QCE Biology · Unit 2
QCE Biology Unit 2 Topic 1: Homeostasis — Flashcards & Quiz
QCE Biology Unit 2 Topic 1 examines how organisms hold their internal conditions within a narrow survivable range while the outside world changes around them. These 20 free flashcards and 20 true/false questions cover the meaning of homeostasis, the stimulus-response model and the roles of receptors, control centres and effectors, negative feedback as the dominant control mechanism, positive feedback and why it is rare, thermoregulation and the responses that gain or shed heat, osmoregulation and the action of antidiuretic hormone, blood glucose regulation by insulin and glucagon, and the comparison between nervous and endocrine coordination.
Key Terms
- Homeostasis
- The maintenance of a relatively stable internal environment within a narrow tolerance range around a set point, despite changes in external conditions. It is achieved by control mechanisms that detect departures and act to reverse them.
- Negative feedback
- A control mechanism in which the response opposes the original change and returns the variable toward its set point. It is the basis of almost all homeostatic regulation because it produces stability.
- Set point
- The value of a regulated variable that control mechanisms act to restore. It can itself be adjusted, as happens during a fever when chemicals released during infection raise the thermoregulatory set point.
- Effector
- A muscle or gland that carries out the response determined by the control centre. Identifying the correct effector for a given scenario, such as sweat glands or skeletal muscle, is routinely assessed.
- Antidiuretic hormone
- A hormone produced in the hypothalamus and released from the posterior pituitary that increases the permeability of kidney collecting ducts to water, so more water is reabsorbed and a smaller volume of concentrated urine is produced.
- Insulin
- A hormone secreted by beta cells of the pancreatic islets when blood glucose rises. It increases glucose uptake by cells and promotes its conversion to glycogen for storage, lowering blood glucose toward the set point.
- Glucagon
- A hormone secreted by alpha cells of the pancreatic islets when blood glucose falls. It stimulates the breakdown of liver glycogen into glucose and the formation of new glucose, raising blood glucose toward the set point.
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.
Q6: Which structure acts as the control centre for thermoregulation, and what does it monitor?
The hypothalamus is the thermoregulatory control centre. It monitors the temperature of blood flowing through it, giving it direct information about core temperature, and it also receives input from thermoreceptors in the skin that report the temperature of the surroundings. Comparing both against the set point allows it to trigger responses before core temperature has shifted far.
Q7: Describe the responses that reduce body temperature when it rises.
Vasodilation widens the arterioles supplying skin capillaries so more warm blood flows near the surface and heat is lost by radiation and conduction. Sweat glands secrete sweat onto the skin, and the evaporation of that water removes a large amount of heat. Metabolic heat production is reduced and behavioural responses such as seeking shade or reducing activity add to the effect.
Q8: Describe the responses that raise body temperature when it falls.
Vasoconstriction narrows the arterioles supplying skin capillaries, diverting blood away from the surface and conserving heat in the core. Shivering causes rapid involuntary contraction of skeletal muscle, which releases heat as a by-product of respiration. Metabolic rate may be increased by hormonal action, hairs are raised to trap an insulating layer of air, and behavioural responses such as adding clothing reduce loss.
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.
Q4: Blood clotting is an example of positive feedback.
Answer: TRUE
Correct. Activated platelets recruit further platelets, reinforcing the original stimulus so the process is driven rapidly to completion.
Q5: Departures beyond the tolerance range matter largely because enzyme function is disrupted.
Answer: TRUE
Correct. Enzymes are sensitive to temperature and pH, so conditions outside the tolerance range reduce reaction rates and can denature them.
Why It Matters
Homeostasis is the idea that ties the whole of physiology together, because every organ system is ultimately in the business of holding some variable within the range that cells can tolerate. Once the receptor, control centre and effector pattern is secure, the same reasoning transfers to temperature, water balance, glucose and pH, which makes this topic one of the highest-leverage pieces of the course.
Key Concepts
The Stimulus-Response Model
Homeostatic control follows a consistent sequence: a stimulus moves a variable away from its set point, a receptor detects it, a control centre compares the value with the set point and decides the response, and an effector carries that response out. Being able to name each component for an unfamiliar scenario is the single most transferable skill in the topic.
Negative and Positive Feedback
Negative feedback opposes the original change and restores stability, and accounts for virtually all routine regulation. Positive feedback reinforces the change and is reserved for processes that must run rapidly to completion, such as clotting and childbirth, where an external event terminates the cycle.
Thermoregulation and Osmoregulation
Temperature is controlled by the hypothalamus through vasodilation, vasoconstriction, sweating and shivering, with surface-area-to-volume ratio shaping the challenge an organism faces. Water balance is controlled by the kidneys under the influence of antidiuretic hormone, which adjusts collecting-duct permeability and therefore urine concentration.
Blood Glucose Regulation
Beta and alpha cells of the pancreatic islets secrete insulin and glucagon respectively, producing opposing effects that hold blood glucose within range from both directions. Glycogen is the storage form because it is large and relatively insoluble, avoiding the osmotic consequences of storing free glucose.
Common Mistakes to Avoid
- Defining homeostasis as keeping conditions constant. The correct idea is a relatively stable internal environment fluctuating within a narrow range around a set point.
- Writing that capillaries constrict or dilate. Capillary walls contain no muscle, so it is the arterioles supplying them that change diameter.
- Saying that sweat cools the body, rather than that the evaporation of sweat cools it. The distinction explains why humid conditions reduce the effectiveness of sweating.
- Swapping insulin and glucagon, or attributing both to the same pancreatic cells. Beta cells secrete insulin and alpha cells secrete glucagon, with opposing effects.
- Stating that antidiuretic hormone is released from the hypothalamus. It is produced there but released from the posterior pituitary.
Study Tips
- Draw every feedback loop in the same four-box format of stimulus, receptor, control centre and effector, so an unfamiliar scenario can be slotted straight into a structure you already know.
- For each regulated variable, learn the response in both directions. Examiners routinely ask about the opposite case to the one most students revise.
- Say evaporation of sweat rather than sweating when explaining cooling, and arterioles rather than capillaries when explaining blood flow to the skin.
- Keep insulin and glucagon separated by their cell of origin: beta cells for insulin, alpha cells for glucagon, with opposite effects on glycogen.
- Finish every extended-response feedback answer by stating that the variable returns toward the set point and naming the feedback type, since that sentence often carries its own mark.
- Use fever as a self-test of understanding: if you can explain why a person shivers while their temperature is already rising, you have grasped what a set point is.
Related Topics
Exam Prep & Study Notes
Frequently Asked Questions
What does QCE Biology Unit 2 Topic 1 cover?
It covers homeostasis and the maintenance of the internal environment: the stimulus-response model of receptors, control centres and effectors, negative and positive feedback, thermoregulation, osmoregulation and the role of antidiuretic hormone, blood glucose regulation by insulin and glucagon, and the comparison of nervous and endocrine coordination.
What is the difference between negative and positive feedback?
Negative feedback produces a response that opposes the original change and returns the variable toward its set point, which stabilises the system and accounts for almost all homeostatic control. Positive feedback produces a response that reinforces the original change and drives the variable further from the starting point, which is useful only where a process must be driven rapidly to completion, such as blood clotting or childbirth.
Why do insulin and glucagon have opposite effects?
They are secreted by different cells of the pancreatic islets in response to opposite stimuli. Beta cells release insulin when blood glucose rises, promoting uptake by cells and storage as glycogen, while alpha cells release glucagon when blood glucose falls, promoting the breakdown of glycogen back into glucose. Together they hold blood glucose within its tolerance range from both directions.
Last updated: March 2026 · 20 flashcards · 20 quiz questions · Content aligned to the QCAA Syllabus