QCE Biology — Unit 2
Homeostasis — Flashcards & Quiz
Homeostasis is the maintenance of a relatively stable internal environment and underpins QCE Biology Unit 2. You need to describe negative feedback loops using the stimulus-receptor-control centre-effector-response model, and apply the framework to specific examples such as thermoregulation, osmoregulation and glucose regulation. Strong responses name the hormones and organs involved and explain how they correct disturbances back toward the set point.
Key Points
- Homeostasis = relatively stable internal environment despite external change; "relatively" matters because variables oscillate around a set point.
- Negative feedback loop structure: stimulus → receptor → control centre → effector → response → return to set point.
- Thermoregulation: hypothalamus detects temperature change; effectors include sweat glands, vasodilation/constriction, and shivering.
- Glucose regulation: pancreatic β-cells release insulin when glucose is high; α-cells release glucagon when low.
- Osmoregulation: the hypothalamus triggers ADH release from the pituitary, which increases water reabsorption in kidney collecting ducts.
- Positive feedback is rare but exists — childbirth oxytocin, blood clotting; it amplifies the change rather than correcting it.
Common Mistakes to Avoid
- Saying homeostasis maintains a "constant" internal environment — it's RELATIVELY stable, not fixed.
- Confusing receptors (detect stimuli) with effectors (produce responses).
- Forgetting positive feedback exists — it's rare but examinable.
- Missing the role of the hypothalamus as the main integrator of thermoregulation and osmoregulation.
- Applying the same feedback loop diagram without adapting to the specific variable being regulated.
Exam Strategy
QCAA Unit 2 homeostasis questions give a disturbance (cold, dehydration, hyperglycaemia) and ask you to describe the corrective response. Structure your answer around the feedback loop: identify the stimulus, name the receptor, trace the signal to the control centre, list the effectors and their responses, and explain how the response returns the variable to the set point. Always use specific hormone and organ names — "the pancreas responds" scores less than "pancreatic β-cells secrete insulin, which binds receptors on liver and muscle cells to promote glucose uptake".
Sample Flashcards
Q1: Define homeostasis and explain why the word 'constant' is misleading.
Homeostasis is the maintenance of a relatively stable internal environment despite changes in external conditions. The word relatively matters: internal variables are not held at a fixed value but are allowed to fluctuate within a narrow tolerable range around a set point. Control mechanisms detect departures from that range and act to reverse them, so the true picture is continual small oscillation rather than a flat line.
Q2: Set out the components of the stimulus-response model used in homeostatic control.
A stimulus is a detectable change in a variable away from the set point. A receptor detects that change and sends information to a control centre, which compares the value with the set point and determines the response. The control centre signals one or more effectors, which are the muscles or glands that carry out the response, and the response returns the variable toward the set point.
Q3: How does negative feedback differ from positive feedback?
In negative feedback the response opposes the original change and drives the variable back toward the set point, which stabilises the system. In positive feedback the response reinforces the original change and drives the variable further from the starting point, which amplifies rather than stabilises. Almost all homeostatic control uses negative feedback, because stability is the goal.
Q4: Why is positive feedback uncommon in the body, and where does it occur?
Positive feedback pushes a variable further from its starting value, so it cannot maintain stability and is unsuitable for routine regulation. It is used where a process needs to be driven rapidly to completion rather than held steady, and it is normally terminated by an external event that ends the cycle.
Q5: What is a set point and what does tolerance range mean?
The set point is the value of a variable that control mechanisms act to restore. The tolerance range is the band of values either side of the set point within which cells continue to function normally. Beyond that range enzyme activity and membrane function are disrupted, so the organism experiences stress and, if the departure continues, cell damage or death.
Sample Quiz Questions
Q1: Homeostasis means holding internal conditions at a completely fixed value.
Answer: FALSE
Homeostasis maintains a RELATIVELY stable internal environment, with variables fluctuating within a narrow tolerance range around a set point rather than being held constant.
Q2: In the stimulus-response model, the effector carries out the response.
Answer: TRUE
Correct. Receptors detect the change, the control centre processes it and determines the response, and effectors such as muscles or glands carry that response out.
Q3: Negative feedback amplifies the original change in a variable.
Answer: FALSE
Negative feedback OPPOSES the original change and returns the variable toward the set point. It is positive feedback that amplifies the change.
Revision Tip
Feedback loop diagrams are classic flashcard material — build a Revizi deck with one loop per variable (temperature, glucose, water, blood pressure) and practise filling in the receptor, effector and hormone fields from memory.
Related Concepts
Last updated: March 2026 · 8 flashcards · 8 quiz questions