HSC Exam Practice

Sample response. Blood glucose homeostasis is the physiological process that maintains blood glucose concentration within a normal tolerance range, ensuring adequate glucose supply to cells (especially neurons) while preventing the vascular damage caused by excess glucose. The normal set point range is approximately 4–6 mmol/L.

Marking notes. 1 mark for definition (maintenance of blood glucose within a normal range via corrective mechanisms); 1 mark for stating the set point (4–6 mmol/L or equivalent, e.g. “around 5 mmol/L”). Accept “4–8 mmol/L” only if qualified as “post-meal upper limit”.

Sample response. Alpha cells detect falling blood glucose (below ~4 mmol/L) and secrete glucagon into the bloodstream. Beta cells detect rising blood glucose (above ~6 mmol/L) and secrete insulin into the bloodstream. Both cell types are located within the islets of Langerhans in the pancreas. They act in opposition: insulin lowers blood glucose and glucagon raises it.

Marking notes. 1 mark for alpha cells secreting glucagon when blood glucose is low; 1 mark for beta cells secreting insulin when blood glucose is high; 1 mark for correctly locating both in the islets of Langerhans. No mark for reversing alpha/beta cell roles.

Sample response. The liver is the primary effector organ. In response to insulin (high blood glucose): glycogenesis — the liver converts excess glucose into glycogen for storage, removing it from the bloodstream. In response to glucagon (low blood glucose): glycogenolysis — the liver breaks down stored glycogen into glucose and releases it into the blood, raising blood glucose.

Marking notes. 1 mark for identifying the liver as the primary effector; 1 mark for glycogenesis (glucose → glycogen, insulin-triggered); 1 mark for glycogenolysis (glycogen → glucose, glucagon-triggered). Both processes must be named, not merely described.

Sample response. A single corrective hormone would only be able to push blood glucose in one direction. If the hormone lowered blood glucose, there would be no active mechanism to raise it when glucose fell (and vice versa). Two opposing hormones allow simultaneous push-pull control — as insulin drives glucose down, glucagon is suppressed; as glucagon drives glucose up, insulin is suppressed. This push-pull system produces faster, more precise fine-tuning, maintaining tighter oscillation around the set point than a single bidirectional response could achieve. The two-hormone system also allows the raising and lowering pathways to be independently regulated in response to different physiological demands.

Marking notes. 1 mark for explaining that a single hormone could only correct in one direction; 1 mark for explaining that two opposing hormones provide push-pull control in both directions; 1 mark for explaining that this produces faster/tighter control around the set point (less overshoot/undershoot). Accept analogy to cruise control throttle + brake.

Sample response. Type 1 diabetes: the pathway component that fails is the receptor/secretory step (Step 2). Autoimmune destruction eliminates functional beta cells from the islets of Langerhans, so no insulin can be produced. The feedback loop has no signal. Type 2 diabetes: the pathway component that fails is the effector response (Step 3). Insulin is produced normally by beta cells, but target cells (liver, muscle, adipose) are resistant to the insulin signal — insulin receptor sensitivity is reduced. The feedback loop sends a signal, but the effector does not respond adequately. Both produce chronic hyperglycaemia because in both cases the corrective feedback loop fails to return blood glucose to the set point.

Marking notes. 1 mark for Type 1: identifies beta cell destruction as the mechanism; 1 mark for Type 1: states that no insulin is produced (not just “less insulin”); 1 mark for Type 2: identifies insulin resistance in target cells as the mechanism; 1 mark for Type 2: correctly states that insulin is present but cells do not respond (not that insulin is absent). Deduct if student reverses the two conditions.

Sample response. All three complications arise from the same mechanism: chronic hyperglycaemia causes glucose to attach non-enzymatically to proteins in blood vessel walls throughout the body — a process called glycation. Glycated proteins in capillary walls thicken and lose elasticity, progressively narrowing the lumen and reducing blood flow to target tissues. In retinal capillaries this causes retinopathy; in glomerular capillaries of the kidney it causes nephropathy; in the microvasculature supplying peripheral nerves it causes neuropathy. All three share the same root cause (chronic blood glucose above ~7 mmol/L → glycation) and the same mechanism (vascular damage in small vessels), affecting different tissues because those tissues are all supplied by microvasculature.

Marking notes. 1 mark for identifying glycation as the shared mechanism (glucose attaching to proteins in blood vessel walls); 1 mark for explaining that glycation causes vascular damage (thickening/narrowing of capillary walls); 1 mark for linking the shared vascular mechanism to all three complications (each requires microvasculature; chronic hyperglycaemia damages all small vessels equally). Must use the term “glycation” for full marks.

Sample response (a). Plasma insulin starts at approximately 60 pmol/L at t = 0, rises sharply to a peak of approximately 420 pmol/L at t = 60 min, then falls progressively over the remainder of the test to approximately 70 pmol/L at t = 180 min. Plasma glucagon shows the opposite pattern: it starts at approximately 80 ng/L at t = 0, falls to a minimum of approximately 45 ng/L at t = 60 min, then partially recovers to approximately 65 ng/L by t = 180 min. The two curves are roughly mirror images, with insulin peaking when glucagon is lowest.

Marking notes (a) — 3 marks. 1 mark for describing the insulin trend with at least one specific value (e.g. peak of 420 pmol/L at ~60 min); 1 mark for describing the glucagon trend with at least one specific value (e.g. falls from 80 to ~45 ng/L); 1 mark for identifying the opposing (mirror-image) relationship between the two curves. Accept approximate values (±10%).

Sample response (b). After glucose is absorbed, blood glucose rises, triggering beta cells (which directly detect elevated blood glucose) to secrete insulin. Simultaneously, elevated blood glucose inhibits alpha cells, so glucagon secretion decreases. Insulin rises and glucagon falls — the opposing pattern. At t = 60 min, when insulin is at its peak, the liver is performing glycogenesis at maximum rate: converting the absorbed glucose into glycogen for storage, which progressively lowers blood glucose. As blood glucose returns toward the set point, beta cell insulin secretion decreases and alpha cell glucagon secretion partially recovers, explaining the recovery of glucagon seen between t = 60 and t = 180 min.

Marking notes (b) — 4 marks. 1 mark for explaining that rising blood glucose triggers beta cells to secrete insulin while simultaneously suppressing alpha cells and glucagon; 1 mark for explaining the opposing pattern as simultaneous push-pull from the two cell types; 1 mark for naming glycogenesis as the liver process occurring at t = 60 min; 1 mark for explaining why glucagon recovers after t = 60 min (blood glucose falling toward set point → reduced beta cell stimulus → insulin falls → alpha cells recover).

Sample response (c). In a Type 1 diabetic without insulin injection, the insulin curve would remain near zero or very low throughout (beta cells are destroyed and cannot produce insulin). As a result, blood glucose would not be corrected after the glucose load. Glucagon would not fall as expected (because the suppression signal from high insulin is absent), and may remain elevated or even rise further, as uncorrected hyperglycaemia could stimulate alpha cells. Blood glucose would continue to rise well beyond the normal post-meal peak and remain elevated far beyond 180 min, as no homeostatic mechanism is intact to remove glucose from the blood.

Marking notes (c) — 2 marks. 1 mark for correctly predicting insulin would remain negligible/absent throughout (beta cells destroyed); 1 mark for predicting glucagon would not fall as normal (or would remain elevated) and explaining why (no insulin to suppress alpha cells; or hyperglycaemia stimulus without correction). Accept alternative wording that correctly links the absence of insulin to continued hyperglycaemia.

Sample response. Negative feedback is a corrective mechanism in which the response to a change directly opposes the original stimulus, returning a regulated variable toward its set point. The glucose homeostasis system is a clear example because two opposing negative feedback loops continually oscillate blood glucose around a set point of approximately 5 mmol/L.

High blood glucose scenario: stimulus = blood glucose exceeds ~6 mmol/L (e.g. after a meal). Receptor / control centre = beta cells in the islets of Langerhans within the pancreas directly detect the elevated blood glucose and secrete insulin into the bloodstream. Effector = the liver (primary) and body cells: insulin signals the liver to perform glycogenesis (glucose → glycogen storage) and signals muscle and adipose cells to mobilise GLUT4 glucose transporters, increasing glucose uptake. Response = blood glucose falls toward ~5 mmol/L. Negative feedback is complete because as glucose normalises, beta cells reduce insulin secretion — the response is self-limiting.

Low blood glucose scenario: stimulus = blood glucose falls below ~4 mmol/L (e.g. during exercise or fasting). Receptor / control centre = alpha cells in the islets of Langerhans detect the low glucose and secrete glucagon into the bloodstream. Effector = the liver: glucagon signals glycogenolysis (glycogen → glucose), releasing stored glucose into the blood. Response = blood glucose rises toward ~5 mmol/L. As glucose normalises, alpha cells reduce glucagon secretion — again self-limiting negative feedback.

The liver is the key effector in both pathways because it is the organ that physically changes blood glucose concentration. It can act as a glucose sink (glycogenesis when insulin is high) or a glucose source (glycogenolysis when glucagon is high). The liver stores approximately 100 g of glycogen — enough for ~12–16 hours of fasting. Without a functioning liver, neither insulin nor glucagon can maintain blood glucose homeostasis regardless of how much hormone is present.

The claim that the pancreas is the most important organ is only partially defensible. The pancreatic islets are indispensable as the receptor and signalling gland: they detect blood glucose changes with precision and secrete the corrective hormones that drive the entire system. Without the pancreas, there are no hormonal signals. However, without the liver as a functional effector, those hormonal signals produce no change in blood glucose. The liver is the structure that actually executes the homeostatic correction. A more defensible claim is that the pancreas and liver are jointly essential: the pancreas provides the detection and signal, the liver provides the effector response. Neither alone can maintain glucose homeostasis.

Marking notes.

• 1 mark — Defines negative feedback correctly (response opposes the original stimulus, returning variable to set point) and identifies blood glucose as the regulated variable.
• 2 marks — Describes the complete high-blood-glucose pathway: stimulus (elevated BG after meal), receptor (beta cells, islets of Langerhans), hormone (insulin), effector (liver + glycogenesis named; body cells + GLUT4 named), response (BG falls), self-limiting feedback.
• 2 marks — Describes the complete low-blood-glucose pathway: stimulus (low BG during exercise/fasting), receptor (alpha cells), hormone (glucagon), effector (liver + glycogenolysis named), response (BG rises), self-limiting feedback.
• 1 mark — Explains why the liver is the key effector (it physically changes blood glucose by performing glycogenesis and glycogenolysis; pancreas only signals; without liver, hormones produce no effect).
• 1 mark — Evaluates the claim about the pancreas: concedes the pancreas is essential as receptor and signal source; rejects it as “most important” by arguing the liver is the executory effector; reaches a justified conclusion that both organs are jointly required for blood glucose homeostasis.