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Unit 4 · Topic 4.4 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. Blood glucose is in milligrams per deciliter (mg/dL); where a graph carries error bars, the caption says what they represent.

Question 1

A saltwater fish holds the salt in its blood near 10 mg/mL. When the fish swims into saltier water, the salt in its blood rises to 12 mg/mL. Cells in its gills then pump salt out of the blood until it is back near 10 mg/mL.

Which of these is the stimulus that the gill cells detect in this loop?

A. The rise in blood salt to 12 mg/mL B. The 10 mg/mL the fish holds its blood salt near C. The pumping of salt out by the gill cells D. The saltier water the fish swam into

Question 2

One healthy person's core temperature, measured every two hours for a day. The dashed line is at 37.0 °C. Gridlines every 0.2 °C.

The graph below shows one healthy person's core temperature, measured every two hours for a day. The dashed line is at 37.0 °C.

Which statement describes what the graph shows?

A. The set point moved to each new reading in turn B. Homeostasis failed during the day, because the readings differ from one another C. The temperature wandered a few tenths of a degree from 37.0 °C and was brought back each time D. The temperature was held exactly at 37.0 °C all day

Question 3

A woman sits in a hot bath. Her core temperature rises to 37.8 °C and she begins to sweat heavily. She gets out, and within ten minutes the sweating has slowed and stopped.

Why did the sweating stop?

A. Her sweat glands ran out of water and had to refill B. Sweating cooled her back toward 37 °C, so the rise that triggered it had shrunk away C. Her set point rose to 37.8 °C to match the new temperature D. Getting out of the bath switched the loop from negative to positive feedback

Question 4

A healthy dog has fasted overnight, and its blood glucose is 85 mg/dL. A veterinarian injects glucagon into the dog's blood.

Predict the dog's blood glucose thirty minutes later.

A. Lower than 85 mg/dL, because glucagon makes muscle cells take glucose up B. Still 85 mg/dL, because the dog's set point has not changed C. Higher than 85 mg/dL, but only once the dog eats, because glucagon acts on food in the gut D. Higher than 85 mg/dL, because glucagon makes liver cells break glycogen down and release glucose

Question 5

Model of the three-enzyme pathway that makes the vitamin in a fungus cell. Arrows run from each substance to the next; the line from the vitamin ends in a bar under enzyme 1.

A fungus makes a vitamin in three enzyme steps. The model below shows the pathway and a line that runs from the vitamin and ends in a bar under enzyme 1. The vitamin's shape is nothing like A's.

What does the bar-ended line represent?

A. Enzyme 1 turns the vitamin back into A when the vitamin is plentiful B. The vitamin binds enzyme 1 at a site away from its active site and slows it C. The vitamin competes with A for the pocket on enzyme 1 where A binds D. The vitamin speeds enzyme 1 up, so more vitamin is made

Question 6

A liver cell in a mouse keeps the concentration of potassium ions inside itself near a set value. When potassium ions leak out faster than usual, pumps in the liver cell's membrane bring potassium ions back in faster, and the concentration inside returns to its usual value.

At which level is this negative feedback operating, and why?

A. Cellular, because one cell detects the change and one cell responds to it B. Molecular, because potassium ions are the thing being moved C. Organismal, because the liver cell belongs to a whole animal D. Molecular, because the pumps are protein molecules

Question 7

Loop 1: in a bacterium, the end product of a three-enzyme pathway binds the first enzyme when the product is plentiful, and the pathway slows. Loop 2: when more water flows into an amoeba, its contractile vacuole fills and empties more often. Loop 3: when a bird's core temperature falls, muscles across its body shiver and its feathers fluff up, and its temperature rises back.

Which pairing gives the level of each loop?

A. Loop 1 cellular, loop 2 cellular, loop 3 organismal B. Loop 1 molecular, loop 2 organismal, loop 3 organismal C. Loop 1 molecular, loop 2 cellular, loop 3 organismal D. Loop 1 organismal, loop 2 cellular, loop 3 molecular

Question 8

In a rare condition, a person's pancreas cells make insulin with one changed amino acid. The changed insulin has a different shape and does not fit the ligand-binding domain of the insulin receptor. The person's receptors, liver cells and muscle cells are all normal. The person eats a meal.

Predict the person's blood glucose two hours after the meal, compared with a person whose insulin is normal.

A. Lower, because the pancreas releases extra insulin to make up for the poor fit B. The same, because the set point of about 90 mg/dL is unchanged C. Lower, because the changed insulin acts like glucagon D. Higher, because the insulin is released but the cells never receive its message

Question 9

A growth in a person's pancreas releases glucagon into the blood all the time, whether the blood glucose is high or low. The person's insulin, receptors and liver are normal.

Predict the person's blood glucose between meals, compared with a healthy person's.

A. Higher, because the liver keeps releasing glucose whatever the glucose level is B. Higher only after meals, because glucagon acts only when food arrives C. Lower, because glucagon makes muscle cells take glucose up D. Near 90 mg/dL, because the set point has not changed

Question 10

In the days before an egg is released from a woman's ovary, cells in the ovary release the hormone estrogen. Estrogen makes a gland at the base of the brain release a second hormone, and that second hormone makes the ovary cells release even more estrogen. The estrogen level climbs faster and faster until the egg is released.

Which statement describes this loop?

A. Each round of estrogen release reduces the rise that triggered it: negative feedback B. Each round of estrogen release increases the rise that triggered it: positive feedback C. The estrogen is brought back to a set point each day: negative feedback D. Estrogen release is good for reproduction: positive feedback

Question 11

When a honeybee stings an animal near its hive, it releases an alarm chemical into the air. Nearby bees that detect the chemical fly at the animal and sting it, and each new sting releases more alarm chemical. The animal runs away, and within a minute the stinging has stopped.

Why did the stinging stop?

A. The alarm chemical reached its set point B. The bees ran out of alarm chemical to release C. The animal's leaving removed the stimulus, so no new stings released alarm chemical D. The loop switched to negative feedback once enough bees had stung

Question 12

After a deep cut, a person loses a lot of blood and their blood pressure falls. At the lower pressure the heart muscle itself receives less blood and pumps more weakly, and the weaker pumping lowers the pressure further.

Which statement classifies this loop, with the correct reason?

A. The pressure is falling, so the loop is negative feedback B. The outcome harms the person, so the loop is negative feedback C. The loop stops when a doctor treats the person, so it is negative feedback D. The weaker pumping increases the fall in pressure that triggered it: positive feedback

Question 13

On a hot afternoon a plant loses water through pores in its leaves, and its water content falls. As the water content falls, the pores close, and the loss of water slows. The plant's water content is still falling, more slowly, at the end of the afternoon.

Which statement classifies this loop, with the correct reason?

A. Closing the pores reduces the water loss that triggered it: negative feedback B. The water content keeps falling, so the loop is positive feedback C. Closing the pores is good for the plant, so the loop is positive feedback D. The water content is falling, and a falling quantity means negative feedback

Question 14

Before an egg is released, ovary cells release estrogen; estrogen makes a gland at the base of the brain release a second hormone; and the second hormone makes the ovary cells release more estrogen, so the estrogen climbs faster and faster. In one woman, a drug blocks the estrogen receptors on the cells of the brain gland. Her ovary cells and their receptors are normal.

Predict what happens to her estrogen level over the following days, and why.

A. It climbs faster than usual, because the blocked estrogen builds up in the blood B. It climbs as usual, because the ovary cells release estrogen on their own C. It stays low, because the brain gland never gets the message to drive the next round D. It falls to zero, because blocking a receptor switches the loop to negative feedback

Question 15

Model of the loop that holds the fish's blood salt near 10 mg/mL. The box marked X, with the broken arrow after it, is the step the poison blocks.

The model below shows the loop that holds a saltwater fish's blood salt near 10 mg/mL. A poison blocks the salt pumps in the gill cells, the step marked X. The fish then swims into saltier water.

Predict what happens to the salt in the fish's blood.

A. It rises to 12 mg/mL and returns to 10 mg/mL, because the set point has not changed B. It rises and stays high, because the response that would bring it back is missing C. It falls, because the blocked pumps let salt leak out of the blood D. It stays at 10 mg/mL, because the gill cells still detect the rise

Question 16

Mean core temperature of normal and mutant mice after two hours in a room at 35 °C, eight mice in each group. Each point is the mean; error bars represent ±2SE. Gridlines every 0.5 °C.

Some mice carry a mutation in the cells of the brain that detect core temperature. Eight normal mice and eight mutant mice were kept in a room at 35 °C for two hours, and then their core temperatures were measured. The graph below shows the two means; the error bars represent ±2SE.

Which claim do the data support?

A. Both groups regulated their temperature, because both means are above 37.0 °C B. The mutant mice have a set point of about 39.6 °C C. The mutant mice's loop worked faster, because their temperature moved further from 37 °C D. The mutant mice failed to bring their temperature down: the bars do not overlap

Question 17

When a sperm enters a frog's egg, a little calcium is released into the egg's cytoplasm near the entry point. Released calcium triggers nearby stores to release more calcium, and a wave of calcium sweeps across the egg in about a minute. The wave stops once every store has emptied.

Which statement classifies this loop, with the correct reason?

A. The wave stops, so the loop is negative feedback B. The calcium is brought back to a set point once the wave has passed: negative feedback C. Each release of calcium increases the release that triggered it: positive feedback D. The wave helps the egg develop, so the loop is positive feedback

Question 18

A fungus makes a vitamin in three enzyme steps, and when the vitamin is plentiful it binds enzyme 1 at a site away from the active site and slows it. Researchers add to the fungus a molecule that fits that same site on enzyme 1 and stays bound permanently. The cells then use their vitamin up.

Predict what happens to the pathway once the vitamin is scarce.

A. It speeds up, because the vitamin has left enzyme 1 B. It stays slowed, because the added molecule keeps enzyme 1 in its slowed shape C. It speeds up, because the added molecule competes with the vitamin and cancels it D. It stops completely, because the added molecule blocks the active site

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

Researchers tested whether the glucagon receptor on liver cells is needed for blood glucose to be brought back up when it falls. They bred rats whose liver cells lack the glucagon receptor; these rats' pancreas cells, insulin and insulin receptors are normal. Eight normal rats and eight receptor-lacking rats, all the same age and kept at the same temperature, fasted for 18 hours with water available, and then each rat's blood glucose was measured. The graph below shows the mean blood glucose of each group; the error bars represent ±2SE.

Mean blood glucose of normal rats and of rats whose liver cells lack the glucagon receptor, after an 18-hour fast; eight rats in each group. Error bars represent ±2SE. Gridlines every 10 mg/dL.

(a) Identify the independent variable, the dependent variable, and the control group in this experiment. (1 point)

A full-credit answer: Independent variable: whether the rats' liver cells carry the glucagon receptor. Dependent variable: the blood glucose after the 18-hour fast, in mg/dL. Control group: the normal rats, treated the same way but with the receptor present.

Check the box for each point your answer earns

all three: independent variable, whether the liver cells carry the glucagon receptor (normal rats or receptor-lacking rats); dependent variable, the blood glucose after the 18-hour fast, in mg/dL; control group, the normal rats.

Accept 'the presence or absence of the glucagon receptor' for the independent variable. Do not award the point if the two variables are reversed, or if a condition kept the same for every rat (the 18-hour fast, the temperature, the age) is named as the control.

Common slip: Naming the 18-hour fast as the independent variable or as the control. Every rat fasted for 18 hours; the fast is a condition kept the same, and the thing the researchers changed between the groups is the receptor.

(b) State the null hypothesis for this experiment. (1 point)

A full-credit answer: There is no difference in blood glucose after an 18-hour fast between normal rats and rats whose liver cells lack the glucagon receptor.

Check the box for each point your answer earns

a no-difference statement naming the factor changed and the quantity measured: there is no difference in blood glucose after an 18-hour fast between normal rats and rats whose liver cells lack the glucagon receptor.

Accept 'the glucagon receptor has no effect on fasting blood glucose'. Do not award the point for a prediction of a difference in either direction, or for 'glucagon has no effect on glucose'.

Common slip: Writing the expected result ('the receptor-lacking rats have lower glucose') as the null. The null hypothesis predicts no difference, and it names both the factor changed and the quantity measured.

(c) Justify the claim that the glucagon receptor is needed for blood glucose to be brought back up during a fast, using the error bars. (1 point)

A full-credit answer: The normal rats' bar runs from 80 to 92 mg/dL and the receptor-lacking rats' bar from 50 to 62 mg/dL. The two ±2SE bars do not overlap, so the difference between the means is very unlikely to be chance. During the fast the receptor-lacking rats' glucose fell well below the set point and stayed there, while the normal rats' glucose was held near 86 mg/dL.

Check the box for each point your answer earns

the ±2SE bars do not overlap (normal rats 80–92 mg/dL, receptor-lacking rats 50–62 mg/dL), so the lower glucose in the receptor-lacking rats is very unlikely to be chance; their glucose fell during the fast and was not brought back toward the set point, while the normal rats' glucose was held near it.

Accept 'the bars do not overlap, so the difference is significant', with the two ranges read from the graph. Do not award the point for a comparison of the two means alone (86 against 56 mg/dL).

Common slip: Comparing the two means alone, 86 against 56 mg/dL. The claim rests on the ±2SE bars not overlapping; two means can differ by chance.

(d) The researchers measured the rats after an 18-hour fast rather than one hour after a meal. Explain why the fast was needed to show the effect of the missing receptor. (1 point)

A full-credit answer: Glucagon is released only when glucose falls below its set point, so the glucagon arm of the loop runs during a fast, when the liver should be breaking glycogen down and releasing glucose. One hour after a meal the glucose is above the set point and the insulin arm is running instead; both groups have normal insulin and insulin receptors, so they would look the same. Only a fast makes the loop use the receptor the rats lack.

Check the box for each point your answer earns

the glucagon arm of the loop runs only when glucose falls below the set point (between meals or in a fast), when other pancreas cells release glucagon and the liver breaks glycogen down; one hour after a meal glucose is above the set point and the insulin arm is running, which both groups have intact, so the two groups would look alike and the missing receptor would show no effect.

Accept 'glucagon is released only when glucose is low, so only a fast tests the glucagon arm'. Do not award the point for 'fasting lowers the glucose' with no link to which arm of the loop is running.

Common slip: Saying the fast 'empties the stomach' or 'lowers the glucose' and stopping. The point is which arm of the loop is running: the glucagon arm runs only when glucose is below the set point, and that is the arm with the missing part.

Free-response score: 0 of 4

Free response 2 · Analyze Model · 4 points

A species of bacterium that lives in seawater makes light, but only when its population is dense. Each cell releases a small signal molecule into the water. When the concentration of the signal molecule in the water is high enough, it enters the cells and binds a receptor protein inside each cell; the bound receptor switches on the genes for making light and the gene for making more of the signal molecule. The model below shows the loop. A researcher adds to the water an enzyme that destroys the signal molecule as fast as the cells release it; the point where the enzyme acts is marked X.

Model of the loop in the light-making bacterium. The break marked X on the returning arrow is where the added enzyme destroys the signal molecule.

(a) Describe how the concentration of the signal molecule in the water changes over time once the cells begin to release it, using the model. (1 point)

A full-credit answer: The concentration rises faster and faster. A little signal binds some receptors, those cells release more signal, the higher concentration binds more receptors, and each round adds more signal to the water than the last, until every cell is releasing as much as it can.

Check the box for each point your answer earns

the concentration rises, and rises faster and faster (each rise makes the cells release more, so each round adds more signal than the last), until every cell is releasing at its maximum.

Accept 'it climbs faster and faster'. Do not award the point for 'it rises' alone with no reference to the rise growing, or for 'it is held at a set value'.

Common slip: Describing a steady rise, or a level held at a set point. The returning arrow means each round feeds the next, so the rise speeds up.

(b) Identify the kind of feedback shown in the model, and explain how the model shows it. (1 point)

A full-credit answer: Positive feedback. The stimulus is the rising concentration of signal molecule in the water, and the response, each cell releasing more signal molecule, increases that very concentration. The arrow returning from the release box to the concentration box shows the response feeding its own trigger.

Check the box for each point your answer earns

positive feedback, because the response (a cell releasing more signal molecule) increases the very change that triggered it (the concentration of signal molecule in the water), shown by the arrow that returns from the release box to the signal-concentration box.

Do not award the point for 'positive feedback' alone, or for 'positive because light is useful to the bacteria'.

Common slip: Naming the kind without saying what the response does to its trigger. 'Positive' is earned by the returning arrow: the response increases the change that triggered it.

(c) Predict the effect of the added enzyme on light production by a dense population, and justify your prediction. (1 point)

A full-credit answer: The cells stay dark. The enzyme destroys the signal molecule as fast as it is released, so its concentration in the water never rises high enough to bind the receptors inside the cells. With no bound receptor, the genes for light and for more signal are never switched on, no round of the loop starts, and the dense population makes no light.

Check the box for each point your answer earns

light production stays off (or stops), because the enzyme removes the signal molecule from the water before its concentration can rise, so receptors are not bound, the genes for light and for more signal are not switched on, and the loop cannot feed itself; the prediction must give the direction (no light, or less light) and the reason.

Do not award the point for 'less signal' with no link to light, or for 'the cells make light anyway because they are dense'.

Common slip: Describing the blocked component ('the signal is destroyed') and stopping. The point is downstream: no bound receptor, so no light, and the loop never starts.

(d) A mutant strain of the bacterium lacks the enzyme that makes the signal molecule but has normal receptors. Predict whether a dense culture of the mutant strain makes light when it is grown mixed with a dense culture of normal cells, and justify your prediction. (1 point)

A full-credit answer: Yes. The normal cells release signal molecule into the shared water and, as the loop runs, its concentration climbs. The signal enters the mutant cells and binds their normal receptors, and the bound receptor switches on their light genes. The mutant adds nothing to the signal, but it responds to the signal the normal cells supply.

Check the box for each point your answer earns

yes, the mutant cells make light, because the signal molecule released by the normal cells builds up in the shared water, enters the mutant cells and binds their normal receptors, switching on their genes for light; the loop's signal is in the water, so any cell with the receptor responds to it.

Do not award the point for 'no, because the mutant makes no signal', or for 'yes' with no reference to the normal cells' signal binding the mutant's receptor.

Common slip: Predicting no light because the mutant makes no signal. The signal is in the water, shared by every cell; what a cell needs in order to respond is the receptor, which the mutant has.

Free-response score: 0 of 4

Multiple choice checked: 0 of 18 correct.

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