IB Biology HLChemical SignallingPaper 1 & 2~12 min read
Regulating Cell Signalling
A signal that never stops is not a signal, it is a fault. Most of the body’s control systems exist to cancel out changes. A small number do the opposite on purpose — and knowing which is which is the whole of this page.
📚 What you need to know
Most homeostatic control uses negative feedback to keep physiological factors, such as internal temperature or blood glucose, within certain limits.
A negative feedback loop needs a receptor to detect a stimulus, a coordination system (nervous or hormonal) to transfer information, and an effector (muscle or gland) to respond.
If a factor increases, the body makes it decrease; if it decreases, the body makes it increase. Negative feedback works by reversing a change.
In positive feedback, the response makes the factor deviate even more from the normal range, enhancing the original stimulus.
Examples of positive feedback: dilation of the cervix during labour, and blood clotting.
Positive feedback is useful to quickly activate a process, and the body reverts to negative feedback afterwards.
It can also occur when homeostatic mechanisms break down, for example hypothermia.
Because positive feedback does not maintain a constant internal environment, it is not involved in homeostasis.
Negative feedback
The majority of homeostatic control mechanisms use negative feedback to maintain homeostatic balance — keeping physiological factors within certain limits.
🧩 The parts of a negative feedback loop
A receptor detects a stimulus involving a physiological factor, such as a change in temperature or blood glucose level.
A coordination system transfers information between different parts of the body. This may be the nervous system or the hormonal system.
An effector carries out a response. Effectors are muscles or glands.
The outcome is always a reversal:
If there is an increase in the factor, the body responds to make the factor decrease.
If there is a decrease in the factor, the body responds to make the factor increase.
For example, if body temperature rises, a negative feedback system acts to lower it again. If blood glucose drops, a negative feedback system acts to raise it again.
The factor is monitored continuously, so it never sits perfectly still — it fluctuates around a set point rather than holding one exact value.
Positive feedback
In a positive feedback loop, the original stimulus produces a response that causes the factor to deviate even more from the normal range. These loops enhance the effect of the original stimulus rather than cancelling it.
Dilation of the cervix during labour
Nothing here is being corrected. Stronger contractions push the baby further down, which stretches the cervix more, which produces more oxytocin still.
The cervix stretches as the baby pushes against it.
Stretch receptors in the cervix are stimulated and send impulses to the brain.
The pituitary gland is stimulated to release oxytocin, which increases the intensity of uterine contractions.
This pushes the baby further down the birth canal and stretches the cervix even further.
Blood clotting
Positive feedback loops are useful to quickly activate a process. Clotting is the clearest example:
When the body is injured, platelets become activated.
They release chemicals that activate more platelets, which in turn release chemicals that activate even more platelets.
This ensures the wound is quickly closed by a blood clot before too much blood is lost, or too many pathogens enter the bloodstream.
Once the clot has formed, the body reverts to negative feedback mechanisms.
The bit students miss. Positive feedback loops in the body are temporary. They run hard, achieve something quickly, then hand back to negative feedback. A positive feedback loop that never ended would kill you.
Comparing the two
The green line never sits exactly on the set point. That is normal — the factor is continuously monitored, so it fluctuates around an ideal value rather than holding it.
Feature
Negative feedback
Positive feedback
Effect on the change
Reverses it
Enhances it
Result for the factor
fluctuates around a set point, within normal limits
deviates further from the normal range
Role in homeostasis
the main mechanism of homeostasis
not involved in homeostasis
Examples
body temperature, blood glucose
cervical dilation in labour, blood clotting
Read that last row of the table twice. Positive feedback is not a homeostatic mechanism, because it does not maintain a constant internal environment. Saying “positive feedback helps homeostasis” is a straightforward error that examiners look for.
When homeostasis breaks down
Positive feedback can also kick in when homeostatic mechanisms fail. During prolonged exposure to extreme cold, hypothermia can occur: body temperature drops, which decreases metabolism, which in turn causes body temperature to drop further.
🧠
One question tells them apart
Ask: does the response push the factor back, or push it further? Back means negative. Further means positive. The names have nothing to do with whether the outcome is good or bad — clotting is positive feedback and it saves your life.
Worked examples
WE 1
Outline a negative feedback loop
Outline the components of a negative feedback control loop and the outcome it produces. (4 marks)
Component 1: receptor
A receptor detects a stimulus involving a physiological factor, such as temperature or blood glucose.
Component 2: coordination system
A coordination system, either the nervous system or the hormonal system, transfers information between parts of the body.
Component 3: effector
An effector, a muscle or a gland, carries out a response.
The outcome
The change is reversed: an increase in the factor produces a decrease, and a decrease produces an increase, keeping it within normal limits.
Receptor, coordination system, effector — and the change is reversedname the effector types (muscles or glands) — it is often a separate mark
WE 2
Explain positive feedback in labour
Explain how positive feedback causes dilation of the cervix during labour. (4 marks)
Point 1: the stimulus
The cervix stretches as the baby pushes against it.
Point 2: detectionStretch receptors in the cervix are stimulated and send impulses to the brain.
Point 3: the response
The pituitary gland releases oxytocin, increasing the intensity of uterine contractions.
Point 4: why it is positive
Stronger contractions push the baby further down and stretch the cervix even more, so the original stimulus is enhanced rather than reversed.
Each round of the loop makes the stimulus bigger, not smallerthe final point is the one that earns the “positive” mark — say the deviation increases
WE 3
Evaluate a statement
A student writes: “Positive feedback is a homeostatic mechanism because blood clotting keeps the body healthy.” Evaluate this statement. (3 marks)
Point 1: what is correct
Blood clotting is a genuine example of positive feedback, and it is beneficial, closing the wound quickly before too much blood is lost.
Point 2: what is wrong
Homeostasis means maintaining a constant internal environment. Positive feedback drives a factor further from normal, so it is not a homeostatic mechanism.
Point 3: the correction
Being useful is not the same as being homeostatic; once the clot forms, the body reverts to negative feedback mechanisms.
Useful, yes. Homeostatic, no.“evaluate” means say what is right as well as what is wrong — do not simply write “incorrect”
💡 Exam tips
Always name the three parts: receptor, coordination system, effector.
Say effectors are muscles or glands, and coordination is nervous or hormonal.
Use the verbs reverses for negative feedback and enhances for positive feedback.
State clearly that positive feedback is not involved in homeostasis.
Learn two positive feedback examples: cervical dilation and blood clotting.
Mention that the body reverts to negative feedback once a positive loop has done its job.
⚠ Common mistakes
Saying positive feedback is part of homeostasis. It does not maintain a constant internal environment.
Thinking “positive” means good and “negative” means bad. They describe the direction of the response only.
Saying negative feedback holds a factor perfectly constant. It fluctuates around a set point.
Forgetting the coordination system. Receptor straight to effector misses a mark.
Giving hypothermia as a useful example of positive feedback. It is what happens when homeostasis breaks down.
Saying oxytocin is released by the brain. It is released by the pituitary gland after the brain is stimulated.
That completes Chemical Signalling. The seven notes tell one story: a ligand is released, a receptor with a complementary shape catches it, the message is converted into something the cell can use, an effect is produced — and then, almost always, something switches the whole thing off again.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
