4.1 Cell Communication

1. AA T cell touches the presenting cell, and the touch alone activates it, whatever the cell displays.
   A student who thinks the touch itself is the signal picks this. Contact brings the molecules together, but the T cell is activated only if its receptor recognizes a displayed fragment; touching a cell that displays nothing it recognizes does not activate it.
2. BThe fragments enter the blood, which carries them to T cells waiting throughout the body.
   A student who thinks every chemical signal travels in the blood picks this. The fragments are held on molecules in the presenting cell's membrane; they are not released into the blood, so the T cell must be in contact with the presenting cell.
3. CThe fragments pass into the T cell's cytoplasm, and once inside the T cell they activate it.
   A student who thinks a signal must enter its target cell picks this. The T cell detects the fragment with a receptor on its own surface while the cells touch; the fragment does not have to enter the T cell.
4. DA T cell touches the presenting cell, and a receptor on the T cell binds a displayed fragment. Correct
   The fragments stay on molecules in the presenting cell's membrane, so they can be detected only by a cell in contact with it. When a helper T cell touches the presenting cell and one of its receptors recognizes a displayed fragment, the T cell is activated: communication by direct contact.

CED 4.1.A.1 · Read this in Fix

1. AIt diffuses across the narrow gap between the neuron and the muscle cell next to it. Correct
   Acetylcholine is a local regulator. Released into the narrow gap of the synapse, it diffuses the short distance to the adjacent muscle cell and binds receptors there.
2. BIt is carried in the blood from the neuron to muscle cells all over the body.
   A student who thinks every chemical signal travels in the blood picks this. A neurotransmitter acts on the cell across the synapse; it is a local regulator, not a hormone.
3. CIt crosses the gap as an electric current that jumps from the neuron into the muscle cell.
   A student who thinks a nerve impulse jumps a synapse as electricity picks this. At this chemical synapse the electrical impulse causes release of acetylcholine, and the chemical carries the signal across the gap.
4. DIt moves toward the muscle cell, because the muscle cell needs it in order to contract.
   A student who thinks molecules move toward where they are needed picks this. Acetylcholine diffuses down its concentration gradient by random motion; the muscle cell's needs do not attract it.

CED 4.1.B.1 · Read this in Fix

1. AThe gland sends the hormone straight to its target cells, so other cell types do not receive it.
   A student who thinks hormones are sent directly to their targets picks this. The hormone enters the blood and reaches nearly every cell; the difference lies in whether a cell can recognize it.
2. BThe cell types that respond let the hormone in, and a hormone acts on a cell only from inside it.
   A student who thinks a signal must enter a cell to act picks this. Many hormones act by binding receptors at the cell surface without entering; what matters is whether the cell has a receptor for the hormone.
3. CThe cell types that respond have receptors that recognize the hormone; other cell types lack them. Correct
   A hormone in the blood is carried throughout the body, but a cell responds only if it has a receptor that recognizes that hormone. Cells with such receptors are its target cells.
4. DCells close to the gland receive enough hormone to respond, but cells far away get too little.
   A student who thinks every chemical signal acts mainly near where it is released picks this. The blood carries the hormone throughout the body, so target cells respond whether they are near the gland or far from it.

CED 4.1.B.2 · Read this in Fix

1. AThe signal molecules stay in the membrane of S, and Y does not touch S. Correct
   In the model the signal molecules are part of the membrane of S and are not released, so only a cell touching S, such as W, can detect them with its receptors. Y has fitting receptors but does not touch S, so the signal never reaches it.
2. BTouching S is itself the signal, and Y is not in contact with S.
   A student who thinks touching is itself the signal picks this. Y does not touch S, but contact alone is not the signal: X touches S and cannot respond, because its receptors do not fit the signal molecule.
3. CS releases its signal molecules, but too few of them spread as far as Y.
   A student who thinks every signal is released and spreads to the cells that respond picks this. The model shows the signal molecules in the membrane of S; they are not released, so none spread toward Y at any distance.
4. DA signal must enter its target cell, and S's signal molecules cannot enter Y.
   A student who thinks a signal must enter its target cell to act picks this. W responds without the signal entering it, by detecting the signal molecule with a receptor at its surface; Y cannot respond because it does not touch S.

CED 4.1.A.1 · Read this in Fix

1. APut all of the Q cells in water from a P-cell culture with the P cells removed, and prepare no other dish.
   A student who thinks a control is unnecessary when an effect is expected picks this. Without Q cells in fresh water for comparison, a shape change could not be attributed to anything released by P cells; Q cells might change shape anyway.
2. BPut some Q cells in water that held P cells, now removed, and put the other Q cells in fresh water. Correct
   Water in which P cells have lived contains anything they released, but no P cells, so contact is impossible. Comparing these Q cells with Q cells in fresh water, which differ only in the absence of P-cell products, shows whether a released chemical alone causes the shape change.
3. CGrow P cells and Q cells together in one dish, and then record whether the Q cells change shape.
   A student who thinks producing the effect tests its cause picks this. With P and Q cells together, both contact and released chemicals are possible, so seeing the shape change cannot tell the two explanations apart.
4. DPut Q cells in warm water from a P-cell culture with the P cells removed, and others in cool fresh water.
   A student who thinks changing two factors at once still gives a fair test picks this. The groups differ in both P-cell products and temperature, so any difference in shape could be caused by either.

CED 4.1.A.1 · Read this in Fix

1. ABoth contact and a released chemical act, as every treatment with R cells or their liquid has a higher mean.
   A student who judges differences by the means alone picks this. The separated (13%) and R-cell liquid (8%) means are higher than 6%, but their error bars overlap those of T cells alone, so these differences may be due to chance.
2. BA released chemical is proven to play no part, as two treatments' bars overlap the bars of T cells alone.
   A student who thinks overlapping error bars prove no effect picks this. Overlap means the data do not show a significant effect of released chemicals; a small effect, such as the slightly higher mean of the separated cells, is not ruled out.
3. CDirect contact is supported, as only the bars for T cells mixed with R cells do not overlap the T-alone bars. Correct
   The mixed treatment (51% to 65%) is clear of T cells alone (3% to 9%), so contact with R cells very likely raised production. The bars for the separated treatment (5% to 21%) and the R-cell liquid (4% to 12%) overlap the T-alone bars, so neither shows a significant effect of released chemicals.
4. DNo conclusion can be drawn, as some T cells grown alone also made the protein, so the experiment failed.
   A student who thinks a control must show zero picks this. T cells alone show the baseline (about 6%); treatments are judged by comparison with it, and the mixed treatment is far above it.

Working Read each bar as mean ± 2 SE: T alone 6 (3 to 9); mixed with R 58 (51 to 65); separated from R 13 (5 to 21); R-cell liquid 8 (4 to 12). Compare each with T alone: mixed, 51 > 9, no overlap, so a likely real difference; separated, 5 < 9, overlap, no significant difference shown; R-cell liquid, 4 < 9, overlap, no significant difference shown. Only the treatment that allows contact differs, which supports signaling by direct contact.

CED 4.1.A.1 · Read this in Fix

1. ALight per cell rose in proportion to the signal across the whole range from 1 nM to 40 nM.
   A student who expects every response to be proportional to the signal picks this. A 10-fold rise in signal from 1 nM to 10 nM barely changes light per cell, a doubling from 10 nM to 20 nM multiplies it 30-fold, and doubling again to 40 nM adds only 3 units.
2. BThe cells began to release the signal only once the density reached 2.0 × 10⁸ cells/mL.
   A student who thinks bacteria start making the signal only when crowded picks this. The signal is present at every density, rising from 1 nM at 0.1 × 10⁸ cells/mL to 40 nM at 4.0 × 10⁸ cells/mL in step with density; only the light appears suddenly, at 2.0 × 10⁸ cells/mL.
3. CThe signal molecule gave off the light itself, as light rose wherever the signal rose.
   A student who thinks the signal itself carries out the response picks this. The signal rose 10-fold at low densities with almost no change in light, so the light does not track the signal; the cells produce the light in response to it.
4. DLight per cell rose sharply only once the signal passed a level between 10 nM and 20 nM. Correct
   Light per cell stays at 1 to 2 units while the signal rises from 1 nM to 10 nM, jumps to 60 units at 20 nM, and changes little at 40 nM (63 units). The response depends on a threshold concentration of the signal, as in quorum sensing.

CED 4.1.B.1 · Read this in Fix

1. ACells at every distance develop as type A, as all of them are exposed to the same morphogen.
   A student who thinks one signal always causes one kind of response picks this. The concentration falls below threshold 1 at about 80 µm, so cells farther away respond differently to the same morphogen.
2. BA cell moved early from 250 µm to 50 µm from the source still develops as type C, as its genes set its type.
   A student who thinks cell types differ because their genes differ picks this. In the model, a cell's type is set by the concentration it is exposed to: at 50 µm the morphogen is about 57, above threshold 1, so a cell moved there develops as type A.
3. CCells 150 µm from the source develop as type B, as the concentration there is between the thresholds. Correct
   At 150 µm the curve is at about 19, below threshold 1 (40) and above threshold 2 (12), so cells there develop as type B. One morphogen produces three cell types because cells respond to the concentration they receive, which falls with distance from the source.
4. DCells 50 µm from the source respond nearly twice as much as cells at 100 µm, as they get nearly twice the morphogen.
   A student who thinks a response is always proportional to the signal's concentration picks this. The morphogen is about 57 at 50 µm and 33 at 100 µm, but in the model cells do not respond in proportion: those at 50 µm (above threshold 1) develop as type A and those at 100 µm (between the thresholds) as type B.

CED 4.1.B.1 · Read this in Fix

1. ACells produce light when their culture is dense and produce no light when their culture is sparse.
   A student who takes any agreeing observation as support picks this. Both explanations predict light at high density, where cells are crowded and any released chemical is concentrated, so this observation cannot tell them apart.
2. BSparse cultures produce light when given fluid from a crowded culture filtered free of cells. Correct
   Cells in a sparse culture rarely touch one another, and the filtered fluid contains whatever the crowded cells released but no cells. If the sparse culture then produces light, a released chemical is enough to trigger the response, which the touching explanation does not predict.
3. CProducing light only at high density helps the cells, as light from just a few cells would be wasted.
   A student who treats a benefit as evidence of mechanism picks this. A possible advantage says nothing about how the cells detect density, by a chemical or by touch.
4. DA purified signal molecule from the cells gives off light when it is placed in liquid with no cells in it.
   A student who thinks the signal itself produces the response picks this. Even if a purified molecule glowed, that would not show that cells detect density through it; in quorum sensing the cells produce the light after detecting the signal.

CED 4.1.B.1 · Read this in Fix

1. AThe tail breaks down, as the blood carries H to the tail cells. Correct
   Hormones travel in the blood. Injected H is carried throughout the body, reaches the tail's target cells and causes them to break down, whether H came from the gland or from an injection into the blood.
2. BThe tissue near the injection site breaks down, but the tail is not affected.
   A student who thinks every chemical signal acts on cells near where it is released picks this. H is carried in the blood, so it reaches the tail, which is where its target cells are.
3. CNo tissue breaks down, as the gland is what delivers H to the tail.
   A student who thinks a gland sends its hormone directly to its targets picks this. The gland does not direct H; it releases H into the blood, and H injected into the blood travels the same way.
4. DThe tail breaks down only if nerves connect the injection site to the tail.
   A student who thinks long-distance communication needs nerves picks this. H is a hormone, and the blood carries it to the tail cells; no nerve pathway is needed.

CED 4.1.B.2 · Read this in Fix

1. APituitary cell → blood → cells all over the body → every cell reached responds → growth
   A student who thinks every cell a signal reaches responds to it picks this. Growth hormone reaches nearly every cell, but only target cells, which have receptors for it, respond.
2. BPituitary cell → one vessel to target tissue alone → target cells respond → growth
   A student who thinks a gland sends its hormone directly to its targets picks this. No vessel carries the hormone to its target tissues alone; it enters the general circulation.
3. CPituitary cell → nerve impulse down the spinal cord → target cells respond → growth
   A student who thinks long-distance signaling happens only through nerves picks this. Growth hormone is a hormone carried in the blood, not an impulse carried by neurons.
4. DPituitary cell → blood → cells all over the body → only cells with receptors respond → growth Correct
   Growth hormone is released into the blood and carried throughout the body, so it reaches many cell types. Only cells with receptors for it respond, and their responses lead to growth.

CED 4.1.B.2 · Read this in Fix

1. AGroup 1, considered on its own
   A student who takes any agreeing observation as support picks this. Darkening in intact fish is expected whether the gland acts through the blood, through nerves or not at all, so it cannot support this claim over others.
2. BGroup 3 compared with group 4 Correct
   Both groups lack the gland and differ only in the plasma they received. Plasma from fish with a gland darkens the skin; plasma from fish without one does not. So the blood carries something, dependent on the gland, that darkens the skin, as the claim states.
3. CGroup 1 contrasted with group 2
   A student who thinks showing a structure is needed shows how it acts picks this. Groups 1 and 2 show that the gland is needed for darkening, but not whether it acts through the blood or through nerves.
4. DGroup 3, with no comparison
   A student who thinks no comparison group is needed picks this. Without group 4, darkening in group 3 might be caused by the injection itself or by anything in plasma, not by a gland-dependent chemical.

CED 4.1.B.2 · Read this in Fix