Glucose Regulation — Insulin, Glucagon and the Pancreatic Feedback System

Q1 — Sample Band 6 response (8 marks), annotated

Blood glucose homeostasis is the physiological process that maintains blood glucose concentration within the normal tolerance range of approximately 4–6 mmol/L. This set point represents the balance between glucose entering the blood (from digestion or liver release) and glucose leaving it (into cells or liver storage). [1 — definition + set point]

In the normal insulin pathway: stimulus = blood glucose rises above ~6 mmol/L after a meal. Receptor / control centre = beta cells in the islets of Langerhans within the pancreas directly detect the elevated glucose and secrete insulin into the bloodstream. Effector = the liver: insulin signals glycogenesis (conversion of glucose to glycogen for storage). Body cells also respond by mobilising GLUT4 transporters, increasing glucose uptake. Response = blood glucose falls back toward ~5 mmol/L. As glucose normalises, beta cells reduce insulin secretion — a self-limiting negative feedback. [2 — full pathway with all components named and liver processes named]

In Type 2 diabetes, the pathway failure occurs at Step 3 — the effector response. Insulin is produced and secreted normally by beta cells (the signalling step is intact), but the target cells (liver, muscle, adipose) exhibit insulin resistance — their insulin receptors do not respond adequately. GLUT4 mobilisation is diminished and glycogenesis is impaired, so blood glucose removal is insufficient despite the insulin signal being present. The stimulus triggers the hormone (correct), but the hormone does not produce the required corrective response (incorrect effector response). The result is that blood glucose remains chronically elevated: chronic hyperglycaemia. [2 — identifies correct step of failure, explains insulin resistance mechanism, explains why hyperglycaemia still results]

All three named complications follow from a single homeostatic failure: chronic hyperglycaemia. When blood glucose chronically exceeds ~7 mmol/L, glucose molecules attach non-enzymatically to proteins throughout the body (glycation). In retinal capillaries, glycated proteins in vessel walls cause thickening and narrowing → retinopathy and progressive blindness. In glomerular capillaries of the kidney, the same thickening reduces filtration function → nephropathy and eventual kidney failure. In peripheral nerves, damage to the microvasculature supplying nerve cells reduces oxygen delivery → neuropathy and loss of sensation in the feet. All three share the same mechanism (glycation → vascular damage) because they all require intact small blood vessels, and chronic hyperglycaemia damages all small vessels via the same biochemical process. [2 — glycation defined, all three complications correctly linked to same mechanism]

The stimulus notes compensatory hyperinsulinaemia: beta cells overproduce insulin in an attempt to overcome cellular insulin resistance. This compensates temporarily but fails for two reasons: (a) the insulin resistance tends to worsen as adipose-related inflammatory signals continue to impair receptor function, requiring even more insulin to achieve the same glucose uptake; (b) chronically overworked beta cells eventually exhaust and their number declines (beta cell apoptosis), reducing insulin output. Once beta cell function is significantly impaired, the Type 2 patient’s ability to even produce insulin diminishes — at this stage, exogenous insulin is required, just as in Type 1. This implies that early intervention (diet, exercise, metformin) to reduce insulin resistance is critical before beta cell exhaustion occurs. [1 — explains why compensatory hyperinsulinaemia fails and treatment implication]

Marking criteria.

• 1 mark — Defines blood glucose homeostasis and identifies the set point (~4–6 mmol/L or ~5 mmol/L).
• 2 marks — Describes the full normal insulin pathway: stimulus (high BG), receptor (beta cells + islets of Langerhans), hormone (insulin), effector (liver: glycogenesis named; body cells: GLUT4 uptake), response (BG falls), negative feedback (self-limiting).
• 2 marks — Identifies the correct step of failure in T2D (Step 3: effector response), explains insulin resistance as the mechanism, and explains why chronic hyperglycaemia still results despite insulin being present.
• 2 marks — Defines glycation and correctly links retinopathy, nephropathy, and peripheral neuropathy to the same mechanism (chronic hyperglycaemia → glycation → vascular damage in target tissue).
• 1 mark — Evaluates why compensatory hyperinsulinaemia fails (worsening insulin resistance + beta cell exhaustion) and states the implication (early intervention; eventual need for exogenous insulin).

Q2 — Sample Band 6 source critique (7 marks), annotated

The passage contains four biological errors and a dangerously flawed therapeutic proposal. [1 — overall evaluative judgement]

Error 1: “Glucagon breaks down blood glucose.” This is incorrect. Glucagon does not act on blood glucose directly. It signals the liver to perform glycogenolysis — the breakdown of stored glycogen (a polymer) into glucose monomers. The end result is that more glucose enters the blood, but glucagon acts on glycogen in liver cells, not on blood glucose itself. [1 — correct biology: glucagon → glycogenolysis; glycogen not blood glucose]

Error 2: “Glucagon causes hypoglycaemia.” This is the opposite of the truth. Glucagon raises blood glucose by triggering glycogenolysis in the liver. It is one of the primary counter-regulatory hormones against hypoglycaemia; it is actually used clinically to treat severe hypoglycaemia. Insulin lowers blood glucose; glucagon raises it. [1 — correct direction of glucagon’s effect]

Error 3: The proposed intervention (permanent glucagon receptor blockade). Permanently blocking liver glucagon receptors would prevent the liver from performing glycogenolysis in response to low blood glucose. In fasting, exercise, or any situation where glucose intake is reduced, the body could not mobilise its liver glycogen stores. Blood glucose would fall without correction, leading to severe, potentially fatal hypoglycaemia. The brain has almost no glycogen stores and depends critically on a continuous blood glucose supply; without the glucagon → liver → glycogenolysis pathway, even overnight fasting could cause loss of consciousness. [2 — identifies that blockade would prevent glycogenolysis, explains that fasting hypoglycaemia would be inevitable and dangerous, links to brain glucose dependence]

Error 4: “Insulin does everything; glucagon is redundant.” This ignores the fundamental advantage of the two-hormone push-pull system. Insulin alone could correct high blood glucose, but could not respond to low blood glucose — that is glucagon’s function. Without glucagon, the body has no active mechanism for raising blood glucose when it falls. The two hormones maintain tighter oscillation around the set point because they operate in opposing directions simultaneously. [1 — explains the necessity of the two-hormone system for both sides of the homeostatic response]

Defensible reformulation: Glucagon is an essential counter-regulatory hormone that raises blood glucose by signalling the liver to perform glycogenolysis. It is not redundant; it corrects hypoglycaemia and prevents dangerous blood glucose crashes during fasting and exercise. In Type 2 diabetes, the primary problem is insulin resistance in target cells, not glucagon excess. A permanent glucagon receptor blockade would eliminate the body’s only rapid endogenous mechanism for raising blood glucose, causing severe and potentially fatal hypoglycaemia. [1 — biologically defensible reformulation]

Marking criteria.

• 1 mark — States an overall evaluative judgement (e.g. “the passage contains multiple biological errors and a dangerous therapeutic proposal”).
• 1 mark — Correctly identifies that glucagon acts on glycogen (not blood glucose) via glycogenolysis, and raises (not lowers) blood glucose.
• 1 mark — Correctly identifies the direction of glucagon’s effect (raises blood glucose; is used to treat hypoglycaemia).
• 2 marks — Evaluates the therapeutic proposal: (a) permanent blockade would prevent glycogenolysis during fasting/exercise; (b) this would cause severe hypoglycaemia because blood glucose cannot be raised, endangering the brain.
• 1 mark — Explains why the two-hormone system is not redundant (glucagon corrects low blood glucose; insulin alone cannot raise BG; push-pull provides tighter homeostatic control).
• 1 mark — Provides a biologically defensible reformulation of the claim that correctly states glucagon’s true role and correctly identifies why the proposed intervention is dangerous.