Signal Transduction

Unit 4 · Topic 4.3 end-of-topic test

Suggested time: about 50 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, they represent ±2SE, the range the true mean is likely to lie in.

Question 1

In autumn a hormone reaches the cells at the base of a maple leaf's stalk. Over the next three days these cells build cell-wall-digesting enzymes that were absent from them before, the cell walls between them weaken, and the leaf drops.

Which of the following describes how the hormone produced the new enzymes?

A. The hormone entered the cells and was itself rebuilt into the enzymes B. The receptor released enzymes the cells had stored in vesicles C. The pathway's last activated protein reached the DNA and the cells expressed the enzymes' genes D. Kinases added phosphates to enzymes the cells already held, switching them on

Question 2

A researcher gives a fish one 10-minute dose of a hormone. Two hours later its liver cells contain a protein they lacked before, and the protein is still present two days later, long after the hormone has gone from the blood.

Which of the following best explains both the two-hour delay and the protein outlasting the hormone?

A. Building a protein from its gene takes hours, and the protein made keeps working after the signal has gone B. The hormone needs two hours to cross the membrane, and it stays bound inside the cell for days C. cAMP takes two hours to build up, and it stays high for days after the hormone has gone D. The receptor changes shape slowly, and it stays in its changed shape for days

Question 3

A sea urchin egg releases a small peptide into the water. Within two seconds of the peptide reaching a sperm cell, the sperm's flagellum beats faster; when the peptide is washed away, the beat returns to normal within a minute.

Which of the following describes this response?

A. The sperm expressed the genes for new flagellum proteins within the two seconds B. The peptide crossed into the sperm and bound the flagellum's motor proteins directly C. The peptide was taken up and used as fuel for the faster beat, and the beat slowed when the fuel was used up D. Proteins already in the flagellum were switched on, and switched off again when the signal stopped

Question 4

In a cell, a signal passes along this pathway: hormone → receptor → G protein → cAMP-making enzyme → cAMP → kinase → channel opens.

Which of the following components is upstream of the cAMP-making enzyme?

A. cAMP B. The G protein C. The kinase D. The channel

Question 5

A hormone in the stem of a pea seedling makes the stem cells express genes for enzymes that loosen their cell walls, so each cell lengthens. Over a week the seedling grows 10 cm taller.

Which of the following is the change in the plant's phenotype?

A. The seedling growing 10 cm taller B. The hormone binding its receptors in the stem cells C. The base sequence of the cell-wall-loosening genes in the stem cells D. The relay proteins in each cell changing shape

Question 6

Growth hormone binds receptors on cells in a child's growing bones, and a pathway makes those cells divide, lengthening the bone. A child carries a mutation in the receptor's ligand-binding domain, so the binding site is misshapen and growth hormone floats past unbound. Blood tests show growth hormone at a normal level.

Which of the following predicts the child's height compared with a child whose receptors are normal, and gives the reason?

A. Taller, because unbound growth hormone builds up in the blood B. The same height, because the hormone acts on bone cells directly C. Shorter, because the hormone's message never reaches the bone cells' pathway D. The same height, because more hormone makes up for the weaker fit

Question 7

As a tadpole turns into a frog, a hormone reaches the cells of its tail, and over ten days the tail shrinks and disappears. Tail cells kept in a dish of nutrient medium, with no other tissue present, shrink and break up when a researcher adds the hormone and stay healthy with no hormone. In the tadpole, the tissue around the tail stays healthy throughout.

Which of the following describes how the tail cells were removed?

A. Each cell beside the tail, on receiving the hormone, released enzymes that digested the tail cell next to it from outside B. Each tail cell, on receiving the hormone, cut up its own contents with its own enzymes, and nothing leaked out C. The tail cells swelled until their membranes gave way, and the blood carried their own spilled contents away D. The hormone cut off the tail's blood supply, and the tail cells starved

Question 8

In a chick embryo's wing, the cells between the future fingers dismantle themselves between day 7 and day 8, and the finger cells beside them are unharmed. A dismantling cell's own enzymes cut up its DNA and its proteins.

Which of the following explains why those enzymes do no harm to the finger cell beside the dismantling cell?

A. The finger cells carry no receptor for the enzymes, so the enzymes cannot enter them B. The finger cells destroy any enzyme that reaches them from a dismantling cell C. The dismantling cell bursts, and the blood carries its spilled contents away before they reach the finger cells D. The dismantling cell packages its contents into pieces that neighboring cells clear away, so nothing leaks out

Question 9

Two mutations in the gene for a G protein are studied in cells in which a signal passes along this pathway: hormone → receptor → G protein → cAMP-making enzyme → cAMP. Mutation 1 swaps one amino acid on the G protein's outer surface, far from where it contacts the receptor or the enzyme; these cells respond normally. Mutation 2 swaps one amino acid on the surface that contacts the enzyme; in these cells cAMP stays at its resting level.

Why do the two mutations have different effects?

A. Every swapped amino acid destroys a protein's activity, so mutation 1 must have left the G protein's amino acids unchanged B. Only mutation 2 changed the shape of a part the G protein uses to act on the enzyme C. Mutation 1 changed the hormone's shape, while mutation 2 changed the receptor's D. Mutation 2 removed the G protein from the cell altogether, while mutation 1 made more of it

Question 10

A single base in the gene for a hormone receptor in a lizard's skin cells is changed. The cells' response to the hormone is weaker than normal. The cells make the normal number of receptor molecules.

Which chain of events explains how the changed base weakened the response?

A. Changed DNA sequence → one changed amino acid → changed receptor shape → weaker fit for the hormone B. Changed DNA sequence → changed hormone shape → the hormone fits the receptor's binding site less well C. Changed DNA sequence → the gene is read less often → the receptor is made in smaller numbers → weaker response D. Changed DNA sequence → the receptor is made as before → the receptor moves into the nucleus → the hormone cannot reach it

Question 11

Hormone bound to the receptors, and whether cAMP rises inside the cells after the hormone is added, in the two lines of kidney cells given the same hormone.

Researchers give two lines of kidney cells the same hormone. Line 1 is normal; line 2 carries a mutation in the hormone's receptor. The researchers measure the amount of hormone bound to the receptors, and whether cAMP rises inside the cells after they add the hormone. The table below shows the results.

Which stage of the pathway failed in line 2's cells, and in which domain of the receptor is the mutation?

A. Reception failed, so the mutation is in the ligand-binding domain B. Reception failed, so the mutation is in the intracellular domain C. Transduction failed after binding succeeded, so the mutation is in the ligand-binding domain D. Transduction failed after binding succeeded, so the mutation is in the intracellular domain

Question 12

A fish's gill cells carry a receptor for a salt-regulating hormone. Two mutant lines are found: in line P the mutation is in the receptor's ligand-binding domain, and in line Q it is in the intracellular domain. Cells of both lines show no response to the hormone.

Which single measurement tells the two lines apart?

A. Whether the hormone crosses the plasma membrane into the cells B. Whether the response proteins are present in the cells C. Whether the hormone is found bound to the receptors D. Whether the cells respond when far more hormone is added

Question 13

Measurements of the germination pathway in normal spores and two mutant strains, each given sugar.

A mold's spores germinate when they detect sugar. The pathway is: sugar binds a receptor, which activates protein A; protein A phosphorylates kinase B; kinase B phosphorylates protein C; protein C reaches the DNA and the germination genes are expressed. A researcher gives sugar to normal spores and to two mutant strains and measures each step. The table below gives the results.

Where is the break in mutant strain 1's pathway?

A. At the receptor's ligand-binding domain B. At the step from protein A to kinase B C. At the step from kinase B to protein C D. At the germination genes themselves

Question 14

The same measurements of the germination pathway in normal spores and two mutant strains.

Using the same table, a researcher makes one addition to mutant strain 2 to try to restore germination.

Which addition restores germination in mutant strain 2?

A. An always-active kinase B B. An always-active protein A C. Extra copies of the gene for protein C, so the cells hold more protein C D. One hundred times the sugar concentration

Question 15

Pigment made in 24 hours by normal and mutant shrimp pigment cells, six cultures per treatment. Error bars represent ±2SE. Gridlines every 5 μg per million cells.

In a shrimp's skin, pigment cells make dark pigment when a hormone binds their receptors. A mutant line of pigment cells is studied. The graph below shows the pigment made in 24 hours by normal and mutant cells with and without the hormone, and by mutant cells whose receptor gene has been removed. No hormone is detectable in any culture's medium unless the researchers added it.

Which conclusion do the data support?

A. The mutant cells' receptor cannot bind the hormone, so their pathway is switched off B. The mutant cells' receptor is locked in its active shape, so the signal passes down the pathway with no hormone C. A relay protein downstream of the receptor is stuck on and drives the pathway by itself D. The mutant cells make pigment only when the hormone is present

Question 16

The body makes small peptides that bind receptors on certain brain cells; a pathway in those cells then lessens the sensation of pain. Morphine, a drug, fits the same binding site on the receptor and produces the same shape change that the peptides do.

Predict morphine's effect on these cells.

A. The cells' response to the body's peptides is switched off while morphine is bound B. Morphine is made into a second messenger inside the cells C. Morphine enters the cells and acts on the relay proteins directly D. The cells respond as if the body's own peptide had bound, with no peptide present

Question 17

Adenosine builds up in the brain through the day. It binds receptors on certain brain cells, and a pathway in those cells slows their activity, which is felt as drowsiness. Caffeine fits the adenosine receptor's binding site and produces no shape change in the receptor.

Which of the following explains why a person who drinks coffee feels less drowsy?

A. Caffeine occupies binding sites, so fewer receptors bind adenosine and less signal passes down the pathway B. Caffeine activates the receptor more strongly than adenosine, speeding the cells up C. Caffeine breaks adenosine down in the blood before it reaches the brain D. Caffeine enters the cells and blocks a kinase in the slowing pathway

Question 18

Three measurements on the salamander's mucus-gland cells: whether hormone is found bound to the receptors, whether cAMP rises, and whether mucus is secreted.

In a salamander's skin, gland cells secrete mucus when a hormone binds their receptors. The signal passes along this pathway: receptor → G protein → cAMP-making enzyme → cAMP → kinase → mucus secreted. The table below shows three measurements for normal cells given the hormone, for mutant V cells given the hormone, and for mutant V cells given a chemical that raises cAMP inside the cell directly, with no hormone.

Where does mutant V's change act?

A. At the receptor's ligand-binding domain B. Between hormone binding and the making of cAMP C. Between cAMP and the kinase it switches on D. At the proteins that secrete the mucus

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

When skin is damaged, nearby cells release histamine. Histamine binds a receptor on the cells that line small blood vessels, and a pathway of relay proteins inside those cells, one of them protein K, loosens the junctions between the cells within minutes, so fluid leaks out of the vessel and the skin swells. Researchers grow sheets of vessel-lining cells and measure the volume of dye, in microliters, that leaks through each sheet in 10 minutes. There are six sheets in each of five treatments: (1) no histamine; (2) histamine; (3) histamine and drug X, a molecule that fits the histamine receptor's binding site and produces no shape change in the receptor; (4) cells carrying an always-active protein K, with no histamine; (5) cells carrying an always-active protein K, with drug X and no histamine. The graph below shows the mean volume leaked for each treatment; the error bars represent ±2SE.

Mean volume of dye leaking through a sheet of vessel-lining cells in 10 minutes, six sheets per treatment. Error bars represent ±2SE. Gridlines every 5 μL.

(a) Identify the dependent variable in this experiment. (1 point)

A full-credit answer: The dependent variable is the volume of dye that leaks through each sheet of cells in 10 minutes, in microliters.

Check the box for each point your answer earns

the volume of dye leaking through the sheet in 10 minutes (μL) as the dependent variable.

Accept: the leakiness of the cell sheet, measured as the dye leaked. Do not award the point for the treatment, the presence of histamine or of drug X (independent variables), or for 'swelling', which is not measured here.

Common slip: Naming histamine or drug X as the dependent variable. Those are what the researchers chose to add; the leaked volume is what they measured.

(b) Explain why treatment 1, cells given no histamine, is included. (1 point)

A full-credit answer: Treatment 1 is the control.
It shows how much dye leaks through a sheet with the tested factor, histamine, absent, so the extra leak in treatment 2 can be credited to histamine and its pathway rather than to leakiness the sheets have anyway.

Check the box for each point your answer earns

treatment 1 gives the leak with histamine absent (a control), so the leak in treatment 2 can be compared against it and credited to histamine.

Accept: 'it is the baseline for comparison' with what is being compared. Do not award the point for 'it is the control' alone, or for 'it shows the cells are alive'.

Common slip: Writing 'it is the control' and stopping. The point is earned by saying what the control lets you compare: the leak with histamine absent against the leak with it present.

(c) Evaluate the claim that drug X acts at the receptor rather than downstream of protein K, using the data and the error bars. (1 point)

A full-credit answer: The claim is supported.
With histamine, drug X cuts the leak from 38 μL to 6 μL, level with treatment 1’s 4 μL.
So drug X stops the message.
With an always-active protein K, drug X changes nothing: treatments 4 and 5 give 36 and 35 μL, and their bars overlap.
A drug acting downstream of protein K would have lowered treatment 5’s leak too.
So drug X acts upstream of protein K, at the receptor.

Check the box for each point your answer earns

the judgement (the claim is supported) AND the ground for it (treatments 4 and 5, cells with an always-active protein K without and with drug X, give the same leak, their ±2SE bars overlapping, so drug X changes nothing downstream of protein K and must act upstream, at the receptor; treatment 3 falling to treatment 1's level fits).

Accept a judgement grounded on treatments 4 and 5 with the overlap stated. Do not award the point for the judgement alone, for treatment 3 against treatment 2 alone (that shows the drug works, not where), or for a comparison of means with no reference to the error bars.

Common slip: Giving the judgement with no ground, or citing treatments 2 and 3 only. Only treatments 4 and 5, where protein K is locked on, show where the drug acts.

(d) A person takes drug X before being stung by a bee. Predict the swelling at the sting compared with a person who took no drug, and justify your prediction using the pathway. (1 point)

A full-credit answer: The sting swells less.
Drug X sits in the binding sites of the histamine receptors on the vessel-lining cells, so the histamine released by the sting binds fewer receptors, less signal passes down the pathway that loosens the junctions, less fluid leaks out of the vessels, and the swelling, the feature you can see on the skin, is smaller.

Check the box for each point your answer earns

less swelling, because drug X occupies the receptors' binding sites so histamine binds fewer of them and less signal passes down the pathway that loosens the junctions (and lets fluid leak).

Accept: 'little or no swelling'. Do not award the point for 'less swelling' with no link to the receptor and the pathway, or for 'more swelling because histamine builds up'.

Common slip: Stopping at 'drug X blocks the receptor'. The point needs the chain to the visible feature: fewer receptors bound, less signal passes down the junction-loosening pathway, less fluid leaks, less swelling.

Free-response score: 0 of 4

Free response 2 · Analyze Model · 5 points

A barley seed stores starch. When the seed takes up water, its embryo releases a plant hormone, hormone B, which reaches the cells of the seed's outer layer. The model below shows how these cells respond: hormone B binds a receptor; the receptor activates kinase 1; kinase 1 phosphorylates kinase 2; kinase 2 phosphorylates protein F; protein F reaches the DNA and the cells express the gene for amylase, an enzyme that digests the stored starch into sugar for the growing embryo (word equation: starch → sugar, by amylase). In mutant strain M, the component marked in the model is absent: strain M seeds make no kinase 2.

The proposed pathway in the barley seed's outer-layer cells, from hormone B to starch digestion. The dashed box is the component missing in mutant strain M.

(a) Using the model, describe the response of the outer-layer cells to hormone B, and identify the kind of response (a change in gene expression or a change in cell function). (1 point)

A full-credit answer: The cells start making amylase, an enzyme they were not making before: protein F reaches the DNA and the amylase gene is expressed, so new enzyme is built and the stored starch is digested into sugar.
This is a change in gene expression, which is why it takes hours rather than seconds.

Check the box for each point your answer earns

the cells express the amylase gene and make new amylase, which digests the starch: a change in gene expression.

Accept: 'the cells switch on the amylase gene'. Do not award the point for 'the cells make sugar' with no mention of expressing the gene or making the enzyme, or for 'a change in cell function'.

Common slip: Calling the response a change in cell function. Existing proteins are switched on in seconds; here a new protein, amylase, has to be made from its gene.

(b) A researcher gives strain M seeds hormone B. Predict the amount of amylase these seeds make, and identify one component of the model that is still activated in them. (1 point)

A full-credit answer: Strain M seeds make little or no amylase.
Hormone B still binds the receptor, because the receptor is upstream of the missing kinase 2.
The receptor still activates kinase 1, because kinase 1 is upstream of the missing kinase 2 too.
Kinase 1 has no kinase 2 to phosphorylate.
So protein F is never phosphorylated.
So the amylase gene is never expressed.

Check the box for each point your answer earns

little or no amylase, AND the receptor (or kinase 1) still activated, because it is upstream of the missing kinase 2.

Accept: either the receptor or kinase 1 as the component still activated. Do not award the point for 'kinase 2 is inactive' as the effect (the question asks what happens downstream), or for a prediction of normal amylase.

Common slip: Describing the missing kinase rather than the effect: 'kinase 2 does nothing'. The point is the measured outcome downstream (no amylase) and the upstream component that still works.

(c) A researcher gives strain M seeds an always-active protein F and no hormone. Make a claim about the amount of amylase these seeds make compared with normal seeds given hormone B, and support your claim using the model. (2 points)

A full-credit answer: Strain M seeds given an always-active protein F make about as much amylase as normal seeds given hormone B.
On the model, protein F sits downstream of kinase 2.
Strain M lacks kinase 2, so normally protein F is never phosphorylated.
An always-active protein F needs no phosphate from kinase 2.
So protein F reaches the DNA on its own.
So the amylase gene is expressed and amylase is made, with no hormone B and no kinase 2.

Check the box for each point your answer earns

the claim: about the same amount of amylase as normal seeds given hormone B (accept 'amylase is made even with no hormone'). Do not award the point for 'no amylase'. the support: the evidence from the model (protein F sits downstream of the missing kinase 2, and an always-active protein F needs no phosphate from kinase 2) AND the reasoning (so protein F reaches the DNA on its own, so the amylase gene is expressed with or without hormone B or kinase 2). Do not award the point for naming protein F as always-active with no link to the amylase gene being expressed, or for reasoning that never refers to the model.

Common slip: Claiming no amylase because no hormone was given, or reasoning without the model. The hormone sits above protein F in the pathway, and an always-active F acts without it; name where protein F sits relative to the missing kinase 2, then link that to the amylase gene.

(d) A chemical fits hormone B's binding site on the receptor and produces no shape change in the receptor. Predict the effect on starch digestion in normal seeds given both hormone B and the chemical, and explain how the chemical produces this effect. (1 point)

A full-credit answer: Starch digestion falls: the seeds make little or no amylase.
The chemical sits in the receptor's binding site, so hormone B cannot bind; with no shape change in the receptor, kinase 1 is never activated, the message never reaches protein F, and the amylase gene stays off.

Check the box for each point your answer earns

less (or no) starch digestion, because the chemical occupies the binding site so hormone B binds fewer receptors and, producing no shape change, starts no transduction, so the amylase gene is not expressed.

Accept: 'the chemical blocks the receptor so the pathway is switched off' with the binding-site reasoning. Do not award the point for 'the chemical switches the pathway on' (that needs the shape change) or for 'the chemical breaks down the hormone'.

Common slip: Treating the chemical as a look-alike that switches the pathway on. A molecule that fits the site but produces no shape change blocks the natural ligand; only one that produces the shape change switches the pathway on.

Free-response score: 0 of 5

Feedback and scoring guides appear after you submit.

Multiple choice checked: 0 of 18 correct.

Next: A set point, a stimulus, a response