Signal Transduction

Unit 4 · Topic 4.3 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, 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 wall-digesting enzymes that were absent from them before, the walls between them weaken, and the leaf drops.

Which statement 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 fish is given 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.

Why does the protein appear only after two hours, and why does it outlast 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 statement describes this response?

A. The sperm expressed the genes for new flagellum proteins within the two seconds B. The peptide entered the sperm and pushed the flagellum directly C. The sperm secreted the peptide back out to signal the egg D. Proteins already in the flagellum were switched on, and switched off again when the signal stopped

Question 4

A hormone makes the cilia on a mussel's gill cells beat twice as fast within three seconds. Researchers add a chemical that stops all gene expression in the cells, wait an hour, and then give the hormone again. The cilia still speed up within three seconds.

What does the result with the chemical show about the response?

A. The response needs a gene to be expressed each time the hormone arrives B. The response uses proteins the cells already hold, so no gene has to be expressed C. The chemical is a look-alike of the hormone and switched the pathway on D. The hormone entered the cells and drove the cilia directly

Question 5

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

Which of these 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 wall-loosening genes being switched on 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.

Predict the child's phenotype compared with a child whose receptors are normal.

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. Over ten days the tail shrinks and disappears. Under the microscope the tail cells shrink and break into neat pieces, which nearby cells take up; nothing leaks into the surrounding tissue.

Which statement describes how the tail cells were removed?

A. Nearby cells digested the tail cells from outside with their enzymes B. Each tail cell dismantled itself with its own enzymes in response to the signal C. The tail cells burst from the strain of the change and were washed away D. The tail cells moved into the body and became leg cells

Question 8

During a roundworm's development, 131 particular cells always dismantle themselves and are cleared by their neighbors. In a mutant strain carrying a change in the gene for a protein-cutting enzyme that is expressed in these 131 cells, all 131 cells survive.

What does the mutant strain show about the deaths of these cells in normal worms?

A. The cells are killed by enzymes their neighbors secrete onto them B. The cells die because they run out of nutrients C. The cells burst when the worm's body squeezes them D. The cells carry out their own death using enzymes they make themselves

Question 9

Two mutations in the gene for a G protein are studied in cells whose pathway runs 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. Mutation 1 changed only the DNA, while mutation 2 changed the protein 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 the first relay protein inside the cell is phosphorylated, in the two lines of fat cells given the same hormone.

Two lines of fat cells are given the same hormone. Line 1 is normal; line 2 carries a mutation in the hormone's receptor. Researchers measure how much hormone is found bound to the receptors and whether the first relay protein inside the cell is phosphorylated. The table below shows the results.

What do the measurements show about line 2's receptor?

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 ran but the response proteins failed, so the mutation is downstream of the receptor D. Reception succeeded and transduction failed, 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

The proposed pathway from the sugar signal to the germination genes in the mold's spores. Each box is one component; each arrow is one step, from upstream to downstream.

A mold's spores germinate when they detect sugar. The model below shows the pathway: the 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. In a mutant strain given sugar, protein A is activated and kinase B is phosphorylated as normal, but protein C stays unphosphorylated and the spores stay dormant.

Where is the break in the mutant strain'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 proposed pathway from the sugar signal to the germination genes in the mold's spores. Each box is one component; each arrow is one step, from upstream to downstream.

In the mutant mold strain of the model below, the break lies between kinase B and protein C: protein C's phosphorylation site is altered, so kinase B cannot activate it. Researchers try to restore germination with one addition.

Which addition restores germination?

A. An always-active protein C B. An always-active kinase B C. An always-active protein A D. One hundred times the sugar concentration

Question 15

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

In a fish'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 it was added.

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 pathway runs 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 statement explains why a person who drinks coffee feels less drowsy?

A. Caffeine occupies binding sites, so fewer receptors bind adenosine and the slowing pathway runs less 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 frog's mucus-gland cells: whether hormone is found bound to the receptors, whether cAMP rises, and whether mucus is secreted.

In a frog's skin, gland cells secrete mucus when a hormone binds their receptors. The pathway runs receptor → G protein → cAMP-making enzyme → cAMP → kinase → mucus secreted. The table below shows three measurements for normal cells given the hormone, for mutant P cells given the hormone, and for mutant P cells given an always-active kinase with no hormone.

Where does mutant P's change act?

A. At the receptor's ligand-binding domain B. Between hormone binding and the production of cAMP C. Between cAMP and the kinase 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) Justify 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: With histamine present, drug X brings the leak down from 38 μL to 6 μL, and the 6 μL bar overlaps the 4 μL of treatment 1, so drug X stops histamine's message. In cells with an always-active protein K, drug X changes nothing: treatment 5 (35 μL) overlaps treatment 4 (36 μL), their ±2SE bars sharing the range 31 to 39 μL. A drug acting downstream of K would have lowered the leak in treatment 5 as well. So drug X acts upstream of protein K, at the receptor whose binding site it fits.

Check the box for each point your answer earns

treatments 4 and 5 give the same leak (their ±2SE bars overlap), so drug X changes nothing downstream of an always-active protein K; the drug must act upstream of K, at the receptor, which also fits treatment 3 falling to the level of treatment 1.

Accept: an answer built on treatments 4 and 5 with the overlap stated. Do not award the point for treatment 3 against treatment 2 alone (that shows the drug works, not where it acts), or for a comparison made from the means with no reference to the error bars.

Common slip: Comparing treatments 2 and 3 only. That shows drug X stops the response; only treatments 4 and 5, where protein K is locked on, show that the drug acts upstream of K.

(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, the pathway that loosens the junctions runs less, 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 the pathway that loosens the junctions (and lets fluid leak) runs less.

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, the junction-loosening pathway runs less, less fluid leaks, less swelling.

Free-response score: 0 of 4

Free response 2 · Analyze Model · 4 points

A barley seed stores starch. When the seed takes up water, its embryo releases a plant hormone, hormone G, which reaches the cells of the seed's outer layer. Researchers propose the model below for how these cells respond: hormone G 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. 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 G 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 G, 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) Strain M seeds are given hormone G. 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 G still binds the receptor and the receptor still activates kinase 1, because both are upstream of the missing kinase 2; but kinase 1 has nothing to phosphorylate, so protein F is never activated and 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) Researchers introduce an always-active protein F into strain M seeds and give them no hormone. Predict the amount of amylase these seeds make compared with normal seeds given hormone G, and justify your prediction. (1 point)

A full-credit answer: These seeds make about as much amylase as normal seeds given hormone G. Protein F sits downstream of the missing kinase 2, and an always-active protein F needs no phosphate from kinase 2: it reaches the DNA on its own, so the amylase gene is expressed whether or not any hormone or kinase 2 is present.

Check the box for each point your answer earns

amylase is made (about the normal amount), because protein F is downstream of the break and an always-active F needs no phosphate from kinase 2, so the steps after F are intact and run.

Accept: 'amylase is made even with no hormone'. Do not award the point for 'no amylase, because there is no hormone' or 'because kinase 2 is missing'.

Common slip: Predicting no amylase because no hormone was given. An always-active component acts with no input from upstream; the hormone and kinase 2 sit above it in the pathway.

(d) A chemical fits hormone G'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 G 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 G 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 G 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 4

Multiple choice checked: 0 of 18 correct.

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