AP BiologyUnit 4: Cell Communication and Cell Cycle4.4
AP Biology · Unit 4 · Topic 4.4

Feedback

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson3 min read

Ask one thing: does the response cancel its trigger or feed it? Cancels → negative. Feeds → positive.

In feedback, a process's output loops back and controls the process. Negative feedback cancels the change that started it and holds conditions steady. Positive feedback amplifies that change and races to an endpoint.

Key terms

  • Negative feedback — the response opposes the change that triggered it, pulling a variable back toward its set point.
  • Positive feedback — the response strengthens the change that triggered it. The process builds on itself until something outside the loop stops it.
  • Set point — the value a system defends, such as a core body temperature near 37 °C in humans.
  • Homeostasis — keeping internal conditions in a narrow range while the outside world changes. Negative feedback does most of this work.
  • Feedback inhibition — a pathway's end product inhibits an early enzyme, usually the first, so production slows once enough builds up.

How it works

  1. A variable drifts from its set point. At high altitude, for example, less oxygen reaches the kidneys.
  2. A sensor detects the change. Kidney cells sense the drop in oxygen and release the hormone erythropoietin (EPO).
  3. The hormone reaches an effector. EPO drives the bone marrow to make more red blood cells.
  4. The response removes its own trigger. More red cells → more oxygen delivered → less EPO released, even though the air is still thin.
  5. Positive feedback runs the other way. Activated platelets release chemicals that activate more platelets, so the clot builds faster and faster.
  6. A positive loop needs an outside stop. Clotting ends when the wound is sealed. Labor contractions end when the baby is delivered.

Why it matters

Negative loops work at every level. Inside a cell, feedback inhibition shuts down an early step, so the cell doesn't waste carbon and energy on intermediates it doesn't need. In a plant short of water, a root hormone closes the stomata, and its level falls once the soil is wet. When a loop breaks, the variable drifts out of range and stays there. In type 1 diabetes, the pancreas makes too little insulin, so blood glucose stays high.

Exam traps

TrapActually
Negative feedback is bad and positive feedback is good.The sign is about direction, not value. Clotting is positive feedback, and it keeps you from bleeding out.
A response that keeps getting stronger must be positive feedback.Check direction only. Sweating harder as a room heats up still opposes the rise in body temperature, so it's negative feedback.
Negative feedback always lowers the variable.It opposes the change, in either direction. Low blood glucose triggers glucagon release, and glucagon raises glucose. That's negative feedback too.
End products stop a pathway by blocking its last enzyme or competing with substrate.They usually bind the first enzyme, away from its active site. Extra substrate can't displace them, and intermediates don't build up.
Worked example

Two adults drink the same glucose solution after fasting. In person A, blood glucose rises from 85 to 150 mg/dL, insulin rises with it, and both are back near their starting levels within 2 hours. In person B, glucose starts at 130, rises to 240 mg/dL, and is still 220 at 2 hours, while insulin stays higher than person A's the whole time. Which part of person B's feedback loop is failing?

  1. Name the loop. Glucose rises → the pancreas releases insulin → body cells take up glucose and the liver stores it → glucose falls → insulin falls.
  2. Check the sensor and the signal. Person B's insulin climbs even higher than A's, so the pancreas detects the glucose and sends the message.
  3. Check the effector. Glucose stays high despite plenty of insulin, so the target cells are not responding to the signal.

The target cells' response to insulin. Person B makes the signal, but the cells take up too little glucose, as in type 2 diabetes.

In a working negative loop, the response cancels the stimulus and then the signal drops. In person B the stimulus doesn't clear, so insulin stays high. The near-miss is 'the pancreas makes too little insulin', which describes type 1 diabetes. That would show low insulin, not high.

Now try it on real exam-style questions

Ten questions on Feedback, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 4.4 Feedback

After a meal, insulin signals which organ to store excess glucose as glycogen?

  1. A The pancreas
  2. B The liver
  3. C The kidneys
  4. D The stomach
Show the answer

B — Insulin signals the liver (and muscles) to take up glucose and link it into glycogen, which lowers blood glucose. The pancreas releases insulin, but it is not where the extra glucose is stored.

Question 2 · 4.4 Feedback

What is the maintenance of a relatively stable internal environment in an organism called?

  1. A Metabolism
  2. B Homeostasis
  3. C Positive feedback
  4. D Adaptation
Show the answer

B — Homeostasis is the maintenance of stable internal conditions, such as body temperature and blood glucose, and it depends mostly on negative feedback. Metabolism is the sum of a cell's chemical reactions, which homeostasis regulates but which is a different idea.

Question 3 · 4.4 Feedback

What is a regulatory loop called when its response reverses the change that triggered it?

  1. A Positive feedback
  2. B Signal amplification
  3. C Negative feedback
  4. D Signal transduction
Show the answer

C — In negative feedback the response opposes the original change, so the variable returns toward its set point. Students often pick positive feedback because it sounds like the helpful kind, but positive feedback strengthens the original change instead of reversing it.