Feedback

After lunch, glucose goes 85 → 140 and insulin 9 → 62, both returning by 15:00; glucagon dips from 90 to 70 and comes back.

• Correct: Insulin rises and falls with glucose, while glucagon falls and then recovers.: Insulin tracks glucose, and glucagon moves the opposite way, as the meal peaks show.
• Insulin and glucagon both rise with glucose and both fall as glucose falls back.: Glucagon falls after meals; only insulin rises.
• Glucagon rises with glucose, while insulin stays near its fasting level through the day.: This reverses the two hormones.
• Insulin rises an hour before glucose rises, in step with the time of each meal.: Insulin rises with glucose after each meal; at 7:00, 12:00 and 18:00 it is still at its fasting level.

Glucose falls from 90 to 72 during the run; glucagon rises to 118 and insulin falls to 6; by 17:00 glucose is back to 84, before dinner at 18:00.

• Correct: Running muscles took up glucose; the fall triggered glucagon release and less insulin, so the liver released glucose.: Glucose use lowers glucose; the low level is the stimulus for glucagon, whose response restores it.
• Running used up the insulin in the blood, so glucose fell; glucagon then rose to take the place of the missing insulin.: Insulin falls during the run because glucose falls; glucagon is the opposing hormone, not a replacement.
• The run raised insulin, which lowered glucose; glucagon rose afterward because glucose had become too high.: Insulin falls during the run (15 → 6), it does not rise; and glucose was low, not high, when glucagon rose.
• Glucagon rose first and caused glucose to fall, and glucose recovered at 17:00 because the next meal began.: Glucagon raises glucose; and the recovery at 17:00 comes before the 18:00 meal.

Negative feedback means the response opposes the change and brings the variable back: glucose returns to its set point after meals (insulin up) and after the run (glucagon up).

• Correct: After each rise or fall, glucose returns to about 85-90 mg/dL as the opposing hormone acts.: A return to the set point, driven by a hormone that opposes the change, is what defines negative feedback.
• Glucose reaches about the same peak, 138-142 mg/dL, after each of the three meals of the day.: Similar peaks show similar meals; they do not show what brings glucose back down.
• Insulin is higher at 19:00 than at any earlier time of day.: A single high value does not show a response opposing a change.
• Glucagon is about 90 pg/mL at 6:00 and about 89 pg/mL at 22:00.: Two matching values at the start and end do not show how glucose was corrected during the day.

B's problem is the missing signal, so supplying it works well. C already has plenty of insulin; the problem is the response, so extra insulin helps less.

• Correct: B's glucose falls a lot, and C's falls less, because C's cells respond weakly to insulin.: B lacks insulin but responds to it; C has insulin resistance.
• C's glucose falls a lot, and B's falls less, because B's cells have lost their insulin receptors.: B's beta cells are destroyed, not the receptors on target cells.
• Neither glucose level falls, because diabetes damages the liver rather than the hormone system.: Both types are failures of the insulin loop: a missing signal or a weak response.
• Both fall by the same amount, because injected insulin works the same way in each person.: C's high insulin with high glucose shows C's cells respond weakly, so the same dose does less.

A slow start followed by an accelerating rise is the shape of positive feedback: the product (ethylene) triggers more of its own production, until ripening is complete and production falls.

• Correct: Ethylene binding the fruit's receptors makes the fruit produce more ethylene, so production speeds itself up.: Each increase in ethylene causes a bigger increase: positive feedback.
• The bananas lose water as they age, so the ethylene in them becomes more concentrated each day.: Ethylene is measured as gas given off per hour, not as a concentration in the fruit, and water loss could not explain a 20-fold rise in three days.
• Ethylene production is set by the day of harvest, so each banana starts producing heavily on day 4.: Group 2 bananas, from the same harvest, start on day 1, so timing is not fixed by harvest date.
• Ethylene breaks down cell walls, and the broken walls release ethylene that had been stored in them.: Ethylene is made as the fruit ripens, not stored in the walls; and 1-MCP, which acts on receptors, stops the rise.