Unit 4 · Topic 4.4 end-of-topic test
Suggested time: about 50 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.
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 the following is the stimulus that the gill cells detect in this loop?
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 of the following describes what the graph shows?
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 healthy sheep has fasted overnight, and its blood glucose is 85 mg/dL. A veterinarian injects glucagon into the sheep's blood.
Predict the sheep's blood glucose concentration thirty minutes later.
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 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?
Three negative-feedback loops are described in the table below. A single-celled organism’s loop counts as cellular.
Which of the following gives the level of each loop?
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.
Which of the following predicts the person’s blood glucose concentration two hours after the meal, compared with a person whose insulin is normal, with the correct reason?
A growth in a person's pancreas releases glucagon into the blood all the time, whether the blood glucose concentration is high or low. The person's insulin, receptors and liver are normal.
Predict the person's blood glucose concentration between meals, compared with a healthy person's.
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 of the following classifies this loop, with the correct reason?
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 flees, and within a minute the stinging has stopped.
Why did the stinging stop?
Four events in a person’s body are described.
Which of the following is an example of feedback?
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 of the following classifies this loop, with the correct reason?
A climber walks up a high mountain, where the air holds less oxygen. The oxygen in her blood falls lower than the value her body holds it near. Cells in her neck arteries detect the fall, and she breathes faster and deeper until the oxygen in her blood is back near that value.
Which of the following is the response in this loop?
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.
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?
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 of the following classifies this loop, with the correct reason?
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. A researcher adds to the fungus a molecule that fits that same site on enzyme 1 and stays bound permanently. The cells then use their vitamin up.
Which of the following predicts what happens to the pathway once the vitamin is scarce, with the correct reason?
(a) Identify the independent variable and the dependent variable in this experiment. (1 point)
A full-credit answer: Independent variable: whether the rats’ pancreas cells make glucagon.
Dependent variable: the blood glucose after the 18-hour fast, in mg/dL.
Check the box for each point your answer earns
Accept 'the presence or absence of glucagon' 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 independent variable.
Common slip: Naming the fast, the temperature or the age as a variable. Those were kept the same for every rat; the researchers changed whether the rats make glucagon and measured the blood glucose.
(b) Identify the control group in this experiment and justify your answer. State the null hypothesis for this experiment. (2 points)
A full-credit answer: The control group is the normal rats.
The normal rats were the same age as the glucagon-lacking rats, were kept at the same temperature and fasted for the same 18 hours.
The normal rats differ from the glucagon-lacking rats only in making glucagon, the factor under test.
Null hypothesis: there is no difference in blood glucose after an 18-hour fast between normal rats and rats whose pancreas cells make no glucagon.
Check the box for each point your answer earns
Common slip: Naming the 18-hour fast as the control. The fast is a condition every rat shared. The control is the group that has glucagon and is otherwise treated the same. Writing the null hypothesis as a prediction of a difference ('the glucagon-lacking rats have lower glucose'). The null hypothesis predicts no difference.
(c) Support the claim that glucagon is needed for blood glucose to be brought back up during a fast, using evidence from the error bars. (1 point)
A full-credit answer: The normal rats’ bar runs from 80 to 92 mg/dL.
The glucagon-lacking rats’ bar runs from 50 to 62 mg/dL.
The two ±2SE bars do not overlap, so the difference between the two groups is very unlikely to be chance.
During the fast the glucagon-lacking rats’ glucose fell well below the set point and stayed there.
The normal rats’ glucose was held near 86 mg/dL.
So without glucagon the glucose is not brought back up: glucagon is needed.
Check the box for each point your answer earns
Accept 'the bars do not overlap, so the difference is not chance', with the two ranges read from the graph and the link to glucagon stated. Do not award the point for a comparison of the two means alone (86 against 56 mg/dL), or for the ranges with no link to the claim.
Common slip: Comparing 86 with 56 mg/dL and stopping, or quoting the bars with no link. Supporting the claim needs the non-overlap and what it shows about glucagon.
(d) The researchers measured the rats after an 18-hour fast rather than one hour after a meal. Evaluate that decision as a test of whether glucagon is needed. (1 point)
A full-credit answer: The decision was right.
Glucagon release rises when glucose falls below its set point, and falls after a meal.
So the glucagon arm of the loop is active during a fast.
One hour after a meal the glucose is above the set point, so the insulin arm is active instead.
Both groups have normal insulin and insulin receptors, so after a meal the two groups would look the same.
So only a fast tests the arm the rats lack.
Check the box for each point your answer earns
Accept 'a good decision, because glucagon release rises during a fast and falls after a meal, so only a fast tests the glucagon arm'. Do not award the point for the judgement alone, or for 'fasting lowers the glucose' with no link to which arm of the loop is active.
Common slip: Judging the decision without naming which arm of the loop each measurement would test. After a meal the insulin arm is active, and both groups have it.
(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.
So 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
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 that stays 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
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
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 releases the signal molecule normally, but its receptor protein has a binding site of the wrong shape for the signal molecule. Determine whether a dense culture of the mutant strain, grown on its own, makes light, and justify your decision. (1 point)
A full-credit answer: The dense mutant culture makes no light.
The mutant cells release the signal molecule, so the signal builds up in the water as the culture grows dense.
But the signal cannot fit the mutant receptor, so no receptor is bound.
An unbound receptor switches on no genes.
So the genes for light and for more signal stay off.
So no round of the loop starts, and the culture stays dark.
Check the box for each point your answer earns
Do not award the point for the decision alone, for 'no light, because there is no signal' (the mutant makes the signal), or for 'light, because the culture is dense' with no reference to the receptor.
Common slip: Deciding 'light' because the culture is dense and the signal is present. Density matters only because the signal binds the receptor; here the receptor cannot be bound.