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 quiz 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 and how it differs from positive feedback, sensory receptor types, the structure of neurons and the three types of neuron, action potentials and synaptic transmission, how hormones act on target cells with specific receptors, thermoregulation in humans and the structural, behavioural and physiological mechanisms of endotherms, osmoregulation and the action of antidiuretic hormone, water balance in plants, and the comparison between nervous and endocrine coordination. Aligned to the QCAA Biology 2025 General senior syllabus.
Inside the Revizi QCE Biology deck
Across the whole subject there are 1,427 flashcards spanning 19 topics, including 93 formula cards.
Describes how the cards in this deck are pitched — not a measure of how hard the subject is.
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 part of analysing a feedback-control diagram.
- 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.
- Synapse
- The junction between a neuron and another cell. Neurotransmitter released from vesicles in the presynaptic neuron crosses the synaptic cleft and binds to receptors on the postsynaptic membrane, passing the signal on.
- Abscisic acid
- A plant hormone produced when water is scarce. It causes guard cells to lose turgor so that stomata close, reducing water loss by transpiration.
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: Describe the structural, behavioural and physiological mechanisms endotherms use to regulate body temperature.
Structural: insulation (fur, feathers, fat or blubber) reduces heat loss, and brown adipose tissue generates heat directly. Behavioural: kleptothermy (gaining heat from other animals, e.g. huddling), hibernation (long periods of lowered body temperature and metabolism in cold seasons), aestivation (dormancy during hot, dry conditions) and torpor (short-term drops in body temperature and metabolic rate to save energy). Physiological: evaporative heat loss (sweating, panting, licking), thermogenesis (heat from shivering and non-shivering metabolism) and vasomotor control (vasoconstriction and vasodilation of skin arterioles).
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, and you understand how neurons and hormones carry the signals, the same reasoning transfers to temperature, water balance in animals and water balance in plants, 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, and endotherms add structural (insulation, brown adipose tissue), behavioural (kleptothermy, hibernation, aestivation, torpor) and physiological mechanisms. Water balance is controlled by the kidneys under the influence of antidiuretic hormone, and plants control water balance through stomata, vacuoles, the cuticle and abscisic acid.
Nervous and Endocrine Signalling
Receptors detect stimuli; sensory, inter- and motor neurons carry action potentials, and neurotransmitters carry the signal across synapses. Hormones travel in the blood but act only on cells with specific receptors, and a cell’s sensitivity depends on how many receptors it displays.
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.
- Saying that a nerve impulse travels electrically across the synapse. The impulse is electrical along the axon, but the signal crosses the synaptic cleft chemically, as neurotransmitter binding to receptors on the postsynaptic membrane.
- 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 — too high and too low — so you can handle either case.
- Say evaporation of sweat rather than sweating when explaining cooling, and arterioles rather than capillaries when explaining blood flow to the skin.
- Draw a labelled neuron and a synapse from memory, then trace an impulse from receptor to effector, naming each structure it passes.
- Finish every extended-response feedback answer by stating that the variable returns toward the set point and naming the feedback type, so the loop is complete.
- 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: how the nervous and endocrine systems use negative feedback (the stimulus-response model of receptors, control centres and effectors), types of sensory receptors, neuron structure and the types of neurons, action potentials and synaptic transmission, how hormones act on cells with specific receptors, feedback-control diagrams, thermoregulation in endotherms and in humans, osmoregulation and the role of antidiuretic hormone and the kidney, and water balance in plants through stomata, vacuoles, the cuticle and abscisic acid.
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.
How does a nerve impulse pass from one neuron to the next?
An action potential travels along the axon to the axon terminal, where it causes vesicles to release neurotransmitter into the synaptic cleft. The neurotransmitter diffuses across the cleft and binds to specific receptors on the postsynaptic membrane, opening ion channels and converting the chemical signal back into an electrical one (signal transduction). The neurotransmitter is then broken down or taken back up so the signal stops.
Last updated: 9 October 2026 · 20 sample flashcards · 20 sample quiz questions · Content aligned to the QCAA Syllabus