Practice questions · Topic 4.2

Unit 4 · Practice for the Topic 4.2 end-of-topic test

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

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 free-response questions, write your answer in full sentences and show any calculation. The first free-response question walks you through one investigation one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it. Amplification at a step is the number of molecules made or activated at that step divided by the number of molecules that activated them; counts of molecules are given as measured. Where a graph carries error bars, the caption says what the bars represent.

Question 1

A yeast cell carries a receptor on its surface that binds glucose. When glucose binds, the cell speeds up its growth. A sugar with the same atoms as glucose but a different arrangement of its parts reaches the receptor and is passed by.

Why does the receptor bind glucose and pass the other sugar by?

A. The receptor's binding site fits glucose by shape and charge, and the other sugar fits it poorly B. The receptor binds whichever sugar arrives first, and glucose arrives first C. The receptor breaks the other sugar down before it can bind D. The receptor binds any sugar, and the cell decides afterwards which one to respond to

Question 2

A peptide hormone of an insect is tagged so that each molecule can be found. Fat cells given the tagged hormone begin releasing stored sugar within a minute, and a relay protein inside them carries a new phosphate. Every tagged molecule is still on the outside of the cells.

What crossed the membrane to start the change inside?

A. The hormone itself, through a pore in the receptor B. The tag, which came off the hormone and entered the cell C. A change in the receptor's shape, passed to its intracellular domain D. Sugar, carried in by the hormone

Question 3

A hormone reaches a kidney cell. At 2 seconds it is bound to receptors in the membrane. At 15 seconds the cAMP inside the cell has risen ten-fold. At 3 minutes water channels have reached the cell's surface.

Which option classifies the three events?

A. 2 s: reception; 15 s: cellular response; 3 min: transduction B. 2 s: reception; 15 s: transduction; 3 min: cellular response C. 2 s: reception; 15 s: reception; 3 min: cellular response D. 2 s: transduction; 15 s: reception; 3 min: cellular response

Question 4

An insect's molting hormone is built of four carbon rings and dissolves in oil. The same insect's flight hormone is a chain of ten amino acids. Both travel in the insect's blood.

How is each ligand classified, and where does its receptor sit?

A. Molting hormone: a peptide with a receptor at the surface; flight hormone: a small molecule with a receptor inside the cell B. Both are small molecules with receptors at the surface, because both travel in the blood C. Both are peptides with receptors inside the cell, because both are hormones D. Molting hormone: a small molecule with a receptor inside the cell; flight hormone: a peptide with a receptor at the surface

Question 5

Every cell of the insect carries the gene for the protein that hardens its new outer layer. After the molting hormone arrives, skin cells make large amounts of the protein over the next day; gut cells, reached by the same hormone, make no hardening protein.

Which statement describes the two cell types?

A. The skin cells carry the gene and the gut cells lack it B. Both express the gene; the gut cells break the protein down as fast as they make it C. Both carry the gene; the skin cells express it after the hormone arrives and the gut cells do not D. The hormone is turned into the protein inside the skin cells

Question 6

In a kidney cell, relay protein Y moves water channels to the surface only while Y carries a phosphate. A drug blocks the kinase that phosphorylates Y, and then the hormone arrives.

Predict what happens to the water channels.

A. The water channels stay inside, because Y is never switched on B. The water channels move to the surface as usual, because Y is still present C. The water channels move to the surface faster, because the phosphatase works unopposed D. The water channels stay inside, because the cell runs out of ATP

Question 7

In a snail's gland cell, a signal's receptor switches on kinase 1; kinase 1 phosphorylates kinase 2; kinase 2 phosphorylates kinase 3; and kinase 3 phosphorylates the protein that releases the gland's product. A chemical blocks kinase 1, and then the signal binds its receptor.

Predict which kinases are switched on, and whether the product is released.

A. Kinases 2 and 3; the product is released B. Kinase 3 only; the product is released C. All three kinases; the product is released D. No kinase is switched on; the product stays in the cell

Question 8

In a kidney cell, 8 receptors have the hormone bound, and over the next minute the cell's cAMP rises from 250 molecules to 2,650 molecules.

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

A. 31 cAMP per receptor B. 300 cAMP per receptor C. 331 cAMP per receptor D. 19,200 cAMP per receptor

Question 9

A gill cell of a snail carries a receptor for a neurotransmitter. When the neurotransmitter binds, negative ions flow into the cell within a millisecond. The same current flows just as fast when a drug blocks every kinase in the cell and the enzyme that makes cAMP.

What kind of receptor is this, and why does the drug change nothing?

A. A ligand-gated channel; the receptor's own shape change opens a channel through it, with no relay inside B. A G protein-coupled receptor; the G protein opens the channel without needing cAMP C. An intracellular receptor; the neurotransmitter enters the cell and opens the channel itself D. A cell-surface receptor that phosphorylates the channel before the drug can act

Question 10

The insect's flight hormone binds the same kind of cell-surface receptor on fat cells and on flight-muscle cells, and cAMP rises in both. The fat cells release stored sugar into the blood; the muscle cells burn fuel faster.

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

A. The flight hormone changes shape inside each kind of cell B. The fat cell's receptor makes a different second messenger from the muscle cell's C. The muscle cells receive more flight hormone than the fat cells do D. The kinases switched on by cAMP reach different response proteins in the two cell types

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 · 5 points

When the body is short of water, a peptide hormone from the brain reaches the cells lining the kidney's tubes and makes them save water. The hormone binds a G protein-coupled receptor on the cell surface; the G protein switches on an enzyme in the membrane; the enzyme makes cAMP from ATP; cAMP switches on a kinase; and the kinase phosphorylates the proteins that move water channels to the cell's surface. Researchers give cultured kidney cells one of four treatments, six dishes each: no hormone; the hormone; the hormone together with a drug that blocks the enzyme that makes cAMP; or the hormone for five minutes, then washed away, in cells first treated with a phosphatase inhibitor. Ten minutes after each treatment they count the water channels at the cell surface. The graph below shows the means for the first three treatments, and the error bars represent ±2SE; the fourth has still to be counted. In the dishes given the hormone alone, 10 receptors per cell have the hormone bound after one minute, and the cAMP in each cell has risen from 250 molecules to 3,250 molecules.

Water channels counted at the surface of kidney cells 10 minutes after each treatment, six dishes per treatment. Error bars represent ±2SE. Gridlines every 20 channels per cell. The fourth treatment has still to be counted.

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

Hint: Which thing did the researchers change between dishes, and which thing did they count to see the effect?

A full-credit answer: The independent variable is the treatment each dish receives (no hormone, the hormone, the hormone with the enzyme blocker, or the hormone then a wash in inhibitor-treated cells), and the dependent variable is the number of water channels at the cell surface after ten minutes.

Check the box for each point your answer earns

both: independent variable, the treatment each dish receives (no hormone, hormone, hormone with the enzyme blocker, hormone then wash with the phosphatase inhibitor); dependent variable, the number of water channels at the cell surface after ten minutes.

Do not award the point if the two are reversed, or if cAMP or the receptor is named as a variable.

Common slip: Naming cAMP as a variable. cAMP is a molecule of the relay; the researchers change the treatment and count channels.

(b) Identify the stage of the pathway that the rise in cAMP belongs to, and justify your choice. (1 point)

Hint: Where in the cell does the rise happen, and is it what the cell finally does differently?

A full-credit answer: The rise in cAMP is transduction, because cAMP is a second messenger made inside the cell that carries the message on from the receptor to the kinase. Reception is the hormone binding its receptor, and the cellular response is the water channels reaching the surface.

Check the box for each point your answer earns

transduction, because cAMP is a molecule inside the cell relaying the message from the receptor toward the response (the ligand binding is reception, and the channels reaching the surface is the response).

Do not award the point for 'reception' or for 'the response'.

Common slip: Calling the rise in cAMP the response. The response is what the cell finally does differently, the channels moving; cAMP is a step of the relay that gets there.

(c) Explain why the dishes given the hormone with the enzyme blocker show about the same number of channels as the dishes given no hormone. (1 point)

Hint: Which molecule does the blocked enzyme make, and what does that molecule normally switch on next?

A full-credit answer: With the enzyme blocked, no cAMP is made, so the kinase that cAMP would switch on stays off, the proteins that move the channels are never phosphorylated, and the channels stay inside the cell. The hormone still binds and the G protein is still switched on, but the message stops at the enzyme, so the cells look like cells given no hormone.

Check the box for each point your answer earns

the blocked enzyme is the one that makes cAMP, so no cAMP is made; with no cAMP the kinase stays off, the proteins that move the channels are never phosphorylated, and the channels stay inside, as in cells given no hormone (the hormone still binds and the G protein is still switched on).

Do not award the point for 'the hormone cannot bind' or for 'the drug destroys the channels'.

Common slip: Saying the drug stops the hormone binding. The block is inside, at the enzyme; binding and the G protein are unchanged, and everything after the enzyme stays off.

(d) Calculate the amplification at the cAMP step in the dishes given the hormone alone, as cAMP molecules made per bound receptor. (1 point)

Hint: Amplification is the number of new molecules made divided by the number of molecules that made them. Which of the two cAMP counts is the level before the hormone acted?

cAMP per receptor

Write down the values in the question:

bound receptors = 10

cAMP before = 250 molecules

cAMP after = 3,250 molecules

Write down the equation:

amplification = \frac{activated molecules}{activating molecules}

Substitute the values into the equation:

amplification = \frac{activated molecules}{activating molecules}

new cAMP = 3,250 - 250 = 3,000 molecules

amplification = \frac{3,000}{10}

amplification = 300 cAMP per receptor

A full-credit answer: The amplification at this step is 300 cAMP molecules per bound receptor.

Do not award the point for 325 (the final count divided by 10, with the starting 250 molecules counted as new) or for 25 (the starting count divided by 10).

(e) Predict the number of water channels at the surface in the fourth treatment (hormone for five minutes, then washed away, in cells treated with the phosphatase inhibitor), compared with cells given no hormone, and justify your prediction. (1 point)

Hint: What do phosphatases normally do to the relay proteins once the hormone has gone, and what does the inhibitor do to that?

A full-credit answer: After the wash, the treated cells still hold many channels at the surface, far more than the 12 per cell in cells given no hormone, because the phosphatases that would normally remove the phosphates the kinase added are inhibited. The proteins that move the channels stay phosphorylated and switched on although the hormone is gone, so the response continues.

Check the box for each point your answer earns

many channels stay at the surface (far more than the 12 per cell in cells given no hormone), because phosphatases normally switch the relay off by removing the phosphates the kinase added; with the phosphatases inhibited, the proteins that move the channels stay phosphorylated and switched on after the hormone is gone, so the response continues.

Do not award the point for 'the channels go back inside because the receptors are empty' or for a prediction with no reason.

Common slip: Predicting that the channels go back inside as soon as the hormone is washed away. The receptors are empty, but the relay is switched off by phosphatases, and those are blocked.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

An insect's molting hormone is a small nonpolar molecule built of four carbon rings. When it reaches the insect's skin cells, the hormone is found inside them within minutes, bound to a protein in the nucleus, and over the next day the cells make large amounts of the protein that hardens the new outer layer. The same insect's flight hormone, a chain of ten amino acids, reaches its fat cells in the blood, and within a minute the cells begin releasing stored sugar.

(a) Predict where the receptor for the molting hormone sits in a skin cell, and justify your prediction from the hormone's chemistry. (1 point)

A full-credit answer: The receptor sits inside the cell, in the cytosol or the nucleus. The molting hormone is small and nonpolar, so it dissolves into the oily middle of the plasma membrane and passes through; it is met inside by an intracellular receptor.

Check the box for each point your answer earns

inside the cell (in the cytosol or the nucleus), because a small nonpolar molecule dissolves into the membrane's oily middle and passes through, so it is met inside by an intracellular receptor.

Do not award the point for 'at the surface, because it arrives in the blood' or for the location with no reason from the hormone's chemistry.

Common slip: Putting the receptor at the surface because the hormone arrives in the blood. Arriving in the blood says nothing about crossing the membrane; a small nonpolar molecule crosses and is met inside.

(b) Describe what the bound receptor does to change what the skin cell makes. (1 point)

A full-credit answer: With the hormone bound, the receptor itself moves into the nucleus and attaches to the DNA. There the bound pair changes the expression of particular genes: the cell starts making large amounts of the hardening protein from that protein's gene. An intracellular receptor needs no relay of other molecules; the receptor is the molecule that reaches the DNA.

Check the box for each point your answer earns

the hormone binds its receptor, the bound pair moves into the nucleus and attaches to the DNA, and the cell changes the expression of particular genes, making more of the hardening protein; the receptor itself reaches the DNA, with no relay of other molecules.

Do not award the point for 'the receptor switches on a cascade of kinases' or for 'the hormone attaches to the DNA on its own'.

Common slip: Adding a relay of kinases. An intracellular receptor carries the message to the DNA itself; the relay belongs to cell-surface receptors.

(c) Explain why the new hardening protein takes many hours to appear. (1 point)

A full-credit answer: The hardening protein is a new protein: the cell had none of it before. The response is a change in gene expression, so the gene has to be expressed and the protein built from its instructions, and making new protein takes hours. Responses that only switch on proteins the cell already has are the fast ones.

Check the box for each point your answer earns

the response is a change in gene expression: the protein did not exist in the cell before, so its gene has to be expressed and the protein built, which takes minutes to hours (or longer); responses that use proteins the cell already has are the fast ones.

Do not award the point for 'the hormone crosses the membrane slowly' or for 'the hormone has far to travel'.

Common slip: Blaming the hormone's journey or its crossing of the membrane. The hormone is inside the cell within minutes; the slow part is making a protein that did not exist before.

(d) Explain why the fat cells' release of stored sugar in response to the flight hormone is so much faster. (1 point)

A full-credit answer: The flight hormone binds a receptor on the fat cell's surface, and the relay inside switches on, with phosphates from ATP, enzymes the cell already holds. Those enzymes release the stored sugar at once. Nothing new has to be made, so the response comes within a minute, while the skin cell's response waits for a new protein to be built.

Check the box for each point your answer earns

the flight hormone, a peptide, binds a cell-surface receptor, and the relay inside switches on, by phosphorylation, enzymes the cell already holds, which release the stored sugar; no new protein has to be made, so the response takes seconds to a minute.

Accept 'the enzymes are already there and are switched on by a phosphate'. Do not award the point for 'the flight hormone is smaller so it acts faster' or for 'the fat cells carry more receptors'.

Common slip: Saying the flight hormone acts faster because it is smaller. Both hormones reach their receptors within a minute; the difference is that one response uses proteins already present and the other needs new protein made.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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