Epinephrine is a hormone.

How does a hormone travel from the cell that releases it to the cells it acts on?

1. A. ✓ The blood carries it
2. B. It moves along a nerve
   A nerve carries an electrical signal along itself.
   A hormone is a molecule released into the blood, and the blood carries it.

Why: A hormone is a signal molecule carried in the blood to cells elsewhere in the body.

A yeast cell releases a small molecule into the liquid around it. A second yeast cell, some distance away, detects the molecule and stops dividing.

Which of the following is the chemical signal?

1. A. The liquid around the yeast cells
   The liquid carries the molecule from one yeast cell to the other.
   The chemical signal is the molecule the liquid carries.
2. B. ✓ The molecule the first yeast cell released
3. C. The second yeast cell’s stopping dividing
   Stopping dividing is what the second yeast cell does after the molecule reaches it.
   The chemical signal is the molecule that reached it.

Why: A chemical signal is a molecule one cell releases that carries information to other cells.
The first yeast cell released the molecule into the liquid.
The second yeast cell detected that molecule.
So the molecule is the chemical signal.

A student says: “The epinephrine molecule carries instructions inside it that tell the heart to beat faster.”

Is the student correct?

1. A. Yes — the instructions travel inside the molecule
   Epinephrine is the same small molecule wherever it goes.
   The heart cell is built to detect it, and the heart cell decides the response.
2. B. ✓ No — the heart cell detects it and responds in its own way

Why: Epinephrine is the same small molecule in every part of the blood.
It carries no instructions inside it.
Heart cells are built to detect epinephrine, and they respond in their own way.
So the heart cell, not the molecule, decides the response.

A yeast cell releases a small molecule into the liquid around it. A second yeast cell, some distance away, detects the molecule and stops dividing.

Which of the two yeast cells is the signaling cell?

1. A. ✓ The first yeast cell
2. B. The second yeast cell
   The second yeast cell detects the molecule and responds.
   The signaling cell is the cell that releases the molecule, and that is the first yeast cell.

Why: The signaling cell is the cell that releases the molecule.
The first yeast cell released the molecule into the liquid.
So the first yeast cell is the signaling cell.
The second yeast cell detected the molecule and responded.

What is a chemical signal?

1. A. The change a cell makes when a molecule reaches it
   The change is the cell’s response.
   The chemical signal is the molecule that reached the cell.
2. B. The blood or liquid that carries a molecule between cells
   The blood carries the molecule.
   The chemical signal is the molecule itself.
3. C. ✓ A molecule one cell releases that carries information to other cells

Why: A chemical signal is a molecule one cell releases.
The molecule carries information to other cells.
Epinephrine, released by the adrenal cells into the blood, is one.

What is a signaling cell?

1. A. ✓ The cell that releases the chemical signal
2. B. The cell that detects the chemical signal
   The cell that detects the signal responds to it.
   The signaling cell is the cell that released it.
3. C. Any cell the blood carries the chemical signal past
   The blood carries the signal past many cells that never released it.
   The signaling cell is the one cell type that released it.

Why: The signaling cell is the cell that releases the chemical signal.
The adrenal cells released the epinephrine, so the adrenal cells are the signaling cells.

A leaf cell chewed by a caterpillar releases a molecule into the plant’s sap. Cells throughout the plant detect it and make a bitter compound.

Which of the following is the chemical signal?

1. A. The sap that carries the molecule
   The sap moves the molecule through the plant.
   The chemical signal is the molecule the sap carries.
2. B. The bitter compound the cells make
   The bitter compound is what the detecting cells make after the molecule reaches them.
   The chemical signal is the molecule that reached them.
3. C. ✓ The molecule released into the sap

Why: The chemical signal is the molecule one cell released that carries information to other cells.
The chewed leaf cell released a molecule into the sap.
So that molecule is the chemical signal.

An egg cell releases a molecule into the water around it. A sperm cell detects the molecule and swims toward the egg.

Which of the two cells is the signaling cell?

1. A. The sperm cell
   The sperm cell detects the molecule and swims.
   The signaling cell is the cell that releases the molecule.
   The egg cell released the molecule.
2. B. ✓ The egg cell

Why: The signaling cell is the cell that releases the molecule.
The egg cell released the molecule into the water.
So the egg cell is the signaling cell.
The sperm cell detected the molecule and responded.

A nerve ending releases a molecule into the gap beside a muscle cell. The muscle cell contracts.

Which of the following is the chemical signal?

1. A. ✓ The molecule released into the gap
2. B. The fluid in the gap
   The fluid in the gap carries the molecule from the nerve ending to the muscle cell.
   The chemical signal is the molecule itself.
3. C. The contraction of the muscle cell
   The contraction is what the muscle cell does after the molecule reaches it.
   The chemical signal is the molecule that reached it.

Why: The chemical signal is the molecule one cell released that carries information to another cell.
The nerve ending released a molecule into the gap.
So that molecule is the chemical signal.

Cells of a gland in the neck release a molecule into the blood. Bone cells throughout the body grow faster.

Which of the two cell types is the signaling cell?

1. A. The bone cell
   The bone cells grow faster after the molecule reaches them.
   The signaling cell is the cell that releases the molecule.
   The gland cells released the molecule.
2. B. ✓ The gland cell

Why: The signaling cell is the cell that releases the molecule.
The gland cells released the molecule into the blood.
So the gland cell is the signaling cell.
The bone cells detected the molecule and responded.

A damaged skin cell of a minnow releases a molecule into the water. Other minnows detect the molecule and flee.

Which of the following is the chemical signal?

1. A. The damage to the skin
   The damage made the skin cell release the molecule.
   The chemical signal is the molecule.
2. B. The fleeing of the other minnows
   The fleeing is what the other minnows do after the molecule reaches them.
   The chemical signal is the molecule that reached them.
3. C. ✓ The molecule released into the water

Why: The chemical signal is the molecule one cell released that carries information to other cells.
The damaged skin cell released a molecule into the water.
So that molecule is the chemical signal.

A channel protein is set through a cell’s plasma membrane.

Which of the following does the channel protein do?

1. A. ✓ It lets a substance cross the membrane
2. B. It stores the cell’s energy as glucose and fat
   A cell stores energy in glucose and fat molecules in the cytosol, not in a membrane protein.
   A channel protein lets a substance cross the membrane.

Why: A membrane protein is a protein set through the plasma membrane.
A channel or carrier protein lets a substance the bilayer blocks cross the membrane.

A hormone in the blood reaches a kidney cell, a muscle cell and a skin cell at the same concentration. Only the kidney cell responds.

Which of the three cells carries a receptor for the hormone?

1. A. The muscle cell
   The muscle cell showed no response.
   A cell responds only when a receptor binds the signal, so the muscle cell carries no receptor for this hormone.
2. B. All three cells
   Only the kidney cell responded.
   The muscle cell and the skin cell carry no receptor for this hormone.
3. C. ✓ The kidney cell

Why: A receptor is a protein that binds the signal.
A cell responds only when its receptor binds the signal.
Only the kidney cell responded.
So only the kidney cell carries a receptor for the hormone.

What is a receptor for a chemical signal?

1. A. The molecule one cell releases to carry information
   The released molecule is the chemical signal.
   The receptor is the protein that binds it.
2. B. The blood vessel that carries the signal to the cell
   The blood carries the signal to the cell.
   The receptor is the protein that binds the signal when it arrives.
3. C. ✓ A protein on or in a cell that binds that signal

Why: A receptor is a protein on or in a cell.
It binds one particular chemical signal by fitting it in a pocket.

Epinephrine reaches a liver cell, a heart muscle cell and a lens cell at the same concentration. The liver cell releases glucose and the heart muscle cell contracts harder. The lens cell shows no change.

Which of the following cells are target cells for epinephrine?

1. A. The heart muscle cell only
   The liver cell released glucose when the epinephrine reached it.
   So the liver cell responded, and a cell that responds to the signal is a target cell.
2. B. ✓ The liver cell and the heart muscle cell
3. C. All three: the heart muscle cell, the liver cell and the lens cell
   A target cell is a cell that responds to the signal.
   The lens cell showed no change.
   So the lens cell is not a target cell.

Why: A target cell is a cell that responds to the signal.
The liver cell responded: it released glucose.
The heart muscle cell responded: it contracted harder.
So both are target cells for epinephrine.
The lens cell showed no change, so the lens cell is not a target cell.

A student looks at the same three cells and says: “The lens cell is ignoring the epinephrine, because the lens cell has no use for it.”

Is the student correct?

1. A. Yes — a cell responds only to the signals it needs
   A cell cannot weigh what it needs.
   A cell responds only when a receptor binds the signal.
   The lens cell has no receptor for epinephrine, so it cannot detect it.
2. B. ✓ No — nothing in the lens cell can detect the epinephrine

Why: A cell detects a signal only through a receptor, a protein that binds the signal.
The lens cell carries no receptor for epinephrine.
So nothing in the lens cell can detect the epinephrine; the epinephrine washes past it.

A nerve ending releases a molecule into the gap beside a muscle cell, and the muscle cell responds. A researcher adds a blocker to the gap. The nerve ending still releases the molecule, the molecule still fills the gap at its usual concentration, and the muscle cell stops responding.

Which of the following has the blocker done?

1. A. The blocker has broken the molecule down in the gap
   A molecule that is being broken down falls in concentration.
   The molecule still fills the gap at its usual concentration.
   So nothing broke the molecule down.
2. B. The blocker has occupied the receptors on the nerve ending’s surface
   The nerve ending still releases the molecule, so nothing at the nerve ending has changed.
   The muscle cell is the cell that stopped responding, and its receptors bind the molecule.
3. C. ✓ The blocker has occupied the receptors on the muscle cell’s surface

Why: The molecule still fills the gap at its usual concentration.
The nerve ending still releases the molecule, so nothing there has changed.
The muscle cell stopped responding, so the change is in the muscle cell.
The muscle cell detects the molecule through its surface receptors, so the blocker occupied them.

What is a target cell for a chemical signal?

1. A. The cell that releases the signal into the blood
   The cell that releases the signal is the signaling cell.
2. B. Any cell the blood carries the signal past
   The blood carries the signal past many cells that carry no receptor for it.
   Those cells do nothing, so they are not target cells.
3. C. ✓ A cell that carries a receptor for the signal

Why: A target cell carries a receptor for the signal.
So when the signal arrives, the receptor binds it and the cell responds.

When food reaches the small intestine, cells lining the small intestine release a molecule into the blood. Cells in the pancreas detect it and release digestive enzymes. Liver cells, reached by the same blood, show no change.

Which of the following is the signaling cell?

1. A. The pancreas cell
   The pancreas cell detects the molecule.
   The signaling cell is the cell that released the molecule.
2. B. The liver cell
   The liver cell neither released nor detected the molecule.
3. C. All three cells
   Only the cell that releases the molecule is the signaling cell.
   The blood just carries the molecule.
4. D. ✓ The cell lining the small intestine

Why: The signaling cell is the cell that releases the chemical signal.
The cell lining the small intestine released the molecule into the blood.
So the cell lining the small intestine is the signaling cell.

Cells in the pancreas release hormone G into the blood. Liver cells release glucose when G reaches them. Leg muscle cells, reached by the same blood, show no change.

Which of the following cells carry a receptor for G?

1. A. ✓ The liver cells
2. B. The leg muscle cells
   The leg muscle cells showed no change.
   A cell responds only when its receptor binds the signal, so the leg muscle cells carry no receptor for G.
3. C. Both the liver cells and the leg muscle cells
   The leg muscle cells showed no change when G reached them.
   So the leg muscle cells carry no receptor for G.

Why: A cell responds to a signal only when it carries a receptor that binds the signal.
The liver cells responded: they released glucose.
The leg muscle cells showed no change.
So the liver cells carry a receptor for G, and the leg muscle cells do not.

Two cell types respond to the same hormone in different ways: liver cells release glucose, and heart muscle cells contract harder.

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

1. A. The liver cells receive a different form of the hormone from the heart cells
   The blood carries one hormone to both.
   The difference lies in the cells.
2. B. The heart cells receive the hormone first and use up its message
   The hormone reaches both cell types, and each responds in its own way.
3. C. The hormone carries a different set of instructions to each cell type
   The hormone is the same small molecule everywhere.
   It carries no instructions inside it.
4. D. ✓ Each cell type responds in its own way once its receptor binds the hormone

Why: The hormone carries no instructions inside it.
Each cell type carries the receptor and responds in its own way.
So the same molecule makes a liver cell release glucose and a heart muscle cell contract harder.

When food reaches the small intestine, cells lining it release a molecule into the blood. The molecule reaches the pancreas cells and the liver cells. The pancreas cells release digestive enzymes; the liver cells show no change.

Which of the following cells are target cells for the molecule?

1. A. The liver cells
   A target cell responds to the signal.
   The liver cells did not respond.
2. B. Both the pancreas cells and the liver cells
   Reaching a cell is not enough.
   A target cell responds to the signal, and the liver cells showed no change.
3. C. ✓ The pancreas cells

Why: A target cell is a cell that responds to the signal.
The pancreas cells responded: they released digestive enzymes.
So the pancreas cells are the target cells.
The liver cells showed no change, so the liver cells are not target cells.

A skin cell carries no receptor for hormone K. A researcher gives the skin cell one hundred times the usual concentration of K.

Which of the following does the skin cell do?

1. A. It responds weakly
   A cell responds only when a receptor binds the signal.
   The skin cell has no receptor for K, so nothing in it binds K at any concentration.
2. B. It responds as strongly as a bone cell
   A cell responds only when a receptor binds the signal.
   The skin cell has no receptor for K, so nothing in it binds K at any concentration.
3. C. ✓ It does nothing

Why: A cell detects a signal only through a receptor.
The skin cell carries no receptor for K.
So nothing in the skin cell binds K, however much K reaches it.
The skin cell does nothing.

A root cell of a bean plant, attacked by a fungus, releases a molecule into the soil water. Bacteria nearby detect the molecule and move toward the root.

Which of the following is the chemical signal?

1. A. The fungus attacking the root
   The fungus made the root cell release the molecule.
   The chemical signal is the molecule.
2. B. ✓ The molecule released into the soil water
3. C. The bacteria moving toward the root
   The movement is what the bacteria do after the molecule reaches them.
   The chemical signal is the molecule that reached them.

Why: A chemical signal is a molecule one cell releases that carries information to other cells.
The root cell released a molecule into the soil water.
The bacteria detected that molecule.
So the molecule is the chemical signal.

A cell carries no receptor for a signal. The signal reaches the cell.

Which of the following does the cell do?

1. A. It responds weakly
   A cell responds only when a receptor binds the signal.
   This cell has no receptor, so nothing in it binds the signal.
2. B. ✓ It does nothing

Why: A cell detects a signal only through a receptor.
This cell carries no receptor for the signal.
So nothing in the cell binds the signal, and the cell does nothing.

Skin cells carry a receptor for signal Q. Bone cells carry none. Signal Q reaches both.

Which of the following cells respond?

1. A. ✓ The skin cells
2. B. The bone cells
   The cells that carry the receptor respond.
   The skin cells carry the receptor for Q. The bone cells carry none.
3. C. Both the skin cells and the bone cells
   A cell responds only when it carries a receptor that binds the signal.
   The bone cells carry none.
   So the bone cells do nothing.

Why: Only cells that carry the receptor respond.
The skin cells carry the receptor for Q, so the skin cells respond.
The bone cells carry none, so the bone cells do nothing.

The graph below shows how much of the receptor for hormone H three cell types carry. Cells that lack the receptor read 0.3 units. Hormone H reaches all three cell types.

Do the kidney cells respond to H?

1. A. ✓ Yes
2. B. No
   The kidney cells carry 7.5 units of the receptor for H, far above the 0.3 units of cells that lack it.
   So H binds the kidney cells and they respond.

Why: The kidney cells carry 7.5 units of the receptor for H. That is far above the 0.3 units of cells that lack the receptor.
So H binds the kidney cells’ receptors, and the kidney cells respond.

The graph below shows how much of the receptor for hormone H three cell types carry; cells that lack the receptor read 0.3 units. Hormone H reaches all three cell types.

Do the skin cells respond to H?

1. A. Yes
   The skin cells carry 0.2 units of the receptor, below the 0.3 units of cells that lack it.
   So the skin cells carry almost none: nothing in them binds H.
2. B. ✓ No

Why: The skin cells carry 0.2 units of the receptor for H.
That is below the 0.3 units of cells that lack the receptor.
So the skin cells carry almost no receptor.
Nothing in them binds H.
So the skin cells do nothing.

The graph below shows how much of the receptor for hormone H three cell types carry; cells that lack the receptor read 0.3 units. Hormone H reaches all three cell types. A researcher then gives the skin cells ten times as much H.

Do the skin cells respond now?

1. A. Yes
   More hormone changes how strongly receptor-carrying cells respond, never which cells respond.
   The skin cells carry almost no receptor, so there is still almost nothing to bind H.
2. B. ✓ No

Why: More hormone changes how strongly the cells that carry the receptor respond.
It never changes which cells respond.
The skin cells carry almost no receptor for H.
So at ten times the concentration there is still almost nothing in the skin cells to bind H.

The graph below shows how much of the receptor for hormone M three cell types of a mouse carry. Cells that lack the receptor read 0.3 units. Hormone M reaches all three cell types.

Do the bone cells respond to M?

1. A. ✓ Yes
2. B. No
   The bone cells carry 4.4 units of the receptor for M, far above the 0.3 units of cells that lack it.
   So M binds the bone cells and they respond.

Why: The bone cells carry 4.4 units of the receptor for M. That is far above the 0.3 units of cells that lack the receptor.
So M binds the bone cells’ receptors, and the bone cells respond.

The graph below shows how much of the receptor for hormone M three cell types of a mouse carry. At a first concentration of M the bone cells responded weakly. A researcher gives the bone cells ten times as much M.

Which of the following do the bone cells do now?

1. A. The bone cells respond as before
   At the first concentration the bone cells responded only weakly.
   More M means more of their receptors bind M, so the bone cells respond more strongly.
2. B. The bone cells stop responding
   The bone cells carry the receptor for M.
   More M means more of their receptors bind M, so the response grows.
3. C. ✓ The bone cells respond more strongly

Why: The bone cells carry the receptor for M, and at the first concentration they responded weakly.
More hormone changes how strongly the cells that carry the receptor respond.
So at ten times the concentration more of the bone cells’ receptors bind M, and the bone cells respond more strongly.

A student looks at the guard cells (9.2 units of the receptor) and the pavement cells (0.4 units, near the 0.3 units read by cells that lack the receptor) and says: “If the plant made ten times as much hormone, the pavement cells would respond too.”

Is the student correct?

1. A. Yes — more hormone would reach more of the pavement cells
   The hormone already reaches every pavement cell.
   A pavement cell carries almost no receptor, so more hormone has almost nothing more to bind.
2. B. ✓ No — the pavement cells carry almost no receptor

Why: The amount of hormone changes how strongly the cells that carry the receptor respond.
It never changes which cells respond.
The pavement cells carry 0.4 units of the receptor, almost none.
So at ten times the concentration the hormone still has almost nothing to bind in a pavement cell.

A mouse’s liver cells carry 6.0 units of the receptor for signal S. At a low concentration of S the liver cells respond weakly. A researcher raises the concentration of S one hundred times.

Which of the following do the liver cells do at the higher concentration?

1. A. ✓ The liver cells respond more strongly
2. B. The liver cells respond as weakly as before
   The liver cells carry 6.0 units of the receptor.
   At one hundred times the concentration more of their receptors bind S. So the liver cells respond more strongly.
3. C. The liver cells stop responding
   The liver cells carry the receptor for S.
   More S means more of their receptors bind S, so the response grows.

Why: The amount of signal changes how strongly the cells that carry the receptor respond.
The liver cells carry 6.0 units of the receptor.
So at one hundred times the concentration more of the liver cells’ receptors bind S, and the liver cells respond more strongly.

The graph below shows how much of the receptor for hormone M three cell types of a mouse carry; cells that lack the receptor read 0.3 units. A student predicts that the gut cells respond to M.

Which of the following justifies the prediction?

1. A. The gut cells need M to digest food
   A cell cannot weigh what it needs.
   A cell responds only when its receptor binds the signal.
2. B. The blood carries M to the gut cells first
   Every cell the blood reaches gets M.
   Only a cell that carries the receptor responds.
3. C. ✓ The gut cells read 5.8 units of the receptor

Why: A cell responds to a signal only when it carries a receptor that binds the signal.
The gut cells carry 5.8 units of the receptor for M, far above the 0.3 units of cells that lack it.
So the receptor is the justification.

A mouse’s fat cells carry 0.1 units of the receptor for signal S, below the 0.3 units measured in cells that lack the receptor. At a low concentration of S the fat cells showed no change. A researcher raises the concentration of S one hundred times.

Which of the following do the fat cells do at the higher concentration?

1. A. The fat cells now respond weakly
   The fat cells carry almost no receptor, so they have almost nothing to bind S at any concentration.
   They show no response at all, not a weak one.
2. B. The fat cells now respond as strongly as the liver cells
   More signal changes how strongly receptor-carrying cells respond, never which cells respond.
   The fat cells carry almost no receptor, so at any concentration almost nothing in them binds S.
3. C. ✓ The fat cells still stay as they are

Why: The amount of signal changes how strongly the cells that carry the receptor respond.
It never changes which cells respond.
The fat cells carry 0.1 units of the receptor: almost none.
So however much S arrives, almost nothing binds it, and the fat cells stay as they are.

Cells lining a frog’s gut carry 6.3 units of the receptor for hormone R. The frog’s skin cells carry 0.2 units. Cells that lack the receptor read 0.3 units. R reaches both cell types.

Which of the following cells respond to R?

1. A. ✓ The gut cells
2. B. The skin cells
   The skin cells carry 0.2 units of the receptor: almost none.
   The gut cells carry 6.3 units, so R binds the gut cells.
3. C. Both the gut cells and the skin cells
   The skin cells carry 0.2 units of the receptor, below the 0.3 units of cells that lack it.
   Nothing in the skin cells binds R.

Why: Only cells that carry the receptor respond.
The gut cells carry 6.3 units of the receptor for R, so R binds the gut cells and they respond.
The skin cells carry 0.2 units, almost none, so the skin cells do nothing.

A frog’s skin cells carry 0.2 units of the receptor for hormone R; cells that lack the receptor read 0.3 units. A researcher doubles the concentration of R around the skin cells.

Which of the following do the skin cells do?

1. A. The skin cells now respond weakly
   The skin cells carry almost no receptor for R.
   Doubling R leaves almost nothing in the skin cells for R to bind.
2. B. ✓ The skin cells still do nothing

Why: The amount of hormone changes how strongly the cells that carry the receptor respond.
It never changes which cells respond.
The skin cells carry 0.2 units of the receptor: almost none.
So at twice the concentration R still has almost nothing to bind, and the skin cells do nothing.

Kidney cells carry 7.5 units of the receptor for hormone H and respond to it. A student says: “The kidney cells respond because the kidney needs H to make urine.”

Is the student correct?

1. A. Yes — a cell responds to the signals it needs
   A cell cannot weigh what it needs.
   A cell responds only when its receptor binds the signal.
2. B. ✓ No — the kidney cells respond because they carry the receptor for H

Why: A cell responds to a signal only when it carries a receptor that binds the signal.
The kidney cells carry 7.5 units of the receptor for H.
So H binds the kidney cells’ receptors, and the kidney cells respond.
What the kidney needs plays no part.

What is a receptor for a chemical signal?

1. A. The molecule a cell releases to carry a message
   The molecule a cell releases is the chemical signal itself.
   The receptor is the protein that binds it.
2. B. ✓ A protein on or in a cell that binds that signal
3. C. A cell that changes what it does when the signal arrives
   A cell that responds is a target cell.
   The receptor is the protein on or in it that binds the signal.

Why: A receptor is a protein on or in a cell.
It binds one particular chemical signal by fitting it in a pocket.
A cell that carries the receptor responds when the signal arrives.

A cell lining the gut begins to make mucus when an immune cell presses against it. With a fine mesh between the two cells, one that lets molecules through but keeps the cells apart, the gut cell makes no mucus.

Which of the following is how the message passes from the immune cell to the gut cell?

1. A. By a molecule the immune cell releases into the fluid around both cells
   A released molecule would cross the mesh and reach the gut cell.
   With the mesh in place, the gut cell made no mucus.
2. B. By a molecule that the mesh stops from reaching the gut cell’s surface
   The mesh lets molecules through.
   Only the two cell surfaces are kept apart.
3. C. ✓ By a protein on the immune cell’s surface binding a receptor on the gut cell

Why: The mesh lets molecules through, so a released molecule would still reach the gut cell.
The gut cell made mucus only when the immune cell pressed against it.
So the message passes where the cells touch: a surface protein binds a receptor.

What is cell-to-cell contact?

1. A. A cell releases a molecule that spreads through the fluid to the cells beside it
   In cell-to-cell contact, nothing is released into the fluid.
   The two cell surfaces bind.
2. B. ✓ A protein on one cell’s surface binds a receptor on the surface of a cell it touches
3. C. The blood carries a molecule from one cell to cells far away
   In cell-to-cell contact, the two cells touch.
   No blood carries anything between them.

Why: In cell-to-cell contact, two cells touch.
A protein on the surface of one binds a receptor on the surface of the other.
Nothing is released into the fluid between them.

Two cells communicate by cell-to-cell contact.

What is released into the fluid between them?

1. A. A small molecule
   A released molecule would spread through the fluid.
   In cell-to-cell contact, the message passes only where the surfaces touch.
2. B. The surface protein
   The surface protein stays fixed in the cell’s membrane.
   It binds the receptor on the cell it touches.
3. C. ✓ Nothing

Why: The protein on one cell’s surface binds the receptor on the other cell’s surface.
Both proteins stay in their membranes.
So nothing is released into the fluid between the cells.

In an embryo, a cell changes what it becomes only when a neighboring cell’s surface is pressed against it. Fluid the neighbor grew in has no effect on it.

Did the message need the two cells to touch?

1. A. ✓ Yes
2. B. No
   The neighbor’s fluid had no effect.
   So no released molecule carries the message; the two surfaces had to touch.

Why: The cell responds only when the neighbor’s surface presses against it.
The neighbor’s fluid does nothing.
So a protein on one surface binds a receptor on the other: cell-to-cell contact.

Cells at the edge of a wound release a molecule into the fluid. Cells a few cell-widths away, with fluid between them and the wound cells, begin dividing within a day.

Did the message need the two cells to touch?

1. A. Yes
   Fluid lies between the responding cells and the wound cells.
   The molecule reached them through that fluid.
2. B. ✓ No

Why: The wound cells released a molecule into the fluid.
Cells a few cell-widths away, with fluid between, responded.
So the message crossed the fluid, and no touch was needed.

A killer T cell presses against an infected cell, and within minutes the infected cell begins to die. Infected cells a few cell-widths away from the T cell are unaffected.

Did the message need the two cells to touch?

1. A. ✓ Yes
2. B. No
   A released molecule would spread through the fluid to the infected cells a few cell-widths away, and they would begin to die too.
   They are unaffected.

Why: Only the infected cell the T cell presses against dies.
The infected cells a few cell-widths away are unaffected.
So no released molecule carries the message through the fluid.
So a protein on the T cell’s surface binds a receptor on the infected cell where the two touch.

A cell releases a molecule into the fluid around it. Near the cell, the molecule is concentrated; further away, it is dilute.

Which way does the molecule spread?

1. A. ✓ From where it is more concentrated to where it is less concentrated
2. B. From where it is less concentrated to where it is more concentrated
   Particles move at random, so more leave the crowded region than arrive in it.
   The molecule spreads away from the cell, where it is concentrated.
3. C. It stays where the cell released it
   Particles move constantly and at random.
   So the molecule spreads out from where it is concentrated.

Why: Particles move constantly and at random.
More leave the region where the molecule is concentrated than arrive there.
So the molecule spreads from where it is more concentrated to where it is less concentrated: diffusion.

When skin is cut, the damaged cells release a molecule. Cells within a fraction of a millimeter begin dividing faster within a day.

How does the molecule reach the nearby cells?

1. A. ✓ It spreads through the fluid by diffusion
2. B. The blood carries it to them
   Cells a fraction of a millimeter away are beside the damaged cells.
   The molecule diffused to them through the fluid; no blood carried it.
3. C. The damaged cells press against them
   The damaged cells released the molecule into the fluid.
   It reached cells they do not touch.

Why: The damaged cells released the molecule into the fluid.
The molecule spread by diffusion to the cells a fraction of a millimeter away.
A signal that spreads by diffusion to nearby cells is a local regulator.

What is a local regulator?

1. A. A signal the blood carries through the whole body
   A local regulator reaches only the cells in its immediate neighborhood.
   No blood carries it.
2. B. A protein on one cell’s surface that binds a receptor on a cell it touches
   A local regulator is released into the fluid.
   A surface protein that binds on touch is cell-to-cell contact.
3. C. ✓ A signal a cell releases that spreads by diffusion to the cells beside it

Why: A local regulator is a signal a cell releases into the fluid.
It spreads by diffusion.
It reaches the target cells in its immediate neighborhood.

A cell releases a local regulator.

Which cells does it reach?

1. A. Only the cell it touches
   A local regulator is released into the fluid, so it reaches cells the sender does not touch.
2. B. ✓ The cells in its immediate neighborhood
3. C. Cells throughout the body
   A local regulator reaches only the cells in its immediate neighborhood.
   No blood carries it through the body.

Why: The cell releases the local regulator into the fluid.
The local regulator spreads by diffusion.
So it reaches the cells beside the sender.

A student says: “The signal from the cut acts only on the nearby cells because the signal knows to stop once it reaches them.”

Is the student correct?

1. A. Yes — the signal stops spreading once it has reached its target cells
   A molecule spreading by diffusion has no way to stop.
   Enzymes break the molecule down within minutes.
   So the molecule is gone before it spreads further.
2. B. ✓ No — enzymes destroy the signal before it can spread further

Why: A molecule has no way of knowing where to go.
The molecule spreads by diffusion in every direction.
Enzymes break the molecule down within minutes.
So the molecule is destroyed before it can spread further than the nearby cells.

Cells at the edge of a wound release a local regulator. Cells within 0.2 mm respond. Cells 2 mm away carry the receptor and show no change. Now imagine a drug blocks the enzyme that breaks the local regulator down.

Which cells respond now?

1. A. None of the cells, within 0.2 mm or beyond
   Blocking the enzyme does not stop the cells from releasing the local regulator.
   The cells within 0.2 mm still respond.
2. B. Only the cells within 0.2 mm, as before
   With the enzyme blocked, the local regulator is no longer destroyed within moments.
   It keeps spreading.
3. C. ✓ The cells within 0.2 mm and cells further away too

Why: The enzyme destroyed the local regulator within moments, so it reached only the cells within 0.2 mm.
With the enzyme blocked, the local regulator is not destroyed.
It keeps spreading by diffusion.
So cells further away, which carry the receptor, now respond too.

A nerve ending sits a fraction of a micrometer from a gland cell, with a gap between them. Each time a signal reaches the nerve ending, a molecule crosses the gap. The gland cell responds within two milliseconds, and the molecule is gone from the gap within one millisecond.

Which of the following kinds of signal is the molecule?

1. A. A hormone
   A hormone is carried in the blood through the whole body.
   This molecule crossed a gap a fraction of a micrometer wide by diffusion; no blood carried it.
2. B. A local regulator released by the gland cell
   The gland cell is the target: it responds.
   The nerve ending released the molecule.
3. C. ✓ A neurotransmitter
4. D. A surface protein bound where the two cells touch
   A surface protein binds only where two cells touch, and nothing is released.
   Here a molecule crossed a gap.

Why: The nerve ending released the molecule into the gap.
The molecule diffused across the gap and was gone within a millisecond.
A signal that diffuses to the next cell and is destroyed at once is a local regulator.
A local regulator released by a nerve cell is a neurotransmitter.

What is a neurotransmitter?

1. A. A hormone a nerve cell releases into the blood to reach distant cells
   A neurotransmitter is released into a gap, not into the blood.
   It reaches only the cell beside the nerve ending.
2. B. ✓ The local regulator a nerve cell releases into the gap beside its target
3. C. The receptor on a muscle cell that binds a nerve cell’s signal
   The receptor is the protein that binds the neurotransmitter.
   The neurotransmitter is the molecule the nerve cell releases.

Why: A nerve cell releases a molecule into the gap beside its target.
The molecule diffuses across the gap and is destroyed within a millisecond, so it is a local regulator.
A local regulator a nerve cell releases is a neurotransmitter.

Which kind of cell releases a neurotransmitter?

1. A. A gland cell
   A gland cell can be a nerve cell’s target.
   It receives the neurotransmitter.
2. B. A muscle cell
   A muscle cell can be a nerve cell’s target.
   It receives the neurotransmitter.
3. C. ✓ A nerve cell

Why: A neurotransmitter is the local regulator a nerve cell releases.
Muscle cells and gland cells are targets that receive it.

Cells in an inflamed joint release a molecule into the fluid. Within minutes, cells a few cell-widths away begin making a protective protein. The molecule is gone from the joint within ten minutes.

Which of the following kinds of signal is the molecule?

1. A. A signal passed by cell-to-cell contact
   Cells a few cell-widths away responded.
   So the molecule crossed the fluid between cells.
2. B. ✓ A local regulator
3. C. A hormone
   The molecule reached only cells a few cell-widths away and was gone within ten minutes.
   No blood carried it.

Why: The molecule diffused to cells a few cell-widths away and was destroyed within ten minutes.
A signal that acts only in its immediate neighborhood and is quickly destroyed is a local regulator.

A nerve ending releases a molecule into the gap beside a cell in the gut wall. The gut cell contracts within a millisecond, and the molecule is gone from the gap within a millisecond.

Which of the following kinds of signal is the molecule?

1. A. A hormone
   A hormone is carried in the blood.
   This molecule diffused across a gap a fraction of a micrometer wide.
2. B. A local regulator released by the gut cell
   The gut cell is the target: it responds.
   The nerve ending released the molecule.
3. C. ✓ A neurotransmitter

Why: The nerve ending released the molecule into the gap beside its target.
The molecule diffused across the gap and was gone within a millisecond.
A local regulator a nerve cell releases into the gap beside its target is a neurotransmitter.

A cell lining the lung releases a local regulator.

How does the local regulator reach the cells beside it?

1. A. ✓ By diffusion through the fluid
2. B. In the blood
   The blood carries hormones through the whole body.
   A local regulator spreads only through the fluid around the cell that released it.
3. C. Through the two cells touching
   A local regulator is released into the fluid.
   It reaches cells the sender does not touch.

Why: The lung cell releases the local regulator into the fluid around it.
The local regulator spreads by diffusion to the cells beside the lung cell.

A local regulator released by a kidney cell reaches only cells a few cell-widths away.

Why does it travel no further?

1. A. It is too large to diffuse further
   A larger molecule diffuses more slowly, but it still spreads.
   Enzymes destroy the local regulator within moments.
2. B. ✓ Enzymes destroy it, or cells take it up, within moments
3. C. It stops spreading once it reaches a cell that carries its receptor
   A molecule spreading by diffusion has no way to stop.
   Enzymes destroy it before it spreads further.

Why: The local regulator spreads by diffusion in every direction.
Within moments, enzymes destroy it, or cells take it up.
So it is gone before it has spread more than a few cell-widths.

A student says: “A neurotransmitter is a hormone, because a nerve cell releases it into the fluid.”

Is the student correct?

1. A. Yes — a nerve cell releases it into the fluid, so it is a hormone
   A hormone is carried in the blood through the whole body.
   A neurotransmitter diffuses across a gap to one cell and is destroyed within a millisecond.
2. B. ✓ No — a neurotransmitter is a local regulator, not a hormone

Why: A nerve cell releases the neurotransmitter into the gap beside its target.
The neurotransmitter diffuses across the gap and is destroyed within a millisecond.
A signal that reaches only the cell beside the sender is a local regulator, not a hormone.

A nerve ending releases a neurotransmitter into the gap beside a muscle cell. Now imagine a drug blocks the enzyme in the gap that destroys the neurotransmitter.

What happens to the muscle cell’s response?

1. A. ✓ It lasts longer
2. B. It ends sooner
   The enzyme ended the response by destroying the neurotransmitter.
   With the enzyme blocked, the neurotransmitter stays in the gap.
3. C. It does not change
   The enzyme was what removed the neurotransmitter from the gap.
   Blocking it lets the neurotransmitter keep binding the receptors.

Why: The enzyme destroys the neurotransmitter within a millisecond.
With the enzyme blocked, the neurotransmitter stays in the gap and keeps binding the muscle cell’s receptors.
So the muscle cell’s response lasts longer.

Testosterone is made in the testes. The blood carries it to muscle and bone cells, which respond by growing.

What kind of chemical signal is testosterone?

1. A. ✓ A hormone
2. B. A neurotransmitter
   A neurotransmitter is released into the gap beside one cell.
   Testosterone is carried in the blood to cells throughout the body.

Why: Testosterone is made in one part of the body.
The blood carries it to cells elsewhere, which respond.
A chemical signal made in one part of the body that changes what happens elsewhere is a hormone.

Cells in the kidney release a molecule into the blood. Over the next few days, cells in the bone marrow, a body’s length away, make more red blood cells.

Which of the following kinds of signal is the molecule?

1. A. A signal passed by cell-to-cell contact
   The bone marrow lies a body’s length from the kidney.
   The two kinds of cell never touch.
2. B. A local regulator
   A local regulator is destroyed within moments and reaches only its neighbors.
   This molecule reached cells a body’s length away.
3. C. ✓ A hormone

Why: The kidney cells released the molecule into the blood.
The blood carried it a body’s length to the bone marrow.
Target cells there responded.
A signal the blood carries to distant target cells is a hormone.

A cell releases a local regulator.

How does the local regulator reach the cells beside it?

1. A. The blood carries it to them
   The blood carries hormones.
   A local regulator spreads only through the fluid around the cell that released it.
2. B. ✓ It spreads through the fluid by diffusion
3. C. The two cells touch each other
   A local regulator is released into the fluid.
   It reaches cells the sender does not touch.

Why: The cell releases the local regulator into the fluid around it.
The local regulator spreads by diffusion to the cells in its immediate neighborhood.

A neurotransmitter makes a muscle cell respond within a millisecond of its release. Insulin makes liver cells respond about a minute after its release.

Why does the hormone take so much longer to act?

1. A. Insulin is a larger molecule and diffuses more slowly across the gap
   There is no gap to cross.
   Insulin rides the blood a body’s length, and the blood is slow.
2. B. Liver cells carry fewer receptors than muscle cells
   The delay is in the journey, not the target.
   The blood carries the hormone far before it arrives.
3. C. ✓ The blood has to carry insulin all the way to the liver cells
4. D. Insulin has to be broken down before the liver cells can respond
   Breakdown ends a signal.
   The delay comes from the blood carrying insulin to distant cells.

Why: A neurotransmitter diffuses across a gap a fraction of a micrometer wide.
Insulin rides the blood a body’s length before it reaches the liver cells.
That is a far slower journey.
So the response starts later.

A student says: “A hormone acts on every cell the blood carries it to.”

Is the student correct?

1. A. ✓ No — a hormone acts only on cells that carry its receptor
2. B. Yes — the blood carries the hormone to every cell
   The blood does carry the hormone to every cell.
   But a cell responds only when it carries a receptor that binds the hormone.

Why: The blood carries a hormone to every cell.
But a cell responds only when it carries a receptor that binds the hormone.
Cells with no receptor for the hormone do nothing.
So a hormone acts only on its target cells.

Cells lining the gut release a molecule after a meal. Over the following hours, bone cells throughout the skeleton, from the skull to the toes, respond to it.

Which of the following kinds of signal is this?

1. A. Cell-to-cell contact
   The bones lie far from the gut.
   Only the blood could carry a molecule from the gut to every bone in the skeleton.
2. B. A local regulator
   A local regulator is broken down within moments and reaches only its neighbors.
3. C. ✓ A hormone

Why: One cell type released a molecule.
Target cells of another type far away, throughout the skeleton, responded hours later.
Only the blood carries a signal that far to that many cells.
So the molecule is a hormone.

A damaged cell in the gum releases a molecule. Within seconds the cells touching it and those a few cell-widths away begin making a protective protein. The molecule is gone from the tissue within a minute.

Which of the following kinds of signal is this?

1. A. Cell-to-cell contact
   Cells a few cell-widths away also responded.
   So the molecule crossed the fluid between cells.
2. B. ✓ A local regulator
3. C. A hormone
   The molecule reached only cells a few cell-widths away and was gone within a minute.
   No blood carried it.

Why: The molecule diffused to cells a few cell-widths away and was destroyed within a minute.
A signal that acts only in its immediate neighborhood and is quickly destroyed is a local regulator.

In an embryo, a cell changes what it becomes only when a neighboring cell’s surface is pressed against it. Fluid the neighbor grew in has no effect on it.

Which of the following kinds of signal is this?

1. A. ✓ Cell-to-cell contact
2. B. A local regulator
   A local regulator acts through the fluid.
   The neighbor’s fluid had no effect.
   So no released molecule carries the message.
3. C. A hormone
   No blood carries a signal between two touching cells in an embryo.
   The response needed the two cell surfaces to touch.

Why: The cell responds only when the neighbor’s surface presses against it.
The neighbor’s fluid does nothing.
So a protein on one surface binds a receptor on the other: cell-to-cell contact.

A nerve ending releases a molecule into the gap beside a heart muscle cell. The cell slows its beat within a millisecond, and the molecule is gone from the gap within a millisecond.

Which of the following kinds of signal is this?

1. A. Cell-to-cell contact
   A molecule crossed the gap.
   In cell-to-cell contact, nothing is released.
2. B. ✓ A local regulator
3. C. A hormone
   The molecule crossed a gap beside one cell and was gone within a millisecond.
   No blood carried it.

Why: The nerve ending released a molecule into the gap beside its target.
The molecule diffused across the gap and was gone within a millisecond.
A signal that acts on the cell beside it and is destroyed at once is a local regulator: here, a neurotransmitter.

Fat cells release a molecule into the blood. About an hour later, cells in the brain, a body’s length away, respond to it.

Which of the following kinds of signal is this?

1. A. Cell-to-cell contact
   The brain lies a body’s length from the fat.
   Only the blood could carry the molecule there.
2. B. A local regulator
   A local regulator is destroyed within moments and reaches only its neighbors.
3. C. ✓ A hormone

Why: The fat cells released the molecule into the blood.
The blood carried it a body’s length to the brain.
Target cells there responded about an hour later.
A signal the blood carries to distant cells is a hormone.

A killer T cell presses against an infected cell, and within minutes the infected cell begins to die. Infected cells a few cell-widths away from the T cell are unaffected.

Which of the following kinds of signal is this?

1. A. ✓ Cell-to-cell contact
2. B. A local regulator
   A released molecule would spread through the fluid to the infected cells a few cell-widths away, and they would begin to die too.
   They are unaffected.
3. C. A hormone
   The blood would carry a molecule to every infected cell.
   Only the infected cell the T cell presses against dies.

Why: Only the infected cell the T cell presses against dies.
The infected cells a few cell-widths away are unaffected.
So no released molecule carries the message through the fluid.
So a protein on the T cell’s surface binds a receptor on the infected cell where the two touch: cell-to-cell contact.

Cells in the stomach wall release a molecule into the blood before a meal. About 20 minutes later, cells in the brain respond to it, and you feel hungry.

Which of the following kinds of signal is this?

1. A. Cell-to-cell contact
   The brain lies far from the stomach.
   The two kinds of cell never touch.
2. B. A local regulator
   A local regulator is destroyed within moments and reaches only its neighbors.
   This molecule reached the brain.
3. C. ✓ A hormone

Why: The stomach cells released the molecule into the blood.
The blood carried it to the brain.
Target cells there responded.
A signal the blood carries to distant target cells is a hormone.

An immune cell that has captured part of a bacterium presses against a second immune cell, which begins dividing within a day. With a fine mesh between them that lets molecules through but keeps the cells apart, the second cell stays as it was.

Which of the following kinds of signal is this?

1. A. ✓ Cell-to-cell contact
2. B. A local regulator
   A local regulator would cross the mesh through the fluid.
   With the mesh in place, the second cell stayed as it was.
3. C. A hormone
   The blood plays no part between two cells pressed together.
   The response needed the two surfaces to touch.

Why: The mesh lets molecules through, so a released molecule would still reach the second cell.
The second cell divided only when the first pressed against it.
So a surface protein binds a receptor where the cells touch: cell-to-cell contact.

The blood carries growth hormone to every cell in the body.

Which cells respond to it?

1. A. All the cells in the body
   A cell responds only when it carries a receptor that binds the hormone.
   Cells with no receptor do nothing.
2. B. ✓ Only the cells that carry its receptor

Why: The blood carries growth hormone to every cell.
A cell responds only when it carries a receptor that binds it.
So only the cells that carry the growth hormone receptor respond.

A molecule released by one cell reaches only the cells beside it and is gone within a minute.

What carries the message to those cells?

1. A. ✓ Diffusion through the fluid
2. B. The blood
   The blood carries a molecule through the whole body.
   This molecule reached only the cells beside the sender.
3. C. Surface proteins binding where the cells touch
   The molecule was released, so it left the cell’s surface.
   It reached cells the sender does not touch.

Why: The molecule was released into the fluid and reached only the cells beside the sender.
It spread to them by diffusion through the fluid.
It is a local regulator.

A nerve ending releases a neurotransmitter into the gap beside a muscle cell. A gland releases a hormone into the blood at that very moment.

Which response starts first?

1. A. The distant cells’ response to the hormone
   The blood has to carry the hormone a long way before it arrives.
   The neurotransmitter has only a gap to cross.
2. B. ✓ The muscle cell’s response to the neurotransmitter
3. C. The two responses start at the same moment
   The neurotransmitter crosses its gap in a millisecond.
   The hormone is still being carried in the blood.

Why: The neurotransmitter diffuses across a gap a fraction of a micrometer wide, so the muscle cell responds within a millisecond.
The blood has to carry the hormone to its distant target cells.
So the muscle cell’s response starts first.

After a meal, cells lining the small intestine release a molecule into the blood. Within an hour, cells in the pancreas, several centimeters away, respond to it.

Which of the following kinds of signal is this?

1. A. Cell-to-cell contact
   The intestine cells and the pancreas cells never touch.
   A molecule was released into the blood.
2. B. A local regulator
   A local regulator spreads through the fluid only to the cells beside the sender.
   This molecule traveled several centimeters in the blood.
3. C. ✓ A hormone

Why: The intestine cells released the molecule into the blood.
The blood carried it several centimeters to the pancreas.
A signal the blood carries to target cells elsewhere is a hormone.

A cell releases a signal molecule into the fluid around it, and the molecule spreads by diffusion.

Which cells respond to the molecule?

1. A. Every cell the molecule reaches
   A cell with no receptor for the molecule does nothing, however much of the molecule reaches it.
2. B. ✓ Only the cells that carry a receptor for it

Why: A cell responds to a signal only through a receptor that binds it.
So only the cells that carry the receptor respond.

Suppose a culture of the light-producing bacterium at 20,000 cells per mL is left to multiply for a day. By the next day it holds 200,000,000 cells per mL.

Which of the following happens to the concentration of the signal molecule in the water?

1. A. ✓ The concentration rises
2. B. The concentration stays the same
   Every cell releases the signal molecule.
   By the next day, 10,000 times as many cells release it into the same water.
3. C. The concentration falls
   More cells release more of the molecule into the same water.

Why: Each cell releases the signal molecule into the water.
By the next day, 10,000 times as many cells release it into the same volume of water.
So the concentration of the signal molecule rises.

Two flasks hold the same light-producing bacterium. One holds 30,000 cells per mL. The other holds 300,000,000 cells per mL.

Which flask glows?

1. A. The flask at 30,000 cells per mL
   At 30,000 cells per mL the signal molecule diffuses away and stays dilute.
   The cells stay dark.
2. B. ✓ The flask at 300,000,000 cells per mL

Why: The cells make light only when the signal molecule has built up enough.
At 300,000,000 cells per mL, many cells release the molecule into the same water, so it builds up.
So only the flask at 300,000,000 cells per mL glows.

What is quorum sensing?

1. A. ✓ Bacteria sensing the crowd through a molecule they all release
2. B. Bacteria touching their neighbors to count how many there are
   The count works through a molecule released into the water, not through touch.
3. C. Bacteria detecting the light their neighbors give off
   Each cell detects a released signal molecule, not light.

Why: Each bacterium releases a signal molecule into the water.
When cells are dense, the molecule builds up.
Bacteria detecting the crowd through that built-up signal is quorum sensing.

A student says: “The bacteria wait until enough of them are present, then they decide together to switch on the light.”

Is the student correct?

1. A. ✓ No — each cell switches on by itself once the signal is concentrated enough
2. B. Yes — the bacteria sense their neighbors and then decide together when to act
   A bacterium has no way to weigh its neighbors.
   Each cell switches on automatically once the signal molecule binds its receptor.

Why: Each cell carries a receptor for the signal molecule.
Each cell switches on its light automatically once the signal molecule’s concentration is high enough to bind that receptor.
The concentration is high enough only when many cells are releasing the molecule.

A mutant strain of the bacterium carries the receptor but releases no signal molecule. A researcher adds fluid from a dense culture of the normal strain, with every cell removed, to a dense culture of the mutant.

Which of the following does the mutant culture do?

1. A. The mutant culture glows only once its own cells are denser
   Density matters only because it raises the signal molecule’s concentration.
   The added fluid supplies the molecule directly.
2. B. The mutant culture stays dark
   The mutant cannot make the signal molecule.
   But the mutant carries the receptor, and the added fluid supplies the molecule.
3. C. ✓ The mutant culture glows

Why: The mutant carries the receptor and lacks only the signal molecule.
The fluid from the dense normal culture carries the molecule above its threshold concentration.
So the molecule binds the mutant’s receptors, and the culture glows.

What does the control in an experiment show?

1. A. ✓ The result when the tested factor is absent
2. B. The result when every factor is present
   The control lacks the one factor under test.
3. C. The largest result the experiment can produce
   The control is a comparison, not a maximum.

Why: The control is given the same treatment as the other tubes or dishes but lacks the factor under test.
So it shows the result with that factor absent.

Cell-free fluid from a dense culture of the light-producing bacterium raises a sparse culture from 1 to 140 light units. Cell-free fluid from a sparse culture leaves it at 1 light unit, and so does fresh fluid with its nutrients removed.

Which of the following do the results show causes the light?

1. A. Contact between bacteria packed closely together
   Fluid with every cell removed switched the light on.
   So no cell needed to touch another.
2. B. ✓ A released molecule that has reached a high enough concentration
3. C. Light given off by neighboring bacteria
   The sparse culture was dark until it received fluid from the dense culture.
   A molecule in that fluid, not light, set it glowing.
4. D. Nutrients used up in the crowded culture
   Fresh fluid with its nutrients removed left the sparse culture dark.
   The dense culture’s fluid switched the light on: something it contained, not something the culture lacked.

Why: Fluid from the dense culture, with every cell removed, made a sparse culture glow.
Fluid from the sparse culture did nothing.
Fluid with its nutrients removed did nothing.
So a molecule the dense cells released, at a high enough concentration, causes the light.

A soil bacterium releases a signal molecule into the water around it and makes a slime coat only when it is crowded. One cell of this bacterium sits alone in a drop of water.

Which of the following does the cell do?

1. A. ✓ The cell makes no slime coat
2. B. The cell makes a thin slime coat
   One cell’s molecule diffuses away and stays dilute.
   Below the threshold concentration the receptor stays empty, and nothing is switched on.
3. C. The cell makes a full slime coat
   One cell alone cannot build the molecule up to its threshold concentration.

Why: One cell releases the signal molecule into the drop.
The molecule diffuses away and stays dilute, below its threshold concentration.
So the molecule does not bind the cell’s receptor.
So the cell makes no slime coat.

A researcher dilutes a dense, glowing culture of the light-producing bacterium a thousand-fold with fresh growth medium. Once the signal molecule stops binding their receptors, these cells stop making light within about an hour.

Which of the following happens to the light over the next hour?

1. A. The light stays on
   The light depends on the signal molecule binding the receptors.
   Diluted a thousand-fold, the molecule is below its threshold concentration, so the receptors are empty.
2. B. The light brightens
   The light depends on the signal molecule’s concentration.
   Dilution lowers the concentration a thousand-fold.
3. C. ✓ The light goes out

Why: The light depends on the signal molecule’s concentration.
Diluting a thousand-fold drops the molecule below its threshold concentration.
So the molecule no longer binds the receptors.
So the cells stop making light, and within about an hour the light is out.

A student says: “In the sparse flask the cells have no receptors. That is why it stays dark.”

Is the student correct?

1. A. Yes — a cell in a sparse flask makes no receptor for the signal
   Every cell of the bacterium carries the receptor, crowded or not.
   In the sparse flask the signal molecule is too dilute to bind it.
2. B. ✓ No — every cell carries the receptor; the signal is too dilute to bind it

Why: Every cell carries the receptor.
In the sparse flask, few cells release the signal molecule, so it stays dilute, below the threshold concentration.
So the molecule does not bind the receptors, and the flask stays dark.

A mutant strain of the light-producing bacterium releases the signal molecule but carries no receptor for it. A researcher grows the mutant to 500,000,000 cells per mL.

Which of the following does the mutant culture do?

1. A. The mutant culture glows
   The signal molecule builds up, but nothing in the cells binds it.
2. B. The mutant culture glows dimly
   With no receptor, nothing in a cell binds the molecule.
   So nothing is switched on at all.
3. C. ✓ The mutant culture stays dark

Why: The mutant cells release the signal molecule, so at 500,000,000 cells per mL it builds up past its threshold concentration.
But the mutant carries no receptor for the molecule.
So nothing in the cells binds it.
So no cell switches on its light, and the culture stays dark.

A researcher adds fluid from a dense culture of the light-producing bacterium, with every cell removed, to one sparse culture. She adds fresh fluid to a second sparse culture.

Which of the following is the control?

1. A. ✓ The sparse culture given fresh fluid
2. B. The sparse culture given the cell-free fluid
   The cell-free fluid carries the factor under test.
   The control lacks that factor.
3. C. The dense culture the fluid came from
   The dense culture is where the fluid came from, not a culture given a treatment.

Why: The factor under test is the fluid the dense cells grew in.
The control is the culture given the same treatment but lacking that factor.
The fresh-fluid culture receives fluid in which no cell grew.
So the sparse culture given fresh fluid is the control.

The graph below shows the light produced by cultures of a second light-producing bacterium at four densities.

Between which two densities does the light first switch on?

1. A. Between 20,000 and 200,000 cells per mL
   Light went from 2 to 3 light units there.
   The culture is still almost dark.
2. B. ✓ Between 200,000 and 2,000,000 cells per mL
3. C. Between 2,000,000 and 20,000,000 cells per mL
   The culture was already glowing at 2,000,000 cells per mL, at 40 light units.
   The switch from dark to glowing came one step earlier.
4. D. At every step
   Below the threshold concentration the light barely changes.
   The switch happens where the signal molecule first binds the receptors.

Why: The culture is almost dark at 20,000 and 200,000 cells per mL.
It is glowing at 2,000,000 cells per mL, at 40 light units.
So the signal molecule passes its threshold concentration between 200,000 and 2,000,000 cells per mL.

In an enzyme experiment, a tube of hydrogen peroxide with no tissue in it sits beside the tubes of peroxide with tissue.

Which of the following is the no-tissue tube?

1. A. A condition kept the same in every tube
   A condition kept the same in every tube is a control variable, not a tube.
2. B. ✓ The control
3. C. The condition the experimenter changes
   The condition the experimenter changes is the independent variable: here, whether tissue is present.

Why: The no-tissue tube is given the same peroxide for the same time but lacks the factor under test, the tissue.
So the no-tissue tube is the control.

A fungus growing in a dish stops a second fungus 3 mm away from growing. A researcher sets up two more dishes of the second fungus: one given fluid the first fungus grew in, filtered to remove every cell; one given fresh fluid.

Which dish is the control?

1. A. The dish with both fungi
   The dish with both fungi carries the factor under test, the first fungus.
   The control lacks that factor.
2. B. The filtered-fluid dish
   The filtered fluid carries what the first fungus released.
   The control lacks that factor.
3. C. ✓ The fresh-fluid dish

Why: The factor under test is the first fungus and what it releases.
The fresh-fluid dish is given the same treatment but has neither.
So the fresh-fluid dish is the control.

A yeast cell releases a mating signal into the fluid around it. A researcher adds fluid that a mating yeast culture grew in, with every cell removed, to one dish of fresh yeast cells. She adds fresh fluid to a second dish of fresh yeast cells.

Which dish is the control?

1. A. ✓ The dish given fresh fluid
2. B. The dish given the cell-free fluid
   The cell-free fluid carries the factor under test, what the mating cells released.
3. C. The mating culture the fluid came from
   The mating culture is where the fluid came from, not a dish given a treatment.

Why: The factor under test is the fluid the mating culture grew in.
The dish given fresh fluid receives the same treatment but lacks that fluid.
So the dish given fresh fluid is the control.

A researcher tests whether wounded leaf cells release a molecule that makes healthy leaf cells produce a defense protein. One dish holds healthy cells given fluid the wounded cells sat in, filtered to remove every cell. A second dish holds healthy cells given fresh fluid. A third dish holds healthy cells touching wounded cells.

Which dish is the control?

1. A. The filtered-fluid dish
   The filtered-fluid dish carries the factor under test, the fluid the wounded cells sat in.
2. B. ✓ The fresh-fluid dish
3. C. The dish where healthy cells touch wounded cells
   The dish where healthy cells touch wounded cells carries the wounded cells themselves.
   The control lacks the factor under test.

Why: The factor under test is the fluid the wounded cells sat in.
The fresh-fluid dish receives fluid in which no wounded cell sat.
So the fresh-fluid dish is the control.

A nerve ending releases a molecule into the narrow gap beside a muscle cell, and the muscle cell contracts.

Which of the following carried the message?

1. A. A hormone carried in the blood
   The molecule crossed a narrow gap to the cell beside it, not the whole body in the blood.
2. B. Touch between the two cells
   The nerve ending released a molecule into the gap; the two cells did not need to touch.
3. C. ✓ A molecule released to the cells nearby

Why: The nerve ending released a molecule into the gap.
The molecule crossed the gap to the muscle cell beside it.
So a molecule released to the cells nearby carried the message.

A fungus growing on one side of a dish makes a second fungus on the other side stop growing, across a gap of 3 mm. No thread of either fungus touches the other.

Which of the following does the 3 mm gap show about the message?

1. A. The message passes where the two fungi touch
   No thread of either fungus touches the other, and the second fungus still stopped growing.
   So the message crossed the gap without contact.
2. B. ✓ The message crosses the gap as a released molecule

Why: No thread of either fungus touches the other.
The second fungus still stopped growing.
So the message crossed the 3 mm gap without contact.
A message that crosses a gap through fluid is a released molecule.

Activated donor immune cells release a molecule into the fluid around them. Recipient immune cells that carry a receptor for the molecule respond within hours to fluid the donor cells grew in. A researcher adds the same fluid to recipient cells that carry no receptor for the molecule.

Which of the following do these recipients do?

1. A. The recipients respond
   A cell responds only through a receptor, and these recipients carry none.
2. B. The recipients respond weakly
   With no receptor, nothing in the cell binds the molecule.
   So nothing changes at all.
3. C. ✓ The recipients stay as they were

Why: Only cells that carry the receptor respond.
These recipients have no receptor for the molecule.
So the fluid reaches them and nothing in them binds the molecule.
So they stay as they were.

A gland in the neck releases a molecule into the blood, and bone cells all over the body respond to it.

Which of the following carried the message?

1. A. Touch between the gland cells and the bone cells
   The bone cells sit all over the body, far from the gland.
   No gland cell touches them.
2. B. A molecule released to the cells nearby
   A molecule released to nearby cells reaches only its own neighborhood.
   These bone cells sit all over the body.
3. C. ✓ A hormone carried in the blood

Why: The gland released the molecule into the blood.
The blood carried the molecule through the whole body.
Bone cells all over the body responded.
So a hormone carried in the blood carried the message.

Activated liver cells sit on one side of a membrane that stops cells but lets molecules through. Fat cells sit on the other side. The fat cells begin releasing their stored fat.

Which of the following does the result show about the message?

1. A. ✓ The message crossed the membrane as a released molecule
2. B. The message passed where liver cells touched fat cells
   The membrane stops cells.
   No liver cell could touch a fat cell.

Why: The membrane stops cells, so no liver cell touched a fat cell.
The fat cells still responded.
The membrane lets molecules through.
So the message crossed as a molecule the liver cells released.

A strain of the second fungus lacks the receptor for the molecule the first fungus releases. A researcher adds filtered fluid the first fungus grew in to this strain.

Which of the following does this strain do?

1. A. ✓ The strain keeps growing
2. B. The strain stops growing
   With no receptor, nothing in the cells binds the molecule.
   So nothing in them changes.
3. C. The strain grows more slowly
   A cell without the receptor does nothing at all.
   The molecule reaches it and nothing binds it.

Why: The fluid carries the first fungus’s molecule to the cells.
These cells have no receptor for the molecule.
So nothing in them binds it.
So the strain keeps growing.

In the mixed dish, the activated donor cells were touching some of the recipient cells. A student says: “Because the donors were touching the recipients, the message must have passed by touch.”

Is the student correct?

1. A. Yes — cells that touch pass their message by touch
   Touching cells can still send a released molecule.
   The membrane test showed the message crossing with no contact at all.
2. B. ✓ No — the recipients also responded across a membrane no cell could cross

Why: Cells that touch may still release a molecule into the fluid.
Recipients responded across a membrane that stopped every donor cell.
So the message traveled as a released molecule, not by touch.

A researcher sets up only one dish: recipient cells given fluid the donor cells grew in. The recipients respond.

Which of the following is still needed before the response can be credited to a molecule from the donors?

1. A. A dish of donors given fresh fluid
   The question is whether the recipients’ response comes from the donors’ molecule.
   The control lacks that molecule and keeps everything else.
2. B. A second dish of recipients given the donors’ fluid
   Repeating shows the result is steady.
   It does not show that the fluid alone would have done nothing.
3. C. A dish of donors and recipients touching
   Contact is a different question.
   What is missing is the control that lacks the tested factor, the donors’ molecule.
4. D. ✓ A dish of recipients given fresh fluid

Why: The control is given the same treatment but lacks the factor under test.
Recipients given fresh fluid show what fluid alone does.
Only with that dish can the response to the donors’ fluid be credited to a molecule the donors released.

A researcher adds fluid the activated donor immune cells grew in, with every cell removed, to one dish of recipient cells. She adds fresh fluid to a second dish of recipients.

Which of the following does the fresh-fluid dish show?

1. A. ✓ That fluid on its own produces no response
2. B. That the donors release a molecule
   The fresh-fluid dish has no donor fluid in it.
   It shows what happens with the donors’ molecule absent.
3. C. That the recipients carry the receptor
   The fresh-fluid dish carries no signal for a receptor to bind.
   It shows what fluid alone does.

Why: The fresh-fluid dish is the control: the same treatment minus the fluid the donors grew in.
Its recipients stay as they were.
So the fresh-fluid dish shows that fluid on its own produces no response.

A substrate fits into an enzyme’s active site.

What decides whether a molecule fits the active site?

1. A. ✓ Its shape and charge
2. B. Its size alone
   Two molecules of the same size can differ in shape and in where their charges sit; only the one whose shape and charge match the site fits it.

Why: A substrate fits an enzyme’s active site when its shape and its charges match the site.

A plant hormone reaches a root cell and binds a pocket on a protein set in the root cell’s membrane.

Which is the ligand?

1. A. The pocket on the protein
   The pocket is where the ligand fits; the ligand is the molecule that fits it.
2. B. The membrane protein
   The receptor holds the pocket; the ligand is the signal molecule it binds.
3. C. The root cell
   The root cell is the target cell; the ligand is the signal molecule that binds the cell’s receptor.
4. D. ✓ The plant hormone

Why: The ligand is the signal molecule that binds the receptor: the plant hormone.
The protein is the receptor and the pocket is where the hormone fits.

What is a ligand?

1. A. The receptor’s pocket
   The pocket is part of the receptor; the ligand is the molecule that fits the pocket.
2. B. The cell that carries the receptor
   The cell that carries the receptor is the target cell; the ligand is the molecule that binds the receptor.
3. C. ✓ The signal molecule that binds a receptor

Why: A ligand is the signal molecule that binds a receptor.
Insulin is the ligand of the insulin receptor.

The model below shows a receptor set in a membrane with its ligand. Four positions are marked 1 to 4.

Which numbered position marks the receptor’s ligand-binding domain?

1. A. Position 1
   Position 1 marks the ligand itself, which is not part of the receptor.
   The ligand-binding domain is the region of the receptor that holds the pocket: position 2.
2. B. ✓ Position 2
3. C. Position 3
   Position 3 marks the region of the receptor within the membrane.
   The ligand-binding domain faces outward and holds the pocket the ligand fits.
4. D. Position 4
   Position 4 marks the region of the receptor facing the inside of the cell.
   The ligand never reaches that region; the ligand-binding domain is the outer region with the pocket.

Why: The ligand-binding domain is the region of the receptor that holds the pocket the ligand fits: the part above the membrane, position 2.
Position 1 is the ligand itself.

What is a binding site?

1. A. The signal molecule that fits a receptor’s pocket
   The signal molecule that fits a receptor’s pocket is its ligand; the binding site is the pocket the ligand fits.
2. B. The whole folded receptor protein
   The binding site is one pocket on the protein, not the whole protein.
3. C. ✓ The pocket on a protein that one molecule fits

Why: A binding site is the pocket on a protein that one molecule fits by shape and charge.
An enzyme’s active site is its binding site for its substrate.
A receptor’s pocket is its binding site for its ligand.

What is a receptor’s ligand-binding domain?

1. A. The ligand, the molecule that fits the pocket
   The ligand is the signal molecule; it is not part of the receptor.
2. B. ✓ The region of the receptor that holds the pocket
3. C. The middle part, set within the membrane
   The part within the membrane holds no pocket; the ligand-binding domain is the region that does.

Why: A domain is a region of a protein with one job.
The ligand-binding domain is the region of the receptor that holds the pocket its ligand fits.

A molecule of insulin binds the pocket of an insulin receptor, stays for a few seconds, and is released again.

Which of the following has happened to the insulin molecule by the time it is released?

1. A. ✓ The insulin molecule is unchanged
2. B. The insulin molecule has gained a phosphate group from the receptor
   The receptor is the molecule that changes shape while the insulin is bound.
   The insulin leaves as it arrived.
3. C. The insulin molecule has been converted into a different signal molecule
   A receptor makes no product.
   The receptor releases the same insulin molecule.
4. D. The insulin molecule has been split into two smaller products
   An enzyme makes products from its substrate.
   A receptor is not an enzyme.
   The receptor holds the insulin, changes its own shape, and lets the insulin go as it arrived.

Why: Insulin is the receptor’s ligand.
A ligand is not a substrate.
The receptor makes no product from its ligand.
The receptor holds the insulin, changes its own shape while the insulin is bound, and releases the same insulin molecule unchanged.

Insulin binds the pocket of its receptor on a muscle cell and is released a few seconds later. A student says: “The receptor is an enzyme, and insulin is its substrate, so the insulin comes out changed.”

Is the student correct?

1. A. Yes — the receptor changes the insulin into a product
   An enzyme makes a product from its substrate; a receptor makes no product from its ligand.
   The receptor holds the insulin, changes its shape, and releases the insulin unchanged.
2. B. ✓ No — the receptor changes its own shape and releases the insulin unchanged

Why: A receptor is not an enzyme, and a ligand is not a substrate.
The receptor holds the insulin.
The receptor changes its own shape while the insulin is bound.
Then the receptor releases the same insulin molecule, unchanged.

A liver cell carries two kinds of receptor. One kind has a small pocket, sized for epinephrine, a molecule of a few dozen atoms. A molecule of insulin, far larger than epinephrine, reaches the small pocket.

Predict what happens.

1. A. ✓ The small pocket does not hold the insulin
2. B. The small pocket holds the insulin
   A pocket holds only a molecule whose shape and charge match it.
   Insulin is far too large for a pocket sized for epinephrine.
3. C. The small pocket widens and holds the insulin
   A receptor’s pocket has one shape.
   A molecule that does not fit that shape is not held.

Why: A receptor’s pocket holds only a molecule whose shape and charge match it.
Insulin is far too large for a pocket sized for epinephrine.
So the small pocket does not hold the insulin, and the insulin passes by.

Amino acids join one after another to make a chain.

Which bond joins each amino acid to the next?

1. A. A hydrogen bond
   Hydrogen bonds hold a folded chain in shape; they do not join one amino acid to the next.
2. B. An ionic bond
   An ionic bond joins two ions; amino acids in a chain are joined by a covalent bond.
3. C. ✓ A peptide bond

Why: Two amino acids join by a peptide bond.
A chain of amino acids is held together by peptide bonds, one between each pair.

A hormone of the adrenal gland is built of four fused carbon rings with a few attached atoms.

Which kind of ligand is this molecule?

1. A. A peptide
   A peptide is a chain of amino acids.
   This hormone has no amino acids: four fused carbon rings with a few attached atoms, a few dozen atoms in all.
2. B. ✓ A small molecule

Why: A peptide is a chain of amino acids.
This hormone is four fused carbon rings with a few attached atoms, and it has no amino acids.
A few dozen atoms make a small molecule.
So this hormone is a small molecule.

A signal molecule released by gut cells is a folded chain of 120 amino acids.

Which kind of ligand is this molecule?

1. A. ✓ A peptide
2. B. A small molecule
   Folding packs the chain into a compact shape, but every one of the 120 amino acids is still there.
   A chain of amino acids is a peptide, whatever its length.

Why: A peptide is a chain of amino acids, short or long.
This signal molecule is a chain of 120 amino acids.
A chain long enough to be a whole protein is still a chain of amino acids.
So this signal molecule is a peptide.

A signal molecule released by nerve cells is a chain of 5 amino acids.

Which kind of ligand is this molecule?

1. A. ✓ A peptide
2. B. A small molecule
   A small molecule is a few dozen atoms with no amino acids.
   A chain of 5 amino acids is still a chain of amino acids, so it is a peptide.

Why: A peptide is a chain of amino acids.
This signal molecule is a chain of 5 amino acids.
A short chain is still a chain of amino acids.
So this signal molecule is a peptide.

A signal molecule made by the cells lining a blood vessel is a gas of two atoms.

Which kind of ligand is this molecule?

1. A. A peptide
   A peptide is a chain of amino acids.
   This gas has no amino acids.
   Two atoms make a very small molecule.
   So this signal molecule is a small molecule.
2. B. ✓ A small molecule

Why: A peptide is a chain of amino acids.
This signal molecule is two atoms, with no amino acids.
So this signal molecule is a small molecule.

A plant signal molecule is a single carbon ring with a short tail, about 20 atoms in all.

Which kind of ligand is this molecule?

1. A. A peptide
   A peptide is a chain of amino acids.
   This molecule has no amino acids.
   About 20 atoms make a small molecule.
   So this plant signal is a small molecule.
2. B. ✓ A small molecule

Why: A peptide is a chain of amino acids.
This plant signal molecule is one carbon ring with a short tail, about 20 atoms, and it has no amino acids.
So this plant signal molecule is a small molecule.

Epinephrine reaches a heart muscle cell and binds a pocket on a protein set in the cell’s membrane.

Which is the ligand?

1. A. The heart muscle cell
   The heart muscle cell is the target cell; the ligand is the molecule that binds its receptor.
2. B. The protein set in the membrane
   The protein is the receptor; the ligand is the signal molecule it binds.
3. C. The pocket on the protein
   The pocket is the binding site; the ligand is the molecule that fits it.
4. D. ✓ The epinephrine

Why: The ligand is the signal molecule that binds the receptor.
Epinephrine binds the protein’s pocket, so epinephrine is the ligand.

A hormone molecule fits a pocket on a receptor protein.

What is the pocket called?

1. A. ✓ The binding site
2. B. The ligand
   The ligand is the hormone molecule that fits the pocket, not the pocket.
3. C. The ligand-binding domain
   The ligand-binding domain is the region of the receptor that holds the pocket; the pocket itself is the binding site.

Why: The pocket a molecule fits is the binding site.
The region of the receptor that holds the pocket is the ligand-binding domain.

A thyroid hormone is two carbon rings with a few attached atoms, about 35 atoms in all.

Which kind of ligand is this molecule?

1. A. A peptide
   A peptide is a chain of amino acids.
   This hormone has no amino acids.
2. B. ✓ A small molecule

Why: A peptide is a chain of amino acids.
This thyroid hormone is two carbon rings with a few attached atoms, and it has no amino acids.
About 35 atoms make a small molecule.
So this hormone is a small molecule.

A cell carries receptors for a growth signal and no other receptors. Insulin reaches the cell.

Which of the following happens?

1. A. Insulin binds the growth-signal receptors
   The growth-signal receptors’ pockets fit the growth signal by shape and charge; insulin does not fit them.
2. B. The cell makes a new receptor for insulin
   A cell’s receptors are the proteins it already carries; a signal arriving does not make the cell build a receptor for it.
3. C. ✓ Insulin passes by and binds nothing

Why: A receptor’s pocket holds only the ligand whose shape and charge match it.
The cell’s receptors fit the growth signal, not insulin.
So insulin passes by and binds nothing.

A molecule of growth hormone binds its receptor on a bone cell and is released a few seconds later.

Which of the following describes the released molecule?

1. A. A smaller molecule, with part cut off
   A receptor is not an enzyme; it cuts nothing off its ligand.
2. B. A different signal molecule
   A receptor makes no product from its ligand.
3. C. ✓ The same molecule, unchanged

Why: Growth hormone is the receptor’s ligand.
A receptor makes no product from its ligand.
The receptor holds the growth hormone, changes its own shape, and releases the same molecule unchanged.

A receptor set in a membrane has a region that holds the pocket its ligand fits.

What is that region called?

1. A. The binding site
   The binding site is the pocket itself; the region of the receptor that holds the pocket has its own name.
2. B. ✓ The ligand-binding domain
3. C. The ligand
   The ligand is the signal molecule that fits the pocket; it is not part of the receptor.

Why: A domain is a region of a protein with one job.
The region of the receptor that holds the pocket is the ligand-binding domain.

A molecule binds a protein at one site.

What can that binding do to the rest of the protein?

1. A. ✓ Bend the protein’s shape elsewhere
2. B. Nothing beyond the site it binds
   A protein is one folded chain.
   A molecule bound at one site pulls on the fold, so the shape changes elsewhere too.

Why: A molecule bound at one site can bend a protein’s shape elsewhere, and a protein’s shape is its job.

Insulin binds the pocket of its receptor on a liver cell, and the receptor changes shape.

Which region of the receptor acts on the next molecule inside the cell?

1. A. The ligand-binding domain
   The ligand-binding domain faces the outside of the cell; it holds the insulin, and nothing inside the cell touches it.
2. B. The binding site
   The binding site is the pocket the insulin fits, on the outer face of the receptor.
3. C. ✓ The intracellular domain

Why: With insulin bound, the receptor holds a different shape all the way to its intracellular domain, the region facing the cytosol.
That region is the part inside the cell, so it is the part that acts on the next molecule.

What is a receptor’s intracellular domain?

1. A. The pocket that the ligand fits
   The pocket the ligand fits is the binding site, on the outer face of the receptor.
2. B. ✓ The region of the receptor that faces the cytosol
3. C. The middle part, set within the membrane
   The part within the membrane is surrounded by the phospholipid tails; the intracellular domain is the region below it, in the cytosol.

Why: A domain is a region of a protein with one job.
The intracellular domain is the region of the receptor that faces the cytosol.
While the ligand is bound, the receptor holds a different shape all the way to this region.

A researcher gives fat cells a peptide hormone tagged with a label that shows where each molecule goes. Ten minutes later every labeled molecule is still outside the cells, and the cells have begun breaking down their stored fat.

Which of the following carried the message from the outside of the membrane to the inside of the cell?

1. A. The hormone molecule, passed through the receptor’s pocket into the cytosol
   Every labeled molecule is still outside the cells after ten minutes, so no hormone molecule crossed the membrane.
   The pocket holds the hormone; it does not pass the hormone through.
2. B. A piece of the hormone molecule, cut off in the pocket and passed into the cytosol
   A receptor is not an enzyme: it makes no product from the hormone.
   The receptor holds the hormone, changes shape, and releases it unchanged; no piece is cut off.
3. C. ✓ A change in the shape of the receptor protein

Why: Every labeled molecule is still outside, so no hormone crossed the membrane.
Yet the cells changed, so something else carried the message.
The bound hormone holds the receptor in a different shape, right through the membrane.
So the receptor’s change of shape carried the message across and started the relay.

A researcher gives fat cells a labeled peptide hormone. After ten minutes every labeled molecule is still outside the cells, and the cells have begun breaking down their stored fat. A student says: “The cells changed, so some of the hormone must have entered the cells.”

Is the student correct?

1. A. ✓ No — the hormone stays outside, and the receptor’s change of shape enters
2. B. Yes — a cell changes only when the signal molecule enters it
   Every labeled hormone molecule is still outside, so no hormone entered.
   The receptor changes shape through to its intracellular domain; that change of shape is what enters the cell.

Why: Every labeled molecule is still outside, so no hormone entered.
The hormone binds its receptor at the surface.
The receptor changes shape through to its intracellular domain.
That change of shape is what enters the cell, and it starts the relay that breaks down the fat.

Two cultures of muscle cells: one carries normal insulin receptors; the other carries receptors whose intracellular domain is altered but whose pocket is unchanged. A researcher adds insulin to both cultures.

Predict which culture moves glucose transporters to the cell surface.

1. A. ✓ The normal culture only
2. B. The altered culture only
   Insulin binds the altered receptor’s pocket, but its altered intracellular domain cannot change shape.
   So the relay never starts, and the transporters stay inside.
3. C. Both cultures
   Both receptors bind insulin.
   The normal receptor changes shape through to its intracellular domain; the altered receptor passes nothing on.
   So the normal culture alone moves its transporters.
4. D. Neither culture
   The normal receptor has a working pocket and a working intracellular domain.
   Insulin binds, the receptor changes shape through to its inner face, and the relay starts.

Why: Insulin binds both kinds of receptor.
A normal receptor then changes shape through to its intracellular domain.
That inner change starts the relay that moves the transporters.
The altered receptor cannot change shape inside, so its relay never starts and its transporters stay inside.

What is signal transduction?

1. A. The ligand binding its receptor at the cell surface
   The ligand binding its receptor happens at the surface, before the relay inside begins.
2. B. ✓ The relay of the message through the inside of the cell
3. C. The signal molecule crossing the membrane into the cytosol
   The signal molecule stays outside; what moves inside is a change of protein shape.

Why: Signal transduction is the relay of the message through the inside of the cell.
It starts from the change in the receptor’s shape, and one molecule changes the next.

A signal transduction pathway has three stages.

In which order do the three stages happen?

1. A. ✓ Reception, transduction, cellular response
2. B. Transduction, reception, cellular response
   The ligand must bind its receptor before any molecule inside can relay the message.
3. C. Reception, cellular response, transduction
   The relay inside must reach the end before the cell does anything differently.

Why: The ligand binds its receptor first: reception.
Then molecules inside the cell relay the message: transduction.
Last, the cell does something differently: the cellular response.

What is reception?

1. A. ✓ The ligand binding its receptor
2. B. One molecule inside the cell changing the next
   One molecule inside the cell changing the next is transduction, the stage after reception.
3. C. What the cell finally does differently
   What the cell finally does differently is the cellular response, the last stage.

Why: Reception is the first stage of a signal transduction pathway.
Reception is the ligand binding its receptor.

What is the cellular response?

1. A. The ligand binding its receptor
   The ligand binding its receptor is reception, the first stage.
2. B. One molecule inside the cell changing the next
   One molecule inside the cell changing the next is transduction, a step of the relay.
3. C. ✓ What the cell finally does differently

Why: The cellular response is the last stage of a signal transduction pathway.
The cellular response is what the cell finally does differently, such as contracting harder or releasing glucose.

What is a signal transduction pathway?

1. A. The pocket on the receptor that a ligand fits
   The pocket a ligand fits is the binding site, one part of the receptor.
2. B. ✓ The whole relay from the receptor to the cellular response
3. C. The first stage only, the ligand binding its receptor
   The ligand binding its receptor is reception, the first stage; the pathway is the whole relay from receptor to response.

Why: A signal transduction pathway is the whole ordered relay.
It starts with the receptor binding its ligand and ends with the cellular response.

A stomach cell begins releasing acid.

Which stage of the pathway is this event?

1. A. Reception
   Reception is the ligand binding its receptor.
   Releasing acid is what the stomach cell finally does differently.
   So releasing acid is the cellular response.
2. B. Transduction
   Transduction is the relay inside the cell, one molecule changing the next.
   Releasing acid is what the stomach cell finally does differently.
   So releasing acid is the cellular response.
3. C. ✓ Cellular response

Why: The cellular response is what the cell finally does differently.
The stomach cell begins releasing acid.
So releasing acid is the cellular response.

A hormone fits the pocket of a receptor on a bone cell.

Which stage of the pathway is this event?

1. A. ✓ Reception
2. B. Transduction
   Transduction is the relay inside the cell.
   The hormone fitting the receptor’s pocket is the ligand binding its receptor.
   So this event is reception.
3. C. Cellular response
   The response is what the cell finally does differently.
   The hormone fitting the receptor’s pocket is the ligand binding its receptor.
   So this event is reception.

Why: Reception is the ligand binding its receptor.
The hormone fitting the pocket of the receptor is the ligand binding its receptor.
So this event is reception.

With its ligand bound, a receptor in a kidney cell’s membrane changes shape.

Which stage of the pathway is this event?

1. A. ✓ Reception
2. B. Transduction
   The receptor changing shape with its ligand bound is the last event of reception.
   Transduction begins when the changed receptor acts on the next molecule inside the cell.
3. C. Cellular response
   The response is what the cell finally does differently.
   The receptor changing shape with its ligand bound is the end of reception.

Why: Reception is the ligand binding its receptor, and reception ends when the receptor has changed shape.
The receptor changing shape with its ligand bound is the last event of reception.
So this event is reception.

A guard cell narrows the pore beside it.

Which stage of the pathway is this event?

1. A. Reception
   Reception is the ligand binding its receptor.
   Narrowing the pore is what the guard cell finally does differently.
   So narrowing the pore is the cellular response.
2. B. Transduction
   Transduction is the relay of molecules inside the cell.
   Narrowing the pore is what the guard cell finally does differently.
   So narrowing the pore is the cellular response.
3. C. ✓ Cellular response

Why: The cellular response is what the cell finally does differently.
The guard cell narrows the pore beside it.
So narrowing the pore is the cellular response.

One relay protein inside a kidney cell changes the shape of a second relay protein.

Which stage of the pathway is this event?

1. A. Reception
   Reception is the ligand binding its receptor at the surface.
   One relay protein changing the shape of the next is the relay inside the cell.
   So this event is transduction.
2. B. ✓ Transduction
3. C. Cellular response
   The response is what the cell finally does differently.
   One relay protein changing the next is a step of the relay inside the cell.
   So this event is transduction.

Why: Transduction is the relay inside the cell, one molecule changing the next.
One relay protein changing the shape of a second relay protein is exactly that.
So this event is transduction.

A signal binds a receptor on a gland cell. A relay protein inside the cell changes shape. Then the cell releases a stored hormone into the blood. A student says: “The relay protein changing shape is the cell’s response to the signal.”

Is the student correct?

1. A. Yes — the relay protein changing shape is the cellular response
   The relay protein changing shape is one molecule inside the cell changing: that is transduction.
   The cellular response is what the cell finally does differently: here, releasing the stored hormone.
2. B. ✓ No — that change is transduction, and the response is releasing the hormone

Why: The response is what the cell finally does differently, not a step of the relay.
The relay protein changing shape is transduction.
The cell releasing the stored hormone is the cellular response.

A hormone binds a receptor on a liver cell. The receptor changes shape. Then the receptor’s intracellular domain acts on a relay protein. A student says: “Transduction began when the receptor changed shape.”

Is the student correct?

1. A. ✓ No — transduction began when the receptor acted on the relay protein
2. B. Yes — transduction begins as soon as the receptor changes shape
   Reception ends when the receptor has changed shape.
   Transduction begins when the changed receptor acts on the next molecule inside the cell: here, the relay protein.

Why: Reception is the ligand binding its receptor, and reception ends when the receptor has changed shape.
Transduction begins when the changed receptor acts on the next molecule inside the cell.
Here that molecule is the relay protein.
So transduction began when the receptor acted on the relay protein.

In a yeast cell, a mating signal binds its receptor at 0 seconds; a relay protein inside changes shape at 5 seconds; the cell stops dividing at 60 seconds.

Which event is transduction?

1. A. ✓ The relay protein changing shape at 5 seconds
2. B. The mating signal binding its receptor at 0 seconds
   The ligand binding its receptor is the first stage; transduction is the relay inside that follows.
3. C. The cell stopping its division at 60 seconds
   Stopping division is what the cell finally does differently; transduction is the relay that gets there.
4. D. The mating signal being released by the other cell
   Transduction happens inside the target cell, after reception.

Why: Transduction is the relay inside the cell, one molecule changing the next.
The relay protein changing shape at 5 seconds is a step of that relay.

In a liver cell, epinephrine binds a receptor; the receptor changes shape; a relay molecule rises; an enzyme is switched on; the cell releases glucose.

What is the name for this whole ordered relay, from receptor to response?

1. A. Reception
   Reception is the first stage only: the ligand binding its receptor.
   The whole relay has a name of its own.
2. B. ✓ A signal transduction pathway
3. C. A cellular response
   The cellular response is the last stage only: what the cell finally does.
   The whole relay from receptor to response has a name of its own.

Why: The whole ordered relay, from the receptor binding its ligand to the cell’s response, is a signal transduction pathway.

The model below shows three steps of a signaling pathway in a liver cell, numbered 1 to 3.

Which numbered step is the cellular response?

1. A. Step 1
   Step 1, epinephrine bound to its receptor, is reception, the first stage, not what the cell finally does.
2. B. Step 2
   Step 2, the relay molecule rising, is transduction; the response is the change in what the cell does.
3. C. ✓ Step 3
4. D. Steps 1 and 2 together
   Reception and transduction together are still not what the cell finally does differently; step 3, releasing glucose, is.

Why: The cellular response is what the cell finally does differently: step 3, the cell releasing glucose.
Step 1 is reception and step 2 is transduction.

Insulin binds its receptor on the surface of a fat cell.

Which is the first change inside the cell?

1. A. Insulin appears in the cytosol
   Insulin stays outside; the change that enters is a change of the receptor’s shape.
2. B. Glucose transporters reach the surface at once
   The transporters move only after the relay inside has passed the message on, minutes later.
3. C. The receptor’s pocket changes shape on the outer face
   The pocket holds the ligand; the change that starts the relay is on the receptor’s inner face.
4. D. ✓ The receptor’s intracellular domain changes shape

Why: With insulin bound, the receptor holds a different shape all the way to its intracellular domain, the region facing the cytosol.
That change on the inner face is the first change inside the cell.

A peptide hormone binds its receptor on the surface of a kidney cell, and the cell begins moving water channels to its surface.

Which of the following crossed the membrane?

1. A. The hormone molecule
   The hormone binds the pocket on the outer face and stays outside.
2. B. A piece of the hormone molecule
   A receptor makes no product from its ligand; no piece is cut off.
3. C. ✓ A change in the receptor’s shape

Why: The hormone stays outside, held in the receptor’s pocket.
The receptor changes shape through to its intracellular domain.
That change of shape is what crosses the membrane and starts the relay inside.

A plant signal binds a receptor on a root cell. A relay protein inside the root cell changes shape. Then the root cell begins to grow toward water.

Which event is the cellular response?

1. A. The signal binding its receptor
   The signal binding its receptor is reception, the first stage.
2. B. The relay protein changing shape
   The relay protein changing shape is transduction, a step of the relay inside.
3. C. ✓ The root cell growing toward water

Why: The cellular response is what the cell finally does differently.
The root cell growing toward water is what it finally does differently.
So that event is the cellular response.

A membrane’s oily middle lets some molecules through on their own and holds others back.

Which of these crosses the oily middle of a membrane on its own?

1. A. A large polar molecule
   The oily middle holds a large polar molecule back.
2. B. An ion
   The oily middle holds back a charged particle, however small.
3. C. ✓ A small nonpolar molecule

Why: Only small nonpolar molecules cross the oily middle on their own.
Ions, large molecules and polar molecules are held back.

A researcher gives cells two ligands, P and S. The model below shows where each ligand is found after ten minutes.

Which of the following describes ligand S?

1. A. A peptide
   A peptide is a chain of amino acids, large and polar, and the membrane’s oily middle holds it back.
   S was found in the nucleus, so S crossed the membrane.
2. B. A large polar molecule
   The membrane’s oily middle holds a large polar molecule back.
   S was found in the nucleus, so S crossed the membrane.
3. C. ✓ A small nonpolar molecule

Why: S was found inside the nucleus, so S crossed the plasma membrane.
The membrane’s oily middle lets only small nonpolar molecules through.
So S is small and nonpolar, and S binds an intracellular receptor.
P stayed outside, so P cannot cross: a peptide or a polar molecule.

Testosterone is a small nonpolar hormone that slips through the plasma membrane. A student says: “Because testosterone gets inside the cell, it can act on the cell’s DNA by itself, with no receptor.”

Is the student correct?

1. A. ✓ No — testosterone changes the cell only through its receptor
2. B. Yes — a hormone that enters the cell needs no receptor
   Inside the cell, testosterone binds an intracellular receptor; the bound pair moves into the nucleus and acts on the DNA.
   A hormone never acts without its receptor.

Why: A hormone acts only through its receptor.
Testosterone does enter the cell.
Inside the cell, testosterone binds an intracellular receptor.
The bound pair moves into the nucleus and attaches to the DNA.
So testosterone changes the cell through its receptor, never on its own.

What is a cell-surface receptor?

1. A. ✓ A receptor set in the plasma membrane with its binding site facing outward
2. B. A receptor set in the plasma membrane with its binding site facing the cytosol
   A cell-surface receptor binds a ligand that stays outside the cell, so its binding site faces outward.
3. C. A receptor waiting in the cytosol or the nucleus
   A receptor in the cytosol or the nucleus is an intracellular receptor.

Why: A cell-surface receptor is set in the plasma membrane.
Its ligand cannot cross the membrane, so its binding site faces outward.

What is an intracellular receptor?

1. A. A ligand that has crossed the membrane into the cell
   A ligand is the signal molecule; the receptor is the protein that binds it.
2. B. ✓ A receptor inside the cell, in the cytosol or the nucleus
3. C. A receptor set in the plasma membrane with its binding site facing outward
   A receptor set in the plasma membrane with its binding site facing outward is a cell-surface receptor.

Why: An intracellular receptor is a receptor inside the cell, in the cytosol or the nucleus.
It binds a small nonpolar ligand that has slipped through the membrane.

A plant signal molecule is small and nonpolar, and it dissolves in oil.

Which kind of receptor does this signal molecule bind?

1. A. A cell-surface receptor
   A small nonpolar molecule dissolves into the membrane’s oily middle and crosses into the cell.
   A ligand that crosses the membrane binds an intracellular receptor.
2. B. ✓ An intracellular receptor

Why: A small nonpolar molecule dissolves into the membrane’s oily middle and crosses into the cell.
So this plant signal binds a receptor inside the cell.
Its receptor is an intracellular receptor.

A hormone is a chain of 84 amino acids.

Which kind of receptor does this signal molecule bind?

1. A. ✓ A cell-surface receptor
2. B. An intracellular receptor
   A chain of amino acids is large and polar, and the membrane’s oily middle holds it back.
   So this hormone stays outside and binds a cell-surface receptor.

Why: A chain of 84 amino acids is a peptide, large and polar.
The membrane’s oily middle holds a large polar molecule back.
So this hormone binds a receptor at the cell surface.
Its receptor is a cell-surface receptor.

A signal molecule is an ion carrying a full charge.

Which kind of receptor does this signal molecule bind?

1. A. ✓ A cell-surface receptor
2. B. An intracellular receptor
   The membrane’s oily middle holds back a charged particle, however small.
   So the ion stays outside and binds a cell-surface receptor.

Why: An ion carries a full charge.
The membrane’s oily middle holds back a charged particle, however small the ion is.
So this signal molecule binds a receptor at the cell surface.
Its receptor is a cell-surface receptor.

A hormone of the ovary is a steroid, built of four fused carbon rings.

Which kind of receptor does this signal molecule bind?

1. A. A cell-surface receptor
   A steroid is small and nonpolar, so it dissolves into the membrane’s oily middle and crosses into the cell.
   This hormone binds an intracellular receptor.
2. B. ✓ An intracellular receptor

Why: A steroid is small and nonpolar.
A small nonpolar molecule dissolves into the membrane’s oily middle and crosses into the cell.
So this hormone binds a receptor inside the cell.
Its receptor is an intracellular receptor.

A signal molecule is small and polar, like a sugar.

Which kind of receptor does this signal molecule bind?

1. A. ✓ A cell-surface receptor
2. B. An intracellular receptor
   The membrane’s oily middle holds a polar molecule back, even a small one.
   So this signal molecule stays outside and binds a cell-surface receptor.

Why: This signal molecule is polar.
The membrane’s oily middle holds a polar molecule back, even a small one.
So this signal molecule binds a receptor at the cell surface.
Its receptor is a cell-surface receptor.

A signal molecule made by the cells lining a blood vessel is a gas of two atoms, small and nonpolar.

Which kind of receptor does this signal molecule bind?

1. A. A cell-surface receptor
   A small nonpolar molecule dissolves into the membrane’s oily middle and crosses into the cell.
   So this gas binds an intracellular receptor.
2. B. ✓ An intracellular receptor

Why: This gas is small and nonpolar.
A small nonpolar molecule dissolves into the membrane’s oily middle and crosses into the cell.
So this signal molecule binds a receptor inside the cell.
Its receptor is an intracellular receptor.

Signal molecule Z is a chain of 14 amino acids released into the blood by gut cells.

Predict where the receptor for Z sits on its target cells.

1. A. In the nucleus, attached to the DNA
   A chain of amino acids is large and polar, so it cannot cross the membrane.
   A receptor at the cell surface binds it.
2. B. In the cytosol, near the nucleus
   The oily middle of the membrane holds a chain of amino acids back.
3. C. In the membrane, with the binding site facing the cytosol
   The ligand stays outside, so the binding site must face outward to bind it.
4. D. ✓ In the membrane, with the binding site facing outward

Why: A chain of 14 amino acids is a peptide, large and polar, so it cannot cross the membrane.
Its receptor is a cell-surface receptor with the binding site facing outward.

A researcher gives cells a small nonpolar hormone, and the cells respond.

Predict where the hormone molecules are found ten minutes later.

1. A. Outside the cell, bound to a receptor in the membrane
   A small nonpolar molecule crosses the oily middle and binds a receptor inside the cell.
2. B. ✓ Inside the cell, bound to their receptor in the nucleus
3. C. Outside the cell, free in the fluid
   The hormone is not released after binding.
   Within ten minutes the bound pair has moved into the nucleus, and the hormone is still bound.
4. D. Inside the cell, free in the cytosol, acting on the DNA alone
   A hormone acts only through its receptor; the pair attaches to the DNA together.

Why: A small nonpolar hormone crosses the membrane, binds its intracellular receptor, and the bound pair moves into the nucleus.
Ten minutes later the hormone molecules are there, bound to the receptor.

Every cell of your body carries DNA.

Where does DNA hold its information?

1. A. ✓ In the order of its bases
2. B. In the length of its strands
   A longer strand holds more bases; the information is in which base comes where.
3. C. In the number of its strands
   DNA has two strands in every cell; the information is in the order of the bases along them.

Why: DNA is a chain of bases.
The information is held in the order of those bases along the chain.

Keratin is the protein hair is built from. Cells at the base of a hair make keratin.

Which of the following is the gene for keratin?

1. A. ✓ The stretch of DNA that carries the instructions for making keratin
2. B. The keratin protein itself, folded and built into the hair
   Keratin is the protein the gene’s instructions make; the gene is the instructions.
3. C. The ribosome that makes keratin from a copy of the instructions
   A ribosome makes the protein from a copy of the instructions; the gene is the instructions.

Why: A gene is a stretch of a cell’s DNA that carries the instructions for making one protein.
The stretch of DNA that carries the instructions for making keratin is the keratin gene.

What is a gene?

1. A. A protein a cell has made from a copy of its instructions
   A protein is what the gene’s instructions make.
2. B. ✓ A stretch of a cell’s DNA that carries the instructions for making one protein
3. C. The whole of a cell’s DNA, every stretch of it together
   A cell’s DNA carries many genes; one gene is one stretch of it.

Why: A gene is one stretch of a cell’s DNA.
That stretch carries the instructions for making one protein.

A stretch of a liver cell’s DNA carries the instructions for making the enzyme catalase.

Is this stretch of DNA a gene?

1. A. ✓ Yes
2. B. No
   This stretch of DNA carries the instructions for making one protein, catalase.

Why: This stretch of DNA carries the instructions for making one protein, catalase.
So this stretch of DNA is a gene: the catalase gene.

A catalase molecule, a folded protein, sits in the cytosol of a liver cell.

Is this catalase molecule a gene?

1. A. Yes
   A gene is a stretch of DNA; catalase is the protein made from the catalase gene’s instructions.
2. B. ✓ No

Why: A gene is a stretch of a cell’s DNA.
A catalase molecule is a protein, not DNA.
So the catalase molecule is not a gene.

A ribosome in a liver cell is making a catalase molecule.

Is this ribosome a gene?

1. A. Yes
   A gene is a stretch of DNA; a ribosome makes the protein from a copy of the gene’s instructions.
2. B. ✓ No

Why: A gene is a stretch of a cell’s DNA.
A ribosome is the structure that makes the protein.
So the ribosome is not a gene.

After a signal arrives, a skin cell makes four times as much of a protective protein as before.

Which of the following changed in the skin cell?

1. A. The cell gained a new gene for the protein
   The cell already had the gene; the cell changed how fast it used the gene.
2. B. The signal molecule became the protein
   The cell built the protein from the gene’s instructions; the signal only changed how fast.
3. C. The order of bases in the gene changed
   The order of its bases stayed the same; the cell sped up making the protein from it.
4. D. ✓ The cell changed the expression of the gene for the protein

Why: The cell already carried the gene and sped up making the protein from it.
Speeding up the making of a protein is a change in gene expression.

A pancreas cell makes the enzyme amylase. A skin cell from the same body makes no amylase at all.

Which statement explains the missing amylase?

1. A. The skin cell’s copy of the gene has a different order of bases
   The skin cell carries the same gene as the pancreas cell; it is not using it.
2. B. The skin cell makes amylase and then breaks it down
   The skin cell never makes amylase: the gene is present and not expressed.
3. C. ✓ The skin cell has the amylase gene and does not express it

Why: Every cell of the body carries the same genes.
A cell carries a protein only if it expresses that protein’s gene.
The skin cell has the amylase gene and does not express it.

What is gene expression?

1. A. A cell copying its DNA before it divides
   Copying DNA is not making a protein from a gene.
2. B. A cell carrying a gene in its DNA without using it
   Every cell carries the gene; expressing it is using its instructions to make the protein.
3. C. ✓ A cell using a gene’s instructions to make that gene’s protein

Why: Gene expression is a cell using a gene’s instructions to make that gene’s protein.

A stomach cell makes the enzyme pepsin.

Is the stomach cell expressing the pepsin gene?

1. A. ✓ Yes
2. B. No
   The stomach cell is making pepsin, so it is using the pepsin gene’s instructions.

Why: The stomach cell is making pepsin.
So the stomach cell is using the pepsin gene’s instructions.
The stomach cell is expressing the pepsin gene.

A muscle cell carries the pepsin gene and makes no pepsin.

Is the muscle cell expressing the pepsin gene?

1. A. Yes
   Carrying a gene is not expressing it; the muscle cell is not making pepsin.
2. B. ✓ No

Why: The muscle cell carries the pepsin gene.
The muscle cell is not making pepsin.
So the muscle cell is not expressing the pepsin gene.

After a signal arrives, a bone cell makes twice as much of one protein as before.

Has the bone cell changed its gene expression?

1. A. ✓ Yes
2. B. No
   The bone cell sped up the making of a protein; speeding up is a change in gene expression.

Why: The bone cell sped up the making of one protein.
A cell changes gene expression when it starts, stops, speeds or slows the making of a protein.
So the bone cell has changed its gene expression.

Cortisol is a steroid: small and nonpolar.

Which kind of receptor does cortisol bind?

1. A. A cell-surface receptor
   A small nonpolar molecule slips through the membrane’s oily middle and binds a receptor inside the cell.
2. B. ✓ An intracellular receptor

Why: Cortisol is small and nonpolar.
So cortisol slips through the membrane into the cell.
Its receptor is an intracellular receptor.

A steroid hormone has entered a cell and bound its intracellular receptor.

Which of the following describes how the bound receptor changes what the cell makes?

1. A. The bound receptor switches on a relay of proteins that carry the message to the nucleus
   An intracellular receptor needs no relay.
   The receptor itself is the molecule that reaches the DNA.
2. B. The hormone leaves the receptor and attaches to the DNA on its own
   The hormone acts only through its receptor.
   The bound pair goes to the DNA together.
3. C. ✓ The bound receptor itself moves into the nucleus and attaches to the DNA
4. D. The bound receptor moves to the cell surface and lets more hormone in
   The bound pair moves inward, into the nucleus, where the genes are.

Why: An intracellular receptor needs no relay.
With the hormone bound, the receptor itself moves into the nucleus.
In the nucleus the bound receptor attaches to the DNA.
Attached to the DNA, the bound receptor changes the expression of particular genes.

A cell line carries a cortisol receptor that binds cortisol normally but cannot move into the nucleus. A researcher gives the cells cortisol and measures the output of a cortisol-controlled protein after an hour.

Predict the protein output after the hour.

1. A. More of the protein than before cortisol
   The receptor changes gene expression only by attaching to the DNA in the nucleus.
   This receptor cannot enter the nucleus, so the bound pair never reaches the DNA.
2. B. ✓ The same amount of the protein as before cortisol
3. C. Less of the protein than before cortisol
   A bound pair that cannot enter the nucleus never reaches the DNA.
   So nothing changes the gene’s expression, up or down.

Why: Cortisol enters the cells and binds the receptor.
The receptor changes gene expression only by entering the nucleus and attaching to the DNA.
This receptor cannot enter the nucleus.
So the bound pair never reaches the DNA, and the output stays at its level before cortisol.

A researcher holds every relay protein in muscle cells switched off with a drug, then gives the cells testosterone. Ten minutes later the receptor, with testosterone bound, sits in the nucleus attached to the DNA. Over the following hours the cells make more of a testosterone-controlled protein, as untreated cells do.

What does this result show about how the message reaches the DNA?

1. A. A relay of proteins carried the message before the drug had time to switch them off
   Every relay protein was already switched off when testosterone arrived, yet the message still reached the DNA and the protein output rose.
   So no relay protein carried the message.
2. B. ✓ The bound receptor itself carried the message to the DNA
3. C. Testosterone reached the DNA on its own, and the receptor followed it into the nucleus
   A hormone acts only through its receptor.
   The receptor was found in the nucleus with testosterone bound, so the bound pair is what attached to the DNA.
4. D. The receptor released a small molecule, and that small molecule carried the message to the DNA
   The receptor itself was found in the nucleus, attached to the DNA.
   With the hormone bound, the receptor is the molecule that reaches the DNA: no messenger, no relay.

Why: Every relay protein was held off, yet the bound receptor reached the DNA and the protein output rose.
So no relay was needed.
Testosterone bound its receptor, and the bound pair moved into the nucleus and attached to the DNA.
The receptor itself carries the message to the DNA.

After weeks of training, a muscle cell makes more of one contractile protein than before.

Which of the following changed in the muscle cell?

1. A. ✓ The expression of the protein’s gene
2. B. The order of bases in the protein’s gene
   The gene’s bases stayed in the same order; the cell made the protein from it faster.
3. C. The number of copies of the protein’s gene
   The cell carries the same copy of the gene as before; the cell sped up making the protein from it.

Why: The muscle cell already carried the gene for the contractile protein.
The muscle cell sped up making the protein from that gene.
Speeding up the making of a protein is a change in gene expression.

Estrogen is a steroid hormone. A researcher gives estrogen to cells of the womb lining.

Predict where the estrogen receptor is found ten minutes later.

1. A. In the plasma membrane, with its binding site facing outward
   A steroid is small and nonpolar, so it crosses the membrane; its receptor waits inside the cell.
2. B. In the cytosol, with no estrogen bound
   Estrogen binds its receptor inside the cell.
   Ten minutes later the bound pair is in the nucleus, on the DNA.
3. C. ✓ In the nucleus, with estrogen bound, attached to the DNA

Why: Estrogen is a steroid, small and nonpolar, so it crosses the membrane and binds its intracellular receptor.
The bound pair sits in the nucleus, attached to the DNA.
Ten minutes later the receptor is there, with estrogen bound.

Epinephrine makes a heart cell contract harder within seconds. Cortisol changes a liver cell’s output of a protein over an hour or more.

Why does the cortisol response take so much longer?

1. A. Cortisol has further to travel in the blood than epinephrine
   Both hormones ride the blood.
   The difference is what the cell does after the hormone arrives.
2. B. Cortisol is a larger molecule and crosses the membrane slowly
   Cortisol is small and nonpolar and crosses the membrane quickly.
   The slow part comes after.
3. C. ✓ Cortisol changes gene expression, and making new protein takes time
4. D. The liver cell carries fewer receptors than the heart cell does
   The delay is in the response.
   Building new protein from a gene takes far longer than making a cell contract.

Why: Epinephrine’s response is a quick change in what the cell is already doing.
Cortisol’s response is a change in gene expression.
The cell makes more or less of particular proteins, and building them takes an hour or more.

A protein inside a cell is phosphorylated.

What happens to the protein?

1. A. ✓ It gains a phosphate group from ATP and changes shape
2. B. It loses a phosphate group and is broken down
   Phosphorylation adds a phosphate group to the protein.
   Nothing is removed, and the protein is not broken down.

Why: In phosphorylation, ATP’s outer phosphate moves onto the protein.
The protein then changes shape.

Relay protein R moves glucose transporters to a muscle cell’s surface only while R carries a phosphate. A chemical blocks the kinase that phosphorylates R. Then the signal arrives.

Predict what happens to the glucose transporters.

1. A. The transporters move to the surface as usual
   Only the kinase adds the phosphate that switches R on.
   The kinase is blocked, so R never gains a phosphate.
2. B. ✓ The transporters stay inside the cell
3. C. The transporters move to the surface faster than usual
   No kinase is adding phosphates, so R never gains one.
   R stays in its resting shape.

Why: R is switched on by a phosphate.
Only the kinase can add that phosphate.
The kinase is blocked.
So R never gains a phosphate, and R stays in its resting shape.
So the transporters stay inside the cell.

A relay protein in a liver cell gains a phosphate from a kinase. A student says: “Adding a phosphate always switches a relay protein on.”

Is the student correct?

1. A. ✓ No — for some relay proteins, removing the phosphate is what switches them on
2. B. Yes — a phosphate added by a kinase always switches the protein on
   Adding a phosphate changes a protein’s shape.
   For some relay proteins the phosphate-free shape is the active one.

Why: The phosphate changes the protein’s shape.
For some relay proteins that new shape is active.
For others the phosphate-free shape is active, so removing the phosphate switches them on.
Which way the switch works is read from what the cell does.

In a plant cell, relay protein S keeps a growth response switched on only while S carries no phosphate. A hormone normally switches on the kinase that phosphorylates S, and the growth stops. A chemical blocks that kinase. Then the hormone arrives.

Predict what happens to the growth response.

1. A. The growth response stops
   The hormone stops growth only through the kinase.
   The kinase is blocked, so S keeps its phosphate-free, active shape.
2. B. ✓ The growth response continues
3. C. The growth response stops, then restarts
   No kinase phosphorylates S at any point.
   S never leaves its active shape.

Why: S is active while S carries no phosphate.
The hormone works by switching on the kinase that phosphorylates S.
The kinase is blocked.
So S never gains a phosphate and stays active.
So the growth response continues.

A kinase acts on a relay protein.

What does the kinase transfer onto the protein?

1. A. ✓ A phosphate group
2. B. A molecule of ATP
   The ATP gives up only its outer phosphate group; the rest leaves as ADP.
3. C. An amino acid
   A kinase moves a phosphate group, not an amino acid.

Why: A kinase transfers a phosphate group from ATP onto a protein.

A kinase phosphorylates a relay protein.

Where does the phosphate group come from?

1. A. From the receptor
   The receptor gives up no phosphate.
2. B. ✓ From ATP
3. C. From the relay protein itself
   The relay protein gains the phosphate; the phosphate comes from elsewhere.

Why: A kinase takes the phosphate group from ATP and transfers it onto the protein.

A kinase has just put a phosphate group onto a relay protein.

What happens to the relay protein’s shape?

1. A. ✓ The shape changes
2. B. The shape stays the same
   The added phosphate changes the protein’s shape.

Why: The added phosphate changes the protein’s shape.
The new shape is what switches the protein’s activity.

A kinase takes a phosphate group from a molecule of ATP.

What is left of the ATP?

1. A. Nothing
   Only one phosphate group leaves the ATP.
2. B. ✓ ADP
3. C. A second ATP
   No new ATP is made here.

Why: ATP loses its outer phosphate group.
ADP is left.

Relay protein T is active only while T carries a phosphate. A kinase phosphorylates T.

Is T now switched on or off?

1. A. ✓ Switched on
2. B. Switched off
   T is active while T carries a phosphate, and T now carries one.

Why: T is active while T carries a phosphate.
The kinase put a phosphate onto T.
So T is switched on.

Relay protein U is active only while U carries no phosphate. A kinase phosphorylates U.

Is U now switched on or off?