You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
This lesson covers the principles of homeostasis and the mechanism of negative feedback as required by the Edexcel A-Level Biology specification (9BI0), Topic 9 -- Control Systems. You need to understand what homeostasis is, why it is important, and how negative and positive feedback loops work.
Homeostasis is the maintenance of a constant internal environment despite changes in external conditions. It is the process by which the body keeps physiological parameters within narrow limits.
Key physiological parameters that are maintained by homeostasis include:
| Parameter | Typical Value / Range | Why it Must be Controlled |
|---|---|---|
| Core body temperature | ~37°C | Enzymes denature at high temperatures; low temperatures slow enzyme activity |
| Blood glucose concentration | ~4-6 mmol/L (fasting) | Too high → osmotic damage, glycosylation of proteins; too low → brain cells cannot function |
| Blood water potential | Around -300 kPa | Cells swell or shrink if water potential is not controlled (osmosis) |
| Blood pH | 7.35-7.45 | Enzymes and proteins are pH-sensitive; deviation disrupts metabolic reactions |
| Blood CO₂ concentration | ~5.3 kPa | High CO₂ lowers blood pH (forms carbonic acid); affects oxygen transport |
Exam Tip: The definition of homeostasis must include the phrase 'maintenance of a constant (or stable) internal environment'. Do not say 'keeping things the same' -- the internal environment fluctuates around a set point and is maintained within a narrow range.
Enzyme function: Enzymes have an optimum temperature and pH. Deviations reduce the rate of enzyme-catalysed reactions, and extreme changes cause denaturation.
Osmotic balance: If the water potential of the blood changes, cells may gain or lose water by osmosis. Animal cells can lyse (burst) if they gain too much water or crenate (shrink) if they lose too much.
Electrical signalling: The resting potential of neurones depends on precise ion concentrations. Changes to Na+, K+, or Ca²+ levels disrupt nerve impulse transmission.
Protein function: Many proteins (not just enzymes) are sensitive to temperature, pH, and ion concentrations. Haemoglobin, antibodies, and membrane proteins all rely on precise conditions.
Every homeostatic mechanism involves the same three basic components:
| Component | Role | Example (Body Temperature) |
|---|---|---|
| Receptor (sensor) | Detects a change (deviation from the set point) | Thermoreceptors in the skin and hypothalamus |
| Control centre (coordinator) | Processes information and coordinates the response | The hypothalamus |
| Effector | Carries out the corrective response | Sweat glands, blood vessels, skeletal muscles |
The system works as a feedback loop: the effector's response changes the internal condition, which is detected by the receptor, which feeds this information back to the control centre.
Negative feedback is the primary mechanism of homeostasis. It works by reversing (counteracting) any deviation from the set point.
This creates a self-correcting cycle that keeps the parameter oscillating around the set point.
| Deviation | Receptor | Control Centre | Effector | Response | Result |
|---|---|---|---|---|---|
| Temperature rises above 37°C | Thermoreceptors in hypothalamus | Hypothalamus | Sweat glands, arterioles in skin | Sweating increases; vasodilation occurs | Heat is lost; temperature decreases |
| Temperature falls below 37°C | Thermoreceptors in hypothalamus | Hypothalamus | Skeletal muscles, arterioles in skin | Shivering; vasoconstriction occurs | Heat is conserved/generated; temperature increases |
Exam Tip: When describing negative feedback, always use the phrase 'the response counteracts the deviation from the set point' or 'the response brings the parameter back towards normal'. This is what makes it 'negative' -- the response opposes the change.
Positive feedback occurs when a change in a physiological parameter triggers a response that amplifies (increases) the change further. This moves the parameter further away from the set point.
Positive feedback is relatively rare in the body because it is inherently destabilising. However, it is useful in situations where a rapid, escalating response is needed:
| Example | Mechanism |
|---|---|
| Childbirth (labour) | The baby's head presses on the cervix → oxytocin is released → uterine contractions increase → more pressure on cervix → more oxytocin → stronger contractions. The cycle ends when the baby is born. |
| Blood clotting | Platelets at a wound release clotting factors → attract more platelets → more clotting factors released → cascade continues until the clot is formed |
| Action potential generation | Na+ enters the axon → membrane depolarises → more voltage-gated Na+ channels open → more Na+ enters → rapid depolarisation to +40 mV |
| Fruit ripening | Ethylene gas promotes ripening → ripe fruit produces more ethylene → accelerates ripening of nearby fruit |
Exam Tip: Positive feedback always has a defined end point or it would be fatal. In labour, the end point is the birth of the baby. In blood clotting, the end point is the formation of a stable clot. In action potentials, the end point is the inactivation of Na+ channels at +40 mV.
Many homeostatic systems use antagonistic effectors -- two effectors that have opposite effects. This allows finer control:
| Condition | Hormone | Source | Effect |
|---|---|---|---|
| Blood glucose too high | Insulin | Beta (β) cells of islets of Langerhans | Stimulates glycogenesis, increases glucose uptake by cells |
| Blood glucose too low | Glucagon | Alpha (α) cells of islets of Langerhans | Stimulates glycogenolysis and gluconeogenesis |
The two hormones act in opposition (they are antagonistic), providing precise control of blood glucose concentration around the set point.
The set point is the normal (optimal) value for a physiological parameter. In most cases, the set point is relatively fixed, but it can be adjusted in some circumstances:
The diagram below summarises the general feedback loop:
Normal level → Deviation detected by receptor → Signal to control centre → Control centre activates effector → Effector counteracts change → Return to normal level → (cycle repeats as needed)
In negative feedback, the loop is self-limiting: once the parameter returns to the set point, the corrective response is switched off.
In positive feedback, the loop is self-amplifying: the response increases the deviation, which triggers a stronger response, and so on until an external event or limiting factor ends the cycle.
This lesson sits in Edexcel 9BI0 Topic 8 — Grey Matter (Coordination, Response and Gene Technology) and provides the conceptual scaffold for the case studies that follow (thermoregulation, blood glucose, osmoregulation). The content statements paraphrase to: define homeostasis as the maintenance of a constant internal environment; describe the generic feedback loop comprising sensor, integrator (control centre) and effector; distinguish negative feedback (output opposes the deviation → stability) from positive feedback (output amplifies the deviation → instability or rapid switching); explain why antagonistic effectors give finer control than a single regulator (refer to the official Pearson Edexcel 9BI0 specification document for exact wording). The material is examined on Paper 2 — Energy, Exercise and Coordination and is heavily synoptic: hormones (Lesson 1) supply the slow effector arm of homeostatic loops; thermoregulation (Lesson 7), blood glucose regulation (Lesson 8) and osmoregulation (Lesson 9) are the three named case studies; Topic 5 (mitochondrial respiration) provides the heat source for thermogenesis; Topic 7 (cardiovascular physiology) supplies the medullary baroreflex and the chemoreflex as further homeostatic exemplars.
Question (8 marks): Blood glucose concentration in a healthy adult is held close to a set point of ~5 mmol/L despite large meal-related disturbances.
(a) Describe the negative-feedback loop by which a rise in blood glucose after a carbohydrate-rich meal is corrected. (5)
(b) Explain why blood glucose regulation uses two antagonistic hormones rather than a single regulator. (3)
Solution with mark scheme:
(a) Step 1 — disturbance and sensor. After a meal, intestinal absorption raises plasma glucose above the ~5 mmol/L set point. β-cells in the islets of Langerhans of the pancreas sense this rise: glucose enters via the GLUT2 transporter, is phosphorylated by glucokinase, and the resulting ATP rise closes ATP-sensitive K⁺ channels, depolarising the β-cell.
M1 (AO1) — names β-cells / islets of Langerhans as the sensor for high glucose. "The pancreas detects glucose" without cell type does not score the M1.
Step 2 — integration. The β-cell depolarisation opens voltage-gated Ca²⁺ channels; Ca²⁺ entry triggers insulin exocytosis. The pancreatic islet integrates the signal — its output (insulin) is graded with the size of the disturbance.
M1 (AO1) — explicit Ca²⁺-triggered insulin secretion proportional to the rise.
Step 3 — effector response. Insulin binds insulin receptors on muscle and adipose cells, triggering GLUT4 translocation to the membrane so glucose enters those tissues. In the liver, insulin activates glycogen synthase and inhibits glycogen phosphorylase, driving glycogenesis (glucose → glycogen).
A1 (AO2) — names GLUT4 translocation in muscle/fat and glycogenesis in liver as the two main effector arms.
Step 4 — return to set point and switch-off. As tissue uptake and hepatic storage exceed intestinal supply, plasma glucose falls back toward 5 mmol/L. Falling glucose at the β-cell reverses the depolarisation; insulin secretion stops. This switch-off is the defining feature of negative feedback — the corrective signal is removed once the disturbance is corrected.
A1 (AO2) — explicit identification of the switch-off as the feedback closing.
Step 5 — synthesis. The system oscillates around — never sits exactly at — the set point. Stability is maintained because the direction of the response always opposes the direction of the disturbance.
A1 (AO3) — names the principle: output opposes deviation → dynamic equilibrium.
(b) Single-regulator systems can only push the variable in one direction (insulin can lower glucose; nothing in that loop alone can raise it). A second loop — α-cells secreting glucagon in response to low glucose, driving glycogenolysis and gluconeogenesis in the liver — pushes the variable in the opposite direction.
M1 (AO1) — names α-cells / glucagon as the antagonistic arm.
A1 (AO2) — links antagonism to bidirectional control: the variable can be pushed up or down independently.
A1 (AO3) — synthesis: two opposing loops give finer, faster correction than a single regulator because each can be tuned independently and the resting state is a balance of both, not the absence of one.
Total: 8 marks (5 + 3).
Question (6 marks): Compare and contrast negative feedback and positive feedback as regulatory architectures. Refer to the direction of the response, the typical end state, and one named biological example of each.
Mark scheme decomposition by AO:
| Mark | AO | Awarded for |
|---|---|---|
| 1 | AO1 | Defining negative feedback: the response opposes (counteracts) the deviation, returning the variable toward the set point. |
| 2 | AO1 | Defining positive feedback: the response amplifies the deviation, driving the variable further from the starting state. |
| 3 | AO2 | Linking negative feedback to stability / dynamic equilibrium: variable oscillates around the set point; the system is self-limiting because once corrected, the corrective signal is removed. |
| 4 | AO2 | Linking positive feedback to rapid switching / escalation: the system is self-amplifying and requires an external end point (e.g. birth, clot completion, AP repolarisation) or it would run unbounded. |
| 5 | AO1 | Naming a credible example of each: negative — thermoregulation, blood glucose, osmoregulation, baroreflex; positive — parturition (oxytocin), blood clotting, depolarising phase of the action potential, follicular surge of LH at ovulation. |
| 6 | AO3 | Synthesis / evaluation — explicit linking of mechanism to function: negative feedback is rare-as-failure (the body uses it everywhere homeostasis matters); positive feedback is rare-as-design (used only where rapid, terminating switching is the desired output). Equivalent: noting that positive feedback without a defined end point is pathological (e.g. fever spiral, septic shock cascade, eclamptic seizures). |
Total: 6 marks (AO1 = 3, AO2 = 2, AO3 = 1). AO3 is reserved for tying architecture to function (stability vs switching), not restating the definitions.
Connects to:
Homeostasis questions on 9BI0 typically split AO marks toward AO1 and AO2, with AO3 reserved for synthesis or evaluation:
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.