AP Biology · Unit 4 · Topic 4.4
Two loops with opposite signs. One brings a value back to where it started; the other drives it as far from the start as it can get.
Learning objective 4.4.A Explain how feedback mechanisms maintain homeostasis.
Both loops detect a departure and respond to it. The only difference is whether the response pushes against the departure or with it. Knock the value away from where it started and watch what each one does about it.
Drag the starting value above 37 and then below it without switching loops. The same mechanism cools in one case and warms in the other, because the direction of the correction comes from the direction of the error. That is why one loop handles both.
Notice that the curve never turns round. There is nothing in the loop that would make it. Every real positive feedback loop is stopped by an event outside itself, and the next tab lists what that event is for each one.
An exam answer that names the sensor, the effector and the direction of the response has said everything the mark scheme is looking for. An answer that says only “negative feedback brings it back to normal” has named the outcome and none of the mechanism.
Three of these hold something steady and three drive something to completion. Work out which is which from the response before you read the label.
All three positive feedback examples here are things a healthy body or plant does on purpose. Childbirth needs contractions that get stronger, not contractions that settle at a comfortable level. A wound needs a clot that forms fast, not one that stabilizes half-formed. Positive feedback is the right tool when the job is to finish something quickly rather than to hold something steady.
Look at the set point row as you step through the six systems. Three of them have one and three of them have nothing in that row at all — and it is not that the number is unknown, it is that there is no such thing.
A positive feedback loop is not aiming at a value. It has a starting point it is being driven away from, which is a different kind of thing entirely. The presence or absence of a set point is the structural difference between the two kinds of loop.
A response takes time to arrive. By the time it does, the value has already moved — so the correction is always slightly out of date, and the variable overshoots. Add the delay and watch a smooth approach turn into an oscillation.
Set the delay to 0. The value slides onto the set point and stops, and never crosses it once. That is a system with no delay in it, and no real biological system is one: a hormone has to be released, travel in the blood, bind a receptor and produce a response, and all of that takes time.
So a healthy core temperature is quoted as a range, and a healthy blood glucose is quoted as a range. The range is the oscillation, and its existence is evidence that the loop is working rather than that it is not.
With a long delay and a strong response, each correction is not only late but too big for the error that is actually there by the time it arrives — so the next error is larger than the last, and the swings grow instead of shrinking.
Biological loops mostly avoid this by responding gently. It is a useful thing to know when you meet a graph of a regulated variable that is swinging more and more widely: that is not a loop doing its job badly, it is a loop whose gain and delay do not match.
Every negative feedback loop has a sensor, an effector and a connection between them. Break any one and the variable stops being regulated: it drifts wherever the outside conditions push it, and the cells that depend on those conditions being steady stop working properly.
Answer these in your own words before you open the answers.
1.
2.
3.
4.
5.
6.
7.
8.
| Negative feedback | Positive feedback |
|---|
If you can fill in this table from memory you can answer almost any question on this topic, including the ones that give you an unfamiliar example and ask which kind it is.