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 blood glucose regulation and the causes and consequences of diabetes mellitus as required by the Edexcel A-Level Biology specification (9BI0), Topic 9 -- Control Systems. You need to understand the roles of insulin and glucagon, the cellular mechanisms involved, and the differences between Type 1 and Type 2 diabetes.
Blood glucose concentration must be maintained within a narrow range (approximately 4-6 mmol/L when fasting, rising after a meal):
| Condition | Blood Glucose Level | Consequences |
|---|---|---|
| Hyperglycaemia (too high) | >7 mmol/L (fasting) | Increases blood osmolarity → water leaves cells by osmosis → dehydration; long-term damage to blood vessels, nerves, kidneys, retina |
| Hypoglycaemia (too low) | <3.5 mmol/L | Brain cells cannot function (glucose is their primary energy source) → confusion, seizures, coma, death |
Glucose is the primary respiratory substrate for most cells, and the brain relies almost exclusively on glucose for energy. Precise regulation ensures a constant supply to cells while preventing the damaging effects of excess glucose.
The islets of Langerhans in the pancreas contain two key cell types:
| Cell Type | Hormone Produced | Stimulus for Secretion | Effect |
|---|---|---|---|
| Beta (β) cells | Insulin | High blood glucose concentration | Lowers blood glucose |
| Alpha (α) cells | Glucagon | Low blood glucose concentration | Raises blood glucose |
The beta and alpha cells act as both receptors (detecting blood glucose changes) and effectors (secreting hormones).
Exam Tip: Remember alpha cells produce glucagon (both start with letters near the beginning of the alphabet: A and G). Beta cells produce insulin (B and I come after A and G). This mnemonic helps in exams.
When blood glucose concentration rises (e.g. after a meal):
| Response | Tissue | Mechanism |
|---|---|---|
| Increased glucose uptake | Muscle and adipose | Insulin stimulates translocation of GLUT4 glucose transporters from vesicles to the cell membrane, increasing glucose entry |
| Glycogenesis | Liver and muscle | Insulin activates the enzyme glycogen synthase, which catalyses the conversion of glucose → glycogen (storage) |
| Increased glycolysis | Most cells | Insulin stimulates enzymes of glycolysis, increasing the rate at which glucose is broken down in respiration |
| Lipogenesis | Liver and adipose | Excess glucose is converted to fatty acids and glycerol, then stored as triglycerides |
| Increased protein synthesis | Muscle | Insulin stimulates amino acid uptake and protein synthesis |
The net effect is a decrease in blood glucose concentration back towards the set point.
When blood glucose concentration falls (e.g. between meals or during exercise):
| Response | Tissue | Mechanism |
|---|---|---|
| Glycogenolysis | Liver | Glucagon activates glycogen phosphorylase, which catalyses the hydrolysis of glycogen → glucose |
| Gluconeogenesis | Liver | Glucagon stimulates the conversion of amino acids, glycerol, and lactate into glucose |
| Inhibition of glycogenesis | Liver | Glucagon inactivates glycogen synthase |
The net effect is an increase in blood glucose concentration back towards the set point.
Exam Tip: Glucagon primarily acts on the liver (not muscle). Muscle glycogen cannot be converted back to blood glucose because muscle cells lack the enzyme glucose-6-phosphatase. Liver glycogen is the main store used to maintain blood glucose.
Blood glucose regulation is a classic example of negative feedback with antagonistic hormones:
Blood glucose rises → Beta cells secrete insulin → Glucose uptake and glycogenesis → Blood glucose falls back to set point
Blood glucose falls → Alpha cells secrete glucagon → Glycogenolysis and gluconeogenesis → Blood glucose rises back to set point
The two hormones act in opposition, providing fine and continuous control. As one increases, the other decreases -- they do not simply switch on and off.
The following diagram illustrates negative feedback in blood glucose regulation:
graph TD
A["Blood Glucose<br/>RISES"] -->|"Detected by<br/>β cells"| B["Insulin Released"]
B --> C["Glucose uptake<br/>Glycogenesis"]
C --> D["Blood Glucose<br/>Returns to Normal"]
E["Blood Glucose<br/>FALLS"] -->|"Detected by<br/>α cells"| F["Glucagon Released"]
F --> G["Glycogenolysis<br/>Gluconeogenesis"]
G --> D
Adrenaline, released from the adrenal medulla during stress or exercise, also raises blood glucose:
Diabetes mellitus is a condition in which blood glucose concentration is chronically elevated (hyperglycaemia) because the body cannot regulate it effectively.
| Feature | Detail |
|---|---|
| Cause | Autoimmune destruction of beta cells in the islets of Langerhans |
| Age of onset | Usually childhood or adolescence |
| Insulin production | Little or none |
| Treatment | Regular insulin injections (or insulin pump); careful monitoring of blood glucose; carbohydrate counting |
| Proportion of diabetics | ~10% |
In Type 1 diabetes, the immune system (specifically T lymphocytes) attacks and destroys the beta cells. Without beta cells, the body cannot produce insulin.
| Feature | Detail |
|---|---|
| Cause | Insulin resistance -- target cells become less responsive to insulin; may also involve reduced insulin secretion over time |
| Risk factors | Obesity (especially visceral fat), sedentary lifestyle, age (>40), family history, ethnicity |
| Insulin production | Initially normal or high (beta cells compensate); declines over time |
| Treatment | Lifestyle changes (diet, exercise, weight loss); oral medications (e.g. metformin); insulin in advanced cases |
| Proportion of diabetics | ~90% |
In Type 2 diabetes, insulin is produced but the target cells have fewer insulin receptors or the receptors have a reduced response to insulin. This means the cells do not take up glucose efficiently even when insulin is present.
Exam Tip: The key distinction: Type 1 = no insulin produced (autoimmune destruction of beta cells). Type 2 = insulin is produced but target cells are resistant (receptors don't respond properly). In extended-response questions, explain the mechanism, not just the name.
| Feature | Type 1 | Type 2 |
|---|---|---|
| Insulin production | None (beta cells destroyed) | Produced but ineffective (resistance) |
| Onset | Rapid (weeks) | Gradual (months to years) |
| Age | Typically young | Typically older (but increasingly in young people) |
| Body type | Often normal weight | Often overweight/obese |
| Treatment | Insulin injections essential | Lifestyle changes; oral drugs; insulin if needed |
| Genetic link | Some genetic predisposition | Strong genetic component |
| Autoimmune | Yes | No |
Chronic hyperglycaemia causes damage throughout the body:
| Complication | Mechanism |
|---|---|
| Cardiovascular disease | Excess glucose damages blood vessel walls; accelerates atherosclerosis |
| Peripheral neuropathy | Damage to peripheral nerves; numbness, tingling, pain |
| Diabetic retinopathy | Damage to capillaries in the retina; can cause blindness |
| Nephropathy | Damage to kidney glomeruli; reduced filtration; can lead to kidney failure |
| Poor wound healing | Reduced blood supply to tissues; increased risk of infection |
| Method | Description |
|---|---|
| Blood glucose monitoring | Finger-prick test using a glucometer; measures current blood glucose |
| HbA1c test | Measures glycosylated haemoglobin; indicates average blood glucose over the past 2-3 months |
| Insulin injections | Subcutaneous injection of recombinant human insulin (genetically engineered); required for Type 1 |
| Insulin pump | Continuous subcutaneous delivery of insulin via a small device |
| Oral medications | Metformin (reduces glucose output from liver; increases insulin sensitivity); for Type 2 |
This lesson sits in Edexcel 9BI0 Topic 8 — Grey Matter (Coordination, Response and Gene Technology) and is the canonical worked example of homeostatic control by antagonistic hormones. The content statements paraphrase to: explain how blood glucose concentration is maintained near a set point (~5 mmol L⁻¹) by the opposing actions of insulin (β-cells of pancreatic islets, lowers glucose) and glucagon (α-cells, raises glucose); describe the cellular mechanism of insulin secretion (GLUT2 + glucokinase as glucose sensors → ATP-sensitive K⁺ channel closure → β-cell depolarisation → voltage-gated Ca²⁺ channel opening → vesicular insulin release); describe insulin action on muscle and adipose (insulin receptor tyrosine kinase → signalling cascade → GLUT4 translocation to plasma membrane → glucose uptake) and on liver (glycogen synthesis, suppression of gluconeogenesis); describe glucagon action (cAMP-mediated activation of glycogen phosphorylase → glycogenolysis; activation of gluconeogenesis); and contrast Type 1 diabetes (autoimmune β-cell destruction, absolute insulin deficiency) with Type 2 diabetes (insulin resistance with relative insulin deficiency, often with obesity) — 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 deeply synoptic: the loop architecture comes from Lesson 6 (homeostasis principles); insulin and glucagon are peptide hormones (Lesson 1) acting through cell-surface receptors; glucose entering cells fuels glycolysis (Topic 5); chronic hyperglycaemia damages the capillary endothelium (Topic 7) producing diabetic retinopathy, nephropathy and neuropathy.
Question (8 marks): A healthy adult eats a carbohydrate-rich meal. Blood glucose concentration rises from 5 mmol L⁻¹ to 8 mmol L⁻¹ within 30 minutes.
(a) Describe the cellular mechanism by which pancreatic β-cells sense the rise in glucose and secrete insulin. (4)
(b) Explain how insulin acting on skeletal muscle lowers blood glucose, naming the key transporter and the molecular event by which it reaches the cell surface. (4)
Solution with mark scheme:
(a) Step 1 — glucose enters the β-cell. Glucose is absorbed at the ileum, drains through the hepatic portal vein, and enters the systemic circulation. Pancreatic β-cells of the islets of Langerhans express GLUT2, a high-Km, low-affinity glucose transporter that allows intracellular glucose concentration to track plasma glucose closely. Inside the β-cell, glucokinase (hexokinase IV) phosphorylates glucose to glucose-6-phosphate; glucokinase has a high Km (~10 mmol L⁻¹) so its rate scales with glucose over the physiological range — it is the true molecular glucose sensor.
M1 (AO1) — names GLUT2 and glucokinase as the sensor pair. "Glucose enters the cell" without naming the transporter does not score the M1.
Step 2 — ATP rises, K_ATP closes. Glucose-6-phosphate enters glycolysis and ultimately oxidative phosphorylation, raising the cytoplasmic ATP:ADP ratio. ATP-sensitive K⁺ channels (K_ATP) in the β-cell membrane close in response to rising ATP.
M1 (AO1) — links rising ATP to K_ATP closure.
Step 3 — depolarisation and Ca²⁺ entry. With K⁺ no longer leaving the cell, the membrane depolarises. Depolarisation opens voltage-gated Ca²⁺ channels; Ca²⁺ flows into the cytoplasm down its electrochemical gradient.
A1 (AO2) — explicit identification of voltage-gated Ca²⁺ entry as the trigger.
Step 4 — exocytosis. The Ca²⁺ rise triggers fusion of insulin-containing secretory vesicles with the plasma membrane. Insulin is released into the islet capillary network and reaches systemic circulation via the portal venous drainage of the pancreas.
A1 (AO2) — names Ca²⁺-driven exocytosis explicitly.
(b) Step 1 — receptor binding. Insulin (a 51-residue peptide of two chains, A and B, linked by disulfide bonds) binds the insulin receptor on the muscle cell surface. The receptor is a receptor tyrosine kinase: ligand binding triggers receptor autophosphorylation on tyrosine residues.
M1 (AO1) — names the insulin receptor as a tyrosine kinase.
Step 2 — signalling cascade. Phosphorylated receptor recruits insulin receptor substrate (IRS) proteins, activating the PI3K → Akt signalling axis.
M1 (AO1) — names a downstream component (PI3K, Akt, IRS — any one).
Step 3 — GLUT4 translocation. GLUT4 glucose transporters sit constitutively in cytoplasmic vesicles in resting muscle cells. Akt activation drives the translocation of GLUT4-containing vesicles to the plasma membrane, where they fuse and insert GLUT4 into the membrane.
A1 (AO2) — explicit identification of GLUT4 translocation as the immediate insulin-driven event. "Insulin lets glucose into cells" does not score; the GLUT4 translocation step is the examinable mechanism.
Step 4 — glucose uptake and disposal. With GLUT4 now in the membrane, glucose flows into the muscle cell down its concentration gradient by facilitated diffusion. Inside the cell, glucose is phosphorylated by hexokinase, trapping it; it is then either oxidised through glycolysis or stored as glycogen through glycogen synthase activity (also stimulated by insulin via the same Akt axis).
A1 (AO2) — links GLUT4-mediated entry to disposal (glycolysis or glycogen synthesis).
Total: 8 marks (4 + 4).
Question (6 marks): Compare and contrast the cellular mechanisms underlying Type 1 and Type 2 diabetes mellitus. Refer to the role of pancreatic β-cells, the action of insulin at target tissues, and why HbA1c is elevated in both.
Mark scheme decomposition by AO:
| Mark | AO | Awarded for |
|---|---|---|
| 1 | AO1 | Naming Type 1 diabetes as autoimmune destruction of pancreatic β-cells, leading to absolute insulin deficiency — typically childhood/adolescent onset; no endogenous insulin secreted. |
| 2 | AO1 | Naming Type 2 diabetes as insulin resistance at target tissues (muscle, fat, liver) accompanied by relative insulin deficiency as β-cells eventually fail to compensate; strongly associated with obesity and adult onset. |
| 3 | AO2 | Linking T1DM mechanism to clinical consequence: with no insulin, GLUT4 fails to translocate in muscle and fat, hepatic glycogenolysis and gluconeogenesis are unrestrained, and plasma glucose rises uncontrolled — patients require exogenous insulin for survival. |
| 4 | AO2 | Linking T2DM mechanism to clinical consequence: insulin is present (often elevated initially), but post-receptor signalling defects blunt GLUT4 translocation and Akt activation; β-cells initially hyper-secrete to compensate but eventually fail; lifestyle modification, metformin and (latterly) GLP-1 receptor agonists are first-line therapy before insulin is required. |
| 5 | AO2 | Explaining HbA1c: glucose binds non-enzymatically to N-terminal valines of haemoglobin β-chains; the fraction of haemoglobin so glycated reflects average glucose over ~3 months (the lifespan of an erythrocyte). Both T1DM and T2DM elevate HbA1c because both result in chronic hyperglycaemia, even though the underlying mechanisms differ. |
| 6 | AO3 | Synthesis — both conditions converge on the same downstream lesion (failure of insulin's effect at target tissues) but arrive there from opposite directions: T1DM removes the signal; T2DM disables the receiver. The clinical phenotypes overlap (chronic hyperglycaemia, vascular complications) because the output of the loop has failed; the architectural failure is at different points. |
Total: 6 marks (AO1 = 2, AO2 = 3, AO3 = 1). AO3 is reserved for the convergent-failure synthesis, not for restating the mechanisms.
Connects to:
Blood glucose 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.