Biology • Year 12 • Module 8 • Lesson 3
Glucose Regulation — Insulin, Glucagon and the Pancreatic Feedback System
Apply the glucose homeostasis pathway to real CGM data, a pathway sequence task, a Type 1 vs Type 2 comparison, and a clinical scenario.
1. Interpret continuous glucose monitor (CGM) data — post-meal and exercise response
The graph below shows interstitial glucose concentration recorded by a CGM worn by a healthy adult during a 4-hour window. The participant ate a standardised breakfast (75 g carbohydrate) at t = 0 and performed 30 minutes of moderate cycling starting at t = 90 min. Data are modelled on published CGM reference profiles (Ref: Klonoff et al., 2017, Journal of Diabetes Science and Technology). 8 marks
Figure 1.1. CGM trace for a healthy adult after a 75 g carbohydrate breakfast at t = 0 and 30 min moderate cycling starting at t = 90 min. Modelled on Klonoff et al. (2017), J. Diabetes Sci. Technol. 11(6): 1076–1085. Green band = normal range (4–6 mmol/L).
1.1 Describe the trend in blood glucose from t = 0 to t = 90 min. Identify the approximate peak value and the time at which it occurred. 2 marks
1.2 Using lesson content, explain the homeostatic mechanism responsible for the fall in blood glucose from its peak at t = 45 min back to 5.4 mmol/L at t = 90 min. Name the stimulus, receptor, hormone, effector organ, and liver process. 3 marks
1.3 During the cycling bout (t = 90–120 min), blood glucose fell to 4.1 mmol/L. (a) Name the hormone that responds to this fall and the organ it acts upon. (b) Explain, with a named process, how blood glucose is restored to 5.2 mmol/L by t = 150 min without the participant eating anything. 3 marks
2. Sequence the steps — insulin negative feedback pathway
The eight events below describe the response to a high blood glucose reading, but they have been shuffled. Write the correct order (1–8) in the “Order” column. 8 marks
| Order | Shuffled event |
|---|---|
| Blood glucose concentration falls back toward ~5 mmol/L as glucose is removed from circulation. | |
| A large meal is consumed; digested carbohydrate is absorbed into the portal blood, raising blood glucose to 9.1 mmol/L. | |
| As blood glucose normalises, beta cells detect the lower concentration and reduce insulin secretion — the response is self-limiting. | |
| Insulin signals the liver to perform glycogenesis — converting excess blood glucose into glycogen stored within liver cells. | |
| Insulin is secreted by beta cells into the bloodstream and transported to target tissues. | |
| Body cells (muscle, adipose) mobilise GLUT4 glucose transporters to their plasma membranes, increasing glucose uptake from the blood. | |
| Beta cells in the islets of Langerhans detect the elevated blood glucose concentration directly. | |
| Blood glucose stabilises near the set point; no further homeostatic correction is required. |
3. Compare Type 1 and Type 2 diabetes
Complete the comparison table below. Use precise biological terminology. 8 marks (1 per cell)
| Feature | Type 1 Diabetes | Type 2 Diabetes |
|---|---|---|
| Primary cause at the cellular level | ||
| Insulin level in blood | ||
| Which step of the glucose homeostasis pathway fails | ||
| Common management approach |
4. Case study — CGM in a closed-loop insulin pump system
Australia has one of the highest rates of Type 1 diabetes in the world, with approximately 130,000 Australians living with the condition (Diabetes Australia, 2023). In 2023, the Therapeutic Goods Administration (TGA) approved the Omnipod 5 — a hybrid closed-loop insulin delivery system in which a CGM sensor automatically communicates with a wearable insulin pump. When the CGM detects a rising blood glucose trend, the pump delivers a calibrated insulin bolus without any user input. When glucose falls below a threshold, the pump suspends insulin delivery. 6 marks
4.1 Using the stimulus-response model from lesson Cards 1–3, identify which component of the normal pancreatic glucose homeostasis system each part of the Omnipod 5 system replaces: (a) the CGM sensor, (b) the algorithm that processes the reading and determines whether to dose, (c) the insulin pump itself. 3 marks
4.2 The system can suspend insulin delivery when glucose falls below 4.4 mmol/L. Explain which hormone in the normal (non-diabetic) system would ordinarily respond to a glucose drop to this level, and why a Type 1 diabetic still needs dietary carbohydrate (rather than relying on the pump’s suspend function alone) to recover from a hypoglycaemic episode. 3 marks
Q1.1 — Trend description (2 marks)
Blood glucose rises from 5.1 mmol/L at t = 0 to a peak of 8.9 mmol/L at approximately t = 45 min [1], then falls progressively back to 5.4 mmol/L by t = 90 min [1].
Q1.2 — Insulin pathway (3 marks)
Stimulus: blood glucose rising to 8.9 mmol/L, above the ~6 mmol/L upper limit of the normal range [1]. Receptor: beta cells in the islets of Langerhans detect the elevated concentration and secrete insulin into the bloodstream [1]. Effector organ: the liver; insulin signals glycogenesis — conversion of excess glucose into glycogen for storage. Body cells also increase glucose uptake via GLUT4. Together these remove glucose from the blood, returning it to 5.4 mmol/L [1]. (1 mark for naming stimulus + receptor; 1 for naming insulin; 1 for liver + glycogenesis + response.)
Q1.3 — Exercise and glucagon pathway (3 marks)
(a) The hormone that responds is glucagon, secreted by alpha cells in the islets of Langerhans; it acts primarily on the liver [1]. (b) Glucagon signals the liver to perform glycogenolysis — breaking down stored glycogen into glucose and releasing it into the bloodstream. This liver-derived glucose replaces the glucose consumed by exercising muscles, raising blood glucose from 4.1 mmol/L back to 5.2 mmol/L without any dietary intake [2]. (Award 1 mark for glucagon + alpha cells + liver; 1 mark for glycogenolysis correctly defined; 1 mark for linking to the rise in blood glucose.)
Q2 — Correct sequence (8 marks, 1 per step)
Correct order: 2, 7, 5, 4, 6, 1, 3, 8
- A large meal is consumed; digested carbohydrate is absorbed into the portal blood, raising blood glucose to 9.1 mmol/L.
- Beta cells in the islets of Langerhans detect the elevated blood glucose concentration directly.
- Insulin is secreted by beta cells into the bloodstream and transported to target tissues.
- Insulin signals the liver to perform glycogenesis — converting excess blood glucose into glycogen stored within liver cells.
- Body cells (muscle, adipose) mobilise GLUT4 glucose transporters to their plasma membranes, increasing glucose uptake from the blood.
- Blood glucose concentration falls back toward ~5 mmol/L as glucose is removed from circulation.
- As blood glucose normalises, beta cells detect the lower concentration and reduce insulin secretion — the response is self-limiting.
- Blood glucose stabilises near the set point; no further homeostatic correction is required.
Q3 — Type 1 vs Type 2 comparison (8 marks)
| Feature | Type 1 | Type 2 |
|---|---|---|
| Primary cause | Autoimmune destruction of beta cells → no insulin produced | Insulin resistance in target cells → inadequate glucose uptake despite insulin present |
| Insulin level | Very low or absent | Initially normal or elevated; may decline as beta cells exhaust over time |
| Pathway step that fails | Step 2 — no insulin secreted (beta cells destroyed) | Step 3 — insulin present but cells do not respond (insulin resistance) |
| Management | Exogenous insulin injections or pump; cannot survive without external insulin | Lifestyle modification, oral medications (e.g. metformin); insulin only in late stages |
Award 1 mark per accurately completed cell (8 cells total).
Q4.1 — Omnipod 5 stimulus-response mapping (3 marks)
(a) CGM sensor = replaces the receptor function (detects the homeostatic variable — blood glucose concentration — and signals when it deviates from the normal range) [1]. (b) The algorithm = replaces the control centre function (processes the glucose reading and determines whether a corrective response is needed and, if so, how large it should be) [1]. (c) The insulin pump = replaces the effector function (delivers insulin as the response, causing cells to take up glucose and the liver to perform glycogenesis, physically lowering blood glucose concentration) [1].
Q4.2 — Hypoglycaemia in Type 1 (3 marks)
In a non-diabetic, a fall to 4.4 mmol/L triggers alpha cells to secrete glucagon, which signals the liver to perform glycogenolysis, releasing stored glucose into the blood and raising blood glucose back toward the set point [1]. A Type 1 diabetic still has functioning alpha cells and can secrete glucagon, but the pump’s suspend function only stops more insulin being delivered — it does not actively add glucose to the blood [1]. Without glucagon injection or dietary carbohydrate to trigger a glucose source, blood glucose may continue to fall or stabilise at a dangerously low level; consuming carbohydrate provides exogenous glucose directly, the fastest way to raise blood glucose back into the normal range [1].