End-of-topic test: Introduction to Signal Transduction

Unit 4 · Topic 4.2 end-of-topic test

Suggested time: about 51 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. Counts of molecules are given as measured. Where a graph carries error bars, the caption says what the bars represent.

Question 1

A receptor set in a cell's membrane, its binding site lined with negative charges and facing outward; the fluid outside the cell is shaded paler than the fluid inside. The three signal molecules 1, 2 and 3 found in the fluid around the cell are drawn above the membrane, each marked with the charge it carries.

The drawing below shows a receptor set in a cell's membrane with its binding site facing outward, and three signal molecules, 1, 2 and 3, found in the fluid around the cell. The binding site is lined with negative charges.

Which molecule is this receptor's ligand?

A. Molecule 2 B. Molecule 3 C. Molecule 1 D. Molecules 1 and 2

Question 2

Acetylcholine released from nerve endings reaches heart muscle cells, which beat more slowly, and salivary gland cells, which release saliva. Both cell types carry a receptor that binds acetylcholine.

Why do the two cell types respond differently to the same molecule?

A. Each cell type holds its own receptor, relay and response proteins, so the same ligand ends at different proteins B. Acetylcholine changes into a different molecule on its way from the nerve ending to the salivary gland cells C. The heart cells receive their acetylcholine from the blood, and the gland cells receive theirs from a nerve ending D. The salivary cells receive more acetylcholine than the heart cells do, and a stronger signal gives a different kind of response

Question 3

Hormone C, which lowers the calcium in the blood, is a chain of 32 amino acids. Melatonin, the hormone that makes a person sleepy at night, is built of two fused carbon rings with two short chains attached, about 30 atoms in all.

Which of the following classifies the two ligands?

A. Hormone C is a small molecule; melatonin is a peptide B. Both ligands are peptides C. Both ligands are small molecules D. Hormone C is a peptide; melatonin is a small molecule

Question 4

Bone cells carry a cell-surface receptor for hormone C. One cell line carries a shortened receptor that lacks the intracellular domain but keeps the ligand-binding domain and the part set in the membrane. Hormone C binds the normal and the shortened receptors equally well. Within a minute of hormone C binding, a relay protein inside normal cells gains a phosphate; in the shortened-receptor cells the relay protein stays as it was.

What does the comparison show about how the message gets into the cell?

A. Hormone C passes through the receptor into the cytosol, and the shortened receptor is too short to let it through B. The bound receptor changes shape all the way to its intracellular domain, and that change starts the relay inside C. The relay protein gains its phosphate directly from hormone C, which hands the phosphate over at the cell surface D. The shortened receptors hold hormone C less firmly than the normal receptors do, so the relay inside them is weaker

Question 5

Hormone C arrives at a bone cell. At 3 seconds it is bound to receptors in the membrane. At 20 seconds a relay molecule inside the cell has risen ten-fold. At 5 minutes the cell has stopped breaking down bone.

Which of the following classifies the three events?

A. 3 s: reception; 20 s: cellular response; 5 min: transduction B. 3 s: reception; 20 s: reception; 5 min: cellular response C. 3 s: transduction; 20 s: reception; 5 min: cellular response D. 3 s: reception; 20 s: transduction; 5 min: cellular response

Question 6

Thyroid hormone is a small nonpolar molecule that dissolves in oil. Hormone C is a chain of 32 amino acids. Both hormones reach a bone cell in the blood.

Where does each hormone bind its receptor?

A. Thyroid hormone at the cell surface; hormone C inside the cell B. Both at the cell surface C. Thyroid hormone inside the cell; hormone C at the cell surface D. Both inside the cell

Question 7

Cells lining a person's small intestine make large amounts of lactase, the enzyme that digests milk sugar; that person's muscle cells make none. After a signal reaches the intestine-lining cells, they make three times as much lactase over the next few hours.

What is the difference between the intestine-lining cells and the muscle cells, and what changed in the intestine-lining cells after the signal?

A. Intestine-lining cells express the lactase gene and muscle cells do not; the signal changed how fast the gene is expressed B. Intestine-lining cells carry the lactase gene and muscle cells lack it; the signal added more copies of the gene C. Both cell types express the gene, but muscle cells destroy the lactase; the signal stopped that destruction D. Intestine-lining cells express the gene and muscle cells do not; the signal molecule itself was made into lactase

Question 8

In a tadpole's tail cells, the receptor for thyroid hormone is found in the nucleus once the hormone has arrived. Over the next six hours the cells make far more of an enzyme that breaks down the tail's connective tissue.

How does the hormone change what the tail cells make?

A. A relay of kinases carries the message from a cell-surface receptor to the nucleus B. The bound receptor moves to the DNA and changes which genes the cell expresses C. The hormone attaches to the DNA on its own, and the receptor follows it in D. The receptor converts the hormone into the tissue-digesting enzyme

Question 9

In a fruit fly cell, relay protein W is switched on only while it carries a phosphate group. Two minutes after a growth factor binds the cell's receptor, W carries a phosphate and is active.

Which enzyme switched W on, and where did the phosphate come from?

A. A kinase, which transferred a phosphate from ATP onto W B. A kinase, which transferred a phosphate from the growth factor onto W C. A kinase, which transferred a phosphate from the receptor onto W D. A phosphatase, which removed a phosphate from W

Question 10

In a plant cell, relay protein V is switched off while it carries a phosphate and switched on when the phosphate is removed. A researcher adds a drug that blocks the cell's phosphatases, and then the signal arrives.

Predict what happens to V's activity compared with untreated cells.

A. V is switched on more strongly B. V's activity is unchanged C. V is switched on faster D. V stays mostly switched off

Question 11

In a clam's egg cell, the receptor for a maturation hormone switches on kinase A; kinase A phosphorylates kinase B; kinase B phosphorylates kinase C; and kinase C phosphorylates the proteins that start the egg's maturation. One egg carries a changed form of kinase B that no kinase can phosphorylate. The hormone binds this egg's receptors.

Predict which kinases are switched on in this egg.

A. Kinase A only B. Kinase C only C. Kinases A and C D. Kinases A, B and C

Question 12

Hormone S reaches a fat cell and binds a receptor on the cell's surface. Within a minute the cAMP inside the cell has risen from 420 molecules to 3,540, and kinases throughout the cytosol are switched on.

Which statement describes the role of cAMP here?

A. cAMP is the ligand: hormone S is turned into cAMP as it crosses the membrane into the cytosol B. cAMP is a second messenger: a small molecule made from ATP inside the cell that switches on the kinases C. cAMP is a protein of the relay, phosphorylated by the receptor and passed from one kinase to the next along the cytosol D. cAMP is the cellular response: the fat cell releases it into the blood as the product that hormone S called for

Question 13

In the fat cell, 12 receptors have hormone S bound, and over the next minute the cell's cAMP rises from 420 molecules to 3,540 molecules.

What is the amplification at this step, as cAMP molecules made per bound receptor?

A. 35 cAMP per receptor B. 260 cAMP per receptor C. 295 cAMP per receptor D. 37,440 cAMP per receptor

Question 14

In a salivary gland cell, 4 bound receptors lead to 180 molecules of the cAMP-making enzyme being switched on, and those enzyme molecules go on to make 10,800 molecules of cAMP.

Which of the following explains why 4 bound receptors end in 10,800 molecules of cAMP?

A. Each bound receptor itself makes 2,700 molecules of cAMP from ATP before the hormone leaves it B. Once made, each cAMP molecule makes copies of itself, so the count doubles again and again C. Each activated enzyme molecule makes cAMP molecule after molecule until it is switched off, so each step multiplies the count D. The 4 bound receptors let 10,800 hormone molecules into the cell, and the cell turns each one into a cAMP

Question 15

In fat cells, the response to hormone S ends within minutes of the hormone being washed away. In one dish a researcher first treats the cells with a drug that blocks the enzyme that breaks cAMP down. The researcher then adds hormone S to the dish and, after five minutes, washes the hormone away.

Predict what happens to fatty acid release from the treated cells over the next ten minutes, compared with untreated cells.

A. The treated cells stop releasing fatty acids at once B. The treated cells keep releasing fatty acids C. The treated cells release fatty acids more slowly D. The treated cells stop at the same time as the untreated cells

Question 16

cAMP inside salivary gland cells one minute after each treatment, in molecules per cell, six dishes per treatment. Error bars represent ±2SE. Gridlines every 1,000 molecules per cell.

Researchers grew two kinds of salivary gland cell: normal cells, and cells whose changed G protein is permanently switched on. Three treatments, six dishes each: normal cells with no signal; normal cells given signal D; changed cells with no signal. After one minute they measured cAMP; the graph shows the means with ±2SE error bars. In the dishes given no signal, no signal D was bound to any receptor.

Which of the following claims about the G protein's job in this pathway is best supported by the results?

A. The G protein stops the cell making cAMP until signal D arrives B. The G protein binds signal D at the surface and pulls it into the cell C. The G protein carries the switch from the bound receptor to the cAMP-making enzyme D. The G protein breaks cAMP down once the signal is gone

Question 17

Cells of the electric organ of a ray carry a receptor for acetylcholine. When acetylcholine binds, positive ions flow into the cell within a millisecond. Researchers purified the receptor protein and set it, on its own, into an artificial membrane that holds no other protein. When the researchers added acetylcholine, positive ions crossed this membrane within a millisecond.

What kind of receptor is this, and how does it produce the current?

A. A G protein-coupled receptor; cAMP made inside the cell opens a separate channel B. An intracellular receptor; acetylcholine enters the cell and opens a channel from inside C. A cell-surface receptor whose inner part is a kinase; it phosphorylates a channel, which then opens D. A ligand-gated channel; the receptor's own shape change opens a channel through it

Question 18

Three responses to signals are recorded. A cell of the adrenal gland releases stored epinephrine within two seconds of its signal arriving. A liver cell holds twice as much of a new enzyme six hours after its signal arrives. Cells in a root tip begin dividing two days after a plant hormone arrives.

Which of the following classifies the three responses, in order?

A. A change in gene expression; secretion; growth and division B. Secretion; a change in gene expression; growth and division C. Secretion; growth and division; a change in gene expression D. Growth and division; a change in gene expression; secretion

Question 19

The growth factor pathway of a fruit fly cell drawn as boxes 1 to 6 joined by arrows; the shaded band is the plasma membrane.

The model below shows the growth factor pathway of a fruit fly cell as boxes numbered 1 to 6. A researcher adds the growth factor and then counts the phosphate groups on kinase B (box 4).

Which stage of the pathway does this measurement belong to, and at which box does the message cross the membrane?

A. Transduction; box 2 B. Transduction; box 3 C. Transduction; box 4 D. Reception; box 2

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

Plant cells defend themselves against fungi. A short chain of sugars broken from a fungus's cell wall binds a receptor on the surface of a plant cell. Inside the cell the activated receptor phosphorylates kinase 1, kinase 1 phosphorylates kinase 2, kinase 2 phosphorylates kinase 3, and the cell then makes a defense protein over the following hours. Researchers grow plant cells in eighteen dishes and give six dishes each of three treatments: fresh medium; the sugar fragment; or the sugar fragment together with a drug that stops kinase 2 from being phosphorylated. After six hours they measure the defense protein in each dish, in units. The graph below shows the mean for each treatment, and the error bars represent ±2SE.

Defense protein made by cultured plant cells in six hours, in units, six dishes per treatment. Error bars represent ±2SE. Gridlines every 5 units.

(a) Identify the independent variable and the dependent variable in this investigation. (1 point)

A full-credit answer: Independent variable: the treatment each dish receives (fresh medium, the sugar fragment, or the sugar fragment with the kinase 2 blocker).
Dependent variable: the amount of defense protein the cells make in six hours, in units.

Check the box for each point your answer earns

both: independent variable, the treatment the dishes receive (fresh medium, the sugar fragment, or the fragment with the kinase 2 blocker); dependent variable, the amount of defense protein made in six hours, in units.

Accept 'whether the fragment and the blocker are present' for the independent variable. Do not award the point if the two are reversed, or if kinase 2 or the receptor is named as a variable.

Common slip: Naming kinase 2 as a variable. Nothing about kinase 2 is measured or set as a level; the researchers change the treatment and measure the protein.

(b) State the null hypothesis for the comparison between the dishes given the sugar fragment and the dishes given fresh medium. (1 point)

A full-credit answer: The sugar fragment makes no difference to the amount of defense protein the plant cells make in six hours, compared with cells given fresh medium.

Check the box for each point your answer earns

a no-difference statement that names the tested factor and the measured result: the sugar fragment makes no difference to the amount of defense protein the cells make in six hours (compared with fresh medium).

Accept 'the fragment has no effect on defense protein output'. Do not award the point for a prediction of a difference in either direction, for a null about the blocker, or for a statement that names neither the fragment nor the protein.

Common slip: Writing the researchers' own prediction ('the fragment raises the defense protein') as the null. The null hypothesis predicts no difference, and it names both the factor changed and the quantity measured.

(c) Predict how the amount of phosphate carried by kinase 1 in the dishes given the fragment with the blocker compares with the dishes given the fragment alone, and justify your prediction. (1 point)

A full-credit answer: Kinase 1 carries about the same amount of phosphate in both sets of dishes.
The receptor switches on kinase 1 first; kinase 1 then phosphorylates kinase 2.
The drug stops kinase 2 from being phosphorylated, a step after kinase 1.
So kinase 1 is switched on as usual, and only kinase 2, kinase 3 and the defense protein after them are affected.

Check the box for each point your answer earns

about the same amount of phosphate on kinase 1 in both, because kinase 1 is switched on by the receptor before kinase 2 in the chain, and the drug acts at kinase 2; each kinase is switched on by the one before it, so blocking a later step leaves the earlier steps as they were.

Do not award the point for 'less phosphate on kinase 1 because the pathway is blocked' or for 'more phosphate on kinase 1 because it cannot pass it on' (each kinase keeps its own phosphate and takes a fresh one from ATP for the next).

Common slip: Predicting less phosphate on kinase 1 because 'the pathway is blocked'. A block stops everything after it and nothing before it; kinase 1 is switched on by the receptor, which the drug leaves alone.

(d) Support the claim that the rise in defense protein produced by the sugar fragment needs kinase 2, using evidence from the error bars. (1 point)

A full-credit answer: The bars represent ±2SE.
The fragment bar runs from 34 to 42 units and the fresh-medium bar from 3 to 5 units.
They do not overlap, so the rise from 4 to 38 units is unlikely to be chance.
So the fragment raised the defense protein.
The fragment-plus-blocker bar runs from 3 to 7 units and overlaps the fresh-medium bar.
So with kinase 2 blocked the fragment made no difference these data can show.
Therefore the rise needs kinase 2.

Check the box for each point your answer earns

the evidence (the fragment bar, 34 to 42 units, does not overlap the fresh-medium bar, 3 to 5 units; the fragment-plus-blocker bar, 3 to 7 units, overlaps the fresh-medium bar) AND the reasoning (the fragment raised the protein, and with kinase 2 blocked the data show no rise, so the rise needs kinase 2). Both bar readings and the link are needed.

Do not award the point for the readings with no link to the claim, for a comparison of means alone, or for 'the blocker lowered the protein' (the blocker dishes are compared with fresh medium, and their bars overlap).

Common slip: Quoting the bars and stopping, or reasoning with no readings. Supporting a claim needs the evidence and the link that ties it to the claim.

Free-response score: 0 of 4

Free response 2 · Analyze Model · 5 points

Some frogs darken their skin in dim light. Hormone J, a peptide released into the blood by a gland at the base of the brain, reaches the skin cells, and within minutes their pigment granules spread through the cell, so the skin darkens. The model below shows the pathway as boxes 1 to 7; hormone J binds a G protein-coupled receptor. Researchers treated skin cells with a drug: the X on the model marks the step the drug blocks, between box 5 and box 6.

The pathway by which hormone J darkens a frog skin cell, drawn as boxes 1 to 7 joined by arrows. The shaded band is the plasma membrane. The X marks the step the drug blocks: box 5 switching on kinase K.

(a) Describe what happens at the receptor (box 2) when hormone J binds, and how this passes the message to the G protein (box 3). (1 point)

A full-credit answer: Hormone J fits the receptor's binding site by shape and charge, and while it is bound the receptor holds a different shape all the way to its intracellular domain, the part facing the cytosol.
That new shape switches on the G protein sitting beside it on the inner face of the membrane.
Hormone J itself stays outside the cell; what crosses the membrane is the change in the receptor's shape.

Check the box for each point your answer earns

hormone J fits the receptor's binding site by shape and charge, and while it is bound the receptor holds a different shape; the change reaches its intracellular domain, whose new shape switches on the G protein on the inner face of the membrane, while hormone J itself stays outside.

Accept 'the receptor changes shape on its inner side and that switches on the G protein'. Do not award the point for 'hormone J passes through the receptor to the G protein' or for 'the receptor phosphorylates the G protein'.

Common slip: Sending hormone J into the cell. A peptide cannot cross the membrane; the message crosses as a change of protein shape.

(b) Identify the second messenger in the model, and describe what it does. (1 point)

A full-credit answer: The second messenger is cAMP, box 5.
Enzyme E, switched on by the G protein, makes many molecules of cAMP from ATP; cAMP is a small molecule, so it spreads quickly through the cytosol and switches on kinase K.

Check the box for each point your answer earns

cAMP (box 5), a small molecule made from ATP by enzyme E when the G protein switches the enzyme on; it spreads through the cytosol and switches on kinase K.

Do not award the point for naming the G protein or enzyme E as the second messenger, or for 'cAMP is a protein'.

Common slip: Naming enzyme E as the second messenger. The enzyme makes the second messenger; the small molecule it makes, cAMP, is the messenger.

(c) Make a claim about the level of the box 5 molecule and the darkening of the skin when hormone J reaches the drug-treated cells, compared with untreated cells given hormone J, and support your claim using the model. (2 points)

A full-credit answer: cAMP at box 5 rises in the drug-treated cells as in untreated cells.
The skin of the drug-treated cells stays pale.
The X sits between box 5 and box 6, kinase K.
Hormone J still binds the receptor, so the G protein switches on enzyme E.
Enzyme E makes cAMP, so cAMP rises.
The X stops cAMP switching kinase K on, so kinase K stays off.
So the pigment granules stay clumped and the skin stays pale.

Check the box for each point your answer earns

the claim: the box 5 molecule (cAMP; accept 'the box 5 molecule' or 'the second messenger' without the name, since the identification is scored in (b)) rises as it does in untreated cells, AND the skin stays pale. Both halves are needed. Do not award the point for 'cAMP stays low' or for 'the skin darkens as usual'. the support: the evidence from the model (the X sits between cAMP, box 5, and kinase K, box 6) AND the reasoning (everything before the X, boxes 1 to 5, works as before, so cAMP rises; cAMP cannot switch kinase K on, so kinase K stays off and the pigment granules stay clumped). Do not award the point for naming the position of the X with no link to the claim, or for reasoning that never refers to the model.

Common slip: Claiming that the cAMP concentration stays low, or giving the reasoning without pointing to where the X sits on the model. The block sits after cAMP, so cAMP is still made; supporting a claim means naming the evidence and linking it to the claim.

(d) Cells of the frog's adrenal gland carry the same kind of receptor for hormone J, the same G protein, enzyme E and kinase K, and respond to hormone J by releasing a steroid hormone. Explain why the two cell types respond differently to hormone J. (1 point)

A full-credit answer: Hormone J is the same molecule at both cells, and in both cells the signal passes along the same pathway as far as kinase K.
The difference is in the response proteins each cell holds.
In a skin cell, kinase K reaches the proteins that move pigment granules; in an adrenal cell it reaches the proteins that release the steroid hormone.
So the same ligand ends at whatever proteins the cell has.

Check the box for each point your answer earns

the response depends on the proteins each cell holds; the pathway is identical to kinase K, and kinase K then reaches whatever response proteins that cell has: the proteins that move pigment granules in the skin cell, the proteins that release the steroid in the adrenal cell. Specificity lives in the receiving cell's proteins, not in hormone J.

Do not award the point for 'the adrenal cells receive more hormone J', 'hormone J is different in the adrenal gland', or 'the adrenal cells have a different receptor' (the stimulus says the receptor is the same kind).

Common slip: Putting the difference in the hormone or in the amount of it. More hormone makes a response stronger; it cannot turn pigment movement into steroid release. The proteins at the end of the pathway differ.

Free-response score: 0 of 5

Feedback and scoring guides appear after you submit.

Multiple choice checked: 0 of 19 correct.

Next: Bananas in a paper bag