What homeostasis is, and the negative-feedback framework
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explain what is meant by homeostasis and the importance of homeostasis in mammals; explain the principles of homeostasis in terms of internal and external stimuli, receptors, coordination systems (nervous system and endocrine system), effectors (muscles and glands) and negative feedback.
Why a steady internal environment matters
A cell works best in a narrow range of conditions. Enzymes have an optimum pH and temperature; membrane transport rates depend on the concentration gradient across the membrane; protein structure depends on the ionic composition of the cytoplasm. If a cell's environment drifts — say, temperature rises or pH falls — enzymes denature, transport rates change, and the cell stops working efficiently. In a multicellular animal, every cell is bathed in tissue fluid, and the composition of tissue fluid is kept constant by exchanges with the blood. Homeostasis is the maintenance of the internal environment (the tissue fluid) within narrow limits, despite changes in the external environment or in the body's level of activity.
The three quantities that the syllabus highlights in mammals are:
- Core body temperature — kept close to 37 °C in humans so that enzymes work near their optimum.
- Blood glucose concentration — kept around 5 mmol dm⁻³ in the fasting state, so that cells have a steady energy supply.
- Blood water potential — kept around −7 kPa (i.e. slightly less negative than pure water, but close to it), so that cells neither gain nor lose water by osmosis.
None of these is held absolutely constant. They are all held within a narrow range around a set point. The mechanism that does the holding is the same in every case: a negative-feedback loop.
The four components of a negative-feedback loop
Every homeostatic reflex — whether the variable is temperature, glucose or water potential — is built from the same four components, and the mark schemes test that the student can name them and place them in the right order:
- Stimulus — a change in the internal environment, away from the set point. (The stimulus can also be an external change that the body detects indirectly — e.g. cold air cools the skin, which then drops core temperature, which is the internal stimulus that the thermoregulatory centre actually responds to.)
- Receptor — a cell or group of cells that detects the stimulus. Receptors in homeostasis are usually specific to one variable: thermoreceptors detect temperature, osmoreceptors detect water potential, glucoreceptors detect glucose. The receptor sends a signal to the coordination centre.
- Coordination centre — receives the signal from the receptor, compares it to the set point, and decides what to do. In mammals the coordination centre is usually part of the nervous system (a region of the brain, e.g. the hypothalamus, or a reflex centre in the spinal cord) or the endocrine system (a hormone-secreting gland such as the pancreas or the posterior pituitary).
- Effector — a muscle or gland that carries out the response. Effectors are always muscles or glands; the mark scheme will not accept "liver" or "kidney" without saying "the liver cells" or "the cells of the kidney tubule" — an effector is a tissue, not an organ.
A response is the change the effector makes. The defining feature of a negative-feedback response is that it reverses the stimulus — bringing the variable back towards the set point. (A positive feedback would amplify the stimulus; that exists in biology — childbirth, the action potential, blood clotting — but it is not the basis of homeostasis.)
Fig 14.1 The four components of a negative-feedback loop. The dashed red arrow on the bottom shows the negative part: the response opposes the stimulus and so returns the variable to the set point.
Negative feedback: a worked introduction
Imagine the blood glucose concentration rises after a sugary meal. The four steps of the negative-feedback loop play out as follows.
- Stimulus — blood [glucose] rises above the set point of ~5 mmol dm⁻³.
- Receptor — glucoreceptors in the pancreas detect the rise. (Some of the cells in the islets of Langerhans act as glucoreceptors — they are specialised endocrine cells whose firing rate depends on the local glucose concentration.)
- Coordination centre — the same islet cells, now acting as the coordination centre, increase the secretion of the hormone insulin into the blood.
- Effector — insulin travels in the blood to its target cells (muscle cells and liver cells), which respond by taking up glucose from the blood and storing it as glycogen.
The response — glucose uptake and glycogen synthesis — drives the blood [glucose] back down towards the set point. As it does, the glucoreceptors detect the falling value and reduce their firing rate; insulin secretion falls; glucose uptake slows. The system settles at the set point. The whole loop is "negative" because the response opposes the stimulus: glucose was rising, the response brings it down.
Every homeostatic reflex in this note is a worked example of this same shape. The receptors and effectors change (osmoreceptors in §06, glucoreceptors in §07, guard cells in §09); the coordination centre changes (hypothalamus in §06, pancreatic islets in §07, the guard cell itself in §10); but the four-component structure is the same.
Why the 'negative' is the key word
A positive-feedback response would amplify the stimulus — making the deviation larger, not smaller. A negative-feedback response opposes it. The mark scheme is strict: any answer that says "negative feedback" without explaining that the response opposes the stimulus loses the mark. The phrase to write is "the response reverses the change in the stimulus and returns the variable to the set point", not "the response makes the stimulus negative".
A second common loss is to name a coordination system wrongly. The mark scheme expects one of: nervous system (fast, electrical, short-lived — e.g. reflex arc, hypothalamus → pituitary), or endocrine system (slow, hormonal, longer-lived — e.g. insulin from the pancreas, ADH from the posterior pituitary, glucagon from the pancreatic α-cells). The same loop can use both: the hypothalamus receives the nervous input from the receptor and responds by secreting a hormone (ADH) into the blood — that is a neuro-endocrine reflex, and the mark scheme accepts either label as long as it is precise.
Identifying the components of a negative-feedback loop
A flow chart shows blood glucose concentration being detected by a receptor in the pancreas, which signals a coordination centre that secretes insulin, which acts on muscle cells to take up glucose. The flow chart is described as an example of a control mechanism.
State the type of homeostatic control mechanism operating in Fig. 6.1.
Show full working
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The mechanism is negative feedback. The receptor (in the pancreas) detects that blood [glucose] has risen above the set point. The coordination centre (also in the pancreas) responds by secreting insulin. The effector (muscle cells) takes up glucose. The response reverses the stimulus — glucose was rising, the response brings it down.
Mark 1. The MS credits the literal phrase 'negative feedback' (and rejects 'feedback' alone, or 'homeostatic control' alone).
Negative feedback.
On a one-mark 'state' question the answer is one or two words, never a sentence. 'Negative feedback' is the term; do not write 'it is feedback that is negative in direction' or 'homeostasis' — the MS is looking for the technical name of the mechanism.
Your turn — negative feedback
- 16 marks
A person exercises vigorously on a hot day. Core body temperature rises above 37 °C. Describe the negative-feedback response that brings core body temperature back to the set point.
In your answer you should name the stimulus, the receptor, the coordination centre, the effector(s) and the response.
Stuck? Show hint
Receptors for temperature in mammals are in the skin and in the hypothalamus. Effectors for heat loss are skin arterioles, sweat glands and hair-erector muscles. The coordination centre is the hypothalamus.
Show solution
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Stimulus: core body temperature rises above 37 °C. This is the change in the internal environment, detected by the body.
Mark 1. The MS credits the stimulus as a rise in core temperature.
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Receptors: thermoreceptors in the skin and in the hypothalamus detect the rise. The skin thermoreceptors detect the external signal (the hot day), and the hypothalamic thermoreceptors detect the internal change (the rise in core temperature). The hypothalamus is the one that drives the response.
Mark 2. The MS credits thermoreceptors in the skin AND in the hypothalamus (or 'in the brain').
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Coordination centre: the hypothalamus. The hypothalamus receives the input from the thermoreceptors, compares it to the set point of 37 °C, and decides what to do.
Mark 3. The MS credits the hypothalamus by name.
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Effectors: (i) arterioles in the skin, (ii) sweat glands, (iii) hair-erector muscles (in furry mammals). The mark scheme expects at least two named effectors, with the tissue type (muscle or gland) made clear.
Mark 4. The MS credits any two of the three effectors.
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Response: (i) vasodilation of skin arterioles — more blood flows near the skin surface, more heat is lost by radiation; (ii) increased sweating — evaporation of sweat cools the skin; (iii) hair-erector muscles relax, hairs lie flat, less insulation. All three responses oppose the stimulus (heat loss when temperature is high). The result is that core temperature falls back to 37 °C.
Marks 5 and 6. The MS credits the responses linked to the effectors, with the explicit idea that the responses reverse the stimulus.
AnswerStimulus: core body temperature rises above 37 °C. Receptor: thermoreceptors in the skin and in the hypothalamus. Coordination centre: the hypothalamus. Effectors: arterioles in the skin, sweat glands, hair-erector muscles. Response: vasodilation of skin arterioles (more heat lost by radiation), increased sweating (evaporative cooling), hair-erector muscles relax. The responses oppose the stimulus, so core temperature falls back to 37 °C.
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- 22 marks
A student says: "Homeostasis keeps the internal environment constant, no matter what."
(a) State one way in which this statement is correct.
(b) State one way in which this statement is incorrect.Stuck? Show hint
(a) Think about what homeostasis actually achieves. (b) Think about what 'constant' literally means, and what really happens in the body.
Show solution
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(a) Correct: homeostasis keeps the internal environment within narrow limits around a set point. The body does keep temperature, blood [glucose] and blood water potential close to a fixed value, which is what the student is describing.
Mark 1. The MS credits the idea of 'narrow limits around a set point'.
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(b) Incorrect: the internal environment is not held constant — it oscillates around the set point, and the set point itself can shift (e.g. during exercise, fever, sleep). The mark-scheme-precise wording is "the internal environment fluctuates within a narrow range, not constant". A 24-hour temperature trace, for example, varies by about 0.5–1 °C even in health.
Mark 2. The MS credits the idea of 'fluctuation' or 'narrow range' rather than literal constancy.
Answer(a) Homeostasis does keep the internal environment within narrow limits around a set point, which is what the student is describing. (b) The internal environment is not held absolutely constant — it fluctuates around the set point, and the set point itself can change (e.g. during fever, exercise or sleep). The mark-scheme-precise wording is "fluctuates within a narrow range".
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The rest of this note
Can you do all of these?
Define homeostasis: maintenance of the internal environment within narrow limits around a set point, despite changes in the external environment or in the body's level of activity
Explain the four components of a negative-feedback loop: stimulus, receptor, coordination centre (nervous or endocrine), effector (muscle or gland); the response reverses the stimulus
State that urea is the main nitrogenous excretory product of mammals and is made by deamination of excess amino acids in the liver, via the ornithine (urea) cycle
Identify and describe the six features of a kidney LS: fibrous capsule, cortex, medulla, renal pelvis, ureter, and the branches of the renal artery and renal vein
Identify and locate the parts of a nephron: glomerulus, Bowman's capsule, PCT, loop of Henle, DCT, collecting duct, plus the afferent and efferent arterioles and the peritubular capillaries
Describe ultrafiltration at the glomerulus: high hydrostatic pressure (afferent wider than efferent) forces fluid through the three filtration barriers (endothelium, basement membrane, podocytes) into Bowman's capsule; cells and large proteins are retained
Describe selective reabsorption in the PCT: useful solutes (glucose, amino acids, Na⁺) and most of the water are reabsorbed back into the peritubular capillaries; urea and excess ions remain in the filtrate
Relate PCT ultrastructure to function: microvilli (large surface area), tight junctions (force through-cell transport), co-transporters (Na⁺ with glucose/amino acids), many mitochondria (ATP for active transport), folded basal membrane (Na⁺/K⁺ pumps)
Describe the loop of Henle and the medullary osmotic gradient: ascending limb actively pumps Na⁺/Cl⁻ into the medulla; descending limb loses water by osmosis; the deeper the medulla, the lower the water potential
Describe the ADH pathway: low blood water potential → osmoreceptors in the hypothalamus → ADH released from the posterior pituitary → ADH acts on collecting duct → insertion of aquaporins → more water reabsorbed → small volume of concentrated urine
Explain the cAMP cascade triggered by glucagon: glucagon binds receptor → G-protein activates adenylyl cyclase → cAMP made → protein kinase A activated → enzyme cascade → glycogen phosphorylase activated → glycogen broken down to glucose → glucose released into blood
Explain how insulin lowers blood glucose: increases glucose uptake (via GLUT4) by muscle and adipose cells; increases glycogen synthesis in the liver; the response opposes the stimulus (negative feedback)
Explain the principle of a glucose test strip (glucose oxidase + peroxidase, colour change) and a glucose biosensor (immobilised glucose oxidase, partially permeable membrane, transducer, amplified digital reading)
Describe the structure of guard cells: variable cell-wall thickness (inner thicker than outer), no plasmodesmata, many chloroplasts (with few grana), many mitochondria
Describe the daily rhythm of stomatal opening: open in the light (CO₂ uptake for photosynthesis), close in the dark (reduce water loss by transpiration)
Describe the ABA signalling pathway in water stress: ABA binds receptor → Ca²⁺ released (second messenger) → K⁺ channels open → K⁺ leaves the cell → water potential of the cell rises → water leaves by osmosis → guard cell loses turgor → stoma closes