How does the body keep blood glucose concentration within narrow limits?
The principles of homeostasis and negative feedback in maintaining a constant internal environment. The control of blood glucose concentration by insulin and glucagon, including the roles of the liver in glycogenesis, glycogenolysis and gluconeogenesis, the action of insulin through the second messenger model involving adenylate cyclase and cyclic AMP, and the causes and control of types 1 and 2 diabetes mellitus.
A focused answer to the AQA 3.6 dot point on homeostasis and blood glucose. Explains negative feedback, the roles of insulin and glucagon, glycogenesis, glycogenolysis and gluconeogenesis, the second messenger model, and types 1 and 2 diabetes.
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What this dot point is asking
AQA wants you to explain homeostasis and negative feedback, describe how insulin and glucagon control blood glucose (with the liver's three processes), explain the second messenger model of insulin action, and outline types 1 and 2 diabetes.
The answer
Homeostasis and negative feedback
Homeostasis works mainly by negative feedback: a change in a factor is detected by receptors, and the body responds to reverse the change and return the factor to its set point. If the factor rises, the response lowers it; if it falls, the response raises it.
Using separate mechanisms to raise and to lower a factor gives more sensitive control, because each can be adjusted independently.
Control of blood glucose: insulin and glucagon
Blood glucose is controlled by two hormones from the islets of Langerhans in the pancreas:
- Beta cells secrete insulin when blood glucose is too high.
- Alpha cells secrete glucagon when blood glucose is too low.
When blood glucose is too high (after a meal):
- Beta cells detect the rise and secrete insulin.
- Insulin binds to receptors on liver and muscle cells, increasing the number of glucose transporter proteins in their membranes, so they take up more glucose.
- Insulin activates enzymes that carry out glycogenesis (conversion of glucose to glycogen for storage).
- It also increases the rate of respiration and conversion of glucose to fat.
These effects lower blood glucose to normal.
When blood glucose is too low (fasting or exercise):
- Alpha cells detect the fall and secrete glucagon (and the adrenal glands release adrenaline in stress).
- Glucagon binds to receptors on liver cells and activates enzymes for glycogenolysis (breakdown of glycogen to glucose).
- It promotes gluconeogenesis (production of glucose from non-carbohydrate sources such as amino acids and glycerol).
These effects raise blood glucose to normal.
The second messenger model of glucagon and adrenaline
Glucagon (and adrenaline) act on the liver through the second messenger model, which you should be able to describe:
- The hormone (the first messenger) binds to a receptor on the liver cell-surface membrane.
- This activates the enzyme adenylate cyclase inside the membrane.
- Adenylate cyclase converts ATP into cyclic AMP (cAMP), the second messenger.
- cAMP activates a cascade of enzymes (protein kinases) inside the cell.
- These enzymes catalyse glycogenolysis, releasing glucose into the blood.
Diabetes mellitus
Diabetes is a condition in which the body cannot control blood glucose concentration properly.
Type 1 diabetes
- Cause: the beta cells cannot produce insulin (usually because the immune system destroys them); it often begins in childhood.
- Effect: blood glucose rises uncontrollably after meals (hyperglycaemia); glucose appears in the urine.
- Control: regular insulin injections (matched to diet and activity) and monitoring of blood glucose.
Type 2 diabetes
- Cause: the receptors on target cells lose their responsiveness to insulin (insulin resistance), or insufficient insulin is made; linked to obesity, poor diet, age and inactivity.
- Effect: blood glucose stays high because cells do not take up glucose properly.
- Control: a low-sugar diet, weight loss and exercise, sometimes with drugs that increase insulin sensitivity or secretion.
Examples in context
Example 1. Continuous glucose monitors and insulin pumps. People with type 1 diabetes increasingly use continuous glucose monitors linked to insulin pumps that deliver insulin automatically. The system mimics the body's negative feedback loop: rising glucose triggers insulin delivery, lowering it again. This technology directly applies the principle of detecting a change and responding to reverse it.
Example 2. The glucose tolerance test. To diagnose diabetes, a patient drinks a glucose solution and blood glucose is measured over two hours. In a healthy person insulin returns glucose to normal within about two hours; in a diabetic person it stays high. This clinical test reveals whether the insulin-driven negative feedback control of blood glucose is working.
Try this
Q1. Explain what is meant by negative feedback in homeostasis. [2 marks]
- Cue. A change in a factor is detected and a response is triggered that reverses the change, returning the factor to its set point.
Q2. Describe the roles of insulin and glucagon in controlling blood glucose. [4 marks]
- Cue. Insulin (beta cells) lowers glucose by increasing uptake and glycogenesis; glucagon (alpha cells) raises glucose by glycogenolysis and gluconeogenesis in the liver.
Q3. Describe the second messenger model by which glucagon raises blood glucose. [4 marks]
- Cue. Glucagon binds a receptor on the liver cell membrane; activates adenylate cyclase; this converts ATP to cyclic AMP; cAMP activates enzymes (protein kinases) that catalyse glycogenolysis to release glucose.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 AQA4 marksDescribe how a rise in blood glucose concentration is returned to normal.Show worked answer →
A 4-mark answer needs the detection, the hormone, the target cells and the effects that lower glucose.
- The rise in blood glucose is detected by beta cells in the islets of Langerhans in the pancreas, which secrete insulin.
- Insulin binds to receptors on liver and muscle cells, increasing the number of glucose transporter (channel) proteins in their membranes, so cells take up more glucose.
- Insulin activates enzymes that convert glucose to glycogen (glycogenesis) in the liver and muscle.
- It also increases the use of glucose in respiration and its conversion to fat. These effects lower blood glucose back to normal, an example of negative feedback.
Markers reward detection by beta cells, secretion of insulin, increased glucose uptake, and glycogenesis lowering glucose.
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