Regulation of blood glucose: WACE Year 12 Human Biology Unit 3
“Explain how blood glucose concentration is regulated by negative feedback, including the roles of the pancreas, insulin and glucagon and the liver”
Blood glucose is held near its set point by two antagonistic hormones from the islets of Langerhans in the pancreas. When glucose is high, beta cells release insulin, which makes the liver and body cells take up glucose and store it as glycogen, lowering glucose. When glucose is low, alpha cells release glucagon, which makes the liver break glycogen down into glucose, raising it. Both arms are negative feedback.
What this dot point is asking
WACE wants you to apply the negative-feedback model to blood glucose in full detail, naming the gland, the two cell types, the two hormones, the target organ and the storage molecule. Glucose must be regulated because cells need a steady supply for respiration, while too much glucose damages tissues and too little starves the brain.
Why blood glucose must be controlled
Glucose is the main fuel for cellular respiration. After a meal, glucose floods in from digestion; between meals and during exercise, glucose is used up. Without regulation the concentration would swing wildly. The body keeps it within a narrow tolerance range so that cells always have fuel and the blood does not become dangerously concentrated.
The pancreas: detector and effector gland
The pancreas contains clusters of endocrine cells called the islets of Langerhans. These cells both monitor blood glucose and respond to it, so the pancreas is both the receptor and the hormone-secreting effector in this loop.
- Beta cells detect a rise in blood glucose and secrete insulin.
- Alpha cells detect a fall in blood glucose and secrete glucagon.
Antagonistic hormones have opposite effects on the same variable, so the body can move it in either direction. Insulin and glucagon are antagonistic: insulin lowers blood glucose while glucagon raises it. Controlling a variable with two opposing hormones gives precise, two-way regulation around the set point.
Lowering blood glucose: insulin
When blood glucose rises above the set point (for example after eating), beta cells secrete insulin into the blood. Insulin acts mainly on the liver and muscle cells, making them take up glucose from the blood and convert it to glycogen for storage (glycogenesis). It also makes body cells take up and use more glucose. As glucose moves out of the blood and into storage, the concentration falls back toward the set point, which switches off insulin secretion. This is negative feedback because the response (lowering glucose) opposes the stimulus (raised glucose).
Raising blood glucose: glucagon
When blood glucose falls below the set point (for example between meals or during exercise), alpha cells secrete glucagon. Glucagon acts on the liver, making it break stored glycogen back down into glucose (glycogenolysis) and release it into the blood, and promoting the production of new glucose from other molecules. Blood glucose rises back toward the set point, which switches off glucagon secretion.
Insulin and glucagon both act mainly on the liver, but in opposite directions: insulin makes the liver store glucose as glycogen, while glucagon makes the liver release glucose by breaking glycogen down. Naming the liver as the target organ and glycogen as the storage molecule is what turns a vague answer into a full-mark one.
Response to a rise and then a fall
Rise: after a meal, blood glucose rises above the set point (stimulus). Beta cells in the islets of Langerhans detect the rise and secrete insulin (response). The liver and muscle cells take up glucose and store it as glycogen, and body cells use more glucose (effectors). Blood glucose falls toward the set point (negative feedback). Fall: hours later, blood glucose falls below the set point (stimulus). Alpha cells detect the fall and secrete glucagon (response). The liver breaks glycogen down into glucose and releases it (effector). Blood glucose rises toward the set point (negative feedback). A marker wants both arms, with the correct cell, hormone, target organ and effect named in each.
How this maps to the exam
Expect to be given a stimulus (a meal, fasting, exercise) and asked to write the hormonal negative-feedback loop, or to read a graph of blood glucose over time and explain the hormonal responses at the peaks and troughs. This loop is the foundation for the diabetes content in the disruption-of-homeostasis topic, where the loop fails.
The most common error is swapping the two hormones or their effects. Anchor it: insulin is released when glucose is high and stores glucose as glycogen (in goes glucose); glucagon is released when glucose is low and releases glucose from glycogen. A useful cue is that glucaGON makes glucose GO into the blood. Also avoid saying a hormone tells the liver what to do; state that the hormone binds to the liver cells and changes their activity.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style7 marksAfter a carbohydrate-rich meal, blood glucose concentration rises sharply. Describe the negative feedback response that returns blood glucose toward the set point, naming the gland, the cells, the hormone, the target organ and the storage molecule involved.Show worked answer →
A 7 mark describe response needs the full pathway in order, with each named component.
- Stimulus
- Blood glucose rises above the set point as glucose is absorbed from the gut.
- Receptor and gland
- Beta cells in the islets of Langerhans of the pancreas detect the rise.
- Hormone
- Beta cells secrete insulin into the blood.
- Target organ and effect
- Insulin acts mainly on the liver and on muscle and body cells, increasing their uptake of glucose and stimulating the liver and muscle to convert glucose to glycogen (glycogenesis) for storage. Body cells also increase glucose use.
- Return and feedback
- As glucose moves out of the blood, the concentration falls back toward the set point, which switches off insulin secretion. The response opposes the stimulus, so it is negative feedback.
Markers reward the correct gland, cell type, hormone, target organ and storage molecule, plus an explicit statement that the response opposes the change.
WACE 2024-style4 marksExplain why insulin and glucagon are described as antagonistic hormones and why two-way control of blood glucose is necessary.Show worked answer →
A 4 mark explain answer needs the meaning of antagonistic plus the reason two-way control matters.
Antagonistic. Antagonistic hormones have opposite effects on the same variable. Insulin lowers blood glucose by promoting glucose uptake and glycogen storage, while glucagon raises blood glucose by promoting the breakdown of glycogen to glucose in the liver.
Why two-way control is needed. Blood glucose can move in either direction: it rises after meals and falls between meals or during exercise. A single hormone could only push the variable one way, so two opposing hormones allow the body to correct both rises and falls and hold glucose precisely around the set point.
Markers reward the opposite-effects definition and the point that glucose must be corrected in both directions.