7.3.1Unit 6: Practical Skills in Biology II
Negative and Positive Feedback — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 29 of 48 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Parts of a Control Loop
1
A receptor detects a stimulus, which is a change in a physiological factor such as temperature or blood glucose concentration.
2
A coordination system, either the nervous system or the hormonal system, carries the information to the control centre.
3
An effector, which is a muscle or a gland, carries out a response.
4
In negative feedback the response reverses the original change, so a rise in the factor leads to a fall and a fall leads to a rise.
5
As the factor approaches its normal value, the size of the correction is reduced and the factor settles around its set point.
Part 2 of 4Negative Feedback
Purpose
- Purpose Keeps factors such as temperature and blood glucose fluctuating around a normal level, giving dynamic equilibrium.
Two mechanisms
- Two mechanisms There is usually one corrective response for the factor being too low and a different one for it being too high.
Temperature example
- Temperature example If body temperature rises, effectors such as sweat glands cool the body, and if it falls, mechanisms warm the body.
Glucose example
- Glucose example If blood glucose falls, the response raises it, and if it rises, the response lowers it again.
Stability
- Stability The reversal of the change gives a stable internal environment that supports enzyme action.
Part 3 of 4Positive Feedback
Positive Feedback
Definition
- Definition The response increases the change that caused it, so the factor moves further from its starting value.
Childbirth
- Childbirth Stretching of the cervix sends impulses that cause release of oxytocin, which intensifies contractions and stretches the cervix further.
Blood clotting
- Blood clotting Activated platelets release chemicals that activate more platelets, quickly sealing a wound.
Hypothermia
- Hypothermia In extreme cold falling temperature slows metabolism, which lowers temperature further, so this is a breakdown, not homeostasis.
| Feature | Negative feedback | Positive feedback |
|---|---|---|
| Effect on change | Reverses it | Amplifies it |
| Result | Stability | Rapid change |
| Role | Homeostasis | Completes a process |
Part 4 of 4Exam Checklist & Tips
7.3.1 – Key Points for the Exam
- Negative feedback A response that reverses a change and returns a factor toward its normal level.
- Positive feedback A response that amplifies the original change.
- Receptor A cell or structure that detects a stimulus.
- Effector A muscle or gland that carries out a response.
- Set point The normal level around which a factor fluctuates.
Common Mistakes
- Describing negative feedback as bad because it is negative; it restores stability.
- Saying positive feedback is used to maintain homeostasis.
- Forgetting the sequence of receptor, coordination system and effector in a loop.
- Stating that the factor stays exactly constant; it fluctuates around a set point.
Last updated:
\n