AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/4/4-4)
Unit 4 · Topic 4.4
4.4 Feedback
Feedback is how organisms respond to changes inside and outside their bodies. Negative feedback pushes a variable back toward its set point, keeping conditions stable (homeostasis), as insulin and glucagon do for blood sugar. Positive feedback pushes a change even further, amplifying a process until it's complete, as in childbirth, lactation and fruit ripening.
Key terms
- homeostasis
- set point
- negative feedback
- positive feedback
- insulin
- glucagon
Homeostasis and set points
Homeostasis is keeping internal conditions, such as body temperature, blood sugar and water balance, within a narrow, stable range. The target value is the set point. Conditions are always being disturbed, by a meal, a hot day or exercise, so organisms need ways to detect changes and respond.
A feedback loop usually has a sensor that detects a change (the stimulus), a control center that compares it to the set point, and an effector that responds. What makes it feedback is that the response changes the original stimulus.
Negative feedback
In negative feedback, the response reduces the original stimulus, bringing the variable back toward the set point. Most homeostasis works this way. It works at every level: molecules (feedback inhibition of an enzyme pathway, 3.2), cells and whole organisms.
- Blood sugar rises after a meal → the pancreas releases insulin → cells take up glucose, and the liver stores it as glycogen → blood sugar falls back toward the set point → insulin release slows.
- Blood sugar falls between meals → the pancreas releases glucagon → the liver breaks down glycogen and releases glucose → blood sugar rises back toward the set point → glucagon release slows.
- Insulin and glucagon are antagonistic hormones: they have opposite effects, and together they keep blood glucose in a narrow range.
Positive feedback
In positive feedback, the response strengthens the original stimulus, pushing the variable further from where it started. Each round of the response triggers more of the same response. Positive feedback isn't used to keep things steady. It's used to drive a process quickly to completion, and then something outside the loop ends it.
- Childbirth: the baby's head pushes on the cervix → this triggers release of the hormone oxytocin → oxytocin causes stronger uterine contractions → the head pushes harder → more oxytocin. The loop ends when the baby is born and the pressure stops.
- Lactation: a baby suckling stimulates release of hormones that cause milk production and release → milk flows, and the baby keeps suckling → more hormone release. The loop continues as long as the baby nurses.
- Fruit ripening: ripening fruit gives off the gas ethylene → ethylene speeds up ripening in that fruit and its neighbors → they give off even more ethylene. That's why one ripe banana can speed the ripening of a whole bunch.
When feedback fails
Disrupting a feedback loop can cause disease. In type 1 diabetes, the immune system destroys the pancreatic cells that make insulin, so blood glucose stays high after meals. In type 2 diabetes, cells respond weakly to insulin, so glucose isn't taken up well even when insulin is present. In both cases, the negative feedback loop can't bring blood sugar back to its set point.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Negative or positive?
Classify each as negative or positive feedback and justify: (a) When body temperature rises, sweating increases until temperature drops back to normal. (b) When a blood vessel is damaged, platelets stick to the site and release chemicals that attract even more platelets, until a plug forms.
Show the solutionHide the solution
- Step 1: Ask the key question: does the response reduce the original change, or increase it?
- Step 2: (a) The change is a temperature rise. The response (sweating, which cools by evaporation, 1.1) lowers the temperature. It reduces the change, so it's negative feedback.
- Step 3: (b) The change is platelets gathering. The response (chemicals released) brings more platelets. It amplifies the change, so it's positive feedback.
- Step 4: Note that (b) ends with a completed product, the plug, which is typical of positive feedback.
Answer: (a) Negative feedback: the response reverses the temperature change. (b) Positive feedback: the response amplifies platelet gathering until the plug forms.
- Example 2
Reading a blood glucose graph
Two people drink a glucose solution at time 0. In person A, blood glucose rises from 90 to 140 mg/dL by 45 minutes and returns to about 90 mg/dL by 2 hours. In person B, blood glucose rises from 90 to 250 mg/dL and is still about 230 mg/dL at 2 hours. Person B's blood insulin stays very low throughout. Explain both results.
Show the solutionHide the solution
- Step 1: Person A: glucose rises after the drink (the stimulus). The pancreas releases insulin, cells take up glucose and the liver stores it. Glucose returns to the set point: normal negative feedback.
- Step 2: Person B: glucose rises much higher and stays high, so the loop isn't bringing it back.
- Step 3: Person B's insulin stays low, so the problem is that insulin isn't being made or released, not that cells ignore it.
- Step 4: That pattern fits a failure to produce insulin, as in type 1 diabetes. Without insulin, cells don't take up glucose well, and the liver doesn't store it as glycogen.
Answer: A shows normal negative feedback via insulin. B can't bring glucose back down because little insulin is released, consistent with type 1 diabetes.
Common mistakes
- Thinking 'negative' means bad and 'positive' means good. Negative feedback reduces the stimulus; positive feedback amplifies it.
- Saying positive feedback maintains homeostasis. It moves a variable away from its starting point; it's used to complete processes.
- Mixing up insulin and glucagon. Insulin lowers blood glucose; glucagon raises it.
- Leaving out what the response does to the stimulus. That's the part that decides whether it's negative or positive feedback.
On the exam
- Expect scenarios where you classify a loop and justify it by saying whether the response reduces or amplifies the original stimulus.
- Graphs of blood glucose, hormone levels or temperature over time are common. Describe the trend, then explain it with the feedback loop.
Connected topics
Videos
Check yourself
4 questions on 4.4 Feedback. Pick an answer to see if you got it, and why.
| Time after meal (min) | Blood glucose (mg/dL) | Blood insulin (µU/mL) |
|---|---|---|
| 0 | 85 | 8 |
| 30 | 150 | 60 |
| 60 | 130 | 55 |
| 90 | 105 | 30 |
| 120 | 90 | 15 |
| 180 | 85 | 9 |
Experimental data: mean values for 12 healthy adults after eating a meal containing carbohydrates
Which of the following best describes the relationship shown in the data?
A person whose pancreas cannot make insulin eats the same meal. Which result is most likely?
Several hours after the meal, the person has not eaten and blood glucose begins to fall below the set point. Which response would most likely restore it?
Which of the following is an example of positive feedback?
0 of 4 answered