Explain negative feedback around a set point
Show how a response opposes a departure from a reference level.
Section B13.3 · Homeostasis
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall hormone production and blood transport, respiration, liver storage and electrical impulses along neurones.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a set point in this model?
What you will learn
- Identify the departure and reference value
- Explain how the response reduces the difference
Explain negative feedback around a set point
| Feature | Meaning |
|---|---|
| Reference | Supplied set point |
| Departure | Value differs from the reference |
| Correction | Opposes the departure |
| Return | Smaller stimulus; reduced corrective response |
Identify the departure and reference value
A set point is a reference level around which an internal condition is regulated in a simplified homeostatic model. A sensor detects a departure from that level, a coordinating system responds, and effectors change activity. The important negative-feedback feature is that the response reduces the original departure rather than making it larger.
If the regulated value is above the supplied set point, an appropriate response moves it down towards that point. If it is below the set point, an appropriate response moves it up. Negative therefore does not mean the value must always decrease; it refers to opposing the change that moved the condition away from its regulated level.
Explain how the response reduces the difference
As the value returns towards its set point, the original stimulus becomes smaller and the corrective response is reduced. Without this reduction, a controller could continue driving the variable beyond the intended region. Real biological control operates within ranges and can have delays; the model is not a claim that a body value is perfectly fixed at a single number.
For a supplied set point of 5 model units, a value of 7 has a departure of +2 and a value of 3 has a departure of −2. The correct response direction differs, but both can reduce the magnitude of the departure. Always name the regulated variable and reference level before deciding whether a proposed response is negative feedback.
Sources for this lesson
Pause and explain
Which response is negative feedback for a value below its set point?
Worked example
Predict the response directions around set point 5.
Show the worked solution
- At 7, a downward response reduces the positive departure.
- At 3, an upward response reduces the negative departure.
Answer 7→5 requires a downward correction; 3→5 requires an upward correction. Both oppose departure from the set point.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain negative feedback with reference to a set point.
Give me a hint
Include departure, opposing response and reduced stimulus.
Compare my reasoning
- A departure from the reference level is detected.
- The response opposes it and declines as the value returns.
Negative feedback produces a response opposing departure from a set point; as the value returns, the stimulus and corrective response are reduced.
Look for these in your work
- Name the set point and departure
- Explain opposition and response reduction
For set point 5, predict the corrective directions for values 7 and 3.
Give me a hint
Compare each value with the reference.
Compare my reasoning
- Seven is above, so correction is downward.
- Three is below, so correction is upward.
At 7: decrease towards 5. At 3: increase towards 5.
Look for these in your work
- Compare both departures
- Give both corrective directions
Why should the original corrective response decrease as the set point is approached?
Give me a hint
The detected departure becomes smaller.
Compare my reasoning
- The original stimulus is reduced.
- Continued unchanged correction could drive a new departure.
The departure and stimulus diminish as the value returns, so the corrective response is reduced rather than continuing unchanged and driving a new departure.
Look for these in your work
- Connect correction with reduced stimulus
- Explain why response strength declines
Common mistakes
- Saying negative feedback always decreases the value.
- Continuing an unchanged corrective response after the departure is removed.
What can you explain now?
Which response is negative feedback for a value below its set point?
Compare with the explanation
An upward response towards the set point
It reduces the original downward departure.
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Make it stick
Explain the feedback chain and why negative does not mean always decreasing.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.