Practice questions · Topic 4.2

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

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Watch first: Topic 4.2 summary: across the membrane and inside

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The ligand stays outside and the receptor's shape change crosses the membrane; inside, a relay of switches and second messengers amplifies the message; the cell decides what it does with the message.

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 gives the reasoning. For the 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. The first free-response question walks you through the reasoning 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 is an enzyme that changes glucose into a product, and the other sugar is not a substrate it can change D. The receptor binds any sugar, and the cell decides afterwards which one to respond to

Question 2

A peptide hormone reaches a mosquito’s salivary gland cells, and the cells begin releasing an enzyme. Researchers follow a labeled form of the hormone over the first minute: at 2 seconds it is bound at the cell surface; at 5 seconds the receptor's intracellular domain has changed shape; at 30 seconds a relay protein inside the cell carries a new phosphate. At every time, the amount of labeled hormone inside the cells is zero.

What crossed the membrane to start the change inside?

A. The hormone itself, through a pore in the receptor B. A phosphate group, carried in by the hormone from outside C. A change in the receptor's shape, passed to its intracellular domain D. Sugar, carried in by the hormone

Question 3

A receptor set in a cell's membrane; positions 1 to 4 are marked.

The drawing below shows a receptor set in a cell's membrane, with four positions numbered 1 to 4.

Which numbered position marks the site where the ligand binds?

A. Position 1 B. Position 2 C. Position 3 D. Position 4

Question 4

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

Which of the following classifies each ligand and places its receptor?

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

Question 5

After the molting hormone arrives, an insect's skin cells make large amounts of the protein that hardens its new outer layer.

Which of the following is the gene for the hardening protein?

A. The hardening protein itself, stored in the skin cells until the hormone arrives B. The molting hormone, which carries the instructions for the protein into the skin cells C. The stretch of the skin cell's DNA that carries the instructions for making the hardening protein D. The chain of amino acids that the skin cell joins together to build the hardening protein

Question 6

In a kidney cell, relay protein Y moves water channels to the surface only while Y carries a phosphate. One cell line carries a changed form of Y: the amino acid that the kinase would phosphorylate has been replaced, so no kinase can add a phosphate to Y. The hormone arrives at these cells.

Which of the following best predicts what happens to the water channels, and why?

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 as usual, because the kinase adds the phosphate to a different part of Y D. The water channels stay inside, because the hormone can no longer bind its receptor

Question 7

When a fish's fin is cut, cells at the wound release molecule N into the fluid around them. Fin cells that carry the receptor for molecule N begin dividing within a day, and the fin regrows.

Which of the following is the growth factor in this case?

A. Molecule N B. The receptor on the fin cells C. The dividing fin cells D. The kinases that relay the message inside the fin cells

Question 8

In a kidney cell, 6 receptors have the hormone bound, and over the next minute the cell's cAMP rises from 330 molecules to 2,430 molecules.

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

A. 55 cAMP per receptor B. 350 cAMP per receptor C. 405 cAMP per receptor D. 12,600 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. Throughout, the cell's cAMP stays at its resting level, and no relay protein inside the cell gains a phosphate.

What kind of receptor is this, and why does the current need no change inside the cell?

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 whose inner part is a kinase; it phosphorylates the channel, which then opens

Question 10

The insect's flight hormone binds the same kind of cell-surface receptor on fat cells and on flight-muscle cells. The fat cells release stored sugar; the muscle cells burn fuel faster. When cAMP itself is injected into fat cells with no hormone present, they release stored sugar; when cAMP is injected into flight-muscle cells, they burn fuel faster.

What does the injection result show about where the two pathways differ?

A. Before cAMP, at the receptor: the two cell types bind the hormone differently B. At cAMP: the two cell types make different second messengers C. Before cAMP, at the G protein: the two cell types' G proteins switch on different enzymes D. After cAMP: 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 (word equation: ATP → cyclic AMP + two linked phosphates); cAMP switches on a kinase; and the kinase phosphorylates the proteins that move water channels to the cell's surface. Researchers give 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 cells first treated with a phosphatase inhibitor, then given the hormone for five minutes, then washed free of hormone. 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 280 molecules to 2,180 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: The blocked enzyme is the enzyme that makes cAMP, and cAMP is what switches on the kinase.
The kinase is what phosphorylates the proteins that move the channels.
With no cAMP made, the kinase stays off, so those proteins are never phosphorylated and the channels stay inside.
The hormone still binds and the G protein is still switched on, but the message stops at the enzyme.
So these 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 the increase in cAMP molecules 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 = 280 molecules

cAMP after = 2,180 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 = 2,180 - 280 = 1,900 molecules

amplification = \frac{1,900}{10}

amplification = 190 cAMP per receptor

A full-credit answer: The amplification at this step is 190 cAMP molecules per bound receptor.

Do not award the point for 218 (2,180 divided by 10, with the resting 280 molecules not subtracted) or for 28 (the resting count divided by 10).

(e) Support the claim that the cells of the fourth treatment (hormone for five minutes, then washed away, in cells treated with the phosphatase inhibitor) still hold many channels at their surface when the researchers count, using the pathway. (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: Phosphatases normally switch the relay off by removing the phosphates the kinase added.
In these cells the phosphatases are inhibited.
So the proteins that move the channels keep their phosphates.
So those proteins stay switched on although the hormone is gone.
So the channels stay at the surface, far more than the 12 per cell of the no-hormone dishes.

Check the box for each point your answer earns

the evidence (in these cells the phosphatases are inhibited; phosphatases normally switch the relay off by removing the phosphates the kinase added) AND the reasoning (so the proteins that move the channels keep their phosphates and stay switched on after the hormone is gone, so the channels stay at the surface, far more than the 12 per cell in the no-hormone dishes).

Do not award the point for 'the phosphatases are blocked' with no link to the channels, or for 'the receptors are empty, so the channels go back inside'.

Common slip: Naming the inhibitor and stopping. Supporting the claim needs the link: what the phosphatases would have done, and what their inhibition leaves switched on.

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, 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. The flight hormone binds a receptor on the fat cell's surface; inside the cell, cAMP rises and a kinase phosphorylates the enzymes that release the stored sugar.

(a) Determine where the receptor for the molting hormone sits in a skin cell, using 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.
So it binds a receptor inside the cell: an intracellular receptor.

Check the box for each point your answer earns

the decision (inside the cell, in the cytosol or the nucleus) AND the reasoning it rests on (a small nonpolar molecule dissolves into the membrane's oily middle and passes through, so it binds a receptor inside the cell, an intracellular receptor).

Do not award the point for the location alone, or for 'at the surface, because it arrives in the blood'.

Common slip: Deciding on the surface because the hormone arrives in the blood. Arriving in the blood does not decide whether a hormone crosses the membrane; a small nonpolar molecule crosses and binds a receptor inside the cell.

(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 cell's gene expression: 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 its gene expression, making more of the hardening protein; the receptor itself reaches the DNA, with no relay of other molecules.

Accept: the receptor already waits in the nucleus, binds the hormone there, and the bound receptor attaches to the DNA.

Do not award the point for 'the receptor switches on a phosphorylation cascade' 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.
Building a protein takes hours.

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 hours.

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) Within a minute of the flight hormone leaving the blood, the fat cells stop releasing sugar. Describe two changes inside a fat cell that end the response. (1 point)

A full-credit answer: An enzyme breaks the cAMP down, so the cAMP concentration falls and the kinase switches off.
Phosphatases remove the phosphates the kinase added, so the enzymes that release the sugar return to their resting shape and stop.

Check the box for each point your answer earns

two of: the flight hormone leaves the receptor and is removed, so the receptor returns to its resting shape; an enzyme breaks cAMP down, so the kinase switches off; phosphatases remove the phosphates the kinase added, so the sugar-releasing enzymes stop.

Do not award the point for one change alone, or for 'the cell has no sugar left' or 'the receptor is destroyed'.

Common slip: Naming one change and stopping. The task asks for two: the cell ends a response by removing the ligand, destroying the second messenger and removing the phosphates.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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