Homeostastis, Negative and Positive Feedback
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of homeostasis from GCSE Biology. You can test your knowledge on this below.
What is homeostasis?
The maintenance of a stable internal environment within set limits, even when the external environment is changing.
Which two body systems are involved in homeostasis?
The nervous system and the endocrine system.
Give two examples of conditions controlled by negative feedback in the human body.
Blood glucose concentration and body temperature.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains homeostasis, negative and positive feedback or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Overview of Homeostasis and Feedback Mechanisms
What Is Homeostasis?
- Homeostasis refers to the maintenance of a stable internal environment within set limits, despite external changes.
- Key controlled variables include:
- Core body temperature.
- Blood glucose concentration.
- Water potential of blood.
- Blood pH.
Why Homeostasis Matters
- Ensures optimum conditions for enzyme activity.
- Prevents damage to cells due to extreme fluctuations (e.g., osmotic swelling/shrinkage).
- Helps maintain metabolic efficiency and survival in changing environments.
Negative Feedback
What Is Negative Feedback?
- A corrective mechanism that restores a factor back to normal when it deviates from its ideal value.
- Involves a receptor, coordinator, and effector.
Examples of Negative Feedback:
Blood Glucose Regulation
- Rise in glucose → insulin released → glucose converted to glycogen.
- Fall in glucose → glucagon released → glycogen converted to glucose.
Thermoregulation
- High body temp → vasodilation, sweating.
- Low body temp → vasoconstriction, shivering.
Water Potential
- Low water potential → ADH release → more water reabsorbed.
- High water potential → less ADH → more water lost in urine.
Positive Feedback
What Is Positive Feedback?
- A mechanism that amplifies the change, moving the system further away from the set point.
- Less common than negative feedback in healthy systems.
Examples of Positive Feedback:
Blood clotting:
- Platelets release chemicals → attract more platelets → clot formation escalates.
Childbirth (labour):
- Stretching of cervix → release of oxytocin → stronger contractions → more stretching.
Nerve impulse generation:
- Initial stimulus opens Na⁺ channels → Na⁺ enters → more Na⁺ channels open (voltage gated) → more Na⁺ enters → further depolarisation.
Positive feedback is not involved in homeostasis, but still biologically important.
Key Terms
- Homeostasis: The maintenance of a stable internal environment within set limits, even when the external environment is changing.
- Negative feedback: A mechanism that reverses a change to restore a variable to normal.
- Positive feedback: A mechanism that amplifies a change, moving the variable further from the norm.
- Effector: A muscle or gland that brings about a response.
- Receptor: Detects the change in the internal environment and converts the stimulus into an electrical impulse.
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Exam Tip
Avoid saying that negative feedback ‘prevents’ change – that’s incorrect. Negative feedback allows change, but reverses it to restore normal conditions.
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Explain the role of negative feedback in the control of blood glucose concentration. (4 marks)
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A receptor (e.g. in pancreas) detects a change in blood glucose concentration.
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If glucose rises, insulin is released by β cells of pancreas.
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Insulin increases glucose uptake by cells and conversion to glycogen in the liver.
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If glucose falls, glucagon is released by α cells of pancreas.
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Glucagon stimulates breakdown of glycogen to glucose in the liver.
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These responses bring glucose back to normal level – this is negative feedback.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!