Unit 4 · Topic 4.2 end-of-topic test
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. A ligand is held only if it fits the site and is attracted by its lining.
Which molecule is this receptor's ligand?
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?
Hormone C, which lowers the calcium in the blood, is a chain of 32 amino acids. Thyroid hormone is built of two carbon rings with iodine atoms attached, a few dozen atoms in all.
How are the two ligands classified?
Bone cells carry a cell-surface receptor for hormone C. In one cell line the receptor's intracellular domain has been removed, leaving the ligand-binding domain and the part set in the membrane. Hormone C binds the normal and the shortened receptors equally well. In normal cells a relay protein inside gains a phosphate within a minute of hormone C binding; in the cells with the shortened receptor it stays as it was.
What does the comparison show about how the message gets into the cell?
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 option classifies the three events?
Thyroid hormone is a small nonpolar molecule that dissolves in oil. Hormone C is a chain of 32 amino acids. Both reach a bone cell in the blood.
Where does each hormone meet its receptor?
Every cell in a person carries the gene for amylase, the enzyme that digests starch. Salivary gland cells make large amounts of amylase; muscle cells make none. After a signal reaches the salivary cells, they make three times as much amylase over the next few hours.
What is the difference between the salivary cells and the muscle cells, and what changed in the salivary cells after the signal?
In a tadpole's tail cells, the receptor for thyroid hormone is found in the nucleus, attached to the DNA, 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?
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?
In a plant cell, relay protein V is switched off while it carries a phosphate and switched on when the phosphate is removed. A drug that blocks the cell's phosphatases is added, and then the signal arrives.
Predict what happens to V's activity compared with untreated cells.
In the fruit fly cell, the growth factor's receptor switches on kinase A; kinase A phosphorylates kinase B; kinase B phosphorylates kinase C; and kinase C phosphorylates the proteins that start cell division. A chemical blocks kinase B, and then the growth factor binds its receptor.
Predict which kinases are switched on.
A fat cell is given hormone S, which binds a receptor on the cell's surface. Within a minute the cAMP inside the cell has risen from 300 molecules to 6,300, and kinases throughout the cytosol are switched on.
Which statement describes the role of cAMP here?
In the fat cell, 20 receptors have hormone S bound, and over the next minute the cell's cAMP rises from 300 molecules to 6,300 molecules.
What is the amplification at this step, as cAMP molecules made per bound receptor?
In a salivary gland cell, 6 bound receptors lead to 240 molecules of the cAMP-making enzyme being switched on, and those enzyme molecules go on to make 12,000 molecules of cAMP.
What is the amplification at the second step, as cAMP molecules made per enzyme molecule?
In fat cells, the response to hormone S ends within minutes of the hormone being washed away. In one dish the cells are first treated with a drug that blocks the enzyme that breaks cAMP down. Hormone S is added to the dish, and after five minutes it is washed away.
Predict what happens to fatty acid release from the treated cells over the next ten minutes, compared with untreated cells.
Salivary gland cells were given one of three treatments, six dishes each: signal S alone; signal S together with a drug that holds G proteins switched off; or no signal. After one minute the cAMP in the cells was measured; the graph below shows the means, and the error bars represent ±2SE. In every dish given signal S, the same amount of S was bound to the receptors. Tested on its own, the cAMP-making enzyme from the drug-treated cells works normally.
What do the results show about the G protein's job in this pathway?
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. A patch of membrane torn from the cell, holding the receptor and nothing from inside the cell, shows the same current when acetylcholine is added, and a drug that blocks every kinase in the cell changes nothing.
What kind of receptor is this, and how does it produce the current?
Three responses to signals are recorded. A gland cell in the gut releases stored mucus 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 option classifies the three responses, in order?
The model below shows the growth factor pathway of a fruit fly cell as boxes numbered 1 to 6. Researchers count the phosphate groups on kinase B (box 4) after the growth factor is added.
Which stage of the pathway does this measurement belong to, and at which box does the message cross the membrane?
(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
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
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 and kinase 3, and the defense protein after them, are affected.
Check the box for each point your answer earns
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) Using the error bars, justify the claim that the rise in defense protein produced by the sugar fragment needs kinase 2. (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, and 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: the rise needs kinase 2 to be phosphorylated.
Check the box for each point your answer earns
Do not award the point for a comparison of means alone with no use of the bars, or for 'the blocker lowered the protein' (the blocker dishes are compared with fresh medium, and their bars overlap).
Common slip: Comparing the blocker dishes with the fragment dishes and stopping. That shows the blocker removed most of the rise; the claim that the rise needs kinase 2 is made by showing the blocker dishes are no different from fresh medium.
(a) Describe what happens at the receptor (box 2) when hormone M binds, and how this passes the message to the G protein (box 3). (1 point)
A full-credit answer: Hormone M 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 M 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
Accept 'the receptor changes shape on its inner side and that switches on the G protein'. Do not award the point for 'hormone M passes through the receptor to the G protein' or for 'the receptor phosphorylates the G protein'.
Common slip: Sending hormone M 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
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) Predict the level of cAMP and the darkening of the skin when hormone M reaches the drug-treated cells, compared with untreated cells given hormone M, and justify your prediction using the model. (1 point)
A full-credit answer: cAMP rises in the treated cells just as in untreated cells, because everything before the X works: hormone M binds, the receptor switches on the G protein, the G protein switches on enzyme E, and enzyme E makes cAMP. But the skin stays pale. The X sits between cAMP and kinase K, so the cAMP that piles up can no longer switch kinase K on, and the pigment granules stay clumped.
Check the box for each point your answer earns
Do not award the point for 'cAMP stays low' (the blocked step is after cAMP is made) or for a prediction with no reference to the position of the block in the model.
Common slip: Predicting that cAMP stays low. A block stops everything after it and nothing before it; the X is after cAMP, so the second messenger is made and then goes nowhere.
(d) Cells of the frog's adrenal gland carry the same kind of receptor for hormone M, the same G protein, enzyme E and kinase K, and respond to hormone M by releasing a steroid hormone. Explain why the two cell types respond differently to hormone M. (1 point)
A full-credit answer: Hormone M is the same molecule at both cells, and both run 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. The same ligand, through the same kind of receptor, ends at whatever proteins the cell has, so what a cell does with the message is decided by the cell.
Check the box for each point your answer earns
Do not award the point for 'the adrenal cells receive more hormone M', 'hormone M 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.