Learning focus
Use precise biological vocabulary, trace control pathways, explain cause-and-effect mechanisms, interpret diagrams and apply ideas to unfamiliar contexts.
Need for control
Glucose is an important respiratory substrate. If blood glucose falls too low, cells, especially brain cells, may not receive enough fuel. If it remains too high, water balance is disturbed and tissues may be damaged. Concentration must therefore be regulated within limits.
Response to high glucose
After a carbohydrate-rich meal, the pancreas detects rising blood glucose and releases insulin. Insulin promotes uptake of glucose by cells and conversion of glucose to glycogen in the liver. These processes lower blood glucose towards the normal level.
Response to low glucose
Between meals or during prolonged exercise, falling blood glucose stimulates the pancreas to release glucagon. Glucagon causes the liver to convert glycogen to glucose and release it into blood. Blood glucose rises towards the set point.
Liver as an effector
The liver stores carbohydrate as glycogen and releases glucose when required. It therefore acts as an effector in the negative-feedback loop. Insulin and glucagon are antagonistic hormones because they have opposite effects.
Practical or data skill
Plot a glucose-concentration curve before and after a meal and identify the periods of insulin and glucagon action. Explain why the curve returns gradually rather than instantly.
Examination tip
Insulin lowers blood glucose; glucagon raises it. Do not confuse glucagon with glycogen.
Review questions and suggested answers
Question 1
Which hormone is released when blood glucose is high?
Suggested answer
Insulin.
Question 2
What does glucagon cause the liver to do?
Suggested answer
Convert glycogen to glucose and release glucose into the blood.
Question 3
Why are insulin and glucagon described as antagonistic?
Suggested answer
They have opposite effects on blood glucose concentration.
