are different heights'.

Common slip: Comparing dish 1 with dish 2 only and stopping. That comparison shows the mesh changed nothing; the released molecule is shown by dish 2 against dish 3, the fluid with no S cells against fresh fluid.

(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, but a cell responds to a signal only through a receptor that binds it. This line carries no receptor for the molecule, so nothing in the cells binds it, nothing inside them changes, and 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 0.3 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, hours after its release.

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), starting more slowly than a local signal and reaching 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: 0.4 units per cell against 7.6 in marrow cells. The hormone reaches them in the blood, but 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 it changes.

Check the box for each point your answer earns

liver cells carry almost none of the receptor (0.4 units against 7.6 in marrow cells), 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 propose raising the concentration of the hormone in the blood twenty-fold. Predict how the liver cells respond, and justify your prediction. (1 point)

A full-credit answer: The liver cells still show no change. Twenty times the hormone changes how strongly the marrow cells, which carry the receptor, respond; it changes nothing in liver cells, which carry almost none. However much hormone surrounds a liver cell, there is almost nothing in it to bind the hormone.

Check the box for each point your answer earns

the liver cells still show no change, because the amount of signal changes how strongly the cells that carry the receptor respond, never which cells respond; with almost no receptor there is almost nothing for the extra hormone to bind.

Do not award the point for 'a small response in proportion to the extra hormone' or for 'they respond once enough hormone is present'.

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) Using the error bars, justify the claim that marrow cells carry more of the receptor than liver cells. (1 point)

A full-credit answer: The bars represent ±2SE, the range each true mean is likely to lie in. The marrow bar runs from 7.0 to 8.2 units and the liver bar from 0.2 to 0.6 units; they do not overlap, so the difference between 7.6 and 0.4 is unlikely to be chance, and the claim is supported. (The liver and skin bars overlap, so those two are not shown to differ.)

Check the box for each point your answer earns

the marrow bar (7.0 to 8.2 units) does not overlap the liver bar (0.2 to 0.6 units), so the difference between the means is unlikely to be chance and the null hypothesis of no difference is rejected.

Accept a note that the liver and skin bars (0.2 to 0.6 and 0.1 to 0.5) overlap, so those two cell types are not shown to differ. Do not award the point for a comparison of the means alone (7.6 against 0.4) with no use of the bars.

Common slip: Justifying the claim from the two means alone. The means differ, but the claim needs the bars: only when the ±2SE bars are apart is the difference unlikely to be chance.

Free-response score: 0 of 4

Multiple choice checked: 0 of 19 correct.

Next: The message crosses, the molecule stays outside