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].