3.6.5Inquiry Process
Control Mechanisms — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 145 of 160
Part 1 of 5Negative Feedback
Set point
- The normal value of a variable, such as body temperature near 37 °C, around which it is held within narrow limits.
Receptor and control centre
- Receptors detect deviations and send information to a control centre,
- such as the hypothalamus,
- which compares it with the set point.
Effector
- Muscles or glands bring about a response that reverses the change and restores the variable to its set point.
Stability
- Because the response opposes the change, negative feedback keeps conditions steady, which is the basis of homeostasis.
Multiple loops
- Many variables are controlled by several loops at once, which increases precision and provides a back-up if one loop fails.
| Variable | Rise detected: response | Fall detected: response |
|---|---|---|
| Blood glucose | Insulin released, glucose stored | Glucagon released, glucose released |
| Temperature | Sweating, vasodilation | Shivering, vasoconstriction |
Part 2 of 5Positive Feedback
1
A stimulus causes a response that increases the original stimulus further.
2
The change therefore grows quicker and quicker instead of being corrected.
3
In childbirth, stretching of the cervix triggers release of oxytocin, which increases contractions and stretches the cervix more.
4
The loop continues until an end event, the birth of the baby, removes the stimulus.
5
Positive feedback cannot hold conditions steady and is used only for processes that must be completed quickly.
Part 3 of 5Comparing the Two
Comparing the Two
Direction
- Negative feedback reduces a change, whereas positive feedback increases it.
Examples
- Negative
- thermoregulation and blood glucose;
- positive: oxytocin in labour and ethylene during fruit ripening.
- thermoregulation and blood glucose;
Role
- Negative feedback maintains internal conditions, while positive feedback drives a process to completion.
Time lag
- There is a delay between detection and response, so variables oscillate slightly around the set point.
Part 4 of 5Worked Example
Question
- A person’s core temperature is 37.0 °C and the hypothalamus detects a rise to 37.4 °C.
- Describe the feedback response and calculate the percentage change from the set point.
- Change = 37.4 − 37.0 = 0.4 °C.
- Percentage = 0.4 ÷ 37.0 × 100 = 1.08%, about 1.1%.
- Receptors detect warming, so the hypothalamus triggers sweating and vasodilation, which lose heat and return temperature to 37 °C.
Answer Rise of about 1.1%, corrected by negative feedback that cools the body.
Part 5 of 5Exam Checklist & Tips
3.6.5 – Key Points for the Exam
- Negative feedback A response that reverses a change and returns a variable to its set point.
- Positive feedback A response that amplifies the original change.
- Set point Target value for a physiological variable.
- Homeostasis Maintenance of a stable internal environment.
Common Mistakes
- Calling any response that makes a number go up positive feedback;
- what matters is the direction relative to the change.
- Stating that positive feedback maintains homeostasis; it moves conditions away from the set point.
- Forgetting the receptor and control centre stages of a loop.
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