4 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 4
In humans, the hormone epinephrine causes liver cells to break down glycogen and release glucose, and it causes heart muscle cells to contract more strongly. Which statement best explains how the same hormone produces different responses in these two cell types?
Answer and reasoning
AThe two cell types differ in the receptor, relay or target proteins that link epinephrine binding to a response.Correct A signal transduction pathway links reception to a response. Liver cells and heart muscle cells contain different receptor, relay or target proteins, so binding of the same hormone leads to glycogen breakdown in one and stronger contraction in the other.
BEpinephrine changes its shape in a different way in each tissue, so it carries a different message to each tissue. A student who thinks the ligand changes shape to carry the message picks this. Binding changes the receptor, not the hormone; the difference lies in the proteins of each cell type.
CEpinephrine enters each cell and acts directly on the different proteins that are found in each cell type. A student who thinks signals enter cells and act directly picks this. Epinephrine binds receptors at the cell surface; relay molecules inside each cell carry the signal to that cell's targets.
DDifferent amounts of epinephrine reach the two tissues, and the amount sets which response will occur. A student who thinks the signal alone sets the response picks this. The kind of response depends on the proteins in the receiving cell; different cell types respond differently even to the same concentration.
Many signal transduction pathways include a phosphorylation cascade. Which statement correctly describes each step of such a cascade?
Answer and reasoning
AAn activated kinase adds a phosphate group to the next protein, and the phosphate's energy powers the response. A student who thinks the phosphate supplies energy for the response picks this. Adding a phosphate changes the target protein's shape and activity; that change, not a delivery of energy, passes the signal on.
BAn activated kinase hands its own phosphate group on to the next protein, and after that it is no longer active. A student who thinks one phosphate is handed down the chain picks this. Each kinase uses ATP to add a new phosphate group, and an activated kinase can phosphorylate many target molecules.
CAn activated kinase moves a phosphate group from ATP to the next protein, and this changes the target's shape.Correct A protein kinase adds a phosphate group, taken from ATP, to a specific protein. The phosphate changes the protein's shape and so its activity; in a cascade, the protein phosphorylated is often the next kinase, which then phosphorylates the next protein.
DAn activated kinase passes the signal molecule itself to the next kinase, and that kinase passes it on. A student who thinks the ligand itself travels along the pathway picks this. The signal molecule stays bound to the receptor; each kinase passes on activation by phosphorylating the next protein.
Oxytocin is a peptide hormone made of nine amino acids. Epinephrine is a hormone that is a small molecule made from a single amino acid. Which statement about these two hormones is correct?
Answer and reasoning
AOnly oxytocin can bind a receptor, as all chemical messengers must be peptides or proteins. A student who thinks every signal molecule must be a protein picks this. Small molecules such as epinephrine are also chemical messengers recognized by receptors.
BEach binds its own receptor, as that receptor's binding domain matches the hormone's shape and chemistry.Correct A ligand-binding domain recognizes a specific chemical messenger, which can be a peptide, such as oxytocin, or a small molecule, such as epinephrine. Each hormone binds the receptor whose binding site complements its shape and chemical properties.
CEither hormone can bind the other's receptor, as receptors can bind any molecule that reaches them. A student who thinks any molecule can bind any receptor picks this. A receptor's binding domain matches one particular messenger; oxytocin and epinephrine differ greatly in shape and chemistry.
DEach is broken down by its receptor, just as an enzyme breaks down its substrate when it binds. A student who thinks receptors act like enzymes picks this. Binding changes the receptor's shape; the receptor does not chemically change the hormone.
After a meal, cells in the pancreas release the peptide hormone insulin, which causes muscle cells throughout the body to take up more glucose from the blood. Which statement best describes how insulin reaches and acts on muscle cells?
Answer and reasoning
AIt is sent along a vessel that leads from the pancreas directly to the muscle cells alone. A student who thinks hormones are sent directly to their targets picks this. Insulin enters the general circulation and reaches cells throughout the body; only cells with insulin receptors respond.
BIt travels in the bloodstream and binds receptors in the plasma membrane of muscle cells.Correct Insulin is a hormone: it is carried in the bloodstream over long distances to target cells, binds receptors at their surface, and the resulting signaling pathway leads the cells to take up more glucose.
CIt binds glucose in the blood and then carries this glucose along with it into muscle cells. A student who thinks insulin carries glucose picks this. Insulin is the signal; glucose enters the muscle cells through transport proteins after the signaling pathway acts.
DIt enters the muscle cells, and inside them it acts directly to pull glucose in from outside. A student who thinks a signal must enter a cell and act directly picks this. Insulin binds a receptor at the cell surface; relay molecules inside carry the signal on.
In preparation: 0 of 4 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
4.2.A.1 Signal transduction pathway Fix
Signal transduction pathway
The series of molecular events that links the binding of a signal to a receptor with a response by the cell. The signal is passed on (transduced) by molecules inside the cell, often in several steps, so the signal molecule itself need not enter the cell.
Reception, transduction and response
The three stages of cell signaling: a receptor recognizes and binds the signal (reception); relay molecules pass the signal on inside the cell (transduction); and a cell activity changes, such as the activity of an enzyme, secretion, or the expression of genes (response).
Students often think A signal molecule causes a response by entering the cell and acting directly on the cell's proteins or genes. In fact No. Many signals, such as peptide hormones, bind a receptor at the cell surface and never enter; the receptor and the relay molecules inside the cell carry the information to the proteins that respond. Some small nonpolar signals do enter, but they act by binding an intracellular receptor.
Students often think The signal molecule itself is handed along the pathway, from the receptor to each relay protein in turn, until it reaches the target. In fact No. The ligand stays bound to the receptor. What is passed on is a change: each activated molecule changes the shape or activity of the next, or makes a new molecule such as cAMP.
4.2.A.2 Protein kinase Fix
Protein kinase
An enzyme that transfers a phosphate group from ATP to a specific protein (phosphorylation). Adding the phosphate changes the protein's shape and so its activity.
Phosphorylation cascade
A series of protein kinases in which each activated kinase phosphorylates, and so activates, the next. Because each kinase can phosphorylate many molecules, the cascade can amplify the signal; other enzymes (phosphatases) remove the phosphate groups, which helps switch the pathway off when the signal ends.
Students often think The phosphate group added by a kinase supplies the energy that powers the next step of the response. In fact No. Adding a phosphate group changes the shape, and so the activity, of the protein it is added to; that change, not a release of energy, is what passes the signal on.
Students often think Once a signaling pathway has been switched on, its components stay active even after the signal has gone. In fact No. In most pathways, components return to their resting state when the signal is removed; for example, phosphatases remove phosphate groups, and ligand-gated channels return to their resting state, so the response stops.
4.2.B.1 Ligand Fix
Ligand
A molecule that binds specifically to another molecule. In cell signaling, the ligand is the chemical messenger that binds a receptor protein.
Receptor protein
A protein in a target cell that recognizes and binds a specific ligand; binding changes the receptor's shape, which starts the cell's response. A cell responds to a signal only if it has a receptor for it.
Ligand-binding domain
The region of a receptor that recognizes and binds the ligand. Its shape and chemical properties, such as the charges and polarity of its amino acids, complement those of one particular messenger, which may be a peptide (protein) or a small molecule.
G protein-coupled receptor (GPCR)
A eukaryotic receptor protein whose single chain crosses the plasma membrane seven times. When its ligand binds outside the cell, the receptor changes shape and activates a G protein on the cytoplasmic side of the membrane.
G protein
A protein on the cytoplasmic side of the plasma membrane that is activated by a ligand-bound G protein-coupled receptor; when active it binds GTP, and it then activates other proteins, such as an enzyme that makes the second messenger cAMP.
Cell-surface and intracellular receptors
Receptors for ligands that cannot cross the plasma membrane, such as peptide hormones, are in the plasma membrane with their binding domain outside the cell. Small nonpolar ligands, such as steroid hormones, can diffuse through the membrane and bind receptors in the cytoplasm or the nucleus.
Students often think A receptor recognizes its ligand by size, so any molecule small enough to fit the binding site binds it. In fact No. A ligand-binding domain recognizes its ligand by a close match in shape and in chemical properties, such as charge and polarity; molecules of similar size that do not match are not bound.
Students often think Signal molecules must be peptides or proteins. In fact No. A ligand can be a peptide or protein, such as oxytocin or insulin, or a small molecule, such as epinephrine or a steroid hormone.
4.2.B.2 Signal amplification Fix
Signal amplification
The increase in the number of active molecules at successive steps of a signaling pathway when one active molecule activates or makes many molecules of the next kind, so that a few ligand molecules can produce a large response.
Cellular response
The change in a target cell produced by a signaling pathway, such as cell growth, secretion of molecules or a change in gene expression. Some responses, such as activation of enzymes already present, occur within seconds; responses that need new proteins take longer.
Intracellular domain of a receptor
The part of a membrane receptor inside the cell. When a ligand binds outside, the intracellular domain changes shape, which lets it activate relay molecules and so starts transduction of the signal.
Second messenger
A small, nonprotein molecule or ion, such as cyclic AMP (cAMP), that is made or released inside a cell in response to a signal and spreads through the cytoplasm, relaying the signal and amplifying it.
Cyclic AMP (cAMP)
A second messenger made from ATP by an enzyme that is activated, for example, by a G protein. One active enzyme molecule makes many cAMP molecules, and cAMP typically activates a protein kinase.
Hormone (long-distance messenger)
A signaling messenger that, in animals, is released into the bloodstream and travels long distances to target cells, where it binds receptors and starts a signal transduction pathway or, for some hormones, acts through an intracellular receptor.
Ligand-gated channel
A receptor protein that is also an ion channel. Binding of its ligand changes the protein's shape, opening or, for some channels, closing the channel and so changing the flow of specific ions across the membrane by diffusion.
Students often think Amplification means that the cell makes extra copies of the signal molecule, so more signal is present inside the cell. In fact No. Amplification means that each activated molecule in the pathway activates or makes many molecules of the next kind, so the number of active molecules grows at each step; the number of ligand molecules does not change.
Students often think When a ligand binds, the ligand changes shape, and the changed ligand carries the message on to the rest of the pathway. In fact No. Binding changes the shape of the receptor, including its intracellular domain, and that change starts transduction; the ligand is not the part that passes on the message.
20 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 20
The model shows a signal transduction pathway in a eukaryotic cell. The signal molecule binds the receptor, and later the expression of a gene in the nucleus changes. Based on the model, which statement best explains how binding at the receptor leads to this change?
Answer and reasoning
AThe signal molecule is handed from relay protein to relay protein until the signal molecule reaches the gene. A student who thinks the ligand itself is handed along the pathway picks this. The model shows the signal molecule bound to the receptor outside the cell; what passes along the pathway is activation of one protein by the next.
BThe receptor changes shape when bound and activates relay proteins in turn until a transcription factor reaches the gene.Correct In the model, the signal molecule stays outside the cell, bound to the receptor. Binding changes the receptor, which activates relay protein 1, then relay protein 2, and then a transcription factor that enters the nucleus and changes the gene's expression: the pathway links reception to the response.
CThe receptor lets the signal molecule into the cytoplasm, and then the signal molecule moves to the nucleus. A student who thinks receptors let their ligands into the cell picks this. In the model, the signal molecule stays outside the cell, at the outer end of the receptor.
DThe signal molecule gives energy to the receptor, and that energy is then passed along the proteins to the gene. A student who thinks the signal supplies energy for the response picks this. The signal carries information: its binding changes the receptor's shape, and each relay protein changes the activity of the next.
Kinases K1, K2 and K3 are part of a phosphorylation cascade in a hypothetical cell. The graph shows the percentage of each kinase in its phosphorylated form after a ligand was added at time 0 and removed at 5 minutes. Which statement is best supported by the data?
Answer and reasoning
AK1, K2 and K3 all stayed phosphorylated after the ligand was removed, so the response kept going. A student who thinks a pathway stays on once activated picks this. After the ligand was removed at 5 minutes, all three kinases fell to 5% to 7% phosphorylated by 10 minutes.
BEach kinase lost its phosphate as the next one gained it, so only one kinase was active at any time. A student who thinks one phosphate group is passed down the cascade picks this. From 3 to 5 minutes all three kinases are more than 50% phosphorylated at the same time.
CK1, K2 and K3 became phosphorylated at the same moment, as the ligand reached all of them at once. A student who thinks every component of a pathway is activated at once picks this. At 1 minute K1 is 70% phosphorylated while K3 is only 5%; K3 rises later.
DK1 was phosphorylated first, then K2, then K3, which fits a cascade running from K1 to K2 to K3.Correct At 1 minute K1 is 70% phosphorylated, K2 25% and K3 5%; K2 reaches its plateau by 3 minutes and K3 by 4 to 5 minutes. Each kinase becomes phosphorylated after the one before it, as expected if K1 phosphorylates K2 and K2 phosphorylates K3.
The model shows a signaling pathway in a hypothetical cell. A drug binds the active site of kinase Z and prevents it from phosphorylating other proteins. If the drug is present when the signal binds the receptor, which outcome is predicted?
Answer and reasoning
AKinase X, kinase Y and the response are all blocked, as the drug stops every step. A student who thinks blocking one step stops the whole pathway picks this. Kinase X is activated by the receptor, before kinase Z in the sequence, so the drug does not prevent its phosphorylation.
BKinase Z alone is affected; kinase Y and the response are activated as usual. A student who considers only the blocked component picks this. Kinase Y is activated by kinase Z, so when kinase Z cannot phosphorylate it, kinase Y and every later step stay inactive.
CKinase X is phosphorylated as usual, but kinase Y and the response are not activated.Correct In the model, the receptor activates kinase X, kinase X phosphorylates kinase Z, and kinase Z phosphorylates kinase Y. Blocking kinase Z leaves the steps before it working, so kinase X is phosphorylated, but kinase Y, the transcription factor and the response are not activated.
DKinase X activates kinase Y directly in place of kinase Z, so the response still occurs. A student who thinks a pathway reroutes because the cell needs the response picks this. In the model, kinase Y is activated only by kinase Z; nothing in the pathway lets kinase X take its place.
Cells of type P respond to hormone X, and cells of type Q do not. Both cell types are exposed to the same concentration of X. When Q cells are given a gene that makes them produce the receptor protein for X found in P cells, the Q cells respond to X. Which claim is best supported by these observations?
Answer and reasoning
AA cell responds to X only if it has a receptor protein that recognizes X, which Q cells lacked.Correct Q cells did not respond until they made the receptor protein from P cells; nothing else was changed. So recognition of X by a specific receptor protein in the target cell is needed for signaling to begin.
BThe new protein carries X into the Q cells, where X itself then brings about the whole response. A student who thinks receptors let their ligands into the cell picks this. The observations show only that the receptor is needed; a receptor's role is to recognize X and change shape, not to carry X in.
CHormone X changes the genes of Q cells, so they turn into P cells with P cells' responses. A student who thinks signals change a cell's genes picks this. The Q cells were given a gene by the experimenters; hormone X does not alter DNA, and nothing indicates that Q cells became P cells.
DAny receptor protein added to the Q-cell membrane would have worked, as X can bind to any receptor. A student who thinks any molecule can bind any receptor picks this. Binding is specific: only a receptor whose binding domain matches X recognizes it, which is why the receptor from P cells was needed.
Gland cells of a hypothetical animal secrete a protein when hormone X binds its receptor. Cells were given no hormone, X, X together with an antibody that binds the receptor's ligand-binding domain, or the antibody alone. The graph shows mean protein secretion (n = 6 cultures for each treatment; error bars, ±2 SE). Which conclusion is best supported?
Answer and reasoning
AThe antibody only partly blocked X, as X with antibody had a higher mean than cells given no hormone. A student who judges differences by the means alone picks this. The mean for X with antibody (17) is above 10, but the error bars overlap, so this difference may be due to chance.
BThe antibody is proven to stop all binding of X, as the X-with-antibody bars overlap the no-hormone bars. A student who thinks overlapping error bars prove no effect picks this. Overlap means no significant effect of X was shown with the antibody present; it does not prove that no X at all could bind.
CNo conclusion can be drawn, as the cells given no hormone still secreted some of the protein during the test. A student who thinks a control must show zero picks this. The no-hormone cells show the baseline secretion (about 10 units/h), and the treatments are judged against it.
DThe antibody blocked X's effect, as X with antibody, unlike X alone, overlaps the no-hormone bars.Correct X alone (57 to 71 units/h) is clear of no hormone (6 to 14), so X increases secretion. X with antibody (9 to 25) overlaps no hormone, so with the binding domain covered, X had no significant effect: recognition by the receptor is needed for signaling to begin.
Working Read each bar as mean ± 2 SE: no hormone 10 (6 to 14); X 64 (57 to 71); X + antibody 17 (9 to 25); antibody alone 9 (5 to 13). X versus no hormone: 57 > 14, no overlap, so a likely real increase. X + antibody versus no hormone: 9 < 14, overlap, so no significant increase shown. Antibody alone versus no hormone: overlap, so the antibody by itself has no effect shown. The antibody, which covers the ligand-binding domain, removes the effect of X.
The model shows the ligand-binding domain of a receptor and four molecules, A to D. Charges on the walls of the binding site and on the sides of each molecule are shown. Based on the model, which molecules are expected to bind the receptor?
Answer and reasoning
AA and B, as both of them have the same shape as the binding site A student who thinks binding depends on shape alone picks this. In B the negative side meets the pocket's negative wall and the positive side its positive wall, so like charges repel even though the shape fits.
BA, B and D, as each of them is small enough to fit inside the site A student who thinks receptors recognize ligands by size alone picks this. A, B and D are all small enough to enter the pocket (C is too wide), but B's charges face like charges on the walls and D matches neither the shape nor the charges.
CAll four, as any molecule that reaches a binding site is able to bind it A student who thinks any molecule can bind any receptor picks this. A binding domain recognizes a specific messenger; B, C and D each fail to match in shape, charge or both.
DA only, as only A matches the site in both its shape and its chargesCorrect Molecule A has the pocket's shape, and its positive side faces the pocket's negative wall while its negative side faces the positive wall, so both shape and charges are complementary. B has the right shape but like charges facing each other; C has complementary charges but the wrong shape and is too wide; D has neither.
The model represents a G protein-coupled receptor in the plasma membrane of a eukaryotic cell. Which statement correctly describes the model?
Answer and reasoning
AThe G protein binds the ligand outside the cell and carries it across into the cytoplasm. A student who thinks the G protein is the receptor picks this. In the model the ligand is bound by the receptor's loops outside the cell; the G protein is on the cytoplasmic side.
BThe receptor spans the membrane, and the G protein is on its cytoplasmic side.Correct The model shows the receptor's single chain crossing the membrane seven times, with the ligand bound outside the cell, and the G protein inside the cell, on the cytoplasmic side, next to the receptor and the enzyme it activates.
CThe receptor is a channel that lets the ligand pass through the membrane into the cytoplasm. A student who thinks receptors let their ligands into the cell picks this. The model shows the ligand outside, bound to the receptor; a G protein-coupled receptor is not a channel.
DThe receptor breaks the ligand down and passes the pieces on to the G protein inside. A student who thinks receptors act like enzymes on their ligands picks this. The ligand is bound, not broken down; binding changes the receptor's shape, which activates the G protein.
In human liver cells, both epinephrine, a small molecule, and glucagon, a peptide of 29 amino acids, cause the level of cAMP inside the cell to rise. Each acts through a G protein-coupled receptor. Which reasoning best explains how two such different ligands lead to the same change inside the cell?
Answer and reasoning
AEach binds its own receptor, and both receptors activate the same kind of G protein and relay pathway.Correct Epinephrine and glucagon differ in shape and chemistry, so each is recognized by a different receptor. Both receptors are G protein-coupled receptors that, when bound, activate the same kind of G protein, which activates the enzyme that makes cAMP: different receptions feed one transduction pathway.
BEach ligand is turned into cAMP once it has bound its receptor at the surface of the liver cell. A student who thinks the ligand becomes the second messenger picks this. cAMP is made from ATP by an enzyme that the G protein activates; the hormones remain outside the cell, bound to their receptors.
CEach ligand bypasses its receptor and binds directly to the G protein on the inner side of the cell. A student who thinks the G protein binds the ligand picks this. The G protein is inside the cell; each hormone binds its receptor outside, and the receptor activates the G protein.
DEach ligand can bind and activate any of the G protein-coupled receptors present in the liver cell's membrane. A student who thinks any molecule can bind any receptor picks this. Receptors are specific: a small molecule and a 29-amino-acid peptide cannot both match the same binding site.
Estrogen is a small, nonpolar steroid hormone that binds a receptor inside its target cells. Insulin is a peptide hormone of 51 amino acids whose receptor is in the plasma membrane. Which statement best explains this difference in where the receptors are located?
Answer and reasoning
AInsulin's receptor lets insulin into the cell, so insulin acts inside the cell, just as estrogen does. A student who thinks receptors let their ligands in picks this. Insulin binds its receptor outside the cell and does not need to enter to signal; its receptor's job is to recognize it and change shape.
BEstrogen is carried in by a transport protein, as every hormone needs one to cross the membrane. A student who thinks every substance needs a transport protein picks this. Small nonpolar molecules such as estrogen diffuse directly through the lipid bilayer.
CEstrogen can diffuse through the lipid bilayer, but insulin is too large and too polar to cross it.Correct Small nonpolar molecules such as steroid hormones pass through the hydrophobic interior of the membrane and can reach receptors in the cytoplasm or nucleus. Insulin is a large polar peptide that cannot cross the bilayer, so its receptor must be at the cell surface.
DEstrogen alters gene expression, and only a receptor inside a cell can lead to that kind of change in it. A student who thinks only intracellular receptors affect gene expression picks this. Pathways that start at cell-surface receptors can also change gene expression; receptor location depends on whether the ligand can cross the membrane.
Hormone H causes a response in cells of a hypothetical tissue. A student wants to find out whether H's receptor is on the cell surface or inside the cell. H can be attached to beads that are too large to cross the plasma membrane but that leave H able to bind a receptor. Which procedure would best answer the student's question?
Answer and reasoning
AGive some cultures H attached to beads and measure each one's response, with no other cultures used. A student who thinks no comparison group is needed picks this. Without free H for comparison, a weak or absent response to bead-attached H could not be interpreted.
BCompare free H at 37 °C with H attached to beads at 25 °C, using matching cultures. A student who thinks two factors can be changed at once picks this. The groups differ in temperature as well as in whether H can enter, so a difference could be caused by either.
CGive cultures increasing amounts of free H and record how strongly each one of the cultures responds. A student who thinks measuring the effect in more detail tests its cause picks this. Free H can reach receptors on the surface and inside, so a dose–response measurement cannot tell where the receptor is.
DCompare the response to free H with the response to H attached to beads, using matching cultures.Correct Bead-attached H can reach only receptors on the cell surface, while free H can reach receptors anywhere. If bead-attached H works as well as free H, the receptor is on the surface; if only free H works, the receptor is inside. The matching cultures make the form of H the only difference.
In a hypothetical gland, binding of hormone G to its receptor starts a signaling cascade that reduces the cells' secretion of a protein. Secretion was 80 units per hour before G was added and 52 units per hour after. What is the percent change in secretion?
Answer and reasoning
A54% decrease A student who divides by the final value picks this: (80 − 52) ÷ 52 × 100 = 54%. The base for a percent change is the original value, 80.
B65% decrease A student who takes the new value as a percentage of the original picks this: 52 ÷ 80 × 100 = 65%. That is the fraction remaining; the change is 35%.
C35% decreaseCorrect Percent change = (52 − 80) ÷ 80 × 100 = −35%, a 35% decrease, using the original value, 80 units per hour, as the base.
D28% decrease A student who treats the difference as a percent picks this: 80 − 52 = 28 units per hour, which must still be divided by 80 to give a percent.
Working Percent change = (new − original) ÷ original × 100 = (52 − 80) ÷ 80 × 100 = −28 ÷ 80 × 100 = −35%, a 35% decrease. Distractors: dividing by the final value, 28 ÷ 52 × 100 = 54%; new as a percentage of original, 52 ÷ 80 × 100 = 65%; difference written as a percent, 28%.
The model shows four levels of a signal transduction pathway. A very low concentration of the hormone that binds this receptor can produce a large response in the cell. Based on the model, which statement best explains why?
Answer and reasoning
AEach step of the pathway makes more copies of the hormone, so more hormone is present in the cell. A student who thinks amplification means making more signal molecules picks this. The model shows more relay proteins, enzymes and products at each level, not more hormone.
BEach active molecule acts on several molecules at the next level, so their number rises at every step.Correct In the model, one activated receptor activates 3 relay proteins, these activate 9 enzymes, and the enzymes make 27 product molecules. Because each active molecule acts on several molecules of the next kind, a few hormone molecules lead to many active molecules: the signal is amplified.
CThe receptor breaks each hormone molecule into pieces, and each piece goes on as a new signal. A student who thinks receptors act like enzymes on their ligands picks this. The receptor binds the hormone without breaking it down; the increase comes from activation of many molecules at each level.
DEach hormone molecule carries enough energy to power the reactions at every level of the pathway. A student who thinks signals supply the energy for the response picks this. The hormone carries information; the cell's own molecules, such as ATP, supply energy, and the model shows the number of active molecules growing.
When hormone H binds receptors on liver cells, the cells begin breaking down stored glycogen and releasing glucose within seconds. Which statement best explains how such a fast response is possible?
Answer and reasoning
ARelay molecules activate enzymes that are already present in the cytoplasm, and no new proteins are needed.Correct A signaling cascade can activate enzymes the cell has already made, for example by phosphorylating them, so glycogen breakdown starts within seconds. Responses that need new proteins made through gene expression take longer.
BThe signal switches on the genes for enzymes that break down glycogen, and these are made within seconds. A student who thinks every response needs new gene expression picks this. Making new proteins from genes takes far longer than a few seconds; this fast response uses enzymes already present.
CHormone H enters the liver cells and breaks the glycogen down itself, without any relay molecules. A student who thinks a signal enters the cell and acts directly picks this. H binds receptors at the cell surface, and relay molecules inside the cell activate the enzymes that break down glycogen.
DHormone H delivers the energy that liver cells need to break their glycogen down into glucose. A student who thinks the signal supplies energy for the response picks this. H carries information; it changes the activity of the cell's enzymes rather than supplying energy.
Which sequence best represents the first events of signal transduction at a receptor protein in the plasma membrane?
Answer and reasoning
ALigand binds outside cell → the ligand passes through to the intracellular domain → the ligand then activates the relay protein A student who thinks receptors let their ligands into the cell picks this. The ligand stays outside, bound to the receptor; the change in the receptor's shape, including its intracellular domain, carries the signal inward.
BLigand binds outside cell → the ligand changes its shape → the changed ligand then activates the relay protein A student who thinks the ligand changes shape and carries the message picks this. It is the receptor whose shape changes when the ligand binds.
CLigand binds outside cell → the receptor breaks the ligand down → the pieces of it activate the relay protein A student who thinks receptors act like enzymes on their ligands picks this. The receptor binds the ligand without changing it chemically; the receptor itself changes shape.
DLigand binds outside cell → receptor's intracellular domain changes shape → it activates the relay proteinCorrect After the ligand binds the receptor's binding domain outside the cell, the receptor changes shape, including its intracellular domain, and the changed intracellular domain activates the first relay protein, starting transduction.
Researchers made a version of a hypothetical receptor that lacks its intracellular domain; its ligand-binding domain is unchanged. The graphs show ligand binding and the rise in the second messenger cAMP in cells with the normal or the shortened receptor, each as a percentage of the value for the normal receptor. Which statement is best supported by the data?
Answer and reasoning
AThe shortened receptor bound the ligand almost normally but caused almost no rise in cAMP.Correct Ligand binding by the shortened receptor is 96% of normal, but the rise in cAMP is only 4% of normal. Binding alone is not enough: the intracellular domain, which changes shape when the ligand binds, is needed to start transduction.
BThe shortened receptor neither bound the ligand nor caused any rise in cAMP inside the cells. A student who thinks a change to one part of a protein destroys all its functions picks this. Binding by the shortened receptor is 96% of normal; only the cAMP response is lost.
CLigand binding alone caused the rise in cAMP, whichever form of the receptor was present. A student who thinks ligand binding by itself produces the response picks this. Both receptors bound the ligand, but only the normal receptor caused a rise in cAMP.
DThe shortened receptor's slightly lower binding caused its much lower rise in cAMP. A student who takes any link between two variables as cause picks this. A 4% drop in binding cannot account for a 96% drop in cAMP; the missing intracellular domain explains the difference.
In a hypothetical pathway, one ligand-bound receptor activates 12 G proteins. Each G protein activates one enzyme molecule, and each enzyme molecule makes 500 molecules of cAMP per minute. Four cAMP molecules are needed to activate one protein kinase molecule. How many protein kinase molecules can be activated per minute as a result of one ligand-bound receptor?
Answer and reasoning
A2.4 × 10⁴ A student who thinks every step amplifies multiplies by 4 at the last step: 12 × 500 × 4 = 24,000. Four cAMP molecules are needed per kinase, so that step divides by 4.
B5.2 × 10² A student who adds the numbers for each step gets 12 + 500 + 4 = 516 ≈ 5.2 × 10². Each active molecule acts on many at the next step, so the factors multiply.
C1.5 × 10³Correct 12 G proteins × 1 enzyme each × 500 cAMP per minute = 6,000 cAMP per minute. Four cAMP activate one kinase, so 6,000 ÷ 4 = 1,500 = 1.5 × 10³ kinase molecules per minute.
D1.3 × 10² A student who thinks a signal is shared out among the molecules it is passed to treats the 12 G proteins as splitting one receptor's signal, so together they activate only one enzyme's worth: 500 ÷ 4 = 125 ≈ 1.3 × 10². Each G protein activates a whole enzyme molecule, so the factor of 12 multiplies.
Working cAMP made per minute = 12 G proteins × 1 enzyme per G protein × 500 cAMP per enzyme per minute = 6,000 cAMP per minute. Kinases activated = 6,000 ÷ 4 = 1,500 = 1.5 × 10³ per minute. Distractors: multiplying by 4 at every step, 12 × 500 × 4 = 2.4 × 10⁴; adding, 12 + 500 + 4 = 516 ≈ 5.2 × 10²; sharing the receptor's signal among the 12 G proteins, 500 ÷ 4 = 125 ≈ 1.3 × 10².
Liver cells of a hypothetical mammal were broken open and separated into a membrane fraction and a cytoplasm fraction; the cytoplasm contains an enzyme that breaks down glycogen. The table shows the activity of this enzyme after each treatment. Which conclusion best relates these results to how the hormone's signal is relayed?
Answer and reasoning
AThe hormone activates the enzyme directly, and it needs nothing else from the liver cell in order to act. A student who thinks a signal acts directly on the cell's proteins picks this. In tube 1 the hormone and the enzyme are together, yet the activity (4 units) is no higher than with no hormone (5 units).
BThe hormone's binding at the membranes makes a messenger that, once in solution, activates the enzyme.Correct The hormone does not activate the enzyme in cytoplasm alone (tube 1, 4 units, like tube 4). Liquid from membranes treated with hormone activates it strongly (tube 2, 85 units), but liquid from untreated membranes does not (tube 3, 6 units). So hormone binding at the membrane produces a dissolved messenger that relays the signal to the enzyme, as cAMP does.
CThe hormone's receptor is in the cytoplasm, which is where the responding enzyme is found. A student who thinks a receptor must be where the response happens picks this. Cytoplasm plus hormone gives no response; membranes are needed, so the receptor is in the membrane fraction.
DThe hormone passes through the membranes into the liquid, where it then activates the enzyme. A student who thinks the signal molecule itself is passed along picks this. If the hormone itself activated the enzyme, tube 1, where they are mixed directly, would show high activity.
A student observes that when liver cells are exposed to a hormone, their cAMP level rises and they then break down glycogen. Which question could be tested to find out whether cAMP relays the hormone's signal to the glycogen-breakdown response?
Answer and reasoning
AWhy do liver cells need to break down glycogen when this hormone is present in their blood? A student who thinks questions about need are testable picks this. A cell's 'need' is not a variable an experiment can change or measure, so this question cannot test the relay.
BIs cAMP a more important molecule in liver cells than the hormone that causes its level to rise? A student who thinks questions about importance are testable picks this. 'More important' is not a measurable variable, so no experiment can answer the question in this form.
CDoes a form of cAMP that can enter cells cause glycogen breakdown when no hormone is present?Correct If cAMP relays the signal, raising cAMP inside the cells should produce the response even without the hormone. The question names a variable that can be changed (cAMP added or not) and one that can be measured (glycogen breakdown).
DDo the cAMP level and the rate of glycogen breakdown rise together after the hormone is added? A student who thinks showing that two variables change together tests cause picks this. The student has already seen them rise together; a rise together cannot show that cAMP causes the breakdown.
Two ligand-gated ion channels, A and B, from a hypothetical neuron were studied in separate experiments. The graph shows the ion flow through each channel before, during and after exposure to its ligand, which was present from 2 to 6 seconds. Which statement is best supported by the data?
Answer and reasoning
ALigand binding opened both of the channels, as binding of a ligand switches a channel on. A student who thinks ligand binding always switches a protein on picks this. Flow through channel B fell from about 60 to about 5 while its ligand was present, so binding closed it.
BEach channel stayed in its new state after its ligand was removed, so the effect continued. A student who thinks a pathway stays on once activated picks this. After 6 seconds, channel A's flow fell back to about 3 and channel B's rose back to about 60.
CThe flow recorded was the ligand itself passing through each channel while it was open. A student who thinks the ligand passes through the channel picks this. Flow through channel B fell while its ligand was present, which could not happen if the flow were the ligand itself; the flow is of ions.
DLigand binding opened channel A but closed channel B, and both returned to their earlier state.Correct Ion flow through channel A rose from about 3 to about 80 while its ligand was present, so binding opened it; flow through channel B fell from about 60 to about 5, so binding closed it. After 6 seconds both returned to their resting levels: ligand binding can open or close a channel.
At the synapse between a motor neuron and a muscle cell, acetylcholine binds a receptor protein in the muscle cell's plasma membrane, and Na⁺ then flows into the muscle cell. A student claims that this receptor is itself a ligand-gated ion channel, rather than a receptor that acts through a separate channel. Which observation would best support the student's claim?
Answer and reasoning
AThe purified receptor protein, alone in an artificial lipid membrane, lets Na⁺ cross when acetylcholine binds.Correct With no other cell proteins present, ion flow can only pass through the receptor protein itself, and it occurs only when acetylcholine binds. The binding site and the channel must be parts of the same protein: a ligand-gated channel.
BNa⁺ starts to flow into the muscle cell soon after the neuron releases acetylcholine at the synapse. A student who takes any agreeing observation as support picks this. Both explanations, a ligand-gated channel and a receptor acting on a separate channel, predict Na⁺ flow after acetylcholine is released.
CAcetylcholine is found inside the muscle cell's cytoplasm once the flow of Na⁺ has begun. A student who thinks the ligand passes through its channel picks this. A ligand-gated channel lets ions, not its ligand, cross the membrane, so finding acetylcholine inside would not support the claim.
DThe receptor protein spans the plasma membrane, with its acetylcholine-binding site facing the synapse. A student who thinks any protein that spans the membrane must be a channel picks this. Receptors that act through separate proteins, such as G protein-coupled receptors, also span the membrane with their binding sites outside, so both explanations predict this observation.
Compiled from the AP Biology Course and Exam Description (effective Fall 2025) and our question bank · Specialist review in progress. How these pages are made · Free, no account
