Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Homeostasis keeps internal conditions near a set point. It is dynamic: the value moves and is pulled back.
- A loop has a stimulus, sensor, control centre, effector and response. It is feedback because the response acts on the stimulus.
- Negative feedback reduces the original change. It works at molecular level (end-product inhibition of the first enzyme in a pathway), cellular level and organism level (blood glucose, body temperature).
- Positive feedback amplifies the original change: childbirth (oxytocin), lactation (suckling → oxytocin and prolactin), fruit ripening (ethylene → more ethylene), blood clotting.
Key relationships
| Variable | Sensor / control | Signal | Effector response | Type |
|---|---|---|---|---|
| glucose rises | pancreas β cells | insulin | cells take up glucose; liver makes glycogen | negative |
| glucose falls | pancreas α cells | glucagon | liver breaks down glycogen, releases glucose | negative |
| end product D builds up | first enzyme in the pathway | D binds allosteric site | enzyme slows, less D made | negative |
| cervix stretched | stretch receptors, brain | oxytocin | stronger uterine contractions, more stretch | positive |
| fruit ripens | fruit cells | ethylene | more ripening, more ethylene | positive |
| Disruption | Effect on blood glucose | Effect on insulin |
|---|---|---|
| β cells destroyed (type 1) | high | very low |
| target cells resist insulin (type 2) | high | normal or high |
Assumptions behind the reasoning
- A loop works only if every link works; a break anywhere stops the correction.
- If the variable never returns, the stimulus stays, so the control centre usually keeps signalling (often more strongly).
- Simple models (halving a difference each round) show the shape of a response, not real values.
Mistakes to avoid
- Saying “negative” means harmful.
- Saying positive feedback keeps a variable stable.
- Mixing up glucagon (hormone) and glycogen (storage carbohydrate).
- Saying type 2 diabetes always means no insulin.
- Forgetting to say what ends a positive feedback loop.
Quick self-check
- Why does insulin release fall as glucose returns to normal? (The stimulus, high glucose, is shrinking: negative feedback switches itself down.)
- What stops the positive feedback in childbirth? (The birth removes the pressure on the cervix, so oxytocin release stops increasing.)
- Is fruit ripening controlled by negative or positive feedback? (Positive: ethylene triggers ripening, and ripening fruit release more ethylene, so the change grows.)
Next: practice questions.