In January 1922, doctors in Toronto injected a teenage boy named Leonard Thompson with an extract of pancreas. He had type 1 diabetes, and his blood glucose was dangerously high because his body had lost one half of a feedback loop. The first, impure batch lowered his blood glucose only slightly and caused an abscess. A purified extract, given from 23 January, put back the missing signal, and his blood glucose came down. That loop, and loops like it, are what this topic of Unit 4 is about: how organisms use positive and negative feedback to maintain homeostasis.
Homeostasis and set points
Homeostasis means keeping internal conditions, such as body temperature, blood glucose and water balance, within a narrow range. AP Biology calls it dynamic homeostasis because nothing stays perfectly still. A variable drifts, gets corrected, and drifts again around a set point, the target value. Your core temperature sits close to 37 °C, but it rises and falls a little through the day.
The parts of a feedback loop
Most loops you'll meet have the same parts:
- A stimulus: a change, such as blood glucose rising after lunch.
- A sensor, or receptor, that detects it.
- A control center that compares the reading with the set point. For temperature, that's the hypothalamus in the brain.
- An effector, a muscle, gland or group of cells that makes the response.
This is where Unit 4 connects. The messages between these parts are often hormones, and every hormone works through cell signaling: reception, transduction, response.
Negative feedback: steering back
In negative feedback, the response reduces the original stimulus, so the variable returns toward its set point. This happens at the molecular, cellular and organism level.
- Blood glucose. When glucose rises, beta cells in the pancreas release insulin. Liver, muscle and fat cells take up glucose, and the liver stores it as glycogen. When glucose falls, alpha cells release glucagon, and the liver breaks glycogen down and releases glucose. Insulin and glucagon are antagonistic hormones.
- Body temperature. Too hot: you sweat, and blood vessels near the skin widen. Too cold: you shiver, and those vessels narrow to keep heat in your core.
- Molecular level. In feedback inhibition, the end product of a metabolic pathway slows an enzyme early in that pathway (it builds on enzyme regulation from Unit 3).
Positive feedback: pushing further
In positive feedback, the response intensifies the stimulus, moving the variable further from where it started. It's not for staying steady; it's for finishing a job quickly. An endpoint shuts it off.
- Labor. The baby pressing on the cervix triggers oxytocin release, which strengthens contractions, which push harder on the cervix. Birth ends the loop.
- Lactation. Suckling triggers prolactin (milk production) and oxytocin (milk release). The loop keeps milk coming as long as the baby keeps feeding.
- Fruit ripening. Ethylene triggers ripening, and ripening fruit makes more ethylene. That's why a ripe banana in a paper bag speeds up the fruit sharing the bag.
- Blood clotting. Platelets at a wound release chemicals that attract more platelets until the break is sealed.
When a loop breaks
The AP course asks you to predict what happens when one part of a system is disrupted. Take each part in turn. No sensor or no signal means no response. In type 1 diabetes, the immune system destroys the beta cells, so there's no insulin. No working receptor on the target cell has a similar effect. In type 2 diabetes, cells respond to insulin less, and the pancreas may not make enough. Either way, glucose stays high. This is the simple version: the full disease has several causes.
Common mistakes students make
- Reading "negative" as "bad". Negative feedback is what keeps you stable. The word describes the direction of the response, not its value.
- Calling a single response a loop. It's feedback only if the response changes the stimulus that started it.
- Mixing up the control center and the effector. The hypothalamus decides; sweat glands and muscles act.
- Saying insulin "turns glucose into glycogen in the blood". Glucose is taken into cells, and the conversion happens inside liver and muscle cells.
Frequently asked questions
What is the difference between positive and negative feedback?
Negative feedback reduces a change and returns a variable to its set point. Positive feedback amplifies a change until an endpoint, such as birth, stops it.
What are examples of positive feedback in AP Biology?
Labor contractions driven by oxytocin, milk release during lactation, fruit ripening driven by ethylene, and blood clotting.
Is blood sugar regulation positive or negative feedback?
Negative. Insulin lowers high blood glucose and glucagon raises low blood glucose, so both push it back toward the set point.
Why is positive feedback not used to maintain homeostasis?
Because it moves a variable further away from its starting point. It suits processes that need to run to completion, then stop.
Try it
Name the sensor, control center and effector for each example as you flip the cards. Then predict: what would you expect to see if a person's pancreas stopped making glucagon and they skipped a meal?
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