SPEC 3.6.4.1 - 3.6.4.2 - RESPONSES
Homeostasis & Blood Glucose
Principles of negative feedback, why blood glucose matters, and the roles of insulin and glucagon in the islets of Langerhans
What is Homeostasis?
Homeostasis
The maintenance of a constant internal environment within narrow limits, despite changes in the internal and external environment.
AQA requires you to understand why homeostasis is necessary — not just what it is:
Core Temperature
Enzyme activity is highly temperature-sensitive. A stable core temperature maintains optimal enzyme activity for metabolic reactions. Too high → enzymes denature; too low → reaction rates fall dramatically.
Blood pH
Enzymes and proteins have an optimum pH. Changes in blood pH alter the ionisation of amino acid R-groups, disrupting enzyme tertiary structure and activity.
Blood Glucose Concentration
Must be maintained for two reasons: (1) glucose is the primary respiratory substrate — cells need a constant supply for ATP production; (2) blood glucose affects the water potential of blood — too high and osmosis draws water out of cells, causing crenation and cell damage.
Negative Feedback
Negative Feedback
A mechanism in which a deviation from the set point triggers a response that reverses the change and restores the system to its original level.
Set Point
Deviation detected by receptor
Effector responds
Change reversed → Set point restored
↖ Negative feedback loop — the response feeds back to reduce the deviation ↙
Greater Control: Two Opposing Mechanisms
Having separate mechanisms for correcting deviations in both directions from the set point gives greater control. For blood glucose: insulin corrects a rise; glucagon corrects a fall. This prevents overshooting in either direction.
Positive Feedback — Not Homeostatic
Positive feedback amplifies a deviation rather than reversing it. Examples: action potential generation, blood clotting, uterine contractions during labour. These are useful for producing rapid, all-or-nothing responses — but they are not homeostatic.
Why Blood Glucose Must Be Controlled
Consequences of Too HIGH
Lowers water potential of blood
Water moves out of cells by osmosis → cells crenate (shrink)
Dehydration of tissues
Long-term: damages blood vessels, kidneys, nerves and eyes
Glucose excreted in urine (glucosuria)
Consequences of Too LOW
Insufficient glucose as respiratory substrate
The brain is critically dependent on glucose for ATP — unlike most other tissues, it cannot use fatty acids
Leads to confusion, unconsciousness, and in severe cases, death
Exam Technique
When asked why blood glucose homeostasis is important, give two reasons: (1) availability of respiratory substrate, and (2) effect on water potential of blood. Both are mark points in extended answers.
The Pancreas & Islets of Langerhans
Blood glucose is regulated by hormones secreted by the islets of Langerhans — clusters of endocrine cells within the pancreas:
Cell typeHormone secretedStimulus for secretionEffect on blood glucose
β (beta) cellsInsulinRise in blood glucose concentrationLowers blood glucose
α (alpha) cellsGlucagonFall in blood glucose concentrationRaises blood glucose
Memory Aid
INsulin IN-creases glucose uptake (from β cells — think "B for Blood glucose lowering"). GLUcagon GLUcose-releasing (from α cells — think "A for Above needs pushing up"). Or simply: β → insulin → blood glucose falls; α → glucagon → blood glucose rises.
Examiner Alert
Students frequently swap α and β cells. β cells → insulin (lowers glucose); α cells → glucagon (raises glucose). Alpha and glucagon both begin with letters that come early in the alphabet — one memory trick.
Action of Insulin
Insulin is released when blood glucose is above normal. It acts on liver and muscle cells to lower blood glucose:
1
Blood glucose rises (e.g. after a meal) → β cells in islets of Langerhans detect rise → secrete insulin into the blood
2
Insulin binds to receptors on the surface membranes of target cells (liver, muscle, adipose)
3
Controls the uptake of glucose by regulating the inclusion of channel proteins (GLUT4) in the surface membranes of target cells — more channel proteins inserted → more glucose enters cells by facilitated diffusion
4
Activates enzymes for glycogenesis — conversion of glucose to glycogen for storage in liver and muscle
5
Blood glucose falls → negative feedback → β cells secrete less insulin → set point restored
Action of Glucagon
Glucagon is released when blood glucose is below normal. It acts primarily on liver cells to raise blood glucose:
1
Blood glucose falls (e.g. between meals, during exercise) → α cells in islets of Langerhans detect fall → secrete glucagon into the blood
2
Glucagon binds to receptors on the surface membranes of liver cells
3
Activates enzymes for glycogenolysis — hydrolysis of glycogen to glucose → glucose released into blood
4
Activates enzymes for gluconeogenesis — synthesis of glucose from non-carbohydrate sources (glycerol and amino acids) in the liver
5
Blood glucose rises → negative feedback → α cells secrete less glucagon → set point restored
The Three Glucose Processes
AQA requires you to know all three, distinguish them precisely, and state which hormone triggers each:
ProcessDefinitionTriggered byLocation
GlycogenesisConversion of glucose → glycogen (storage)InsulinLiver and muscle
GlycogenolysisHydrolysis of glycogen → glucoseGlucagonLiver (mainly)
GluconeogenesisSynthesis of glucose from non-carbohydrate sources (glycerol, amino acids)GlucagonLiver
Memory Aid
Genesis = making/building (glycogenesis = making glycogen; gluconeogenesis = making new glucose). Lysis = breaking down (glycogenolysis = breaking glycogen). The suffix tells you the direction.
Examiner Alert
Do not confuse glycogenolysis (breakdown of glycogen) with gluconeogenesis (synthesis of new glucose from non-carbohydrate sources). Both raise blood glucose but by different mechanisms — and the examiner will distinguish between them.
The Full Negative Feedback Loop
This diagram shows both directions of control operating simultaneously as opposing negative feedback mechanisms:
TriggerDetectorHormoneResponse in liver/muscleResult
Blood glucose rises (e.g. after meal)β cellsInsulin ↑↑ Glucose uptake (GLUT4 inserted); ↑ glycogenesis in liver & muscleBlood glucose falls back to set point
Blood glucose falls (e.g. exercise/fasting)α cellsGlucagon ↑↑ Glycogenolysis in liver; ↑ gluconeogenesis in liver; ↑ glucose release into bloodBlood glucose rises back to set point
Why Two Mechanisms Are Better than One
A single corrective mechanism would lead to oscillation around the set point — overshooting in both directions. Having separate insulin and glucagon mechanisms allows finer and more rapid correction from both above and below the set point. This is a key AQA spec point.
Insulin — Mechanism at the Cell
The AQA spec specifies three precise points about how insulin acts — all three are mark points:
Point 1: Receptor Binding
Insulin attaches to specific receptor proteins on the surface membranes of target cells. This binding initiates an intracellular signalling cascade.
Point 2: Glucose Channel Insertion (GLUT4)
Insulin controls the uptake of glucose by regulating the inclusion of channel proteins (GLUT4 carriers) in the surface membrane. These allow glucose to enter by facilitated diffusion. Without insulin, GLUT4 is stored in vesicles inside the cell — insulin causes these vesicles to fuse with the cell membrane.
Point 3: Enzyme Activation for Glycogenesis
Insulin activates enzymes involved in glycogenesis — the conversion of glucose to glycogen for storage. This reduces the intracellular glucose concentration, maintaining the concentration gradient for further uptake.
Exam Technique
The AQA mark scheme gives credit specifically for "insertion/inclusion of channel proteins" — not just "increases permeability." Use the precise language: channel proteins inserted into the cell surface membrane.
DEEP Chain: Blood Glucose Regulation
D
Define Homeostasis is the maintenance of a constant internal environment. Blood glucose is regulated by negative feedback, using the hormones insulin (from β cells) and glucagon (from α cells) in the islets of Langerhans.
E
Explain the Mechanism HIGH glucose: β cells secrete insulin → binds receptors on target cells → GLUT4 channel proteins inserted into membrane → glucose uptake increases → glycogenesis activated (glucose → glycogen) → blood glucose falls. LOW glucose: α cells secrete glucagon → binds liver cell receptors → glycogenolysis activated (glycogen → glucose) + gluconeogenesis (glycerol/amino acids → glucose) → glucose released → blood glucose rises. Both operate as opposing negative feedback loops.
E
Evidence / Example Having two separate mechanisms (insulin and glucagon) gives greater control than a single mechanism — it allows correction from both directions without overshooting. Blood glucose that is too high reduces the water potential of blood (osmotic effects); too low deprives the brain of its primary respiratory substrate.
P
Practical / Quantitative Normal blood glucose concentration is approximately 4–6 mmol/L. Failure of this system results in diabetes mellitus — Type I (autoimmune destruction of β cells, no insulin) or Type II (insulin resistance of target cells). Blood glucose can be measured using colorimetric techniques (Required Practical 11).
Examiner Alerts
Error 1: Swapping α and β cells
β cells → insulin (lowers blood glucose). α cells → glucagon (raises blood glucose). This is one of the most common errors in AQA biology exams.
Error 2: Confusing glycogenolysis and gluconeogenesis
Glycogenolysis = breakdown of glycogen already stored. Gluconeogenesis = new synthesis of glucose from glycerol and amino acids. They are both triggered by glucagon but are different processes.
Error 3: "Insulin opens glucose channels"
Insulin does not simply open pre-existing channels. It causes new GLUT4 channel proteins to be inserted into the cell surface membrane. The distinction matters — AQA mark schemes specifically credit "insertion/inclusion of channel proteins."
Error 4: Glucagon acts on muscle
Glucagon acts primarily on liver cells, not muscle. Muscle cells lack glucagon receptors. Insulin acts on both liver and muscle.
Synoptic Links
🔗 Synoptic Link
Carbohydrates (1.02): Glycogen is a polysaccharide of α-glucose. Glycogenesis and glycogenolysis are condensation and hydrolysis reactions respectively — directly linking to Module 1 biochemistry. This is a common Paper 3 synoptic connection.
🔗 Synoptic Link
Enzymes (1.05): Both insulin and glucagon work by activating intracellular enzymes. Temperature and pH homeostasis are important because enzyme activity depends on these conditions — linking homeostasis directly to enzyme biochemistry.
🔗 Synoptic Link
Cell membranes (2.04): Insulin triggers insertion of GLUT4 carrier proteins into the cell surface membrane — a direct application of membrane transport and membrane protein function from the Cells module.
🔗 Paper 3 Essay Trigger
Blood glucose regulation features in synoptic essays on "The importance of cell communication in maintaining homeostasis" and "The role of proteins in biological systems" — covering insulin/glucagon as hormone proteins binding to receptor proteins.
Practice Question
A student eats a large meal containing carbohydrates. One hour later, blood glucose has returned to its normal concentration.
Explain, in terms of negative feedback, how blood glucose concentration is restored to its normal level after the meal. [5 marks]
D:
Negative feedback: a rise in blood glucose above the set point triggers a corrective response that reverses the change and restores blood glucose to its normal level.
E:
After the meal, blood glucose rises. β cells in the islets of Langerhans detect this rise and secrete insulin into the blood. Insulin binds to receptor proteins on the surface membranes of target cells (liver and muscle). GLUT4 channel proteins are inserted into the cell surface membrane, increasing glucose uptake by facilitated diffusion.
E:
Insulin also activates enzymes for glycogenesis — converting glucose to glycogen for storage. These combined effects reduce blood glucose concentration back toward the set point. As blood glucose falls, β cells reduce insulin secretion — this is the negative feedback completing the loop.
P:
Normal blood glucose is approximately 4–6 mmol/L. The system is maintained by two opposing mechanisms: insulin corrects rises, glucagon corrects falls — giving greater precision than a single corrective mechanism.
Summary
Blood Glucose Regulation — At a Glance
Blood glucoseCell typeHormoneKey responses
↑ (too high)β cellsInsulinGLUT4 insertion; glycogenesis in liver & muscle
↓ (too low)α cellsGlucagonGlycogenolysis; gluconeogenesis in liver
Why control is needed
1. Respiratory substrate availability (brain depends on glucose) 2. Water potential of blood
Processes
Glycogenesis (insulin) → glycogen storage. Glycogenolysis (glucagon) → glycogen breakdown. Gluconeogenesis (glucagon) → new glucose from glycerol/amino acids.
Exam Tip
β cells → insulin → lowers glucose. α cells → glucagon → raises glucose. Insulin inserts GLUT4 channel proteins — not just "opens channels." Glucagon acts on liver cells — not muscle.
Next: What is Homeostasis?