4.4 Feedback

1. AThey keep each internal condition at one exact value, which does not vary from moment to moment.
   A student who thinks homeostasis holds conditions exactly constant picks this. Regulated variables fluctuate around a set point, and these fluctuations are what trigger feedback responses.
2. BThey act at the level of the whole organism, but not between molecules or within cells.
   A student who thinks feedback is only a whole-body process picks this. Feedback also operates at the molecular and cellular levels, for example when the end product of a pathway inhibits an enzyme early in the pathway.
3. CThey respond to changes caused by the organism's own activity and to changes in its surroundings. Correct
   Feedback mechanisms respond to internal changes, such as heat produced by working muscles, and external changes, such as a fall in air temperature. In each case the change is detected and triggers a response.
4. DThey are called negative when they harm the organism and positive when they help it.
   A student who reads 'negative' and 'positive' as 'bad' and 'good' picks this. The words describe whether the response reduces or intensifies the stimulus; both kinds occur in healthy organisms.

CED 4.4.A.1 · Read this in Fix

1. AAs the amino acid builds up, it inhibits more of the first enzyme, slowing production until its level falls. Correct
   This is feedback inhibition, negative feedback at the molecular level. A rise in the end product inhibits more molecules of the first enzyme, slowing the pathway; as the amino acid is used and its concentration falls, fewer enzyme molecules are inhibited and production speeds up again.
2. BThe amino acid is made until the first enzyme is used up, and then production starts again once more enzyme is made.
   A student who thinks enzymes are used up in reactions picks this. Enzymes are not consumed; the amino acid slows the pathway by binding to the first enzyme and inhibiting it.
3. CAs the amino acid builds up, it denatures the first enzyme, and production restarts when new enzyme replaces it.
   A student who thinks inhibitors work by denaturing enzymes picks this. The amino acid binds to an allosteric site and inhibits the enzyme without unfolding it; when the amino acid leaves the site, the enzyme is active again.
4. DHormones from other organs hold the amino acid level steady, as feedback works only at the level of the whole body.
   A student who thinks feedback works only at the level of the whole organism picks this. Here the feedback is within the cells: the end product acts directly on an enzyme of its own pathway.

CED 4.4.A.1.i · Read this in Fix

1. AThe insulin loop is negative feedback, and the glucagon loop is positive feedback, as glucagon raises glucose.
   A student who thinks negative feedback always lowers a variable picks this. Glucagon raises glucose only after glucose has fallen, so its effect opposes the initial change: this is negative feedback.
2. BBoth loops are positive feedback, as each response benefits the person by keeping glucose in a healthy range.
   A student who reads 'positive' as 'beneficial' picks this. Positive feedback intensifies the initial change; in both loops the response opposes the change, which is negative feedback.
3. CTogether, the loops hold blood glucose at exactly one value, so its level does not change at all.
   A student who thinks homeostasis holds conditions exactly constant picks this. The model shows blood glucose rising after a meal and falling between meals; the loops return it toward the set point after each change.
4. DIn each loop, the response moves blood glucose back toward the set point, so both are negative feedback. Correct
   In the upper loop, a rise in glucose leads to insulin release and a fall in glucose; in the lower loop, a fall leads to glucagon release and a rise. Each response reduces the initial change and returns glucose toward the set point, so both loops are negative feedback.

CED 4.4.A.1.i · Read this in Fix

1. AGlucose stayed at about the same level throughout, as homeostasis prevented any rise.
   A student who thinks homeostasis holds conditions exactly constant picks this. Glucose rose by more than 60 mg/dL after the drink before it returned toward its starting level.
2. BInsulin rose as glucose rose, then fell as glucose returned to near its starting level. Correct
   Glucose rose from 88 to 150 mg/dL by 30 minutes, and insulin rose from 6 to 52 μU/mL over the same period. Both then fell, glucose to 86–90 mg/dL and insulin to 6–10 μU/mL by 120–180 minutes. Insulin is released in response to high glucose and falls as glucose returns toward the set point.
3. CInsulin was highest when glucose was lowest, as insulin acts to lower blood glucose.
   A student who expects insulin and glucose to move in opposite directions at every moment picks this. Insulin was highest at 30 minutes, when glucose was also highest; high glucose triggers insulin release.
4. DThe rise in insulin caused the rise in glucose, since both rose over the first 30 minutes.
   A student who treats a correlation as showing that one variable causes the other picks this. Glucose rose because of the drink; the rise in glucose triggered insulin release, which then lowered glucose.

CED 4.4.A.1.i · Read this in Fix

1. A45% Correct
   Percent decrease = (160 − 88)/160 × 100 = 45%. Negative feedback through insulin returned blood glucose most of the way to its starting value of 85 mg/dL.
2. B72%
   A student who reports the change in concentration, 160 − 88 = 72 mg/dL, as the percent change picks this. The change must be divided by the original value: 72/160 × 100 = 45%.
3. C82%
   A student who divides the change by the new value picks this: 72/88 × 100 = 82%. Percent change is calculated relative to the original value, 160 mg/dL.
4. D55%
   A student who gives the new value as a percentage of the original picks this: 88/160 × 100 = 55%. The concentration at 120 minutes is 55% of the value at 30 minutes, which is a decrease of 45%.

Working From the table, blood glucose is 160 mg/dL at 30 min and 88 mg/dL at 120 min. Change = 160 − 88 = 72 mg/dL. Percent decrease = 72/160 × 100 = 45%. Distractors: reporting the change, 72 mg/dL, as a percentage gives 72%; dividing the change by the new value gives 72/88 × 100 = 82%; giving the new value as a percentage of the original gives 88/160 × 100 = 55%.

CED 4.4.A.1.i · Read this in Fix

1. AIt returns to the set point as fast as normal, as the body adjusts to meet its own need.
   A student who thinks the body makes up for a missing part of a feedback mechanism because it needs to picks this. No other hormone takes over insulin's role of lowering blood glucose, so the rise is not corrected normally.
2. BIt rises and stays high longer than usual, as the response that lowers it is missing. Correct
   Insulin is the part of the negative feedback loop that lowers blood glucose after it rises. Without insulin, the rise after a meal is not corrected as it normally is, so blood glucose stays high for longer.
3. CIt stays at exactly the set point, as homeostasis keeps blood glucose at a fixed value.
   A student who thinks homeostasis holds conditions exactly constant picks this. Blood glucose rises after a meal even in a healthy person; without insulin, it rises further and stays high.
4. DIt falls below the set point, as glucagon, which lowers blood glucose, is still released.
   A student who thinks glucagon lowers blood glucose picks this. Glucagon raises blood glucose by causing the liver to release glucose, so it cannot lower glucose in the absence of insulin.

CED 4.4.A.1.i · Read this in Fix

1. AOnce started, it keeps on increasing, whether or not the infant continues to suckle.
   A student who thinks positive feedback continues without end picks this. The loop depends on suckling: without the stimulus, oxytocin release falls and milk release stops.
2. BIt is held at a fixed set point, as oxytocin keeps milk flow steady while the infant suckles.
   A student who thinks positive feedback keeps a variable near a set point picks this. Positive feedback moves the process further from its starting point, increasing milk release, rather than holding it at a set point.
3. CIt is amplified while suckling goes on, then ends shortly after the suckling has ceased. Correct
   Suckling triggers oxytocin release and milk release, and milk flow stimulates further suckling, so the loop amplifies milk release. When the infant stops suckling, the stimulus is removed, oxytocin release falls and the loop ends.
4. DIt is switched off when the mother's body decides that enough milk has been released for the infant.
   A student who explains regulation by what the body decides it needs picks this. Nothing in the loop measures how much milk has been released; release continues while the stimulus, suckling, continues, and it is the removal of the stimulus that ends the loop.

CED 4.4.A.1.ii · Read this in Fix

1. AIt is positive feedback because its outcome, the birth of a baby, benefits the organism.
   A student who reads 'positive' as 'beneficial' picks this. The loop is positive feedback because each response intensifies the stimulus, not because its outcome is good; negative feedback loops also benefit organisms.
2. BIt is positive feedback because it raises a hormone's level, which negative feedback cannot do.
   A student who thinks negative feedback can only lower a variable picks this. Negative feedback can raise a hormone's level too, as when glucagon release rises after blood glucose falls.
3. CEach response returns the cervix toward its original set point, so labor stays steady.
   A student who thinks positive feedback keeps a variable near its set point picks this. In the model, each response increases the pressure on the cervix, moving it further from its starting state.
4. DEach response intensifies the stimulus that caused it, so contractions grow stronger until the loop ends. Correct
   In the model, stronger contractions push the baby's head harder against the cervix, which increases the signals to the brain and the release of oxytocin, which strengthens contractions further. The response amplifies the stimulus until birth ends the loop: this is positive feedback.

CED 4.4.A.1.ii · Read this in Fix

1. AAn identical sealed bag kept at 20 °C that contains no fruit at all
   A student who thinks the control is a setup with nothing in it picks this. An empty bag contains no unripe fruit, so it cannot show how long unripe fruit takes to ripen without a ripe fruit nearby.
2. BThree unripe fruits sealed in an identical bag at 20 °C, with no ripe fruit Correct
   This setup differs from the experimental bag only in lacking the ripe fruit, the source of the ethylene being tested. If the unripe fruits in the experimental bag ripen sooner, the difference can be attributed to the ripe fruit.
3. CThree unripe fruits and one ripe fruit, sealed together in a second bag at 20 °C
   A student who thinks the control should receive the treatment picks this. This bag repeats the experimental setup, ripe fruit included, so it cannot show what happens without the ripe fruit.
4. DThe experimental bag itself, checked daily until its unripe fruits ripen
   A student who thinks a treated setup alone can show an effect picks this. Watching the experimental bag gives no unripe fruits kept without a ripe fruit, so the ripening time has nothing to be compared with.

CED 4.4.A.1.ii · Read this in Fix

1. AThey make less ethylene and ripen more slowly, as ethylene can no longer trigger more of its own production. Correct
   Ripening is amplified by positive feedback: ethylene, acting through its receptors, triggers the production of more ethylene. With the receptors blocked, ethylene cannot trigger a response, so the loop is broken; the fruits make less ethylene and ripen more slowly than untreated fruits.
2. BThey make more ethylene than usual, so as to make up for the receptors on their cells that have been blocked.
   A student who thinks the body makes up for a blocked part of a feedback mechanism because it needs to picks this. Increased ethylene production is itself a response to ethylene binding its receptors, so blocking the receptors reduces it.
3. CTheir ethylene production rises without limit, as positive feedback cannot be stopped once begun.
   A student who thinks positive feedback, once started, continues without end picks this. Blocking the receptors breaks the loop; it is the response to ethylene that amplifies its production.
4. DThey ripen normally, as ethylene can act on the fruit's cells without binding to the receptors.
   A student who thinks a hormone can act without binding its receptor picks this. Ethylene triggers ripening only by binding its receptors; with the receptors blocked, it has no effect.

CED 4.4.A.1.ii · Read this in Fix